Sorption of Pb(II) on palygorskite and sepiolite in the presence of amino acids: Equilibria and kinetics

The environmental mobility and bioavailability of heavy metals in soil are greatly influenced by sorption reactions on soil minerals, which are, in turn, controlled by the presence of various organic ligands, such as amino acids, in soil solution. In this study, the effects of L-cysteine (Cys) and L-histidine (His) amino acids on lead (Pb) sorption by palygorskite and sepiolite, as the specific silicate clay minerals of arid and semiarid soils, were studied using the batch method. Experimental Pb sorption data were subsequently analyzed by various isothermal and kinetic models. The equilibrium data revealed that both Langmuir and Freundlich models can describe the Pb sorption on palygorskite and sepiolite minerals. The sorption capacities of the minerals for Pb were decreased in the presence of Cys, probably because of the formation of neutral stable complexes of Pb and Cys as [ML] in the solution. The His effects, however, were different for the two minerals. Histidine reduced the Pb sorption capacity of sepiolite, but its diminishing effect was much lower than that of Cys. This result may be because of the formation of cationic soluble His-Pb complexes as [ML2H](+), which can be slightly adsorbed by the clay surfaces via electrostatic attraction. L-Histidine increased the sorption of Pb by palygorskite, probably due to formation of highly charged complexes with Pb, in the form of [ML2H2](2+) and [ML2H3](3+), or ternary surface complexation processes. The kinetics of Pb sorption by the minerals were satisfactorily described by the power, pseudo-first order, pseudo-second order and parabolic diffusion equations, among which the power function provided the best fit. The kinetics of Pb sorption by the palygorskite was rapid, approaching equilibrium within 10 h. Lead sorption on the sepiolite, however, was much slower and equilibrium condition was attained after 10 h. The Pb sorption rate indices, such as the initial sorption rate and diffusion rate constant, were also considerably affected by the presence of amino acids. These findings emphasize the importance of amino acids in evaluating the fate of Pb in arid soils in which fibrous silicate clay minerals are dominated in the clay fraction. (C) 2015 Elsevier B.V. All rights reserved.