Molecular orbital study on the change in surface characteristics of allophane with oxo-acids adsorption.

含氧酸（硫酸、磷酸、硼酸、草酸和低分子量有机酸）在火山灰土壤中活性粘土纳米球水铝英石上的吸附行为对环境保护和农业生产具有重要意义。本文采用理论量子化学计算结合实验方法研究了含氧酸在纳米球铝英石上的吸附机理，实验结果表明，含氧酸在纳米球铝英石上的吸附是通过与铝英石表面羟基、Si-OH和Al-OH的配体交换反应实现的，虽然这类配体交换反应已有报道，但我们进一步利用理论量子化学方法发现了一些新的事实，分子轨道方法新的调查结果说明如下。磷酸盐、硼酸盐、硼酸盐吸附在纳米球水铝英石上，硅从水铝英石中释放出来。这被称为含氧酸与Si的交换反应。在硼酸盐吸附的情况下，出现了一种不寻常的键合AT-OH-B.含氧酸的吸附使水铝英石的阳离子交换容量和表面酸性增大。这是由于吸附位点附近的硅烷醇基团的pKa降低（Brφnsted酸度增加）。硅醇基pKa值的降低是由吸附含氧酸的诱导作用引起的，其中对磷酸根吸附的诱导作用最大.含氧酸的吸附使铝英石中的Al-O键和Si-O键的结合强度降低，使铝英石对盐酸、硝酸和硫酸等酸具有吸附能力。这对最近的酸雨问题很重要。

Adsorption behavior of oxo-acids (sulfuric acid, phosphoric acid, boric acid, molybdic acid and low molecular weight organic acids) on nano-ball allophane, one of active clay mateials in volcanic ash soils, is important in view of environmental conservation and agriculture. We investigated the detailed adsorption mechanism with theoretical quantum chemical calculation in addition to with experimental methods.Experimental results showed that all of the oxo-acids were adsorbed on nano-ball allophane via ligand exchange reaction with surface hydroxy groups of allophane, Si-OH and Al-OH.Although this kind of ligand exchange reactions have been reported, we further found some new facts by utilizing thoretical quantum chemical method, molecular orbital method. The new findings are described below.1. The phosphate, molybdate and borate adsorbed on nano-ball allophane with releasing Si from allophane. This was called exchange reaction of oxo-acids with Si.2. In case of borate adsorption, a unusual bonding, AT-OH-B, was suggested.3. With oxo-acids adsorption, cation exchange capacity and surface acidity of allophane increased. This is due to decrease in pKa (increase in Brφnsted acidity) of silanol groups near the adsorption site.4. The decrease in pKa of the silanol groups was caused by inductive effect of the adsorbed oxo-acids, and the inductive effect was greatest for phosphate adsorption.5. The adsorption of oxo-acids also decresed bond strength of near Al-O and Si-O, and the allophane became suspective for acids such as hydrochloric, nitric and sulfuric acids. This is important in relation to the recent problem of acid rain.