Metal recovery and hydrogen production from acid mine drainage by acid-base imbalance electrolytic cell

An acid-base unbalanced electrolytic cell (AB-EC) was designed to overcome the limitations of slow reaction rate and low hydrogen (H₂) production in the treatment of acid mine drainage (AMD) by microbial electrolysis and photoelectrochemical technology. Taking the coal mine type AMD with a single Fe²⁺ as the model system, the key parameters such as electrode spacing, voltage, urea concentration and pH were systematically optimized to establish the basic operating conditions of AB-EC. Furthermore, the optimization system was extended to the AMD treatment of freeze-thaw pyrite containing Fe²⁺, Al³⁺, Mn²⁺ and Ni²⁺, and the reaction mechanism under the competition of polymetallic ions was deeply explored. Finally, a strategy of two-stage AB-EC collaborative hydrogen production and staged metal recovery is proposed.The results showed that the optimal operating conditions of AB-EC were determined by system optimization tests as follows: electrode spacing of 2.5 cm, applied voltage of 8 V, and 0.5 mol/L urea solution with pH 13 for the anode electrolyte. After treating 230 mL of coal-type AMD for 50 min under these conditions, the Fe²⁺ removal rate reached 70.10%±3.87% and the H₂ production was (54±2)mL, which was significantly better than microbial electrolysis and photoelectrochemical technology in terms of metal removal and H₂ production. For the 230 mL freeze-thawed sulfuric iron pyrite AMD, the average concentrations of Fe²⁺, Al³+, Mn²⁺, Ni²⁺ in the effluent were 0.71, 0.01, 2.81, 0.23 mg/L, respectively, and the removal rate was more than 97% after the AB-EC was operated for 100 min. Simultaneously, the H₂ production amounted to (116±3)mL, and the anode urea and NaOH average consumption were measured at 0.21 and 0.18 g, respectively. Further results showed that the construction of sequential batch two-stage AB-EC process could simultaneously achieve the H₂ production and the selective separation and recovery of Al(OH)₃ and Fe₃O₄.