Immobilization mechanism of Cd2+/HCrO4-/CrO42- ions and carboxin on montmorillonite modified with Rhizobium leguminosarum bv. trifolii exopolysaccharide

The modification of clay minerals by exopolysaccharides (EPSs) may significantly increase their adsorption capacity. Therefore, this study focused on the impact of the EPS synthesised by the soil bacterium Rhizobium leguminosarum bv. trifolii on the adsorptive features of montmorillonite relative to two heavy metal ions (cadmium/Cd(II) and chromium/Cr(VI)) and a pesticide (carboxin). The characterization of montmorillonite was carried out using various methods: X-ray diffraction, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, scanning electron microscopy. The adsorption/desorption data were modelled using theoretical equations (Langmuir-Freundlich, Redlich-Peterson, etc.). The obtained results showed that EPS promoted the accumulation of heavy metals on the clay mineral and, simultaneously, contributed to a lower degree of their desorption. This resulted from complexation between the biopolymer and the Cd(II)/Cr(VI) ions. After montmorillonite modification with 100 mg/L EPS, the maximum noted growth in Cd(II) adsorption was 26.10%, whereas it was 20.30% for Cr(VI). The adsorbed amounts of Cd(II) and Cr(VI) were then 24.24 and 14.45 mg/g, respectively. In the case of carboxin, the EPS effect on its adsorption/desorption rates was opposite - its adsorption level decreasing by 10.80%, was 0.27 mg/g. Thus, the presence of EPS-producing bacteria could reduce the bioavailability of the heavy metals, but not of the selected pesticide.