Positive Effect of Biochar Application on Soil Properties: Solubility and Speciation of Heavy Metals in Non-Acidic Contaminated Soils near a Steel Metallurgical Plant in Southeastern Europe

Neutral and slightly alkaline arable soils from the vicinity of the former and the biggest metallurgical plant in southeastern Europe were analyzed for the status of the water soluble pool of heavy metals in 1–20% w/w biochar (BC)-amended contaminated soils. Heavy metal solubility was monitored over a 6-month period. The metals (Fe, Mn, Cu, Zn, Cd, Pb and Ba) exhibited significant relationships between each other and exceeded the maximum permissible concentrations (MPC) in surface waters for domestic and drinking purposes. In most of the investigated sites and BC treatments, metal concentrations decreased with time due to the transfer to more resistant soil pools. Cation exchange capacity, exchangeable Ca and pH increased after BC application, while electrical conductivity decreased. BC amendment led to the prevalence of humic acids (HAs) over fulvic acids (FAs) and increased the fraction of refractory organic carbon. The share of metal–organic complexes increased for the metals Zn, Cd, Mn and Ba in the BC-amended soils, and the share of free Me2+ species decreased. This trend was especially pronounced in the soils with the lowest pH of 6.4–6.9. In addition to improving soil physicochemical and ecochemical properties, biochar application contributed to metal species in solutions that were less mobile and bioavailable.