Water treatment device and method

Abstract

The present invention relates to a device and method for treating water. In an embodiment, it relates to an adsorbent for treating water, the adsorbent is made of a composite material comprising an organosilicon quaternary salt and a carbon-based material, wherein the composite material removes an amount of boron present in the water that is being treated.

Description

DESCRIPTIONTITLE OF THE INVENTION: WATER TREATMENT DEVICE AND METHODFIELD OF THE INVENTION

[0001] The invention relates to a device and method for treating water. In particular, it relates to a device and method for removing boron and chloride ions from, in an embodiment, brackish water. More particularly, it relates to a device and method for removing boron from brackish water.BACKGROUND OF THE INVENTION

[0002] Boron is a naturally occurring element in the environment. Its presence comes mainly in the form of boric acid (H3BO3) or borate ions (B(OH)4, B4O?2, H3BO2) or salts. Its aqueous solution plays an important role in many application fields, e.g. in a middle-scale semiconductor factory, million tons of pure water with ppb (parts per billion) level of boron is daily consumed during the manufacture process, higher boron concentration might cause defects due to the p-type dopant in semiconductor chip manufacturing, and leak might happen.

[0003] In irrigation water, boron content must not exceed 1 mg / L (parts per million). Its deficiency and excess are harmful to the normal growth of plants. On one hand, boron deficiency may reduce absorption of calcium, magnesium and phosphorus in the growth and functioning of plants. On the other hand, excess boron can result in dwarfing or death of plants.

[0004] For potable water, the World Health Organization (WHO) recommends a guideline concentration of boron up to 2.4 mg / L level in 2011 . Increased boron content causes problems in cardiovascular, coronary, nervous and reproductive systems. It is particularly dangerous for pregnant women to take excess of boron because of the risk of birth pathology.

[0005] The concentration of boron is approximately 5 mg / L in seawater. In seawater desalination, reverse osmotic membrane, capacitive desalination, and electro-dialysis desalination are the most popular technologies. However, none of them can efficiently remove boron from seawater due to its small size and uncharged species of boric acid at pH 8.4. In this case, additional post-treatment processes are needed to remove boron during theseawater desalination. These post-treatment processes include electrocoagulation, chemical precipitation, ion exchange processes, and liquid-liquid extraction. However, most of these methods are inefficient in solutions of low boron concentrations or adjustment of pH is required.

[0006] Accordingly, there remains a need to provide for an improved boron removal medium and method that overcome, or at least alleviate the above drawbacks by controlling and keeping the boron concentration within the applicable limit.

[0007] Due to the increasing scarcity of suitable quality water for drinking, industrial, and agricultural purposes, there is a pressing need to enhance freshwater production from unconventional sources like seawater and brackish water. While reverse osmosis (RO) has been extensively developed for seawater desalination, its energy demand doesn't scale efficiently with decreasing feed concentration, such as in brackish water (second RO) plants.

[0008] Despite brackish water being less concentrated than seawater, the energy input required for desalination remains high. As desalinating brackish water is a promising solution, technologies that have energy demands proportional to feed concentration are more desirable than RO. Electrochemical processes like capacitive deionization (CDI) offer a promising solution due to their energy demand being tied to salt removal, rather than water volume. CDI is scalable and suitable for brackish water desalination or polishing in combination with other treatments.

[0009] One critical challenge in brackish water desalination is the removal of boron, an amphoteric contaminant prevalent in seawater as boric acid (H3BO3). Traditional desalination methods (RO and CDI) struggle with efficient boron removal due to its uncharged state. High levels of boron are detrimental to human health and plant growth, necessitating stringent concentration limits to meet the standards of both drinking water and irrigation water with a minimum limit of 0.5 mg / L. This requirement is still difficult to be reached for several conventional deboronation treatment processes. Specifically, boron removal needs increased pH technology to transform the H3BO3 into B(OH)4' anion, this process often yields high boron concentrations in effluent and an increased capital-operational expenses. Consequently, efficient and sustainable boron removal techniques are needed to control the boron level (<0.5 mg / L).

[0010] Adsorption serves as the primary mechanism and deems to be the most effective method for the boron removal in aqueous solutions, given its simplicity and effectiveness at neutral pH levels even at low B concentrations though adsorbents need to be regenerated as well. During the adsorption, boron removal is achieved through the selective formation ofborate complexes based on covalent or ionic bonds between boron species and functional groups, such as hydroxyl groups (-OH), present on the adsorbent's surface. While ion exchange is effective, it necessitates improved regeneration methods to reduce chemical usage. Similarly, expensive membrane filtration results in concentrated boron in backwash effluent, posing potential discharge issues.

[0011] To address this concern, the focus has shifted towards efficient, boron-specific adsorbents. Notably, functionalized carbon-based materials like graphene oxide (GO) have gained prominence due to their availability, eco-friendliness, and high adsorption capacity in comparison to commercial ion exchange resins.

[0012] There is potential of using nitrogen-doped graphene oxide (NGO) for seawater boron removal. With an impressive adsorption capacity of 6.55 mg / g, NGO demonstrates remarkable performance, as indicated by the Langmuir adsorption isotherm with a capacity of 58.7 mg / g. This enhanced capacity arises from stronger bonding between boron compounds and NGO's adjacent hydroxyl groups of quaternary nitrogen doping. Notably, NGO can be conveniently regenerated through acid treatment, contributing to its appeal as an adsorbent.

[0013] Despite its high potential, NGO faces limitations hindering commercial application. Firstly, as a two-dimensional (2D) material, NGO is prone to stack and block the water transport pathway, reducing the boron removal rate and increasing the cost of the final boron rejection column. Secondly, previous methods of collecting NGO employed repeated cycles of water washing and centrifugation. Nevertheless, this technique presented difficulties in achieving a proper separation between the NGO and the aqueous solution, resulting in a modest yield of approximately 30%. This yield was additionally impacted by elevated material costs linked to the process. Thirdly, the inherent property of NGO to readily expand due to its low density has led to a notable upsurge in the usage of chemicals required for adsorbent regeneration. In addition to the points mentioned earlier, the pronounced superhydrophilicity of pure NGO poses a considerable obstacle in effectively separating water from mixed solutions containing the adsorbent. This challenge consequently hampers the overall production yield of purified water and drives up the expenses related to the adsorbent.

[0014] To counteract these limitations, it becomes crucial to pioneer the development of advanced, cost-effective, and large-scale NGO-based composites, thereby mitigating the need for excessive chemical consumption during the regeneration process.SUMMARY OF THE INVENTION

[0015] The escalating scarcity of high-quality water for potable, industrial, and agricultural use necessitates innovative methods to augment freshwater supply from unconventional sources. This invention aims to treat brackish water by removing or lowering boron (B) and chloride (Cl ) levels in the water to meet water quality standards of B < 0.5 mg / L and Cl' < 250 mg / L.

[0016] In order to achieve this objection, the invention uses an adsorbent, OQAS- AC@NGO, for the boron removal. This adsorbent combines organosilicon quaternary ammonium chloride (OQAS) modified activated carbon (AC) via silane coupling reaction within a self-assembled nitrogen-doped graphene oxide (NGO) structure.

[0017] In an embodiment, before boron removal, a capacitive deionization (CDI) system may be used, employing OQAS-AC as the anode and COOH-modified AC as the cathode for chloride ion electrosorption. Through this cooperation procedure, the treated water meets the required quality standards while significantly reducing chemical costs by approximately 13.5 times when compared to conventional ion exchange resin methods for adsorbent regeneration.

[0018] The OQAS-AC@NGO adsorbent sustains its efficiency through multiple cycles, outperforming non-assembled NGO in terms of yield, performance, and water flux. This underscores the cost-effective adsorbent preparation and enhanced water productivity. Economic analysis endorses the integrated CDI & B adsorption approach, advancing both sustainable water treatment processes and practical industrial applications.

[0019] In one aspect, the present invention provides an adsorbent for treating water, the adsorbent is made of a composite material comprising an organosilicon quaternary salt and a carbon-based material, wherein the composite material removes an amount of boron present in the water that is being treated.

[0020] In various embodiments, the organosilicon quaternary salt is organosilicon quaternary ammonium chloride.

[0021] In various embodiments, the organosilicon quaternary ammonium chloride is modified with activated carbon. By “modified”, it is meant to include any chemical or physical modification to the organosilicon quaternary ammonium chloride, for example its surface. It includes any modification that results in functionalizing the surface of the organosilicon quaternary ammonium chloride.

[0022] In an embodiment, this “functionalization” is achieved by the silane coupling reaction which results in the surface of the organosilicon quaternary ammonium chloride- activated carbon composite being positively charged.

[0023] In various embodiments, the carbon-based material is graphene oxide.

[0024] In various embodiments, the graphene oxide is self-assembled N-doped graphene oxide.

[0025] In various embodiments, the graphene oxide is nitrogen doped.

[0026] In various embodiments, the composite material is an organosilicon quaternary ammonium chloride modified activated carbon and nitrogen-doped graphene oxide composite.

[0027] In various embodiments, the mass ratio of the organosilicon quaternary ammonium chloride modified activated carbon and nitrogen-doped graphene oxide composite is any one selected from the group consisting of 2:1 , 1 :1 , 1 :2 and 1 :3.

[0028] In another aspect of the invention, there is provided a water treatment device comprising an adsorbent according to an aspect of this invention.

[0029] In various embodiments, the water treatment device further comprises a capacitive deionization system for removing chloride ions.

[0030] In various embodiments, the capacitive deionization system comprises an organosilicon quaternary ammonium chloride modified activated carbon anode and COOH- modified activated carbon cathode.

[0031] In yet another aspect of the invention, there is provided a method of removing an amount of boron present in an aqueous solution, the method comprising: (a), contacting a boron removal medium with the aqueous solution; and (b). separating the boron removal medium from the aqueous solution, wherein the boron removal medium comprises a carbonbased material comprising a hydrophilic portion and a hydrophobic portion.

[0032] In various embodiments, the boron removal medium is a medium according to an earlier aspect of this invention.

[0033] In various embodiments, the separating comprises centrifuging or filtering the aqueous solution, or the separating comprises passing water through the boron removal medium.

[0034] In various embodiments, the method further comprises the step of removing chloride ions prior to step (a) by electrosorption.

[0035] In various embodiments, the electrosorption is carried out by providing an organosilicon quaternary ammonium chloride modified activated carbon anode and COOH- modified activated carbon cathode.

[0036] According to one aspect of the invention, there is provided a method of removing or reducing the amount of boron present in an aqueous solution by forming a borate complex. The method comprises contacting a boron removal medium with the aqueous solution to react with the boric or borax species to form the borate complex. The method further comprises separating the boron removal medium from the aqueous solution.

[0037] The boron removal medium may comprise a carbon-based material. The carbonbased boron removal medium may comprise at least one hydroxyl group and at least one pyridinic nitrogen, or pyrrolic nitrogen, or graphitic nitrogen, or amine group. In other words, there is always one or more hydroxyl groups in the carbon-based boron removal medium.

[0038] In various preferred embodiments, the carbon-based material may comprise at least two hydroxyl groups and at least one pyridinic nitrogen, or pyrrolic nitrogen, or graphitic nitrogen, or amine group.

[0039] Preferably, the carbon-based material comprises at least one of graphene, graphite, graphene oxide, carbon nanotube, activated carbon, lonsdaleite, fullerene, carbon fiber, carbon black, charcoal, and amorphous carbon.

[0040] More preferably, the carbon-based material is doped, such as nitrogen-doped.

[0041] In certain preferred embodiments, the carbon-based material may comprise nitrogen-doped (N-doped) graphene oxide or N-doped reduced graphene oxide. In one exemplified embodiment where N-doped graphene oxide is used as the boron removal medium, the boron absorption capacity can be up to 6.154 mg / g for N-graphene oxide synthesized at 50 °C hydrothermal treatment.

[0042] The hydroxyl group and the pyridinic nitrogen, or pyrrolic nitrogen, or graphitic nitrogen, or amine group of the carbon-based material may be directly covalently bound to the carbon-based material. Alternatively, the hydroxyl group and the pyridinic nitrogen, or pyrrolic nitrogen, or graphitic nitrogen, or amine group of the carbon-based material may be covalently bound to the carbon-based material via a linker small molecule.

[0043] In various embodiments, the separating may comprise centrifuging or filtering the aqueous solution, or the separating may comprise passing water through the boron removal medium.

[0044] According to another aspect of the invention, there is provided a method of forming the present boron removal medium for use in removing or reducing the amount of boron present in an aqueous solution, wherein the boron exists in a form of boric acid or borate ions in the aqueous solution.

[0045] In various embodiments where the boron removal medium comprises a nitrogen- doped graphene oxide, the method comprises oxidizing graphite to graphene oxide, followed by doping the graphene oxide with ammonia to thereby form the N-doped graphene oxide. By subjecting the graphene oxide to N-doping, amine groups can be grown on the graphene oxide in order to enhance the boron absorption ability.

[0046] The N-doped graphene oxide may comprise at least one hydroxyl group and at least one pyridinic nitrogen, or pyrrolic nitrogen, or graphitic nitrogen, or amine group. The hydroxylgroup may come from the oxidation of the graphite material. Alternatively, the hydroxyl group may come from the transformation of a function group, preferably a carboxyl group, or carbonyl group. In other embodiments, the hydroxyl group may come from another small molecule comprising a hydroxyl group coupled to a carbon material.

[0047] In various embodiments, the nitrogen doping may come from the hydrothermal treatment of carbon materials with ammonia in an autoclave.

[0048] In further embodiments, the nitrogen doping may come from ammonia or nitrogen plasma treatment of carbon materials.

[0049] In yet further embodiments, the nitrogen doping may come from a direct synthesis of nitrogen doping of carbon materials.

[0050] In other embodiments, the nitrogen doping may come from N+ ion-irradiated carbon materials.

[0051] In still further embodiments, the nitrogen doping may come from a thermal treatment of carbon materials with ammonia.

[0052] Alternatively, the nitrogen doping may come from a chemical treatment of carbon materials, preferably hydrazine, or other small molecule with nitrogen or amine group coupled to a carbon material.

[0053] Another advantage of the present boron removal medium is the ease of regenerating a used medium. Accordingly, another aspect of the invention relates to a method of regenerating a used boron removal medium. The method comprises contacting the used boron removal medium with an acid, followed by rinsing the used boron removal medium with deionized water.

[0054] Suitable acids include, but not limited to, sulfuric acid (H2SO4) and hydrochloric acid (HCI).

[0055] The presently disclosed boron removal medium and method of removing or reducing the amount of boron present in an aqueous solution may be extended to a method of detecting and quantifying the amount of boron present in an aqueous solution.

[0056] In an embodiment of the invention, the invention provides integrating organosilicon quaternary ammonium chloride (OQAS) modified activated carbon (AC) within a selfassembled nitrogen-doped graphene oxide (NGO) matrix, yielding a NGO-based composite (OQAS-AC@NGO) for boron removal from brackish water.

[0057] Advantageously, the invention offers three key benefits: (i) OQAS bonds with AC, introducing positively charged surfactants (-N+) to AC surfaces; (ii) the self-assembly of positively charged AC and negatively charged NGO enhances the NGO layers' spacing and composite density, resulting in a high adsorbent utilization and water flux, as well as areduction in bed volume; (iii) the incorporation of OQAS-AC within the NGO matrix enhances the hydrophobic properties due to the alkyl chains present, enhancing the separation rate between the adsorbent and water. This minimizes toxicity of produced water and reduces environmental damage. The hydrophobic property of the material is due to the introduction of OQAS.

[0058] The surface of NGO linked to AC, bonded through an -N+group attached to an alkyl chain, forms a hydrophobic layer. This layer diminishes the adsorption and retention of water molecules on the NGO's surface, potentially facilitating the creation of water flow channels within the NGO-AC-NGO structure, thereby enhancing water flux during filtration.

[0059] The combination of these factors results in a composite that not only enhances water transport rate and quality without sacrificing adsorption capacity, but also reduces costs by utilizing affordable AC.

[0060] In another embodiment of the invention, a GDI desalination process is introduced as a preliminary stage before the boron adsorption column.

[0061] This CDI process effectively removes chloride ions, addressing issues such as equipment corrosion and environmental contamination. By adhering to public drinking water standards, which mandate chloride levels below 250 mg / L, this approach ensures water safety and quality.

[0062] Advantageously, integrating both layouts (using the OQAS-AC@NGO composite to remove boron and CDI desalination process) enhances plant economics, safety, and flexibility, ensuring compliance with boron and chloride standards while minimizing equipment deterioration and reducing investments.

[0063] Additionally, the combination of the CDI system can be scaled up using the OQAS- AC as the anode and carboxyl group (COOH) modified AC (COOH-AC) as cathode to achieve the salts ion removal. This results in the reduction in required time and cost of materials. The CDI process operates by utilizing electrically charged electrodes to remove salt ions from water, while the boron adsorption system eliminates boron from the water based on the residence time of the solution passing through it.

[0064] Despite both processes being compartmentalised within an integrated system, they can retain their distinct functions. This cooperative approach yields a final water product that satisfies the stringent quality criteria set for a 2nd RO replacement process, namely the boron remains under 0.5 mg / L and chloride levels are kept below 250 mg / L. This integrated process refers to the replacement of second stage of reverse osmosis (RO) treatment, where seawater undergoes further filtration to remove additional salts and impurities after the first RO process and then the effluent is used to this treated process. Or it may be carried out after water isbeing treated by a device or method of this invention. This step is used to improve water quality and achieve higher levels of desalination. Moreover, this system manages to attain these exacting water quality benchmarks while concurrently reducing energy consumption, curtailing the use of chemicals, and minimizing the initial capital investment required.

[0065] Advantageously, this invention contributes to the advancement in the field of water treatment, being both academically insightful and industrially applicable.BRIEF DESCRIPTION OF THE DRAWINGS

[0066] The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:

[0067] Figure 1 shows the synthesis of OQAS-AC@NGO;

[0068] Figure 2 (a-b) FT-IR spectrum of AC, OQAS-AC, GO, NGO, and OQAS-AC@NGO; (c) optimization of OQAS dosage in OQAS-AC@NGO composite with 1 / 2 mass ratio of OQAS- AC to NGO; (d) optimization of mass ratio of OQAS-AC to NGO with optimal OQAS dosage;

[0069] Figure 3. Overview of the integrated CDI & Boron adsorption system;

[0070] Figure 4. The chloride and boron removal performance after CDI & B adsorption system using the optimal materials combination: [OQAS-AC / / COOH-AC as CDI device -> OQAS-AC@NGO as boron adsorbent];

[0071] Figure 5. (a) Evaluating the output water quality after two types of integrated system: CDI & B adsorption system and (b) dual-adsorption system.

[0072] Figure 6. Evaluating the chemical consumption during adsorbent regeneration and multi-cycle regeneration performance for OQAS-AC@NGO at different mass-to-volume ratio of the adsorbent to feed water: (a) 1 mg / L, (b) 2 mg / L and 4 mg / L;

[0073] Figure 7. The synthesis of self-assembled NGO and no-assembled NGO (a) and corresponding to the SEM images: (b and d) no-assembled NGO and (c and e) self-assembled NGO;

[0074] Figure 8. Zeta potential of OQAS-AC and NGO;

[0075] Figure 9. The fitted XPS spectra of the obtained NGO;

[0076] Figure 10. Analyzing the chemical stability of OQAS-AC@NGO composite after 30 cycles regeneration by XPS tests;

[0077] Figure 1 1 . Comparison of the boron removal performance and the quaternary N content between no-assembled NGO (N-GO) and assembled NGO;

[0078] Figure 12. (a-d) A symmetrical CDI device based on AC and OQAS-AC for salt removal to figure out the optimal OQAS-AC samples. Wherein, the feed solution is the 500 mg / L pure NaCI solution;

[0079] Figure 13. (a-b) The asymmetric CDI devices OQAS-AC-2 / / AC for adsorption capacity in multi-ion feed solution; and

[0080] Figure 14. The Na+, Cl , and B concentration from output water after only boron removal process.DETAILED DESCRIPTION OF THE INVENTION

[0081] Other embodiments are within the following claims and non- limiting examples. In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group.

[0082] EXAMPLE

[0083] This example sets out integrating both layouts (using the OQAS-AC@NGO composite) to remove boron and CDI desalination process to remove chloride.

[0084] MATERIAL AND METHODS

[0085] Synthesis of non-assembled NGO

[0086] Nitrogen-doped graphene oxide exhibits excellent boron selectivity in water, yet its commercial use faces challenges such as stacking, low yield (30%), and increased total costs. Additionally, the material's expanded density leads to higher chemical usage in regeneration. To overcome these issues, this invention involves integrating organosilicon quaternary ammonium chloride (OQAS) modified activated carbon (AC) into a self-assembled NGO matrix, forming OQAS-AC@NGO.

[0087] Once again, this invention offers three key advantages: (i) Self-assembly significantly boosts synthesis yield to over 90%, effectively reducing costs; (ii) OQAS-AC enhances NGO layer spacing and density, improving adsorbent utilization, water flux, and cost-efficiency; (iii) The incorporation of OQAS-AC balances the hydrophilicity and hydrophobicity of NGO based composite, creating a water flow channel within the NGO-AC- NGO structure and facilitating better separation between the adsorbent and water resulting in an increased water flux.

[0088] OQAS-AC@NGO not only meets stringent water quality standards but also drastically reduces chemical consumption during regeneration, making it a highly cost- effective solution for brackish water purification.

[0089] Aqueous ammonia (2.5%v / v with 28% concentration) was added to the dispersion of graphene oxide (GO) under stirring for 20 min. Then, the mixture was transferred to a Teflon-lined autoclave to proceed with the hydrothermal reaction at 50 °C for 5 h before natural cooling. Then, the above solution was centrifuged and then freeze-dry to obtained NGO sample with a high yield (-30%).

[0090] Synthesis of self-assembled NGO

[0091] Expanding further from the previous section, Figure 7a shows preparing selfassembled NGO. The collection method during the preparation of NGO to significantly improve the yield from approximately 30% to 90%, while also reducing the operation time. This success was achieved through the implementation of a more efficient self-assembly process, which resulted in a higher separation rate of NGO from the mixture solution. Specifically, aqueous ammonia (2.5%v / v with 28% concentration) was added to the dispersion of graphene oxide (GO) under stirring for 20 min. Then, the mixture was transferred to a Teflon-lined autoclave to proceed with the hydrothermal reaction at 50 °C for 5 h before natural cooling. NGO was collected by adding ethanol to the final mixture solution slowly after the hydrothermal reaction.

[0092] During this process, self-assembly of NGO nanosheets at a water-ethanol interface, which helps form honeycomb NGO with a porous morphology. Then, the assembled solution mixture was centrifuged. Repeat the above steps to remove excess ammonia until the pH of the solution is neutral. Then, the above solution was centrifuged and then freeze-dry to obtained NGO sample with a high yield. In contrast to no-assembled NGO (Figure 7b & 7d), the morphology of self-assembled NGO by SEM analysis showed the well-defined and interlinked nanostructure with rich pores in a state of like-aerogel (Figure 7c & 7e). The selfassembly process effectively circumvents a phenomenon in which smaller and thinner NGO particles are often excluded during centrifugation. Additionally, the interlinked nanostructure further enhances the adsorbent's active sites, leading to improved performance.

[0093] Synthesis of OQAS-AC

[0094] Firstly, disperse AC power in deionized water, and stirred it at 25 °C for 0.5 h and then filtered it. Repeated the process 3-5 times until the effluent conductivity below 10 us / cm. The collected solid was dried at 120 °C for 2 h to obtained pretreated AC. Then, 1.5 g pretreated AC was poured into the beaker, and different dosage (1 , 5, 7, 9 mL) of quaternary ammonium salt (OQAS, SiO3C26H58N+CI_, 42 wt% in methanol) solution and deionized water (40 mL in total) was added in sequence. The beaker was placed in 80 °C water bath with constant stirring at 100 r / min. After 24 h, the resulting product was filtered and washed several times with absolute ethanol and deionized water until there was no obvious foamy substance. The treated product was finally freeze-dried, named as OQAS-AC (named as OQAS-AC-1 ,OQAS-AC-2, OQAS-AC-3, and OQAS-AC-4 corresponding to the different dosage of OQAS) with more cationic groups.

[0095] Zeta potential is a crucial factor in understanding the surface charge and stability of particles suspended in liquids. Figure 8 illustrates the Zeta potential of OQAS-AC and NGO in an aqueous solution at neutral pH. OQAS-AC exhibits a Zeta potential of 20.55 mV, indicating a substantial positive charge on its surface. In contrast, NGO maintains a negative Zeta potential, measuring at -17.30 mV. The opposing charges of OQAS-AC and NGO promote their attraction, facilitating assembly through hydrogen bonding between positive and negative charges. This assembly process is vital for the formation of a stable OQAS-AC@NGO composite.

[0096] Synthesis of OQAS-AC(a)NGO

[0097] 5 mg of OQAS-AC was dispersed in 40 mL of distilled water containing 10 mg ofNGO by ultrasonication for 30 min and freeze-drying after stirring for 6 h. The final composite is named OQAS-modified AC@NGO (abbreviated as OQAS-AC@NGO).

[0098] The synthesis process of OQAS-AC@NGO adsorbents is displayed in Figure 1 , in which the quaternary ammonium group (-N+) is loaded onto the surface of AC through the C- O-Si covalent bond by self-condensation of the silane coupling agents, which makes the surface of the OQAS modified AC permanently positively charged. Then, the self-assembly process between positively charged OQAS-AC and negatively charged NGO takes place, resulting in the formation of the OQAS-AC@NGO composite. The surface of NGO linked to OQAS-AC, bonded through an -N+group attached to an alkyl chain of OQAS, forms a hydrophobic layer. This layer diminishes the adsorption and retention of water molecules on the NGO's surface, potentially facilitating the creation of water flow channels within the NGO- AC-NGO structure, thereby enhancing water flux during filtration. This enhancement facilitates an improved separation rate between the adsorbent and water.

[0099] Synthesis of COOH-AC

[0100] For fabricating the COOH-AC, 5 g pretreated AC powder was added to 40 mL HNOa solution (1 :1 ), and the mixture was stirred and heated at 60 °C for 2 h. The solution was then filtered after cooled down, and the solid was washed with deionized water several times until the effluent pH was the same as that of pure water. Finally, the obtained product was dried, named as COOH-AC with more anionic groups.

[0101] Synthesis of brackish water

[0102] More elements in the synthetic feed water were included to better simulate the brackish water, involving in sodium (Na+, 241 mg / L), chloride (Cl-, 400 mg / L), boron (B, 1 .8 mg / L), calcium (Ca2+, 2.3 mg / L), magnesium (Mg2+, 6.5 mg / L), potassium (K+, 10 mg / L) andsulphate (SO42; 6.0 mg / L). This synthetic brackish water was used in the subsequent experiments as the feed water, which is consistent with the low-pressure reverse osmosis (LPRO) feed water quality.

[0103] CPI device fabrication and desalination test

[0104] The electrochemical deionization experiments were conducted through the assembling of OQAS-AC / / COOH-AC (anodeZ / cathode) device with the optimal mass ratio of 1 :1. Compared with pure AC, the COOH-AC was used as the cathode to have more anionic groups on AC and the OQAS-AC with more cationic groups was used as the anode. Therefore, the negatively charged ions can be easily adsorbed by OQAS-AC whereas the electrosorption of positively charged ions can occur in COOH-AC, resulting in an enhancement of the adsorption capacity.

[0105] For the preparation of electrodes, the prepared active materials, carbon black, and Poly (vinylidene fluoride) (PVDF) at a mass ratio of 8:1 :1 , and the total mass of each electrode is approximately 10 mg with the area of (2 x 2) cm2coating onto the graphite paper substrate.

[0106] The device assembly consists of COOH-AC as cathode with cation exchange membrane (CEM), separator, and OQAS-AC as anode with anion exchange membrane (AEM).

[0107] For the desalination test, pure NaCI solution (500 mg / L) for initial figuring out the optimal OQAS-AC anode material or the synthetic brackish water for the evaluation of chloride removal in the integrated system, is passing the GDI device and circulated back from the GDI outlet to the feed tank by using a pump. The flow rate is controlled at 50 mL / min. A conductivity meter is used to monitor and record the conductivity of the feed solution under the stirring conditions. A constant current is applied through the deionization device (the current density is 0.1 A / g).

[0108] For the pure NaCI solution as feed water, the salt removal capacity of deionization is expressed as: F= (Cf - CO) x V I Mt. Where Cf and CO are the final and initial NaCI concentrations (mg / L), V is the volume of the feed NaCI solution (L) and Mt is the mass of anode in this work.

[0109] Operation of the integrated CPI & boron adsorption system

[0110] The CDI is here much smaller, allowing reducing salinity only, while the boron reduction is taken care of by the adsorption column. As shown in Figure 3, the synthetic brackish water first passes through the CDI device using a peristaltic pump with 50 mL / min, focusing on the Cl ion removal through charge-discharge-charge cycles to achieve the maximum salt adsorption. Specifically, the CDI system involves an anode of OQAS-AC(positive electrode) with net positive surface charge and a cathode of COOH-AC (negative electrode) with net negative surface charge.[0011 1] The electrodes are connected by an external circuit with an applied voltage range of -1.4-1.4 V, and then ions from the feed stream migrate to the interfaces at the water / electrodes. Salt removal in this CDI system was achieved in a manner with cell charging leading to ion sorption to the electrodes while cell discharging by short-circuiting the anode and cathode led to ion desorption.

[0112] Then, the effluent flow continuously into the adsorption system to remove boron. In a typical boron removal experiment, the ratio is always maintained at 1 mg of adsorbent per 1 mL of inlet water for all tests and let the mixture solution stand for 7 h followed by this filtration process. The filtered solution was collected for analyzing the ions’ concentration to evaluate the output water quality by ion chromatography (IC) for Na+, Ca2+, Mg2+, K+, Cl; SCV' and inductively coupled plasma optical emission spectroscopy (ICP) for B.

[0113] Estimation of water flux during the adsorption process

[0114] During the boron adsorption removal steps, following a specific adsorption period, the mixed solution containing the adsorbent undergoes separation of the adsorbent and outlet water using a filtration device. The filtration membrane utilized is a nylon membrane with a pore size of 0.22 pm. The time taken for a certain volume of the mixed solution to pass through the same device (the flow area is n*1 .9*1 .9 cm2) is used to estimate the water flux, measured in L nr2h’1. Following the separation process, the adsorbent is regenerated for reuse, while the filtrate undergoes ICP testing to analyze the water quality.

[0115] Regeneration of adsorbent

[0116] After the CDI & boron adsorption processes, the regeneration of adsorbents (borate bonded OQAS-AC@NGO) needs to be performed. The regeneration process is in the following order: DI water rinsing -»■ HCI soaking -> DI water rinsing -»■ NaOH soaking -> DI water rinsing, which constitute one cycle of adsorbent regeneration. Wherein, the adsorbents were immersed in 5% hydrochloric acid (HCI) for half an hour to break down the formed association of the borate-nanocomposites complex and release the chemically adsorbed boron, and another half-hour immersion in 2.5% sodium hydroxide (NaOH) solution to restore the functional sites. Then, the regenerated adsorbents were ready to be reused in the next boron removal process.

[0117] Above steps retest no less than 60 cycles, and the solid adsorbent obtained by suction filtration of the resulting filtrate, is subjected to XPS analysis to evaluate the adsorbent’s chemical stability. To ensure the accuracy of ion concentration, ultrapure water with pH of 6 is used to prepare the synthetic feed water, and the solution pH then increasedto ~ 6.5 due to production of HCO3 from dissolved CO2. Hence, during the regeneration process, the feed water's pH is consistently maintained at approximately 6.5, ensuring the absence of any residual chemical substances.

[0118] RESULTS AND DISCUSSION

[0119] Optimization of Adsorbent for Boron Removal

[0120] The synthesis process of OQAS-AC@NGO adsorbents is illustrated in Figure 1 , in which the quaternary ammonium group (-N+) is loaded onto the surface of AC through the C- O-Si covalent bond by self-condensation of the silane coupling agents, which makes the surface of the OQAS modified AC permanently positively charged. On the other hand, we optimized the collection method during the preparation of the NGO to significantly improve the yield from approximately 30% to 90%, while also reducing the operation time. This success was achieved through the implementation of a more efficient self-assembly process, which resulted in a higher separation rate of NGO from the mixture solution (Figure 7a).

[0121] In contrast to no-assembled NGO (Figure 7b), the morphology of self-assembled NGO by SEM analysis showed the well-defined and interlinked nanostructure with rich pores in a state of like-aerogel (Figure 7c). The self-assembly process effectively circumvents a phenomenon in which smaller and thinner NGO particles are often excluded during centrifugation. Additionally, the interlinked nanostructure further enhances the adsorbent's active sites, leading to improved performance. Then, the self-assembly process between positively charged OQAS-AC and negatively charged NGO takes place, resulting in the formation of the OQAS-AC@NGO composite. Zeta potential is a crucial factor in understanding the surface charge and stability of particles suspended in liquids. Figure 8 illustrates the Zeta potential of two materials, OQAS-AC and NGO, in an aqueous solution at a neutral pH. Specifically, the Zeta potential of OQAS-AC was 20.55 mV indicating that the OQAS-AC surface carries a significant positive charge. On the other hand, the Zeta potential values of NGO remained negative, measuring at -17.30 mV. The opposite charges of OQAS- AC and NGO made them easily attract each other, facilitating the assembly of the two materials through hydrogen bonding between positive and negative charges. This assembly process is crucial for creating stable OQAS-AC@NGO composite.

[0122] Figure 2a-b displays the FT-IR spectra of AC, OQAS-AC, NGO, and OQAS- AC@NGO to analyze the chemical structure. The strong absorption peak at -3400 cm-1corresponds to the stretching vibration absorption peak of -OH from absorbed water for all samples. Upon covalent linkage of OQAS, which possesses an aliphatic chain and a siloxane- anchoring group, increased C-H stretching (3000-2850 cm-1), and vibrations of Si-O-Si and C- O-Si bonds (1 150-1250 cm1) are observed from OQAS-AC and OQAS-AC@NGO. Thesepeaks confirm the presence of silane on the AC surface, indicating that OQAS forms a covalent bond with the hydroxyl group of AC through dehydration. Besides, a poor new peak corresponding to quaternary ammonium (C-N stretching) groups (1631 cm1) is observed after functionalization. On the other hand, GO, NGO, and OQAS-AC@NGO all exhibit typical adsorption bands at 1726 cm1(v(C=O) from carboxyl groups), 1623 cm1(v(sp2-C=C) from the graphitic region), 1595 cm ’ (v(-NH2)), and 1050-1250 cm1(v(C-O-C)). In the OQAS- AC@NGO spectrum, unlike GO and OQAS-AC, the gradual strong peaks at 3238 cm-1(N-H) and 1401 cm1(from O-H bending in plane, as well as the gradual weak of -NH2 group and the shift of C-O-C group indicate the successful doping of nitrogen. The new peak at 1 166 cm1(C-O-Si) was observed confirming the introduction of OQAS-AC. Notably, the intensity of the carboxyl group peak is significantly decreased, indicating the consumption of these groups during the self-assembly between NGO and OQAS-AC. Above results demonstrate that OQAS-AC was successfully linked onto NGO via the formation of hydrogen bond.

[0123] To further optimize the dosage of OQAS and the mass ratio of OQAS-AC to NGO, the adsorption capacity of boron is a key factor. Theoretically, nitrogen atom on the NGO exhibits three types of sites involving pyridinic N, pyrrolic N and quaternary N, in which the quaternary N atom is bonded with two adjacent hydroxyl groups suggesting the best adsorption performance of boron compounds. X-ray photon spectroscopy (XPS) was used to characterize the chemical contents of the samples to further analyzing the boron adsorption capacity. Based on the fitted XPS spectra (Figure 9), the obtained NGO have three different types of nitrogen doping including pyridinic N (398.5 eV), pyrrolic N (400.2 eV), and quaternary N (401 .3 eV).

[0124] Firstly, the ratio of AC to NGO was fixed to 1 / 2 to figure out the optimal ratio of modifying agent OQAS to AC. The four OQAS-AC samples with different OQAS dosage (1 , 5, 7, 9 mL) were then incorporated into NGO to be evaluated the boron removal performance. It can be found that the OQAS-AC@NGO with 7 mL of OQAS exhibits the highest quaternary N / C and quaternary N / O atomic ratio, resulting in the highest boron adsorption capacity of 7.1 mg / g (Figure 2c). Under the optimal dosage of OQAS, a series of samples with different OQAS-AC / NGO mass ratios (2:1 , 1 :1 , 1 :2, 1 :3) were synthesized and explored the boron removal performance in simulated brackish water to further confirm the optimal mass ratio of OQAS-AC to NGO.

[0125] As shown in Figure 2d the B removal efficiency gradually increased as the proportion of NGO increased. There is a comparable B removal performance at OQAS- AC / NGO mass ratios of 1 / 2 and 1 / 3, namely the B concentrations from output water were all below 0.5 mg / L. However, it is difficult to separate the clean water and the adsorbent due tothe layer-by-layer stacking of NGO at OQAS-AC / NGO mass ratios of 1 / 3. Difficult separation of the adsorbent increases the operating cost in the later stage of adsorption column. Besides, the high material cost of NGO also limited OQAS-AC@NGO (AC: NGO = 1 :3) as the promising adsorbent. It also highlights the significance of using an optimal amount of OQAS-AC, which can balance the hydrophobic properties of the composite, thereby achieving the best separation efficiency between the adsorbent and water.

[0126] Therefore, considering B removal performance and operating cost, the optimal OQAS-AC / NGO mass ratio is 1 :2, referred to as OQAS-AC@NGO in the following description.

[0127] OQAS-AC as Anode and COOH-AC as Cathode for Improved Adsorption Capacity

[0128] To emphasize the significance of introducing OQAS for electrosorption capacity, desalination experiments were initially conducted using a symmetrical GDI device based on AC and OQAS-AC. As can be seen from Figure 12, upon modification with appropriate amount OQAS, an enhanced adsorption capacity of OQAS-AC-2 was measured at 48.13 mg / g, more than 6~7 times higher than that of AC (7.13 mg / g). An asymmetrical device boosts higher charge utilization, leading to greater energy efficiency and reduced energy consumption, while also demonstrating shorter adsorption-desorption times and improved cycling stability compared to its symmetric counterpart. Thus, OQAS-AC-2 was chosen as the anode material to assemble OQAS-AC-2 / / AC device with the mass ratio of 1 :1 to further confirm the adsorption capacity and stability performance in the multi cycles desalination process under the synthetic blackish water. As shown in Figure 13, the adsorption capacity of salt for OQAS- AC-2 / / AC device in the multi-ions’ solution shows a small decrease to 36.95 mg / g, which is due to the selectivity and competition from various ions. Although, the remarkable deionization performance of the OQAS-AC-2 / / AC device can be attributed to two key factors.

[0129] Firstly, it is evident that the more stable binding of quaternary ammonium groups to anions plays a pivotal role. These groups, characterized by positively charged nitrogen atoms, are highly effective in attracting and retaining negatively charged ions (anions) during the deionization process. This stable binding prevents desorption and ensures an efficient and sustained removal of ions, as demonstrated by the OQAS-AC-2 / / AC cell's stable desalination performance over 100 cycles.

[0130] Secondly, OQAS-AC-2 provides a greater number of sorbent sites on its surface compared to pure AC. This is due to its rougher surface, featuring numerous irregular protrusions, as observed in previous research. These findings collectively highlight the importance of both the stable binding of quaternary ammonium groups and the enhanced surface properties of OQAS-AC-2 in achieving superior deionization performance. For thesubsequent experiments, OQAS-AC-2 is denoted as OQAS-AC when used as the anode material in the GDI device for the chloride removal.

[0131] Compared to using pure AC, the incorporation of COOH-AC as the cathode in the GDI device offers an increased number of anionic groups. In parallel, OQAS-AC, enriched with more cationic groups, serves as the anode. This strategic configuration enables efficient adsorption of negatively charged ions by OQAS-AC, while COOH-AC facilitates the electrosorption of positively charged ions, thereby enhancing the overall adsorption capacity of the CDI cathode.

[0132] Analyzing the CL and B Removal Performance After the Integrated CDI & Boron Adsorption System

[0133] As illustrated in Figure s, an integrated CDI desalination and boron removal process was performed. The optimal material combination, namely [OQAS-AC / / COOH-AC as CDI device— >OQAS-AC@NGO as boron adsorbent], were applied for salt and boron removal based on the synthetic feed solution. During the integrated processes, CDI primarily focuses on the deep removal of chloride ion, followed by an adsorption process specifically targeting boron removal.

[0134] The concentrations of the ions before and after treatment process were tested by IC (for the optimization experiments, mainly Na+and CL) and ICP for boron, listed in Figure 4. The integrated CDI & B removal process demonstrated remarkable effectiveness in reducing ion concentrations in the output water, achieving concentrations of 110 mg / L for Na+, 210 mg / L for Cl', and an impressive 0.335 mg / L for B. As expected, the product water meets the required water quality standards, with B and Cl' concentrations below the desired limits of 0.5 mg / L and 250 mg / L, respectively.

[0135] Despite the substantial changes in ion concentrations observed after both CDI and CDI & B removal, it is evident that both physical absorption and electro adsorption mechanisms play a role in the overall process. In the electro adsorption via CDI process, the attraction between Na+and Cl' ions occurs through positive and negative charges, leading to their accumulation on the electrode material's surface. This accumulation is a result of the formation of an electrical double layer induced by the applied electric field. It's crucial to highlight that the CDI process only resulted in a minor change in CL ion concentration (a reduction of 37 mg / L), whereas the B removal process demonstrated significant effectiveness in reducing the concentrations of Na+and CL in the effluent. This reduction was achieved by physically absorbing these ions into the porous boron adsorbent's void spaces during the boron adsorption process. This analysis prompts an exploration of the self-sufficiency of the boron adsorption process in meeting water quality standards. The result of Figure 14underscores the pivotal role played by the integrated CDI & B removal system. Figure 4 shows the efficacy of Cl' removal from the CDI process.

[0136] In the experiments here, the most compact CDI device to treat a specific influent volume was utilized, resulting in an effluent chloride ion concentration of less than 250 mg / L. This invention provides a seamless integration of CDI and boron adsorption processes, ensuring the optimal overall performance of our water treatment system. In short, the reversible stability of the CDI process allows for electrode material recycling, leading to cost savings and aligning with sustainability goals by reducing resource consumption and waste generation.

[0137] Evaluation of the Product Water Quality

[0138] A comprehensive analysis was conducted to assess the water quality, specifically examining the changes in ion concentrations in the treated water. The synthetic feed water consisted of seven types of ions: Na+, Cl', B, Ca2+, Mg2+, K+, and SO?'. To treat the water, a combination of CDI and boron adsorption processes with an optimal material composition was employed, as illustrated in Figure 5a.

[0139] Following each treatment process, IC and ICP tests were carried out to analyze the ion concentrations in the treated water. The results clearly indicate that, besides achieving the primary objectives of reducing chloride ion concentration to below 250 mg / L and boron concentration to below 0.5 mg / L, the treatment process significantly reduced the concentrations of other ions as well. Specifically, Cl' decreased from 400 mg / L to 210 mg / L, B decreased from 1.8 mg / L to 0.4 mg / L, Na+decreased from 241 mg / L to 122 mg / L, Ca2+decreased from 2.3 mg / L to 0.255 mg / L, Mg2+decreased from 6.5 mg / L to 0.817 mg / L, K+decreased from 10 mg / L to 2.72 mg / L, and SO ' decreased from 6 mg / L to 2.62 mg / L. These results not only meet the secondary RO effluent standards mainly for chloride ions and boron concentration but also highlight the robust removal efficiency of the electrode and adsorbent materials for chloride ions and boron, even in the presence of multiple ions. Furthermore, the incorporated processes exhibit the potential for synergistically removing other ions, suggesting broader applicability and effectiveness in enhancing overall water quality.

[0140] Using fresh OQAS-AC@NGO as the adsorbent for the two-step boron removal process, IC and ICP tests were carried out to analyze the ions’ concentration from effluent following each treatment process. As shown in Figure 5b, after a single adsorption process, the concentrations of all ions significantly decreased. Specifically, Cl- was measured at 278 mg / L, B at 0.4 mg / L, Na+at 145 mg / L, Ca2+at 1 .77 mg / L, Mg2+at 2.92 mg / L, K+at 4.85 mg / L, and SO?' at 4.28 mg / L. Despite the concentration of Cl' exceeds 250 mg / L, the boron removal process exhibits remarkable effectiveness by significantly reducing the concentrations ofboron (below 0.5 mg / L) in the effluent, along with other ions. After the second adsorption process with the 1 st treated effluent using fresh adsorbent, the ion concentrations in the output water were as follows: Cl' at 163 mg / L, B at 0.358 mg / L, Na+at 83 mg / L, Ca2+at 0.974 mg / L, Mg2+at 1.10 mg / L, K+at 2.24 mg / L, and SO ' at 3.24 mg / L, respectively. Interestingly, it exhibits lower concentrations of other ions besides B, attributed to the additional physical absorption steps utilizing the porous adsorbent during the B removal process. These results not only meet the water quality standards mainly for chloride ions and boron concentration but also highlight the robust removal efficiency of the adsorbent materials for chloride ions and boron, even in the presence of multiple ions. Above dual-adsorption process can be constructed as a parallel multi-stage adsorption column system.

[0141] Evaluating the Chemical Consumption and Adsorbent Performance After Multiple Regeneration Cycles

[0142] Once OQAS-AC@NGO is saturated with boron ions, it can be easily regenerated via backwashing with acid (5% HCI solution) to break down the formed association of the borate-OQAS-AC@NGO complex, where boron was released. This is followed by saturating the OQAS-AC@NGO with 2.5% NaOH solution for 30 minutes to neutralize the acid added and restore the functional sites. Then, the regenerated adsorbent is ready to be reused in the next boron removal process.

[0143] Here, one of the objectives is not only to meet the water quality target but also to significantly reduce chemical consumption during the regeneration process of the OQAS- AC@NGO adsorbent, aiming to reduce it to only one-sixth of the chemicals amount typically used with the NGO adsorbent. This approach plays a vital role in establishing a sustainable and highly effective water treatment process, emphasizing environmental responsibility and resource efficiency.

[0144] In previous reports, more than three bed volumes (BV), which is the volume of adsorbent per unit mass according to its density, of regenerant are used for regenerating NGO to reuse it in boron removal tests. Despite the large usage of chemicals attributed to the low density of NGO (-450 Kg / m3), 81 % of the boron removal capacity is still maintained after three rounds of regeneration (4 absorption cycles) in the laboratorial testing. In contrast, the OQAS- AC@NGO composite exhibits a denser and more rigid structure, with a measured density of approximately 750 Kg / m3.

[0145] This invention aims to reduce regenerants to only one-sixth of that of nonassembled NGO, that is 3.8 m3of 5% HCI and 166 kg NaOH for 1 MGD of product water. Consequently, 1 mg of adsorbent requires 0.0027 mL of 5% HCI solution and 0.12 mg NaOH or 0.00468 mL of 2.5% NaOH solution after producing 1 mL of product water. These valuesare set as the maximum chemical usage for regeneration of OQAS-AC@NGO in subsequent experiments.

[0146] When comparing adsorbent to product water (details can be seen in Table 1 ), the concentration of boron in the feed solution becomes a crucial factor influencing the removal capacity of the adsorbents (NGO and OQAS-AC@NGO). The maximum capacity of NGO is reported to be 2 mg / g in a 2 mg / L boron feed solution. Based on this, the mass of NGO required for producing 1 MGD (Million Gallons per Day) of product water under a 7-hour operation duration and 1 -hour regeneration duration, with three operation & regeneration cycles per day, is calculated to be 1 136.5 kg. As for the OQAS-AC@NGO composite, it exhibits a conservative removal capacity of 1.4 mg / g with a boron feed concentration of 1.8 mg / L. This results in an amount of adsorbent required per MGD of product water as 1407.1 kg.

[0147] Table 1 . Adsorbent vs. product water.

[0148] The quantity of adsorbent determines the amount of chemicals used in regeneration, which is defined by the Table 2 computing process. Consequently, 1 mg of adsorbent requires 0.0027 mL of 5% HCI solution and 0.12 mg NaOH or 0.00468 mL of 2.5% NaOH solution after producing 1 mL of product water. These values are set as the maximum chemical usage for regeneration of OQAS-AC@NGO in subsequent experiments.

[0149] Table 2. Comparation of chemical consumption for regeneration.Product water: 1 MGDChemicalMass of Effective Amount ofDensity Regeneration consumption*Adsorbent adsorbent one bed regenerant(kg / m3) rate / day 5% HCI NaOH (Kg) volume (m3) required (BV)(in’) . (kg)NGO -1136 5 -450 -2.53 3 22.8 996

[0150] Afterwards, the chemical consumption testing for OQAS-AC@NGO regeneration was conducted, and two ratios of regenerants volume to the adsorbent mass were examined in the experiment, as shown in Figure 6a. Subsequently, the performance of B removal by the adsorbent was evaluated after each regeneration. During the initial six regeneration cycles, 0.0020 mL / mg of fresh 5% HCI and 2.5% NaOH were used.

[0151] The results clearly indicated that the regenerated OQAS-AC@NGO maintained a high boron removal capacity, achieving a B concentration of less than 0.5 mg / L in the output water after six cycles of B removal operation. This represented a significant reduction in chemical consumption compared to the initial study target, where the use of about 2.82 m3of 5% HCI and 70.36 kg NaOH instead of 3.8 m3of 5% HCI and 166 kg NaOH for 1 MGD of product water was possible.

[0152] However, it was observed that the B concentration exceeded 0.5 mg / L after the sixth regeneration cycle, indicating the necessity to evaluate the adsorbent's stability for B removal after multiple regeneration cycles. To address this issue and maintain the desired B concentration in the output water, the amount of regenerants were adjusted to 0.0027 mL / mg of 5% HCI and 0.0047 mL / mg of 2.5% NaOH in the subsequent regeneration cycles. As a result, OQAS-AC@NGO demonstrated the capability to maintain a B concentration of around 0.7 mg / L in the output water after 60 regeneration cycles, highlighting the stable lifecycle of the adsorbent.

[0153] The observed slight increase in effluent concentration compared to the initial cycles can be primarily attributed to the loss of adsorbent during the filtration process after each regeneration in the lab-scale setup. Specifically, a significant portion of the adsorbent loss occurs on the suction filter paper used during the filtration step. To further verify the chemical stability of OQAS-AC@NGO adsorbent after multiple regeneration cycles, elemental analysis of nitrogen was conducted through XPS testing. Figure 10 displayed N1 s spectra of adsorbents both before and after 30 regeneration cycles, which could be fitted into three types of nitrogen sites (quaternary N, pyrrolic N, pyridinic N) with similar peak intensity, further affirming the excellent stability of the chemical bond between OQAS-AC and NGO. In future practical applications utilizing adsorption columns, the transformation of adsorbent powders into granules will play a crucial role in effectively minimizing adsorbent loss during the regeneration process, ultimately leading to the production of high-quality product water.

[0154] Further, the mass-to-volume ratio of the adsorbent (OQAS-AC@NGO) to feed water to improve effluent quality was optimized. Using the same quantity of regenerant, further tests were conducted on the multi-cycle regeneration performance of OQAS-AC@NGO at adsorbent-to-water ratios of 2 mg / mL and 4 mg / mL. As shown in Figure 6b, OQAS-AC@NGO maintained boron concentrations in the outlet water below 0.5 mg / L for over 66 cycles at both ratios than previously 1 mg / mL. Fine-tuning the adsorbent-to-water ratio enhanced boron removal efficiency without requiring additional regenerants. Throughout the process, the effluent boron concentration consistently remained below 0.5 mg / L. These results demonstrate the mechanical and chemical stability of OQAS-AC@NGO as an adsorbent.

[0155] Advantages of OQAS-AC@NGO as Adsorbent Compared With NGO

[0156] Here, the development of a composite adsorbent, OQAS-AC@NGO, has been presented through the optimization of N-GO, offering several advantages over the previously reported non-assembled N-GO. These advantages can be comprehensively discussed based on factors like yield, performance, water flux, and cost-effectiveness, as summarized in Table 3.

[0157] The self-assembled NGO exhibits a remarkable yield of approximately 90%, a substantial improvement compared to the estimated 30% yield of N-GO previously reported. Assembly process of NGO prevents smaller particles from being excluded during centrifugation, resulting in higher yield and efficient production of assembled NGO and its composite, such as OQAS-AC@NGO. As expected, both assembled NGO and OQAS- AC@NGO significantly reduce boron concentration in output water to 0.301 mg / L and 0.404 mg / L, respectively, from a 1.8 mg / L feed solution. While non-assembled N-GO only achieves 0.589 mg / L. This enhanced boron removal performance suggests that the increased ratio of quaternary N content and the honeycomb structure play a crucial role in promoting the boron removal (Figure 1 1 ).

[0158] Notably, the NGO-based composite, like OQAS-AC@NGO, shows improved water flux of about 10241 L nr2h'1while assembled NGO only with -603 L nr2h’1, indicating an enhanced layer spaces between NGO sheets by introducing the OQAS-AC. This increased water flux not only boosts water productivity but also lowers energy-related operational costs and materials’ cost due to the increased separation efficiency between the adsorbent and water.

[0159] Furthermore, the cost-effectiveness of the synthesized materials is evaluated based on estimated materials cost, with price information sourced from the Sigma website for ease of cost comparison. Assembled NGO reduces costs from 1 .25 € / mg to 0.31 € / mg due to better yield. Additionally, OQAS-AC@NGO synthesis combines assembled NGO with affordableOQAS-AC, costing -0.21 € / mg. Even though a small cost reduction, the OQAS-AC@NGO composite maintains cost-effectiveness in materials preparation, while offering enhanced adsorption performance and increased water flux. These advantages underscore the significant progress made in optimizing N-GO-based composite adsorbents, highlighting their potential for practical applications in water treatment and purification processes.Table 3. Comparation of adsorbents.B concentration of output Water flux Estimated materialsAdsorbent Yield , „ water* (mg / L) (L m’2h‘1) cost* (€ / mg)N-GO -30% 0 589 -603 -1 25Self-assembled NGO -90% 0.301 -603 -0 31Table 5. The calculation of materials’ cost for different adsorbents.

[0160] Economic Comparison

[0161] The initial RO process serves as the primary treatment step, producing brackish water with a boron concentration of 1 .8 mg / L and Cl concentration of 400 mg / L. Subsequent treatment methods are designed to meet specific output water quality requirements: a boron concentration of < 0.5 mg / L and CL concentration below 250 mg / L. These methods include the 2nd RO process, ion exchange resin (Amberlite IRA743), single adsorption column, andan integrated CDI & boron adsorption system. Economic comparisons presented in Table 4 incorporate detailed operating conditions and associated costs.

[0163] Analyzing the data provided, it's evident that the 2ndRO process carries the highest estimated operational cost due to its greater energy demands, ranging from €1.97 to €3.60 per m3of water product. Consequently, it appears less economically favorable compared to the alternative methods. In terms of economic applicability, the following observations can be made for the replacements to the 2nd RO process. Firstly, the "Ion Exchange Resin (Amberlite IRA743)" method incurs relatively higher chemical costs and moderate energy costs compared to other options. While its pH requirements might be more manageable in specific scenarios, its high metal selectivity limits its application to boron removal or low-saline water. In contrast, the adsorption process exhibits excellent suitability for diverse feed waters, offering flexibility in both design and operational conditions. The "Single Adsorption Column" method effectively removes boron while maintaining lower chemical costs. However, the elevated chloride level in the output water could be a concern depending on the application. When combined with a CDI process to deeply remove chloride prior to the boron adsorption, the "Integrated CDI & B Adsorption System" delivers the desired output water quality with low boron and chloride levels while boasting the lowest chemical costs and energy costs comparable to the "Ion Exchange Resin (Amberlite IRA743)" method.

[0164] Among the replacements, the ion exchange resin process, although effective, entails higher chemical costs. The integrated GDI & B adsorption approach presents a well- balanced solution that effectively aligns with water quality targets and considerations of costefficiency.

[0165] CONCLUSION

[0166] In response to the escalating demand for alternative water sources, this study addresses the challenge of boron and chloride removal from brackish water through the development of an innovative CDI & B adsorption integrated system. The synthesized OQAS- AC@NGO composite exhibited remarkable performance in efficiently removing boron. Through the synergistic combination of CDI using the device of OQAS-AC / / COOH-AC for chloride electrosorption, the integrated system not only met water quality targets (B < 0.5 mg / L and CP < 250 mg / L) but also significantly reduced chemical consumption during adsorbent regeneration in comparison to the traditional ion exchange resin method. After 66 regeneration cycles, the OQAS-AC@NGO composite still maintained below 0.5 mg / L B in the output water at adsorbent-to-water ratios of 2 mg / mL and 4 mg / mL, suggesting the excellent stability. Additionally, it offers advantages over non-assembled N-GO in terms of yield, performance, and water flux, thereby demonstrating a cost-efficient method of adsorbent preparation and enhancing water productivity. Further, an economic analysis revealed that the integrated CDI & boron adsorption approach emerged as a promising solution due to its ability to achieve stringent water quality standards while minimizing chemical and energy costs, resulting the reduction of operational cost. This approach balances technical efficacy with economic feasibility, making it a strong candidate for addressing water scarcity challenges. As water resources continue to deplete, this work establishes an innovative path forward, showcasing the potential for advanced materials and integrated treatment processes to enhance water treatment efficiency, economic viability, and environmental sustainability.

[0167] Non-limiting examples of the invention and comparative examples will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention.

[0168] While embodiments of the invention have been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.

Claims

CLAIMS1 . An adsorbent for treating water, the adsorbent is made of a composite material comprising an organosilicon quaternary salt and a carbon-based material, wherein the composite material removes an amount of boron present in the water being treated.

2. The adsorbent according to claim 1 , wherein the organosilicon quaternary salt is organosilicon quaternary ammonium chloride.

3. The adsorbent according to claim 2, wherein the organosilicon quaternary ammonium chloride is modified with activated carbon.

4. The adsorbent according to any one of the preceding claims, wherein the carbon-based material is graphene oxide.

5. The adsorbent according to claim 4, wherein the graphene oxide is selfassembled N-doped graphene oxide.

6. The adsorbent according to claim 4, wherein the graphene oxide is nitrogen doped.

7. The adsorbent according to any one of the preceding claims, wherein the composite material is an organosilicon quaternary ammonium chloride modified activated carbon and nitrogen-doped graphene oxide composite.

8. The adsorbent according to claim 7, wherein the mass ratio of the organosilicon quaternary ammonium chloride modified activated carbon and nitrogen- doped graphene oxide composite is any one selected from the group consisting of 2:1 , 1 :1 , 1 :2 and 1 :3.

9. A water treatment device comprising an adsorbent according to any one of claims 1 to 8.

10. The water treatment device according to claim 9, further comprising a capacitive deionization system for removing chloride ions.

11. The water treatment device according to claim 10, wherein the capacitive deionization system comprises an organosilicon quaternary ammonium chloride modified activated carbon anode and COOH-modified activated carbon cathode.

12. A method of removing an amount of boron present in an aqueous solution, the method comprising:(a). contacting a boron removal medium with the aqueous solution; and(b). separating the boron removal medium from the aqueous solution, wherein the boron removal medium comprises a carbon-based material comprising a hydrophilic portion and a hydrophobic portion.

13. A method according to claim 12, wherein the boron removal medium is a medium according to any one of claims 1 to 8.

13. The method according to claim 12, wherein the separating comprises centrifuging or filtering the aqueous solution, or the separating comprises passing water through the boron removal medium.

14. The method according to any one of claims 12 or 13, further comprising the step of removing chloride ions prior to step (a) by electrosorption.

15. The method according to claim 14, wherein the electrosorption is carried out by providing an organosilicon quaternary ammonium chloride modified activated carbon anode and COOH-modified activated carbon cathode.

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