Soil solution chemistry of contrasting soils amended with heavy metals

The soil solution chemistry of heavy metal amended soils is of great import ance in assessing the bioavailability of heavy metals and their toxicity to the soil biota. Three contrasting soils were amended with Cd(II), Cu(II), Ni(II), Pb(II), Zn(II), and Cr(III) nitrate salts at rates of 10-100 mmol/k g. This concentration range was chosen to encompass a wide range of erects on sensitive soil biochemical properties as part of a larger project. Soil solutions were extracted and analysed for pH, and for concentrations of hea vy metals, and major cations and anions. Heavy metal speciation was calcula ted with the GEOCHEM-PC model. Heavy metal concentrations in the soil solutions increased both in absolute terms and as a percentage of added heavy metal as amendment rates increase d. This observation is due to decreasing specific adsorption (caused by dec reasing pH induced by the amendments), and to increasing saturation of cati on exchange sites. For all 3 soils, the percentage increase commonly follow s the order Cr(III) < Pb < Cu < Ni < Cd < Zn. The percentage of each metal held in the soil solution increased from soil to soil as cation exchange ca pacity, and therefore sorptivity, decreased. Both the concentration and activity of free heavy metal ions were substanti ally lower than the corresponding total metal concentration. This was ascri bed to ion-pairing of metal ions with anions, particularly nitrate introduc ed in the amending solutions, as well as to increases in ionic strength as a result of amendment. Metal-anion species were mainly inorganic but where Cu and Pb were relatively low in concentration because of strong adsorption by the soils, organic complexation was likely to be significant. Speciatio n trends were similar for the 3 soils but different in magnitude.