Biopolymer-metal composites for selective removal and recovery of waterborne orthophosphate

Abstract

Orthophosphate (P_i) remediation from effluent serves to address global water security by preventing eutrophication. Herein, chitosan (C), alginate (Alg) and three respective metal systems (Fe³⁺, Al³⁺, Cu²⁺) were used to prepare binary (BMC) or ternary (TMC) metal composite adsorbents. Their physicochemical properties were analyzed through XPS, IR and TGA, while the adsorption properties of the composites were characterized via adsorption isotherms and single-point experiments in saline environmental water. Al-composites formed Al-O complexes, while Fe- and Cu-composites formed in the presence of the biopolymer backbone FeO(OH) and Cu₂(OH)₃NO₃, respectively. While Al-composites showed the highest bound water fraction (up to 16%), the Cu-composites (Cu-TMC-N, CuC-BMC-N; where N = nitrate) revealed the lowest water content. Alginate-based binary composites showed slightly higher water content, as compared to ternary and binary chitosan composites. Among the four materials (Al-TMC-N, Fe-TMC-N, Cu-TMC-N and CuC-BMC-N), the Al-TMC showed the highest P_i selectivity over sulfate, along with high P_i removal-% even in a binary mixture (sulfate + orthophosphate) despite the presence of competitive anion species. Upon spiking saline groundwater samples with low P_i (5 mg/L) that contains 2060 or 6030 mg/g sulfate, Al-TMC-N showed the highest P_i selectivity, followed by Fe-TMC-N. This trend in adsorption of Pi among the various composites is understood based on the HSAB principle for the conditions employed in this study. Removal efficiencies of P_i above 60% in Well 1 (ca. 2000 mg/L sulfate) and above 30% in Well 3 (ca. 6030 mg/L sulfate). Herein, environmentally compatible and sustainable composite adsorbents were prepared that reveal selective P_i recovery from (highly) saline groundwater that can mitigate eutrophication in aqueous media.

Keywords: Alginate; Chitosan; HSAB principle; Non-hofmeister sequence; Orthophosphate; Phosphate recovery; Selective adsorption; Ternary metal composite.