Study On The Promotion Of Weathering And Dissolution Of Coral Sand By Low Molecular Weight Organic Acids

With the rapid development of human society and the massive consumption of resources,the development and utilization of marine resources has become an important development direction for many countries.China's tropical coral reefs are formed by the accumulation of coral sands.They have a unique ecological environment such as high salt,alkali,and barrenness,which is not conducive to plant growth and human settlement.Rapid soil rebuilding of islands and reefs is the key to constructing a livable environment.Plant residues,plant roots and microorganisms can secrete small organic acids,which have a strong promoting effect in the process of dissolution and weathering of primary and secondary minerals.In this study,the effects of different coral sand particle sizes,different water-rock mixing ratios,different pH and different metal cations on Ca²⁺and Mg²⁺release in coral sand were simulated in the natural environment.The acetic acid?monocarboxylic acid?,oxalic acid,malic acid?dicarboxylic acid?and citric acid?polycarboxylic acid?commonly used in soil were selected as representatives of small organic acids.Different concentrations of small organic acids were studied on coral sand.The composition and microstructure of coral sand after being dissolved were analyzed and observed.The dissolution process and mechanism of different kinds of small organic acids on coral sand were discussed.The leaching experiments of coral sand with different kinds of different concentrations of small organic acids were compared to compare the release rate and degree of Ca²⁺and Mg²⁺in coral sand and the quality loss of coral sand.Through the analysis of experimental data,the main conclusions are as follows:?1?With the increase of reaction time,the particle size decreases,the water-rock ratio decreases,and the dissolution efficiency of Ca²⁺and Mg²⁺in the coral sand increases gradually.When pH<3,the amount of Ca²⁺and Mg²⁺dissolved in coral sand increased significantly.When pH>9,the amount of Ca²⁺and Mg²⁺dissolved in coral sand decreased significantly.K+,Na+,Fe³⁺and Al³⁺can promote the dissolution of Ca²⁺in coral sand,and the promotion of Fe3+and Al³⁺is stronger.By fitting the different kinetic equations of Ca²⁺and Mg²⁺dissolution in coral sand,it is found that internal diffusion is the main process to define the dissolution of coral sand.?2?Ca²⁺,Mg²⁺acid-base dissolution reaction and complex precipitation reaction of Ca²⁺,Mg²⁺and AOH3-mainly occur in the dissolution process of citric acid and coral sand.The initial pH of citric acid solution has a great influence on citric acid-corroded coral sand.With the increase of pH,the dissolution of Ca²⁺and Mg²⁺decreases.When pH<7,Ca²⁺and Mg²⁺are mainly dissolved by H⁺and coral sand acid alkali.When 7?pH?11,citrate is mainly combined with Ca²⁺and Mg²⁺to promote its dissolution.?3?During the dissolution of oxalic acid and coral sand,Ca²⁺and Mg²⁺acid-alkali dissolution reactions and Ca²⁺were precipitated by oxalic acid.As the concentration of oxalic acid increased,the equilibrium time of the reaction gradually increased.At the equilibrium,the Ca²⁺concentration gradually decreased and the Mg²⁺concentration gradually increased.High concentrations of oxalic acid inhibit the dissolution of Ca²⁺in coral sands.?4?The main causes of Ca²⁺and Mg²⁺acid-alkali dissolution of acetic acid and malic acid and coral sand,and the concentration of Ca²⁺and Mg²⁺dissolved with the increase of acetic acid and malic acid concentration,and the dissolution of malic acid by malic acid.Stronger than acetic acid.?5?By fitting the dissolution kinetics of Mg²⁺in coral sand with different kinds and different concentrations of small organic acids,it is found that it conforms to the Stumm model and is mainly affected by the diffusion process.The small molecule organic acid preferentially dissolves the magnesium calcite,followed by the aragonite and finally the calcite.?6?Leaching experiments showed that high concentration of citric acid and oxalic acid inhibited Ca²⁺leaching,and the quality of coral sand increased after leaching.Acetic acid and malic acid gradually increased the amount of Ca²⁺leaching with increasing concentration.At high concentration,malic acid,acetic acid,citric acid and oxalic acid reduced the amount of Ca²⁺leaching,and citric acid had the strongest ability to elute Ca²⁺at low and medium concentrations.With the increase of the concentration of small organic acids,the amount of leaching of Mg²⁺gradually increased.In the case of medium and high concentration,the amount of Mg²⁺eluted by citric acid,malic acid,acetic acid and oxalic acid gradually decreased,at low concentration.Under the oxalic acid,the ability of leaching out Mg²⁺is stronger than that of acetic acid.