Agent-assisted electrokinetic treatment of sewage sludge: Heavy metal removal effectiveness and nutrient content characteristics

Sewage sludge (SS) is rich in nutrient elements such as phosphorus (P), nitrogen (N), and potassium (K), and therefore a candidate material for use in agriculture. But high content of heavy metals (HMs) can be a major obstacle to its further utilization. Therefore, an appropriate HM removal technology is required before its land application. In this study, an innovative biodegradable agent (citric acid, FeCl₃, ammonium hydroxide, tetrasodium iminodisuccinate (IDS), and tea saponin) assisted electrokinetic treatment (EK) was performed to investigate the HM removal efficiency (R_HMs) and nutrient transportation. Citric acid, IDS, and FeCl₃-assisted EK showed a preferable average R_HMs (R_ave) reduction of 52.74-59.23%, with low energy consumption. After treatment, the content of Hg (0.51 mg kg^-1), Ni (13.23 mg kg^-1), and Pb (26.45 mg kg^-1) elements met the criteria of national risk control standard, in all cases. Following the treatment, most HMs in SS had a reduced potential to be absorbed by plants or be leached into water systems. Risk assessment indicated that the Geoaccumulation index (I_geo) value of HMs has decreased by 0.28-2.40, and the risk of Pb (I_geo=-0.74) reduced to unpolluted potential. Meanwhile, no excessive nutrient loss in SS occurred as a result of the treatment, on the contrary, there was a slight increase in P content (18.17 mg g^-1). These results indicate that agent-assisted EK treatment could be an environmentally-friendly method for R_HMs and nutrient element recovery from SS, opening new opportunities for sustainable SS recycling and its inclusion into circular economy concepts.