CONICET | Buscador de Institutos y Recursos Humanos

Heavy metals pollution is an important environmental topic due to the potential health problems. In particular, Cr and Co are essential metals but in relatively high concentration are potentially toxics and they are mainly released from several industrial processes. Sorption of metals on clay systems is a frequently used methodology due to its simple application, high sorption capacity and sorbent material´s low cost. Actually, the development of magnetic clay systems allows the pollutant removal in continuous process, promoting the sorbent separation advantage by an external magnetic field. This work analyse Cr and Co sorption on different montmorillonite sorbents: a raw from Argentina (Mt), an organic (OMt) and an organic-magnetic derivative (OMt-Mag), in batch conditions and with initial concentration from 10 to 60 mg/L; ratio solid/solution= 1 g/L, room temperature, contact time 24 h, and pH=6 and 3 for Co and Cr, respectively. Cr and Co in solution after sorption experiments was determined by colorimetric method [1, 2]. Sorption isotherms were fitted to Langmuir, Freundlich and Sips mathematical models. OMt sample was obtained after dispersing 2 g of Mt in 100 ml of distilled water containing 0.6 g of hexadecyl trimethyl ammonium bromide in order to achieve 100% of cationic exchange capacity. The suspension was stirred for 24 h at 60oC. OMt-mag sample was synthetized by Wang Guanghua [3](co-precipitation method). Sorbents were characterized by X-ray diffraction (XRD), zeta potential, Mössbauer spectroscopy and vibrating sample magnetometer (VSM) techniques, while solid resulted from maximal adsorption experiments (Ci = 50 ppm) were also analysed by XRD and zeta potential in order to determine changes in interlayer space (d001) and electric surface charge, respectively. Regarding sorption isotherms, the experimental adsorption points were fitted better to Freundlich model than for the other mathematical models evaluated, for all sorbents analysed. For Co, Kf values were 7.2, 3.0 and 3.0 mg1-nLn/g for Mt, OMt and OMt-Mag, respectively. Although Mt sample showed the highest Co affinity, OMt-Mag sample also presented a substantial sorption capacity between 81 to 75% when the initial concentration of Co ranged from 10 to60 mg/L. For Cr, Kf values were 1.4, 0.4 and 0.2 mg1-nLn/g for OMt-Mag, OMt and Mt sorbents, respectively. In this case, sorption percentage for OMt-Mag did not exceed 30%. Comparing sorption percentage and Kf values it can be concluded that the sorbents were more efficient for Co than Cr removal.XRD results showed that after Cr and Co sorption the d001 value of Mt sample increased (0.30 and 0.32 nm) respect to raw Mt, while for OMt-Mag the d001 value remained constant (1.87 nm). . For OMt sample an increase and decrease of d001 values (0.11 and 0.06 nm) were found for Cr or Co sorption, respectively. Zeta potential measurements showed the classical negative surface charge for Mt sample while OMt and OMt-Mag samples indicated a positive surface charge. After Cr and Co sorption, the negative charge of Mt , as the positive charge OMt and OMt-Mag, decreased, except for Cr sorption on OMt where the positive charge remained constant. Mössbauer spectra from Mt and OMt were similar. They were fitted by 4 paramagnetic Fe sites, 3 corresponding to Fe3+ and one to Fe2+. Mössbauer spectra of OMt-Mag showed, in addition to the same 4 paramagnetic sites, two magnetic Fe environments. The hyperfine parameters obtained for those sites were in agreement with that corresponding to magnetite, although the area ratio did not correspond to the expected for the magnetite ideal crystal. The obtained relative fraction of magnetic sites was 41±3%. Saturation magnetization value of OMt-Mag (5.43 Am2/kg) was higher than that of Mt (1.75 10 -3 Am2/kg) and OMt samples (2.63 10-3 Am2/kg).

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