Comparative assessment of phosphate adsorption properties and mechanisms on Mg/Al-engineered sewage sludge biochar in aqueous solution

This study reported the development of Mg/Al-engineered sewage sludge biochars (SSBCs) with adjustable surface properties and enhanced phosphate PO43− (ad)sorption capacity. These biochars successfully addressed the traditional constraints of electric repulsion interactions with PO43− ions at various pH levels, leading to highly efficient and recyclable PO43− enrichment from aqueous solution. It was determined that a ratio of 1:2 of Mg/Al is the optimal concentration of Mg/Al SSBCs for maximum PO43− (ad)sorption capability. In this study, the pseudo-second-order kinetics and Redlich Peterson isotherm worked well for Mg-SSBC, however, the pseudo-first-order kinetics and Langmuir isotherm explained the Al-SSBC interaction with PO43− ions properly. Both Mg-SSBC and Al-SSBC showed strong adsorption specificity to PO43− ions, with all coexisting anions producing negative effects on the PO43− adsorption. The data demonstrate that the variation in adsorption mechanism viz., including electrostatic attraction, complex formation, cationic bridging, and chemical co-precipitation was observed in between Mg-SSBC and Al-SSBC. Comparison between Mg-SSBC and Al-SSBC showed that Mg-SSBC has a more extensive pH range for PO43− removal and greater stability with compound elimination power, i.e., Mg-SSBC is more effective for the elimination of phosphates from aqueous solution.