Chemical Stabilization and Enhanced Phytostabilization of Heavy Metal Contaminated Acidic Boreal Forest Soils using Biochar and Modified Biochars

Abstract

Heavy metal contamination in acidic boreal forest soils is a significant environmental concern due to the enhanced mobility and bioavailability of metals under acidic conditions. Biochar treatments have gained increasing attention as sustainable remediation materials because of their ability to immobilize heavy metals and improve soil properties. This research evaluated the effectiveness of biochar (BC) and chemically modified biochar for the chemical stabilization and enhanced phytostabilization of cadmium (Cd), copper (Cu), and Zinc (Zn) in acidic multi-metal-contaminated boreal forest soils under moderately and highly contaminated conditions. The first study evaluated the effectiveness of BC and chemically modified biochar, including NaOH-modified biochar (NBC), H2O2-modified biochar (HBC), and KMnO4-modified biochar (KBC), in reducing metal mobility and improving metal stability in acidic contaminated soils. Adsorption experiments, incubation studies, leachate monitoring, toxicity characteristic leaching procedure (TCLP), synthetic precipitation leaching procedure (SPLP), available metal extraction, and sequential fractionation analysis were conducted to assess amendment performance. Chemically modified biochar of NBC and HBC demonstrated greater adsorption capacity and immobilization efficiency than BC. Biochar treatments significantly reduced available and leachable Cd, Cu, and Zn concentrations while increasing soil pH and promoting redistribution of metals from more labile fractions to more stable forms. Although KBC showed the highest adsorption capacities in batch experiments, its stabilization performance under soil conditions was not consistently superior. The second study evaluated the effectiveness of BC and HBC in biochar-assisted phytostabilization using Canada wild rye (Elymus canadensis), little bluestem (Schizachyrium scoparium), and switchgrass (Panicum virgatum). A three-month growth chamber study was conducted to assess dry matter production, metal accumulation in roots and shoots, available metal concentrations, soil pH, and heavy metal stability in phytostabilized soils. Both BC and HBC improved phytostabilization performance by reducing Cd, Cu, and Zn mobility and accumulation in plant tissues while improving metal stability in soil. However, BC produced greater dry matter and showed comparable or greater effectiveness than HBC in reducing metal uptake and translocation in native grasses. Biochar treatments also increased soil pH and reduced TCLP and SPLP extractable metal concentrations, indicating lower metal leaching potential in phytostabilized soils. Overall, the findings demonstrated that biochar treatments effectively reduced heavy metal mobility, availability, and leachability in acidic boreal forest soils. Chemically modified biochar, particularly NBC and HBC, showed greater potential for chemical stabilization, while BC demonstrated stronger overall performance for biochar assisted phytostabilization. These findings highlight the potential application of biochar treatments as sustainable remediation strategies for acidic Cd, Cu, and Zn contaminated boreal forest soils.