Agricultural use of sewage sludges (SSLs) aligns with the principles of sustainable development and circular economy. Nonetheless, SSLs frequently contain higher contents of potentially toxic elements (PTEs), which may limit their environmental use. The aim of this study was to use protic ionic liquids (PILs), characterized by different properties (1-methylimidazolium chloride (H1CimCl), triethylammonium hydrogen sulfate (TEAHSO4), and 1-methylimidazolium hydrogen sulfate (H1CimHSO4)) to remove PTEs from digested SSLs. Apart from assessing removal efficiency, the mechanisms of action of specific PILs were identified as well as the bioavailability of the PTEs and the toxicity of the residue after the removal of the PTEs were determined. H1CimCl was characterized by the highest metal extraction efficiency. By using this solvent, from 11% to 89% of the PTEs was removed, among which Zn, Cd, and Cu were removed most effectively. However, the efficiency of the other PILs was relatively lower. The extraction efficiency was affected by temperature, SSL-to-PIL ratio, ethanol washing, and SSL type. The treatment of the SSLs with H1CimCl significantly reduced the contents of some bioavailable PTEs. The leachates toxicity was influenced by SSL types and organisms. However, the toxicity of the SSL leachates was observed to decrease (by 14-87%) after treating the SSLs with H1CimCl. The statistical analysis found that Ni and Zn could have been the critical factors determining the SSLs’ toxicity to bacteria, whereas As towards plants. The results confirm the high efficacy of the proposed method, which can be a promising solution supporting circular economy-based management of SSLs.
Błaszczyk et al. (Thu,) studied this question.