荞麦皮生物吸附去除水中Cr(Ⅵ)的吸附特性和机理

Characteristic and Mechanism of Cr(Ⅵ) Biosorption by Buckwheat Hull from Aqueous Solutions

农业废弃物荞麦皮作为生物吸附剂去除水中Cr(Ⅵ),研究了荞麦皮对Cr(Ⅵ)的去除动力学以及溶液pH、吸附剂用量和Cr(Ⅵ)初始浓度对去除效率的影响;通过FT-IR,XPS,SEM-EDX对荞麦皮表面组成和结构进行表征,探索荞麦皮去除Cr(Ⅵ)的机理.结果显示:荞麦皮对Cr(Ⅵ)有很高的去除效率.常温下5.0 g·L-1的荞麦皮在pH=2.0下对100 mg·L-1 Cr(Ⅵ)溶液的去除率可达99.87%.荞麦皮对Cr(Ⅵ)的去除率随溶液pH降低而升高,在pH=2.0时达到最大;随吸附剂用量增加而增大;随Cr(Ⅵ)初始浓度增加而减小.单位质量荞麦皮对Cr(Ⅵ)的去除量随吸附剂用量增加而减小;随Cr(Ⅵ)初始浓度增加而增加,最后趋于稳定.在20℃,pH=2.0,吸附用量为5.0 g·L-1时,荞麦皮对Cr(Ⅵ)的最大去除容量约为36.4 mg·g-1.荞麦皮吸附去除Cr(Ⅵ)的过程符合准二级吸附动力学.FT-IR,XPS和SEM-EDX分析结果表明:荞麦皮是一个多孔材料,表面存在羧基、氨基、羟基等活性基团;荞麦皮对Cr(Ⅵ)的去除是一个吸附-还原耦合的过程,包括Cr(Ⅵ)在荞麦皮表面上的静电吸附,以及此后的固相还原和对还原态的Cr(Ⅲ)再吸附;Cr(Ⅲ)的吸附主要是通过与荞麦皮表面的羧基、氨基的配位,以及与其中的阳离子发生离子交换作用实现的.

Buckwheat hull, an agricultural industry waste, was used as a novel biosorbent to remove hexavalent chromium from water. The absorption kinetics was investigated by measuring the change of Cr（VI） and total chromium concentration with time. It showed that the removal of Cr（VI） by buckwheat hull followed the pseudo-second-order adsorption kinetics. Effects of pH, biosorbent dosage and initial concentration of Cr（VI） on its adsorption removal were also examined in the batch mode. The results showed that the buckwheat hull was very effective for removing Cr（VI） from water. At pH 2.0, load of 5.0 g.L- l, buck- wheat hull could almost completely remove Cr（VI） （99.87% of 100 mg·L^-1） from water. The removal of Cr（VI） increased with the decrease of solution pH values, and the maximum percentage removal was attained at pH 2.0. Increase of adsorbent dosage would raise the removal efficiency, while it would simultaneously reduce the adsorption capacity. Moreover, the removal of Cr（VI） was found to decrease with rising the initial concentration of Cr（VI）. Whereas, the Cr（VI） uptake capacity increased with the rise of Cr（VI）concentration until reaching saturation, which was found to be about 36.4 mg·g^-1 at pH 2.0, temperature of 20℃ and adsorbent dose of 5.0 g·L^-1. In addition, Fourier transform infrared spectroscopy （FTIR）, X-ray photoelectron spectroscopy （XPS）, and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy （SEM-EDX） analyses were employed to characterize buckwheat hull, and further to elucidate the mechanisms involved at the molecular level. SEM revealed that the buckwheat hull is a porous material, while FT-IR and XPS displayed that the biomass possess many active groups such as hydroxyl, carboxyl, and amino groups. The removal of Cr（VI） by the buckwheat hull was found to be a adsorption-coupled reduction process based on XPS analysis and batch experiments. Finally, the mechanisms of chromate anions removal from water by the buckwheat hull were proposed, which included adsorption of Cr（VI） onto buck- wheat hull followed by the partial reduction to Cr（Ⅲ）. The reduced Cr（Ⅲ） was rebound to the biomass mainly through the complexation and ion-exchange mechanisms.