Sequential extraction of nickel and zinc in sewage sludge- or biochar/sewage sludge-amended soil.

Fractionation of nickel (Ni) and zinc (Zn) in sewage sludge-amended soil and in sewage sludge/biochar-amended soil was investigated. The results were compared with ecotoxicological tests in order to determine the contribution of metals to the toxicity of amended soils. The study was conducted as a long-term field experiment. Sewage sludge (SL) or sewage sludge with a 2.5, 5 or 10% addition of biochar (BC) was added to the soil. Samples for analysis were taken immediately after experiment establishment as well as after 12 and 18 months from the beginning of the study. The fractionation analysis of Ni and Zn was performed using the BCR (Community Bureau of Reference) three-step sequential extraction procedure. The following forms were determined: mobile (F1); bound to FeMn oxides (F2); bound to organic matter (F3) and residual (F4). The soil, SL and BC differed in the contribution of individual forms of the metals. The application of SL into the soil resulted in an increased soil content of mobile forms of Ni and Zn by 180 and 103%, respectively. The mobility index (MI) significantly increased, which evidences the risk related to the presence of these metals. Biochar in the sewage sludge significantly reduced the content of Ni and Zn in F1 fraction. The study also demonstrated that biochar amendment promotes the transition over time of available forms of Ni and Zn into their residual forms (F4), which leads to a further reduction in the environmental risk related to their presence in the environment. The conducted statistical analysis revealed only intermittent relationships between the individual forms of the metals and soil physicochemical properties and toxicity, which may indicate more complex mechanisms that occur in the experimental systems investigated. Therefore, the use of SL in combination with BC can be an effective method for reducing the environmental risk related to the presence of metals in SL.