Influence of phosphate on hexavalent chromium immobilization by ferrihydrite under light irradiation

Ferrihydrite is an important geosorbent and catalyst that is ubiquitous in natural environments, and plays a critical role in regulating the speciation and migration of hexavalent chromium (Cr(VI)). Phosphate has a strong affinity for ferrihydrite surfaces, but it remains unclear how phosphate influences the immobilization of Cr(VI) by ferrihydrite in sunlight-influenced surface environments. Herein, the surface interactions of ferrihydrite and Cr(VI) in the presence of phosphate were studied under mercury lamp irradiation. Results showed that mercury lamp irradiation significantly attenuated the inhibitory effect of phosphate on Cr(VI) removal by ferrihydrite compared to that under dark conditions. X-ray photoelectron spectroscopic (XPS) analysis revealed that Cr(III) was the predominant chromium species immobilized on ferrihydrite surfaces. Specifically, the reduction of Cr(VI) produced CrPO 4 precipitates, whereas the released Fe(III) can also co-adsorb with phosphate (i.e., FePO 4 ) onto ferrihydrite surfaces. The reduction of Cr(VI) mediated by the generated Fe(II) species and photoelectrons effectively reduces the electrostatic repulsion between Cr(VI) and phosphate, thereby enhancing the immobilization of these two ions on ferrihydrite. Energy dispersive X-ray spectrometer (EDS) results provide direct quantitative evidence that mercury lamp irradiation substantially enhances the immobilization of Cr(VI) and phosphate by ferrihydrite. The above-described results account for the effect of phosphate on the light-induced coupling process of ferrihydrite reductive dissolution and Cr(VI) reductive immobilization, advancing our understanding of the geochemical cycle of Cr(VI) pollution in sunlight-influenced surface environments. • Phosphate strongly inhibited the adsorption of Cr(VI) onto ferrihydrite in dark conditions. • Mercury lamp irradiation significantly attenuated the inhibitory effect of phosphate on Cr(VI) removal by ferrihydrite. • The Cr species immobilized on ferrihydrite are predominantly present in the form of Cr(III),which can further react with coexisting phosphate to form insoluble CrPO 4 precipitates.