Photoinduced Reductive Sequestration of Pertechnetate/Perrhenate by a Sulfur-Rich Cadmium Sulfide

Highly soluble and migratory pertechnetate (⁹⁹TcO₄^-) can be reduced to sparingly soluble TcS₂ favored by conventional sulfidic sequestration, but this sulfidation is relatively sluggish. Herein, we develop an alternative strategy for the remediation of Tc-contaminated natural water via a photoinduced reduction stimulated by sulfur-rich cadmium sulfide (CdS) and clarify the sequestration mechanism through in situ characterization. By regulating the content of the S^2-/Cd²⁺ molar ratio of 4:1 (CdS-4) in the synthesis system to achieve a high photogenerated charge separation and transfer efficiency, the superior reduction efficiency reached 80% for 2 h under conditions of 4 wt % hole scavenger (HCOONa) and 1 mg L^-1 Re(VII) (a chemical analogue of Tc), coinciding with the rapid removal in actual Tc remediation. The introduction of HCOO^-/HCOOH effectively scavenges photogenerated holes and undergoes oxidation to form highly reductive carboxylic radicals, resulting in the reduction of Re(VII) to Re(VI). Under simulated neutral pH conditions, unstable Re(VI) rapidly disproportionates into Re(VII) and Re(IV). As photogenerated holes are consumed, transient S^2- ions from photocorrosion preferentially react with Re(IV) to form the chemically stable ReS₂. Notably, the sulfide can not only be effectively immobilized in natural aqueous environments but also its inherent stability facilitates subsequent separation or sequestration processes.