Biochemical treatment of leachates from hydrometallurgical recycling of spent alkaline batteries

Recycling of waste electrical and electronic equipment including spent batteries focuses on maximizing the material recovery efficiency (of metals, polymers, or solvents) and decreasing the negative environmental impact of leachates that remain after the treatment (i.e. the secondary wastes). The major aim of this study was to develop a novel, low-cost biochemical treatment technology for the management of effluents generated during sulfuric acid–based hydrometallurgical recycling of spent alkaline batteries. We explored the use of various chemical reagents (40% NaOH, 20% Na2CO3, and 1% NH3(aq)) and biogenic ammonia produced by urea-degrading bacteria to increase the pH of the effluents (from pH 0.5 to pH 5.0) and prepare them for further treatment by sulfate-reducing bacteria. Comparisons of the pretreatment efficiency and metal and sulfate removal yields, as well as the characterization of the neutralization products (sediments and effluents) showed that the most promising results were obtained when the raw leachates were treated with 40% NaOH (to reach pH 3.5), followed by the addition of biogenic ammonia (to reach pH 5.0). Sulfate-reducing bacteria (SRB) activity led to a further pH increase (up to ~7.3), almost complete (99%) sulfate removal and metal sulfide (ZnS, MnS) precipitation, as up to ~99% of Zn and Mn were removed in SRB cultures to which appropriately diluted pretreated leachates had been added. The presented study indicated that the pretreatment and neutralization of hydrometallurgical effluents based on the use of urea-degrading and sulfate-reducing bacteria could be an attractive alternative to conventional chemical treatment. •Gradual acidic leachate pretreatment with sodium hydroxide and biogenic ammonia.•Biogenic ammonia synthesized by urea-degrading bacteria.•Combined biological and chemical pretreatment promotes the growth and activity of SRB.•Sulfates and metals were almost completely removed and metal sulfides precipitated.