Multi-step purification of electrolytic manganese residue leachate using hydroxide sedimentation, struvite precipitation, chlorination and coagulation: Advanced removal of manganese, ammonium, and phosphate

Abstract Water pollution caused by the release of manganese (Mn2+) and ammonia nitrogen (NH4+-N) from electrolytic manganese residue (EMR) generated from industrial activities poses a serious threat to ecosystems and human health. In this study, an integrated process consisting sequentially of hydroxide sedimentation, struvite precipitation, breakpoint chlorination, and ferric chloride coagulation was optimized to remove Mn2+ and NH4+-N from EMR leachate, and to address the issue of residual orthophosphate caused by struvite precipitation. The precipitates were characterized using X-ray diffraction, scanning electron microscopy, and thermogravimetric analyses. Results show that Mn2+ ions and the resulting chemical oxygen demand (COD) were mainly removed using hydroxide precipitation at a sedimentation pH of 10.2, with poor-crystalline manganese hydroxide as the main precipitate. NH4+-N was primarily removed and recovered using struvite precipitation with well crystalline struvite as the main product, and then further eliminated using breakpoint chlorination. The residual orthophosphate introduced by struvite precipitation is successfully removed with ferric coagulation, and the effluent pH (7.5) is also lowered to discharge limits by means of hydrolysis of ferric coagulant. The concentration of COD, Mn2+, NH4+-N, and orthophosphate concentrations in the final effluent were 30.52 ± 9.38, 0.026 ± 0.013, 0.87 ± 0.01, and 0.06 ± 0.002 mg/L, respectively, meeting all local discharge standards. This combined process has robust pollutant removal efficiency, high resource recovery potential and few environmental constraints; thus, it is recommended as a potential solution for the treatment of Mn2+- and NH4+-N-rich acid mine drainage.