MgO-modified biochar by modifying hydroxyl and amino groups for selective phosphate removal: Insight into phosphate selectivity adsorption mechanism through experimental and theoretical

Abstract

Metal oxides-modified biochars have been widely studied as promising adsorbents for removing phosphate from wastewater discharge. Yet, the low adsorption selectivity towards phosphate severely limits its potential in practical applications. In this study, MgO-modified biochar modified by hydroxyl and amino groups (OH/NH2@MBC) is developed for selective phosphorus recovery from wastewater. As major results, the OH/NH2@MBC exhibits favorable phosphate adsorption performance is superior to that of MBC resin in the presence of co-existing anions (NO3−, Cl−, HCO3− and SO42−) and natural organic matter (humic acid) even actual wastewater, suggesting its superior selectivity towards phosphate. The OH/NH2@MBC shows an excellent phosphate adsorption capacity (43.27 mg/g) and desorption ratio (82.34 %) after five cycles under the condition of anion coexistence (100 mg/L). The experimental and DFT theoretical study reveals that attaching hydroxyl and amino groups onto the MBC surface, which facilitates to inhibiting the side effects of anions (NO3−, Cl−, HCO3−, and SO42−) through Lewis acid-base sites, hydrogen bonds, and metal affinity, and preferentially select adsorption P, contributing greatly to improve phosphate adsorption selectivity. Importantly, the presence of amino and hydroxyl groups can reduce the Fermi level of OH/NH2@MgO(220) and OH/NH2@MgO(200) and improve the adsorption selection for HPO42−. This study provides an effective strategy for enhancing the adsorption selectivity of metal oxides-modified biochars towards phosphate through modifying functional groups.