[Effects of acid and heating treatments on the structure of sepiolite and its adsorption of lead and cadmium].

The effects of acid and heating treatment on the structure of sepiolite and its adsorption capacities for Pb2+ and Cd2+ were investigated using X-ray powder diffraction, infrared spectroscopy, surface area analyses and batch sorption experiments in this paper. The results showed that the BET surface area of the sepiolite samples grew with increase concentrations of HCl and prolong treatment time and no obvious structural changes were observed. CaCO3 as the impurity in the natural sepiolite can not be removed completely in the treatment process with 0.5 mol x L(-1) HCl. The surface area of sepiolite treated with 6 mol x L(-1) HCl and continuous stirring 72 h reached the maximum 301.47 m2 x g(-1). A decrease in the BET surface area was observed for the samples treated with acid at higher temperature. The BET surface area of samples treated with different acid species arranged in the order HCl > HNO3 > H2SO4. The calcinations process decreased the BET surface area from 21.44 m2 x g(-1) at 100 degrees C to 0.17 m2 x g(-1) at 900 degrees C. The adsorption results indicated that despite increases in the surface areas upon acid activation, improvements in the adsorption were not observed for heavy metal Pb2+ and Cd2+ ions. Pb2+ ions have higher affinity for specific adsorption onto the sepiolite samples than Cd2+ ions. H2SO4 treatment was found to be more effective for the removal of Pb2+ and Cd2+ ions than HCl and HNO3 treatments. It is apparent that PbSO4 and CdSO4 were formed during the sorption process. The calcinations treatment exhibited no significant effects on the adsorption of Pb2+ ions onto sepiolite, but the adsorption of Cd2+ ions obviously increased above 700 degrees C.