Antagonistic Effect of Nitrate Conversion on Photocatalytic Reduction of Aqueous Pertechnetate and Perrhenate

Sustainable photocatalysis can effectively reduce the radioactive ⁹⁹TcO₄^– to less soluble TcO₂·nH₂O(s), but the reduction efficiency is highly susceptible to coexisting nitrate (NO₃^–). Here, we quantitatively investigate photocatalytic remediation conditions for Tc-contaminated water stimulated by the analogue perrhenate (ReO₄^–) in the presence of NO₃^–, and we elucidate the influence mechanism of NO₃^– by in situ characterizations. The interfering NO₃^– can compete with Re(VII) for the carbonyl radical (·CO₂^–) produced by formic acid (HCOOH) oxidation to generate nitrogen-containing products such as NH₄⁺, NO₂^–, and NO_x, resulting in the decrease in the Re(VII) reduction ratio. Under the conditions of 4% (volume ratio) HCOOH and pH = 3, the yield of NO_x is the lowest, and the selectivity of N₂ reaches 93%, which makes the overall reaction more in line with the pollution-free concept. The X-ray absorption fine structure reveals that the redox product Re(IV) mainly exists in the form of ReO₂·nH₂O(s) and is accompanied by a decrease with the increase in NO₃^– concentration. Re(VII)/Tc(VII) reduction suffers from a serious interferential effect of NO₃^–, whereas the higher the concentration of NO₃^–, the more conducive to slowing down the reoxidation of the reduction products, which is advantageous for the subsequent sequestration or separation.