Optimizing the desalination rate in a photoelectrocatalytic desalination cell (PEDC) by altering operational conditions

Nowadays, millions of humans are suffering from a lack of drinkable water (Gorjian & Ghobadian 2015). It is estimated that four billion people (nearly two-thirds of the world population) experience water stress at least one month per year (Davis et al. 2013; Mekonnen & Hoekstra 2016). Unfortunately, 80% of diseases in developing countries are due to low water quality and inefficient wastewater treatment systems. Because of this issue, it is estimated that more than 1.5 billion people have no access to clean water and more than 2.6 million people suffer from malfunctioning sewage systems (Hameeteman 2013). Therefore, the only possible sustainable solution is to use unconventional or non-traditional sources like seawater. However, these sources, such as oceans and seawaters containing 97.5% of the total water volume in the world, need to be desalinated to meet water shortage needs (Hameeteman 2013).

Fortunately, there are different technologies, such as thermal, membrane, evaporation, freezing based on desalination methods (Elimelech & Phillip 2011), multi-stage flash distillation (MFD), multi-effect distillation (MED), and reverse osmosis (RO) as the most common technologies, to solve this devastating problem (Khawaii et al. 2008). Such technologies have one main drawback in producing hypersaline streams. Moreover, such desalination technologies are unsustainable due to the energy consumption that makes their economic feasibility doubtful (Jeon et al. 2018). Among the mentioned methods, the RO method has more tangible benefits; however, this method may cause severe problems for the environment due to produced brackish water in addition to high energy consumption (Khawaji et al. 2008; Mezher et al. 2011). Therefore, an alternative approach needs to be found for solving this problem. One of the most popular and emerging technologies for water desalination is applying the photoelectrochemical method. This technology uses an unlimited resource, i.e., solar energy, as a driver to remove salt, thus saving a lot of energy and lowering the cost of the procedure (Dariani et al. 2016; Jeon et al. 2018). It has been indicated that typical wastewater contains much higher energy content than the energy required for its treatment process.

Kim et al. (2018) developed a three-in-one sunlight-driven electrochemical system to enhance desalination function, organic photodegradation, and H₂ production. In this study, an innovative method was applied and the impact of membrane in the process was evaluated for the first time. Instead of a bio-anode used in microbial desalination cells (MDC) in previous studies, a nanophotocatalyst coated on an electrode is inserted into the anode. In fact, this study developed previous methods by an innovative change in the anode's electrode.

The photoelectrocatalytic (PEC) method is an advanced oxidation process (AOP) method applied to remove recalcitrant compounds (Jeon et al. 2018). The PEC mechanism is just like other photocatalytic systems and uses photo-induced charge transfer made by electron/hole pairs creation (Tian et al. 2020). The electricity generated from the photocatalytic procedure is utilized to desalinate the saline water. Since then, different studies have been performed to investigate the workability of saltwater desalination coupled with the PEC system which is called a photoelectrocatalytic desalination cell (PEDC) (Jeon et al. 2018; Chen et al. 2019; Tian et al. 2020). Unlike MDC, in PEDC, microorganisms do not need to degrade organic materials. Thus, this method does not have the limitations of biological processes. The results of this study show that PEDC desalinates hyper-saline streams. Thus, it is a suitable choice to desalinate the rejected stream part of RO and recycle the desalinated stream to create a zero liquid discharge system.

Moreover, untreated industrial wastewaters can jeopardize water sources; thus, they should be throroughly treated (Dariani et al. 2016). Textile wastewater is one type of industrial wastewater existing in considerable volumes regarding the consumption of 200 L of water per 1 kg textile, containing a high concentration of organic matter and can be a good source for electrochemical degradation in this study (Yaseen & Scholz 2019). Such wastewaters are difficult to biodegrade with biological treatments due to their complex aromatic structure (Yuangpho et al. 2018). So processes such as MDC, which are used for seawater desalination successfully due to toxic compound in textile wastewater, cannot be a feasible method for treatment of this type of wastewater. While, in PEDC systems, overall efficiency does not rely on the performance of microorganisms, so the toxicity of textile wastewater would not interfere with the process. Methylene blue (MB) is one of the most common dye components in textile wastewater and is the main reason for toxic streams (Zhao et al. 2016; Hunge et al. 2017; Yuangpho et al. 2018). Degradation of MB accompanied by carbon reduction has been established to be a suitable electrochemical driving force to stimulate the desalination process and such studies give a good idea for integrating different processes to achieve the best result for MB degradation (Luan et al. 2022).

Therefore, in this work, in a novel design the separation of cations and anions is carried out by inserting a desalination cell in the middle of PEC cells. This development in previous study optimizes the desalination rate and MB degradation significantly (Rahmaninezhad et al. 2019, 2020a, 2020b). By adding resin for the first time, the reduction of ionic resistance for transmission through the membranes and increase in the mobility will lead to a striking improvement in the process. Ultraviolet (UV) light is used as the driving force and excites coated titanium nanotubes (TNTs) on the anode electrode, producing electron/hole pairs that degrade organic matter. We investigated MB removal, and high salt removal with these new emerging technologies. This has, to the best of the authors’ knowledge, not been presented in the literature before.

Electrons are spread all through the surface of the cathode electrode to react with oxygen molecules. High durability and conductivity are the main features of a suit electrode. Thus, a carbon cloth with a resistivity of less than 4 mΩ is used in PEDC. Mediators are usually utilized in biological electrochemical systems (BES) to assist electron movement in the catholyte (Khawaji et al. 2008). However, several mediators have been studied and, in this study, MB and platinum (Pt) wire are used as mediators that ease the transfer of electrons from the surface of cathode into the catholyte . Therefore, 1.7 mg/l MB is poured into the catholyte at the beginning of each step. Moreover, a 2.5 m Pt wire (Nano-Bazar Company, Iran) with a thickness of 40 μm is selected and woven on the surface of the carbon cloth electrode. During the cathodic half-reaction, hydroxide is produced as a result of electron and oxygen reactions.

The produced H⁺ in the anolyte and OH⁻ in the catholyte create an electrochemical polarity between the two sides of the middle chambers. Since there is an ion-exchange membrane (IEM) between the anolyte and the middle chamber, and also, there is CEM between the catholyte and the middle chamber, H⁺ and OH⁻ could not move to the middle chamber. Conversely, anions and cations in the middle chamber stem from polarity and attraction, moving toward the anolyte and catholyte, respectively. Therefore, the brine water is desalinated with the ionic separation process. Anions moved to the anolyte-like Cl⁻ for NaCl to create HCl and cations moved to the catholyte-like Na⁺ to produce NaOH.

In the aforementioned equation, M_O2 is the molecular weight of oxygen, F is the Faraday constant, n_e is the number of required electrons for oxygen reduction in the water, V_anode is the volume of the anode chamber, ΔCOD_MB represents a difference in the initial and final chemical oxygen demand (COD) amount of MB, and I is the electrical current.

Different concentrations of NaCl are poured into the middle chamber as saline water in these experiments. The electric conductivity (EC) of the solution is measured using a conductivity meter to calculate the removal percent of total dissolved solids (TDS). The pH of the chambers is determined using a pH meter to measure the acidity and alkalinity of the electrolytes. The amperage, voltage, and resistance of the external circuit between the electrodes are measured using a digital multimeter. The MB concentration in the anolyte is detected using a spectrophotometer (DR 5000, HACH, USA). The MB removal (%) of the anolyte, TDS removal (%) in the middle chamber, the pH and TDS amount of changes in the anolyte and catholyte, and the produced power between the two electrodes are calculated as dependent variables to investigate the PEDC performance.

High salt concentration reduces the internal resistivity of PEDC; therefore, the increase in ionic movements raises the TDS removal percentage up to the optimum point. After this point, the TDS removal percentage decreases due to the limitation in the external circuit's capacity. When one mole of the electron is produced and then transferred from the anode to the cathode electrodes, this allows one mole cation and one mole anion to move toward the catholyte and anolyte, respectively. When the capacity of the external circuit exceeds the number of cations and anions in the middle chamber, an increase in the salt content of the middle chamber cannot increase the TDS removal percentage. Moreover, it creates a disturbance in the ionic movement and decreases the TDS removal percentage. In previous research about MDCs, we witnessed a reduction in the TDS removal percentage (Mehanna et al. 2010; Zhang & He 2012; Zamanpour et al. 2017) and TDS removal rates by an increase in salt content in the middle chamber (Mehanna et al. 2010; Zhang & He 2012; Zamanpour et al. 2017).

Due to the electrochemical reactions of redox, as time moves forward, the intensity of electrical current in the system between two electrodes slightly falls, as demonstrated in Figure 4 (Chen et al. 2019). It can be inferred that, because the concentration of MB is decreased as the reaction is conducted, the density of electron donation is decreased. The same trend is also observed in similar studies applying these methods (Luo et al. 2012; Tian et al. 2020). It should be noted that in this method, unlike microbial fuel cells or MDC (Jacobson et al. 2011; Zhao et al. 2016), there is no need to have a start-up time before and after operation and recovery time. After passing the first minutes of the experiment, the electrical current rises to its maximum value.

PEDC has the maximum produced power when 1 g of salt is added to the catholyte (Figure 5(b)). Added salt increases the conductivity in the catholyte but, when this amount is more than 1 g, it has an adverse effect on the ion separation because of ionic disturbance and diminishing the dialysis, therefore produced power decreases. Similar results confirmed this effect, which is caused by an increase in the ion concentration augmenting EC so it decreases ohmic resistance and makes the separation and transfer of the photogenerated charge carriers easier and increases the speed of the ion transport from the middle chamber (Kim et al. 2018). When the added salt is increased to 5 g, the maximum amount of CE occurs, 80%. When 1 g of salt is added to the catholyte, an increase to a maximum of produced power is seen, establishing observed current and observed voltage differences, reaching 0.68 (mW/m²), 32.63 (mA/m²), and 20.88, respectively.

The maximum produced power is the highest amount when the pH of the anolyte is 11. It should be noted that the maximum produced power, the maximum observed current and the maximum observed voltage difference are 1.93 (mW/m²), 55.05 (mA/m²), and 35.23 , respectively. Also. F. Chen et al. claimed that electrical current increases in the photoredox desalination method (Chen et al. 2019).

Higher desalination and flow efficiency can be achieved by mixing anion- and cation-exchange resins in desalination chambers (Zhang et al. 2012, 2015). Considering the higher conductivity of the IER than the bulk solution, almost all ions are transferred to the IEM by IER. The existence of these resins results in an increase in the conductivity of the desalination chamber and the facilitated transfer of ions from the bulk solution to the IEM. Similarly, the presence of IER both increases the desalination efficiency and reduces the concentration of salt extracted from the desalination chamber. The lower the initial salt concentration, the higher the efficiency would be, probably due to the limited ion transport capacity of the resins (Zhang et al. 2012, 2015).

IERs can act as a bridge between ions and IEMs through the bulk liquid which decreases the resistance for transferring ions and increases the mobility of these transportations. In Zhang et al. (2012, 2015), MDC cells reduced the salt concentration from 720 mg/l to 50 mg/l within 80 h. However, the concentration decreased from 720 mg/l to 40 mg/l within 30 hours when the desalination chamber was filled with resin. Moreover, the internal ohmic resistance decreased due to the presence of IERs.

According to the report (Zhang et al. 2015), when using the resin, the internal ohmic resistance decreased from about 7,383 to 159 ohms and from 641 to 277 ohms for the salt concentrations 50 and 700 mg/l, respectively.

The maximum produced current density in this process was compared to other similar PEC process studies which are shown in Table 1.

In this study, the degradation of MB was investigated using a desalination method called photoredox desalination. Also, high salt removal was calculated in this method for the first time. A photocatalyst can produce protons in the anolyte and hydroxide ions in the catholyte. A desalination cell is placed in the middle of the anode and cathode chambers, causing a separation of the anions and cations from the middle chamber. These separations cause a movement for anions to the anolyte and for cations to the catholyte. It should be noted that the former creates acid and the latter creates the base. The advantages of this desalination method outweigh the benefits of MDCs. These results show that the concentration of salt has a direct effect on TDS removal and also it increases the produced power. Also, the more acidic the condition in the anolyte, the higher TDS removal will be. In contrast, the existence of OH⁻ in the anolyte increases produced power. Moreover, IERs increase the speed of TDS removal.

The authors would like to express their sincere gratitude to the Hormozgan Water and Wastewater Company for funding this study.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.