Selective and Efficient Phosphate Removal Using Ca–La Layered Double Hydroxide-Functionalized Sludge Biochar

Effective and sustainable removal of low-concentration phosphate is imperative but challenging for advanced wastewater treatment. Additionally, an eco-friendly and cost-effective treatment and reuse of waste activated sludge is critical. To address these problems, sludge-biochar-based phosphate adsorbents were developed by pyrolyzing waste activated sludge and then functionalizing with Ca–La layered double hydroxide (LDH). The LDH’s structure and adsorption characteristics were optimized by substituting the intercalated ion CO32– with NO3–. A maximal phosphorus adsorption capacity of 78.24 mg of P/g was achieved. Adsorption kinetics showed that 0.2 g/L functionalized sludge biochar effectively lowered PO43––P concentration from 7.0 to <0.02 mg/L within 2 h. The functionalized sludge biochar showed desirable phosphate (7 mg/L) selectivity toward coexisting anions such as sulfate (120 mg/L) and carbonate (250 mg/L), resulting in barely impacted adsorption capabilities. After 4 repetitive adsorption-regeneration experiments, 65.1% adsorption capacity remained. The functionalized sludge biochar was used to remove phosphate from a local wastewater treatment plant. The PO43––P concentration effectively decreased from 1.29 to <0.02 mg/L within 1 h. The promising selectivity and stability of the functionalized sludge biochar are due mostly to selective chemisorption, ion exchange, and phosphate complexation in the inner layer.