Study On Solution Structure Of KCl-H₂O-CH₃OH And K₂SO₄-H₂O-CH₃OH Systems

Saline wastewater contains a lot of inorganic salt and organic substances.The study of the interaction between particles in inorganic salt solution containing soluble organic substance can provide a certain theoretical basis for the separation of inorganic salt and organic substance,and the advanced treatment and zero discharge of saline wastewater.There is a close relationship between the microstructure and macroscopic properties（such as solubility,viscosity and conductivity）of solution.Analyzing the reasons for the change of macroscopic properties through the change of microstructure has important theoretical significance for the understanding and exploration of the microstructure and macroscopic properties of solution.In this paper,the microstructure of KCl-H₂O-CH₃OH and K₂SO₄-H₂O-CH₃OH solutions at room temperature were studied by X-ray scattering,Raman spectroscopy and molecular dynamics simulation.On this basis,the reason why CH₃OH reduced the solubility of KCl and K₂SO₄in water was deeply analyzed.The author combined experimental test with computer simulation to quantitatively explore the effect of CH₃OH on the microstructure of KCl and K₂SO₄aqueous solution.The X-ray scattering results show that with the increase of CH₃OH,the peak of2θ=13°moves to a low angle and the intensity of the peak near 2θ=18°decreased gradually in KCl and K₂SO₄aqueous solution.At the same time,the peak of Q=2.2（?）^-1moves to the low Q direction and the intensity of the peak near Q=4.8（?）^-1gradually weakens in the difference structure function,the destructive effect of CH₃OH on the hydrogen bond structure in KCl and K₂SO₄aqueous solution is gradually enhanced.The Raman spectrum results show that CH₃OH weakens the OH stretching vibration in KCl and K₂SO₄aqueous solution,CH₃OH destroys the DDAA type hydrogen bond structure and enhances the DDA type hydrogen bond structure in water molecules.The destruction effect of KCl and K₂SO₄on the DDAA type hydrogen bond structure is greater than that of CH₃OH.Molecular dynamics simulation results show that CH₃OH enhances the interaction between particles in KCl and K₂SO₄aqueous solution,but the interaction distance between particles remains unchanged,r _Ow-Hw=0.97（?）,r _Ow-Ow=2.73（?）,r _K+_-Ow=2.67（?）,r_Cl-Ow^—=3.15（?）,r _SO42—_-Ow=3.45（?）,r _K+_-Cl—=3.11（?）,r _K+_-SO42—=2.55（?）.With the increase of CH₃OH,the coordination numbers of O_w-O_wis decreasing,CH₃OH destroys the tetrahedral structure of water molecules;and the hydration number of K⁺,Cl^—and SO₄^2—is also decreasing;the number of the direct contact ion pairs of K⁺-Cl^—and K⁺-SO₄^2—are increasing;CH₃OH shortens the hydrogen bond length in KCl and K₂SO₄aqueous solution.CH₃OH reduces the solubility of KCl and K₂SO₄in water by reducing DDAA type and increasing DDA type hydrogen bond structure,and increasing the number of the direct contact ion pairs of K⁺-Cl^—and K⁺-SO₄^2—,so as to separate KCl and K₂SO₄from aqueous solution.Cl^—hydration occurs first than SO₄^2—,and Cl^—-O_wis more stable than SO₄^2—-O_w,CH₃OH has a more significant effect on the solubility of K₂SO₄in water.The experimental results and computer simulation results show that CH₃OH destroys the tetrahedral（DDAA type）hydrogen bond structure of KCl and K₂SO₄aqueous solution.