Effect of biochar on heavy metal solubility and speciation in Technogenic soils around Aurubis-Pirdop copper smelter in Bulgaria

Abstract

Abstract Soils from the area of Aurubis-Pirdop Cu smelter in Bulgaria were studied for the effect of biochar (BC) on the heavy metals concentrations and speciation. Maximum permissible lеvels (MPL) were exceeded for Cu, Pb, Zn, As and Mn. Total concentrations of the contaminants varied from 319–2645 mg/kg for Cu and from 92–234 mg/kg for Pb. Incubation experiments were performed with different rates of biochar (BC), 1, 5, 10 and 20% and changes in soluble forms of metals were recorded. Incubation time and increasing BC rates led to a total decrease of Cu and Pb. There was a clear pronounced effect of BC increasing rates, and especially the highest rate of 20%. Dissolved organic carbon (DOC) and pH increased with BC rates at the 3rd sampling period. Ion speciation, e.g. for Cu, Pb, Cd, Zn, Cr, etc. in the non-treated variants and those with pH 4.7-5 was represented by the free Men+ ions, while with increasing incubation time and BC rates, heavy metal species, were represented by organic fulvic acid – metal complexes, such as bidentate (FA2M) and M-FA gel fraction species, e.g. FAM + 2G; FA-Cr + 3G. The total Cu concentration decreased by ~ 70%, while Cu2+ species decreased to ~ 3,7% at the 4th month of incubation for some soils. Biochar incorporation had a positive effect both on Cu, Pb and other metals total concentrations, as well as on metal speciation in favour of increasing metal-organic complexes in soil solution, which are less mobile and bioavailable.