| Research Abstract |
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.
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