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 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 66, 2018 

A publication of 

 

The Italian Association 
of Chemical Engineering 
Online at www.aidic.it/cet 

Guest Editors: Songying Zhao, Yougang Sun, Ye Zhou 
Copyright © 2018, AIDIC Servizi S.r.l. 

ISBN 978-88-95608-63-1; ISSN 2283-9216 

Synergistic Effect of Ca2+ and SO42- Ions and Gypsum Scaling 

in Feed Solution on Forward Osmosis Boron Removal 

Yi Wanga,b, Hao Guoc, Lubin Chenc, Chaoxin Xieb,c,*, Ningyu Zhoub,c, Zhendong 

Fanga,c,* 

aDepartment of Oil, Army Logistics University, 401311 Chongqing, China 
bWater Industry and Environment Engineering Technology Research Centre, 401311 Chongqing, China 
cDepartment of Military Facilities, Army Logistics University, 401311 Chongqing, China 

kakaxi1202@gmail.com 

This paper investigates the synergistic effect of Ca2+ and SO42- ions in feed solution on the forward osmosis 

(FO) removal of boron by a thin-film, composite membrane with embedded supports of polyester screens 

(TFC-ES). Specifically, water flux, reverse solute flux, boron flux and boron rejection rate were examined 

respectively to explore the influence of gypsum scaling on boron removal. The results indicate that gypsum 

scaling in the FO suppresses water flux and boron flux. After the surface of the polluted TFC-ES membrane 

was cleaned by deionized water, the membrane performance was recovered efficiently, as evidenced by the 

water flux recovery rate of 95.05%, and the boron rejection recovery rate of 94.21%. This research 

demonstrated the potential of the FO membrane in boron removal. 

1. Introduction 

Boron, existing in the form of boric acid or borate, is widespread in seawater, underground brine, and saline 

lakes in nature. Generally, the concentration of boron in the seawater ranges from 4 to 6 mg/L (Melnyk et al., 

2005). Boron is an essential trace element in the human body. However, excessive ingestion may cause such 

adverse effects as diseases in the reproductive, neural, digestive, and cardiovascular systems, and the 

deformity of the axial skeleton. According to the WHO Guidelines for Drinking-water Quality (2006), the boron 

content in drinking water should not exceed 0.5mg/L. In China, the Standards for Drinking Water Quality 

(GB5749-2006) also set a limit of 0.5mg/L on boron content (Wang, 2016). Similarly, boron removal is one of 

the important challenges to seawater desalination(Valladares Linares et al., 2014). 

The beginning of 21st century marks the start of an era of energy and water crises due to the tremendous 

population growth across the globe (Alessia Viola, 2016, Cannistraro et al., 2015, Ciampi et al., 2015).In 

recent years, forward osmosis (FO) has emerged as a promising technology to extract potable water, thanks 

to its high salt rejection, low energy intensity and low membrane fouling propensity (Chung et al. 2012, Lau et 

al., 2015, Luo et al., 2014, Xie et al., 2016, Xie et al., 2015b, Zhao et al., 2012). The growing interest in 

seawater desalination and wastewater reclamation has added the importance to the FO removal efficiency for 

a broad spectrum of trace contaminants (Xie et al., 2012). However, very few studies have been performed on 

the removal of contaminants by the FO. Previous studies have demonstrated that CaSO4 scaling of cellulose 

triacetate (CTA) FO membrane is predominately affected by the feed solution composition and the deposition 

of bulk crystals (Liu and Mi, 2014). According to recent studies, the mechanism of reverse salt diffusion bears 

directly on the organic/biological fouling of the FO, as the fouling is intensified by the interaction between the 

foulants in the feed solution and the divalent ions in the draw solution that diffuse through the FO membrane  

(Zhang et al., 2014). So far, no detailed research has been done on the contaminant rejection rates under the 

synergistic effect of Ca2+ and SO42- ions in feed solution. 

In a previous study, the author studied boron transport through FO membranes and demonstrated that the 

draw solution-oriented active layer (AL-DS) has a much lower boron rejection rate than the feed solution-

oriented active layer (AL-FW) because the AL-DS mode faces severer internal concentration polarization 

(ICP) of boron (Wang, 2016). Herein, the author focused on how the feed solution containing both Ca2+ and 

                                

 
 

 

 
   

                                                  
DOI: 10.3303/CET1866048 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Please cite this article as: Wang Y., Guo H., Chen L., Xie C., Zhou N., Fang Z., 2018, Synergistic effect of ca2+ and so42- ions and gypsum 
scaling in feed solution on forward osmosis boron removal, Chemical Engineering Transactions, 66, 283-288  DOI:10.3303/CET1866048   

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SO42- ions affects water flux, boron flux and boron rejection rate before and after FO membrane scaling in the 

AL-FW mode, seeking to shed new light on gypsum scaling on boron removal in the FO. 

2. Experimental 

2.1 Materials 

As shown in Figure 1, the experimental facilities contained a membrane module, a peristaltic pump, a feed 

solution container, a draw solution container and a data acquisition system consisting of an electronic balance 

and a computer. The circular flow of draw solution and feed solution was established on both sides of the 

membrane through the peristaltic pump, and the water molecules in the feed solution entered into the draw 

solution through the FO membrane in the circular flow. The data acquisition system automatically acquired the 

variation in feed solution quality based on the feedbacks from the electronic balance. NaCl solutions with 

different concentrations were used as the draw solutions and a digital balance was connected a computer to 

monitor weight change of the permeated water at 5 s intervals. DI water was used as the feed solutions and a 

conductivity meter was used to monitor the conductivity change at 30 s intervals for calculating the reverse 

solute flux of the membrane. Membranes were tested under two different modes: active layer facing draw 

solution (AL-DS) mode, and active layer facing feed solution (AL-FS) mode. Each test was conducted for 60 

min in triplicate. 

 

Figure 1: Schematic diagram of the FO experimental facility 

2.2 Material  

The FO membrane was produced by Hydration Technology Innovations (HTI) with an available area of 

47.57cm2. The asymmetric membrane was denoted as the TFC-ES, for it is made up of a thin-film, composite 

(TFC) active layer, and a porous support layer with embedded supports (ES) of polyester screens. The 

material of the active layer was polyamide and that of the support layer was polysulfone and polyester. The 

TFC-ES membrane was about 115 μm-thick, and pH ranged from 2 to 11 (Wei et al., 2011a, Wei et al., 

2011b).  

                    
(a) Active Layer of TFC-ES Membrane   (b) Structure Layer of TFC-ES Membrane 

Figure 2: The SEM images of three kinds of FO membranes 

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Figure 2 is the scanning electron microscopy (SEM) chart of the active layer and the support layer of the TFC-

ES membrane. It can be seen that the membrane was porous and meshed in the support layer, but compact 

and non-porous on the top. 

2.3 Measurement Index and Method. 

Water flux is an important index representing the efficiency of forward osmosis, whose value exerts significant 

impact on the application popularization of osmosis water purification technology. In the forward osmosis 

process, pure water is permeated to the sides of draw solution from the feed solution, which leads to the 

decrease of feed solution quality. The quality of osmosis pure water is acquired by the real-time monitoring of 

quality change of feed solution by electronic balance and the water flux is calculated based on formula: 

Jw=Δm/ρst. Apart from water flux, the solute concentration difference exists between the draw solution and 

feed solution in the process of forward osmosis, which causes small amount of draw solution solute may 

diffuse into the feed solution. This phenomenon is called reverse solute diffusion and its flux is called reverse 

solute flux. The researches indicate that on the one hand, this reverse solute flux will aggravate concentration 

polarization effect and membrane pollution, reducing the stability of forward osmosis system; on the other 

hand, it will accelerate the consumption of draw solution, adding processing costs. Multi-parameter water 

quality analyzer is used to measure the conductivity of feed solution after the experiment. The concentration of 

KCl can be calculated through the standard curve of concentration on conductivity of KCl. (Figure 3). The fitted 

equation of the standard curve of KCl concentration on conductivity is y=1926.83251x+8.1004 and the degree 

of fitting is R2=0.99973.  The value of specific reverse solute flux was calculated by the ratio of reverse solute 

flux and water flux representing the efficiency of forward osmosis. The specific reverse solute flux calculation 

formula of forward osmosis is as follows: Fs=Js/Jw, where Fs is specific reverse solute flux (mMol/L); Js is 

solute flux (mmol/m2 h); where JW is the water flux (L/m2 h) (Wang, 2016).  

 

Figure 3: The standard curve of solution concentration about its conductivity, KCl. 

3. Result and Discussion 

3.1 Synergistic effect of Ca2+ and SO42- ions on water flux  

Figure 4a shows that, during the 960min running cycle, the water flux fell by 44.69% from 11.86LMH to 

6.56LMH in the baseline test. The decline may be attributed to the net osmotic pressure reduction resulted 

from the dilution of the draw solution and the concentration of the feed solution. In the pollution test, the water 

flux decreased by 80.86% from 12.07LMH to 2.31 LMH, 36.17% more than the drop in the baseline test. The 

additional decline in water flux of the FO process is caused by gypsum scaling, a unique process in the 

pollution test other than the dilution of the draw solution and the concentration of the feed solution. To confirm 

this explanation, the active layer of the TFC-ES membrane after the pollution test was sampled and scanned 

by the SEM Figure 4b. Then, an element analysis was conducted based on the SEM chart and the membrane 

contamination of the active layer Figure 4c. The results show that there were three main elements: Ca 

(27.5%), S (25.67%) and O (46.59%), which signifies the existence of CaSO4 crystals. 

Moreover, the water flux decrease in the pollution test was roughly divided into two phases. The first phase 

lasted 270min from 500min to 770min, and the second phase lasted from 770min to the end of the test. The 

two-phase pollution phenomenon was observed by Xie et al. (2015a, 2013) in an organic pollution test. The 

following is a detailed description of the sharp decline in water flux. In the first phase, the feed solution was 

continuously enriched, causing the precipitation of CaSO4 crystals; at 500min, the crystals began to appear 

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on the active layer, blocking the water passage through the membrane; then, the water flux gradually 

stabilized despite the further crystallization on the membrane surface. Under the action of water flux, the 

crystals precipitated in the feed solution continued to accumulate on the active layer, forming a compact layer 

of crystals as of 770min. The further pollution led to drastic drop in the water flux in the second phase. 

In light of the high density and compactness of the active layer of FO membrane, the water flux decline was 

not triggered by film-hole plugging. To sum up, the pollution of the active layer had the following impacts on 

water flux: First, the existence of pollution layer hindered the reverse diffusion of the draw solute into the feed 

solution, exacerbating the concentration polarization; Second, the fouled layer increased the penetration 

resistance, i.e. the space resistance to the water flux, which indirectly lowered the driving force of the water 

flux. 

   
(a)                                                    (b)                                                       (c) 

Figure4: (a) Effects of coexistence of Ca2+ and SO42- in raw water on water flux, (b) SEM image of pollutants 

on the active layer surface of membrane after fouling, (c) EDS image of fouling layer 

3.2 Synergistic effect of combined Ca2+ and SO42- ion on Boron flux and Rejection 

Table 1 shows that the baseline test of boron flux is 20.69 mg/m2 h, the corresponding pollution test boron flux 

is 16.49 mg/m2h, illustrate the foulant test produced fouled layer of boron flux has certain effect. As mentioned 

previously , boron  in feed solution in the forward osmosis process will pass through the active layer surface 

caused the enrichment of concentration polarization, but as a result of membrane fouling after the water flux 

fallen sharply, lead to weakened dramatically boron concentration degree, while pollution layer itself 

exacerbated boron concentration polarization, but due to water flux of the boron concentration degree decline, 

thus reducing boron concentration difference effectively, which reduced the boron flux. Although the baseline 

test boron flux was higher than the foulant test’s, but the table 1 shows the baseline test boron intercept rate 

and the foulant test boron intercept rate similar to that of 47.58% and 49.02%, respectively, this is mainly due 

to the average of the baseline test water flux is greater than the average foulant test water flux, which leads to 

a more intense the dilution effect (Xie et al., 2016). 

Table1: Effects of coexistence of Ca2+ and SO42- in raw water on boron flux and rejection rate 

Experiment Boron Rejection % Boron Flux mg/(m2·h) 

Baseline  47.58 20.69 

Foulant 49.02 16.49 

3.3 Effects of interfacial polymerization reaction time on TFC membranes FO performance  

Figure 5a shows that after cleaning the contaminated membrane at the beginning of the corresponding flux 

was 11.44LMH, the new film at the beginning of the corresponding flux was 14.04 LMH, membrane flux is 

smaller in the early after cleaning. This is mainly due to the fault current cleaning cannot remove contaminants 

on the surface of the active layer completely, leading to positive after the start of the feed solution infiltration 

process cannot be fully formed on the surface activity of pure water in the membrane layer, thus leading to a 

lower initial flux. Figure5b shows the membrane surface after cleaning residue contaminants on the surface of 

scanning electron microscopy (SEM), the figure shows that the crystalline structure of the crystal structure of 

pollutants loose than that before cleaning (Figure 4). 

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(a)                                                                    (b) 

Figure 4: (a) Water flux recovery after cleaning, (b) SEM image of residual pollutants on the active layer 

surface of membrane after cleaning 

Besides, the fouled membrane after cleaning exhibited a water flux of 10.57 LMH, the new film the 

corresponding average water flux was 11.12 LMH, indicates that the crystallization of fouled osmosis 

membrane water flux performance recovered well after cleaning, the recovery rate is 95.05%. 

Table 2 shows the comparison of Boron rejection and product water Boron concentration of the fresh 

membrane and after cleaning membrane. The table shows that corresponding boron intercept contaminated 

membrane after cleaning rate was 57.74%, the new film the corresponding boron intercept rate was 61.29%, 

the recovery rate of 94.21%.This is mainly due to the fouled membrane cleaning after the corresponding new 

membrane water flux and the corresponding water flux gap was intense enough, didn’t remove the 

contamination layer added boron concentration polarization, leaded to the boron intercept rate was less than 

the corresponding rate of boron intercept new film. In addition, the fouled membrane cleaning after the 

corresponding water production boron concentration was less than 0.5 mg/L, to China’s "drinking water health 

standards" (GB5749-2006) of boron concentration is lower than 0.5 mg/L (Wang, 2016). 

Table 2: Boron rejection rate recovery after cleaning 

Membrane Boron Rejection % Product Water Boron Concentration mg/L 

Fresh Membrane 61.29 0.3380 

Fouled Membrane after cleaning 57.74 0.3540 

4. Conclusions 

1. Boron flux and water flux by the effect of Ca2+ and SO42- coexist in the feed solution. Gypsum scaling in the 

process of forward osmosis prevented water flux as well as the flux boron; 

2. The fouled TFC - ES membrane was cleaned by DI water at the membrane surface and efficient 

performance obtained after cleaning, the water flux recovery rate was 95.05%, and the rate of boron intercept 

recovery rate was 94.21%, indicating a promising application of Forward Osmosis membrane in boron 

removal. 

Acknowledgments  

The authors acknowledge the collective support granted by the Graduate Student Research Innovation Project 

in Chong Qing in 2015. The first author thanks China scholarship Council for providing financial support. 

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