Extraction Heavy Metals from Contaminated, Water using Chelating Agents

Introduction

Pollution of water from industrial activities and domestic has not been regularly monitored and recorded as a problem. The development of industry and many activities such as mining, electroplating, refinishing, munitions  , metalworking and manufacturing produce metal-contaminated by products that are responsible for a large amount of metal polluted sites in soils [1] .Soil and water pollution is becoming a significant industrial activities in the whole world[2] .Heavy metal pollutionhave been subjectedto both surface and subsurface environments [3].The most hazardous heavy metals:lead, Cadmium,copper, arsenic,mercury, zinc and chromium are contaminated soil and water[4] . Water maintains the life on the earth because it is the most vital liquid. About 97% water is not suitable for drinkingexists in oceans and about 3% is fresh water were 2.97% of it is glaciers and ice caps and the other 0.3% is available as a ground and a surface water for human use [5]  .

Safe drinking water is a basic right of humans and a simple need for good health. In many part of the world fresh water resources is already limited. Because of the population increasing, urbanization and climate change, in the following century, fresh water will become  even more limiting[6] . Because of thedischarge of untreated domestic and industrial wastewater into these resources, many cities in Asia facing increase in chemical material in drinking water[7]. Situation in South Asia is more aggravated, because of poor water quality and bad sanitation more than 0.5 million deaths of newborns happened per year with additional health threats. For example, in India (West Bengal) and some areas of Bangladesh, groundwater is polluted with arsenic (As) at levels as much as 70 times higher than the national drinking water quality standard of 0.05mg/l [8] . Worldwide, about 26% of all deaths are a result from infectious diseases caused by pathogenic bacteria and more people are dying from bad quality of water every year than from all types of physical violence including war[9,10].In India,lack of access to clean and safe water generated waterborne diseases unnecessarily by the residents of slums there[11] .Polluted water leads to death with waterborne diseases for infants and children such as diarrhea, while every fifth citizen suffers from illness and disease caused by this water [12] .

Toxic element pollution is a global problem, the common toxic element likePb, Cd, Hg, Cu and Coetc. are phytotoxic at both very high concentration as well as low concentration are detected in polluted water[13].These toxic elements reach the food chain through plants and aquatic animals when they presented in sediments and water[14]. In recent years there has been increasing need for estimating effect done by contamination of water. In this regard, there is growing interest in the use of Chelating agents like Ethylene diamine tetra acetate (EDTA), citric acid (CA) for detecting heavy metal ions, because of their sensitivity and selectivity [15] .The  objective of the study was optimization of Experimental conditions for selected heavy metals (Cu and Hg) extraction from wastewater (polluted water) using aqueous solutions of oxalic acid ,  EDTA (ethylenediaminetetraacetic acid) and citric acid at different concentration and different pH.

Material and Methods

Chemical and Reagents

Al chemicals were pure (analytical grade) and were used without purification. oxalic acid (OA), ethylene di amine tetra acetic acid (EDTA) , citric acid,NaOH and HCl were obtained from BDH. The polluted water containing Cu and Hg ,Stock solution of Hg(ll) and Cu(II) (1,000 mg/l) were prepared to synthetic polluted water . All prepared solutionkept under acid conditions by adjusting the pH below2 using o.1 N HCl.The mixed heavy metals concentration was (10mg/l each). A total content of the analyzed heavy metals  in contaminated water was determined and Content of heavy metals was detected by atomic absorption spectrometry (Model Z-8100 polarized Zeeman).

Extraction Procedure

The effect of removingheavy metals from water contaminated with mercury and copper (Prepared in the laboratory 10 mg /L of each)  was studied. Four factors were identified for their effect on the extraction of heavy metals from contaminated water : 1) chelating agent; 2) reaction time; 3) chelating agent concentration; and 4) effect of pH on heavy metals extraction efficiencies.  Three chelating agents (chelating ligands)  were chosen: oxalic acid (OA) as a  bidentate ligand : ligand withtwo donor atoms. citric acid(CA) as a tridentate ligand : ligand with three donor atoms.Ethylene di amine tetra acetic acid (EDTA) as a hexadentateligand : ligand with six donor atoms.

Batch extractions of heavy metals contaminants using a common extracting concentration of 0. 1M were conducted. The extraction experiments were placed in 25O mL flasks .The flasks containing 50 ml of polluted water by selected heavy metals and 50 ml of 0.1 M ( OA or CA or EDTA) were shaken in rotary shaker at agitation rate of 200 rpm and in an isothermal (25⁰C)  for 120 minutes.

The exclusionwere centrifuged at 3000 rpm for 10 min and the exclusion were then filtered through a Watman-42 filter paper for heavy metal analysis. Then the flasks returned to the shaker. Additionally, the effect of another concentration (0.05 M and 0.01) of chelating agents (EDTA, OA, CA) on the removal efficiency of selected heavy metals was investigated. Finally, the pH of the solutions after mixing was measured using a pH meter. The pH of solution was changed by adding 0.1N HCl or 0.1N NaOH solution in the rang (2-10).The pH of the solution was continuously measured and adjusted to 10 using 1.0N sodium hydroxide.The pH of the solution was adjusted to below 3 using 1.0N hydrochloric acid. The percent of metals removed was calculated using an equation similar to the one earlier reported [16] as:

Percent metal removed (%) = ( C_i – C_o/ C_o) × 100

where C_o (mg /l) is the initial concentration, and C_i isthefinal concentration of  element present in wastewaterafter extraction.

Results and Disscussen

Stability Metal Ion Complex Formation

Chelating agents were introduced into industrial applications such as ethylene diamine tetra acetic acid (EDTA) , citric acid (CA) and oxalic acid (OA). EDTA changes the metal ion removal ability.EDTA can form stable complexes with the transition elements due to EDTA was hexadentate ligand. In the reaction between metal ion and EDTA forms very stable complex. Because EDTA molecule contains of six donor atomsshield metal ion as a substitute for six water molecules. It means thatthe result was aqua complex to EDTA metal complex. The geometry of EDTA complex with metal becomes octahedral and the hybridization orbitals were (sp³d²) since six donor atoms. The bonding will be through nitrogen and oxygen (N. and O.) which is a donor atom with unshared electron paircan coordinate a metal ion. The value of pH increasesas each complexhydrogen of carboxylate ion (–COOH) or aqua ion (–OH₂) is removed. EDTA reacts with metal ions, and forms a metal EDTA complex . The complex structures of EDTA and M (II) are shown in Fig. 1 [17].

In the same behavior the reaction between OA and CA with metal ions, donor atoms of OA and CA are oxygen (O) contain unshared electron pair, which can coordinate a metal ion. Citric acid consist of three donor atoms (O.) with unshared electron pair can coordinate a metal ion. Therefore, citric acid was considered to be tridentate ligand. Oxalic acid was classified as bidentate ligand because it contains tow donor atoms (O.).

Mⁿ⁺  +       EDTA      →      Metal – EDTA  Complex

Mⁿ⁺  +         OA         →       Metal – OA  Complex

Mⁿ⁺  +       CA     →       Metal – CA  Complex

Research show high stability constants of some metal EDTA, CA and OA complexes .The values of stability constants are indicators of stability of metal complex; if the value was higher the stability of metal complex was high and it was difficult to break the metal bond with chelating agents and remove the metal [18].

Figure 1: The M-EDTA complexstructure. Click here to View figure

 

Extraction Heavy Metals from Contaminated Water

Effect of Chelating Agent Concentration

The contaminated water by mercury and copper was prepared to see the ability of chelating agents to remove toxic heavy metals. EDTA is excellent chelating agent  and low cost, so it is the most widely used in industrial applications. Large quantities of EDTA have been released into the environment, it is difficult to remove it.Removal of metals from environmental is a problem facing metal related industries.

The results of the 2-h batch removing of heavy metals from the contaminated water with EDTA, CA and OA are presented in Table.1.As concentration of EDTA increased, the percentage of Hg removal from polluted water  increased reaching the highest value (60.3 , 63.2, 64.1 %) in ( 0.01, 0.05 ,0. 1 M) of EDTA. Relationship between percentage of Hg removed changes and EDTA concentration is almost linear. The behavior is similar in the case of citric acid(CA) and oxalic acid ( OA), as concentration of chelating agents increased, the percentage of Hg removed from polluted water  increased reaching the highest value (74.3 , 75.1, 76.6 %) in ( 0.01, 0.05 ,0. 1 M) of OA and (69.9 , 71.1, 72.8 %) in ( 0.01, 0.05 ,0. 1 M) of CA . Relationship between percentage of Hg removal changes and chelating agents concentration is almost linear. Similarly to the behavior described for cupper are presented in Table. 1.

As concentration of chelating agents increased, the percentage of Cu removed from polluted water  increased reaching the highest value (55.3 , 56.2, 59.5 %) in ( 0.01, 0.05 ,0. 1 M) of EDTA, (64.8 , 65.2, 66.6 %) in ( 0.01, 0.05 ,0. 1 M) of OA and (58.2 , 60.4, 62.8 %) in ( 0.01, 0.05 ,0. 1 M) of CA . Relationship between percentage of Hg removal changes and chelating agents concentration is almost linear.

Effect of Chelating Agent

The results of Experimental tests performed at different chelating agents  concentrations show that oxalic acid and citric acid were more effective with respect to EDTA in removing mercury and copper from polluted water presented in Table. 1. The order of chelating agents in effective to remove heavy metals from contaminated water by mercury and copper were: OA >CA > EDTA. As concentration was (0.1M) and pH=4.00, the percentage of Hg removed from polluted water was 64.1% in the case of EDTA.While the percentage of Hg removed from polluted water was 72.8% in the case of CA and 76.6% in the case of OA. The reason for ordering the effectiveness of the chelating agents may be that it depends on the classification of the chelating agents as a ligands : OA (bidentate ligands) > CA (tridentate ligands) > EDTA(hexadentate ligands).

Studied metal removals from polluted water sample were in the order: Hg >Cu, as using EDTA,as concentration was (0.1M), the percentage of Hg removed from polluted water was 64.1% in the case of EDTA .While the percentage of Cu removed from polluted water was 59.9%.

Table 1: Values of percentage metals removal from polluted water by different  chelating agents at pH=4.00 and room temperature.

Chelating Agents Co.	Hg			Cu
	EDTA	OA	CA	EDTA	OA	CA
0.01M	60.3	74.3	69.9	55.3	64.8	58.2
0.05M	63.2	75.1	71.1	56.2	65.2	60.4
0.1M	64.1	76.6	72.1	59.5	66.6	62.8

 

The Effect of Extraction Time

The effect of extraction time on selected heavy metals removal is shown in Fig.2. and Fig.3. indicates that the rate of metals removal were very rapid during first 50 min, and thereafter, the rate of metal removal remained stable. There were no significant increases found after 60 minutes of experiment and eventually it was the equilibrium time [19].

Figure 2: Effect of Extraction Time on Removal  ofmercury by 0. 1M of EDTA , OA and CA at pH=4.0 and room temperature. Click here to View figure

 

Figure 3: Effect of Extraction Time on Removal  of copper by 0. 1M of EDTA, OA and CA at pH=4.0 and room temperature. Click here to View figure

 

Effect of pH

pH as a main factor has a dominant role in the removal of heavy metals. The removal of Hg and Cu from polluted water decreased with the rising pH value of the  chelating agents solution. When the pH values were (2- 4), the removal of selected heavy metals were high .When the pH values were above 4, the removing heavy metals decreases as solution pH increased.Inmany previous studies have been shown that the pH value is an important factor controlling the removal of heavy metals from contaminated solutions [20].

Figure 4: Effect of pH on Removal  of mercury by 0. 1M of EDTA, OA and CA at room temperature. Click here to View figure

 

Figure 5: Effect of pH on Removal  of copper by 0. 1M of EDTA , OA and CA at room temperature. Click here to View figure

 

The contents of Cu and Hg extracted decreased Whenthe pH value of chelating agents solution increased from 5 to 10. Generally, when pH value of the solutionincreasesthe removal of heavy metals will decrease, as many metal complex has lower value of solubility at high value of pH.when pH value of EDTA, OA and CA solution increased,the extraction amount of the two metals decreased.

Coucluion

The studied contaminated water by  high concentration of mercury and cupper. Both analyzed chelators: EDTA, oxalic acid and citric acid were efficient for selected heavy metals removal from polluted water . Removal of analyzed metals was dependent on the concentration of extracting agent and raised with increasing concentration of chelators. The results of experimental tests tested at different chelators concentrations show that oxalic acid was more effective in removing selected heavy metals thancitric acid and EDTA.The removal of the tested metals depended on the pH value of the solution. The removal of heavy metals from polluted water decreased with the increasing pH value of the chelating agents solution.

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