Immobilization of copper, lead, and nickel in two arid soils amended with biosolids: effect of drinking water treatment residuals

This study was conducted to determine the potential of drinking water treatment residuals (DWTRs) land application in altering the distribution of Cu, Pb, and Ni among soil fractions. The main objectives of the study were to (1) assess the mobility and availability of biosolid-born Cu, Pb, and Ni in two arid zone soils differing widely in soil properties and (2) evaluate the efficiency of inexpensive DWTRs at various rates on immobilization and chemical transformation of Cu, Pb, and Ni in the biosolid-amended soils. Two agricultural arid soils (calcareous and sandy) were collected and subsamples were amended with a combination of biosolids (3 %, w/w) and DWTRs (2, 4, 6, and 8 %, w/w). The amended and unamended soils were incubated for 2 months and sampled for metal bioavailability using diethylene triamine penta acetic acid (DTPA) extractant. The sequential extraction technique was performed to determine the effect of DWTRs on the distribution of Cu, Pb, and Ni among exchangeable, carbonate, oxides, organic, and residual fractions. Application of DWTRs at different rates to the biosolid-amended soils greatly increased residual (RS) Cu, Pb, and Ni fractions and concurrently decreased nonresidual (NORS) fractions in both calcareous and sandy soils. The greatest reduction in NORS fractions were obtained at the highest application rate (8 %). The NORS-Cu, Pb, and Ni fractions in biosolid-amended soils (control) decreased significantly from 51.79 to 20.93 %, from 54.5 to 39.82 %, and from 50.93 to 17.85 % respectively in calcareous soil and from 78.64 to 32.53 %, from 66 to 55.33 %, and from 55.89 to 22.63 % respectively in sandy soils. Thus, DWTRs contributed in Cu, Pb, and Ni fractions conversion from labile forms to more stable forms. Addition of DWTRs to biosolid-amended soils significantly reduced the availability of Cu, Pb, and Ni by chemical modification of their chemical speciation into less available forms. Our results suggest that DWTRs application could relieve the heavy metals risk of sewage sludge and spotlight the benefit of application of biosolid-amended soils with DWTRs.