Lanthanum-loaded geopolymer for phosphate removal from agricultural runoff

The removal of phosphate from agricultural runoff is of great importance to mitigate an overabundance of nutrients discharging into receiving water bodies, which are susceptible to eutrophication. In this study, a La-loaded geopolymer was produced by adding metakaolin within an alkaline medium, consisting of sodium silicate (Na₂SiO₃) and sodium hydroxide (NaOH). The ratio of Na₂SiO₃:NaOH within the geopolymer slurry was manipulated to evaluate its effect on phosphate adsorption capacity. The 1.54 ratio yielded the highest adsorption capacity of 33.65 mg g^-1. However, due to structural strength, safety, and economic considerations, the 2.0 ratio was used for the isotherm and kinetic adsorption testing. The La-loaded geopolymer exhibited higher phosphate removal in batch adsorption experiments at an acidic level (pH 4) and elevated temperature (40 °C). Moreover, ionic strength (3.5-20 mM) had a negligible impact on phosphate removal, indicating inner-sphere complexation as the main mechanism of adsorption. Additionally, bicarbonate and humic acid increased phosphate removal, whereas sulfate slightly decreased adsorption capacity. The La-loaded geopolymer was further evaluated using a synthetic agricultural solution, which yielded a Type III adsorption isotherm, demonstrating unrestricted multilayer phosphate adsorption. Results from this study verified the La-loaded geopolymer is a promising adsorbent for phosphate removal from agricultural runoff and underlined the importance of matrix effects on treatment performance.