Multicycle Calcined Regeneration of Nano-MgO and Phosphate Accumulation from Humic Acid–Contaminated Wastewater

Nano-MgO is an effective adsorbent for various pollutants’ removal from water with low cost. However, the adsorption process suffers inhibition of humic acid (HA). In this study, the influence of HA on the phosphate adsorption performance of nano-MgO and its annealing regeneration was studied for multiple cycles. Nano-MgO was found to be able to simultaneously remove more than 95% of HA and phosphate in 1 h. The presence of 35 mg/L HA led to a decline in phosphate adsorption properties with a Langmuir maximum capacity decreasing from 463 to 408 mg-P/g and a decreased removal speed. This adverse HA effect was simply solved by annealing the sludge at 500°C while the adsorption activity of nano-MgO was reactivated. The phosphate content was accumulated gradually after each regeneration and reuse of nano-MgO by calcination and reached 92.4 mg-P/g or 211.6 mg/g calculated as P2O5 content after seven cycles. Thus, a high-quality P-fertilizer was successfully produced from HA-contaminated wastewater containing 20 mg-P/L and 35 mg-HA/L while reducing the P content in water under 0.5 mg-P/L and removing more than 95% HA. This process based on regenerated nano-MgO provides a feasible strategy on reducing sledge amount and producing high-quality P-fertilizer from HA-contaminated wastewater.