Mechanism Of Interfacial Water Controlling The Crystallization Of Oxysalt Minerals

In the atmosphere,ocean,soil,desert and even vivo,the water-mineral interface is ubiquitous.Trillions of square kilometers of water-mineral interface cover the planet where human beings live.The water on these interfaces can play an important role in life activities and natural environment by regulating the crystallization behavior of the minerals they wrap.For example,affecting the crystallization process of soil minerals to regulate the distribution and evolution of minerals in the earth’s soil system;promoting the formation of minerals with high activity and natural catalytic properties;rich landforms are created by influencing geochemical processes such as solution erosion and particle transport;subtly control the morphology,composition and structural evolution of minerals in life;improve beneficiation efficiency by regulating the selective secondary deposition of natural minerals.Among the minerals forming this water mineral interface,the most typical minerals are oxygenated salt minerals.Their number and mass fraction determine that they are the most abundant and important minerals in the earth’s crust.The interaction between interface water and oxygenated salt minerals generally occurs in the processes closely related to human production and life,such as mineral deposition,atmospheric particle formation,biomineralization and beneficiation in the ocean,and can regulate many thermodynamic characteristics such as the structure,morphology and crystallization rate of these oxygenated salt mineral particles.Therefore,it has attracted extensive attention from materials science,geochemistry,mineralogy,mineral processing engineering and other disciplines.However,little is known about the specific mechanism of interfacial water regulating the crystallization of oxysalt minerals.This thesis discusses two typical oxyhalite minerals,hydrated mineral calcium sulfate and anhydrous mineral chromite.In order to solve the specific problems of high dehydration energy consumption and difficult to accurately determine the crystal form in the production and treatment of these two minerals,the role of interfacial water in their crystallization process was studied in detail.Calcium sulfate is a typical hydrated oxygenated mineral.In industry,bassanite（Ca SO₄·0.5H₂O）,as an important bulk building raw material,is usually prepared by heating dihydrate gypsum（Ca SO₄·2H₂O）to an incompletely dehydrated product.This is because bassanite is unstable at room temperature and easily hydrated in water environment to form dihydrate gypsum,so it is not terrestrial abundance.In order to produce stable bassanite in more energy-saving way,we designed a route to form micronsized bassanite minerals from solution at room temperature by using the method of interfacial water regulation.Polyvinylpyrrolidone（PVP）was introduced as the stabilizer to disperse and stabilize the bassanite nanoparticles in n-propanol colloid at room temperature.The multi-stage self-assembly of bassanite nanorods was induced by adding an appropriate amount of water to the colloid.The bulk bassanite was gained at room temperature for the first time.We found the multistage assembly of bassanite in PVP-n-propanol system as the following steps:i.homogeneous nucleation to form bassanite nanoparticles;ii.These nanoparticles were assembled into calcium sulfate nanorods along the[102]crystal direction;iii.These nanorods are laterally aggregated into micron mesocrystals,which are self integrated on the surface to form smooth hexagonal prism bulk bassanite.The amount of water added into the system plays a decisive role in the stable phase after assembly.The added water content is less than 3.85 wt%,all the products are bassanite,while the additional water is more than 19.8 wt%,the gypsum dominate the products.When the water content is in the middle of the two,the composition of gypsum in the product increases gradually with the increase of water content.In addition,the role of PVP and water in the assembly of bassanite nanorods and the inhibition of phase transition to gypsum were futher studied by ab initio molecular dynamics.Water molecules mainly play two roles,one is displacing PVP molecules that play the role of stabilizing and dispersing nanocrystals on the surface of bassanite.The other is adsorbing on the surface of hemihydrate calcium sulfate to form weak interaction pairs,promoting the water film growth.These significantly increase the surface energy of bassanite nanorods,thus driving the assembly of bassanite from nanorods to bulks.As discussed above,water plays a significant role in the regulation of the growth process of calcium sulfate minerals.However,the synthesis of bulk minerals with inorganic ions often has high material cost.Therefore,this thesis attempts to take gypsum with large natural reserves and low cost as the precursors of crystallization regulation.Firstly,the conditions for complete dehydration of gypsum are studied theoretically.For these systems containing restricted water molecules,water activity is defined as the amount of"free water"currently available,which has no direct correlation with water concentration.The range of water activity values is between 0（absolute drying）and 1（100%relative humidity）.Water activity is usually expressed by the ratio of the vapor partial pressure of water in the specific environment to the vapor pressure of pure water.Water can immigrate from high water activity to low water activity,so it is very convenient to use water activity to judge the direction and limit of water containing reaction.However,the water activity of hydrated minerals can still not be directly obtained according to the structure of hydrated minerals.We extend the concept of water activity to hydrate and calculate the water activity in hydrate by ab initio method.We descript the water activity in molecule scale and confirmed that the distribution of OH vibration frequency in a single water molecule in time dimension is closely related to water activity.The environmental water activity required to induce dehydration of gypsum at room temperature is also predicted.Based on the above theoretical prediction,we designed a dehydration path at room temperature,so that anhydrite can precipitate from the nano thick sulfuric acid liquid film covering the surface of gypsum,so as to spontaneously transform gypsum into anhydrite.Due to the dissolution of gypsum,the anhydrite in the membrane reaches supersaturation,forms crystal nuclei,further grows and forms nano sheets,and then forms anhydrite mesomorphic crystals through self-similar assembly.We also modeled these pathways and proved that anhydrite in the membrane was more stable than gypsum at room temperature and reduced the kinetic barrier to 4%of the membrane free pathway.In addition,we also introduced and verified that the limited solution space determines the nucleation in the membrane.This mechanism may exist widely in solid-state reactions,and our description will contribute to a better understanding of natural and controlled dehydration of mineral systems.The above research shows that for hydrated minerals,the change of interfacial water has a significant impact on their structure and size.In order to further expand the universal applicability of this regulation.We chose anhydrous lead chromate as another model mineral and controlled the crystallization of lead chromate by using interfacial water confined in microemulsion.Fourier transform infrared spectroscopy（FTIR）was used to quantitatively determine the content of interfacial water and bulk water in the reaction matrix microemulsion.Based on the FTIR results,the nucleation and growth of Pb Cr O₄nanoparticles in microemulsion were studied.By controlling the proportion of water in micellar droplets,amorphous nanoparticles are obtained in interfacial water and nanocrystals are generated in bulk water.Amorphous nanoparticles with specific shape are very stable in microemulsion solutions and do not change into microcrystals within a few days.This work provides a precise method for controlling metastable amorphous precursors for subsequent crystallization.In conclusion,the behavior of interfacial water plays a significant role in the regulation of the crystallization process of the two model minerals.Although the direction and limit of this regulation is depending on the mineral structure.However,under mild conditions,the crystallization regulation of interface water on oxygenated salt minerals,whether hydrated or anhydrous,mainly changes its stability by affecting its solubility,interface energy,monomer diffusion and aggregation,and manipulate its crystallization path and rate.