Simultaneous adsorption of phosphate and phosphonate by zirconium-modified biotite: implications for reverse osmosis concentrate treatment

Phosphonates are among the most used antiscalants for preventing membrane scaling in reverse osmosis (RO) and other filtration processes. These antiscalant chemicals end up in RO retentate brine or concentrate, and if not properly treated or managed, could cause negative environmental impacts to receiving waters. Although zirconium (Zr)-modified natural minerals have been widely used for adsorbing phosphate, their effectiveness in adsorbing phosphonates from RO concentrate is limited and inadequately explored. This study investigated the modification of a phyllosilicate, biotite, with Zr to enable simultaneous adsorption of phosphate and phosphonates. The results show that Zr-modification significantly enhanced adsorption of both phosphate and phosphonate compared to raw biotite. The Zr-modified biotite exhibited slower adsorption kinetics but greater capacity for phosphonate compared to phosphate. Different adsorption isotherms were observed for biotite modified with higher levels of Zr, likely due to transition from single layer to multilayer adsorption. Moreover, Zr-modified biotite demonstrated strong adsorption for phosphate and phosphonates simultaneously and effectively removed over 98 % of the total phosphorus from RO concentrate. Combined adsorption analyses suggested that the primary adsorption mechanism of both phosphate and phosphonates on Zr-modified biotite may involve the formation of inner-sphere surface complexes. Hence, this study presented a promising approach for removing phosphorus from RO concentrate, contributing to its effective treatment and management.