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 (similar to 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.