Stabilization of Cd2+/Cr3+ During Aqueous Fe(II)-Induced Recrystallization of Al-Substituted Goethite

The aqueous Fe2+ (Fe-aq(2+))-induced recrystallization of iron minerals is an important geochemical process with critical effects on the environmental behavior of metal pollutants in soils. However, the effects of different heavy metals with distinctive physicochemical properties on the Fe-aq(2+)-induced recrystallization of iron minerals and the different stabilization behaviors of these heavy metals remain unclear. This study examined the effects of Cd2+ and Cr3+ on the Fe-aq(2+)-induced recrystallization of Al-substituted goethites and the simultaneous stabilization of Cd2+/Cr3+ ions by the recrystallized Al-substituted goethites. Results from stable Fe isotopic tracer experiments and Mossbauer characterization show that Fe atoms were exchanged between Fe-aq(2+) and structural Fe(III) in Al-substituted goethites with coexisting Cd2+/Cr3+, although both Cd2+ and Cr3+ decreased the exchange rates. During the Fe atom exchange, Al-substituted goethites were recrystallized and a portion of Cd2+/Cr3+ ions were stabilized by the resulting goethite products. Compared with Cd2+, more Cr3+ ions were immobilized by the recrystallized Al-substituted goethites due to the lower hydrolysis constant of Cr3+ than Cd2+, as well as the closer ionic radius of Cr3+ with Fe3+ and Al3+. Al-goethites with higher Al contents further decreased the Fe atom exchange rates while increasing the amounts of immobilized metal ions when in the presence of Cd2+ or Cr3+. The findings of the present study suggest that the physicochemical properties of metal ions play critical roles in affecting their environmental behavior and fates during Fe-aq(2+)-induced recrystallization of iron minerals in soils.