Extracellular polymeric substances immobilized on microspheres for removal of heavy metals from aqueous environment

Extracellular polymeric substances (EPS) extracted from Rhodococcus opacus were immobilized on synthetic microspheres BES.DM-GMA-TETA, obtained via copolymerization of bis[4(2-hydroxy-3-methacryloyloxypropoxy) phenyl]sulfide (BES.DM) with glycidyl methacrylate (GMA) and modified with triethylenetetramine (TETA). Both the pristine synthetic sorbent and the sorbent with immobilized EPS were used for adsorption of Pb (II) and Cd(II) from aqueous solutions. The optimum pH for adsorption was 5 for Pb(II) and 6.5 for Cd(II). Adsorption of both ions followed the Langmuir model. The monolayer capacity towards Pb(II) and Cd(II) increased after EPS immobilization by 47% (from 58.82 to 86.96 mg/g) and 30% (from 37.45 to 46.73 mg/g), respectively. It was already achievable by a low coverage of EPS on the synthetic microspheres, since the weight ratio of EPS to the microspheres was 1:15. HNO3 and HCl were efficient for a complete desorption of Pb(II), while desorption of Cd(II) was incomplete (60% by HCl and below 50% by HNO3). SEM micrographs showed distinct differences between pristine microspheres, those with immobilized EPS and with the adsorbed metal. Xray microanalysis of the EPS-bound microspheres proved the presence of additional elements such as Na, K, Ca, Mg, P and Si, together with the basic components of the synthetic microspheres (C, N, O, S). XPS technique was used to reveal the binding mechanism (surface precipitation and chelate formation).