Wastewater Treatment And Desalination Of Salt As A Solution To Water Crisis
These days, fast growing of energy and water consumption have been the serious concern since the limitation of recognized natural sources [1, 2]. Water crisis is one of the most important human problems in the last decade as well as in the future [3, 4]. Wastewater treatment and desalination of salt water are known as the suitable solutions for this challenge [5, 6]. The developments of water purification and desalination approaches are still ongoing and according to the research's highlights the membrane based methods because of high efficiency, easy operation conditions and low cost in comparison to other methods, is the most effective way of making fresh water [7-9].
According to previous studies, the potential use of membrane processes in water treatment has been due to the development of materials used in membranes . Some materials such as carbon nanotube (CNT) and graphene family after their discovery, showed powerful performance in membrane based water purification process because of their unique properties including high mechanical strength, antifouling nature and fast permeation of water molecules [11, 12]. Among all in the graphene family, graphene oxide (GO) nanoparticles because of their hydrophilic nature, exclusive orientation in water contact and flexible nanochannels have attracted the attention of many researchers in recent decades [13-16]. These unique properties have been achieved from the present of various oxygen containing functional groups on GO surface during synthesis protocol . Nair et al.  were the first indicated water transport mechanism of the pristine GO membrane. They demonstrated the fast water transport rate compared to negligible penetration rate of other components such as acetone and helium through GO membrane. Based on their results, fast water transport of GO membrane relates to its high hydrophilicity and exclusive orientation in water environments.
The ion permeation through GO layers was investigated by Sun et al.  and Joshi et al. . Their results indicated that, the ions with hydrate radiuses less than 4. 5 could pass through the GO layers. They also showed among various solvents only water could pass the ions between GO layers. The pristine GO membrane with fast and selective water transport mechanism has a few weaknesses including swelling of GO layers in water, low mechanical strength and stacking of GO nanosheets which all can decrease the performance of lamellar GO membranes . To solve these weakness, researchers used some polymers such as poly vinyl alcohol (PVA) with hydroxyl functional groups which can form strong hydrogen bonding with oxygen functional groups on GO surface and avoids GO layers from swelling and stacking together; therefore, the separation performance in some area, including solvent filtration and heavy metals removal was improved [22-24].
On the other hand, the reverse effect is also possible, which means the graphene oxide with its superior properties act as a filler and enhances water transport and salt rejection of polymeric membranes [25-28]. In 2012 Wang et al.  fabricated GO blended PVDF membrane by phase inversion method and investigated membrane performance. Their results represented that both hydrophilicity and water flux were enhanced compared with non-blended PVDF membrane. Ganesh et al.  improved hydrophilicity, salt rejection and water flux of PSF membrane through addition of GO nanoparticles. Zinadini et al.  embedded GO nanoparticles into PES membrane to enhance hydrophilicity and water transport ability of PES membrane. The results indicated that the membrane surface hydrophilicity is improved by blending of the GO nanoplates. In order to avoid the GO layers imperfections, we selected PVA as a base polymer because of its natural hydroxyl functional groups. They make powerful H-bonding with oxygen functional groups of the GO membrane which can improve the desalination performance of membrane.
On the other hand, the GO nanosheets may improve the desalination performance of PVA membrane. Therefore, the novel PVA/GO mixed matrix membranes with high content nonofiller (from 0 to 100 wt. % GO loading) were prepared by the pressure assisted self-assembly (PASA) technique in a wide range of compositions. Salt rejection and water transport performance of as prepared membranes were estimated using self-designed membrane nanofiltration cell in dead-end mod. In order to show the effects of the salt's charge in the membrane performance, three different salts with the charge ratio (Z-/Z+) from 0. 5 for MgCl2, 1 for NaCl and 2 for Na2SO4 were selected as feed solutions.