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

Sustainable photocatalysis can effectively reduce the radioactive 99 TcO 4 – to less soluble TcO 2 · n H 2 O(s), but the reduction efficiency is highly susceptible to coexisting nitrate (NO 3 – ). Here, we quantitatively investigate photocatalytic remediation conditions for Tc-contaminated water stimulated by the analogue perrhenate (ReO 4 – ) in the presence of NO 3 –, and we elucidate the influence mechanism of NO 3 – by in situ characterizations. The interfering NO 3 – can compete with Re(VII) for the carbonyl radical (·CO 2 – ) produced by formic acid (HCOOH) oxidation to generate nitrogen-containing products such as NH 4 +, NO 2 –, 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 N 2 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 ReO 2 · n H 2 O(s) and is accompanied by a decrease with the increase in NO 3 – concentration. Re(VII)/Tc(VII) reduction suffers from a serious interferential effect of NO 3 –, whereas the higher the concentration of NO 3 –, the more conducive to slowing down the reoxidation of the reduction products, which is advantageous for the subsequent sequestration or separation.