PATH-ANALYSIS OF HEAVY-METAL ADSORPTION BY SOIL

Path analysis, a statistical technique that differentiates between correlation and causation, was used to describe heavy metal adsorption by soil. A path analysis model that evaluates the effect of soil pH, CEC, organic carbon content (OC), and clay content on adsorption of Cd, Cu, Ni, Pb, and Zn by soils from two long-term cropping experiments in Sutherland and Kanawha, IA, was investigated. In general, direct effects of soil properties on metal adsorption were pH > OC > CEC. Path analysis direct effects of pH on Cd (0.83), Cu (0.93), and Pb (0.96) by Galva (fine-silty, mixed, mesic Typic Hapludoll) soils from the Sutherland experiment were highly significant (P < 0.01). Significant (P < 0.01) direct effects of pH on Cd (0.93), Cu (0.95), and Pb (0.85) adsorption by Webster (fine-loamy, mixed, mesic Typic Haplaquoll) soils from the Kanawha experiment. Significant (P < 0.01) simple correlation coefficients were found between soil pH and Cd (0.64), Cu (0.89), and Pb (0.84) adsorption by Galva soils and between soil pH and Cd (0.82), Cu (0.74), and Pb (0.62) adsorption by Webster soils. Both path analysis and correlation analysis showed that soil pH was the predominant property affecting Cd, Pb, and Cu adsorption by soil. However, path analysis revealed three additional relationships not found by correlation analysis. They included direct effects of CEC on Ni adsorption (0.72), OC on Pb adsorption (0.35) by Galva soils, and OC on Cd adsorption (0.59) by Webster soils. Partitioning of OC direct and indirect effects suggested that adsorption of Pb and Cd by soil organic matter through complexation reactions was more important than adsorption by organic matter CEC sites.