Fe-La二元氢氧化物对水中As(Ⅴ)和As(Ⅲ)的吸附行为与机制

Adsorption behaviours and mechanisms of As(Ⅴ)and As(Ⅲ)from water by a Fe-La binary(hydr)oxide

采用共沉淀法制备了铁-镧二元氢氧化物(Fe-La)吸附剂,通过吸附动力学、pH的影响和吸附等温线等吸附批实验研究了其对水溶液中砷的去除行为,并通过扫描电子显微镜-X射线能谱分析(SEM-EDS)、傅里叶红外光谱分析(FTIR)、X射线衍射分析(XRD)等方法对铁-镧二元氢氧化物进行了表征,研究其对砷的去除机制.吸附批实验表明,Fe-La二元氢氧化物对As(Ⅴ)具有较强的去除能力,而对As(Ⅲ)的去除能力较弱.吸附动力学实验表明,Fe-La二元氢氧化物对As(Ⅴ)和As(Ⅲ)在前2 h内吸附速率较快,其去除过程均符合伪二级动力学模型.溶液的pH值会显著影响Fa-La二元氢氧化物对As(Ⅴ)和As(Ⅲ)的去除效率,其中,对As(Ⅴ)的吸附量在pH=3.0~4.0时升高,在pH=5.0~11.0时随pH升高而降低;对As(Ⅲ)的吸附量在pH=3.0~6.0时升高,在pH=6.0~10.0范围内保持不变,在pH=10.0~11.0时吸附量减少.Langmuir吸附等温线拟合结果表明,pH=4.0时Fe-La二元氢氧化物对As(Ⅴ)的最大吸附量为303.03 mg·g^(-1),pH=7.0时对As(Ⅲ)的最大吸附量为24.69 mg·g^(-1).共存阴离子对Fe-La二元氢氧化物去除As(Ⅴ)和As(Ⅲ)的影响大小顺序分别为SO_(4)^(2-)<CO_(3)^(2-)<柠檬酸根<HPO_(4)^(2-)和SO_(4)^(2-)(CO_(3)^(2-))<柠檬酸根<HPO_(4)^(2-).3次循环后,As(Ⅴ)的去除率可以达到初始去除效率的79.67%,As(Ⅲ)的去除率可以达到初始去除效率的88.60%左右.SEM-EDS、FTIR和XRD分析表明,通过表面羟基配体交换反应形成砷酸镧(LaAsO4)以及溶液中的La3+与H2AsO4-共沉淀生成砷酸镧是Fe-La二元氢氧化物去除水中As(Ⅴ)的主要机理,而二元氢氧化物中的铁氢氧化物对As(Ⅲ)的固定是As(Ⅲ)去除的主要机制.上述结果表明,Fe-La二元氢氧化物对水中As(Ⅴ)的去除具有潜在的应用价值,但对As(Ⅲ)的去除还需要结合其他的方法,如预先将As(Ⅲ)氧化成As(Ⅴ).

In this study,a Fe-La binary(hydr)oxide was prepared via a co-precipitation method.Its removal behaviours and removal mechanisms for arsenic(As)were studied by batch adsorption experiments,scanning electron microscopy and X-ray energy dispersive spectroscopy(SEM-EDS),Fourier transform infrared spectroscopy(FTIR)and X-ray diffraction(XRD)analyses.Batch adsorption experiments showed that the removal capacity of As(Ⅴ)onto the Fe-La binary(hydr)oxide was significantly higher than that of As(Ⅲ).The adsorption kinetics indicated that the adsorption rates were rapid within the first 2 h,and the adsorption process could be fitted to the pseudo-second-order model.Solution pH can significantly affect the removal efficiency of As(Ⅴ)and As(Ⅲ).The adsorption capacity for As(Ⅴ)increased with the initial solution pH increasing from 3.0 to 4.0,and decreased with the pH increasing in the range of 5.0~11.0.The adsoption capacity for As(Ⅲ)increased with the pH increased from 3.0 to 4.0,almost unchanged with the pH increased from 6.0 to 10.0,and decreased with the pH increased from 10.0 to 11.0.Langmuir isotherm model revealed that the maximum adsorption capacity was 303.03 mg·g^(-1)for As(Ⅴ)at pH=4.0,and 24.69 mg·g^(-1)for As(Ⅲ)at pH=7.0.The effect of competitive anions on the removal of As(Ⅴ)and As(Ⅲ)by the Fe-La binary(hydr)oxide was as follows:SO_(4)^(2-)<CO_(3)^(2-)<citrate<HPO_(4)^(2-)and SO_(4)^(2-)(CO_(3)^(2-))<citrate<HPO_(4)^(2-),respectively.After 3 recycles,the adsorption capacities for As(Ⅴ)and As(Ⅲ)were over 79.67%and approximately 88.60%of the original adsorption capacity,respectively.SEM-EDS,FTIR and XRD analyses suggested that the formation of LaAsO4 through the surface hydroxyl ligand exchange reaction and the co-precipitation of La3+and H2AsO4-was the main mechanism for As(Ⅴ)removal,while the main mechanism for As(Ⅲ)removal was the immobilization by the iron(hydr)oxide in the Fe-La binary(hydr)oxide.These results indicated that the synthesized Fe-La binary(hydr)oxide might have a potential application for As(Ⅴ)removal from water,whereas the removal of As(Ⅲ)should be combined with other pretreatment processes,such as pre-oxidation of As(Ⅲ)to As(Ⅴ).