WO2018235106A1

WO2018235106A1 - WASTEWATER METAL ELIMINATING APPARATUS AND METHOD THEREOF

Info

Publication number: WO2018235106A1
Application number: PCT/IN2018/050412
Authority: WO
Inventors: Prof.Raghuram CHETTY; Prof.Indumathi M NAMBI; Saranya SRIRAM
Original assignee: Indian Institute of Technology Madras
Current assignee: Indian Institute of Technology Madras
Priority date: 2017-06-23
Filing date: 2018-06-23
Publication date: 2018-12-27
Anticipated expiration: 2019-12-23

Abstract

An electrochemical cell configuration to simultaneously treat a nitrogenous animal waste on one hand and to reduce a carcinogenic metal ion on the other. Apparatus and methods are disclosed to reduce hexavalent chromium (Cr⁺⁶) to a lower nontoxic state i.e. Cr⁺³, with cow urine which is an immoderate resource as an anolyte fuel.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to wastewater treatment, particularly to wastewater treatment methods and apparatus based on electrochemical cell. More specifically, to a self- sustaining electrochemical cell to reduce chromium to lower nontoxic form using urea as anolyte.

BACKGROUND OF THE INVENTION

[0002] The toxicity of heavy metals like arsenic, lead, mercury, chromium etc. in the water bodies pose a major threat to the environment and human life. In the midst of the above mentioned heavy metals, hexavalent chromium (Cr (VI)) in its various speciation form is carcinogenic and is widely used in electroplating, textile dyeing, paints, pigments and tannery industries. These industrial discharges ultimately find their way to contaminate the environmental matrices such as surface water, ground water and the soil phase. Traditional remediation techniques include, physico-chemical and biological reduction, electrochemical methods like electro-coagulation with different electrodes, photo-catalytic degradation etc. all of which has been well established. Despite the availability of many in- situ technologies, the electrochemical method offers advantages of no chemical requirement and ease of operation with no residual contamination. The limitations to scale up the existing technique are that they are either energy intensive or the process involves only phase transfer of pollutants rather than the complete metal ion destruction, thus attributing them unsustainable for real scale applications .

[0003] Electrochemical cell to reduce Cr (VI) with animal excretion, specifically cow urine as an anolyte fuel has been reported in the art.

[0004] Dutta et al, 2006 investigated Cr (VI) and Mn0₂ for genotoxic and clastogenic effects on human lymphocytes. Though literatures are fewer on applications of cow urine, the authors have reported the cow urine to possess anticlastogenic and antigenotoxic effects. The authors have reported Cr (VI) to undergo cellular reduction to form Cr (III) with 8 - oxo-guanine which is excreted along the human urine. Also it is reported that Cr (VI) reacts with H₂0₂ to form Cr (V) .

[0005] Ojedokun et al, 2016 investigated the adsorption of heavy metals such has Pb, Cr using cow dung as a bio sorbent. The effect of pH, sorption contact time, adsorbate concentration, adsorbent dosage and temperature is studied. Adsorption may not be a sustainable method of treating metals as it involves only phase transfer of the pollutant rather than complete metal ion reduction. Treating the cow dung with the metal is again challenging as the heavy metals would now be adsorbed onto the cow dung from wastewater. This study demonstrates as Cr (VI) is not reduced or degraded as adsorption involves only mass transfer of pollutant from one other.

[0006] Xu et al, 2016 investigated the reduction of Cr (VI) with human urine as oxidant along with power generation. The authors have developed a three chambered cell such as AEM - KCl - CEM. In the study, the anion exchange membrane (AEM) is placed closed to anode and the cation exchange membrane (CEM) is placed closed to cathode. The anode and cathode compartments are separated by KCl solution. Their intention to place CEM close to the cathode is to prevent the migration of Cr (VI) from the cathode. The AEM is placed near anode to prevent any NH₄ ⁺ion from the anode chamber and the KCl completes the circuit. Ni nano catalyst coated over the carbon cloth was their anode and bare carbon cloth was their cathode. With human urine as the anolyte fuel and 300 ppm of Cr (VI) at cathode and an external resistance of 200 ohm, the reduction experiments were performed. The authors have reported a reduction of 93 % after 71 hours.

[0007] C. Modrogan et al, 2010 explored the reduction of nitrates and Cr (VI) contaminated groundwater with Fe°. The simultaneous oxidation of Fe° to Fe⁺² gives electrons in the acidic medium. These electrons are accepted by nitrates and/ or Cr (VI) present in the water. Although the technique has a broader applicability and is cost effective, the possible limitation of this technique would be the post treatment of the iron particles leached in the system. Also, partial adsorption of the Cr (VI) could happen as the dichromate anions may have an electrostatic attraction towards the positively charged Fe particles. Moreover, the stability of the iron particles in extreme pH is of a great concern, as Fe is highly prone to corrosion, which may also reduce the efficiency of the reduction. As both aerobically and anaerobically the Fe° particle undergo corrosion and the dissolution of the particle in the adjoining water matrices can be a secondary problem, which also requires treatment. The authors have considered nitrate contamination along with Cr (VI) which replicates the field condition. Also, the reduction of nitrates to ammonia by this redox method again leads to the secondary environmental pollution as NH₄ ⁺ should further be converted to N2.

[0008] Jackson, 1994 investigated the production of alkali or alkaline earth metal chlorates and chlorine dioxide. The authors have reported the addition of Cr (VI) in the electrolyte to increase the current efficiency of cells, especially for the production of sodium chlorate. It is reported that for the production of every ton of sodium chlorate, about 2 - 10 kg of sodium chromate is utilized in electrolyte cells and the solution is left back with no efforts of removing the pollutant with chromium. Despite several prior methods being well reported for the removal of Cr (VI) from such industrial discharges, the existing techniques are mostly proven uneconomical on field scale conditions. The authors here present to remove Cr (VI) with a reducing agent and precipitated thereafter and recycled back to the electrochemical cell for the production of metal chlorate solutions The reported study is a chemical reduction process where Cr (VI) is reacted with hydroxylamine sulphate, which is a chemical oxidizing agent which has to be carefully assessed after the reaction with Cr (VI) .

[0009] Another study reported by Tripathi et al,

2011 isolated a native bacterial strain for simultaneous microbial degradation of Cr (VI) and pentachlorophenol . Cow urine is employed as a source of nitrogen for the bacterial growth. The authors have reported that the use of cow urine has significantly increased the growth response than urea or ammonium chloride or ammonium nitrate. A Cr (VI) reduction of 74.5 % was observed with the isolated native bacterial strain. The reported study involves "microorganisms" as the key player involved in the metal ion reduction, whereas in our system "electrons" are the key components. The authors have employed cow urine as a source of nitrogen to enhance the growth of bacterial cells.

[00010] Dasgupta et al, 2013 investigated an embodiment to describe (a) Electrodialytic buffer generator i.e., such as desalination system and (b) electrodialytically generating buffer i.e., generation of buffer through electrolysis. The Electrodialytic buffer generator is a 3 electrode buffer system operated in a flow through mode, with an Anode (+), cathode (-) and a third electrode in the central buffer chamber. The outlet of the chamber 1 is connected to the inlet of chamber 2 and the outlet of chamber 2 is connected to inlet of chamber 3 respectively. The central buffer unit is separated from the anode and cathode chambers with ion exchange membranes namely CEM and AEM respectively. An aqueous electrolyte (0.5 M NaH₂P0₄ or NaHPC^) is filled at the anode and cathode chambers (with pH between 5 and 8) and the central buffer chamber is filled with deionized water. Bipolar plates have been employed as current collectors for the proper current flow between chambers 1 & 2 and chambers 1 & 3 respectively. The ion exchange membranes and the electrodes are sandwiched to form a membrane electrode assembly

(MEA) , typically an orientation of a fuel cell on continuous / flow through mode. Gasket screens are selectively placed between the electrode and the membrane in order to prevent any further leaking and also to aid in preventing any detrimental effect to the MEA. A load of 2 x 10 ohm is applied across the electrodes and the current flowing between the anode and central electrode is termed as CEM current

(i.e. current eventually across the CEM) and similarly, the current flowing between central and third electrode is termed as AEM current (i.e. current flowing across AEM) . Although, the authors have not mentioned the potential that is applied to buffer generator cell, the chief principal of the present study is that, on the application of potential between the electrodes (which are either connected to the positive, negative or the grounded terminal), the ions in the respective electrolytic chamber drift towards to either anode or cathode based on their charge. This evidently demonstrates that electrolysis happens at all the three electrodes. By varying the power source, the CEM current generated is controlled to either be ≥ to AEM current. The primary mechanism is that, depending on the current generated by the CEM or AEM, determines the amount of OH^~ and H⁺ ions in the systems are formed. As the potential is continually applied, these ions tend to transfer to their respective electrodes with the associated electrolysis of water. Upon this, the O₂ and H₂ gas is liberated with water oxidation and reduction. This causes an effective pH gradient in the system with prolonged electrolysis. Additionally, the anionic and cationic electrolyte split as Na⁺ and ¾Ρθ₄ ^~ or ΗΡ0₄ ^~ . The selective transportation of these ions relies on the ion selective membrane to pass through. When the CEM current is greater than the AEM current, the authors have reported to have an evident formation of ¾ and OH^~ at CEM and H⁺ and O₂ at AEM. The liberation of either the oxygen or hydrogen at anode and cathode has periodically been removed through a degassing unit. The authors have also mentioned on several combinations of these electrodes to achieve and operate the same cell to either desalinate the high brackish water or to regenerate the buffer within the same system. Moreover, by altering the applied potential or current, the pH gradient or a concentration gradient can be achieved. The authors thus claim for the following that; An electrodialytic 3 chambered cell is designed on flow through mode. Three electrodes in three chambers have been simultaneously used. Ion selective membranes such as AEM & CEM are employed. The AEM passes anions and blocks the cations; CEM passes the cations and blocks the anion. An aqueous cationic electrolyte is filled between chamber 1 and the central buffer chamber. An aqueous anionic electrolyte is filled between central chamber and 3^rd chamber. The potency of the designed cell relies on either generation of buffer or to deionize the electrolyte. A degassing unit to remove the gases formed to reconfirm on the claimed hypothesis with the generation of H⁺ or OH^~ and / generation of ¾ or O₂.

[00011] Even though several literatures have been reported to handle Cr (VI) by physical, chemical or biological means; there does exist limitation for each of them. The major issue of concern in all of them being the secondary pollution or the intermediates formed due to the pollutant reduction as they coexist in actual scenario.

[00012] Many of the known techniques fail to perform on real scale of either being expensive or leaving behind residual contamination. Thus, an alternate, sustainable technology mandates the need to handle pollutants. We have herein disclosed a metallurgical in-situ self-sustained apparatus and method, extracting the potential from the wastewater .

References

[00013] (1)W. Xu, H. Zhang, G. Li, Z. Wu, J.

Electroanal. Chem, A urine/ Cr fuel cell - Electric power from processing heavy metal and human urine 764 (2016) 38-44.A.

(2) Daramola, D. Singh, G. Botte, J. Phys . Chem. Dissociation Rates of Urea in the presence of NiOOH catalyst: A DFT analysis 114 (2010) 11513-11521.

(3) Wang, G., Huang, L. & Zhang, Y. Biotechnology Lett (2008) 30: 1959. doi : 10.1007/sl0529-008-9792- 4

(4) H.M Zhang, W. Xu, G. Li, Z.M Liu, Z.C Wu ,B.G Li, Sci Rep , Assembly of coupled redox fuel cells using copper as electron acceptors to generate power and its in-situ retrieval (2016) 1-8.

(5) D.Dutta, S.S.Devi, K. Krishnamurthi , T. Chakrabarti, J. BES, Anticlastogenic effect of redistilled cow's urine distillate in human peripheral lymphocytes challenged with manganese dioxide and hexavalent chromium 19 (2006) 487-494.

(6) A.T. Ojedokun, O.S. Bello, Water Resources and Industry 13 (2016) 7-13.

(7) C. Modrogan, A.R. Miron, O.D. Orbulet, CM. Costache, Bulletin UASVM Agriculture 67 (2010) 80- 86.

(8) J.R. Jackson, Integrated process for the production of alkali and alkaline earth metal chlorates and chlorine dioxide, United States Patent .

(9)M. Tripathi, S. Vikram, R.K. Jain, S.K. Garg, Indian J Microbiol 51 (2011) 61-69.

OBJECTS OF THE INVENTION

[00014] Therefore, it is an object of the invention to disclose apparatus for complete reduction of Chromium (VI ) electrochemically to Chromium (III) which is non-toxic.

[00015] It is yet another object of the invention to disclose the present method to reduce a high concentration (400 ppm) of Cr with a reduction efficiency of 99.98 % within 45 minutes.

[00016] It is yet another object of the invention to disclose an electrochemical cell configuration to simultaneously treat an immoderate nitrogenous animal waste on one hand and to reduce a carcinogenic pollutant on the other.

[00017] It is yet another object of the invention to disclose apparatus and methods with zero energy input, ease of operation, catalyst free anode and cathode as electrodes and no residual contamination achieved with alternate membranes, electrodes and electrolytes .

SUMMARY OF THE INVENTION

[00018] To meet the objects of the invention and overcome the disadvantages of the prior art it is disclosed here an apparatus for removal of metals from waste water comprising of: an electrochemical cell for reduction of a metal in waste water comprising of:

i. an anodic chamber with an anode in contact with an anolyte;

ii. a cathodic chamber with a cathode in contact with the metal containing waste water; and

iii. a buffer chamber containing a buffer solution positioned between the said anodic and cathodic chambers, the anodic chamber connected to the buffer chamber through a cation exchange membrane and the cathodic chamber connected to the buffer chamber through an anionic exchange membrane, the buffer chamber allowing the transport of ions between the anodic and cathodic chambers, characterized in that, an open circuit voltage (OCV) developed between the electrodes and electrolytes drives the spontaneous oxidation of the anolyte at anode and metal reduction at cathode; and b. a means to extract the reduced metal.

[00019] Also disclosed herein a method of removing a metal from waste water comprising the steps of:

i . providing an anode chamber having an anode adapted to be in contact with an anolyte ; providing a cathode chamber having a cathode adapted to be in contact with hexavalent containing waste water;

positioning a buffer chamber containing a buffer solution between the said anodic and cathodic chambers, the anodic chamber connected to the buffer chamber through a cation exchange membrane and the cathodic chamber connected to the buffer chamber through an anionic exchange membrane, the buffer chamber allowing the transport of ions between the anodic and cathodic chambers, characterized in that, an open circuit voltage (OCV) developed between the electrodes and electrolytes drives the spontaneous oxidation of anolyte at anode and metal reduction at cathode; and extracting the reduced metal

BRIEF DESCRIPTION OF THE DRAWINGS

[00020] Fig.l (a) is the schematic representation of three chambered cell with electrodes, electrolytes and membranes

Fig.l (b) is the graphical representation of removal efficiency of 400 ppm Cr(VI) dissolved in 0.5M H₂S0₄ at a constant load of 1000 ohm at room temperature. Fig.2 is the schematic representation the 3 chambered cell with ion separators, anolyte fuel and catholyte at a constant load.

Fig.3 (a) is the graphical representations of the comparison of cathodic efficiencies for different membrane configurations and 3 (b) the reduction at different initial concentrations of Cr with urea as anolyte fuel

Fig.3(c) is the graphical representations of comparison between influence of Milli Q water and 0.5 M phosphate buffer solution (PBS) for Cr (VI ) reduction at cathode and 3 (d) reduction kinetics at different concentrations with cow urine as anolyte fuel in anode-CEM-PBS-AEM-cathode system and a constant load of 1000 Ω.

DETAILED DESCRIPTION OF THE INVENTION

[00021] The invention and its various embodiments is better understood by reading the description along with the accompanying drawings which appear herein for purpose of illustration only and does not limit the invention in any way.

[00022] To meet the stated object, referring to FIG.l and FIG 2, it is disclosed here a three- chambered cell namely anode chamber (A) , middle (buffer) (B) and a cathode chamber (C) .

[00023] Urea and cow urine has been employed as anolyte to reduce Cr(VI) to Cr(III) . The buffer channel separates the anode and cathode compartments maintaining the overall ionic balance in the system. The major highlight of the present invention includes the fundamental understanding for the need of membrane designs to primarily curtail the fuel crossover between the anode and cathode chamber which enhances the carcinogenic metal ion reduction. A CEM placed next to anode and an AEM placed next to anode have effectively shown to improve the efficiency of Cr reduction. Ni foam as anode and catalyst free carbon felt as cathode was used. The technology can thus be established in actuality as a self-driven alternate method to simultaneously oxidize the animal excretion on the anode and to reduce the Cr (VI) at the cathode to its lower nontoxic form.

Experimental section:

[00024] The electrodes: Ni foam and carbon felt were procured from Sigma Aldrich and AVCARD respectively. The AEM and CEM were procured from FuMa-Tech, Germany and DuPont, USA. The fresh electrodes were rinsed with MilliQ water, oven dried at 80°C and used. The anolyte used for study was 0.1 M urea dissolved in 1 M KOH and the pH of the anode chamber was maintained between 10 and 12. The phosphate buffer solution was prepared with K2HPO4 and KH2 PO4 in distilled water with pH 7 and 8. The Cr (VI) as the catholyte was prepared using K₂Cr₂0₇ (oven dried for 2 h at 110 °C) in 0.5 M H₂S0₄ with pH maintained between 1 and 2. Standard Cr solution of desired concentrations was prepared from the stock solution. Followed by the experiment with urea, fresh cow urine was also employed as the anolyte fuel and experiments were performed with different initial concentrations of Cr (VI) . All the experiments were carried out connecting a 1000 ohm resistance as a constant load with copper wire used as current collector at ambient temperature and pressure in triplicates and the mean values are reported. All the chemicals used were of analytical grade .

[00025] The reduction in Cr (VI) concentrations were studied using calorimetric 1, 5 diphenylcarbazide method for absorption at 540 nm (Xu et al, 2016) . At periodical sample intervals, 1 mL of the sample was withdrawn from the cathode compartment. The cathodic efficacy was calculated on the subtraction of Cr (VI) measured from the initial concentrations in close circuit conditions, as represented in the following equation:

Cathodic efficiency = {(Initial concentration - Final concentration) / (Initial concentration)} X 100 %

Electrochemical cell fabrication:

[00026] Five different membrane combinations namely (i) without membrane (ii) with only AEM (iii) with only CEM (iv) Sandwiched membrane (with both AEM & CEM. (v) A three chambered H cell with AEM, CEM and a phosphate buffer chamber was investigated as mentioned earlier. The cell configurations from (i to iv) have been performed in dual chambered H cell, where the anolyte was 0.1 M urea dissolved in 1M KOH (pH 10 to 12) and the catholyte was 100 mg/L Cr (VI) dissolved in 0.5 M H₂S0₄ (pH 1 to 2) . A similar extension of the dual chambered was constructed as a three chambered cell (v) with the anolyte chamber (A) , a phosphate buffer chamber (B) and a catholyte chamber (C) . Ni foam and Carbon felt electrode were employed as the anode and cathode as shown schematically in Figure 2. Results and discussion:

[00027] Series of experiments were performed to primarily understand the self-oxidation of urea/cow urine at the anode in each of the membrane systems using a two chambered cell. The transfer of electrons generates a current which is indirectly harvested by the Cr (VI) at the cathode. To validate the design aspects of each of these membrane configurations, the Cr (VI) concentration was monitored as a function of time by applying a constant load of 1000 ohm. Our results revealed that the sandwiched membrane performed better over the other individual membrane configurations (Figure 3 (a)) with a 100 mg/L Cr (VI) reduction efficiency of 98.21 % in 52 h. Further we designed a three chambered cell to explore the reduction efficiency of a similar Cr concentration. We hypothesize that when the individual membranes were employed there is undoubtedly an increase in the ionic movement in the system due to the crossover of the ions from both the compartments. The ionic diffusion predominated resulting in prolonged time for the reduction of same 100 mg/L Cr (VI) . Even though the open circuit voltage (OCV) was relatively high at all these membrane systems, it did not significantly reflect in Cr reduction. On the other hand, when the membranes were sandwiched such as the CEM facing the anode chamber and AEM facing the cathode chamber, we could significantly perceive that the ionic movements between the two chambers were greatly curtailed. The OCV observed here was 0.9 V. This could also serve the purpose of distinctly sustaining the pH at both chambers, limiting the fuel and catholyte crossover and thereby enhancing the reduction.

[00028] The major highlight of the sandwiched membrane is that, the appropriate positioning of the CEM and AEM have served the purpose not to allow the H⁺ and OH ions primarily which has aided in faster reduction. The membranes functions to separate the ionic interference from the individual chambers and thereby maintain pH throughout the reduction period. The anode and cathode chambers entail two extreme pH conditions to independently perform their oxidation and reduction. Thus, we emphasize here that an ideal separator is required to simultaneously utilize a waste to decontaminate a toxic pollutant. Hence, to improve the efficiency of the metal ion reduction, a novel three chambered cell was configured with a phosphate buffer as separating channels between the anode and cathode compartments was designed. With the combination of a phosphate buffer channel in between the anolyte and catholyte, the efficiency was appreciably improved to 99.27% with a reduction 100 mg/L Cr (VI) in 120 min with urea as anolyte fuel and an applied load of 1000 ohm as shown in figure 3 (b) . When the PBS chamber was replaced with Milli Q water, the concentration of Cr remained almost the same as initial metal ion concentration at 2 hrs (2.87 %), whereas the reduction was drastically improved with PBS chamber having a reduction efficiency of 99.27 % ( figure 3 (c) ) .

[00029] Wang et al, 2008 has observed a reduction of 100 mg/1 Cr (VI) in 150 h in a Microbial Fuel Cell at pH 2. The membrane configurations in the present study have significantly reduced the reduction time for the same metal ion concentration in a redox fuel cell. The reduced Cr⁺³ species were prevented from leaving the cathode chamber by the

2-

AEM. The SO 4 ion moves from cathode to chamber [B]

3- and P0₄ from chamber [B] to [C] . The CEM prevents the diffusion of OH to [C] and the overall ionic balance is maintained by the buffer addition. So far to our knowledge, literatures have not reported this efficacy in Cr (VI) reduction on such 3 chambered configuration electrochemical cell mode. To confirm the data reproducibility, the experiments were repeated in triplicate. A pronounced reduction in Cr (VI) is reported in our study when compared with the previous reported literatures. Zhang et al, 2016 reported the reduction of 99.9 % 400 mg/L Cu⁺² with NaBH₄ over 24 h in a four chambered cell employing two cationic exchange membranes.

[00030] As satisfactory results with urea as the anolyte fuel were established, the study was further extended with cow urine as anolyte fuel to have realistic picture as shown in figure 3 (d) . Cow urine is an immoderate resource which is scarcely attempted for enhancing the reduction of carcinogenic pollutants. The electro oxidation of urea / cow urine on Ni foam is the key player for the metal ion reduction at the cathode. The quasi

+2 reversible oxidation of the transition metal Ni to

Ni⁺³ in the alkaline medium marks the spontaneous oxidation of urea (electron donor) to render electron flow from the anode compartment (Daramola et al, 2010) . The electron generated at the anode compartment, is readily consumed up by the Cr (VI) as electron acceptor at the cathode and is reduced ( Figure 2 ) .

[00031] Herein, to mark a beginning of its usage to resolve practical difficulties in handling such Cr (VI) industrial discharges, the invention has attempted to simultaneously handle the dual wastes which are disposed into the environment. Thus, the present invention has strived to reduce carcinogenic metal pollutant on one hand and as well oxidize a nitrogenous waste resource on the other in principal by electrochemical technique understanding the contribution of membranes and their orientation. The present invention is sustainable in nature, which can definitely be commercialized as prototype and is absolutely eco- friendly .

[00032] It will be obvious to a person skilled in the art that with the advance of technology, the basic idea of the invention can be implemented in a plurality of ways. The invention and its embodiments are thus not restricted to the above examples but may vary within the scope of the claims [00033] Further the above-described embodiments of the present invention are intended to be examples only. Alterations, modifications and variations may be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto.

Claims

We Claim:

1. An apparatus for removal of metals from waste water comprising of:

an electrochemical cell for reduction of a metal in waste water comprising of:

an anodic chamber with an anode in contact with an anolyte;

a cathodic chamber with a cathode in contact with the metal containing waste water; and

a buffer chamber containing a buffer solution positioned between the said anodic and cathodic chambers, the anodic chamber connected to the buffer chamber through a cation exchange membrane and the cathodic chamber connected to the buffer chamber through an anionic exchange membrane, the buffer chamber allowing the transport of ions between the anodic and cathodic chambers, characterized in that, an open circuit voltage (OCV) developed between the electrodes and electrolytes drives the spontaneous oxidation of the anolyte at anode and metal reduction at cathode; and a means to extract the reduced metal .

An apparatus as claimed in claim 1 wherein the said anode may be Nickel foam.

An apparatus as claimed in any of the preceding claims wherein the said anolyte may be urea.

An apparatus as claimed in any of the preceding claims wherein the said anolyte may be cattle urine.

5. An apparatus as claimed in any of the preceding claims wherein the said cathode may be a catalyst free carbon felt.

6. An apparatus as claimed in any of the preceding claims wherein the said metal is hexavalent Chromium.

An apparatus as claimed in any of the preceding claims wherein the said reduced metal is Chromium (III)

An apparatus as claimed in any of the preceding claims wherein the said buffer solution is a solution containing phosphate prepared with K2HPO4 and KH2 PO4 in distilled water.

A method of removing a metal from waste water comprising the steps of:

providing an anode chamber having an anode adapted to be in contact with an anolyte;

providing a cathode chamber having a cathode adapted to be in contact with hexavalent chromium containing waste water;

positioning a buffer chamber containing a buffer solution between the said anodic and cathodic chambers, the anodic chamber connected to the buffer chamber through a cation exchange membrane and the cathodic chamber connected to the buffer chamber through an anionic exchange membrane, the buffer chamber allowing the transport of ions between the anodic and cathodic chambers, characterized in that, an open circuit voltage (OCV) developed between the electrodes and electrolytes drives the spontaneous oxidation of anolyte at anode and metal reduction at cathode; and

extracting the reduced metal.

10. A method as claimed in claim 9 wherein the said anode is Nickel foam.

11. A method as claimed in claims 9 or 10 wherein the said anolyte may be urea.

12. A method as claimed in claims 9, 10 or 11 wherein the said anolyte may be cattle urine.

13. A method as claimed in claims 9, 10, 11 or 12 wherein the said cathode may be a catalyst free carbon felt.

14. A method as claimed in claims in 9, 10, 11, 12 or 13 wherein the said metal is hexavalent Chromium.

15. A method as claimed in claims in 9, 10, 11, 12, 13 or 14 wherein the said reduced metal is Chromium (III) A method as claimed in claims 9, 10, 11, 12, 13, 14 or 15 wherein the said buffer solution is a solution containing phosphate prepared with K2HPO4 and KH2 PO4 in distilled water.