Treatment of Process Water Containing Heavy Metals with a Two‐Stage Electrolysis Procedure in a Membrane Electrolysis Cell

Abstract

The capability of a two‐stage electrochemical treatment for the regeneration of acidic heavy‐metal containing process water was examined. The process water came from sediment bioleaching and was characterized by a wide spectrum of dissolved metals, a high sulfate content, and a pH of about 3. In the modular laboratory model cell used, the anode chamber and the cathode chamber were separated by a central chamber fitted with an ion exchanger membrane on either side. The experiments were carried out applying a platinum anode and a graphite cathode at a current density of 0.1 A/cm2. The circulation flow of the process water in the batch process amounted to 35 L/h, the electrolysis duration was 5.5 h at maximum and the total electrolysis current was about 1 A. In the first stage, the acidic process water containing metals passed through the cathode chamber. In the second stage, the cathodically pretreated process water was electrolyzed anodically. In the cathode chamber the main load of dissolved Cu, Zn, Cr and Pb was eliminated. The sulfuric acid surplus of 3–4 g/L decreased to about 1 g/L, the pH rose from initially 3.0 to 4–5, but the desired pH of 9–10 was not achieved. Precipitation in the proximity to the cathode evidently takes place at a higher pH than farther away. The dominant process in the anode chamber was the precipitation of amorphous MnO2 owing to the oxidation of dissolved Mn(II). The further depletion of the remaining heavy metals in the cathodically pretreated process water by subsequent anodic treatment was nearly exhaustive, more than 99 % of Cd, Cr, Cu, Mn, Ni, Pb, and Zn were removed from the leachate. The high depletion of heavy metals might be due to both the sorption on MnO2 precipitates and/or basic ferrous sulfate formed anodically, and the migration of metal ions through the cation exchanger membrane via the middle chamber into the cathode chamber. In the anode chamber, the sulfuric acid content increased to 6–7 g/L and the pH sank to 1.7. All heavy metals contained, with the exception of Zn, were removed to levels below the German limits for discharging industrial wastewaters into the receiving water. Moreover, the metal‐depleted and acid‐enriched process waters could be returned to the leaching process, hence reducing the output of wastewater. The results indicated that heavy metals could be removed from acidic process waters by two‐stage electrochemical treatment to a large extent. However, to improve the efficiency of metal removal and to establish the electrochemical treatment in practice, further work is necessary to optimize the operation of the process with respect to current density, energy consumption, discharging of metal precipitates deposited in the electrode chambers and preventing membrane clogging.

Metal recovery from spent hydrodesulfurization catalysts using a combined acid-leaching and electrolysis process

“…Among these techniques, electrolysis is attractive for its versatility, energy efficiency, simple equipment, easy operation, and low operating cost [17,18]. Electrolysis processes have been utilized in heavy metal recovery from municipal incinerator fly ash [19], spent Ni-Cd batteries [15], printed circuit boards [20], and waste waters [16,21]. However, little information is available about recovering metals from spent HDS catalyst acid-leachate.…”

Electrolytic recovery of chromium salts from tannery wastewater

“…They also mentioned that the exact speciation of the chromium ions depends upon pH, temperature, concentration and the presence of other ions. A two-stage electrochemical treatment using a platinum anode and a graphite cathode at a current density of 0.1 A cm −2 has been described elsewhere [12]. In first stage, the acidic process water containing metals passed through the cathode chamber, where the main load of dissolved Cu, Zn, Cr and Pb was eliminated.…”

A Selective Bioreduction of Toxic Heavy Metal Ions from Aquatic Environment by Saccharomyces cerevisiae

“…These industries produce large quantities of toxic wastewater effluents 2 . The physical and chemical treatment of wastewater may involve one or more of the following: ion exchange 3 , reverse osmosis 4 , electrolysis 5 , precipitation 6 and reduction 7 . Unfortunately, these methods are expensive and require the use of contaminating products for desorption of metals for cleaning up of the inorganic matrix.…”