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 Ni_xFe_(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 Fe^III_(1+2x/3)Fe^II_(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.