Boosting the phosphorus uptake of La2(CO3)3·8H2O based adsorbents via sodium addition: Relationship between crystal structure and adsorption capacity

Excess phosphate contents in water bodies triggers eutrophication, which posts significant challenges to the aquatic ecosystem. Lanthanum-carbonate based adsorbents exhibit excellent phosphate binding properties for remediating eutrophication. However, they suffer from significant adsorption-capacity loss (>85 %) at high pH. Little has been done on understanding this behavior for improving the phosphorus adsorption of lanthanum-carbonate adsorbents in alkaline environments (e.g. eutrophic water bodies). Here, we discover that La₂(CO₃)₃·8H₂O, when produced by a conversion reaction from NaLa(CO₃)₂·xH₂O, exhibits high phosphate adsorption capacity in a wide pH window. Under alkaline conditions (e.g. pH = 10), its adsorption capacity decreases by only 8 % compared to the value under neutral pH. By isolating three different lanthanum-carbonate based compounds and analyzing their molecular structures, we find that the trace amount of Na⁺ residual in our La₂(CO₃)₃·8H₂O alters the chemical environment surrounding the La³⁺ ions, which may significantly boost the phosphate uptake at high pH. Our results provide molecular insights for further tuning the material structure of phosphate adsorbents to achieve robust performances.