Phosphorus and humic acid extraction from fermentation liquor of ferric phosphate sludge via layered double hydroxides: Efficiency and interaction mechanism

Abstract Iron salts are increasingly employed in wastewater treatment worldwide, generating ferric phosphate sludge with low phosphorus (P) bioavailability. The fermentation of ferric phosphate sludge effectively transfers P from the solid phase to the liquid phase and simultaneously releases high contents of ferrous cations (Fe2+) and humic acids (HA) into the sludge liquor. This study aimed to extract P from simulated and real sludge fermentation liquor via Mg/Al layered double hydroxide (LDH), evaluate the impact of co-existed Fe and explore the interaction mechanism between phosphate and HA adsorption. The Fe influence experiments explored the suitable organic matter concentration range for the LDH adsorbent to extract P without the concurrent precipitation of iron compounds. Thus, the residual ferrous species in sludge liquor can be reused as a flocculant. Moreover, the analysis of the adsorption kinetics of phosphate at 25, 35, and 55 °C showed that increasing temperature accelerated the phosphate adsorption rate but had minor effects on the P adsorption capacity. HA and phosphate were jointly adsorbed onto Mg/Al LDH. Investigations on isotherm, kinetics, and mechanism suggested that phosphate anions have a competitive advantage in the ion exchange process but are hindered by large-molecular-weight HA in surface adsorption because of limited adsorption sites. Subsequent adsorption experiments in sludge liquor and nutrient release test confirmed the possibility of using Mg/Al LDH to simultaneously recover P and organic matters from sludge fermentation liquid based on its stable P adsorption performance and intercalated anion slow-release property.