01

Results Overview

 

 

There is an urgent need for economical and efficient desalination technologies to recover industrial saline wastewater and desalinate seawater and brackish water. Desalination technologies based on adsorption principles often suffer from low adsorbent adsorption capacity, susceptibility to pollution, difficult regeneration, and secondary pollution.

In this paper, magnetic reduced graphene oxide (mrGO) was successfully prepared as a magnetic medium to construct a novel magnetic adsorption deionization and capacitive deionization coupled system (MDI-CDI). The internal magnetic field and external magnetic field of the magnetic medium form a superimposed magnetic field with consistent direction.

The relationship between various salt solutions, initial concentrations, operation methods, and deionization effects was investigated using KCl solution, and the MDI system was optimized. The magnetoelectric coupling effect of the MDI-CDI system was investigated using actual petrochemical circulating wastewater (0.933 mS/cm), with average desalination and COD removal rates of 96.9% and 84.8%, respectively. In addition, a three-stage series MDI system was used to investigate the enhanced effect of magnetic adsorption deionization, with an enhancement effect of 79.3% (37.4 mS/cm) for catalytic cracking wastewater and 84.0% (3.68 mS/cm) for petrochemical wastewater.

The results show that, The external magnetic field enhances the main deionization mechanisms of the MDI system, including physical adsorption, magnetic attraction, electrostatic attraction, and surface complexation/deposition effects. The MDI-CDI coupled deionization system can alleviate membrane fouling, and achieve low-consumption, high-efficiency, and stable online regeneration without secondary pollution, providing a new technological approach for the reuse of saline wastewater.

 

 

 

 

02

Background Introduction

 

 

The unplanned discharge of large amounts of high-salinity wastewater has increased the ion concentration in surface water, exacerbating the global freshwater crisis. Researchers are working to find new ways to recover freshwater resources from seawater, brackish water, and industrial saline wastewater to alleviate pressure on freshwater resources.

Among them, saline water usually contains complex pollutants such as TDS, hardness, heavy metals, and organic matter. Most organic matter can be removed through biological processes, but various anions and cations increase treatment costs and hinder the recycling of freshwater resources. In particular, industrial saline wastewater contains a large number of harmful salt ions, such as heavy metals and eutrophication ions, which are harmful to aquatic organisms.

Common wastewater desalination technologies include membrane separation, ion exchange, and electrochemical technologies. Membrane separation technology is efficient and reliable, represented by reverse osmosis and nanofiltration, with research focusing on membrane fouling, blockage, and aging.

Ion exchange is a simple and stable process, but resin life and regeneration wastewater hinder its application. Currently, capacitive deionization (CDI) has become a research hotspot in electrically driven seawater desalination technology due to its advantages of being green, low-energy consumption, high efficiency, and no secondary pollution. The desalination performance of the CDI process is mainly affected by the properties of the capacitor material and the reactor structure.

Finding low-cost and high-efficiency electrode materials remains a key focus and challenge in current research. Common capacitive adsorption materials include traditional carbon-based materials and selective materials such as conductive polymers.

However, due to the mutual electroresistance effect of newly entered heterocharge ions in the pores, capacitive materials are difficult to regenerate. Graphene is a two-dimensional adsorption material with excellent conductivity, and its surface ions can be quickly adsorbed and desorbed. With the development of magnetic capacitive materials with unique capacitive properties derived from spinel ferrimagnetic materials such as Fe ₃ O ₄ Magnetically assisted CDI technology is beginning to emerge.

 

 

 

03

Introduction

 

 

Preparation of Adsorption Materials and Construction of Deionization System

 

As shown in Fig. 1a, the preparation process of mrGO was carried out using the optimized Hummers method. (1) Preparation of graphene oxide (GO) suspension: 60.0 mL of concentrated sulfuric acid was placed in a three-necked flask and stirred for 15 minutes in an ice bath. Then, 2.50 g of high-purity flake graphite powder and 1.50 g of sodium nitrite were added and stirred for 30 minutes. After that, 10.0 g of KMnO4 was slowly added and the temperature was raised to 35 ℃, stirring for 2 hours. ₄ During this period, 300 mL of deionized water was added to the flask in 6 portions. The temperature was further raised to 95 ℃, and the solution was stirred for 1 hour. Then, the temperature was lowered to below 60 ℃, and 7.5 mL of H2O2 (W/W 30%) was added dropwise, and the reaction continued for 15 minutes. Finally, the product was washed 3-5 times with distilled water and centrifuged to obtain a brownish-yellow GO precursor.

A graphene oxide (GO) suspension was obtained by diluting the above precursor with a solid content of 0.5 g in 100 mL of deionized water and ultrasonic dispersion for 10 minutes. An MDI system (Fig. 1b) was constructed based on a nickel foam and magnetic reduced graphene oxide (NF+mrGO) composite adsorption material. The MDI device has a hollow sandwich structure, consisting of a desalination chamber in the middle and two NF+mrGO pieces on both sides, with a pair of permanent magnets fixed on the external non-magnetic 304 stainless steel plate. The effective volume of the desalination chamber is 22 mL. A peristaltic pump pumps the inflow into the desalination chamber for deionization. After a certain cycle of treatment, the salt concentration of the outflow is analyzed. The MDI-CDI system (Fig. 1c) is used for continuous desalination.

Coated titanium electrodes (Ti/SnO2-Sb mesh anode) are placed in the two polar chambers and filled with activated carbon fiber. In the first 30 minutes, the MDI-CDI continuous deionization process is the magnetic adsorption stage, and in the subsequent 60 minutes, the magnetoelectric coupling deionization (1.2 V) stage is carried out.

At this stage, the salt ions remaining in the desalination chamber migrate to the two polar chambers through the exchange membrane to be enriched. Therefore, mrGO is regenerated, and salt ions are concentrated, ensuring the continuous operation of the MDI-CDI desalination process.

Screening and Surface Characterization of Adsorption Materials

 

In Fig. 2a, when CAC and rGO were used, the removal rate of KCl was close to 6.66%. Due to nano Fe

 

在Fig. 2a中，使用CAC和rGO时，KCl的去除率接近6.66%。由于纳米Fe₃ O ₄ The particles facilitate ion attachment, significantly improving the desalination rate of self-made mrGO to 9.78%. In addition, due to magnetic adsorption, the desalination rate of magnetized mrGO is the highest (11.7%). Therefore, magnetized mrGO was selected in this paper, and mrGO mentioned later refers to magnetized mrGO. The FTIR spectrum of mrGO (Fig. 2b) shows the presence of -OH (3377.71 cm ^-1 ), C=C (1564.81 cm ^-1 ) and Fe ₃ O ₄ (574.68 cm ^-1 ) absorption signals. After adsorption, a KCl absorption peak (1363.43 cm ^-1 ) appeared, indicating successful adsorption.

The X-ray diffraction spectra of rGO and mrGO are shown in Fig. 2c. The sharp diffraction peaks indicate that the magnetic particles have high crystallinity. rGO has distinct characteristic diffraction peaks at 2θ of 26.5° and 42.8°, corresponding to PDF#41-1487 (graphite-2H) (200) and (001) crystal planes, respectively. mrGO has five characteristic diffraction peaks at 2θ of 30.3°, 35.7°, 43.5°, 57.2°, and 62.9°, corresponding to the (220), (311), (400), (440), and (511) crystal planes of Fe ₃ O ₄ nanoparticles, which is consistent with the standard card of Fe ₃ O ₄ (Fe ₃ O ₄ , PDF#19-0629, PDF#65-3107).

SEM shows the surface morphology of mrGO (Fig. 3d). The rGO component exhibits a clear two-dimensional structure with distinct wrinkles and layering. A large number of Fe ₃ O ₄ nanoparticles were successfully loaded onto the rGO surface. Analyzing the HRTEM image of mrGO, the size of the magnetic particles is approximately 15-17 nm (Fig. 3e). The composition of the magnetic particles is Fe ₃ O ₄ (311), with an interplanar spacing of 0.253 nm, and clear diffraction rings of Fe ₃ O ₄ particles were observed from the selected area electron diffraction pattern (Fig. 3f), consistent with the results reported in the literature.

 

Electrochemical characteristics of the adsorption material

 

The results of the electrochemical performance of different adsorption materials are shown in Figure 3. The CV curves of different adsorption materials do not have oxidation/reduction peaks (Fig. 3a). At a scan rate of 5 mV/s, the specific capacitances of NF, NF+GO, NF+rGO, and NF+mrGO are 0.0732 F/g, 104.28 F/g, 120.98 F/g, and 181.32 F/g, respectively (Fig. 3a-I).

The EIS results of different electrode materials (Fig. 3a-II) show that the NF+mrGO composite material has the lowest resistance under the action of a magnetic field and has a good ion response in the high-frequency range. Under the action of an external magnetic field, the NF+mrGO composite material coated with Fe ₃ O ₄ nanoparticles has an additional magnetic field within its constituent parts, which can improve the double-layer capacitance, reduce the charge transfer resistance, and enhance the discharge performance. Interactions such as electrostatic interaction, metal coordination, and surface coprecipitation.

Due to the presence of Fe ²⁺ 、Fe ³⁺ and captured H ₂ O on the mrGO surface, this is possible. On the other hand, the specific capacitance of NF+mrGO gradually decreases with the increase in scan rate (Fig. 3b and 3b-I), because a lower scan rate provides sufficient time for salt ions to migrate from the solution to the electrode material. The GCD curve of NF+mrGO is triangular at different current densities, indicating excellent electrochemical reversibility (Fig. 3c).

As the current density decreases, the discharge time increases, and the specific capacitance of the material increases (Fig. 3c-I), which is attributed to concentration polarization at the electrode interface. In addition, the CV curve shows that the magnetic field effect significantly improves the specific capacitance of NF+mrGO (Fig. 3d), from 181.3 F/g to 222.5 F/g (Fig. 3d-I). The increase in specific capacitance is related to the decrease in charge transfer resistance in the magnetic field.

 

Optimization of the magnetic adsorption deionization system

 

 

The dosage of mrGO significantly affects the desalination effect (Fig. 4a). When the dosage (mrGO dry weight) increased from 0.35 g to 0.65 g, the removal rate of KCl increased from 30.9% to 49.4%. However, when the dosage increased to 0.80 g, the removal rate decreased to 43.8% due to local blockage and short-circuiting. Therefore, the dosage of mrGO was determined to be 0.65 g. The effect of the initial concentration (Fig. 4b) shows that the higher the concentration, the higher the desalination efficiency, consistent with the literature results. When KCl is 2.0 g/L, SAC reaches 38.0 mg/g. As shown in the results of Fig. 4c, the removal of KCl is comparable when the flow rate ranges from 3.8-11.4 mL/min. When the flow rate increases to 15.2 mL/min, the desalination effect of MDI decreases due to the discharge of mrGO with the effluent. A flow rate of 7.6 mL/min was selected for subsequent experiments.

The external magnetic field strength was measured using a Gauss meter (F.W. BELL 5180, Bell USA) at the center of the desalination chamber (Fig. 4d). Analyzing the results in Fig. 4e, the removal rate of KCl by MDI was 30.3% and 49.4% in 30 minutes without and with a 300 G magnetic field, respectively. This indicates that the external magnetic field enhances the magnetic adsorption performance of MDI.

In addition, the Fe

content is key to regulating magnetic adsorption. When the Fe ₃ O ₄ content of loaded mrGO increased from 0 wt% to 20.72 wt%, the desalination performance of the MDI system improved by 25.4% (Fig. 4f). This is attributed to the synergistic effect of the external magnetic field and the additional magnetic field of mrGO. However, when the proportion of Fe ₃ O ₄ in mrGO is too high, the content of rGO decreases, leading to a decrease in the total specific surface area and adsorption sites. ₃ O ₄ Application of magnetic adsorption capacitive deionization system

 

The principle of magnetoelectric coupled deionization of MDI-CDI is shown in Fig. 5a, including the magnetically enhanced adsorption of NF+mrGO, the ion electromigration of the external electric field, and the capacitive deionization of ACF.

 

 

MDI-CDI的磁电耦合去离子原理在Fig. 5a中示出，NF+mrGO的磁增强吸附、外部电场的离子电迁移和ACF的电容去离子。

The results in Fig. 5b show that the desalination rate of the MDI-CDI system based on magnetically enhanced adsorption is comparable to that of the single MDI system in the first 30 minutes. The desalination rate of MDI-CDI treating PCW wastewater is 54%.

In Fig. 5c, in the next 30-90 minutes, the conductivity decreased to 29.8 μS/cm, and the total desalination rate of MDI-CDI increased to 96.8% due to the magnetoelectric coupled deionization process. Meanwhile, in both electrode chambers, the conductivity of the mixed solution continuously increased from 46.1 μS/cm to 280 μS/cm, indicating that ions were enriched on the activated carbon fiber, and mrGO was regenerated.

In Fig. 5d, the mrGO regeneration rate is defined as the ratio of desalination efficiency after regeneration to that before regeneration. Using PCW as the experimental object, a 100% mrGO regeneration rate was achieved at 1.2 V, avoiding hydrolysis side reactions.

The data in Fig. 5e and 5f demonstrate that the MDI-CDI system operated continuously for 5 cycles. After 30 min of magnetic adsorption, the average desalination rate and COD removal rate were 54.7% and 54.2%, respectively. After 60 min of magnetoelectric coupling, the average desalination rate and COD removal rate reached 96.9% and 84.8%, respectively. This indicates that the stability and continuity of the MDI-CDI online regeneration deionization system are satisfactory. Using a single MDI system, a three-stage series MDI process can improve the total desalination rate in 90 min.

The results in Fig. 5g show that the conductivity of the CCW effluent decreased from 37.4 mS/cm to 7.75 mS/cm, and the total desalination rates of the three-stage device were 38.2%, 64.0%, and 79.3%, respectively. This indicates that as the influent salt concentration decreases, the concentration gradient between the solid and liquid phases decreases, leading to a decrease in the desalination rate in the multi-stage MDI device.

The results in Fig. 5h show that the conductivity of the PW effluent decreased from 3.68 mS/cm to 1.86 mS/cm, 1.11 mS/cm, and 0.588 mS/cm, and the desalination rates of the three-stage device were 49.5%, 69.9%, and 84.0%, respectively. The COD removal rate of the three-stage PW effluent is consistent with the desalination rate trend, being 59.1%, 72.2%, and 80.4%, respectively (Fig. 5i).

 

04

Results Outlook

 

 

Magnetic adsorption deionization-capacitive deionization (MDI-CDI) coupled system. The NF+mrGO composite material is magnetized under the action of an external magnetic field, thereby increasing the specific capacitance to 222.5 F/g.

Using 1 mol/L KCl as the electrolyte, the cyclic voltammetry curve of the NF+mrGO composite material did not show any redox peaks, indicating that it is a double-layer adsorption process. In the MDI process (0-30 min), the external magnetic field and the additional magnetic field of the internal NF+mrGO composite material enhanced the adsorption of salt ions on mrGO. Using an MDI device to treat KCl solution (2.0 g/L), the SAC value is 38.0 mg/g, and the deionization efficiency is 49.4%.

In the MDI+CDI process (30-90 minutes), a 1.2 V external electric field is applied, and the deionization process includes magnetic adsorption of NF+mrGO, electromigration, and electric adsorption of ACF.

The MDI-CDI coupled system continuously operated 5 times to treat actual industrial saline wastewater from PCW, and the average desalination rate and COD removal rate reached 96.9% and 84.8%, respectively. This indicates that the stability and continuity of the MDI-CDI online regeneration deionization system are satisfactory. This new type of MDI-CDI coupled system is expected to solve the problem of recycling industrial saline wastewater and contribute to the protection of freshwater resources.

(Source: Environmental Frontier New Youth)