Removal of Cd(II) from aqueous solution by sulfur-functionalized walnut shell: adsorption performance and micro-structural morphology

The sulfur-containing functional group was grafted onto walnut shell with xanthate to synthesize a biosorbent (SWM) for removal of cadmium in water. The synthesized adsorbent was characterized by scanning electron microscopy, Brunauer-Emmett-Teller (BET), Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. And the mechanism was further analyzed by pH(PZC) determination. The effects of pH, dosage, contact time, Cd(II) concentration and temperature on adsorption were investigated. Moreover, adsorption kinetics, adsorption isotherms and adsorption thermodynamics were studied. The results showed that the adsorption performance of SWM was better than unmodified walnut shell due to ion exchange and high-density sulfur-containing functional group on surface. The Cd(II) adsorption on SWM was found to follow Langmuir isotherm model with maximum adsorption capacity of 17.79 mg g(-1). And the adsorption was well described by pseudo-second-order kinetic model, the adsorption process was completed by three sections and the intraparticle diffusion process was not the only control step. Furthermore, the adsorption process was spontaneous and exothermic. All the results showed that the high adsorption performance of SWM make it a potential biosorbent in the treatment of cadmium contaminated water.