Three Water-soluble Keggin-GaAl₁₂ Single Crystal Structures And Exploration Of Two Hydrolysis Modes

Solution chemistry of aluminium is a hotspot in many academic fields.Polyaluminium compounds formed by hydrolysis of aluminium are the focus of many disciplines,such as geochemistry,biochemistry and environmental chemistry.At present,they have been widely used as antiperspirants,mordants,deodorants,surfactants,catalysts and aluminium-based high-efficiency flocculants and adsorbents in water treatment.Typical polyaluminium compounds include Keggin-Al₁₃(K-Al₁₃)and Plant-Al₁₃(P-Al₁₃),which are also the best coagulation-flocculation morphology of polyaluminium.Because the preparation of K-Al₁₃ by forced hydrolysis is affected by many factors?including alkalinity,alkali concentration,aging time,pH,temperature,ionic strength and temperature,etc.?,the preparation methods?such as solid alkalization,electrolysis and slow drip alkaline method,etc.?are complex,and it is difficult to prepare pure K-Al₁₃.Subsequent separation and purification process are always needed,such as sulfuric acid precipitation and barium salt replacement method.Firstly,in this Master's thesis the preparation methods of K-GaAl₁₂ compound crystals were explored,and three kinds of crystals were obtained.A direct preparation method of K-GaAl₁₂ chloride crystal was obtained.The first crystal formula is[GaO₄Al₁₂?OH?₂₄?H₂O?₁₂]Cl₇·10H₂O,which is crystallized in the tetragonal P4₁2₁2 space group.Its cell parameters are a=19.0310?6??,b=19.0310?6??,c=36.4071?13??.In the synthesis process,P-Al₁₃ was dissolved in water and aged for 10h,and the morphology of P-Al₁₃ was changed into K-Al₁₃.GaCl₃ solution was added,and crystals grew after 20 days at room temperature.It is the first time that K-GaAl₁₂ chloride crystals were obtained directly from aqueous solution.However,the repeatability of this method is low.The second crystal formula is[GaO₄Al₁₂?OH?₂₄?H₂O?₁₂]Cl(C₁₀H₆SO₃)₆·19H₂O(C₁₀H₆SO₃ hereinafter referred to as NS),which is crystallized in triclinic system,P1space group.Cell parameters are a=9.6625?6??,b=14.2903?9??,c=20.3964?13??,?=90.92°,?=90.47°,and?=97.79°.In this synthesis process sodium2-naphthalenesulfonate is introduced into the first crystal preparation method for capture.The solution system is a mixture of water and methanol,and the crystal is obtained 15 days later.The third crystal molecular formula is[GaO₄Al₁₂?OH?₂₄?H₂O?₁₂]Cl₇·10.5H₂O.The crystal is crystallized in trigonal system,R-3 space group.Cell parameters are a=17.7200?2??,b=17.7200?2??,c=32.1759?3??.Firstly,pure PAC was prepared by spontaneous hydrolysis reaction,and dissolved into methanol solvent,then gallium chloride solid was added into solution,several days later K-GaAl12 chloride crystal was abtained at room temperature.This method is easy to repeat.The prepared solid K-GaAl₁₂ has the following advantages:the solid does not contain Na₂SO₄,BaCl₂,AlCl₃,NaOH and other impurities;It is a pure product of Keggin structured polymer;The preparation method is simple and does not need any precipitant or trapping agent;The solid product obtained by this method is very soluble in water and can be used as a solid flocculant directly in water treatment.The effects of two different hydrolysis modes,forced hydrolysis and spontaneous hydrolysis,on the morphology of aluminium hydrolysis were also investigated in this thesis.Compulsory hydrolysis increases the local alkalinity of solution by directly supplying OH^-,and then polymerizes with Al³⁺to form polyaluminium compounds.Spontaneous hydrolysis releases hydrogen and OH^-,which indirectly provides OH^-and then polymerizes with Al³⁺to form polyaluminium compounds by adding aluminium powder into the system and reacting with water.Keggin-Al₁₃ structure was the dominant form of forced hydrolysis products,while plant-Al₁₃ was the dominant form of spontaneous hydrolysis products.NaCl exists in the forced hydrolysis system because of the direct supply of OH^-,and the commonly used raw material is strong alkali NaOH.Does NaCl limit the production of Plant-Al₁₃ in the forced hydrolysis process?This paper investigates the effect of NaCl on the synthesis and crystallization of P-Al₁₃ prepared by forced hydrolysis,and also explores the influence of other weak alkalis?indirectly providing OH^-?in water system,such as NaAlO₂ and NaHCO₃,on the two hydrolysis modes,so as to determine the relationship between the two hydrolysis modes.In the principle of polyaluminium hydrolysis,different alkalinity?OH/Al?B corresponds to different phase regions,using the preset alkalinity of forced hydrolysis in the range of P-Al₁₃preparation.NaCl phase was first crystallized in the system and P-Al₁₃ phase was finally crystallized.It shows that hydrolysis mode is not the decisive factor limiting hydrolysis products.