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AN OVERVIEW OF RICE HUSK APPLICATIONS
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AND MODIFICATION TECHNIQUES IN
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WASTEWATER TREATMENT
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Insert Running head: …………………………………………….
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Sadon, F.N. (please insert full name), Ahmmed Saadi Ibrahem*,
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and Kamariah Nor Ismail
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Chemical Engineering Department, Universiti Teknologi MARA (UiTM)
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40450 Shah Alam, Selangor – Malaysia. E-mail: ahmadsaadi1@yahoo.com
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* Corresponding author
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Abstract
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Activated carbon is the most common adsorbent used for adsorption process
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of removing various types of organic and inorganic materials. Thus, it has
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been revealed that, rice husk which is a low–value agricultural by-product
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can be made into adsorbent materials which are utilized to adsorb water
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pollutants mainly heavy metals and textile dyes from wastewater. The heavy
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metals being studied are: Fe, Mn, Zn, Cu, Cd, Pb, and Cr(VI). The types of
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textile dyes being studied are: Direct F. Scarlet, Everdirect Orange–3GL,
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Direct Blue–67, Direct Red–31, Direct Orange–26, and Crystal Violet.
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Moreover, other removal studies include removal of surfactants, phenol, and
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paraquat. In this overview, an extensive list of previous and current
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literature of rice husk studies and their modification techniques were
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compiled to provide information on rice husk as potential adsorbents in
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wastewater treatment.
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Keywords: Rice husk, adsorption, heavy metals, dyes, adsorbents
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Introduction
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Industrialization activities for nation development contribute to global
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environmental deterioration as these activities caused depletion and degradation
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of natural resources and biodiversity. In addition, these industrial activities
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indirectly overload water body with thousands of water pollutant and
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subsequently polluting the environment. Major water pollutants are heavy metals,
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textile dyes as well as other organic and inorganic compounds.
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Heavy metals such as Iron (Fe), Manganese (Mn), Zinc (Zn), Copper (Cu),
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Chromium (Cr), Cadmium (Cd), Mercury (Hg) as well as Lead (Pb) have some
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common characteristics which are susceptible to biological degradation, do not
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degrade into harmless end products, toxic to many life forms, and accumulate in
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living tissues which then becoming concentrated throughout the food chain.
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On the other hand, significant losses occurred during the manufacture and
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processing of coloured dyes also contribute to serious water pollution problems as
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these lost chemicals are illegally discharged to water body such as rivers. This is
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because; some dyes and their degradation products such as aromatic amines are
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highly carcinogenic and toxic in nature (Mohamed, 2004). For instance, Abdel
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Wahab et al. in 2005 pointed out that, Direct Red 23 (DR-23) doses will not allow
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sunlight to pass through and thereby affects the photosynthesis of aquatic plants.
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Water contaminations by dyes are usually reported from industrial effluents such
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as textile, tannery, paper, soap, cosmetics, polishes, as well as wax.
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Apart from that, matter became worst when the presence of anionic and
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non – ionic surfactants in sewage, industrial effluents as well as raw domestic
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wastewater poses the same environmental problems as heavy metal and textile
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dyes did. Accordingly, it has been reported by Adachi et al. in 2001 that, the
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toxicity of linear sodium alkyl benzene sulfonate (LABS) to aquatic organisms is
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of concern when concentrations exceed 0.1 mg/L. Besides that, recent evidence
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also proved that, alkyl phenol ethoxylates (APE) breakdown products which are
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weakly estrogenic has intensified concern over environmental and human health
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effects. It also has been noted that, surfactants are used in high volumes in
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detergents, personal care and household cleaning products. Besides that,
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surfactants are also utilized by oil, textile, food and mining industries.
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The characteristics of other identified water pollutants such as phenol and
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paraquat also similar with previously described water pollutant, in which they are
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toxic due to their carcinogenicity. It has been revealed that, phenol is considered
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as notorious contaminant in water environment as it is designated as priority
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pollutant by the US Environmental Protection Agency (USEPA) (Nayak and
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Singh, 2007). According to Busca et al. in 2008, the most important sources of
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phenol pollution are the wastewater from the iron–steel, coke, petroleum,
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pesticide, paint, solvent, pharmaceutics, wood preserving chemicals, paper and
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pulp industries. On the other hand, paraquat is extensively utilized as herbicide,
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primarily for wood and grass. It has been identified that, even a minute amount of
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paraquat can cause fatal or severe physiological damage. This hazardous
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compound is found in agricultural wastewater and the blood of poisoned human
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bodies.
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Accordingly, examples of the critical issues in history caused by
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wastewater problem particularly heavy metal contamination were Itai–Itai and
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Minamata diseases. In relation to Malaysia scenario, most of rivers and seas are
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contaminated with wastewater from agro-based and manufacturing industries, as
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reported by Department of Environment Malaysia in 2011.
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In Malaysia, some regulations have been promulgated under the
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Environmental Quality Act (EQA) 1974 for environmental control of effluent
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discharge. The most important regulation is known as Environmental Quality
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(Industrial Effluents) Regulations, 2009, amendment on EQA 1974 which was
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gazetted on 12 October 2009 by the Minister of Natural Resources and the
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Environment, Datuk Douglas Uggah Embas. The previous Environmental Quality
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(Industrial Effluent) Regulations, 1979 are annulled.
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It is therefore an objective to remove such water pollutants from
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wastewater in order to comply with environmental regulations as well as for the
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sake of environmental preservation and human health and safety as well. Thus,
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various treatment techniques have been employed to achieve such objective such
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as filtration, flocculation, chemical precipitation, ion exchange, membrane
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separation, activated sludge, trickling filter and adsorption (Tarley and Arruda,
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2003). For instance, Chang and Page in 1982 pointed out that, primary treatment
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of surfactant results in low removal rates (0 to 26%). Specifically, current
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methods for removing phenolic from wastewater include microbial degradation,
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adsorption on activated carbon, chemical oxidation, deep–well injection,
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incineration, solvent extraction and irradiation. Among such treatment methods,
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adsorption is determined to be the most desirable and promising treatment as well
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as highly effective technology to remove water pollutants from wastewater due to
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its simple design and produce sludge free environment. In terms of economical
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aspects, it consumes low initial investment and requires small installation space.
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Activated carbon is the most common adsorbent used for adsorption process of
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removing various types of heavy metals. Its extended surface area and micro-
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porous structure which then provides high adsorption capacity have been made
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activated carbon is the most efficient adsorbent in this particular process.
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However, it has been identified that, activated carbon suffers from major
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drawbacks which are not practical in small and medium industries as well as
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expensive materials. As a result, the manufacturing cost of the process will be
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gradually high. Other than that, it has been identified that, 10–15% loss occurred
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during regeneration of activated carbon adsorbent (Hashem, 2007).
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In view of that situation, it is a need to seek for other low–cost and natural
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materials which have strong capability to adsorb heavy metals in wastewater
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streams as mentioned by Tengerdy and Szakacs in 2007. Their sorption
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characteristics are derived from their constituent polymers which are cellulose,
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hemicelluloses, as well as lignin. An adsorbent can be termed as a low-cost
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adsorbent if it requires little processing, is abundant in nature, or is a by-product
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or waste material from another industry.
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Generally, cellulosic surface of plant biomass becomes partially negative
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charged when immersed in water, and therefore possess columbic interaction with
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cationic species (i.e. heavy metals) in water (McKay et al., 1987). The high
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binding capacities of cationic species on the adsorbents are mainly the results of
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columbic interactions (Weixing et al., 1998).
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Thus, due to their strong properties and characteristics of sorption, a range
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of agricultural products have been used as adsorbent for removal of various
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organic and inorganic compounds such as heavy metals and dyes from aqueous
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solution. The removal of water pollutants by agricultural waste or by–products
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have been extensively reviewed by Khan et al. (2004), Kumar (2006), as well as
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Mtui (2009). For instance, Gong et al. in 2009 indicated that, tartaric acid
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esterified wheat straw (EWS) can be used to adsorb basic dyes of methylene blue
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(MB) as well as crystal violet (CV). They have characterized that, the maximum
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sorption capacity of EWS for MB and CV was 129.87 and 112.36 mg/g,
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respectively. The equilibriums of dye sorption were respectively reached about 13
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and 18 h for MB and CV. On top of that, a study on removal of Cu (II), Fe (III)
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and Pb (II) ions from mono-component simulated waste effluent by adsorption on
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coconut husk was also done by Abdulrasaq and Basiru in 2010. They have been
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revealed that, the adsorption of Pb (II) was found to be maximum (94%±3.2) at
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pH 5, temperature of 100°C, metal ion concentration of 30 ppm and contact time
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of 30 min. The adsorption of Cu (II) and Fe (III) were at maximum (92%±2.8 and
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94%±1.4) at pH range of 5-7, metal ion concentration of 50 ppm, temperature of
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50°C but at different times of 30 and 90 min respectively.
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Compared to conventional adsorbent, this type of natural adsorbents pose
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a lot of advantages such as inexpensive, effective, readily and local available,
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technical feasibility, as well as engineering applicability. On top of that, since
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they are low–cost adsorbents, the utilization of agricultural residues can
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simultaneously improve profitability of particular industry, and consequently
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conserve natural environment. Moreover, the usage of natural adsorbents can also
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contribute to the worldwide sustainable development.
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Accordingly, one of the most potential waste materials in the world as
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reported in the literature is rice husk, due to its high availability (Chuah et al.,
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2005). It has been revealed that, rice is the second largest produced cereal in the
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world. Production is geographically concentrated in Western and Eastern Asia.
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Asia is the biggest rice producer, accounting for 90% of the world's production
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and consumption of rice. On the other hand, rice is also a strategically important
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industry in Malaysia, due to the suitability of temperature regime and rainfall
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distribution in the country. From the statistical data of the Malaysia Ministry of
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Agriculture and Agro-Based Industries, 2011, annual production of rice in
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Malaysia was observed to be increased significantly between 2.35 to 2.51 x 106
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Mt/year from 2008 to 2009. The rice production in Malaysia was then reported to
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be increased year by year from 2009 to 2011. Besides that, according to Mansaray
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and Ghaly in 1998, rice husk is responsible for approximately 20-25% of the total
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grain weight depending on the variety. Therefore, it can be approximated that,
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5.33 x 105 and 5.09 x 106 Mt of rice husks were produced in 2011 for Malaysia
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and 2009 for all over the world, respectively.
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Therefore, the potential of rice husk application for wastewater treatment
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should be highlighted due to its special and exceptional features as adsorbent.
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Specifically, rice husk has granular structure as well as insoluble in water.
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Moreover, it also has remarkable chemical stability and high mechanical strength.
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The most significant characteristics of rice husk is regarding to its local
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availability at almost no cost (El - Azab, 1992). Hence, there is no need to
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regenerated spent rice husk due to its low production costs. Moreover, rice husk is
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also tough and woody in nature, as well as poses abrasive inherent resistance
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behaviour. Other than that, rice husk also rich of silica–cellulose structural
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arrangement, which make it as a good adsorbent, compared to other agricultural
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wastes. In view of that situation, an overview of some of the literature on rice
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husk as adsorbent for water pollutant removal from wastewater is presented as
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follows. The report will focus on nature of adsorption studies for heavy metals,
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dyes, surfactants, and organic compounds removal. It also covers the information
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on modification techniques of rice husk that are commonly employed by previous
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researchers.
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Anatomy and Properties of Rice Husk
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Rice husk has the following dimensions: 8–10 mm long, 2.0–2.5 mm wide, and
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0.1–0.15 mm thick (Daifullah et al., 2002). Furthermore, chemical properties of
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rice husk are studied by Rahman et al. in 1993 and 1997 and it is tabulated in
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Table 1. Moreover, Table 2 shows the reported values of rice husk physical
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properties.
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Rice Husk as Adsorbent for Heavy Metal Removal
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Rice husk used as adsorbent in removing heavy metals has been intensively
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studied and reported. Among the metal studied are: Fe, Mn, Zn, Cu, Cd, and Pb.
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The removal efficiencies for each metal ion from the selected previous works are
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summarized in Table 3.
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Rice Husk as Adsorbent for Textile Dyes Removal
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Rice husk used as adsorbent in removing variety types of dyes has been
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intensively studied and reported. Among the dyes studied are: Direct F. Scarlet,
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Indigo Carmine, Congo Red, Everdirect Orange–3GL, Direct Blue–67, Direct
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Red–31, Direct Orange–26 as well as Crystal Violet dyes. The maximum of
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sorption capacity for each type of dyes from the selected previous works are
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summarized in Table 4.
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Rice Husk as Adsorbent for Other Water Pollutants Removal
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Apart from removal of heavy metals and dyes, the study of rice husk capability to
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adsorb other types of water pollutants have also been investigated by previous
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researches. For instance, Mahvi et al. in 2004 have investigated the removal of
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phenol in aqueous systems by using rice husk as adsorbents. On the other hand,
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Hosseinnia et al. in 2007 have studied the adsorption of anionic surfactants
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(Linear sodium alkyl benzene sulfonate–LABS 50% and Alkyl ether sulphate–
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AES 70%) as well as non–ionic surfactants (Nonyl phenol ethoxylate–NPE). The
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studies were continued by Hsu and Pan in 2007, where they carried out
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experiment to adsorb paraquat by using also rice husk as derived material for the
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adsorbents. Furthermore, a study was also done by Daffalla et al. in 2010 on the
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characterization of adsorbent developed from rice husk by investigating the effect
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of surface functional group on phenol adsorption. The maximum of sorption
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capacity of other types of water pollutants from the selected previous works are
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summarized in Table 5.
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Experimental Modes of Previous Studies
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Accordingly, one of the most important factor that distinguish the studies of rice
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husk as adsorbent for wastewater treatment among researchers is the mode of the
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studies, whether in batch or continuous mode.
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Batch Studies Using Rice Husk as Adsorbent
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Batch studies using rice husk as adsorbent have been chosen by most of
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researchers. In particular, the studies were focusing on adsorption and desorption
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studies, in which kinetics, mechanisms, isotherms, thermodynamics and
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physiochemical of such studies are comprehensively investigated. Apart from
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that, some of the studies also emphasizing on the effects of operating parameters
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towards adsorption efficiency of rice husk such as adsorbent amount, contact
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time, initial pH as well as initial water pollutant concentration.
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Adsorption Kinetics
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Theoretically, the analysis of adsorption kinetics can be modelled by
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various kinetics model principally first–order, pseudo–second–order and Elovich
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kinetic models. For instance, Lakshmi et al. in 2008 have been investigated that,
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the sorption can be approximated more appropriately by the pseudo–second–order
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kinetic model than the first–order kinetic model for the adsorption of Indigo
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Carmine (IC) dye by rice husk ash. On the other hand, the studies made by Ong
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and his co–workers in 2009 also observed that, the experimental data of Congo
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Red dye removal by ethylenediamine–modified rice hulls showed a better
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compliance with the pseudo–second–order model than the pseudo–first–order
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model, suggesting that the rate limiting step may be chemical sorption or
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chemisorption involving valency forces through sharing or exchange of electron
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between sorbent and sorbate. Recently, the same results also observed by Safa and
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Bhatti in 2011. They successfully identified that, both pseudo–second–order and
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Elovich kinetic model illustrated good fitness to all types of biomasses showing
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chemisorptions nature of biosorptions for removing Everdirect Orange–3GL and
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Direct Blue–67 dyes. Moreover, Chakraborty et al. in 2011 revealed that, the
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study of Crystal Violet adsorption from aqueous solution onto NaOH–modified
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rice husk followed the pseudo–second–order kinetic model. It has been noted that,
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pseudo–second–order is worked well for textile dyes removal from wastewater. In
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the case of other removal studies, Mahvi et al. in 2004 pointed out that, first order
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rate expression showed a better compliance for adsorption of phenol using raw
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rice husk and rice husk ash. These results are in accordance with the study made
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by Abdel Wahab et al. in 2005 where the best model to express the adsorption
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activity of citric acid–modified rice husk is first order rate expression. The
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significance of adsorption kinetic studies is such that, the rate determining step of
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the adsorption systems can be determined, whether chemisorptions (Ong et al.
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(2009), Safa and Bhatti (2011) or particle diffusions (Abdel Wahab et al., 2005).
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Adsorption Isotherms
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The equilibrium between the concentration of a water pollutant in the
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wastewater and its concentration on the rice husk adsorbent resembles somewhat
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the equilibrium solubility of a gas in a liquid. These data are plotted as adsorption
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isotherms. The concentration in the rice husk adsorbent is expressed as q, kg
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adsorbate (solute)/kg adsorbent (solid), and in the wastewater as c, kg
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adsorbate/m3 fluid. The Freundlich isotherm equation, which is empirical, often
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approximates data for many physical adsorption systems and is particular useful
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liquids:
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q  Kc n
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where K and n are constants and must be determined experimentally. If a log–log
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plot is made for q versus c, the slope is dimensionless exponent n. The dimensions
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of K depend on the value of n.
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The Langmuir isotherm on the other hand, has a theoretical basis and is
given by the following, where qo and K are empirical constants:
q
q0 c
K c
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where qo is kg adsorbate/kg solid and K is kg/m3. The equation was derived by
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assuming that there are only a fixed number of active sites available for
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adsorption, that only a monolayer is formed, and the adsorption is reversible and
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reaches an equilibrium condition. By plotting 1/q versus 1/c, the slope is K/qo, and
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the intercept is 1/ qo.
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Therefore, it has been noted that, adsorption isotherms are of fundamental
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importance for the description of how molecules of water pollutant interact with
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the rice husk surface. Accordingly, Tables 6 and 7 show the reported Langmuir
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and Freundlich isotherms parameters from previous studies of using rice husk as
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adsorbent for wastewater treatment. It has been revealed that, the Langmuir model
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fitted most of the experimental results more closely than did the Freundlich
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model. This is in accordance with the results obtained by Hsu and Pan in 2007,
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where they identified that, the Langmuir model fitted the experimental results
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more closely than did the Freundlich model. The same outcome also reported by
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Aluyor et al. (2009), Wongjunda and Saueprasearsit (2010).
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One advantage of the Langmuir equation in its linear form is that the
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maximum adsorption can be calculated from the regression. This parameter and
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the equilibrium water pollutant concentrations are useful in describing the
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adsorbent capacity. According to Abdel Wahab et al. in 2005, the high fitness of
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the Langmuir model for the adsorption system of rice husk indicates the
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monolayer coverage of water pollutant on the outer surface of rice husk, in which
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the adsorption occurs uniformly on the active part of the surface. This is
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represented by the value of maximum sorption capacity at equilibrium. Other than
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that, Langmuir isotherms also provide information on adsorption energy by the
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value of Langmuir constants, K.
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Besides that, modified rice husk is also observed to have higher adsorption
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capacity than the unmodified rice husk in the removal of water pollutant from
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wastewater. This was reported by Ong et al. in 2009 where the maximum
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adsorption capacity for Congo Red dye was enhanced by 2-fold as compared to
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unmodified rice husk.
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On the other hand, for Freundlich isotherms, the higher value of K (the
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Freundlich constant) showed easy uptake of water pollutant from wastewater.
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Furthermore, higher the value of 1/n, the higher will be the affinity between the
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water pollutant and rice husk, and the heterogeneity of the adsorbent sites. The 1/n
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value indicates the relative distribution of energy sites and depends on the nature
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and strength of the adsorption process; for example, 1/n = 0.90 refers to the fact
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that 90% of the active adsorption sites have equal energy level (Lakshmi et al.,
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2008).
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Adsorption Thermodynamics
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Thermodynamic behaviour of adsorption of water pollutant from
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wastewater on rice husk surface was evaluated by the thermodynamic parameters–
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Gibbs free energy change ( G ), enthalpy ( H  ) and entropy ( S  ). These
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parameters were calculated using the following equations (Anirudhan and
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Radhakrishnan, 2008):
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G   RT ln KC
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KC 
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G  H   TS 
Ca
Ce
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where Ca is an equilibrium solute concentration on the adsorbent (mg/L), Ce is
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an equilibrium solute concentration in wastewater (mg/L), R is the gas constant
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(8.314 J/molK) and T is the absolute temperature (K). A plot of G versus
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T will be linear with the slope and intercept giving the values of H  and S  .
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In view of that situation, it can be concluded that, the negative values of
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ΔG° indicate the feasibility and spontaneity of the adsorption process, as reported
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by Lakshmi et al. (2008), Chakraborty et al. (2011) and Safa and Bhatti (2011).
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According to Chakraborty et al. in 2011, increase in value of ΔG° with increase in
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temperature suggests that lower temperature makes the adsorption easier.
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Moreover, Safa and Bhatti in 2011 identified that, adsorption of direct dyes were
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spontaneous at high temperature. However, at lower temperature, the adsorption
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processes were observed to be not spontaneous due to positive ΔG° values.
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Besides that, Lakshmi et al. in 2008 also revealed that, the adsorption process in
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the solid liquid system is a combination of two processes: (a) the desorption of the
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molecules of solvent (water) previously adsorbed, and (b) the adsorption of
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adsorbate species. The Indigo Carmine (IC) dye molecules have to displace more
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than one water molecule for their adsorption and this result in the endothermicity
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of the adsorption process. The same results were obtained by Safa and Bhatti in
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2011, where they concluded that, the positive value of ΔH° indicated that the
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biosorption of Everdirect Orange-3GL and Direct Blue-67 on the rice husk was
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endothermic in nature. In contrast, Chakraborty and his co–workers in 2011 found
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that, their adsorption phenomenon is exothermic in nature due to negative value of
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ΔH°. Moreover, the positive value of ΔS° as studied by Lakshmi et al. (2008) and
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Safa and Bhatti (2011) showed the increase in randomness at the solid/liquid
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interface and reflect some structural changes in the adsorbate and adsorbent as
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well as a good affinity of biomass towards water pollutant in wastewater. Also,
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positive ΔS° value corresponds to an increase in the degree of freedom of the
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adsorbed species.
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Effect of Adsorbent Amount
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The investigation of the effect of adsorbent amount on adsorption activity
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of rice husk is also part of batch studies, as was done by Mahvi et al. in 2004.
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They indicated that, the removal efficiency of phenol by rice husk and rice husk
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ash increases up to optimum dosage beyond which the removal efficiency is
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negligible. Besides that, Abdel Wahab et al. in 2005 also identified that, the
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increase in the citric acid–modified rice husk concentration resulted in an increase
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of Direct F. Scarlet dye removal capacity, which then may be attributed to the
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increase of biomass of rice husk which gives more surface area for adsorption of
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the dye molecule on the surface. Other than that, Lakshmi et al. in 2008 have been
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investigated that, the Indigo Carmine (IC) dye removal onto rice husk ash
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increases up to a certain limit and then it remains almost constant. Optimum
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adsorbent dosage was found to be 10g/L. The results obtained by Wongjunda and
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Saueprasearsit in 2010 also demonstrated similar observation as previous studies
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did. They have reported that, increase the adsorbent dose (rice husk ash and
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sodium hydroxide–modified rice husk ash) resulted in the increase amount of
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adsorbed Cr (VI). Recently, the results obtained by Chakraborty et al. in 2011 also
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in accordance with previous studies. They observed that, the adsorption efficiency
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of crystal violet by sodium hydroxide–modified rice husk increased from 96.78%
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to 98.17% as the adsorbent dose increased from 0.5 to 1 g. Safa and Bhatti in
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2011 however noted that, adsorption efficiency of Direct Red–31 and Direct
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Orange–26 textile dyes removal from aqueous solution by rice husk decreases
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with increase in the adsorbent dose. Hence, it can be concluded that, increase in
379
the adsorbent dose resulted increase in the amount of adsorbed water pollutant due
380
to the increase in the number of available adsorption sites. However, the
381
adsorption density (amount adsorbed per unit mass) will be decreased when
382
adsorbent dose increased. This is due to unsaturation of adsorption sites through
17
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the adsorption process. Another reason may be due to the particle interaction, such
384
as aggregation, resulting from high adsorbent dose. Such aggregation would lead
385
to decrease in total surface area of the adsorbent and an increase in diffusion path
386
length.
387
Effect of Contact Time
388
Other than that, the effect of contact time on the adsorption efficiency of
389
rice husk is also comprehensively investigated by researchers. This is because; the
390
contact time is the most crucial parameter in batch studies. Thus, Hosseinnia et al.
391
in 2007 noted that, there was no great difference in the adsorption values of Nonyl
392
phenol ethoxylate (NPE) with the time changes. However, desorption was
393
observed after 5 h. On top of that, Ong et al. in 2009 pointed out that, initial
394
adsorption of Congo Red dye by ethylenediamine–modified rice hulls was rapid
395
in the first 60 min followed by a slower process. The same observation was then
396
demonstrated by Wongjunda and Saueprasearsit in 2010. They identified that, the
397
rate of Cr(VI) adsorption is found to be gradually increase in the period time of 0–
398
180 min and thereafter, the removal of Cr(VI) ions is almost constant. Through
399
the studies made by them, it has been revealed that, during the initial stage of
400
adsorption, a large number of vacant surface sites are available for adsorption.
401
After a lapse of some time, the remaining vacant surface sites have difficulty in
402
becoming occupied due to repulsive forces between the adsorbate molecules on
403
the solid surface and in the bulk phase. Besides that, another good reason is such
404
that, the rice husk surface is already saturated with water pollutant solute during
405
the initial stage of adsorption. On the other hand, in parallel to determination of
18
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optimum contact time, this study can also provide information on the equilibrium
407
time of the adsorption processes. Theoretically, equilibrium time is the time when
408
the equilibrium occurred between the water pollutant in solution of wastewater
409
and in the adsorbed state. Table 8 shows the reported equilibrium time from
410
previous batch studies using rice husk as adsorbent. Accordingly, it is identified
411
that, modified rice husk surface would require less equilibrium compared to rice
412
husk without modification treatment.
413
414
Effect of pH
415
It is known that, the adsorption process is dependent on the pH of the
416
solution. Mahvi et al. in 2004 revealed that, the adsorbed amount of phenol by
417
rice husk and rice husk ash decreases with increasing the pH value. This is
418
because, phenol which is a weak acid (pKa = 10), will be adsorbed to a lesser
419
extent at higher pH values due to the repulsive force prevailing at higher pH
420
value. Besides that, Hosseinnia et al. in 2007 also indicates that, adsorption values
421
increase in lower pH. This is true as the results revealed that, 93% of a 100-mL
422
solution containing 1 mg of LABS at pH 2 was adsorbed by 2 g of husks. On the
423
other hand, the adsorption value was less than 11% at pH 6 in the same condition.
424
This is due to the weak basicity of LABS; its acidic form (R-C6H4-SO3H) is
425
predominant in acidic solutions (i.e. in the presence of H3O+) and better adsorbed
426
by rice husk. On the other hand, pH changes were of no great importance in the
427
adsorption amounts for NPE removal because of the nonionic nature of NPE.
428
Lakshmi et al. in 2008 identified that, the Indigo Carmine dye removal by rice
19
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husk ash is maximum and constant for pH greater than or equal to 4.0. The same
430
results also obtained by Wongjunda and Saueprasearsit in 2010. They reported
431
that the chromium (VI) adsorption by rice husk ash and sodium hydroxide–
432
modified rice husk ash decreased with an increase of pH. The maximum value of
433
chromium (VI) retention occurred approximately at pH 2. At lower pH, the
434
adsorbent is positively charged due to protonation and dichromate ion exists as
435
anion leading to an electrostatic attraction between them. As pH is increases,
436
deprotonation starts and thereby results in decrease of adsorption capacity. Safa
437
and Bhatti (2011) also identified the maximum adsorption was recorded at lower
438
pH values (less than 3). However, Chakraborty et al. in 2011 observed that, the
439
percentage removal of Crystal Violet (CV) by sodium hydroxide–treated rice husk
440
increases with the increase in pH of the dye solution, appreciably up to pH 7.0.
441
With further increase in pH from 7.0 to 10.0, the percentage CV removal
442
increases but the difference in the percentage increase is not very significant.
443
Generally, it can be said that, adsorption of water pollutants increase in lower pH.
444
445
Effect of Initial Water Pollutant
446
In terms of the effect of initial water pollutants concentration, the
447
adsorption value trend varied among studies. Abdel Wahab et al. in 2005 reported
448
that, the amount of adsorbed dye increased with increase in concentration and
449
remained nearly constant after equilibrium time. Besides that, Hosseinnia et al. in
450
2007 studied that the adsorption values trends varied to the initial concentration of
451
surfactants. The adsorption value was considerably increased in 10 mg/L while
20
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decreased in 100 mg/L for AES solution. On the other hand, the adsorption value
453
was considerably decreased with an increase in the concentration and no
454
adsorption was observed in the 80 mg/L solution of NPE. However, Ong et al. in
455
2009 identified that, the percentage uptake of Congo Red dye by
456
ethylenediamine–modified rice hulls decrease with the increase in Congo Red dye
457
initial concentration. Then, Wongjunda and Saueprasearsit in 2010 have been
458
reported that, the suitable initial Cr (VI) concentration is 10 mg/L for removal by
459
rice husk ash and sodium hydroxide–modified rice husk ash. At higher
460
concentrations, the available sites of adsorption become decreased and hence the
461
percentage adsorption of Cr (VI) decreases.
462
463
Continuous Studies Using Rice Husk as Adsorbent
464
It has been identified that, only a few studies have been undertaken under
465
the continuous flow (column) conditions, compared to batch conditions.
466
Accordingly, studies that have been carried out under continuous mode only
467
reported by Kumar and Bandyopadhyay (2005), Hosseinnia et al. (2007) and Ong
468
et al. (2009). Major themes that are usually studied under continuous system are
469
adsorption column behaviour and design, column regeneration and reuse as well
470
as Bed–Depth–Service–Time (BDST) model.
471
472
Adsorption Column Behaviour
473
Adsorption column behaviour is investigated by plotting breakthrough
474
curve of S-shaped. The breakthrough curves for Cd(II) at bed depth of 10, 20, and
21
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30 cm have been plotted by Kumar and Bandyopadhyay in 2005 to observe the
476
adsorption behaviour by Sodium carbonate treated rice husk. It was observed that,
477
time to achieve breakthrough was increased with the increase of bed depths. It has
478
been noted that, similar result was reported by Ong et al. in 2009 for the
479
adsorption of Congo Red dye by Ethylenediamine–modified rice hulls at the bed
480
depth of 7, 10, and 15 cm. This was attributed to the increase in binding sites on
481
the adsorbent. In addition to that, Ong and his colleagues also studied the effect of
482
influent concentrations (15 to 25 mg/L) on the adsorption characteristics of Congo
483
Red dye. Thus, they have identified that, the percentage removal of dye decreased
484
with increasing dye concentrations whereby a sharper breakthrough curve was
485
obtained at a higher dye concentration.
486
487
Bed–Depth–Service–Time (BDST) model
488
Data collected during laboratory and pilot plant tests serve as the basis for
489
the design of full-scale adsorption columns. A number of mathematical models
490
have been developed for the use in adsorption column design. Among various
491
design approach, bed depth service time (BDST) approach based on Bohrat and
492
Adams equation is widely used. The equation of Bohrat and Adams (1920), which
493
is based on surface reaction rate theory, can be represented as follows:
494
495
C

ln  0  1  ln e KN 0  X /V   1  KC0t
 CB 


496
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with the assumption of e KN 0 ( X / V ) is much greater than unity, above equation can be
498
rearranged for t
499
t
500
C

N0
1
X
ln  0  1
C0V
C0 K  CB 
501
502
where C0 is the initial concentration of solute (mg/L), CB is the desired
503
concentration of solute at breakthrough (mg/L), K is the adsorption rate constant
504
(L/mg h), N 0 is the adsorption capacity (mg/L), X is the bed depth of column
505
(cm), V is the linear flow velocity of feed to bed (cm/h), t is the service time of
506
column under above conditions (h).
507
Besides that, Hutchins (1973) presented a modification of Bohart–Adams
508
equation, which requires only three fixed bed tests to collect the necessary data
509
instead of nine columns. This is called the bed depth service time (BDST)
510
approach. The Bohart–Adams Equation can be expressed as:
511
t  aX  b
512
513
where
514
515
516
a = slope =
N0
C 0V
and
b = intercept = 
C

1
ln  0  1
KC0  CB 
23
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A BDST correlation can be plotted by recording the operating time to
518
reach a certain removal at each bed depth. The slope of the BDST line is equal to
519
the reciprocal velocity of the adsorption zone and the intercept on abscissa is the
520
critical depth defined as the minimum bed depth required obtaining the desired
521
effluent quality at time zero. If the adsorption zone is arbitrarily defined as the
522
adsorbent layer through which the effluent concentration varies from 90 to 10% of
523
the feed concentration, then this zone is defined as the horizontal distance
524
between these two lines in the BDST plot.
525
Accordingly, a plot of BDST at 10% breakthrough have been developed
526
by Kumar and Bandyopadhyay in 2005, and the depth of adsorption zone which
527
also known as mass transfer zone (MTZ) was determined to be 12 cm. The values
528
of K , N 0 , and X were found to be 823.85 cm3/mg h, 0.889 mg/cm3 and 0.544
529
cm, respectively. The value of K and N 0 indicated that, the sodium carbonate
530
treated rice husk is highly efficient for removal of Cd(II) from water environment.
531
On the other hand, Ong et al. in 2009 developed a plot of BDST at 50%
532
breakthrough for their adsorption system. The plot of t against H at 50%
533
breakthrough for Congo Red–Ethylenediamine–modified rice hulls systems is a
534
straight line that, however, does not pass through the origin. The nonconformity
535
of the BDST model may be due to the presence of more than one rate limiting step
536
in the adsorption process and the complex adsorption mechanism as suggested
537
and observed in their batch study.
538
539
Adsorption Column Design
24
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The column design parameters as obtained from the BDST model could
541
then be used for the design of adsorption column in practical use. According to
542
BDST, if the value of a is determined from one flow rate, values for other flow
543
rates can be computed by multiplying the original slope by the ratio of the original
544
and new flow rates, and the change of b value is insignificant with respect to the
545
changing flow rates (Hutchins, 1973). It is also proposed that, data collected at
546
one influent solute concentration can be adjusted by BDST technique and used to
547
design systems for treating other influent solute concentrations. Purposely, the
548
studies of adsorption column design were done for different flow rate, initial
549
concentration, and numbers of stages and its bed depth. These studies have been
550
conducted by Kumar and Bandyopadhyay in 2005. For example, they indicated
551
that, the breakthrough times (corresponding to 0.1 mg/L effluent concentration)
552
were found to be 27.1 and 8.7 h for 5 and 15 mL/min flow rates, respectively. The
553
revised values of a were calculated from flow rate ratio and the values were
554
found to be 0.931 and 0.310 for 5 and 15 ml/min flow rate, respectively. The
555
value of intercept was 0.2667. From these values of a and b , the service times for
556
30 cm column were calculated and these are 27.67 and 9.03 h for 5 and 15
557
ml/min, respectively.
558
559
Column Regeneration and Reuse
560
Besides that, by using data from adsorption breakthrough curve, the
561
studies on column regeneration and reuse can also be done as reported by also
562
Kumar and Bandyopadhyay (2005). It was observed that, at about 13 h the
25
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column was exhausted. Desorption was carried out by 0.01 mol/l HCl solution
564
through the bed in the downward direction at a flow rate of 8.5 mL/min. The
565
application of counter - current operation generally reduces regeneration costs and
566
regenerant volume as well as increases effluent quality. It has also been noted
567
that, the flow rate of eluting solution must be slightly less that the adsorption flow
568
rate so that volume of regenerant is less which helps in easy handling and high in
569
concentration so that economical solute recovery is possible.
570
It is therefore in theoretical aspect suggested that, the studies under
571
continuous flow conditions are more useful compared to batch conditions, due to
572
its suitability to be applied in large scale wastewater treatment. On top of that,
573
continuous flow studies can also provide useful information on the adsorption
574
process before extending it to commercial systems. Therefore, it can be concluded
575
that, the studies under continuous system is much more practical and easily
576
adopted in real industrial environment. Figure 1 portrayed a summary of works
577
reported in terms of themes and nature of studies.
578
579
Modification Techniques of Rice Husk
580
Although rice husk is shown to be an effective adsorbent for a wide range of
581
solutes particularly water pollutants, they actually suffer from at least two major
582
drawbacks, which are low exchange or sorption capacity as well as poor physical
583
stability (i.e. partial solubility) (Laszlo and Dintzis, 1994). This is due to the inert
584
nature of polymer inside cellulose structure of rice husk. The polymer is relatively
585
inert as the three hydroxyl groups of each cellulose unit responsible for most of
26
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the interactions with organic and inorganic substances are involved in extensive
587
inter- and intramolecular hydrogen bonding (Mulinari et al., 2010).
588
In addition, both lignin and silica constitutes a major obstacle in using rice
589
husk as an adsorbent material. This is mainly because lignin acts as a cementing
590
matrix between cellulose fibrils and hemicelluloses molecules, while silica is
591
present on the outer surface of rice husks in the form of silicon–cellulose
592
membrane (Ndazi et al., 2007). As such, both lignin and silica can reduce the
593
binding between accessible functional groups on rice husks’ surfaces and
594
adsorbate ions/molecules. Furthermore, the inner surface of rice husk is smooth
595
and contains wax and natural fats that provide good shelter for the grain but the
596
presence of these impurities also affects the adsorption properties of rice husk
597
chemically and physically (Chowdhury et al., 2011).
598
Therefore, in order to overcome the associated problems, it is necessary
599
for rice husk to be modified by several treatments to alter or remove structural and
600
compositional impediments to hydrolysis and subsequent degradation processes in
601
order to enhance digestibility, improve the rate of enzyme hydrolysis, and
602
increase yields of intended products (Hendriks and Zeeman, 2009). Moreover,
603
Wan Ngah and Hanafiah (2008) reported that, the treatments of rice husk can
604
increase the cellulose content of the solid fraction by virtue of lignin removal and
605
hemicelluloses solubilization. On top of that, rice husk modification can also
606
reduce cellulose crystallinity as well as increase adsorbent porosity nature.
607
Thus, due to the advantages of rice husk modification in terms of
608
adsorption efficiency, many researchers began to explore this particular area of
27
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study. For example, Hsu and Pan in 2007 found that, the adsorption capacity of
610
methacrylic acid–modified rice husk was 14 times higher than that of the
611
unmodified rice husk and more than three times higher than those of traditional
612
adsorption of paraquat, by Fuller’s earth, activated carbon, cationic exchange
613
resin and bentonites. This is in accordance with the results identified by Safa and
614
Bhatti in 2011. They pointed out that, the hydrochloric acid treatment of the rice
615
husk biomass enhanced the adsorption capacity of Everdirect Orange-3GL and
616
Direct Blue-67 dyes, when compared with free biomass. It is therefore revealed
617
that, modification of rice husk can increase adsorption capacity of water pollutant
618
from wastewater. Accordingly, it has been identified that, the modification of rice
619
husk can be done via three different routes of techniques, which are mechanical,
620
physical and chemical treatment.
621
622
Mechanical Treatment
623
According to de Sousa et al. in 2004, the best mechanical performance is
624
when the reduction of biomass below 20-mesh sieves. The purposes of
625
mechanical treatment of rice husk are mainly for size reduction, as well as
626
increasing digestibility of cellulose and hemicelluloses. In addition, mechanical
627
treatment of rice husk can also increase specific surface area for solute–surface
628
interaction. This is due to the fact that, larger surface area will increase adsorption
629
capacity at equilibrium. The use of mechanical chopping (de Sousa et al., 2004);
630
hammer milling (Iñiguez- Covarrubias et al., 2001; Mani et al., 2004); grind
631
milling (Mtui and Nakamura, 2005); roll milling (Qi et al., 2005); vibratory
28
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milling (Guerra et al., 2006) and ball milling (Inoue et al., 2008) have proved
633
success as a low cost treatment strategy. Table 9 shows types of mechanical
634
treatment used in order to modify rice husk surface prior to using it as adsorbent.
635
636
637
Physical Treatment
638
The techniques of modification by physical treatment have been
639
investigated intensively by previous researchers. It has been revealed that,
640
elevated temperatures (i.e pyrolysis, combustion, burning) are the most successful
641
physical treatments in rice husk applications as adsorbent (Mtui, 2009). The
642
physical treatment enables moisture loss as well as lignin decomposition of rice
643
husk (Lapuerta et al., 2004). On top of that, the treatment of rice husk by physical
644
treatment also reduced the content of hemicellulose, lignin and cellulose
645
crystallinity which leads to an increase of the specific surface area compared to
646
raw rice husk (Daffalla et al., 2010). For example, Nakbanpote et al. in 2007
647
reported that, heating rice husk at 300°C resulted in a loss of C–H stretching
648
bands (2910 cm−1), C–C and C–H (1021 cm−1), C–O and C–O–C (1060 cm−1 and
649
1115 cm−1), and C–O–H (899 cm−1). These were replaced by the primary
650
functional groups of C = O (1715 cm−1), C = O and/or aromatic ring (1611 cm−1)
651
and dominated the silica functional groups of Si–O–Si (1096 cm−1), Si–H (801,
652
469 cm−1) and–OH and Si–OH (3000–3700 cm−1). Besides that, Daffalla et al. in
653
2010 employed the thermal treatments of rice husk by burning at the temperatures
654
of 300, 400, and 600°C for optimum burning temperature determination. They
29
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pointed out that, at 300°C, considerable amount of carbon produced. On the other
656
hand, at 400°C, amounts of carbon are decreased and considerably substituted by
657
an increase amount of silica. Apart from that, high amorphous silica produced is
658
formed at 600°C. In addition, based on surface morphology analysis by Field
659
emission scanning electron microscope (FSEM) and Scanning electron
660
microscope (SEM), the pores of different sizes and shapes could be observed
661
compared to the raw rice husk. The pores were developed from the decomposition
662
of the raw rice husk structure by heat and convert it to small particles with large
663
surface area. Nevertheless, the micrograph corresponding to calcinations at high
664
temperature (600°C) shows the loss of micropore volume, possibly attributed to
665
pores collapses. Table 10 shows types of physical treatment used in order to
666
modify rice husk prior to using it as adsorbent.
667
668
Chemical Treatment
669
Chemical treatments of rice husk able to reduce the content of
670
hemicelluloses, lignin and cellulose crystallinity. The reduction in crystallinity
671
leads to an increase of the specific surface area for treated rice husk compared to
672
raw rice husk (Daffalla et al., 2010). Table 11 shows types of chemical treatment
673
used in order to modify rice husk prior to using it as adsorbent.
674
Modification of rice husk by using powerful oxidizing agents such as
675
ozone and hydrogen peroxide can effectively remove lignin; does not produce
676
toxic residues; and the reactions are carried out at a room temperature and
677
pressure (Sun and Cheng, 2002). On the other hand, treatment with acid is
30
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basically suited for hydrolysis of rice husk. Moreover, the acid not only able to
679
hydrolyze cellulose and hemicelluloses, but also capable in separating lignin and
680
other organic components from rice husk (Rahmanet et al., 2007). Daffalla et al.
681
in 2010 reported that, the porosity of sulphuric acid (H2SO4) treated rice husk
682
increased compared to the raw rice husk, as consequences from the removal of the
683
inorganic compounds such as carbonate and silica from the surface of adsorbent.
684
Moreover, purposely for dye removal, the enhancement of adsorption capacity of
685
acid treated rice husk might also be due to protonation of the adsorbent surface.
686
The surface became positively charged and electrostatic attraction developed
687
between positively charged surface and negatively charged dye molecule which
688
can further increased the amount of dye adsorbed. Common acids uses for the
689
purpose of chemical treatment of rice husk are hydrochloric acid (HCl), sulphuric
690
acid (H2SO4) and nitric acid (HNO3). As a conclusion, treatment with acids
691
generally create higher specific surface area and micropores area of rice husk
692
adsorbent compared to non–activated adsorbent.
693
Apart from that, alkali (base) treatment is also viewed as one of the widely
694
employed chemical treatment techniques for modification of rice husk for the
695
purpose of improving its adsorption properties. For example, treatment of rice
696
husk with aqueous sodium hydroxide (NaOH) solutions breaks the covalent
697
association between lignocelluloses components, hydrolyzing hemicelluloses and
698
de–polymerizing lignin. Other than that, NaOH also improves mechanical and
699
chemical properties of cellulose such as structural durability, reactivity and
700
natural ion–exchange capacity (Ndazi et al., 2007). Besides that, particularly for
31
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heavy metals removal, the applications of base treatment enable removal of
702
surface impurities and then subsequently increase the available binding sites
703
exposure for adsorption process. Notice that, the theories reported by Ndazi and
704
his co–workers are relatively consistent with the results reported by Daffalla et al.
705
in 2010. After the treatment of raw rice husk with 0.5 M NaOH for phenol
706
removal, the silica reacts with NaOH to form sodium silicate (Na2SiO3). The
707
Na2SiO3 is soluble in water and is removed by adequate water–washing. As a
708
result, some large holes remain on the husks outer epidermis. In contrast, alkali
709
treatments such as NaOH and Ammonium hydroxide (NH4OH) decreased the
710
adsorption capability of rice husk for textile dyes removal due to deprotonation of
711
functional groups on the adsorbent surface creating a negative. This might be
712
attributed to electrostatic repulsion between negatively charged dye and rice husk
713
surface (Safa and Bhatti, 2011).
714
On the other hand, modification of rice husk with certain salts such as
715
sodium chloride (NaCl), calcium chloride (CaCl2) and magnesium sulphate
716
(MgSO4) increased the amount of dye adsorbed (Safa and Bhatti, 2011). This
717
might be due to activation of interior adsorbent surface and production of more
718
binding sites for dyes (Batzias and Sidiras, 2007). Other than that, cationic
719
surfactant (CTAB) able to increase the adsorbent capacity due to impregnation of
720
positive charge on the rice husk surface and produced an electrostatic attraction
721
with negative dye molecules (Baskaralingam et al., 2006). However, non–ionic
722
surfactants showed no effect on the adsorbent capacity of rice husk. Accordingly,
723
it can be concluded that, chemical modification is not necessarily improving
32
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adsorption capacity, but at some cases it can reduce the capacity of adsorbent to
725
adsorb water pollutant. It is dependent upon the types of chemical used and its
726
interaction between solute and rice husk surface.
727
728
Conclusions
729
Rice husk as an agricultural by–product has been made into a potentially low–cost
730
adsorbent material used in wastewater treatment in wide area of applications such
731
as heavy metals, textile dyes, surfactants, phenol, and paraquat removals.
732
Generally, two modes of experiments were carried out by previous studies, either
733
by batch or continuous studies. This paper summarizes recent studies that have
734
been carried out by some researchers on the adsorption of water pollutants from
735
wastewater. It was found that, techniques used by researchers were diverged
736
based on types of rice husk used either, raw and unmodified rice husk or modified
737
rice husk. The modification of rice husk can be done via mechanical, physical,
738
and chemical treatments.
739
activation) of raw rice husk significantly affect the surface area development, pore
740
structure evolution, and changes in surface functional groups. Apart from that,
741
some new treatment methods such as by physicochemical (combination of
742
physical and chemical) as well as biological systems are also valuable and
743
interesting to investigate in near future for rice husk applications in wastewater
744
treatment. Figure 2 illustrated a summary of works reported in literature showing
745
authors and area of studies.
It also has been revealed that, the treatment (i.e.
746
33
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923
924
41
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Figure 1: A Summary of Works Reported in Terms of Themes and Nature of Studies.
42
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43
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Figure 2: A Summary of Works Reported in Literature Showing Authors and Area of Studies.
44
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Table 1. Chemical properties of rice husk (Rahman et al., 1993 and 1997)
Composition
Percent (%)
Cellulose
32.24
Hemicellulose
21.34
Lignin
21.44
Extractives
1.82
Water
8.11
Mineral ash
15.05
45
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Table 2. Physical properties of rice husk (Malik, 2003)
CHARACTERISTICS
VALUES
Bulk density (g/ml)
0.73
Solid density (g/ml)
1.5
Moisture content (%)
6.62
Ash Content (%)
45.97
Particle size (mesh)
200 – 16
Surface area (m2/g)
272.5
Surface acidity (meq/gm)
0.1
Surface basicity (meq/gm)
0.45
46
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Table 3: Reported Heavy Metals Removal Efficiencies by Rice Husk.
HEAVY METAL
RESEARCHERS
REMOVAL
EFFECIENCY (%)
Fe
Daifullah et al., 2002
100
Chockalingam and Subramaniam, 2005
99 (Fe3+), 98 (Fe2+)
Mn
Daifullah et al., 2002
100
Zn
Daifullah et al., 2002
100
Chockalingam and Subramaniam, 2005
98
Daifullah et al., 2002
100
Chockalingam and Subramaniam, 2005
95
Cd
Daifullah et al., 2002
100
Pb
Daifullah et al., 2002
100
Cu
Aluyor et al., 2009
Cr(VI)
Wongjunda and Saueprasearsit, 2010
59.12 (Rice husk ash),
81.39 (Modified rice
husk ash)
47
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Table 4: Reported Dyes Adsorption Capacities for Rice Husk.
TYPE OF DYE
RESEARCHERS
CAPACITIES (mg/g)
Direct F. Scarlet
Abdel Wahab et al., 2005
13.00
Everdirect Orange – 3GL
Safa and Bhatti, 2011
29.98
Direct Blue – 67
Safa and Bhatti, 2011
37.92
Direct Red – 31
Safa and Bhatti, 2011
57.88
Direct Orange – 26
Safa and Bhatti, 2011
36.14
Crystal Violet
Chakraborty et al., 2011
43.00
48
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Table 5: Reported Other Water Pollutants Adsorption Capacities for Rice Husk.
WATER POLLUTANTS
Surfactants:
RESEARCHERS
CAPACITIES (mg/g)
Hosseinnia et al., 2007
(a) LABS 50%
93*
(b) AES 70%
90*
(c) NPE
70*
Phenol
Mahvi et al., 2004
0.886
Daffalla et al., 2010
Paraquat
Hsu and Pan,2007
317.7 (modified rice
husk)
*Values correspond to removal efficiency in %.
49
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Table 6: Reported Langmuir Isotherms from Previous Batch Studies Using Rice Husk as
Adsorbent.
WATER
POLLUTANTS
TYPES OF RICE
HUSK (RH)
Direct F.
Scarlet
Unmodified
Paraquat
Pb
Congo Red dye
Cr (VI)
Everdirect
Orange – 3GL
dye
Direct Blue –
67 dye
Citric acid –
modified RH
methacrylic acid
– modified RH
hydrogen
peroxide modified RH
Unmodified
ethylenediamine
– modified RH
MAXIMUM
CONSTANT
SORPTION
(L/mg)
CAPACITY AT
EQUILIBRIUM
(mg/g)
2.415
0.17
R VALUE
RESEARCHERS
0.99
Abdel Wahab et
al., 2005
4.35
0.14
0.97
317.7
-
-
11.88
-4.07
0.979
13.26
26.39
0.0187
0.4427
0.9663
0.9980
Ong et al., 2009
rice husk ash
0.49
12.09
0.9962
Wongjunda and
Saueprasearsit,
2010
sodium hydroxide
- modified rice
husk ash
Unmodified
0.84
2.55
0.9947
28.41
0.061
0.986
CMC
Immobilized
PVA-alginate
immobilized
HCl – modified
Unmodified
21.74
0.0333
0.975
8.77
0.036
0.997
30.96
52.63
0.201
0.016
0.999
0.989
CMC
Immobilized
PVA-alginate
immobilized
HCl – modified
23.42
0.029
0.990
3.022
0.039
0.992
71.43
0.075
0.989
.
50
Hsu and Pan,
2007
Aluyor et al.,
2009
Safa and Bhatti,
2011
Safa and Bhatti,
2011
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Table 7: Reported Freundlich Isotherms from Previous Batch Studies Using Rice Husk as
Adsorbent.
WATER
TYPES OF
ADSORPTION
ADSORPTION
POLLUTANTS
RICE HUSK
INTENSITY, 1/n
CAPACITY, K
(RH)
Phenol
Indigo Carmine
(mol/g)
Unmodified
0.195
0.00092
RHA
0.57
0.00092
RHA
0.4453
1.6018
(IC) dye
Crystal Violet
RESEARCHERS
Mahvi et al., 2004
Lakshmi et al.,
2008
NaOH –
0.1554
modified rice
-
Chakraborty et
al., 2011
husk
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Table 8: Reported Equilibrium Time from Previous Batch Studies Using Rice Husk as
Adsorbent.
WATER
POLLUTANTS
Phenol
TYPES OF RICE
HUSK
Unmodified
Rice husk ash
Direct F. Scarlet dye
Unmodified
Citric acid – modified
rice husk
LABS
Unmodified
NPE
Unmodified
Indigo Carmine dye
Rice husk ash
Congo Red dye
Ethylenediamine
–
modified rice hulls
Cr(VI)
Rice husk ash and
sodium hydroxide modified rice husk ash
Everdirect Orange – Unmodified
and
3GL
Hydrochloric acid –
modified rice husk
CMC
immobilized
rice husk
PVA
–
alginate
immobilized rice husk
Direct Blue – 67
Unmodified
CMC
immobilized
rice husk and PVA –
alginate immobilized
rice husk
Hydrochloric acid –
modified rice husk
Crystal Violet
Sodium hydroxide modified rice husk
EQUILIBRIUM
TIME (hr)
6
3
2
1.5
RESEARCHERS
Mahvi et al., 2004
Abdel Wahab et al., 2005
3
5
8
4
Hosseinnia et al., 2007
Hosseinnia et al., 2007
Lakshmi et al., 2008
Ong et al., 2009
3
Wongjunda and
Saueprasearsit, 2010
3
Safa and Bhatti, 2011
5
6
4
6
Safa and Bhatti, 2011
3
1.5
52
Chakraborty et al. in 2011
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Table 9: Rice Husk Surface Modification by Mechanical Treatment.
TYPE OF MECHANICAL
MESH SIEVER
RESEARCHERS
Crushing
30
Mahvi et al., 2004
Grinding
20 – 30
Aluyor et al., 2009.
Milling
500 – 250*
Daffalla et al., 2010
Grinding (food processor)
-
Safa and Bhatti, 2011
Grinding (food processor)
-
Safa and Bhatti, 2011
TREATMENT
*Unit is in µm.
53
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Table 10: Rice Husk Surface Modification by Physical Treatment.
TYPE OF PHYSICAL TREATMENT
RESEARCHERS
Heating up to:
(a) 100°C
Safa and Bhatti, 2011
(b) 120°C
Abdel Wahab et al., 2005
(c) 300°C
Nakbanpote et al., 2007
Daffalla et al., 2010
(d) 350°C
Daifullah et al., 2002
(e) 400°C
Daffalla et al., 2010
(f) 500°C
Nakbanpote et al., 2007
(g) 600°C
Daffalla et al., 2010
(h) 750°C
Hosseinnia et al., 2007
Burning at:
(a) 650°C (pyrolysis)
Daifullah et al., 2002
(b) 400°C
Mahvi et al., 2004
Boiling:
(a) 30 minutes
Safa and Bhatti, 2011
(b) 30 minutes + KOH
Daifullah et al., 2002
54
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Table 11: Rice Husk Surface Modification by Chemical Treatment.
TYPE OF CHEMICAL TREATMENT
RESEARCHERS
Acid:
(a) Citric acid
Abdel Wahab et al., 2005
(b) Methacrylic acid
Hsu and Pan, 2007
(c) Sulphuric acid
Daffalla et al., 2010
Safa and Bhatti, 2011
(d) Nitric acid
Safa and Bhatti, 2011
(e) Hydrochloric acid
Safa and Bhatti, 2011
Alkali/Base:
(a) Potassium hydroxide
Daifullah et al., 2002
(b) Sodium hydroxide
Daffalla et al., 2010
Safa and Bhatti, 2011
Chakraborty et al., 2011
(c) Ammonium hydroxide
Safa and Bhatti, 2011
Salts:
(a) Sodium carbonate
Kumar and Bandyopadhyay, 2005.
(b) Calcium chloride
Safa and Bhatti, 2011
(c) Sodium chloride
Safa and Bhatti, 2011
(d) Magnesium sulphate
Safa and Bhatti, 2011
Oxidizing agents:
(a) Hydrogen peroxide
Aluyor et al., 2009
Organics:
(a) Ethylenediamine
Ong et al., 2009
(b) Formaldehyde
Daffalla et al., 2010
Surfactants:
(a) Cetyl trimethyl ammonium bromide
Safa and Bhatti, 2011
(b) Triton – X100
Safa and Bhatti, 2011
55