Protic ionic liquid-based extraction of potentially toxic elements from digested sewage sludges with ecotoxicological and bioavailability assessment of residues.

Agricultural use of sewage sludges (SSLs) supports circular economy objectives but is often limited by high levels of potentially toxic elements (PTEs). The aim of this study was to use protic ionic liquids (PILs), characterized by different properties (1-methylimidazolium chloride ([H1Cim]Cl), triethylammonium hydrogen sulfate ([TEA][HSO4]), and 1-methylimidazolium hydrogen sulfate ([H1Cim][HSO4])) 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. [H1Cim]Cl was characterized by the highest PTE 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 hydrogen sulfate-based PILs was relatively lower. The extraction efficiency was influenced by temperature, SSL-to-PIL ratio, ethanol washing, and SSL type. The treatment of the SSLs with [H1Cim]Cl significantly reduced the contents of some bioavailable PTEs, including Zn and Ni, while retaining key nutrients such as phosphorus. The leachates toxicity varied by SSL types and organisms, it decreased overall by 14-87 %, particularly for bacteria (78-87 %). 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.