The Effects of Compost-Derived Dissolved Organic Carbon on Cd, Zn, and Cu Release From Metal-Spiked Soils

Abstract

Dissolved organic carbon (DOC) can affect the mobility or bioavailability of heavy metals in a soil system. We investigated the effects of two compost-derived DOC solutions on the release of Cd, Zn, and Cu from two metal-spiked agricultural soils via repeated extraction experiments under acidic conditions. The two soils used in this study were acidic (pH 5.1-5.7) and differed primarily in soil texture (Sankengtzu [Sk] sandy loam vs. Potu [Pu] silty clay loam). The soils were spiked with individual metals (Cd, Zn, or Cu) to levels close to the limits imposed by local regulations. The metal-spiked soils were treated with two acidified DOC solutions (10 mmoUL DOC; pH 3; Na + exchanged), one derived from sewage sludge compost and the other from swine manure compost, followed by repeated extraction experiments. The results showed that the addition of DOC in the extraction solution can accelerate the acidification and the release of Cd, Zn, and Cu from the two soils. This effect was more obvious for the Sk sandy soil than for the Pu clay soil. The two DOC solutions, which were extracted from different sources, showed different behaviors of metal release. The differences might be related to the acid strength, metal-binding strength of acid sites, and the presence of other anions. The release of Cd from the soils was controlled mainly by the pH of the soil. Neither an inhibition effect of chloride ions nor an improvement effect of DOC on the Cd release was found. The release of Zn was slightly inhibited (~10%) by the DOC solution extracted from the sewage sludge compost (or Neihu secondary sludge compost [NH2SC]-DOC), especially from the Sk sandy soil, which was assumed to be the result of the formation of sulfide. The use of DOC extracted from the swine manure compost (or SMC-DOC) can result in the release of more Cu (up to 35%-40%) from both soils. The DOC solutions can be used for enhancing the efficiency of acid-washing techniques used for metal-contaminated soil remediation (especially for Cu).