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 NOx, resulting in the decrease in the Re(VII) reduction ratio. Under the conditions of 4% (volume ratio) HCOOH and pH = 3, the yield of NOx 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.