Effects of chelating ligands on the kinetics of Pb release from montmorillonite as a prevalent clay mineral in arid and semi-arid regions

Contamination with heavy metals such as leads resulting from human catastrophic excavation activities is one of the serious soil problems in arid and semi-arid region. So, remediation of these kinds of contaminations is one of priorites to protect the soil of these areas. In soil and sediment environments, the potential bioavailability and mobility of metals are predominantly regulated by adsorption/desorption reactions at the mineral-water interface. Although numerous studies have been devoted to understanding the adsorption of metals onto minerals, less is known about desorption processes, especially their kinetics, under the influence of organic ligands. In this study, desorption of Pb from montmorillonite (MMT) was studied as effected by biodegradable chelating ligands, i.e. methylglycine diacetic acid (MGDA) and glutamic-N,Ndiaceticacid (GLDA), at two concentrations (0.25 and 1.0 mM) were compared with that of ethylenediaminetetraaceticacid (EDTA). The results showed that Pb desorption was rapid, reaching equilibrium in a relatively short time (within 3 h). The degree of Pb desorption was proven to be governed by the presence and concentration of the chelating agents. The capacity of the chalants for removing Pb from MMT was in the order EDTA>MGDA>GLDA. Approximately 12%, 19%, and 23% of the sorbed Pb was desorbed by 0.25 mM of GLDA, MGDA, and EDTA, respectively. The corresponding desorptions in the presence of 1.00 mM of the same ligands were 47%, 59%, and 93%, respectively. Therefore, it can be predicted that the presence of biodegradable ligands, i.e. GLDA and MGDA compared to EDTA will create less mobility of Pb in the environment and consequently we will have less pollution in soil and water environments. Owing to soil alkalinity in arid and semi-arid region, it seems that using biodegradable ligands can desorbtion of lead from soil, minimizing the risk of under water contamination.