Abstract

The primary goals of this study are to compare the efficiency of multiple oxidants that are produced using different commercially available anodes and separators and to optimize the reaction conditions for the recovery of multiple oxidants from brine. The brine produced in the desalination plants in Taiwan is the concentrated seawater that is recovered after the reverse osmosis process. The main component in the solution is NaCl. On average, chlorine concentration is approximately 3–5% by weight, which is slightly higher than the concentration for normal seawater. This concentrated brine can be used as raw material for the electrolyte to extract mixed disinfectant solutions. This study uses different catalytic electrolyzers to compare the efficiency with which multiple oxidants are produced using anodes that are coated in precious metal. A ruthenium-coated titanium anode generates the largest amount of active chlorine (chlorine dioxide). In terms of the diaphragms that are tested, the DuPont Nafion NE-2030 ion film produces active chlorine most efficiently. If no other chemicals are added to the brine (salinity 11.3%), Cl2 (302–376 mg L−1) is the primary oxidant generated from the original brine, and ClO2 (3.7–7.2 mg L−1) is the minor product in batch electrolysis.

This article has been made Open Access thanks to the kind support of CAWQ/ACQE (https://www.cawq.ca).

INTRODUCTION

Membrane technologies including reverse osmosis (RO) and ultrafiltration (UF) are the most widely used technologies in desalination plants (Belmont 1998; Khawaji et al. 2007; Oh et al. 2007; Oh et al. 2010). The application of membranes in desalination has increased as membrane materials have improved dramatically and their costs have been reduced (Morillo et al. 2014). When they are applied in desalination processes, the major waste product is concentrated brine. The concentrated brine that is generated from desalination is usually discarded into the surrounding sea and has a negative impact on the marine environment because it endangers native marine life and ecosystems (Bergmann & Rollin 2007; Jung et al. 2007; Oh & Oh 2010). The disposal of concentrated brine can cause a rapid increase in salinity in specific areas, resulting in dead seas. Recovering valuable components and chemical derivatives from brine has the potential to resolve both environmental and economic concerns (Thiel et al. 2017).

Regardless of the technology that it uses, a desalination plant usually requires large amounts of disinfectant to treat raw seawater, clean treatment devices and to sterilize the freshwater product of the desalination processes. This study uses concentrated brine that is derived from the Nankan Third Stage Desalination Plant in Taiwan as the source of multiple oxidants, which are recovered using electrolysis. Disinfection processes are required in water treatment plants in general and in desalination systems in particular. The major component of concentrated brine is NaCl, which is the electrolyte that is used to produce chlorine. This can be used as a source of multiple oxidants that can be used for disinfection. These oxidants include chlorine dioxide (ClO2), chlorine (Cl2), ozone (O3) and hypochlorous acid (HOCl). The electrolyzer hardware components and the operating parameters must be controlled. Waste concentrated brine that is generated after desalination can be used as the raw material to produce disinfectants using electrolysis. These disinfectants can also be applied in many sterilizing processes. In this study, we aimed to investigate the efficiency of the production of multiple oxidants using different diaphragms in the electrolysis process and to explore the potential application of the optimized diaphragm electrolysis process for the production of disinfectants in desalination plant in Taiwan.

MATERIALS AND METHODS

The diaphragm electrolysis method that is used in this study is a modification of the Hooker S-3 Type Salt Diaphragm Electrolysis Method. Two patents have been obtained: Production Equipment for Multi-Oxidant (Chang 2014) and Multifunctional Electrolyzer (Chang 2007). The formulae for the electrolysis reaction are as follows.

Major reaction: 
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Minor reaction:

The oxygen that is yielded from water in the electrolysis process breaks down to singlet oxygen because of the voltage. Some O3 is present and H2O2 is generated by a self-decomposition reaction.  
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Figure 1 illustrates the self-assembled electrolysis equipment. Parameters such as the voltage intensity and different catalysis species are employed for optimization of the production of multiple oxidants, including chlorine dioxide, chlorine, ozone and hypochlorous acid.

Figure 1

Schematic diagram of the self-assembled electrolysis equipment.

Figure 1

Schematic diagram of the self-assembled electrolysis equipment.

RESULTS AND DISCUSSION

Experimental efficiency of anodes that are coated in precious metal

Three anodes were compared: a pure titanium plate, a ruthenium-coated titanium plate and a platinum-coated titanium plate. The results are given in terms of the total oxidation strength, in mg-ClO2 L−1. No significant differences are observed in the total oxidation strength of each of the three plates but different types of diaphragm have an effect on the total oxidation strength. The ruthenium-coated titanium plate produces active chlorine (chlorine dioxide) as the major oxidant (Pillai et al. 2009) but the platinum-coated anode generates more active oxygen (ozone) than the others (Figure 2). The reaction parameters were: DuPont Nafion NE-2030; pH 2; current, 50 A; voltage, 8 V; saturated brine; temperature, 40 °C; inflow rate, 30 mL/min; reaction time, 30 min).

Figure 2

The yields of various oxidants for different anodes.

Figure 2

The yields of various oxidants for different anodes.

Because chlorine dioxide has a higher oxidation and disinfection strength than chlorine and its byproducts have a lower impact on human health than chlorine, it is used as a disinfectant in tap water, as a reagent to prevent epidemics and as a food additive in many countries, including Taiwan. It provides highly efficient disinfection and has commercial value in public health applications. Thus the efficiency of the production of chlorine dioxide is a major concern for the evaluation of the electrolysis process. A ruthenium-coated titanium plate was used to produce the multi-oxidant containing chlorine dioxide in subsequent experiments (Munichandraiah & Sathyanarayana 1987; Withers 2005; Yi et al. 2009).

Efficiency of the experimental production of multi-oxidant using different diaphragms

To determine the production efficiency for different diaphragms, four types were used. DuPont Nafion NE-2030, DuPont Nafion N-424 and PE-Graphite film were purchased from China and fluorine resin film was prepared in the lab (nonwoven fabric coated with 10% fluorine resin). The efficiency with which they produce a solution that contains chlorine dioxide was compared. The reaction conditions were: pH 2; current, 50 A; voltage, 8 V; saturated brine; temperature, 40 °C; inflow rate, 30 mL/min; reaction time, 30 min.

The results are listed in Figure 3. DuPont Nafion NE-2030 ion film produces chlorine dioxide with the greatest efficiency. Figure 4 shows that a comparison of the yield of ClO2 for different diaphragms. The products that are derived from DuPont Nafion NE-2030 ion film exhibit the highest concentration of chlorine dioxide (83%) in solution. Other oxidants include 10% chlorine, 1.7% ozone and only 0.3% H2O2. The products that are derived from DuPont Nafion N-424 ion film are: 47.1% chlorine dioxide, 46% chlorine, 5.6% ozone and 1.3% H2O2. The products that are derived from fluorine resin film are: 46% chlorine dioxide, 50% chlorine (the highest proportion), 3.8% ozone and 0.2% H2O2. The products that are derived from PE-Graphite film that was purchased from China are: 61% chlorine dioxide, 31% chlorine, 7% ozone and only 1% H2O2.

Figure 3

A comparison of the yield of ClO2 for different diaphragms (reaction conditions: 50 A, 8 V, 40 °C, pH 2, 27% NaCl, 30 mL min−1).

Figure 3

A comparison of the yield of ClO2 for different diaphragms (reaction conditions: 50 A, 8 V, 40 °C, pH 2, 27% NaCl, 30 mL min−1).

Figure 4

The proportion of chlorine dioxide in mixture solution that is generated using different diaphragms (reaction condition: 50 A, 8 V, 40 °C, pH 2, 27% NaCl, 30 mL min−1).

Figure 4

The proportion of chlorine dioxide in mixture solution that is generated using different diaphragms (reaction condition: 50 A, 8 V, 40 °C, pH 2, 27% NaCl, 30 mL min−1).

DuPont Nafion NE-2030 ion film was used for in situ experiments to determine the efficiency with which a solution that contains chlorine dioxide and multiple oxidants can be produced using waste brine from the Matsu Nankan Desalination Plant, Taiwan. Ozone and H2O2 account for a relatively small proportion of the products so only chlorine dioxide and chlorine are analyzed.

Production of multi-oxidant that contains chlorine dioxide from brine

The concentrated brine that is derived from the Matsu Nankan Desalination Plant was used to determine the efficiency with which multi-oxidants that contain chlorine dioxide are produced. A diaphragm electrolyzer with a ruthenium-coated titanium anode, a titanium cathode and a DuPont Nafion NE-2030 ion film diaphragm were used to perform the analysis.

Table 1 shows the basic properties of the concentrated brine that is derived from the outflow of the Matsu Nankan Desalination Plant. The brine is the product of the end of the RO process. Because it has a higher chlorine ion concentration than normal seawater (1.5 times higher), the concentrated brine is a suitable electrolyte that can be used to produce multiple oxidants that contain chlorine dioxide by electrolysis.

Table 1

Initial properties of the brine

ItemMeasurement
Conductivity 7,870 μS cm−1 (at 25 °C) 
Chloride 30,600 mg L−1 
Total dissolved solids 61,400 mg L−1 
Na+ 13,500 mg L−1 
ItemMeasurement
Conductivity 7,870 μS cm−1 (at 25 °C) 
Chloride 30,600 mg L−1 
Total dissolved solids 61,400 mg L−1 
Na+ 13,500 mg L−1 

In this experiment, the electrolysis operation parameters were optimized for the diaphragm electrolyzer in standard operating conditions. The disinfecting solution products were sampled at the outflow end of the venturi. The parameters that were measured were: venturi flowrate, operating voltage, the initial temperature of the anode electrolyte and the NaOH concentration in the cathode electrolyte. To accelerate the entire electrolysis reaction and to increase the operational efficiency, NaOH solution was used as the cathode electrolyte. The parameters for the reaction conditions are listed in Table 2.

Table 2

Fixed parameters for electrolysis

ItemParameter setting
Anode electrolyte Brine 
Initial temperature of anode electrolyte 30, 40 °C 
Cathode electrolyte 0.5% NaOH 
Venturi flowrate 1 L min−1 
Operating voltage 8, 10, 12 V 
ItemParameter setting
Anode electrolyte Brine 
Initial temperature of anode electrolyte 30, 40 °C 
Cathode electrolyte 0.5% NaOH 
Venturi flowrate 1 L min−1 
Operating voltage 8, 10, 12 V 

The results for the electrolysis of concentrated brine are shown in Figure 5(a)5(c). The Cl2 concentration reaches a maximum value of 302–376 mg L−1 after 60 minutes of electrolysis and the concentration of ClO2 is 4–6.9 mg L−1. Concentrated brine was used as the anode electrolyte. When the operating temperature is increased to 40 °C, similar results are obtained: chlorine is the major product with concentrations of 313–364 mg L−1 (Figure 5(d)5(f)). On average, the concentration of Cl2 is more than 50 times greater than that of ClO2. It is concluded that Cl2 is the dominant product of the electrolysis of concentrated brine if no other chemicals are added. There is also a small proportion of ClO2.

Figure 5

(a) Electrolysis reaction for concentrated brine at 30 °C and 8 V. (b) Electrolysis reaction for concentrated brine at 30 °C and 10 V. (c) Electrolysis reaction for concentrated brine at 30 °C-and-12 V. (d) Electrolysis reaction for concentrated brine at 40 °C-and-8 V. (e) Electrolysis reaction for concentrated brine at 40 °C and 10 V. (f) Electrolysis reaction for concentrated brine at 40 °C and 12 V.

Figure 5

(a) Electrolysis reaction for concentrated brine at 30 °C and 8 V. (b) Electrolysis reaction for concentrated brine at 30 °C and 10 V. (c) Electrolysis reaction for concentrated brine at 30 °C-and-12 V. (d) Electrolysis reaction for concentrated brine at 40 °C-and-8 V. (e) Electrolysis reaction for concentrated brine at 40 °C and 10 V. (f) Electrolysis reaction for concentrated brine at 40 °C and 12 V.

CONCLUSION

In order to evaluate the efficiency of the production of multiple oxidants as disinfectants using concentrated brine derived from the desalination plant, we conducted various experiments by controlling electrolysis parameters, including composition of diaphragms, voltage, temperature, etc. We demonstrated that the concentrated brine derived from desalination plant can be used as raw material for the electrolysis process to extract multiple oxidants and serve as disinfectants for water treatment. In this study, we applied several different catalytic electrolyzers containing anodes coated in precious metal for the electrolysis process and compared the efficiency of the production multiple oxidants. We confirmed that a ruthenium-coated titanium anode generates the largest amount of active chlorine (chlorine dioxide). In terms of the diaphragms that are tested, the DuPont Nafion NE-2030 ion film produces active chlorine most efficiently. Without the addition of any other chemicals to the brine (salinity 11.3%), Cl2 (302–376 mg L−1) is the primary oxidant generated from the original brine and ClO2 (3.7–7.2 mg L−1) is the minor product in batch electrolysis.

In conclusion, when seawater is desalinated, the remaining brine, which has a high concentration of NaCl, can be reused as the electrolyte to produce chlorine dioxide and other oxidants. Using this method, the concentrated brine can be directly converted into a disinfectant solution by electrolysis. Regardless of the type of electrolysis procedure that is used, an insoluble anode with a precious metal coating as the anode plate may increase the efficiency of the electrolysis process for the production of oxidants.

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