Preparation Of Superhydrophobic And Superhydrophilic Membranes And Study Of Fouling And Wetting In Md Process

Membrane distillation(MD) is a new desalination technology, widely applied in various fields such as desalination, fruit juice condensation and wastewater treatment. However, most of wastewater contained organic substances causing membrane pore wetting or fouling. Therefore, modification of hydrophobic membrane and preparation of anti-fouling composite membranes can effectively relieve membrane wetting or fouling. Systematic research on membrane fouling and wetting is a key to solve the membrane distillation industrialization problem.Two surface modification methods were explored to obtain super-amphiphobic and superdrophilic membranes surface, respectively. The anti-wetting or anti-fouling property of the pristine and modified membranes were examined in a direct contact membrane distillation(DCMD) process using sodium chloride solution containing different organic foulants.Firstly, a facial surface modification method was explored to obtain amphiphobic membranes with mechanical and thermal robustness by dynamically forming perfluorooctyl trichlorosilane(PFTS) and coating Si O2 nanoparticles onto the hydrophobic membrane surface. The effects of different conditions such as the ratio of Si O2 and PFTS, time and pressure on membrane surface hydrophobicity and oleophobicity were investigated in detail. When the ratio of Si O2 and PFTS was 1:1, operating time and pressure were 45 s and 0.10 MPa, respectively. The water and diiodomethane contact angle were over 160° and 140°. the pristine and modified membranes were used in a direct contact membrane distillation(DCMD) process using sodium chloride solution containing different organic foulants(kerosene, humic acid and surfactants). Result showed that three kinds of organic foulants caused serious wetting and fouling, membrane flux declining and the permeate conductivity sharply rising for pristine membrane. And the kerosene fouling was worst. In contrast, the amphiphobic membrane could keep a stable permeate flux and high salt rejection, showing good anti-fouling and anti-wetting property with the presence of three foulants, respectively. These mainly because that the Si O2- PFTS coating of membrane surface made the contact between organic foulants and membrane become more difficult.Secondly, another surface modification method was explored to obtain superhydrophilic membranes surface by attaching a thin layer of PVA hydrogel onto membrane surface. the PVA hydrogel was only 5?m. The long-time DCMD test feeding sodium chloride solution containing different organic foulants showed that the hydrophilic layer of PVA/PVDF membrane surface only delayed the wetting and fouling but not prevented that over time when feeding by kerosene and HA. Due to the repulsion of PVA layer to keroaene, no significant wetting happened at first 40 h, but a little of kerosene would through the PVA layer and adsorbed the PVDF membrane with the extension of time, then membrane weting would slowly happened, illustrating the PVA layer only resist the fast adsorption of kerosene. no kerosene was detected in the permeate because of the strong adsorption of membrane to kerosene. the PVA layer deposited a thick and dense HA layer dute to the stronger adsorption of membrane to HA. The small HA would adsorbed the PVDF membrane and partly membrane pore weting would slowly happened with the extension of time. Thus HA and Na Cl through wetting membrane pore into the permeate. However, the PVA/PVDF membrane could keep good anti-wettability against SDBS in 100 h, presenting good anti-fouling property.Thirdly, the continuous high concentration condensing MD experiments were investigated. It was found that the high concentration surfactant would fast wetted the pristine and modification membranes. the self-cleaning property of superhydrophobic membrane and the delaying fouling of superhydrophilic membrane woule become not obvious. When feeding with high concentration kerosene, kerosene was only adsorded on membrane surface or pore but not permeating the membrane. However, for the high concentration HA, the mass transfer of HA across membrane mainly adopted adsorption-desorption methods.At last, The self-cleaning property and the extended Derjaguin-Landau-VerweyOverbeek(XDLVO) approach were used to predict the mechanisms of membrane anti-fouling. The calculation results showed that the interface free energy between three foulants and PVDF membrane present strong force of attraction, the foulants would easily adsorb on membrane surface and causing fouling. In contrast, the kerosene was most easily adsorbed, but for the SDBS, The hydrophobic moieties preferentially adsorbed on the membrane surface and the hydrophilic moieties in the water phase. the hydrophobic membrane surface gradually became a hydrophilic surface with the increase of adsorption, and the SDBS permeated membrane along the wetting pore. However, for the superhydrophobic surface, due to its better self-cleaning property, the water and all other materials in the solution difficultly contact with the membrane surface. So the superhydrophobic membrane showed the excellent anti-fouling property. For the superhydrophilic PVA/PVDF membrane, the membrane surface showed strong repulsion force on three kinds of organic matters, the contact between the organic matters and membrane surface would became more difficult. the hydrophilic PVA layer only slowed down the wetting and fouling by kerosene and HA, even though it performed comparable anti-wettability against SDBS. But the effect of repulsion was limited, the organic foulants still had access to membrane surface and caused wetting or fouling under the higher concentration.