火山灰土を用いたカラム試験から得られる硝酸態窒素の吸着特性

抄録

The Japanese Ministry of the Environment investigated groundwater quality at sites across the country in 2002. The survey showed that 5.9% of wells exceeded environmental quality standards for nitrogen-nitrate concentration (10mg/L). Ground water with high nitrogen-nitrate concentrations was distributed in areas of diluvial terrace covered by the Kanto loam layer. Kanto loam is a volcanic ash soil containing allophane, which is known to affect the chemical characteristics of this soil type markedly.
This study discusses the adsorptive features of volcanic ash soils on nitrogen-nitrate using a column test. Several soil samples were obtained from two sites on the Kanto plain. The chemical properties of the volcanic ash soils and pure allophane were subjected to physical analysis before conducting the column test. The column was 40 cm in length and 7 cm in diameter. It was packed with 25 g soil and 2800 g silicate sand and percolated with 0.14 mM nitric acid solution to obtain a break-through curve for nitrogen-nitrate under saturated conditions. The factors of dispersion and retardation were determined from the break-through curve.
Marked differences were observed between the pure allophane and soil samples. The break-through curve obtained using the pure allophane sample exhibited long tailing-off characteristics after the inflection point at pV=2.0. The retardation factor of the pure allophane sample was also larger than that of other samples. The pH value of the eluant indicated a range from pH5 to pH7 through the column test, while the percolated nitric acid solution had a pH value of 4.2.
The observed tailing-off characteristics are thought to have been caused by the exchange of H⁺ from the allophane that is bonded by metal ions. In such instances, the dissociation of the allophane-metallic complex is facilitated by H⁺ and is associated with an increase in the specific surface area of the allophane sample surface and the generation of a positive charge. Consequently, it is thought that the adsorption of nitrogen-nitrate is accelerated by exposure to positive ions.
The finding that the adsorption of nitrate by volcanic ash soils was attributed to the H⁺ on fine allophane particles means that, under low pH conditions, volcanic ash soils exhibit the potential for reducing nitrogen-nitrate in groundwater. Volcanic ash soils could therefore potentially be applied to alleviate nitrate contamination in groundwater.