Study On Adsorption And Reduction Of Cr(?) Onto Functionalized Attapulgite By Hydrothermal Carbonization

With the rapid development of economy, water pollution has become more and more serious. Especially, the industrial wastewater is particularly serious, which contains a large number of heavy metal ions, such as chromium ion. The adsorption is considered to be the most favorable processes because of its high efficiency and low operational cost, and it is widely used in chromium- containing wastewater treatment. Therefore, it has become a very urgent and new task to develop a kind of cheap and good adsorption material.Attapulgite is a type of magnesium aluminum silicate mineral that exists widely in nature, and was widely used in wastewater treatment owing to its specific surface area and well adsorption properties. As a result of attapulgite has a special structure,the structure with negative charge, it possesses lower adsorption capacity for Cr(?)with negative charge. In order to improve its adsorption capacity,the attapulgite has been widely modified.Hydrothermal carbonization method is a new kind method of modified the attapulgite. The main contents of this paper are as follows: Firstly, the preparation of composite materials, we prepared attapulgite@C nanocomposites(AT@C) using attapulgite and glucose via one step hydrothermal carbonization process. In order to further improve the adsorption properties of AT@C nanocomposites, we synthesized a new kind carboxylate-rich functionalized AT@C(CA-AT@C) adsorbent via one step hydrothermal carbonization using citric acid as one of carbon sources other than glucose. Secondly, the characterization analysis, the as prepared adsorbents were characterized by Fourier transform infrared(FT-IR), X-ray diffraction(XRD), Zeta potential analysis, and X-ray photoelectron spectra(XPS). Thirdly, the static adsorption experiment, the influence of initial concentration, temperature, time and p H on the adsorption of Cr(?) were discussed, the adsorption kinetics, isothermals were also studied. Lastly, Cr(?) removal mechanism analysis, the Cr(?) removal mechanism was investigated according to the analysis of effect of solution p H and of XPS. The main conclusions are as follows:? The attapulgite@C nanocomposites(AT@C) achieved its best with attapulgite to glucose mass ratio of 1:4 via one step hydrothermal carbonization process. Some organic functional groups existed in the material, such as- COOH,-OH,-CH, and so on. By adding citric acid, the surface carboxyl group contentof CA-AT@C nanocomposites prepared by one-step hydrothermal carbonization was increased by 7.8%. The crystal structure of attapulgite is not destroyed by hydrothermal carbonization. The p Hzpc of AT@C was about 2.8, but p Hzpc of CA-AT@C was about 4.4.? The static adsorption experiments of AT@C nanocomposites showed that the adsorption capacity of total Cr increased with Cr(?) initial concentration increasing and the temperature rising. Adsorption equilibrium time is about 8 h. The adsorption kinetics process meets pseudo-second-order kinetics model. Adsorption isotherms could be well described by Langmuir model, and the calculated maximum adsorption capacity for total chromium could be up to 21.93 ~32.47 mg/g at 293~243 K, respectively.(35)H > 0,(35)G <0 and(35)S >0, which showed that the adsorption is endothermic, spontaneous and entropy increasing process. For the 20 m L concentration of 100 mg/L Cr(?) solution, in the p H range of 1~10, the removal rate of Cr(?) decreased with the increase of p H. The maximum removal rate attained up to 92.7% at p H=1, with adsorption rate 48.5% and reduction rate 42.2%.The adsorption rate of total chromium increased at the beginning and then decreased with the increase of p H. The maximum adsorption rate reached up to 58.2% at p H=2,and the reduction rate was 13.0%, which indicated that both adsorption and reduction resulted in the removal of Cr(?). The more acidic the solution is, the more prone to reduction reactions.? The static adsorption experiments of CA-AT@C nanocomposites. The removal rate of Cr(?) were 89.0%, 68.0% and 1.5% by CA-AT@C, AT@C and raw attapulgite, respectively. which showed that modified attapulgite by adding citric acid can further improve the ability of removing Cr(?). The adsorption equilibrium time is about 12 h. The adsorption kinetics process meets pseudo-second-order kinetics model. Adsorption isotherms could be well described by Langmuir model,and the calculated maximum adsorption capacity for total chromium could be up to46.08 ~75.26 mg/g at 293 ~243 K, respectively.(35)H > 0,(35)G <0 and(35)S >0,which showed that the adsorption is endothermic, spontaneous and entropy increasing process. Cr(?) removal was favored at low p H with maximum removal rate at p H=1. The similar results of total chromium adsorbed were observed, while the maximum adsorption ratio was up to 58.7% at p H=2. Under neutral or alkaline conditions, it was also not conducive to total Cr adsorption.? Cr(?) removal procedure was supposed to AT@C and CA-AT@C whichmay go through certain stages by XPS analysis:(?) Under acidic conditions, the adsorption of Cr(?) by electrostatic attraction and complexation with the oxygen functional groups(such as- OH and- COOH, et al.) on the surface of adsorbents;(?) the reduction of part of adsorbed Cr(?) to Cr(?) by the reducing functional groups such as- OH;(?) the release of the reduced-Cr(?) into the aqueous phase due to electronic repulsion between the Cr(?) and the positively-charged groups(such as-(10)2OH and-(10)2COOH, et al.), or the adsorption of reduced-Cr(?) by complexation(- COOH) and ion exchange(Mg2+,Al3+, and so on). The results showed that the removal of Cr(?) is a coupling process of adsorption and reduction by AT@C and CA-AT@C.