Chelating Agents and Heavy Metal Phytoextraction

Abstract

Phytoremediation is a naturally slow process limited by low availability of target metals in soils. Adsorption−desorption at the particle–solution interface are the most responsible reactions controlling heavy metal bioavailability in soil. Simply put, metal bioavailability and transport are mainly affected by iron and manganese oxides, clay minerals, carbonates, organic matters, and soil microorganisms. In the soils with higher clay content, cation exchange capacity (CEC), organic carbon (OC), biological activity, and amorphous and crystalline Fe and Al, a metal, e.g., Pb, is mostly observed in residual form followed by organic matter-associated and Mn–Fe-associated forms, respectively, whereas in the soils with higher sand content, pH, and equivalent carbonate calcium (EEC), the higher amount of this metal is reported to be in carbonate-associated form. It is worth noting that the existence and absence of soil microorganisms can alter the above distributions by different mechanisms. In order to increase the bioavailability of heavy metals in soil, application of chelating agents in soil is recommended. What chelating agents do is increasing the metal bioavailability and also increasing soil productivity through enhancing soil available nutrients; both result in a more efficient phytoremediation. Chelating agents are mainly categorized into synthetic (e.g., polycarboxylates, hydroxy carboxylates, etc.) and natural (e.g., organic acids, manures, siderophores, etc.) groups. Among all the other chelating agents, EDTA, which is a synthetic chelate, is frequently reported as the most efficient chelant to increase availability of metals in soil and enhance the translocation of them to aerial (harvestable) parts of plant. In terms of biomass production which is important for an efficient phytoremediation, EDTA cannot play its role as successfully as when it is used to enhance metal concentration. Toxicity of EDTA itself on the one hand and the toxicity of increased amount of heavy metals caused by EDTA on the other hand prevent plant from producing much biomass. It may be applied in soil during plant growth with lower concentration and different times. Natural chelating agents, nevertheless, noticeably enhance biomass production, although they are not so effective to improve metal concentration as EDTA is. On the other hand, electrokinetic is another technique used to increase available forms of heavy metals and their movement in soil toward plant roots. The most recent researches, interestingly, were designed based on the combination of electrokinetic and chelating agents, which significantly increases the uptake of metals by plants. Giving the negative charge to plant can lead to more cation taken up by plant roots because the electric field makes biomembranes more permeable, providing a transient exchange of metals across the perturbed membrane structure. More interesting results would be obtained when chelating agents provided more bioavailable metals for plant roots simultaneously. As a result, application of chelating agents along with electric fields amazingly raises the phytoremediation efficiency.