Increasing bromide removal by graphene-silver nanocomposites: Nanoparticulate silver enhances bromide selectivity through direct surface interactions

Bromide forms toxic brominated disinfection by-products during disinfection. Current bromide removal technologies are often non-specific and costly due to naturally occurring competing anions. A silver-impregnated graphene oxide (GO) nanocomposite is reported here that reduced the amount of Ag needed for Br⁻ removal by increasing its selectivity towards Br⁻. GO was impregnated with ionic (GO-Ag⁺) or nanoparticulate Ag (GO-nAg) and compared against Ag⁺ or unsupported nAg to identify molecular level interactions. In nanopure water, Ag⁺ and nAg had the highest Br⁻ removal (∼0.89 mol Br⁻/mol Ag⁺) followed by GO-nAg at 0.77 mol Br⁻/mol Ag⁺. However, under anionic competition, the Ag⁺ removal was reduced to 0.10 mol Br⁻/mol Ag⁺ while all nAg forms retained good Br⁻ removal. To understand the removal mechanism, anoxic experiments were performed to prevent nAg dissolution, which resulted in higher Br⁻ removal for all nAg forms compared to oxic conditions. This suggests that reaction of Br⁻ with the nAg surface is more selective than with Ag⁺. Finally, jar tests showed that anchoring nAg on GO enhances Ag removal during coagulation/flocculation/sedimentation compared to unsupported nAg or Ag⁺. Thus, our results identify strategies that can be used to design selective and silver-efficient adsorbents for Br⁻ removal in water treatment.