Effect of a Hybrid Sorbent Based on Biochar and Metal-Organic Framework MIL-100(Fe) on the Transformation and Bioavailability of Copper and Zinc in Contaminated Soils

Abstract Promising sorbents for soil remediation—biochar from wheat straw and a composite sorbent based on biochar with nanoparticles of the metal-organic framework MIL-100(Fe)—were synthesized. Uniform coating of the biochar surface with MIL-100(Fe) nanoparticles provides a sixfold increase in the specific surface area (up to 419 m2/g) and a hierarchical porous structure. The effects of these sorbents on the transformation of Cu and Zn in Haplic Chernozem with different levels of technogenic contamination and on the translocation of metals to spring barley plants (Hordeum sativum L.) are studied in a model experiment. The transformation of metals in soil is assessed based on the fractional composition determined with the BCR sequential extraction procedure, risk assessment code (RAC) values, and reduced partition index (IR) as indicators of metal binding strength in soils. Characteristic of contaminated soils are high shares of bioavailable Cu (7–11%) and Zn (14–19%) fractions along with significant metal accumulation in plants, inhibiting root growth by 40–64% and shoot growth by 14–27% as compared with uncontaminated soil. Sorbent application (1–2%) promotes a 4–13% shift of the metals from the bioavailable fraction to more stable oxidizable and residual fractions. The RAC values for both metals decreased from 6.8–18.5 to 0.1–15.3%, while IR values increase from 0.55–0.74 to 0.60–0.85. X-ray absorption spectroscopy data (XANES/EXAFS) revealed the transformations of Cu and Zn active centers with the sorbent. Cu and Zn are shown to coprecipitate with mineral components in the sorbent matrix and to form insoluble compounds CuCO3, (ZnOH)2CO3, and Zn3(PO4)2. The application of sorbents reduces the uptake of metals by barley roots and their transport to shoots. In all cases, the nanocomposite at a dose of 2% showed the highest efficiency in stabilizing metals and mitigating toxic effects.