Effective Lead Immobilization in Contaminated Soil with Hydrochar Application

Soil contamination with heavy metals, particularly lead (Pb), poses significant environmental and health risks. Hydrochar, a carbonaceous material derived from biomass via hydrothermal carbonization, has shown potential in mitigating heavy metal bioavailability in contaminated soils. This study investigates the effectiveness of hydrochar derived from apple tree wood waste in immobilizing Pb and improving soil fertility. Two types of hydrochar, produced at 180 °C (H180) and 250 °C (H250), were applied at varying concentrations (0.5 wt.%, 1 wt.%, 1.5 wt.%, and 2 wt.%) to Pb-contaminated soils. Contaminated soil amended with hydrochar at varying concentrations were analyzed through a laboratory incubation experiment, and total and mobile Pb fractions in soil, as well as Pb accumulation in plants (Hordeum sativum), were evaluated. The results revealed that hydrochar amendments did not affect seed germination but influenced root growth, with H180 hydrochar stimulating root development in uncontaminated soil. Hydrochar application significantly decreased the mobile Pb fraction: in contaminated soil, H180 reduced Pb mobility by 37%, while H250 achieved a 52% reduction. Additionally, hydrochar amendment enhanced enzymatic activity in uncontaminated soil but had a mixed impact in Pb-contaminated soil, with some doses reducing dehydrogenase activity. Pb uptake by plants was dose-dependent, with higher hydrochar concentrations resulting in greater reductions in the bioconcentration and translocation factor values. The lowest Pb translocation was observed in soils treated with 2 wt.% of H250 hydrochar, highlighting its greater porosity and adsorption potential. These findings emphasize hydrochar’s potential as a sustainable soil amendment material for heavy metal immobilization and phytostabilization.