Treatment of sewage sludge using choline chloride-lactic acid deep eutectic solvent: Heavy metals removal and resource recovery

Heavy metals (HMs), as non-degradable hazardous substances, will continue to be enriched with the treatment of sewage sludge (SS), causing adverse effects on SS disposal and resource utilization. A green and accessible deep eutectic solvent (DES) based on choline chloride (ChCl) and lactic acid (La) was proposed to extract HMs from SS. The results showed that 80.7 % of Cd, 76.7 % of Zn, 56.9 % of Pb, and 30.9∼45.4 % of As, Cu, and Cr were removed from SS. In addition, DES exhibited a strong ability to destroy and dissolve organic matter like humic substances and extracted 66.6 % of the phosphorus. The treated sludge is more suitable for incineration with an increase of higher heating value and a decrease in ash content. Furthermore, the recyclability of DES was tested. After four recycles of DES, the accumulation of HMs and phosphorus (4.3 g/L) in DES provided potential benefits to recycle phosphorus. Thus, a hydrothermal method was conducted and successfully recycled about 70 % of phosphorus from DES. This research innovatively introduces DES to extract HMs from SS and realize phosphorus recovery, providing new ideas for SS harmlessness and valorization.