Exploring the potential of synthetic and natural chelators to enhance phytoextraction by Indian mustard in metal(loid) (As, Cd, Cu, Ni, Pb, Zn and V)-contaminated soils

Abstract

Soil contamination with metal(loid)s poses significant environmental and human health risks due to their non-biodegradable and highly persistent nature. Phytoextraction is an eco-friendly phytoremediation method that uses plants to extract and accumulate metal(loid)s in harvestable tissues. Chelators can enhance this process by increasing metal(loid) bioavailability through the formation of metal-chelate complexes. Despite the proven effectiveness of synthetic chelators like EDTA, its associated drawbacks, such as excessive metal solubilization, slow degradation rate, phytotoxicity, and the metal leaching risk, have limited its field application. As an alternative, natural chelators such as citric acid (CA) and oxalic acid (OA) offer lower phytotoxicity and greater biodegradability. However, the effectiveness of CA and OA compared to EDTA is poorly investigated under different soil conditions, and a knowledge gap exists in the combined application of these chelators for phytoextraction of acidic multi-metal (loid) contaminated and alkaline V-contaminated soils. Therefore, this thesis examined the effectiveness of chelators alone and in combination for phytoextraction of (i) mining-impacted acidic (pH 5.6) multi-metal (loid) contaminated (As, Cd, Cu, Ni, Pb, Zn) boreal forest soil (first study) and (ii) alkaline soil (pH 7.6) contaminated with V at two different levels (200 and 400 mg/kg V) (second study), using Indian mustard. Separate greenhouse experiments were conducted for 10 weeks (first study) and 13 weeks (second study). Six treatments were applied with triplicates in the first study (control, EDTA, CA, OA at 3 mmol/kg each; EDTA + CA, EDTA + OA at 1.5:3 mmol/kg each) and five (all treatments in study 1 except EDTA) in the second study for each contamination level. Chelators were applied twice at monthly intervals, and soil pore water was collected thrice; before chelator application, after the first application, and after the second application. At the end of the experiment, aboveground biomass (dry weight) and metal(loid) concentrations in pore water, soil, and plant tissues were analyzed using ICP-AES. Results of the first study demonstrated that while EDTA alone or combined with CA and OA was the most effective chelator in increasing the metal(loid) bioavailability in multi-metal(loid)-contaminated soil, it induced severe phytotoxicity in Indian mustard, leading to plant death. Conversely, CA and OA alone had minimal impact on metal(loid) bioavailability and plant uptake but did not induce phytotoxicity. These results suggest that Indian mustard, when used with optimized concentrations of natural chelators alone or in combination with EDTA, could be effective for remediating multi-metal(loid)-contaminated boreal forest soils. In the second study also, EDTA combined applications with CA and OA significantly enhanced the V in pore water, soil, and plant tissues at the 200 mg/kg V-contaminated soils, without inducing phytotoxicity. However, at the 400 mg/kg V-contaminated soils, these treatments were ineffective in increasing V bioavailability and plant uptake. At both V concentrations, natural chelators alone had no significant effect on V phytoextraction. These findings suggest that Indian mustard is suitable for phytoremediation of both acidic multi-metal(loid) (As, Cd, Cu, Ni, Pb, Zn) and alkaline V-contaminated soils. Additionally, the findings highlighted that chelate-assisted phytoextraction is an effective approach to remediate multi-metal(loid) and moderate V-contaminated soils (~ 200 mg/kg). Further investigations are beneficial on optimizing natural chelator concentrations to enhance the phytoextraction efficiency.Master of Science in Bioscience, Technology, and Public Polic