Environmental risk and ecotoxicological assessment of postcoagulation water treatment sludge for the reclamation of anthropogenically degraded soils

This study aims to investigate the physicochemical and ecotoxicological properties of postcoagulation water treatment sludge (PCWTS) in the context of its use for anthropogenic land reclamation. It also attempts to promote the beneficial reuse of post-coagulation sludge as a valuable resource, with the intention of contributing to future research in this field. In the course of the research, PCWTS after the water treatment process by a prehydrolysed coagulant, polyaluminium chloride (PAC, [Al₂(OH)ₙCl 6−n ]ₘ), and soil from three anthropogenically altered areas were subjected to in-depth physicochemical (UV‒VIS, F-AAS, WDXRF, CHNS, and SEM‒EDS) and ecotoxicological analyses and assessments. The potential ecological risk index (PERI), potential ecological risk factor (ER i ), and geoaccumulation index (I geo ) were calculated and interpreted. Statistical analyses (ANOVA and Tukey’s HSD test) were also used to further clarify the ecotoxicological results. The ecological risk assessment of heavy metal pollution indicated low-to-moderate risk levels (ER i <80 and PERI = 206), according to the background of the soils of the Upper Silesia region. The calculated values of ER i were in the order of Ni > Cu> Cd > Cr> Pb > Zn, whereas I geo was Ni > Cu> Cr > Zn> Cd > Pb. The results of the ecotoxicological tests demonstrated the potential suitability of the PCWTS as an amendment for anthropogenically degraded soils. The use of PCWTS as a soil-improving agent does not increase the risk of heavy metal contamination in soils classified as suitable for use in both agricultural and green areas, and contributes to improved soil fertility by providing essential nutrients (N 9.7 g/kg, P 2.3 g/kg, K 22.9 g/kg). Despite the presence of minor heavy metals, PCWTS applied to degraded soils does not pose a high risk of HMs, resulting in nontoxicity and beneficial effects on plant growth. The use of PCWTS for land reclamation in industrially degraded areas is an example of sustainable sludge management in water treatment plants and aptly corresponds with the ideas of circular economy and the principles of sustainable development. While this study provides valuable short-term insights based on a 14-day evaluation, the absence of a long-term assessment remains a limitation that warrants further investigation.