Selective and\nEfficient Phosphate Removal Using Ca–La\nLayered Double Hydroxide-Functionalized Sludge Biochar

Effective\nand sustainable removal of low-concentration phosphate\nis imperative but challenging for advanced wastewater treatment. Additionally,\nan eco-friendly and cost-effective treatment and reuse of waste activated\nsludge is critical. To address these problems, sludge-biochar-based\nphosphate adsorbents were developed by pyrolyzing waste activated\nsludge and then functionalizing with Ca–La layered double hydroxide\n(LDH). The LDH’s structure and adsorption characteristics were\noptimized by substituting the intercalated ion CO<sub>3</sub><sup>2–</sup> with NO<sub>3</sub><sup>–</sup>. A maximal\nphosphorus adsorption capacity of 78.24 mg of P/g was achieved. Adsorption\nkinetics showed that 0.2 g/L functionalized sludge biochar effectively\nlowered PO<sub>4</sub><sup>3–</sup>–P concentration\nfrom 7.0 to <0.02 mg/L within 2 h. The functionalized sludge biochar\nshowed desirable phosphate (7 mg/L) selectivity toward coexisting\nanions such as sulfate (120 mg/L) and carbonate (250 mg/L), resulting\nin barely impacted adsorption capabilities. After 4 repetitive adsorption-regeneration\nexperiments, 65.1% adsorption capacity remained. The functionalized\nsludge biochar was used to remove phosphate from a local wastewater\ntreatment plant. The PO<sub>4</sub><sup>3–</sup>–P concentration\neffectively decreased from 1.29 to <0.02 mg/L within 1 h. The promising\nselectivity and stability of the functionalized sludge biochar are\ndue mostly to selective chemisorption, ion exchange, and phosphate\ncomplexation in the inner layer.