EURECA 2013 – Removal of Zn 2+ from Wastewater Using Membrane Electrolysis
2+
School of Engineering, Taylor’s University, Malaysia
*Corresponding author email: kahkit.ng@sd.taylors.edu.my
Abstract
— Performance of electrolysis with carbon electrode on waste water that contains heavy metal ions (Zn 2+ ), has been studied.
Parameters such as metal ion concentration and temperature were studied to achieve higher removal rates of heavy metals from the waste water. The research shows that electrolysis perform the best when the pH is kept at neutral conditions. Besides that, the removal efficiencies of metal ions is highest at 45 ° C. The research shows that the use of membrane electrolysis has a removal efficiencies of
99.6%.
Keywords
— Electrolysis; Heavy Metals; Membrane; Anion; Cation
Heavy metals are metals that contains more than five times of specific gravity (SG) compare to SG of water [1]. Heavy metals are harmful to human’s body when they are not processed and they will start gather in the human tissue. Heavy metals may enter the human body through digestion, ingestion, inhaling, exposure to skin and eye
[2]. There are a few sources of heavy metals that can leach in to the ecosystems such as industrial discharges, domestic discharges and ore mining activities. Rapid industrialization had also increased the demand for heavy metals and causes them to leach into the ecosystem. Unlike organic substances, heavy metals are not degradable through process such as biological process [3]. Therefore, treatment of heavy metals wastewater are required before discharging.
There are a few treatment process can be done to the wastewater however most of them results in the loss of valuable metals. The ideal way to remove the heavy metals and recovered them back to usable metal is electrolysis. Electrolysis wasn’t the common method used to remove heavy metals due to its low-mass transfer rate between metal ions and cathode surface when the metal ions are in low concentrations. However this can be solved by combining the use of an anion membrane, stirrer and also increase in temperature in order to speed up the rate of reaction. Extraction of heavy metals using electrolysis is selected due to the process does not generate sludge, low operating cost, no reagents are needed for the process and precious heavy metals can be preserve and recycle [4].
The research is separated into two parts. The first part of is to study the optimum parameters for electrolysis of the heavy metals wastewaters. 6 x 200 mm of carbon electrodes purchased from
Japson, India is used in the experiment. Reagents used to prepare the synthetic wastewater (Zinc Sulfate Heptahydrate) is supplied by
Bendosen, Malaysia. Main parameter that need to be considered is the metal ions concentration, initial pH of the electrolyte, temperature of the electrolyre and the current density of the electrolysis process. After the parameters are being optimized, the second part of the research is to use the anion membrane in the electrolysis method and a comparison is made between the conventional electrolysis method and the anion membrane electrolysis method.
All of the solutions used in the experiment is prepared in room temperature with distilled water. Reagents that used to prepare the stock solutions are all under analytical grade and used without purification. The apparatus used in the experiments are all washed and rinsed with distilled water.
Stock synthetic wastewater solution of Zinc Sulfate Heptahydrate
(ZnSO
ZnSO
4
4
.H
.H
2
2
O) were prepared by dissolving required amount of
O powder with distilled water. Further dilution were done for lower concentration of the synthetic wastewater.
The pH of the synthetic wastewater were adjusted to pH 7 by using 1 M of Hydrochloric Acid and Sodium Hydroxide. It is proven by the previous study that the most effective removal capacity were achieved in between pH values 4-8.
Electrolysis was carried out in a laboratory scale setup. The experiment is carried out in a 250ml container. Two carbon electrodes which acted as the anode and cathode were dipped in the synthetic wastewater. The electrodes are supplied with a D.C. current using a laboratory power supply (Meguro Model: MP302-3).
Samples of 3.2ml of the electrolyte were taken periodically and the samples were put in the spectrophotometer in order to determine the residual of the metal ions concentrations. The optical densities of the residual were measure at wavelength, λ=391nm .
The optimum temperature of the electrolyte, initial pH of the electrolyte and metal ions concentration are subjected to the next analysis which is to determine the effect of addition of anion membrane in the electrolysis process.
Figure 1 shows the experimental setup of the analysis.
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EURECA 2013 – Removal of Zn 2+ from Wastewater Using Membrane Electrolysis
Performance of electrolysis the synthetic wastewater is studied in a range of pH from 4 to 8 while keeping the concentration of the metal ions in 100mg/L. The current density is kept at 0.8A as previous research shows that at 0.8A, the removal efficiencies of heavy metal ions can reach up to 99% [5]. The optimum temperature for electrolysis had been studied by conducting the experiment in a range of temperature from 25 ° C to 45 ° C. The experiment repeats by changing the concentration of the heavy metal ions in between the range of 100mg/L to 500mg/L. The optimum parameters will be then used for the experimentation of the membrane electrolysis.
3.
Studies shows that initial pH of the electrolyte will directly influence the results of the electrolysis. In order to determine the optimum pH value, the experiment of effect of pH value on removal of metals in electrolysis is conducted under 25 ° C. Figure 2 shows that the removal efficiencies of the metal ions achieved 98.5% at pH 6 which is slightly acidic.
100
98
96
94
92
90
4 5 6
Figure 2 Effect of pH on removal efficiencies of Zn 2+
From figure 3, results shows that the optimum temperature for electrolysis is 45 ° C. Studies shows that high temperature electrolysis are more efficient and cost effective. High temperature electrolysis is favorable from a thermodynamic point of view because the heat energy can transfer to the metal ions and the activation barrier is lowered [6]. However, there is only a slight increase of removal efficiencies between 40 ° C and 45 ° C.
Considering more cost is needed to increase the initial temperature of the electrolyte, hence 40 ° C is set to the optimum temperature for the membrane electrolysis.
100
99.5
99
98.5
98
97.5
25 30 35 40 45
7 8
Figure 3. Effect of initial temperature of electrolyte on removal efficiencies of Zn 2+
Figure 4 shows that the initial concentration of the metal ions will affect the rate of removal of metal ions. The lower the initial concentration of the metal ions, the rate of removal metal ions will be higher. Therefore, 100mg/L of initial concentration of the electrolyte is selected as the optimum concentration for electrolysis process.
600
400
200
0
Figure 4. Effect of initial concentration of Zn 2+ efficiencies.
on removal
The research proceeds by perform experiment using membrane electrolysis under the optimum parameters such as pH6, temperature at 40 ° C and the initial concentration are kept at 100 mg/L. The removal efficiencies has achieved 99.6% with the use of membrane electrolysis.
This research shown the applicability of removal of zinc metal ions by using electrolysis. This method is suitable to remove and preserve precious heavy metal ions from wastewater. The performance of the experiments achieved a percentage of 99.6 when it is perform under the optimum parameters. The use of membrane electrolysis is to ensure the anion flows in one direction in order to increase the treatment rate and treatment efficiency.
First and foremost, I would like to express my sincere appreciation to my supervisor, for constantly guiding and encouraging me throughout the research module. Besides that, I would like to acknowledge School of
Engineering for their support in providing complete facilities to carry out this research. In addition, I would like to thank my co-supervisor for supporting me in the project by giving me useful advice and information. His technical efforts and suggestions helps me manage to reach the goals of the project.
Besides that, I would like to thank the lab assistant that helps me on the project.
0
500 ppm
200 ppm
10
400 ppm
100 ppm
20 30
TIME/ MIN
300 ppm
40 50
[1] Anne Marie Helmenstine, Ph.D. Heavy Metal Definition from http://chemistry.about.com/od/chemistryglossary/g/Heavy-Metal-
Definition.htm
[2] Howard Hu, M.D., M.P.H., Sc.D. (2002) Human Health and Heavy
Metals Exposure
[3] John H. Duffus (2002), "Heavy metals" a meaningless term? (IUPAC
Technical Report)" Pure and Applied Chemistry, Vol. 74.
[4] Robert W. Peters and Young Ku (1985). Evaluation of recent techniques for removal of heavy metals from industrial wastewaters.
[5] J. Mater. Chem., (2008) Highly efficient high temperature electrolysis
[6] Nafaaˆ Adhoum, Lotfi Monser, Nizar Bellakhal, Jamel-Eddine
Belgaied (2004). Treatment of electroplating wastewater containing
Cu2+, Zn2+ and Cr(VI) by electrocoagulation
[7] Soha A. Abdel-Gawad1*, Amin M. Baraka,Kawther A.
Omran,Mohamed M. Mokhtar (2012) Removal of Some Pesticides from the Simulated Waste Water by Electrocoagulation Method Using
Iron Electrodes
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