Hexavalent Chromium Silicon Material Research In Water And Carbon Adsorption Material

Hexavalent chromium was listed as a priority pollutants in the aquatic environment by U.S. EPA because of its biological toxicity and serious harm to the health and life of the human body and biological, and it was also the objects of focus on prevention and control in the "12th Five-Year" plan in China. We facing the shortage and heavy metal pollution crisis of water during social development nowadays. Four consecutive years of drought since2009and major chromium pollution event happened in Nanpanjiang of Yunnan Province in2011, which led to water of living and industrial production was seriously affected, so the governance of Cr(Ⅵ) in water was urgently needed.Adsorption method were efficient and simple for Cr(Ⅵ) removal in aqueous media, the key lied in the selection of the adsorbent. Mesoporous silica materials had synthetised via one step hydrothermal-template method and applied to the adsorption of Cr(Ⅵ) in our research group, the optimization conditions of adsorption was investigation initially. A part of this study was focused on more information in breadth and depth of Cr(Ⅵ) adsorption using mesoporous silica on the basis of research results of our group members; the other part is a preliminary research of carbon materials as an adsorbent on Cr(Ⅵ) adsorption.In the research we found that the adsorption efficient of Cr(Ⅵ) with mesoporous silica materials increased with increasing chain length of the template, but was not increased with the increasing the amount of template; mesoporous silica with high selectivity for Cr(Ⅵ) and had no adsorption for several conventional anionic(F", Cl-, NO3-, PO43-, SO42-,SiO32-) and five heavy metals cations(Cu2+、Ni2+、Ca2+、Mg2+、Zn2+). Langmuir isotherm model and pseudo-second order kinetic model were suitable for description Cr(Ⅵ) adsorption. Adsorption was spontaneous, endothermic and entropy increased. Adsorbent could be regenerated for several adsorption cycle, and no obvious decrease had discovered after5cycles of adsorption.According to pH changes in the adsorption process, Raman, FT-IR and elemental analysis, we considered that the adsorption mechanism of mesoporous silica for Cr(Ⅵ) removal might be the electrostatic interaction between positively charged ammonium ions in the channels of adsorbent with the HCrO4-and HCr2O7-, hydrogen bond formed between N and H atoms. Cr(Ⅵ) existing species and surface charge of Si in the materials surface were two decisive factors of removal efficient.In the study of carbon materials for Cr(Ⅵ) adsorption, wood activated-carbon(W-AC) and coal activated-carbon(C-AC) had higer adsorption capacities, but multi-walled carbon nanotubes (MWCNTs) and Graphene had poor capacities. The optimal operational conditions is at pH=1.0and sorbent dosage50mg/50ml. Kinetic and thermodynamic investigation shown that W-AC and C-AC adsorbed Cr(Ⅵ) adhered to Langmuir, Freundlich and Tempkin isothermal models and four kinetic models of pseudo-second order, Elovich equation, intraparticle diffusion and Bangham, adsorption was control by chemical reactions, heterogeneous diffusion and pore diffusion. Adsorption was spontaneous, endothermic and entropy increased process. The value of adsorption heat indicate that activated carbon adsorption of Cr(Ⅵ) might be chemical adsorption alone or physical adsorption and chemical adsorption coexist.Cr(Ⅵ) adsorption capacities of several common adsorbent in our research group illustrated that:zeolite and MCM-41had no adsorption for Cr(Ⅵ), SBA-15and mesoporous Al2O3was low (less than50mg/g); however, silica materials and activated carbon materials mainly mentioned in this paper reached more than150mg/g Cr(Ⅵ) adsorption capacity. The maximum adsorption capacity of HMS which had the best removal efficient was215.05mg/g by fitting with Langmuir isothermal model, this value is the highest after aminopropyl-triethoxysilane functionalized mesoporous silica in silicon material adsorbent reported in the literature (226.2mg/g).These findings suggest that mesoporous silica and activated carbon had great potential applications as a alternative adsorbent of Cr(Ⅵ) removal in aqueous sulution.