Effect of lead, cadmium, and mercury co-contaminants on biodegradation in PAH-polluted soils

Contamination of land by persistent organic pollutants has significant implications for human health and for future development potential. Bioremediation is an effective method for reducing the concentrations of such contaminants to below harmful levels, but the presence of co-contaminants may hinder this process. Here, we present the results of a 40-week microcosm study in which the biodegradation of sixteen US-EPA polycyclic aromatic hydrocarbons (PAHs) (total: 2166 mg kg-1) was followed in the presence of three different concentrations of cadmium (up to 620 mg kg-1) and lead (up to 782 mg kg-1) in a high organic matter soil. In the absence of metal treatment, 82% of PAHs were removed during the study period. Lead exerts a greater negative effect on total PAH removal than cadmium at low concentrations (ca. 100 mg kg-1) whilst cadmium exerts the greatest effect at higher concentrations (up to -27.7% reduction). Mercury, intended as the abiotic control (ca. 1150 mg kg-1), exerts the greatest effect overall (-37%). Principal Component Analysis showed that PAH degradation was strongly associated with soil respiration rate, biomass content and Ecoplate Average Well Colour Development. During the initial phase of the experiment, reduced microbial diversity was associated with increased PAH removal, consistent with literature observations for other organic contaminants, though this association was reversed after week 12. Degradation of higher molecular weight PAHs showed the greatest sensitivity to microbial community health. The effect of metal treatments on biotic parameters in microcosms without PAH amendment is also presented.