三种铁氧化物对As(Ⅲ)和As(Ⅴ)的吸附研究

Comparison of Adsorption of As(Ⅲ) and As(Ⅴ) by Three Iron Oxides

采用批量吸附试验,研究实验室制备的水铁矿、磁铁矿和纤铁矿对As(Ⅲ)和As(Ⅴ)的吸附性能。结果表明,在p H中性条件下,结晶差的水铁矿吸附砷的能力明显优于结晶较好的磁铁矿和纤铁矿,准二级动力学方程拟合效果较好,说明存在多种吸附过程,并可能涉及专性吸附作用;在酸性条件下,三种铁氧化物对As(Ⅲ)和As(Ⅴ)的吸附能力大小顺序均为水铁矿>磁铁矿>纤铁矿,铁氧化物吸附As(Ⅴ)的能力优于As(Ⅲ),Langmiur方程和Freundlich方程拟合效果均较好,说明铁氧化物对砷的吸附为均匀介质表面的单层吸附或非均匀介质表面的多层吸附;在弱酸至弱碱范围内,三种铁氧化物对As(Ⅲ)的吸附效果均较好且吸附量变化不大,强酸和强碱条件都会引起铁氧化物吸附As(Ⅲ)量的降低,三种铁氧化物对As(Ⅴ)的吸附均随着p H升高而逐渐减少;由于竞争吸附作用,磷酸盐浓度的增加使三种铁氧化物对As(Ⅲ)和As(Ⅴ)的吸附显著减少;随着腐殖酸浓度的增加,三种铁氧化物对As(Ⅲ)的吸附能力逐渐减小,低浓度的腐殖酸微弱地促进铁氧化物对As(Ⅴ)的吸附,高浓度的腐殖酸抑制As(Ⅴ)的吸附。

Batch experiments were performed to study the adsorption of As（Ⅲ） and As（Ⅴ） by ferrihydrite, magnetite and lepidocrocite prepared in the laboratory. The results showed the adsorption capacity of ferrihydrite with poor crystallinity towards As was significantly higher than that of magnetite and lepidocrocite with high crystallinity in the neutral condition. The pseudo-second-order kinetic equation could fit the experimental data well, which indicated that several adsorption processes existed and specific adsorption were involved. In the acidic condition, the adsorption capacities of iron oxides decreased in the following order： ferrihydrite 〉 magnetite 〉 lepidocrocite. The adsorption capacities of three iron oxides towards As（Ⅴ） were higher than As（Ⅲ）. The adsorption isotherms were well represented by both the Langmuir and Freundlich isotherm models, indicating that monolayer adsorption on the surface of the homogeneous medium and multilayer adsorption on the surface of heterogeneous media were involved in the adsorption process. Three iron oxides had high and constant efficiencies of adsorption towards As（Ⅲ） from weakly acidic to weakly alkaline condition. However, the adsorption amount of three iron oxides towards As（Ⅲ） would decrease under strongly acidic or strongly alkaline condition. The adsorption amount of three iron oxides towards As（Ⅴ） declined with the increasing p H value. Phosphate significantly restrained the adsorption of three iron oxides towards As by competitive adsorption. The adsorption amount of three iron oxides towards As（Ⅲ） decreased with the increasing dosage of humic acid. Low concentration of humic acid weakly increased the adsorption of As（Ⅴ）, while high concentration of humic acid restrained the adsorption of As（Ⅴ）.