Mechanism Studies Of Magnetic Effect On Transport Behavior In Reverse Osmosis Seawater Desalination

Fresh water shortage is one of the main factors that restraint the sustainabledevelopment of socio-economy in many regions and countries of the world. Due tothe vast reserves of seawater available, seawater desalination is an important way tosolove the problem. Any improvement of the currently used seawater desalinationtechnologies can improve the quality of people’s lives. Recently, the most efficientseawater desalination technology is reverse osmosis, whereas, it requires highpressures and frequent replacement of membrane module. It is therefore an urgentneed to improve the current seawater desalination method. Considering theadvantages of magnetic fields in water treatment, we proposed a seawaterdesalination method based on the rotating electromagnetic effect. It has the merits ofscale and corrosion inhibition, sterilization and removal algae, high energyefficiency and low device power loss, etc. But the magnetic effect on themicrostructure and dynamics properties of seawater and the transport behavior ofwater molecules and salt ions in the reverse osmosis membrane during seawaterdesalination process, and the influence mechanism, as well as the influence ofrelated main factors of rotating electromagnetic effect on the magnetic effect isunknown. Therefore, it is studied in this paper by molecular dynamics simulationsand experiments.Molecular dynamics simulation is used to choose the proper water model andpotential, then SPCE model is adopted because the corresponding diffusioncoefficient is close to the experiment value. The effect of magnetic field on diffusioncoefficients, pair correlation functions and running integration numbers of purewater and NaCl solution is investigated. With the effect of magnetic field, thehydrogen network between water molecules and the relevant order are bothenhanced, and the mean sizes of water clustes increase in pure water. In NaClsolution, the interaction between ions and water molecules is weakened. The meansizes of both Na+and Cl-ions and the corresponding hydration numbers aredecreased. The hydrogen interaction between water molecules is weakened and themean sizes of water clustes decrease. Besides, Magnetic field makes the contactpairs increase and the solvent separated ones decrease.The influence of magnetic intensity, concentration, temperature, salt speciesand the type of magnetic field on the magnetic effect is performed. With the increaseof magnetic intensity, concentration and temperature, the magnetic effect isenhanced. The magnetic effect on various solutions is different and it depends on thesalt species. The magnetic effect of pulse and alternating field is less significant than that of static field, and it weakens with the frequency of magnetic feld.Magnetic effect on the infrared spectra, ultraviolet spectra, surface tension,osmosis pressure and viscosities of NaCl solution is studied. The results show that,under the influence of magnetic field, the interaction between ions and watermolecules is weakened. The mean sizes of both Na+and Cl-ions and thecorresponding hydration numbers are decreased. The structure–ordering propertyof Na+ions and the structure–disordering property of Cl-ions are both weakened.the magnetic effect on the Cl-ions is more obvious than that on the Na+ions.Magnetic field disrupts the hydrogen network in the solution and the molar volumeof the solvate water increases. The mean sizes of the water clusters decrease and thehydrogen interaction between water molecules is weakened. These agree well withthe molecular dynamics simulation results of magnetic effect on NaCl solution. Itreveals that molecular dynamics simulation can be used to ayalyze the magneticeffect and predict the influence of magnetic intensity, concentration, temperature,salt species and the type of magnetic field on the magnetic effect.Molecular dynamics simulation is carried out to investigate the polymerizationprocess of MPD and TMC monomers to form the PA. The crosslinking density,density and pore diameter of the modled PA all agree well with the experimentresults. The diffusion behavior of water molecules and salt ions in the membrane issimulated. With the effect of magnetic field, the interaction between polymer andthat between water molecules and polymer is weakened, while the interactionbetween ions and polymer is enhanced. These lead to increased diffusioncoefficients of water molecules and weakened mobility of ions in the PA. Inseawater desalination, the water molecules and salt ions would permeate membranemore readily and hardly, respectively. Magnetic field is beneficial to improve thewater production and salt rejection.Based on the transport behavior of water molecules and salt ions in themembrane, molecular dynamics simulation is performed to study the influence ofmagnetic intensity, temperature, salt species and the type of magnetic field on themagnetic effect. With the increase of magnetic intensity, the diffusion coefficientincreases and the asymptotic behavior is observed. Magnetic effect on the diffusioncoefficients of water molecules and mobility of ions is more obvious at highertemperatures. For the simulation systems composed with different salt species, themagnetic effect on the transport behavior of the system composed of divalent ionsand in which the water molecules interact strongly with polymer is more obvious,while it is less on the transport behavior of the system composed of monovalent ionsand in which the water molecules interact weakly with polymer. Compared with thestatic magnetic field, the magnetic effect of pulse and alternating field is enhancedand weakened, respectively. Moreover, both the effect turns significantly with the frequency of magnetic feld. In seawater desalination, the water production and saltrejection can be further improved by increasing magnetic intensity and properlyregulating rotating rate to raise temperture and increase magnetic frequency.Osmosis and reverse osmosis devices are designed to investigate the magneticeffect on the transport processes of BW30, LE and XLE membranes. The magneticeffect on the three membranes is different from each other and the diversities aredue to the various membrane surface characters. With the influence of magneticfield, the adsorption coefficients Kwaterand Ksaltboth increase, and the transport rateof both water and salt is increased in the osmosis process. However, the diffusioncoefficients Dwaterincrease and Dsaltdecrease. The transport rate of water and salt isincreased and decreased respectively, and water flux and salt rejection are bothincreased in the reverse osmosis process. Thses are in accordance with the moleculardynamics simulation results of magnetic effect on water and salt transport in the PAmembranes. This indicates that molecular dynamics simulation can effectivelyanalyze the effect of magnetic field on the reverse osmosis seawater desalinationand the influence machnism, as well as predict the influence of magnetic intensity,temperature, salt species and the type of magnetic field on the magnetic effect. Inthe seawater desalination method based on the rotating electromagnetic effect,magnetic field is beneficial to the seawater desalination process. Desalinationefficiency can be improved by increasing magnetic intensity and properly regulatingrotating rate to raise temperture and increase magnetic frequency.