The mixed chelators (MC) of N, N-bis(carboxymethyl) glutamic acid (GLDA) and citric acid (CA), which were biodegradable chelating ligand, were employed to remove heavy metals from sewage sludge. The extraction of Cd, Cu, Zn, Mn, and Cr from sludge was studied under different experimental conditions. Results showed that the removal efficiencies of heavy metals from both sludges were the highest with MC of GLDA and CA at the molar ratio of 1:5. For the sludge of plant A, the extraction efficiencies increased with increasing contact time, but only slowly increase was observed after 24 h. However, for the sludge of plant B, the removal efficiencies were significantly decreased after 16 h, and sharply decreased after 24 h. The removal efficiencies of heavy metals decreased with the increase of the solution pH, while they increased with the increase of the solution concentration. For the sludge of plant A, single washing with 200 mmol·L-1 MC might be favourable to remove heavy metals. However, for the sludge of plant B, duplicate washing with 100 mmol·L-1 MC could remove much more heavy metals. The concentration of Cu in sludge of plant B decreased from 695.79-139.16 mg·kg-1, which was conformed to the standard (GB 4284-2018). These results suggested that MC may be useful and environmentally friendly chelators in the removal of heavy metals from sewage sludge.
Keywords: N, N-bis(carboxymethyl) glutamic acid; citric acid; heavy metals; mixed chelators; sewage sludge.