柠檬酸对珊瑚砂溶蚀过程及机理研究

有机酸在岩石矿物溶蚀风化及土壤形成的过程中具有较强的促进作用，了解柠檬酸对珊瑚砂的溶蚀过程对我国岛礁建设具有重要意义。本文通过不同浓度、不同pH柠檬酸溶液对珊瑚砂的溶蚀实验，研究柠檬酸对珊瑚砂的溶蚀过程，探讨其溶蚀机理。结果表明，在不同浓度柠檬酸溶液中，反应初期珊瑚砂中的Ca2+、Mg2+迅速溶出，随后大量的Ca2+与柠檬酸反应生成柠檬酸钙沉淀。平衡时溶液中Ca2+、Mg2+浓度随着柠檬酸浓度的增加而增加。在不同初始pH柠檬酸溶液中，当pH<7时，溶液中Ca2+、Mg2+的浓度随着初始pH的升高而降低，Ca2+、Mg2+主要由碳酸盐矿物与H+反应而溶出。当初始pH≥7时，Ca2+、Mg2+与柠檬酸生成络合物促进Ca2+、Mg2+的溶出，随着pH的升高，Ca2+、Mg2+浓度略有降低。珊瑚砂的溶蚀与柠檬酸钙的沉淀结晶主要受pH控制。通过XRD和SEM的分析表明，高浓度柠檬酸与珊瑚砂反应生成片状的柠檬酸钙沉淀，且饱和溶液中柠檬酸与钙离子的结晶生成平整规则的条状四水合柠檬酸钙沉淀。珊瑚砂溶蚀过程中镁方解石最先发生溶蚀，其次为文石，最后为方解石。

Study on Corrosion Process of Coral Sand by Citric Acid

Organic acid is important for rock mineral corrosion and soil formation. Understanding the dissolution process of citric acid on coral sand is of great significance to the construction of island reefs. The corrosion process of coral sand by different concentration of citric acid at different pH was studied. The results showed that Ca2+ and Mg2+ in the coral sand were quickly dissolved in the different concentrations of citric acid at first, and then a large amount of Ca2+ was fixed by citric acid to form calcium citrate precipitate. As the concentration of citric acid increases, the concentration of Ca2+, Mg2+ and citric acid in the solution gradually increase. The concentration of Ca2+ and Mg2+ in the solution decreases with the increase of the initial pH of the initial citric acid solution when initial pH of citric acid solutions is lower than 7. Ca2+ and Mg2+ in the solution are from dissolution of carbonate minerals by H+. The complex of Ca2+ and citric acid promotes the dissolution of Ca2+ and Mg2+ when the initial pH of citric acid solutions is more than 7. And the concentration of Ca2+ and Mg2+ decreases slightly with the increase of pH. The corrosion of coral and the precipitation of calcium citrate are mainly controlled by pH. XRD and SEM analysis showed that high concentration of citric acid reacted with coral sand to form a flaky calcium citrate precipitate, and the precipitate from the supersaturated solution showed a flat regular strip of calcium citrate precipitated tetrahydrate. Magnesium calcite is the first to dissolve during the dissolution process, followed by aragonite and finally calcite.