Effect of Bentonite amendment on Lead and Cadmium accumulation and plant 2 growth indicators

The objective of this study was to determine the effect of bentonite on lead (Pb) and cadmium (Cd) accumulation in Indian mustard (Brassica juncea), as well as to evaluate the growth parameters of plants grown in soil contaminated with different levels of Pb and Cd. In this study, a pot experiment was conducted using a (3 × 3) factorial arrangement consisting of three levels of lead (10, 100, and 200 mg kg⁻¹), three levels of cadmium (5, 15, and 25 mg kg⁻¹), two bentonite application rates (25% and 50%), and an untreated control. The results showed significant differences (P ≤ 0.05) in metal concentrations in roots and leaves, as well as in growth parameters such as fresh weight and plant height. A progressive increase in Pb and Cd concentrations was observed in both roots and leaves with increasing contamination levels. In contrast, the application of bentonite at 50% significantly decreased metal concentrations; Pb concentration in roots decreased from 75.03 to 41.20 mg kg⁻¹, while Cd concentration at the highest contamination level decreased from 0.29 to 0.19 mg kg⁻¹, respectively. The results also indicated significant reductions in shoot and root biomass under high Pb and Cd concentrations. In the control treatment, fresh shoot weight was 204.67 g, which decreased by 62.53% under 200 mg Pb kg⁻¹ + 25% bentonite and by 61.56% under 25 mg Cd kg⁻¹ + 25% bentonite. Fresh root weight followed a similar trend, decreasing to 43.67 g and 46.00 g, respectively, under the same treatments. However, bentonite application at 50% improved all growth attributes compared with the control, particularly at lower contamination levels, where the highest fresh shoot weights (225.33 g and 219.00 g) were recorded under 10 mg Pb kg⁻¹ + 50% bentonite and 5 mg Cd kg⁻¹ + 50% bentonite, respectively. The results also demonstrated that cadmium was more toxic than lead; however, bentonite reduced the bioavailability of heavy metals and improved soil properties, which consequently enhanced plant growth. At high Pb and Cd contamination levels, Indian mustard tended toward phytostabilization behavior for Pb and phytoextraction behavior for Cd, as the translocation factor (TF ≈ 1) values were higher than the plant accumulation indices. Overall, the findings confirm that bentonite is an effective soil amendment for heavy metal-contaminated soils under varying environmental behavior of each element.