Preparation, characterization, and Cd(II) sorption of/on cysteine-montmorillonite composites synthesized at various pH

Montmorillonite-cysteine could be used as the immobilizer, detector, and detoxifier of heavy metals. To further the understanding and the application, the interaction between the montmorillonite and cysteine and the adsorption of cysteine on montmorillonite and characterization of the composites need to be studied further. In present work, the effects of pH, contact time and initial concentration of cysteine on the adsorption, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and Cd(II) adsorption on the composites were conducted to characterize the composites synthesized at different pH conditions. The results showed that the adsorption amount of cysteine on montmorillonite decreased with the increase of pH in the range of 2.4–8.0, reached equilibrium in about 1 min and increased with the initial concentration of cysteine and reached the maximum at 160 mg/g. The adsorption data fitted with Langmuir better than Freundlich, fitted with first-order and second-order better than the intraparticle diffusion model. XRD patterns and FTIR spectra showed that the interlayer spacing of the composite synthesized in the range of pH 2.4–4.3 was larger than that at pH 4.5–8.0 and the bonding of cysteine and montmorillonite mainly depended on the action of the amino group. Adsorption of Cd(II) on composites indicated more cysteine loaded (pH < 4.5) composite had greater capacity for Cd(II). The above results demonstrated that the composite synthesized under lower pH could retain more active cysteine, which might be beneficial to its various applications.