Cost-effective and environmental friendly remediation of heavy metal contaminated soils

Soil contamination by heavy metals (HMs) is a worldwide problem for human health. Unlike organic pollutants which can be destroyed, HMs are non-degradable by chemical and microbial decomposition, and they are persistent and hard to remove once released into soils. Therefore, remediation of HM contaminated soils is exigent and imperative. One cost-effective and environmental friendly remediation approach is the application of biochar, which is a solid carbonaceous material. Biochar has been widely documented to effectively immobilize metals in contaminated soils and has received increasing attention for use in soil remediation. The purpose of the study is to analyze the efficiency of biochar for copper immobilization in contaminated soil using a combined fractionation scheme (Minkina et al., 2013), which makes possible to determine the composition of loosely (LB) and strongly bound (SB) HM compounds. This scheme is based on a combination of the results obtained by the Tessier method (Tessier et al., 1979) and parallel extractions (Minkina et al., 2018). The studies were carried out as a model experiment on Calcaric Fluvisol (Loamic) (at the depth 0-20 cm) collected in the Severnyi Donets River floodplain (Rostov region, Russia). The analyzed soil is characterized by following physical and chemical properties: C_org – 4.3%; pH 7.5; exchangeable cations (Ca²⁺+Mg²⁺) – 38.1 cMс/kg; CаCO₃ – 0.6%; content of physical clay (particle <0.001 mm) – 55.8%, silt – 32.0%, Cu – 43.7 mg/kg. In a model experiment the soil samples were artificially contaminated with higher portion of CuO (1320 mg/kg). The metal was incubated in soil samples for 6 months. After incubation of soil with metal, biochar was introduced into the vessels in the dose 2.5%. Biochar for the experiment was produced by pyrolysis (temperature 550°C) of birch wood. The elemental analysis, BET, SEM, FTIR, XRD and TGA were used to characterize the physicochemical properties of carbonized material. The domination of strongly bound Cu compounds (92% of total fractions), which is mainly supported of metal retention by primary and secondary minerals (66 %) was observed in uncontaminated soil (control). Mobility of Cu in soils was low (8%) and represented mainly by specifically adsorbed metal compounds. The content of all forms of Cu increases with artificial soil contamination and their ratio increases (up to 38%) in the content of LB compounds. The majority of residual fraction remains in the composition of SB of Cu compounds, however its relative content decreases until 41 % under anthropogenic load. The introduction of biochar has a significant effect on the transformation of Cu compounds in soil and it decreased the metal mobility due to formation of SB compounds. The relative content of LB metal compounds after use of biochar is almost equal to control variant with simultaneous redistribution of fractional-group composition of metal. Thus, the present study demonstrates the possible remediation of soil contaminated by HMs using biochar and provides a particular strategy for remediation of soils contaminated with Cu.

The reported study was funded by RFBR, project number 19-34-60041.