Biosolid Amendment Modulates Copper Speciation and Availability in a Contaminated Technosol from Southern Brazil

Copper (Cu) is an essential micronutrient, but excess concentrations inhibit plant growth, particularly in soils with low organic matter (OM) and clay contents where its bioavailability is higher. Areas containing Cu mining tailings at the Minas do Camaquã geosite (Caçapava do Sul, RS, Brazil) are contaminated, representing an environmental concern. Biosolids, obtained from the stabilization of domestic wastewater treatment sludge, constitute a source of OM and humic substances capable of modifying Cu chemical speciation in soil solution and, consequently, its bioavailability. This study evaluated the influence of biosolid application on Cu ionic speciation in a contaminated Technosol from the geosite. Biosolid doses equivalent to 0, 45 and 90 kg N ha⁻¹ were tested using Canavalia ensiformis as a test species. Cu speciation was estimated through geochemical modeling considering soil solution properties. Biosolid application, especially at the highest dose, increased dissolved organic carbon and enhanced the formation of Cu–DOM complexes, indicating reduced free Cu available for plant uptake. Increasing biosolid rates also reduced hydroxylated Cu species, reflecting changes in soil solution pH. Nutrient enrichment, particularly phosphorus, promoted CuHPO₄ formation, contributing to Cu complexation. By increasing OM inputs and modifying nutrient status, biosolid incorporation alters the chemical environment of contaminated soils and shifts Cu geochemical equilibria in solution. These results demonstrate that biosolids have significant potential to modulate Cu availability and support sustainable strategies for the remediation of Cu-contaminated soils.