Removal of Heavy Metals from Metal Wastes by Precipitation Technique

Introduction

Presence of heavy metals in the metal wastes is a great concern over the last few years. Elements like mercury and cadmium can cause toxic to aquatic organisms even at extremely low concentrations. The elements like silver, lead, chromium zinc and copper also a great threat to fish. There are various techniques for the removal of heavy metals from metal-containing wastes. Some of these techniques include chemical precipitation (hydroxide, carbonate, sulfide, or combination thereof), ion exchange, coagulation/flocculation, solvent extraction, complexation, cementation, electrochemical operations, filtration membrane process and evaporation. Among these techniques, chemical precipitation is most widely used for the removal of heavy metals from metal wastes. In chemical precipitation, a soluble compound is transformed into insoluble form by the addition of chemicals. The addition of chemicals thus produces a supersaturated environment in which precipitation of metals takes place. This technique include hydroxide, sulphide and carbonate precipitation processes.
Hydroxide precipitation is effective for the removal of arsenic, chromium (III), cadmium, iron, copper, manganese, nickel, zinc and lead. This precipitation process is the cheapest for the removal and recovery of chromium. The optimum pH for maximum removal with lime is 6.6 at which recovery of chromium exceeds 98%. This method, however, poses some limitations as hydroxide precipitates tend to resolubilize when the solution pH changes. Also in the presence of complexing agents, this technique has adverse effect on the metal removal. Carbonate precipitation is effective for the removal of nickel, cadmium and lead at a slightly lower pH than hydroxide or sulphide techniques. Carbonate precipitation has several advantages over the hydroxide precipitation technique. Some of these advantages are:
i)Metal carbonate precipitates are denser than the hydroxide precipitates which causes improved solids separation.
ii)Carbonate sludges are easily removed if compared to the hydroxide sludges.
Sulphide precipitation is highly effective for the removal of cadmium, cobalt, mercury, iron, copper, manganese, nickel, zinc, tin and silver. This technique has several advantages over other techniques. Some of the advantages include attainment of a high degree of metal recovery even at low pH, feasibility of selective metal recovery, sludges with better thickening and dewatering characteristics and requirement of less leaching at a pH of 5 as compared to hydroxide sludges.

Aims/Objectives

The objectives of this experiment are to experiment on metal recovery from wastewater by hydroxide precipitation technique and to study the effect of ferric chloride on the precipitation of heavy metals in the presence of complexing agents.

The Materials, Apparatus Used and the Method

A sample solution, calcium hydroxide, sodium hydroxide, 2 M HCl, 50 g/l Fe Cl3 solution, atomic absorption spectrometer, ph-meter, stirring plate, assorted glassware and whatman filter paper No.40.

Method

Preparation of solutions:

  • Prepare 100 ml of the electrodeless copper plating solution as per given instructions. Prepare 2 litres of rinse water (sample solution) by diluting the copper solution by 100 times. Measure the pH of sample solution and analyze its copper concentration by AA spectrometer.
  • Prepare a 250 grams of 5% w/w slurry of Ca(OH)2.
  • Prepare a 250 grams of 5% w/w slurry of NaOH.

Precipitation of copper from the sample without pretreatment:

Take 100 ml of sample water in a beaker and stir the sample at 100 rpm. Then measure pH of sample and add well-suspended Ca(OH)2 slurry rapidly to increase pH of the sample to 10. Continue the stirring for three minutes and then allow the precipitate to stand for 30 minutes. Filter the supernatant through whatman filter paper and analyze the filtrate for copper concentration. Repeat the above experiment expect add the slurry dropwise. Similarly repeat the above experiment by using NaOH rapidly and dropwise.

Precipitation of copper with prior pH adjustment to 2:

Take 100 ml of sample water in a beaker and stir the sample at 80 rpm. Measure pH of sample and then adjust its pH to 2 with the addition of HCl. Add well-suspended Ca(OH)2 slurry rapidly to increase pH of the sample to 10. Continue the stirring for three minutes and then allow the precipitate to stand for 30 minutes. Filter the supernatant through whatman filter paper and analyze the filtrate for copper concentration. Repeat the above procedure for all the steps as mentioned in procedure (B).

Precipitation with pretreatment with FeCl3 and pH adjustment:

Take 100 ml of sample water in a beaker and stir the sample at 80 rpm. Add FeCl3 solution to the sample to increase its Fe concentration twice than that of EDTA. Measure pH of sample and then adjust its pH to 2 with the addition of HCl. Add well-suspended Ca(OH)2 slurry rapidly to increase pH of the sample to 10. Continue the stirring for three minutes and then allow the precipitate to stand for 30 minutes. Filter the supernatant through whatman filter paper and analyze the filtrate for copper concentration. Repeat the above procedure for all the steps as mentioned in procedure (B).

Results

Monitoring of pH

Sr. ? Sample

?.

pH before adding Ca(OH)2 or NaOH pH of supernatant before adding Ca(OH)2 or NaOH

1

B1

10.8

2

B2

10.8

3

B3

11.2

4

B4

10.97

5

C1

9.98→1.89

10.84

6

C2

9.98→1.96

10.71

7

C3

9.98→2.00

10.52

8

C4

9.98→1.98

10.80

9

D1

3.27→2.00

10.58

10

D2

3.04→2.01

10.69

11

D3

3.00→1.98

10.83

12

D4

2.98→2.00

10.71

Monitoring of concentration of copper in filtrate
Concentration of copper in the given sample = 4.2797 ppm.

Sr. No. Sample No. Concentration of copper in filtrate (ppm)
1 B1

0.0555

2 B2

4.0331

3 B3

4.1955

4 B4

4.0184

5 C1

0.1115

6 C2

0.8029

7 C3

4.1112

8 C4

3.9574

9 D1

0.2098

10 D2

0.1644

11 D3

3.4868

12 D4

2.9936

Effect of adjusting pH to 2 before precipitation

It has been found that by adjusting the pH to 2 before precipitation increases the solubility of copper and other contaminants in the solution. This pretreatment has a great effect on the copper removal by raising the pH to 10 by adding Ca(OH)2 dropwise. On the other hand, no significant improvement in copper removal from sample solution has been noted under the similar conditions when NaOH is used for raising the pH to 10. It is therefore concluded that Ca(OH)2 is better precipitating agent than NaOH when complexing agent is present in the solution as flocculation and sedimentation is more favored by using Ca(OH)2 in the presence of complexing agents.

Effect of rate of addition of alkalines on the copper removal

It has been observed that copper precipitation as Cu(OH)2 increases by increasing the rate of addition of Ca(OH)2. Improvement in the formation of copper precipitate with increased rate of addition of hydroxide is observed only when Ca(OH)2 is used as a precipitating agent. It has been observed that there is no significant effect on copper removal by increasing the rate of addition of NaOH.

Effect of addition of FeCl3

In the presence of EDTA, copper in the solution forms Cu-EDTA complex. This complex poses a great difficulty for removal of copper as Cu(OH)2 by adding of FeCl3.
Cu-EDTA complex is converted to Fe-EDTA complex and copper is precipitated out as Cu(OH)2. It has been found that 20-25% copper recovery from the wastewater can be enhanced by the addition of FeCl3 when EDTA is present.

Method to destroy EDTA

EDTA can be destroyed by oxidation with suitable oxidizing agents. There are many oxidizing agents available for the oxidation of EDTA. Some of these oxidizing agents include chlorine, sodium hypochlorite and ozone. It has been found that EDTA can be also destroyed by heating.

Discussion:

  • In order to experience metal recovery from wastewater by hydroxide precipitation technique and also to study the effect of ferric chloride on the precipitation of heavy metals in the presence of complexing agent, an experiment was conducted. In this experiment it has been observed that:
    Recovery of copper as Cu(OH)2 is more significant when Ca(OH)2 is used (as compared to NaOH) as a precipitating agent for increasing the pH to 10 and Ca(OH)2 is added at an increased rate. This behavior of Ca(OH)2 is observed in all the cases i.e irrespective whether precipitation of copper occurs without pretreatment, prior pH adjustment to 2 or with pretreatment by ferric chloride and pH adjustment. This is because Ca(OH)2 is a better precipitating agent as compared to NaOH and it works equally good whether complexing agents is present in the solution or not. However its activity is significantly low when it is used drop wise without pre adjustment of solution pH to 2. Activity of Ca(OH)2 as a precipitating agent, on the other is highly increased for drop wise addition when it is used after pre adjustment of solution pH to 2. Treatment of sample solution with pretreatment with FeCl3 and prior pH adjustment has no significant effect as far as drop wise addition of Ca(OH)2 is concerned.
  • Recovery of copper as Cu(OH)2 is more significant when NaOH is used as a precipitating agent for increasing the pH to 10 with pretreatment of sample with FeCl3 and prior pH adjustment, especially when NaOH is used dropwise. It means NaOH is ineffective when it is used without FeCl3in the presence of a complexing agent such as EDTA. In the presence of FeCl3, soluble Cu-EDTA complex is converted to Fe-EDTA complex and copper is precipitated as Cu(OH)2. It has been found from the experiment that 20-25% copper recovery from the wastewater can be enhanced by using NaOH when FeCl3 is added in the presence of EDTA.

Conclusion

From the above mentioned experiment, it is concluded that hydroxide precipitation with Ca(OH)2 as a precipitating agent is more effective and economical, even in the presence of a complexing agent without pretreatment with FeCl3 and without prior pH adjustment, when Ca(OH)2 is added rapidly. On the other hand, hydroxide precipitation with NaOH is effective only when the sample solution containing a complexing agent is pretreated with FeCl3 and its pH is adjusted to 2 prior to precipitation during which NaOH is added drop wise.

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