Adsorption of humic acid on goethite: Isotherms, charge adjustments, and potential profiles
Adsorption of humic acid on goethite: Isotherms, charge adjustments, and potential profiles
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DOI：
10.1021/la034806z
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发表时间：
2004-02-03
期刊：
影响因子：
3.9
通讯作者：
Tanaka, S
中科院分区：
文献类型：
--
作者：
Saito, T;Koopal, LK;Tanaka, S
天然有机物(NOM)在氧化氢矿物(H2O)上的吸附在评价环境中有毒金属离子的形态中起着重要作用。因为NOM和矿物氧化物都有随吸附而调节的可变电荷，所以在理解金属离子的结合之前，需要对质子结合有很好的了解。在这项研究中，考察了环境条件(pH、盐浓度和PAHA的游离浓度)对纯化的Aldrich腐植酸(PAHA)在针铁矿上的吸附以及对PAHA、针铁矿及其混合物的质子吸附。根据混合物的电荷/pH曲线与单一组分的电荷/pH曲线的不同，将两个组分的诱导电荷分开。利用吸附的PAHA和针铁矿表面的电荷密度，通过Poisson-Boltzmann方程的数值解得到了吸附层间的静电势分布。通过对两组分上诱导电荷的定量计算，揭示了电荷调节的程度与PAHA链段与针铁矿表面的静电亲和力、PAHA链段之间的静电斥力以及盐离子的静电屏蔽作用有关。考虑到吸附的PAHA在针铁矿表面的电荷分布，得出电荷调节的变化对吸附的PAHA的构象变化敏感的结论。通过对所作假设的详细检验以及与已报道的理论计算的比较，所得到的势分布被认为大致反映了真实的势分布。由于NOM在金属(氢氧化物)表面的吸附通常没有详细考虑电荷调节，所获得的结果可以作为进一步发展NOM在金属(氢氧化物)表面吸附模拟的基础。
The adsorption of natural organic matter (NOM) on mineral (hydr)oxide plays an important role in the evaluation of the speciation of toxic metal ions in the environment. Because both NOM and mineral oxide have variable charges that adjust upon adsorption, a good understanding of proton binding is required before the binding of metal ions can be understood. In this study, the adsorption of purified Aldrich humic acid (PAHA) on goethite was examined as a function of the environmental conditions (pH, salt concentration, and free concentration of PAHA) together with the proton adsorption to PAHA, goethite, and their mixtures. The induced charges on both components were separated on the basis of the difference between the charge/pH curves of the mixture and those of the single components. The electrostatic potential profile across the adsorbed layer was obtained as a numerical solution of the Poisson-Boltzmann equation using the charge density of the adsorbed PAHA and the goethite surface. From the quantitative evaluation of the induced charge on both components, it is revealed that the degree of the charge adjustment is related to the electrostatic affinity between the PAHA segments and the goethite surface, the electrostatic repulsion between the PAHA segments, and the electrostatic shielding by salt ions. Considering the charge distribution of the adsorbed PAHA at the goethite surface, it is concluded that the change of the charge adjustment is sensitive to that of the conformation of the adsorbed PAHA. From the detailed inspection of the assumptions made and the comparison with the reported theoretical calculations, the obtained potential profiles are considered to broadly reflect the true potential profiles. Because a charge adjustment is not frequently considered in detail in relation to the NOM adsorption on metal (hydr)oxides, the obtained results can form the basis for the further development of modeling of the adsorption of NOM on (hydr)oxide surfaces.