Feasibility study for the production of multi-oxidants from the desalination of seawater brine

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 ) is the primary oxidant generated from the original brine, and ClO2 (3.7–7.2 mg L ) is the minor product in batch electrolysis. doi: 10.2166/wcc.2018.160 om https://iwaponline.com/wqrj/article-pdf/54/3/242/574550/wqrjc0540242.pdf er 2019 Chiung-Ta Wu Yi-Ying Li Po-Hsiung Lin (corresponding author) Department of Environmental Engineering, National Chung-Hsing University, No. 145 Xingda Rd, South District, Taichung City 402, Chinese Taiwan E-mail: pohsiunglin@yahoo.com Chen-Yu Chang Center for General Education, National Taitung College, Chinese Taiwan


INTRODUCTION
. 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 ; Jung et al. ; Oh & Oh ).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. ).
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 (ClO 2 ), chlorine (Cl 2 ), ozone (O 3 ) 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 ) and Multifunctional Electrolyzer (Chang ).The formulae for the electrolysis reaction are as follows.
Major reaction: Minor reaction: The oxygen that is yielded from water in the electrolysis process breaks down to singlet oxygen because of the voltage.Some O 3 is present and H 2 O 2 is generated by a self-decomposition reaction.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.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.
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.
The results for the electrolysis of concentrated brine are shown in Figure 5(a)-5(c).The Cl 2 concentration reaches a maximum value of 302-376 mg L −1 after 60 minutes of electrolysis and the concentration of ClO 2 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 Cl 2 is more than 50 times greater than that of ClO 2 .It is concluded that Cl 2 is the dominant product of the electrolysis of concentrated brine if no other chemicals are added.There is also a small proportion of ClO 2 .

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%), Cl 2 (302-376 mg L −1 ) is the primary oxidant generated from the original brine and ClO 2 (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.
Membrane technologies including reverse osmosis (RO) and ultrafiltration (UF) are the most widely used technologies in desalination plants (Belmont ; Khawaji et al. ; Oh et al. ; Oh et al. ).The application of membranes in desalination has increased as membrane materials have improved dramatically and their costs have been reduced

Figure 1
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.
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 ; Withers ; Yi et al. ).Efficiency of the experimental production of multioxidant 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.

Figure 1 |
Figure 1 | Schematic diagram of the self-assembled electrolysis equipment.

Figure 2 |
Figure 2 | The yields of various oxidants for different anodes.

Figure 5
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.

Table 1 |
Initial properties of the brine