Hydrogen and triple-oxygen isotope effects of water adsorption on KGa-2 kaolinite with implications for pedological separation of soil water

Abstract

A series of detailed, systematic experiments were conducted to determine hydrogen and triple-oxygen isotope effects of water adsorption on KGa-2 kaolinite from the Clay Mineral Society at 30 °C as a function of the relative vapor pressure (p/po). Both hydrogen and oxygen isotope fractionation factors between the adsorbed water and water vapor are smaller than those of the liquid water – water vapor (L-V) system even near the saturation. They gradually decrease with decreasing p/po, approaching the isotopic property of water vapor. The ratio of hydrogen to oxygen fractionation factors increases from that of the L-V system (8.0) with decreasing p/po. The exponent 17θ for the triple oxygen isotope system also deviates from that of L-V (0.529) up to 0.585 well above the high-temperature limit (0.5305). Our experimental results clearly demonstrate that the isotopic property of water confined in pores and adsorbed on the mineral surface differs significantly from that of bulk liquid water with wide-ranging implications for stable isotope tracers of soil waters in the terrestrial and extraterrestrial hydrological systems, including a new concept of the ‘pedological separation.’