Topic: Analysis of Heavy metals from waste disposals- Determination of (Pb & Cr)
concentration in soil by atomic absorption spectrometry (AAS)
Background/Overview
Heavy metals refer to any metallic chemical element that has a relatively high density and is toxic or
poisonous at low concentration. Examples of heavy metals include mercury (Hg), cadmium (Cd), arsenic
(As), chromium (Cr), thallium (Tl), and lead (Pb). In fact, higher concentrations of heavy metals can lead to
poisoning, where as there are many health related risks that are caused by heavy metal that were
accumulated in soils and waters.
Moreover, heavy metals are considered to be one of the main sources of pollution in the environment,
because of their significant effect on the ecological quality. The main sources of heavy metal pollution in the
environment are man-made effects, including combustion of fossil fuels, mining activities, wastewater
discharges of manufacturing industries, and waste disposal. High levels of heavy metals in the sediments
and soils may pass to the aquatic environment, groundwater, and plants through the transfer processes (food
chains/food webs) and reach to the animals and humans. Therefore, the use of simple and accurate methods
for monitoring heavy metals has a great importance among the environmental studies. Thus, collecting and
analyzing toxic metals from samples of soils at a polluted site can encourage good environmental health in
public or settling areas.
Furthermore, before applying one of the analytical methods, sample digestion and preparation must be done
so as to prepare the desired metals in phases for analysis. In terms, the most commonly used methods for the
sample digestion are: dry ashing, wet digestion and microwave digestion. All methods provide similar
results for the heavy metals determination. However, dry and wet digestion, in terms of efficiency, it
requires more time and are more complicated than the microwave digestion.
In fact, there are various analytical methods for heavy metal analysis which includes ICP--MS, graphite
furnace atomic absorption spectroscopy (GF-AAS), flame-atomic absorption spectroscopy (F-AAS) or
(AAS) and Cold- Vapor Atomic Flame Spectrometry (CV-AFS).
Introduction:
Anthropogenic sources of heavy metals such as industrial wastes, automobile emissions, mining activity and
application of chemical fertilizers, have led to their accumulation in soils. Heavy metals levels in soil are
increasing also due to disposal of domestic sewage sludge on agricultural areas. thus, an important aspect in
environmental research is concerned with the forecasting of hazards resulting from the occurrence or
accumulation of heavy metals in various elements of the environment. In contrast, Kamila. M, (2018) states
that heavy metals in solid samples derived from the environment are present in many forms and phases that
vary in terms of their mobility, bioavailability and toxicity. In relation, the assessment of the impact of
heavy metals on the environment needs to involve the identification of the forms of metallic elements in the
material, in addition to their total content.
Besides, a variety of speciation analysis methods can be performed to prepare the desired solution, including
sequential extraction (SE) are commonly used to predict the behavior of metals present in solid samples. As,
this process involves taking subsequent extractions with solutions of an increasing leaching strength.
Gradually, various forms of metals are extracted under the effect of extraction agents. As a matter of fact, R.
Świetlik, M. Trojanowska, (2008), points out that SE provides a manner in which the simulation of
environmental conditions can be performed, both with regard to natural and anthropogenic factors. Until
recently, SE was only applied in the analysis of soils and sludge. However, quite recently, it was applied in
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the investigations concerned with the composition of solid waste. Thus, the use of SE also contributes to the
improvement of waste re-use and reduction of its negative impact on the environment.
Most importantly, sample preparation is a key determinant for a successful and good analysis. Thus,
digestion is performed with the aim of separating the analyte from the matrix, and to avoid interactions of
organic substances with metal ions or chemical reagents. In fact, Kamila Mizerna, (2018) elaborates that the
choice of digestion procedure depends on the nature of organic substance and the content of inorganic
components, as well as the choice of heavy metal which is selected for the analysis. Thus, for a good
analysis the physical and chemical properties of the elements are very convenient, in terms of sample
preparation.
Additionally, concentration of toxic metals increases in ppm and can reach 1000 ppm if there is an extreme
discharge amount of these metals. Actually, the level of contamination for most heavy metals such as lead is
between 1000 –2000 ppm, which indicates that there is a high concentration of that metal present in the soil
sample, as per United States Environmental Protection Agency, (2015). Moreover, the WHO, (1996)
mentioned that toxic metals becomes contaminants if the concentration exceeded the permissible range/
limit such that lead has a permit range of 600 ppm while chromium has a limit of 50 ppm.
Lead occurs naturally in soil, in levels ranging from 10-50 ppm. However, with the widespread use of lead
in paint and gasoline, in addition to contamination by industry, urban soils will often have concentrations of
lead significantly greater than background levels – up to 10,000 ppm in some places. Ongoing problems
arise from the fact that lead does not biodegrade, and instead remains in the soil. Serious health risks are
associated with lead poisoning, where children are particularly at risk. In fact, Dr. Senthamilselvi. P et al,
(2021) announced that millions of children in the U.S. are exposed to soil containing lead. This exposure can
cause developmental and behavioral problems in children. These problems include learning disabilities,
inattention, delayed growth, and brain damage. Moreover, The Environmental Protection Agency has set a
standard for lead in soil at 400 ppm for play areas and 1,200 ppm for non-play areas by United States
Environmental Protection Agency, (2015).
Furthermore, of the heavy metals, cadmium has the most dangerous long-term effects on human health.
Whereas, lead and cadmium level in soil tend to be higher in cities, near road ways and around industrial
source, Kamila Mizerna, (2018). Potential hazards associated with trace elements pertain to their
accumulation in soils which may (1) lead to a plant toxicity condition or (2) result in increased uptake of
metals into the food chain. Moreover, the permissible limit for the cadmium in soil was expected to be at the
targeted value of 0.8 – 12 ppm by WHO, (1996). Hence, the permissible limits recommended by the WH0
(1996) may be served as an important tool/ references for policy makers to make decisions concerning the
environment and to secure the wellbeing of the community or promoting health care services by minimizing
the levels of heavy metals.
Contrary, Atomic Absorption Spectrometry is a useful technique to analyze a wide range of environmental
samples (e.g., water, soil, sludge, and sediment) for a large number of elements (e.g., heavy metals).
Because of its sensitivity, specificity, simplicity and precision, Atomic Absorption Spectrometry (AAS) is
the most widely recommended instrument utilized in analytical procedures for trace heavy metal analysis.
Therefore, this scientific literature review highlights the use of flame AAS to determine the Pb and cadmium
content in soil. As it, involves the necessary procedures from sample collection, preparation and digestion
for the analysis using AAS. Hence, Atomic Absorption Spectroscopy is a very common technique for
detecting metals and metalloids in samples which was introduced by Walsh, Alkemade and Milatz in 1956.
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1. Methodology
1.1 Study Area:
For the present study, soil samples will be examined for heavy metal (lead, cadmium) analysis of soil
samples. The soil samples will be collected from three different locations of the studied zone of Sander
Hyper Mart at Goroka EHP. The actual sampling area is located at the back of the Sander Hyper, where
there are villages and other setting societies leaving nearer to the site. Moreover, the main drainage system
of the Hyper Mart, runs through the settlers‟ and villagers‟ gardens, which is used routinely for washing,
laundry, and even cooking. Furthermore, animal farming such as piggery is very common in the area where
pigs feed often from the surrounding soil. In terms, any of the toxic metals can leach into the soil polluting
the environment. In relation, Aleksandar. Bojić, (2019), specified that the main sources of heavy metal
pollution in the environment are man-made effects, which include combustion of fossil fuels, wastewater
discharges of manufacturing industries, and waste disposal. Thus, High levels of heavy metals in the
sediments and soils may pass to the aquatic environment, groundwater, and plants through the transfer
processes and reach to the animals and humans.
1.2 Soil collection and sampling
Soil samples can be collected from the top soils randomly or at a systematical array at various depths (0- 20
cm) depending on the soil type. For instance, an undisturbed area is preferable for soil collection at a depth
of 2- 3 inches however, Robinson, J.W., Skelly Frame, E.M., Frame II, G.M, (2005) elaborates that, when
sampling vegetable gardens, collect 6-inch deep samples and then use a soil auger to collect a 1-inch
diameter soil core from sample area. Soil auger is used, after manual removal of leaves, roots and stones.
At the sampling site, at least three sub-sites could be selected for the purpose of random sampling and two
soil samples can randomly be collected from each of the three sub-sites and pooled together to obtain a
single composite sample, see fig 1 below.
Fig 1. Soil sample collection
Fig 2. Oven drying of soil samples,
Source: Journal of Emerging Technologies and Innovative Research (JETIR), 2021.
Moreover, the soil samples that will be collected from different places must be kept in clean polyethylene
bags or plastic and labeled as S1, S2, S3, S4, etc. Whereas, the soil samples that are collected separately
should must be sieved using a USS# 10 sieve wire or 2 mm sieve and mixed thoroughly to attain
homogeneity by shaking for 1-2 minutes. This is to eliminate stones and other materials extraneous to soil.
In addition, Sundarrajan, M. & Velmurugan , M.,(2016), points out that wet soils can be placed in air dry to
reduce moisture contents and then it can be dried in a drying oven (as in figure 2 above) at 40⁰ C for
approximately for 30 minutes. Likewise, Aleksandar Bojić, (2019) also states that dry samples should be
refrigerated upon receipt and analyzed as soon as possible. As, it will have difficulties to obtained the
representative samples with wet or damp materials. He also mentioned that wet samples maybe dried at 90
°C. Finally, samples were to be transferred in to polyethylene bag or plastic and glass containers or sample
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jar and transported to the laboratory for analysis. Hence, the resulting fine powder will kept at room
temperature for digestion.
2. Soil Sample preparation
2.1.Instrumentation
Atomic Absorption Spectrometer
ATOMIC ABSORPTION SPECTROSCOPY (AAS) is a simple inexpensive and excellent instrumental
technique that can be used to analyze trace metals. In terms, Mohammad Ali Salik, (2014 ), emphasize that
AAS relies on the atomization of the determination by flame sources to form free atoms which can absorb
radiation at discrete wavelengths. In terms, each wavelength corresponds to only one element and the width
of an absorption line is only of the order of a few pico-meters (pm) which gives the technique its elemental
selectivity. For instance, the peaks shown on the screen represents the concentration of the metal in
determination. Hence, the amount of energy absorbed is proportional to the concentration of the determinant
present in the sample as the Beer Lambert Law applies. (See fig 3.)
Fig 3. Beer Lambert Law
Hence, the absorption (A) is directly proportional to
the concentration (c).
Source: Google image download
2.2 Reagents
Preparation of Aqua-regia for mineralization.
Aquaregia (35% of HCL, 65% of high purity HNO3,in 3:1 ratio )
In relation, Belay K. and Tadesse A. (2014) presented that with the purpose of determining the total content
of heavy metal, ash mineralization was performed by use of concentrated HNO3 and HCl acids in the ratio
of 1:3. Usually, the nitric acid will be added to the hydrochloric acid, rather than adding hydrochloric to
nitric acid! It will result in a fuming red or yellow liquid. In fact, Wodaje Addis & Alemayehu Abebaw,
(2017) elaborate that Aqua regia is used to dissolve gold, platinum, and palladium where, the weighed
samples are transferred to a 250 ml beaker and will digest it with aqua regia acid solution. Hence, the soil
samples will be digested with 15ml of aqua regia (35%HCL and 65% high purity HNO3, in 3:1 ratio), which
then be heated at 95 ⁰C, whereas the resulting solutions will be cooled and then filtered through Whatman
filter paper.
2.3 Sample Digestion
The most commonly used methods for the sample digestion are: dry ashing, wet digestion and microwave
digestion. All methods provide similar results for the heavy metals determination. However, dry and wet
digestion in terms of efficiency require more time and they are more complicated than the microwave
digestion, by the Research Gate, (Oct 4 2022)
Especially digestion procedure is carried out with the aim of separating analyte from the matrix, and to avoid
interaction of organic substance with metal ions or chemical reagents. As a matter of fact, Kamila Mizerna,
(2018) examined that the choice of digestion procedure depends on the nature of organic substance and the
content of inorganic components, as well as the choice of heavy metal for analysis, or the digestion method
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that will be used for determination. Hence, only one of each methods either; dry ashing , wet digestion and
microwave digestion can apply for sample digestion.
Moreover, samples that were digested in a microwave oven are based in accordance with the guidelines
provided by the manufacturer of the microwave mineralizer. However, perchloric and nitric acid can also be
used to digest 1 gram of soil samples in preparation for analysis at the ratio of 3 : 5, (15 ml of HNO3 and 25
ml of H2O2). The determination of the forms of the heavy metals in the waste applied a sequential extraction
based on the method developed by Tessier et al ,(1979).
Wet ashing is primarily used in the preparation of samples for subsequent analysis of specific minerals. It
breaks down and removes the organic matrix surrounding the minerals so that they are left in an aqueous
solution. In fact, a dried grounded sample is usually weighed into a flask containing strong acids and
oxidizing agents (e.g., hydrochloric, nitric, perchloric and/or sulfuric acids) and then heated. Heating is
continued until the organic matter is completely digested, leaving only the mineral oxides in solution. In
relation, Tuzen, (2002) signifies that the temperature and time used depends on the type of acids and
oxidizing agents used. For instance, the main advantages of wet ashing method for heavy metal
determination in medicinal plants are that it eliminates elemental loss by volatilization because the digestion
takes place at a low temperature and it is more rapid than dry ashing. However, (Ibid, 2002) mentioned that
the disadvantages of this method are, it is labor intensive, requires a special fume cupboard, reagent
contamination, it is tedious and requires operator attention and low sample throughout.
Dry ashing procedures use a high temperature muffle furnace capable of maintaining temperatures of
between 200 and 600 °C. Whereas, water and other volatile materials are vaporized and organic substances
are burned in the presence of oxygen in air to CO2, H2O and N2. Most minerals are converted to oxides,
sulfates, phosphates, chlorides or silicates (Tuzen et al., 2004). Conversely, the disadvantage of this method
is, it requires relatively expensive apparatus (platinum crucibles and muffle furnace) and it cannot be used
for the determination of elements that are volatile below the ashing temperature by Tuzen et al., (2004).
2.4 Preparing calibration standards, stock solutions and standard Solutions
The standard solutions for lead, chromium, cadmium, manganese are prepared by using intermediate stock
solution. Thus, the standards can be prepared in accordance with the calibration standards of the stock
solutions (Fig 4- Table 1), which would include the following chemicals to be used for each metal analysis
as per by Dr. Senthamilselvi P et al (2021).
Table 1: Calibration standard and chemicals for each metal analyte.
Metal (Analyte)
Chemicals used to prepare calibration standards
1
Lead (Pb)
1 Lead nitrate (Pb(NO3)2)
2
Chromium (Cr)
2 Potassium di chromate (K2Cr2O7)
3
Manganese (Mn)
3 Manganese sulphate (MnSO4)
4
Cadmium (Cd)
4 Cadmium chloride (CdCl2 .2½H2O)
Fig 4: shows the calibration standards and the chemicals used to prepare standard solutions of Pb, Cr, Mn
and Cd. Source: Journal of Emerging Technologies and Innovative Research (JETIR) 2021
Moreover, a stock solution is prepared by weighing out an appropriate portion of a pure solid or by
measuring out an appropriate volume of a pure liquid and diluting to a known volume. Usually, the stock
solution is prepared before the standard, and exactly how this is done depends on the required concentration
unit of the analyte. For instance, Manoj Kumar T et al, (2015), stated; to prepare a solution with a desired
molarity you weigh out an appropriate mass of the reagent, dissolve it in a portion of solvent, and bring to
the desired volume. In most cases, the standard solution prepared must usually expressed in concentration
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terms (ppm or ppb), which is precise and will be analyzed using an AAS instrument. As atomic absorption
spectrometry is so sensitive that it can measure down to parts per billion of a gram (μg dm–3) or parts per
million (mg/ kg) in a sample.
The actual analysis will proceed after the preparation of the standard/ diluted solutions with calculations
regarding the concentration of the solute solutions. In relation, the prepared solutions must have the solute
concentration expressed as volume percent i. e prepare the stock solutions (eg. 4ppm 6ppm 8ppm etc) either
as (w/w %) or (v/v %) . Hence, this can be done by measuring appropriate volume of the desired solute and
add sufficient volume to obtained the expected total volume.
3. Analysis of soil sample on AAS
AAS makes use of the wavelengths of light specifically absorbed by an element. Moreover, metal ions or
metalloids in an aqueous sample are vaporized by the nebulizer (converting it into fine aerosol) and will
thermally atomised by a flame in the atomizer. Hence, the flame is created by a fuel and oxidizing gas either
acetylene/methane and or air. A specific element related spectral light (source: hollow cathode lamp) is
radiated through the atomization cell, and a certain wavelength is filtered out of the lines of the spectrum of
the element by a monochromator. Furthermore, Wodaje Addis and Alemayehu Abebaw (2017) states that
the difference in the intensity between the sample and reference beams produces a signal which is directly
proportional to the total number of free atoms present of the element under analysis. A calibration curve is
constructed by running samples of known concentrations under the same conditions as the unknown. With
the flame method, a detection limit of a few mg/kg (ppm) is achieved. The main components of AAS
instrument is shown in fig 5.
Fig 5. Block diagram
showing the
components of AAS
instrument
Source: Google image for block diagram of AAS
Additionally, most analysis are expected to run distilled water as blank to eliminate errors or trace elements
detection. Thus, the heavy metal solution will then be analyzed to achieve the standard calibration curve.
Whereas, all other samples prepared, will be analyze for the selected heavy metals, whereas the results/ data
obtained can be used to determine heavy metal concentration in unknown samples. Moreover, the results
obtained are automated and will be displayed on the data processor. These results will be tabulated as shown
in the table 2 (fig. 6). Hence, the observed results will be compared and evaluated based on the permissible
limits (See table 3) of heavy metals in soil as per by WHO, (1996).
Fig 6: Results of heavy metals concentration in soil samples and permissible limits of heavy metals.
Table 2: Table of results for the selected metals in each sample
Sample No.
Lead (Pb)
concentration
range ppm
Chromium(Cr)
concentration
range ppm
Cadmium
(Cd)concentration
range ppm x10-4
Manganese
(Mn)concentration
range ppm
Sample 1
0.00438
0.00401
4.33
0.247
6
Sample 2
0.00168
0.00591
3.17
Sample 3
0.00129
0.00592
3.04
Source: Journal of Emerging Technologies and Innovative Research (JETIR)
0.221
0.272
JETIR June 2021, Volume 8, Issue 6 http:// www.jetir.org (ISSN-2349-5162)
Table 3: Permissible limits of heavy metals in soils as per WHO, (1996)
Sample .No.
Element
Permissible limit (ppm)
1
Lead
600
2
Cadmium
100
3
Chromium
50
4
Manganese
20
Source: Journal of Emerging Technologies and Innovative Research (JETIR), 2021
Conclusion:
The concentrations of heavy metals in soil samples from industrial area at different places can be
determined. In relation, toxic metals can accumulates in soils exceeding the permissible limits set by the
WHO. Moreover, some of the heavy metals present in lower concentration than others, while some can be
far below the maximum tolerable levels set by the WHO. In terms, frequent waste disposals and bottom ash
from industries or manufactures can contribute to the accumulation of toxic metals in the soil, if washed by
water system. Thus, determining the amount of metal present in soil can reduce health risks and hazards in
the society. In fact there are many health hazards relate to toxic metal contamination, especially children are
in great danger. In fact, the study results can be interpreted and communicated to the public to avoid toxic
metal health risks. As the results can reveal the level of contamination of various heavy metals under
analysis, and can be compared with permissible limits of the WHO. Finally, it is highly recommended that
further analysis studies should be carried out with increasing sample size, even if the result is below the
limit, certain precautionary measures must be conducted in order to prevent possible ingestion of soil
containing heavy metals. Hence, the results would serve as a baseline data for determination of metals/
chemicals in similar study area.
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