Reactive halogen radicals in saline water promote photochemically-assisted formation of manganese oxide nanosheets

Halide ions are naturally abundant in oceans and estuaries. Large amounts of highly saline efflux are also made and discharged to surface water from desalination processes and from unconventional oil and gas recovery. These highly concentrated halides can generate reactive halogen radicals. However, the redox reactions of halogen radicals with heavy metals or transition metals have received little attention. Here, we report undiscovered fast oxidation of manganese ions (Mn2+) by reactive halogen radicals. Hydroxyl radicals (OH) are produced by nitrate photolysis. While OH radicals play a limited role in the direct oxidation of Mn2+, OH can react with halide ions to generate reactive halogen radicals to oxidize Mn2+. In addition, more Mn2+ was oxidized by bromide (Br) radicals generated from 1 mM Br- than by chloride (Cl) radicals generated from 500 mM Cl-. In the presence of Br radicals, the abiotic oxidation rate of Mn2+ to Mn(iv)O-2 nanosheets is greatly promoted, showing a 62% increase in Mn2+(aq) oxidation within 6 h of reaction. This study advances our understanding of natural Mn2+ oxidation processes and highlights unexpected impacts of reactive halogen radicals on redox activities with heavy metals and corresponding nanoscale solid mineral formation in brine. This work suggests a new, environmentally-friendly, and facile pathway for synthesizing MnO2 nanosheets.

Automated summary

This study discovered a previously unknown fast oxidation reaction between halogen radicals and manganese ions, and revealed the impacts of reactive halogen radicals on redox activities with heavy metals and nanoscale solid mineral formation in brine.