CO2 can decrease the dissolution rate of ashed phytoliths

Abstract

Phytoliths are silica structures formed in various silicon (Si)-rich plant species. These structures are a fast-reacting source of available Si for soils and crops and act as a platform for the sequestration of organic matter and CO2. The solubility of phytoliths has been intensively studied. However, little is known about the effect of gases, e.g., CO2, which are commonly emitted from soils. It is hypothesized that CO2 can interact and alter the dissolution rate of phytoliths. In this study, we conducted batch experiments for ashed phytoliths obtained from the controlled pyrolysis of rice straw at 300, 500, 700 and 900 °C and evaluated the Si release along with changes in surface charge (zeta potential) under the influence of CO2 flow. It was found that the ashed phytoliths possessed negative surface charges and released soluble alkaline ions, which enabled sequestration of CO2 by shifting the equilibrium of H2CO3, HCO3− and CO32− more towards HCO3− and CO32−. Conversely, the intensifying presence of CO2 acidified the suspensions containing the phytoliths, stimulated the protonation of the phytolith surface, and decreased nucleophilic attacks from OH− and other anions, thereby preserving the phytolith from dissolution. CO2 showed a higher prohibition efficacy for the phytoliths obtained from 300 and 500 °C than those obtained from 700 and 900 °C. It can be highlighted that CO2 acts to protect phytoliths by altering the solution pH; managing CO2 flux is therefore a potentially effective practice to govern the soil phytolith pool.