Effects Of Surfactants On The Direct Contact Membrane Distillation(DCMD) Processes

Membrane distillation?MD?has an extensive application prosperity in water treatment for its advantages of lower operating pressure and temperature.But the wetting and fouling during the MD process will reduce the efficiency of it,which hiders its steps of industrialization.There are many factors that may lead to wetting and fouling,and the existence of surfactant is one of them.Surfactant can lower the surface tension of the solution significantly,and might change the property of the feed solution as well.But the effect that the surfactants have on direct contact membrane distillation?DCMD?hasn't been studied systematically.So in this research,three kinds of surfactant as well as three kinds of membrane were chosen to investigate the effects of surfactants in the DCMD process.In the first group of experiments,three kinds of surfactant,which were sodium dodecyl sulfate?SDS,anionic surfactant?,dodecyl trimethyl ammonium chloride?DTAC,cationic surfactant?and Lauryl alcohol polyoxyethylene ether?LEO,nonionic surfactant?,were added into the salty water with 35g/L sodium chloride as feed solution to study the effects of surfactant on DCMD desalination process.The wetting phenomenon was studied as a function of the surfactant concentration,and the different membranes.The results showed that the increase of surfactant concentration would make the wetting more severe.It can also be concluded that the omniphobic membrane was the first while the PVDF membrane was the last in an order of the resistance to wetting.Since the omniphobic membranes weren't wetted at all in all the experiments while the PVDF membranes were wetted when dealing the feeds with surfactant at or higher than critical micelle concentration?CMC?,even were wetted by LEO with a low concentration.PTFE membranes had great performances when the feed solutions with LEO and SDS at or lower than CMC,but when it comes to DTAC,the advantage of PTFE membrane over PVDF membrane was disappear.When changing NaCl into salts that can provide counter ions with higher valence states,the aggregations of counter ions and surfactants weaken the wetting phenomenon but their deposits on membrane led a fouling.The interaction energies between the aggregations and the membrane surface were calculated through the XDLVO theory.When the fouling was caused by Ca²⁺/SDS,the data manifested that the attraction force between PVDF membrane and the foulant was the largest while that of omniphobic membrane was the smallest.With the distance between the aggregates and the membrane surface decreasing,there would be a primary energy barrier generated.Only if the energy of the particle is higher than the energy barrier can the particle deposit on the surface.The energy barrier value of omniphobic membrane was the biggest while the PTFE membrane was the smallest.When the fouling was caused by CrO₄^2-/DTAC,the data showed that the attraction force value of the PVDF membrane was still the largest,but the lowest one changed into PTFE membrane.And the omniphobic membrane has the highest primary energy barrier value while the PVDF membrane has the lowest value.Those two groups of data were consistent with the experimental results of DCMD processes.