Novel draw solutes of iron complexes easier recovery in forward osmosis process

Forward osmotic (FO) membrane is a new process for producing fresh water from salt water. The study of draw solute is essential in the development and application of FO technology; draw solution should be good at drawing water and easy at recovery. In this paper, three complex draw solutes such as ammonium iron (II) sulfate, ammonium iron (III) sulfate and ammonium iron (III) citrate with different concentrations were studied. The physical properties such as pH, conductivity and total dissolved solids (TDS) have simultaneously been investigated. The pH of most ammonium complexes was decreased whereas TDS increased with increasing of draw solution concentrations. We found that high water ﬂ ux of these iron complexes was reported in the range of 8.88 – 11.24 LMH which was higher than the ammonium bicarbonate draw solution. In addition, more than 90% iron complexes draw solutions were recovered by NF-90 membrane, which plays an important role in the FO process to produce fresh water. This study provides direct evidence for the ability of permeate water from feed solution to draw solution, which shows the potential applications of iron complexes in FO process to produce fresh water from sea-, brackish or river water.


INTRODUCTION
Along with the rise in sea levels, the status of salinity is occurring in many countries in the world including Vietnam, which seriously impacts on agricultural production and life in these areas. As a result, many people will lack clean water to use in their daily life. To cope with these issues, seawater desalination technologies have been found by scientists worldwide. Desalination can be defined as any process that removes salt from water. At present, two types of technologies that are widely used around the world for desalination can be broadly classified as either thermal technology or membrane technology. These technologies need energy to operate and produce fresh water.
According to Zheng et al. (), membrane technology is commonly applied for industrial wastewater treatment in China with around 6.7 million m 3 of wastewater per day

Materials
Laboratory-grade chemical reagents were purchased from Sigma-Aldrich Corporation, Germany. The properties of these complexes are shown in Table 1. De-ionized water was produced by an ultrapure water system (Purelabflex-3, ELGA, UK). The pressure of these iron complexes were calculated as the most suitable concentration of each complex following the formula: is the concentration of complex as 0.1, 0.4, 1.0 and 1.5 M were the most suitable concentrations of ammonium iron (II) sulfate, ammonium iron (III) citrate, ammonium iron (III) sulfate and ammonium bicarbonate complexes in FO and nanofiltration (NF) processes, respectively.

Characterizations of iron complexes
The pH value, total dissolved solids (TDS) and electrical conductivity of the iron complexes solutions were deter-

Experimental setup
The experimental setup is shown in Figure 1. to monitor the weight and volume changes at specified time intervals (Figure 1(a)). After the FO test, the diluted draw solution was recovered for reuse through an NF-90 membrane by using a cross flow module at room temperature (see Figure 1(b)). The NF-90 (DOWN) membrane was made from polyamide materials which can be used in pH ranging from 2 to 11 with 6.68 LMH/bar of pure water permeability.
The filtration experiments were repeated three times using fresh membranes. The selection of the most appropriate experimental concentration for each complex is made before comparing test results between complexes.
Water flux, reserve flux and solute rejection were deter-  respectively, Vo and Vt (L) are the initial volume and the volume of feed solution measured at the time t, respectively.

Properties and characterization of iron complexes
Two different cations such as Fe þ2 and NH 4 þ were found in these iron complexes. A2S is classified as a double salt of ferrous sulfate and ammonium sulfate. It forms monoclinic crystals, and is soluble in water at approximately 269 g/L.  which is shown in Figure 3. In addition, NH 4 HCO 3 was chosen as a benchmark for comparison with iron complexes group. The results in Figure 3 show that the experimental water flux and reserve salt flux is a function of molar concentration in FO mode where de-ionized water is employed as the feed solution. We found that the water flux of A3S (11.24 LMH) was obviously higher than A2S (8.88 LMH), A3C (8.88 LMH) and the control group (NH 4 HCO 3 ) (8.62 LMH). This is directly related to the fact that high water flux can generate a greater osmotic driving force for water transport through the membrane. The results report that 36.0 mL of water volume was found in A3S which was obviously higher than A2S (28.4 mL), A3C (28.4 mL) and the control group AB (27.6 mL). However, the highest reserve salt flux was 2.62700 (GMH) in A3S solution, followed by A3C (1.44450 GMH), the control group (1.31415 GMH), and the lowest at A2S (0.00143 GMH).
This indicated that the molecular sizes of complexes were ranged in the order: A3S < A3C < AB < A2S. Hence, molecular size leads to draw solutes permeating back to the feed solution through the FO membrane, which may affect the next stages of fresh water production. Both water flux and water volume of both A2S and A3C were higher than

Recovery test of iron complexes draw solution via NF membrane
The draw solution is diluted after the FO process, thus the recovery of draw solution is necessary and represents one of the challenges in FO processing (Zhao et al. ). The large molecular weight and size of iron complexes might be separated from draw solution by a membrane with a larger pore size than that of RO. The recovery convenience of iron complex draw solution was studied through a pressure-driven NF-90 process.

CONCLUSIONS
In summary, various iron complexes were investigated as draw solutes in an FO process. The results showed that high water flux of these iron complexes was reported in the range 8.88-11.24 LMH which was higher than the AB draw solution. In addition, more than 90% iron complexes draw solutions were recovered by NF-90 membrane, which plays an important role in the FO process to produce a fresh water substitute AB.