Electrochemical Oxidation of the Sulfide Ion in Synthetic Geothermal Brines in Batch Cells Using Coke Electrodes

The oxidation of the sulfide ion occurs efficiently in batch cells at massive coke electrodes. At all currents studied, the products included a low yield of elemental sulfur, which deposited on the anode; the yields of sulfate were also low, except at the highest current. The remaining products were soluble organosulfur species, indicating that the coke anodes acted sacrificially. The reaction displayed unusual kinetic behavior with respect to the disappearance of sulfide: a two-stage reaction was observed in which the loss of sulfide was faster in the early stages of reaction, while elemental sulfur deposited on the anode. A subsequent slower current-controlled reaction was associated with the formation of the remaining products

Removal of Arsenic from Synthetic Acid Mine Drainage by Electrochemical pH Adjustment and Coprecipitation with Iron Hydroxide

Acid mine drainage (AMD), which is caused by the biological oxidation of sulfidic materials, frequently contains arsenic in the form of arsenite, As(III), and/or arsenate, As(V), along with much higher concentrations of dissolved iron. The present work is directed toward the removal of arsenic from synthetic AMD by raising the pH of the solution by electrochemical reduction of H+ to elemental hydrogen and coprecipitation of arsenic with iron(III) hydroxide, following aeration of the catholyte. Electrolysis was carried out at constant current using two-compartment cells separated with a cation exchange membrane. Four different AMD model systems were studied:  Fe(III)/As(V), Fe(III)/As(III), Fe(II)/As(V), and Fe(II)/As(III) with the initial concentrations for Fe(III) 260 mg/L, Fe(II) 300 mg/L, As(V), and As(III) 8 mg/L. Essentially quantitative removal of arsenic and iron was achieved in all four systems, and the results were independent of whether the pH was adjusted electrochemically or by the addition of NaOH. Current efficiencies were ∼85% when the pH of the effluent was 4−7. Residual concentrations of arsenic were close to the drinking water standard proposed by the World Health Organization (10 μg/L), far below the mine waste effluent standard (500 μg/L)

Electrochemical Oxidation for Denitrification of Ammonia: A Conceptual Approach for Remediation of Ammonia in Poultry Barns

Ammonia in poultry barns produced by microbial action on the birds’ excreta can be removed by scrubbing into aqueous solution at pH ∼4. However, disposal of the resulting solution remains a problem. In this work, ammonia was oxidized electrochemically in the presence of unreactive electrolytes (NaClO4, Na2SO4), but the conditions were not compatible with treatment inside or outside a poultry barn (high pH, closed electrochemical reactor, and high ammonia concentration). Efficient denitrification is possible without pH adjustment of the scrubbed solution when chloride ion is also present in the scrubbing solution. This reaction is based on electrochemical hypochlorination, which is similar to breakpoint chlorination for the chemical elimination of ammonia. This work confirms a recent mechanistic proposal that efficient denitrification at pH ∼3 is the result of concomitant oxidation of water and acidification at the anode, but shows in addition that the mechanisms of both chemical and electrochemical hypochlorination are similar at acidic pH. These results allow us to propose that ammonia scrubbed into acidic brine can be oxidized to elemental nitrogen with high current efficiency without pH adjustment and without chemical additives, providing a “green” solution to the problem at hand

Industrial Coke as an Electrode Material for Environmental Remediation

Industrial coke was evaluated as a low-cost electrode material for environmental remediation, using the dye Orange II as an example substrate. Coke was used as massive pieces in batch cells or in the ground form for use in a packed-bed reactor. The loss of Orange II was faster when the supporting electrolyte contained chloride ion, and under these conditions the reaction involved hypochlorination. In the batch reactor, the current efficiency for mineralization was only modest (4−14%). In the packed-bed reactor, the loss of both starting material and intermediates was fastest at high current and low flow rate, and a near-quantitative current efficiency was achieved. The high current efficiency was explained by the greater surface area of the electrodes in the packed-bed reactor compared with the batch reactor, and better contact between the solution to be remediated and the coke particles. A drawback to the use of coke electrodes for the remediation of aqueous wastes is their tendency to increase the total organic carbon content of an aqueous solution, especially under anodic polarization

Competition between Electrochemical Advanced Oxidation and Electrochemical Hypochlorination of Sulfamethoxazole at a Boron-Doped Diamond Anode

Sulfamethoxazole (SMX) was used as a model substrate for electrochemical oxidation at a boron-doped diamond anode in the presence of chloride ion, which is present in many waste streams. In the absence of chloride, oxidation of SMX involves mineralization, an electrochemical advanced oxidation process (EAOP) that is initiated by attack of anode-derived hydroxyl radicals. The rate of disappearance of SMX increased monotonically upon addition of chloride ion but without inhibiting mineralization in the early stages of oxidation. This demonstrated that electrochemical hypochlorination (EH) and EAOP are not mutually exclusive reaction pathways; products of EH can undergo EAOP and vice versa. Persistent chlorinated byproducts were formed in the presence of chloride ion, indicating that chloride is a significant detriment to the success of EAOP. No mineralization was observed upon chemical hypochlorination of SMX with sodium hypochlorite

Successive Photosubstitution of Hexachlorobenzene with Cyanide Ion

We report a novel nucleophilic polysubstitution reaction of hexachlorobenzene (HCB) with cyanide ion in acetonitrile/water. Successive photocyanations of HCB occur with high quantum yield (φdiss → 0.18) without the need for an electron acceptor, to give as products pentacyanophenol, 4-chloro-2,3,5,6-tetracyanophenol, and a dichlorotricyanophenol. The phenol functional group is introduced by competing hydrolysis of the polycyanochlorinated benzenes. Sensitization and quenching experiments indicate a triplet reactive excited state. Variation of [CN-] at constant [HCB] follows the expected relationship φdiss −1 ∝ [CN-]-1, but variation of [HCB] at constant [CN-] shows that the reaction becomes less efficient with increasing [HCB], consistent with the formation of an unproductive excimer