Insights into the adsorptive separation of iron from pickling waste acid: The overlooked role of FeCl3 species

Elucidating the dominant adsorption species of FeCln-n is fundamental for precisely regulating the adsorption process and thus enhancing the management of waste pickling liquor. In this work, anion resin (D201) exhibits great adsorption for FeCl4-, also exhibits the unexpectedly high adsorption for neutral species FeCl3. The contribution of various iron species was quantitatively evaluated via Ion Species Contribution Evaluation (ISCE). The results illustrate that both FeCl3 and FeCl4- were the dominant species adsorbed by D201 resin and exhibited similar adsorption activities. Experimental and computational results indicate that the adsorption of FeCl4- is accompanied by the shedding of Cl on FeCl4- and FeCl3 recombines with the Cl on D201, which indicate that FeCl3 is an intermediate species in the FeCl4- adsorption process. Accordingly, FeCl3 was revealed to be critical species for iron adsorption in hydrochloric pickling liquor. The concentration of HCl (20 % w/w) required for FeCl3 was sufficient for the removal of iron compared with the higher concentration (25 % w/w) required for FeCl4-, which broadened the concentration boundary for HCl reuse. Our strategy serves for industrial iron removal from hydrochloric pickling liquor and results in 86 % reduction in the amount of hydrochloric acid during 100 days pilot-scale test.