Concentrations of Heavy Metals in Six Municipal Sludges from Guangzhou and Their Potential Ecological Risk Assessment for Agricultural Land Use

Six kinds of municipal sludge samples were collected from wastewater treatment plants (WWTPs) from Guangzhou in southern China. Total concentration of Cu, Pb, Zn, Cr, Ni, Cd, and Mn in the sludge was determined, and used obtained values to appraise the degree of agricultural land pollution on the base of four pollution indixes. Results showed that the concentrations of the heavy metals Cu (4.567 mg/kg), Mn (1.844 mg/kg), Ni (148 mg/kg), Cr (121 mg/kg), and Cd (5.99 mg/kg) in Si sludge were higher than that of the other sludges. In six sludge samples the concentrations of Mn, Zn, and Cu were highest, followed by Ni, Pb, and Cr. Cd had the lowest concentration. All were higher than those recorded in the background data for crop soils. With the exception of Cu and Cd from site S1, and Ni from sites S1, S2, and S5, all other metal concentrations conformed to permissible levels prescribed by the national application standard of acid soil in China (GB18918-2002). The geoaccumulation index (I-geo) assessment results indicate that there are serious risks in agricultural utilization for the existing Cu and Zn in these six kinds of sludges, especially the I-geo, which was 6.53 for Cu, 3.21 for Mn, 2.78 for Cd in the S I sludge. The results of RI showed that sludge had high potential ecological risks, especially caused by Cd, which should give rise to widespread concerns. The potential ecological risk index (RI) of heavy metals in six sludge samples was ranked in the order of S1>S2>S4>S3>S5>S6.