Study On The Selective Control Of Crystal Plane Of Mg（OH）₂ Synthesized By Ammonia Method And Adsorption Performance Of Congo Red

Organic dyes are widely used in textiles,cosmetics,printing and other fields.A large part of these dyes will be discharged with industrial wastewater,causing serious damage to the ecological balance in the water,and even harm to human life and health.Therefore,the synthesis is highly efficient and pollution-free the waste water treatment materials have become the research focus of dye wastewater treatment.Due to its unique properties,magnesium hydroxide is used in the field of environmental protection,especially in the field of dye and heavy metal ion adsorption as an "environmentally friendly"material.However,due to the difference in the polarity of the crystal plane,it has an important influence on the adsorption performance.Therefore,controlling the process conditions,crystallization environment and other factors during the synthesis process to control the selective growth of its crystal planes has important theoretical and practical significance for improving the properties of materials.In this paper,magnesium hydroxide was prepared by the nucleation/crystallization isolation method.Micron magnesium hydroxide with different polar crystal faces and uniform particle size distribution was obtained by adjusting the time,temperature,and the amount of ammonia water.Studies have found that extending the crystallization time and increasing the crystallization temperature can better promote the growth of the(001)crystal plane with a smaller polarity,and make the ratio of the XRD diffraction peak intensity of the(001)crystal plane and the(101)crystal plane,it can be increased from 0.723 to 2.740,which realized the regulation of the polarity of the crystal plane of magnesium hydroxide.At the same time,the results showed that as the surface polarity of magnesium hydroxide decreased,the hexagonal appearance became more obvious,the dispersion was improved,and the agglomeration phenomenon was reduced.When the amount of ammonia water is increased,due to the increase of OH-concentration,it has a shielding effect on the surface of magnesium hydroxide,hindering the growth of(001)crystal plane,and at the same time reducing the crystallinity of magnesium hydroxide.This paper also studied the influence of hydrothermal crystallization solvent media on the selective growth of magnesium hydroxide crystal surface and the morphological particle size.The study found that with the increase of ethanol/water ratio,the growth of magnesium hydroxide(001)crystal plane changed significantly,and the effect on(001)crystal plane was first promoted and then inhibited the growth.In addition,for small molecular weight organic solvents such as ethanol,ethylene glycol,and glycerol,as the content of alcoholic hydroxyl groups increases,the growth of magnesium hydroxide(001)crystal planes is inhibited;for large molecular weight polyethylene glycols,due to the combined effect of hydroxyl and steric hindrance promotes the selective growth of(001)crystal planes,and obtains magnesium hydroxide with better crystal form and relatively uniform morphology and particle size.This paper further studied the adsorption properties of magnesium hydroxide on Congo Red.The effects of the polarity of magnesium hydroxide crystal plane,dosage,adsorption temperature,and pH of the solution on the adsorption effect were explored,and the adsorption isotherm and kinetics of the adsorption process were studied.By studying the adsorption performance of magnesium hydroxide with different crystal plane polarities,it is found that as the proportion of(101)crystal planes with larger crystal plane polarity increases,its adsorption effect on Congo red is improved.Studies on adsorption thermodynamics and kinetics show that the adsorption process is more in line with the Langmuir isotherm adsorption model and the quasi-secondary kinetic model.When the initial pH of the solution is 8,the dosage of magnesium hydroxide is 1.5 g/L,and the temperature of adsorption is 25℃,A good adsorption effect can be achieved after 30 minutes of adsorption,the removal rate of Congo red can reach 95%,and the maximum adsorption capacity is 116.414 mg/g.