Phosphorus Recovery and Heavy Metal Removal Potential from Sewage Sludge-Derived Hydrochar with Activated Alumina

Sludge-derived hydrochar is rich in phosphorus (P), and heavy metals are also present. Hence, fully recovering P while removing heavy metals as much as possible is a key part of ensuring sustainable P resource recycling and utilization. Citric acid, activated alumina, and NaOH were separately selected as acid extraction, adsorbent, and desorbent for the first time to conduct P recovery from hydrochar. The effects of the adsorbent dosage, initial P concentration, and time on P adsorption were investigated. Kinetics and adsorption isotherm models were employed to analyze the adsorption data. Various characterization techniques were comprehensively applied to further elucidate the adsorption mechanisms. The results showed that the P adsorption rate was positively correlated with the dosage but negatively correlated with the initial P concentration during the experimental parameter range. The P adsorption capacity varied inversely with the adsorption rate. Moreover, at 825 mol·L^–1P and 0.5 g adsorbent dosage conditions, the maximum adsorption capacity was achieved (7.3 mg·g^–1) within 30 min with an 80.2% P adsorption rate at equilibrium (48 h). The P recovery rate could reach 72.6%, with the removal of Zn, Mn, and Cr exceeding 99%. The corresponding mechanisms involve ligand exchange, electrostatic attraction, anion exchange, and surface precipitation.