Effects of Background Ionic Composition during Adsorption: Mechanisms and Implications for Phosphate Extraction from Wastewater

Abstract Excess phosphate in water bodies leads to eutrophication, threatening water quality and wasting a critical, nonrenewable resource. Wastewater is a major untapped source of phosphate, but current methods have limited removal and recovery efficiency at realistic concentrations and generate substantial waste. Iron oxide-coated cellulose sponge adsorbents offer a sustainable approach for phosphate recovery, but the effects of the background ionic composition in wastewater on phosphate adsorption are not fully understood. This study investigates phosphate adsorption on an iron oxide-coated cellulose sponge under varying ionic conditions. The presence of 0.2 mM Ca2+ and 0.01 mM Cu2+ increased the fitted Langmuir maximum adsorption capacity of phosphate by 32% and 70%, respectively. In contrast, 0.32 mM NO3– and 1.04 mM SO42– reduced phosphate adsorption by 12% and 10% (not statistically significant), respectively. SEM-EDS, XPS, and ToF-SIMS analyses, together with ζ potential measurements, indicated that Ca2+ and Cu2+ enhanced phosphate adsorption through cation-mediated interactions and surface charge modification. These findings elucidate how background ionic composition influences phosphate adsorption and adsorbent performance, providing mechanistic insights for phosphate removal and recovery from wastewater.