不同分子量胡敏酸对蒙脱石吸附镉的影响及作用机制

Effect and Mechanism of Humic Acid with Different Molecular Weight on Adsorption of Cadmium on Montmorillonite

工业活动和农业活动导致土壤镉污染日益加剧,蒙脱石和胡敏酸(humic acid,HA)作为土壤胶体中重要的功能组成部分,对镉具有良好的吸附性能。以胡敏酸为研究对象,通过切向流超滤对胡敏酸进行了4种分子量分级(<10 kDa,HA1;10~50 kDa,HA2;50~100 kDa,HA3;>100 kDa,HA4),结合傅里叶变换红外光谱分析(FTIR)、元素测定、官能团滴定等表征分析,研究不同分子量胡敏酸对蒙脱石吸附镉的影响及作用机制。研究结果表明:随着胡敏酸分子量的增加,胡敏酸所含的羟基和羧基量减少而酚羟基和甲基的含量增加,芳香度变大,疏水性增强,pH值逐渐增大。三元体系中蒙脱石吸附的镉和胡敏酸的含量随着胡敏酸分子量的增大而增加。镉的初始浓度为50 mg/L时,HA、HA1、HA2、HA3、HA4与蒙脱石的复合体系对镉的吸附量分别为1.99、2.11、2.46、4.12、4.88 mg/g;各分子量胡敏酸作用下体系吸附的镉均以可还原态为主。胡敏酸分子量越小,越容易竞争吸附蒙脱石表面已结合的镉,并由于其强亲水性会将吸附的镉带入溶液中,使体系吸附的镉含量减少;随着胡敏酸分子量的增大,其与蒙脱石表面的竞争吸附作用减小,且此时胡敏酸进入蒙脱石层间的难度增大,在疏水作用下,吸附了镉的大分子胡敏酸会覆盖在蒙脱石表面,从而增加体系对镉的吸附量。

Industrial and agricultural activities lead to increasing soil cadmium(Cd)pollution.As an important functional component of soil colloid,both montmorillonite(Me)and humic acid(HA)have good adsorption properties for Cd.Four molecular weight grades of HA were obtained by tangential flow ultrafiltration.FTIR,element determination,functional group titration were used to study the effect of HA with different molecular weight on the adsorption of Cd on montmorillonite and the mechanism of action.The results showed that with the increase of molecular weight of HA,the content of hydroxyl and carboxyl groups decreased while the phenolic hydroxyl group and methyl group increased,the aromaticity increased,the hydrophobicity became stronger,and the pH value increased.The contents of Cd and HA adsorbed by montmorillonite in the ternary system increased with the increase of molecular weight.When the initial concentration of Cd was 50 mg/L,the adsorption capacity of HA,HA1,HA2,HA3,HA4 montmorillonite system for Cd was 1.99,2.11,2.46,4.12,4.88 mg/g,respectirely.And the Cd adsorbed under the action of each molecular weight HA was mainly reducible state.The smaller HA was easier to compete for the adsorption of the Cd already bound on the surface of montmorillonite,and due to its strong hydrophilicity,the Cd adsorbed by small HA would be brought into the solution,reducing the content of Cd absorbed by the system.With the increase of molecular weight,its competition effect with montmorillonite was reduced,and the HA was difficult to enter into the montmorillonite layers.At this time,under the hydrophobic effect,the large molecule HA adsorbed Cd would cover the surface of montmorillonite,thus increasing the adsorption amount of Cd in the system.