A stepwise processing strategy for treating highly acidic wastewater and comprehensive utilization of the products derived from different treating steps.

A stepwise processing strategy, including initial neutralization, chemical mineralization, and complete neutralization treating steps, was developed to effectively treat and utilize the highly acidic wastewater derived from titanium dioxide production. Approximately 94.6% of SO42-, 100% of Fe, and most of other metals were recovered to produce white gypsum, schwertmannite, and Fe0/Fe3O4@biochar (Fe0/Fe3O4@BC) composite in the corresponding treating steps. The resulting effluent with neutral pH and a small amount of metal ions could be discharged to general sewage treatment plant for further processing. Schwertmannite was applied as a heterogeneous Fenton-like catalyst to stimulate H2O2 to produce active radicals for effective degradation and mineralization of methyl orange (MO) in solution. The MO removal of 100% and total organic carbon removal of 91.1% were achieved in schwertmannite/H2O2 reaction system, and schwertmannite exhibited good stability and reusability. Fe0/Fe3O4@BC composite was applied to remove Cr(VI), with the adsorption capacity of 67.74 mg g-1. The removal of Cr(VI) using Fe0/Fe3O4@BC composite was a chemisorption process, including the adsorption of Cr(VI), reduction of Cr(VI) to Cr(III), and co-precipitation of Cr(III)/Fe(III) oxides/hydroxides. This stepwise treating strategy is a promising technology for effective treatment of highly acidic industrial wastewater and comprehensive utilization of the related products.