Humic Acids Enhance Potentially Toxic Elements (PTEs) Phytoextraction from a Contaminated Soil by Basil Plants

Elevated levels of potentially toxic elements (PTEs) in soil possess a potential threat to human health. Probably, the use of hyper-accumulator plants can be an eco-friendly and cheap approach to attain more desirable levels of these contaminants in soil. The current study is a trial to increase the efficiency of basil (hyperaccumulator) plants to phytoextract some PTEs, namely Cd, Pb and Co from a clay soil artificially contaminated with these PTEs by the aid of elevated doses of humic acids (HAs 0-50 mg kg-1) derived from a mature compost. A greenhouse experiment was conducted in 2021 to attain this aim and lasted for 60 days, following a complete randomized design. During this investigation, soil moisture was kept gravimetrically at around 70% of water holding capacity to avoid the leach out of soluble metal ions or organic complexes from the soil. By the end of this experiment, plants were harvested and the results indicated significant reductions in root and shoot dry weights and their lengths owing to the application of humic acids at a rate of 20 mg kg-1 versus the control (0 mg HAs kg-1). Increasing the dose of HAs application (up to 50 mg kg-1) led to concurrent significant increases in the abovementioned growth parameters; yet, their values were still below the control plants. Likewise, significant increases were noticeable for PTEs contents within roots and shoots of plants treated with 20 mg HAs kg-1 while increasing the dose of HAs application resulted in significant reductions in their contents versus the lower application dose of HAs. Root-to-shoot translocation factor was >1 for each of Cd and Co, while the corresponding values for Pb were less than one. Unfortunately, all BAF values were below one and this result indicates that basil plants cannot be considered hyperaccumulators for Cd and Co but can be used for successful facilitated phytoextraction of these two contaminants from soil. In conclusion, humic acids although enhanced phytoextraction of the PTEs under study by basil plants, yet they cannot be considered the optimum chelating agent that could facilitate the phytoextraction of soil contaminants under conditions of the soil under study.