High-efficiency leaching of Al and Fe from fly ash for preparation of polymeric aluminum ferric chloride sulfate coagulant for wastewater treatment

Preparation of inorganic polymer coagulants from circulating fluidized bed (CFB) fly ash is one of the most significant value-added and environment-friendly approaches. Nevertheless, CFB fly ash presents relatively low solubility in acidic systems, which limits its utilization efficiency. In this paper, enhanced metal leaching from CFB fly ash using combined HCl-H2SO4 leaching process for polymeric aluminum ferric chloride sulfate (PAFCS) preparation was proposed. PAFCS with the different molar ratios of Al3+/SO42-and Al3+/Fe3+ were prepared from as-obtained leaching liquors and characterized by SEM, XRD, FTIR, and coagulation experiments. The leaching efficiencies of Al and Fe in CFB fly ash were up to 96.75% and 97.64% by combined HCl-H2SO4 leaching process. The optimal molar ratios of Al3+/SO42-and Al3+/Fe3+ for the turbidity and UV254 removal were 10:1 and 10:1, respectively, due to the three-dimensional chain-net structure and the highest [Al + Fe]b content. When applied to biochemical effluent of coking wastewater treatment, PAFCS exhibited excellent removal performance on both humic acid-like and fulvic acid-like matters. The resultant flocs of PAFCS presented the largest recovery factor (Rf = 0.37) and strength factor (Sf = 0.79) in contrast with PAC and PAFC. This work provides a novel and high-efficient Al and Fe leaching strategy from CFB fly ash to prepare Al-Fe coagulant.