Surface properties, solubility and dissolution kinetics of phytoliths, from bamboos of Réunion Island

Although phytoliths, constituted mainly by micrometric opal, have an important control on silicon cycle in superficial continental environments, their physico-chemical properties and their reactivity in solution are still poorly known. The aim of this work is to determine the solubility and dissolution kinetics of bamboo phytoliths from Reunion Island and to characterize their surface properties via electrokinetics measurements, potentiometric titration, and metals adsorption experiments at ambient temperature in a wide range of pH (from 1 to 11). The solubility product of phytoliths (pKsp = 2.74) at 25°C is equal to that of vitreous silica and 17 times higher than that of quartz. Electrophoretic measurements allowed to determine phytoliths pHIEP =1.2 ± 0.1 which is comparable with that of quartz but lower than that of amorphous silica. Phytoliths dissolution kinetics, measured in mixed-flow reactors at far from equilibrium conditions, is intermediate between that of quartz and vitreous silica (see Figure). Similar to quartz and amorphous silica, the dissolution rate dependence on pH (pH ≥ 3) can be modeled within the concept of surface coordination theory using the equation: R = 10-6.5 *{>SiOH0} + 102.9 *{>SiO-}1.7 It follows from the results of this study that phytoliths dissolution rate exhibits a minimum at pH ~ 3. This can explain their good preservation in the acidic soil horizons of Reunion Island. Further study of the physico-chemical properties and reactivity of phytoliths of different origins should improve our knowledge of the biogeochemical cycle of silicon in various terrestrial environments.