Adsorption of Boron by Volcanic Ash Soils Distributed in Japan

Abstract

Core Ideas Boron adsorption increased from pH 5 to 8, peaked at pH 8 to 9, and decreased from pH 9 to 11. Boron adsorption followed a linear adsorption isotherm between 0.4 and 88.4 mg B L–1. Adsorption increased with increasing coexisting anion concentration and occurred via inner sphere complexes. Boron was adsorbed only by amorphous minerals such as allophane, imogolite, and ferrihydrite. Batch adsorption experiments with six volcanic ash soils from Japan were performed under various aqueous chemical conditions to elucidate boron adsorption mechanisms. Boron adsorption increased between pH levels of 5 to 8, remained constant between pH levels of 8 and 9, and then decreased pH levels of 9 to 11. Boron adsorption was closely correlated with borate ion concentrations at pH < 8 and anion exchange capacity at pH > 9. Boron adsorption followed a linear adsorption isotherm between 0.4 and 88.4 mg B L–1. Boron adsorption was influenced by coexisting anion concentration; the greater the coexisting anion concentration, the greater concentration of boron adsorption. This trend suggests that boron adsorbed to the soil by forming inner sphere complexes. No statistically significant correlation coefficients at 95% confidence level were calculated between maximum boron adsorption and allophane, imogolite, ferrihydrite or organic carbon concentrations in the soils. However, multiple regression analysis had a high adjusted R2 value of 0.96 when allophane, imogolite and ferrihydrite concentrations were used to estimate maximum boron adsorption, suggesting these amorphous minerals are especially important for boron adsorption. In summary, these results confirm earlier work demonstrating the importance of amorphous minerals for boron adsorption and provided a statistical description of the relationship between these properties.