Sources, distribution characteristics, and risk evaluation of heavy metals in sludge from industrial parks

Although pollution caused by heavy metals in sludge has recently attracted extensive attention, little is known about the distribution characteristics, potential ecological risks, and sources of heavy metals in sludge collected from industrial parks. In the present study, sludge specimens were gathered from 89 centralized wastewater treatment facilities within the industrial parks of Chongqing, China. An analysis was conducted on the concentrations, spatial distributions, ecological risks, and sources of heavy metals. The concentration ranges of Hg, As, Cd, Pb, Cr, Ni, Cu, and Zn were 0.02-18.55, 0.04-59.80, 1.5-21.85, 25-364, 2.5-57,950, 5-94,509, 5-18,501, and 10.95-83,750 mg/kg, respectively. The mean concentrations of all heavy metals surpassed the local soil background values. Spatial distribution analysis of heavy metals showed relatively higher contents of Cr, Ni, Cu, and Zn in the sludge in the city's central-western regions compared to other areas, which can be correlated with the degree of industrial development. Ecological risk assessment results showed that the potential ecological risk of sludge from 87.64% of the wastewater treatment plants was below the "strong risk" threshold. Notably, the potential ecological risks (E_i) were higher for Hg and Ni than for other heavy metals. The analysis of heavy metal sources indicated that Cr, Ni, Cu, and Zn in the sludge from the city's industrial parks exhibited highly significant pairwise correlations (P < 0.01), originated from the same source, associated with industries such as metal surface treatment.