Filter Cake Biochar Reduces Cadmium Bioavailability and Accumulation in Vegetables Grown in Contaminated Soil

Cadmium (Cd) contamination in agricultural soils poses serious risks to food safety and human health, particularly in regions with acidic soils that favor metal mobility. This study evaluated the effectiveness of filter cake biochar (FCB) in remediating Cd-contaminated soil and mitigating its transfer to vegetables. A greenhouse experiment was conducted using an Ultisol contaminated with 10 mg kg⁻¹ Cd. Four vegetables, Swiss chard (Beta vulgaris), okra (Abelmoschus esculentus), beetroot (B. vulgaris), and bell pepper (Capsicum annuum), were cultivated in pots with and without FCB amendment. FCB application reduced Cd availability in soil, from 4.5 to 0.5 mg kg⁻¹, bringing concentrations below the regulatory guideline (1.3 mg kg⁻¹). This was accompanied by a reduction of up to 87% in Cd concentrations in plant tissues. Improvements in soil fertility were also observed, including increased pH, organic carbon, and availability of P, Ca, Mg, Fe, and Mn. Cd accumulation in plant tissues varied by species, with bell pepper showing the lowest and Swiss chard the highest concentrations. Crucially, only vegetables cultivated in FCB-amended soil yielded edible produce, underscoring the biochar's role in mitigating phytotoxicity and enabling safe consumption. In addition to lowering Cd uptake, FCB enhanced biomass and nutrient accumulation in shoots. Correlation analysis confirmed strong positive relationships between soil fertility parameters and plant nutrient uptake, and negative correlations between Cd availability and plant performance. These results demonstrate that FCB is an effective strategy for reducing Cd risks in contaminated soils and improving the productivity and safety of vegetable crops.