Phosphate Removal from Polluted Water via Lanthanum-Modified Sludge Biochar

Biochar has attracted attention for its capability to remove phosphorus (P) from wastewater. However, the poor dispersion and limited adsorption capacity of unmodified biochar prevent its wide usage in water remediation. Herein, sludge biochar was modified using lanthanum nitrate to improve the removal of P from aqueous solutions. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) were used to elucidate the modification and adsorption mechanisms of biochar. Furthermore, the adsorption performance of the modified biochar was determined through adsorption kinetics and isotherm model fitting. The results showed that the modification process successfully introduced lanthanum-containing functional groups into biochar and considerably improved the complexation performance and ion-exchange capacity. The maximum experimental adsorption capacity for phosphate was 140.237 mg/g at pH 3.0. The adsorption processes of the modified biochar accorded with the Freundlich adsorption isotherm model, which indicates the successful adsorption of phosphate onto the modified biochar via multilayer adsorption. The adsorption mechanism was dominated by chemisorption, which mainly involved inner-sphere complexation, precipitation, and electrostatic attraction. Meanwhile, the adsorption and desorption experiments demonstrated the satisfying recycling performance of the modified biochar and the 72.3% adsorption capacity retention after the sixth desorption cycle. The dynamic adsorption study revealed that the modified biochar had long sustainable treatment durations of 7.58 and 9.08 h at adsorbent dosages of 1 and 2 g, respectively, which proves the feasibility of using biochar as a cost-effective and efficient adsorbent for phosphate-polluted water.