Phosphate and ammonia nitrogen recovery from sewage sludge supernatants by coupled MgO-biomass ash and its potential as heavy metal adsorbent

Abstract

Aiming at the problem of complex treatment for energy solid waste—biomass ash (BA) from thermal power plant and difficult recovery for phosphorus and ammonia nitrogen in sewage sludge supernatants, a coupled MgO-biomass ash (CMBA) was established. CMBA composites were synthesized by in situ thermal-precipitation modification method, which simultaneously achieved recovery of phosphorus and ammonia nitrogen and BA resource utilization, and explored the potential of recovered CMBA (RCMBA) containing phosphate and ammonia nitrogen to remove heavy metal from wastewater. The results showed that magnesium oxide could be effectively loaded on the surface of BA at an MgCl2 concentration of 1.25 M and calcination temperature of 400 °C. pH, adsorbent dosages and initial concentration affected the adsorption capacity of phosphate and ammonia nitrogen by 400CMBA. The existence of K+, Ca2+ and Mg2+ ions promoted the removal of phosphate by 400CMBA, but hindered the removal of ammonia nitrogen. The existence of Cl-, NO2- and SO42- ions affected the removal of phosphate by 400CMBA. The adsorption processes of 400CMBA for phosphate and ammonia nitrogen can be used with the Langmuir-Freundlich model and the pseudo-second-order model reaching a maximum adsorption capacity of 262.88 mg/g and 122.39 mg/g, respectively. The adsorption behavior was controlled by several processes, spontaneous and thermogenic chemisorption. The saturated adsorbent can be effectively regenerated in 0.1 M HCl solution. After the fifth regeneration, the phosphate and ammonia nitrogen removal rates were less than 50% and 25%, respectively. The adsorption mechanisms of phosphate and ammonia nitrogen by 400CMBA were mainly physical adsorption, electrostatic attraction, ion exchange, complexation reaction and chemical precipitation, among which chemical precipitation was dominated by struvite crystallization was dominant. In addition, RCMBA had a specific removal effect on heavy metal in solutions. Overall, this work provides a feasible way for the resource utilization of energy solid waste-BA from thermal power plant.