Aqueous nickel sequestration and release during structural Fe(ii) hydroxide remediation: the roles of coprecipitation, reduction and substitution†
Abstract
Aqueous Ni2+ removal by structural Fe(II) hydroxides (SFH) under well-controlled experimental conditions was investigated in this study, and a possible mechanism for Ni2+ release from solid products was revealed. Experiments with SFH and Ni2+ showed the excellent reactivity of SFH towards aqueous Ni2+ when the molar ratio of [Fe(II)]/[OH−] was below 1 : 2. The reaction started with adsorption and precipitation of Ni2+, followed by reduction of Ni(II) and substitution of Fe(II) with the formation of Ni(II)/Fe(III) layered double hydroxides. However, at long reaction time, Ni(II) release from Ni(OH)2 and NixFe(1−x)(OH)2 precipitates was observed due to the delivery of dissolved Fe2+ and Fe3+, which were determined to substitute Ni(II) by forming Fe(OH)2, Fe(OH)3 and FeIII(1+2x/3)FeII(1−x)(OH)5. The presence of O2 and NO3− reduced the removal efficiency of Ni2+ and promoted its release by consuming Fe(II) and promoting Fe3+ delivery. However, CO32− and PO43− might enhance the removal of Ni2+ and inhibit its release. For wastewater containing a high Ni2+ concentration, using SFH is also beneficial to Ni2+ recycling, as removed Ni(II) could be released and enriched, followed by future utilization.