Chelating Agents and Heavy Metal Phytoextraction

Abstract

Elements with density higher than 5 g cm À3 have been called heavy metals. From the metallic elements that naturally exist, 53 elements belong to the heavy metal group. Some of these heavy metals, such as iron, manganese, copper, zinc, and molybdenum, are known as micronutrients in agriculture, since their amount in soil is low and the little quantity of them is necessary for plant growth. Another group of these elements, such as cobalt and vanadium, has been shown to be useful for plant growth. The other category, such as nickel, chromium, selenium, lead, and cadmium, has toxic effects on plant and other organisms even in low level. The mobility and uptake of these elements and the appearing of their negative effect in the environment depend on soil, water, and plant relations.The term "chelating agents" refers to ligands occupying multiple positions in the inner coordination of a metal ion. Heavy metals combine with one or more reactive groups or ligands like oxygen (-OH, -COO, -POO), nitrogen (-NH 2 , -NH), or sulfur (-SH, -S-S). The result of this phenomenon is the formation of soluble multidentate metal-chelant complexes called "chelates" (Fig. 20.1) which came from a Greek word "chele" meaning the claw of a crab or scorpion (Tsang et al. 2012).To know the increase of metal solubility in soil, it is important to figure out the metal ion-ligand interactions. The complex formation can be considered as equilibrium reaction between the ligand and the metal ion:

Application of manures to mitigate the harmful effects of electrokinetic remediation of heavy metals on soil microbial properties in polluted soils

“…The EK was applied with two intensities including 1.1 V/cm and 3.3 V/cm (Safari Sinegani et al 2015). Intensities of electric fields were chosen to be within the range of other studies Jackman et al 1999;Li et al 2010;Safari Sinegani 2014, 2016).…”

“…Manures were also reported to improve soil microbial properties through increasing the organic substrate and soil physiochemical conditions (Elzobair et al 2016;Pu et al 2017;Usman et al 2013;Zhang et al 2017). Our complementary studies have also shown that the combination of manures and EK (manure-enhanced EK) increased the diethylenetriaminepentaacetic acid (DTPA)-extractable (available) form of heavy metals in the contaminated soils (Safari Sinegani et al 2015;Tahmasbian Ghahfarokhi 2012;. However, effects of the application of manure-enhanced EK on soil microbial properties remain uncertain.…”

“…Electrokinetic remediation (EK) is a newly developed method and has shown to successfully remediate heavy metal polluted soils (Chirakkara et al 2015;Dong et al 2013;Falciglia et al 2017;Giannis et al 2010;Mu'azu et al 2016). In this method, direct electric current is applied into soil leading to the migration of soil solutions, ions and colloids, through different mechanisms including electroosmosis, electromigration and electrophoresis (Acar and Alshawabkeh 1993;Safari Sinegani et al 2015).…”

“…Ethylenediaminetetraacetic acid (EDTA) is one of the most used chelates applied with EK for the remediation of heavy metals (Alcántara et al 2012;Dasgupta et al 2016;Dong et al 2013;Ryu et al 2017;Song et al 2016;Zhang et al 2014). Although the application of chelates with EK has been reported to improve the EK efficiency, both the EK and chelates have shown to negatively influence soil biochemical properties (Alkorta et al 2004;Baek et al 2009;Epelde et al 2008;Safari Sinegani et al 2015).…”

“…metals via forming a heavy metal-chelate complex whereas EK increases the solubility of heavy metals by H + generation at anode(Safari Sinegani et al 2015;Sah and Che 1998;Virkutyte et al 2002;Zhao et al 2014). The solubilized heavy metals are then transported from anode to cathode by electroosmosis and electromigration, which increase the total concentrations of heavy metals in cathodic soilSafari Sinegani et al 2015;Virkutyte et al 2002;Zelina and Rusling 1998). The increased available forms of heavy metals by various chelates and EK as well as heavy metal accumulation in cathodic soil have been also reported in other studies…”