Adsorption Behavior Of CO₃^2-/HCO₃^- On The Surface Of Aeolian Sand Particles Based On Molecular Dynamics Simulation

Dissolved inorganic carbon（DIC）is a major component of the soil carbon pool in arid and semi-arid region,and is characterized by short turnover time and high renewal rate.Aeolian sand soil is widely distributed in arid and semi-arid regions of China,and the interaction between DIC and aeolian sand soil particles during the inorganic carbon cycle is very common.Therefore,it is important to study the adsorption behaviour of DIC on the surface of aeolian sand soil particles and elucidate its adsorption mechanism to understand the carbon cycle process and carbon balance dynamics in this region.Previous studies have mainly focused on revealing the vertical distribution characteristics of DIC in soil and predicting the inorganic carbon storage in specific areas.However,there are few studies on the adsorption behavior of DIC on aeolian sandy soil particles.In this paper,based on molecular dynamics simulation and experimental methods,the adsorption dynamcis,cluster structure and adsorption site distribution of typical DIC(HCO₃^-and CO₃^2-)on the surface ofα-quartz and amorphous silica minerals at different p H values were systematically analyzed.By comparing with the adsorption experiment results,the optimal model for studying the adsorption of typical DIC on the surface of aeolian sandy soil particles was determined.The coupling effect of solution p H values and ion concentration on the adsorption of DIC on the surface of aeolian sandy soil particles was clarified,and the adsorption mechanism was finally revealed.The main research results are as follows:（1）The main mechanisms of typical DIC adsorption on quartz surfaces were hydrogen bonding and cationic bridges.p H value was the most important factor affecting the adsorption behavior,which was achieved by the coupling of the adjusted HCO₃^-/CO₃^2-ratio and the protonation state of the mineral surfaces.At p H 7.5,the proportion of HCO₃^-adsorbed on the quartz surface（λ（HCO₃^-））decreased with decreasing the carbonate concentration,andλ（HCO₃^-）was 22.5%for 0.28 M total carbonate concentration.As the carbonate concentration decreased to 0.14 M and 0.07 M,λ（HCO₃^-）decreased to 12%and 8.3%,respectively.HCO₃^-and CO₃^2-coexisted at p H 9.5 and p H 11,andλ(CO₃ ^2-)was significantly higher thanλ（HCO₃^-）at the same concentration of total carbonate.HCO₃^-tended to be adsorbed on the quartz surface as a single molecule by hydrogen bonding,while CO₃^2-formed cluster structure under the action of Na⁺and adsorbed on the quartz surface as a cationic bridge.（2）The adsorption mechanism of typical DIC on the surface of amorphous silica was consistent with that of quartz,but there were significant differences in the adsorption amount and adsorption configuration,which were related to the flexible structure of amorphous silica and the existence of rough sites on the surface.The surface uneveness provided sufficient adsorption sites for cations（Na⁺）in solution,and the adsorption density of Na⁺on the amorphous silica surface increased with the increase in p H values.When the total carbonate concentration was 0.28 M,the adsorption density of Na⁺on the amorphous silica surface increased from 0.62 nm^-2 to 1.14 nm^-2 and then to 2.18 nm^-2with the increase of p H value from7.5 to 9.5 and then to 11,which made it easier for the CO₃^2-clusters to adsorb on the mineral surface via cationic bridges.On amorphous silica surface,CO₃^2-ions were more inclined to aggregrate with other CO₃^2-ions to form clusters under the action of Na⁺.In contrast,HCO₃^-ions were difficult to combine together to form clusters because they only"stuck"to the edge of the CO₃^2-cluster and far away from the mineral surface,thus making it difficult for HCO₃^-to be stably adsorbed on the mineral surface.（3）The adsorption of DIC on the surface of aeolian sand soil and quartz particles was very similar,and both were affected by the p H values and ion concentration.At p H 7.5,the adsorption of HCO₃^-by both exhibited an increasing trend with the increase of total carbonate concentration.At p H 9.5,the presence of CO₃^2-significantly increased with the adsorption of HCO₃^-by both particles.At p H 11,the adsorption of both HCO₃^-and CO₃^2-on the surface of aeolian sand soil and quartz particles gradually increased with the increase of total carbonate concentration.Overall,these trends obtained from the experiments were in consistent with the results of molecular dynamics simulations.The adsorption of DIC by aeolian sand soil was slightly higher as compared to quartz particles,but the difference was not significant because the main mineral composition of aeolian sand soil was quartz sand,and in addition,the main structure of both quartz and wind-sand soils particles was siloxane.In conclusion,theα-quartz model was more suitable for characterizing the adsorption of DIC by aeolian sand soil.The main adsorption mechanisms of typical DIC onα-quartz and amorphous Si O₂ surfaces were hydrogen bonds and cationic bridges,but were regulated by DIC type,p H values and ion concentration.This study contributed to further understanding of the geochemical behaviour of DIC and provided a theoretical basis for assessing the carbon sequestration capacity of soils,etc.