Preparation Of Wheat Straw Amphoteric Chelate Adsorbent And Study Of Its Cu(Ⅱ) And Cr(Ⅵ) Adsorption Properties

With the development of economy in our country, water pollution has become a serious environmental problem, particularly heavy metal pollution. At present, biomass adsorbents have been used to solve this problem by more scholars and became the research hotspot due to its properties of cheapness, biodegradability and so on. Therefore, wheat straw was modified to a novel biomass amphoteric chelate adsorbent (WSACA), and the adsorption properties, adsorption capacities and adsorption mechanism research of two typical heavy metal ions (Cu (II) and Cr (VI)) have been discussed.WS were modified by epichlorohydrin, ethylenediamine (EDA) and triethylamine in the presence of N, N-dimethylformamide (DMF) and then reacted with chloroacetic acid, finally WSACA was obtained. A series of WSACA were made under different modification conditions, such as the dosage of reactant, reaction temperature and reaction time. Then a suitable modification process and modified products were determined according to the removal efficiencies of Cu (II) and Cr (VI) from aqueous solution onto WSACA in single factor experiments and orthogonal experiments. The optimal conditions used in the preparation of WSACA could be concluded as:VNaoH(40wt%),Mchloroacetic acid,Temperature and Time are:5 mL,3.402g,60℃,2.5h, respectively. The dosage of chloroacetic acid was considered as the key influential factor in the preparation of WSACA. The physical and chemical characteristics of WSACA were investigated by varies physical chemistry methods and modern instruments analysis. The results demonstrated that the physical and chemical characteristics of WSACA had significant difference with those of WS. The surface of WSACA was clearer and smoother in comparison with WS. IR spectrums indicated appearance of amino and carboxyl groups in the structure of WSACA and it was validated by the elemental analysis which indicated a significant increase of nitrogen content in WSACA.The adsorption performances of WSACA for Cu (II) and Cr (VI) were tested by batch adsorption experiments, and the dosage of WSACA, pH,time, ionic strength, thermodynamics and the effects of several factors on kinetics were studied in batch experiments. The results showed that WSACA was particularly effective to remove Cu (Ⅱ) and Cr (Ⅵ). Their removal abilities were influenced by initial pH of aqueous solution significantly, and the removal rate for Cu (Ⅱ) increased as pH increased, but Cr (Ⅵ) was just the opposite.The kinetics study indicated that the pseudo-second-order equation was best suitable in describing the process of Cu (Ⅱ) and Cr (Ⅵ) adsorption ontoWSACA. In addition, intraparticle diffussion existed in the adsorption process but was not the only rate-limiting step. The low activation energy of the adsorption suggested a physisorption process for Cu (Ⅱ) and Cr (Ⅵ) adsorption. The equilibrium isotherm showed that the adsorption system for Cu (Ⅱ) was consistent with Langmuir equation and belonged to monolayer adsorption, while the adsorption of Cr (Ⅵ) fitted to Freundlich model well and multilayer adsorption was also existed to a certain extent.The negative values of standard free energy (△G) and positive enthalpy (△H) indicated that the adsorption of Cu (Ⅱ) and Cr (Ⅵ) onto WSACA were a spontaneous and endothermic process.The synchronous adsorption performance of mixed ions onto WSACA was also studied in the binary system, competitive adsorption was exsisted between Cu(Ⅱ) and Cr(Ⅵ) and there was the higher affinity of wheat straw for Cr(Ⅵ),which was consistent with the results in the binary system.The order of by-product modification appears to influence ion binding in most cases, the final modification appeared to favor a product that has strong characteristics of that modification. In a word, there were both physisorption processes for Cu (Ⅱ) and Cr (Ⅵ) onto WSACA, the adsorption mechanism of Cr (Ⅵ) was probably due to the electric attractive of quaternary amino groups and both of chelate of amino groups and static adsorption of carboxyl groups were likely to explain the adsorption of Cu (Ⅱ) onto WSACA.