Removal technology of boron dissolved in aqueous solutions – A review

Reclamation of boron from solid and liquid streams for fertilizer application

Boron (B) is a crucial element for efficient plant growth and development; therefore, B-based fertilisers have been employed in agricultural applications. The need for B-based fertilisers for agricultural uses is continuously increasing as a result of the world's growing population. It is expected that the global market for B-based fertiliser will grow by around $6.3 billion by 2032; hence, demand for B sources will also increase. In addition to being used in fertiliser, B is also employed in the production of neodymium iron B (NdFeB) permanent magnets. The demand for NdFeB magnets is also continuously increasing. Hence, it is of the utmost importance to reclaim B from secondary resources due to the rising demand for B in a wide variety of applications. This review study addresses the recovery of B from various waste streams. The main focus is on the recovery of B from spent NdFeB magnets, borax sludge, and liquid streams such as brine water, seawater, sewage, industrial wastewater, and agricultural effluents. Different technologies for B recovery are discussed, such as sorption, solvent extraction, membrane processes, precipitation, and hydrometallurgical methods. Solvent extraction has been found to be a very effective approach for reclaiming B from spent NdFeB magnet waste and from liquid streams with high B concentration (>1-2 g/L). Further, the application of B-based fertiliser in agriculture application is reviewed. Challenges associated with B recovery from waste streams and future perspectives are also highlighted in this review.

Preparation and Performance Study of Boron Adsorbent from Plasma-Grafted Polypropylene Melt-Blown Fibers

In this study, the plasma graft polymerization technique was used to graft glycidyl methacrylate (GMA) onto polypropylene (PP) melt-blown fibers, which were subsequently aminated with N-methyl-D-glucamine (NMDG) by a ring-opening reaction, resulting in the formation of a boron adsorbent denoted as PP-g-GMA-NMDG. The optimal conditions for GMA concentration, grafting time, grafting temperature, and the quantity of NMDG were determined using both single factor testing and orthogonal testing. These experiments determined the optimal process conditions to achieve a high boron adsorption capacity of PP-g-GMA-NMDG. Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), energy dispersion spectrum analysis (EDS), and water contact angle measurements were performed to characterize the prepared adsorbent. Boron adsorption experiments were carried out to investigate the effects of pH, time, temperature, and boron concentration on the boron adsorption capacity of PP-g-GMA-NMDG. The adsorption isotherms and kinetics of PP-g-GMA-NMDG for boron were also studied. The results demonstrated that the adsorption process followed a pseudo-second-order kinetic model and a Langmuir isothermal model. At a pH of 6, the maximum saturation adsorption capacity of PP-g-GMA-NMDG for boron was 18.03 ± 1 mg/g. In addition, PP-g-GMA-NMDG also showed excellent selectivity for the adsorption of boron in the presence of other cations, such as Na+, Mg2+, and Ca2+, PP-g-GMA-NMDG, and exhibited excellent selectivity towards boron adsorption. These results indicated that the technique of preparing PP-g-GMA-NMDG is both viable and environmentally benign. The PP-g-GMA-NMDG that was made has better qualities than other similar adsorbents. It has a high adsorption capacity, great selectivity, reliable repeatability, and easy recovery. These advantages indicated that the adsorbents have significant potential for widespread application in the separation of boron in water.

Understanding boron toxicity in aquatic plants (Salvinia natans and Lemna minor) in the presence and absence of EDTA

Even though boron is a widely used element in various industries and a contributor to water pollution worldwide, few studies have examined the toxicity of boron in aquatic plants. EDTA is used to maintain aquatic plants cultures, however it is possible to modify the toxicity of metals. The objective of this study is to assess the toxicity of boron in aquatic plants and explore the impact of EDTA presence on the resulting toxic responses. Floating watermoss Salvinia natans and duckweed Lemna minor were exposed to concentrations ranging from 5 to 100 mg/L for 7 days and 1 to 60 mg/L for 3 days, respectively. Growth and photosynthetic activity parameters were investigated in the presence and absence of EDTA. Growth inhibitions in both aquatic plants were observed in a concentration-dependent manner, irrespective of the presence or absence of EDTA. For instance, based on the specific growth rate (leaves coverage), EC10 values for S. natans were calculated as 12.7 (9.9-15.3) mg/L and 8.0 (5.8-10.3) mg/L with and without EDTA, respectively. In the case of L. minor, EC10 values were calculated as 1.3 (0.8-1.89) mg/L and 2.0 (0.4-4.3) mg/L with EDTA without EDTA, respectively. Significant effects were also observed on the photosynthetic capacity, however there was no change in the increase of boron concentration. Generally, negligible effects of EDTA to the toxicity of boron were observed in the present study. By comparing toxicity results based on the presence and absence of EDTA, which is an essential element in the test medium, the results of this study are expected to be utilized for the ecological risk assessment of boron in aquatic ecosystems.

Effective Removal of Boron from Aqueous Solutions by Inorganic Adsorbents: A Review

Increasing levels of boron in water exceeding acceptable thresholds have triggered concerns regarding environmental pollution and adverse health effects. In response, significant efforts are being made to develop new adsorbents for the removal of boron from contaminated water. Among the various materials proposed, inorganic adsorbents have emerged as promising materials due to their chemical, thermal, and mechanical stability. This review aims to comprehensively examine recent advances made in the development of inorganic adsorbents for the efficient removal of boron from water. Firstly, the adsorption performance of the most used adsorbents, such as magnesium, iron, aluminum, and individual and mixed oxides, are summarized. Subsequently, diverse functionalization methods aimed at enhancing boron adsorption capacity and selectivity are carefully analyzed. Lastly, challenges and future perspectives in this field are highlighted to guide the development of innovative high-performance adsorbents and adsorption systems, ultimately leading to a reduction in boron pollution.

Urbanization-driven soil degradation; ecological risks and human health implications

Urban soils contaminated with heavy metals and pesticide residues are of great concern because of their adverse impact on human health. A total of 66 agricultural topsoil samples (15 cm) were collected to represent the study area and determine how anthropogenic activities adversely affect soil quality and human health. Sampling was conducted in the summer, when it was dry and hot, and in the winter, after atmospheric deposition. Seventeen potentially hazardous metals/metalloids (Ag, As, Al, B, Ba, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Mo, Pb, Se, Zn, and V) were measured in the soils. The mean concentrations of metals ranged between 0.05 and 8080 mg/kg, and their distribution was site-specific, with high pollution at the sampling sites owing to proximity to human activities. In agricultural areas, the greatest arsenic concentration was recorded at 48 mg/kg. The potential ecological risk index (PERI) and health hazard index (HI) were calculated, as well as metal contamination indices including contamination factor (Cf), geo-accumulation index (Igeo), and pollution load index (PLI). The mean PLI was calculated to be 4.89, indicating that the area is highly polluted. The potential ecological risk index showed remarkably high risks for As, Cd, and Hg, and moderate risks for Ni and Pb. The arsenic hazard index (HI) was greater than one (2.41) in children, indicating a risk of exposure through ingestion. Pesticide residue analyses were performed in areas where the metal intensity was high. Banned or restricted organochlorine pesticide (OCPs) residues, including, dieldrin, endrin ketone, endosulfan I, II, heptachlor, heptachlor epoxide, lindane (γ-HCH), PP-DDD, and methoxychlor, were detected between 0.002 and 1.45 mg/kg in the soil samples.

Boron Adsorption Kinetics of Microcrystalline Cellulose and Polymer Resin

Tailoring boron-polysaccharide interactions is an important strategy for developing functional soft materials such as hydrogels, fire retardants, and sorbents for environmental remediation, for example, using lignocellulosic biomass. For such applications to be realized, it is paramount to understand the adsorption kinetics of borate anions on cellulose and their local structures. Here, the kinetic aspects of boron adsorption by microcrystalline cellulose, lignin, and polymeric resin are investigated and compared. Borate anions interact with the vicinal diols in the glucopyranoside moieties of cellulose to yield chemisorbed boron chelate complexes. In contrast to cellulose, technical lignin contains fewer cis-vicinal diols, and it does not have a tendency to form such chelate complexes upon treatment with the aqueous boric acid solution. The formation kinetics and stability of these chelate complexes strongly depend on nanoscale structures, as well as reaction conditions such as pH and concentration of the sorbate and sorbent. Specifically, insights into the distinct boron adsorption sites were obtained by solid-state one-dimensional (1D) ¹¹B magic-angle spinning NMR and the local structures and intermolecular interactions in the vicinities of boron chelate complexes are elucidated by analyzing two-dimensional (2D) ¹H-¹³C and ¹¹B-¹H heteronuclear correlation NMR spectra. The total boron adsorption capacity of cellulose is estimated to be in the 1.3-3.0 mg range per gram of sorbent, which is lower than the boron adsorption capacity of a polystyrene-based resin, ∼17.2 mg of boron per gram of Amberlite IRA 743. Our study demonstrates that the local backbone and side chain flexibility as well as the structures of polyol groups play a significant role in determining the kinetic and thermodynamic stability of chelate complexes, yielding to different boron adsorption capabilities of lignocellulosic polymers.

An overview of the application of electrocoagulation for mine wastewater treatment

One of the challenges of the twenty-first century is related to the discharge and disposal of mine effluents and wastewater resulting from mine dewatering, precipitation, and surface runoff in mines, especially acidic effluents that contain a variety of toxic and heavy metals and are the main sources of surface and groundwater pollution. Various physical, chemical, and biological methods have been developed and used to treat mine effluents. All proposed methods have their own disadvantages that make their use challenging. One of the new methods used for wastewater treatment is the electrical coagulation process, which has attracted the attention of researchers in recent years due to its advantages such as simplicity, environmental friendliness, and low cost. The present review focused on the applications of electrocoagulation for mine wastewater treatment as well as metals recovery. In addition, the main mechanisms, advantages, and weaknesses of electrocoagulation were reviewed.

Boron recovery from desalination seawater brines by selective ion exchange resins

The European Union (EU) depends on third markets to supply many important raw materials. Increasing the circularity of critical raw materials within the EU is important not only from an environmental perspective, but also as a competitive advantage for the EU economy. In the case of boron, the EU's import dependency is about 100%. This work aims to evaluate the boron recovery from seawater desalination plants (SWDP) brines using ion-exchange resins in a circular economy approach. Commercial boron selective resins Purolite S108, DIAION CRB03 and CRB05 were tested and compared on batch and dynamic experiments. Thermodynamic and kinetic experiments were performed, and results were fitted by linear and non-linear models. After a comparison, results showed a good fit to the Langmuir isotherm and the pseudo-second order model, respectively, for all the commercial resins tested. The DIAION CRB03 resin presented higher sorption capacity and percentage of boron sorbed than the other resins and was selected as the best option for boron recovery from SWDP brine. Dynamic experiments in fixed bed column using DIAION CRB03 resulted in a sorption capacity of 13 mg/g of resin, a boron recovery of 98% and a concentration factor of 30, for an initial boron concentration of 50 mg/L. In addition, an economic analysis was carried out as a preliminary estimate of the revenues obtained from the production of boric acid from the brine produced by El Prat desalination plant.

Microalgae tolerant of boron stress and bioresources accumulation during the boron removal process

Boron (B) industry and consuming produce large amounts of B-containing wastewater. Low tolerance of microorganisms and plants resulted in the biological removal of B was limited. Microalgae show high adaptability in adverse environments. Whether microalgae able to be utilized in B removal meanwhile produce bioresources, and the B tolerant mechanisms and regulation pathway of microalgae are unclear. In this study, the cell growth, B removal, and lipid/starch production of Chlorella regularis under different levels of B stress (0.5, 10, 25, and 50 mg/L) were examined. The mechanisms of signal perception and response were explored by transcriptome and network analysis. Microalgae tolerated 25 mg/L high B stress, cell growth showed no decline and biomass reach up to 4.5 g/L. Microalgae took in B with 3.35 mg/g and bonded them to protein and carbon components in cells, the B removal capability was higher than some special adsorbents. Microalgae produced 188.65 mg/(L∙d) lipids and 305.35 mg/(L∙d) starch. The mitogen-activated protein-kinase signaling pathway was involved in the B tolerance of microalgae and regulated B efflux, glycolysis, and lipid/starch accumulation to relieve B stress. This study provides potential biological technique for B removal in wastewater and promotes new insight into signal role in toxic pollutants biological treatment.

Electrospun Composite Nanofiltration Membranes for Arsenic Removal

In recent years, significant attention has been paid towards the study and application of mixed matrix nanofibrous membranes for water treatment. The focus of this study is to develop and characterize functional polysulfone (PSf)-based composite nanofiltration (NF) membranes comprising two different oxides, such as graphene oxide (GO) and zinc oxide (ZnO) for arsenic removal from water. PSf/GO- and PSf/ZnO-mixed matrix NF membranes were fabricated using the electrospinning technique, and subsequently examined for their physicochemical properties and evaluated for their performance for arsenite–As(III) and arsenate–As(V) rejection. The effect of GO and ZnO on the morphology, hierarchical structure, and hydrophilicity of fabricated membranes was studied using a scanning electron microscope (SEM), small and ultra-small angle neutron scattering (USANS and SANS), contact angle, zeta potential, and BET (Brunauer, Emmett and Teller) surface area analysis. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were used to study the elemental compositions and polymer-oxide interaction in the membranes. The incorporation of GO and ZnO in PSf matrix reduced the fiber diameter but increased the porosity, hydrophilicity, and surface negative charge of the membranes. Among five membrane systems, PSf with 1% ZnO has the highest water permeability of 13, 13 and 11 L h−1 m−2 bar−1 for pure water, As(III), and As(V)-contaminated water, respectively. The composite NF membranes of PSf and ZnO exhibited enhanced (more than twice) arsenite removal (at 5 bar pressure) of 71% as compared to pristine PSf membranes, at 43%, whereas both membranes showed only a 27% removal for arsenate.

Efficient sequestration of boron from liquid phase by amidoxime-functionalized poly(acrylonitrile-co-acrylic acid): experimental and modelling analyses

This study aims to produce amidoxime-modified poly(acrylonitrile-co-acrylic acid) using an optimized method and to investigate the performance of amidoxime-modified poly(acrylonitrile-co-acrylic acid) on the adsorption of boron ions in batch operations. Batch adsorption was conducted at the physiochemical parameters of pH, adsorbent dosage, and initial boron concentration. The isotherms and kinetics of adsorption data were studied at various initial boron concentrations. The renewed synthesis process gave a production yield of 77%, and the conversion of nitrile group to amidoxime was 78%. The adsorption reached its optimum point at pH = 8, adsorbent dosage = 4 g·L^-1, and initial adsorbent concentration at 40 ppm. The best model fits for isotherm adsorption was the Sips model with heterogeneity factor (n) = 0.7611. In the kinetic study, the adsorption data fitted best with pseudo-second-order model. The synthesized polymeric adsorbent could be recycled with little decline in its boron entrapment capacities. Hence, amidoxime-modified poly(acrylonitrile-co-acrylic acid) exhibited high adsorption capacity and could be potentially explored as an alternative to commercial resin in the removal of boron from wastewater.

A review of boron removal from aqueous solution using carbon-based materials: An assessment of health risks

Carbon-based compounds have gained attention of researchers for use in boron removal due to their properties, which make them a viable and low cost adsorbent with a high availability, as well as environmental friendliness and high removal efficiency. The removal of boron utilizing carbon-based materials, including activated carbon (AC), graphene oxide (GO), and carbon nanotubes (CNTs), is extensively reviewed in this paper. The effects of the operating conditions, kinetics, isotherm models, and removal methods are also elaborated. The impact of the modification of the lifetime of carbon-based materials has also been explored. Compared to unmodified carbon-based materials, modified materials have a significantly higher boron adsorption capability. It has been observed that adding various elements to carbon-based materials improves their surface area, functional groups, and pore volume. Tartaric acid, one of these doped elements, has been employed to successfully improve the boron removal and adsorption capabilities of materials. An assessment of the health risk posed to humans by boron in treated water utilizing carbon-based materials was performed to better understand the performance of materials in real-world applications. Furthermore, the boron removal effectiveness of carbon-based materials was evaluated, as well as any shortcomings, future perspectives, and gaps in the literature.

Ion-capture electrodialysis using multifunctional adsorptive membranes

Technologies that can efficiently purify nontraditional water sources are needed to meet rising global demand for clean water. Water treatment plants typically require a series of costly separation units to achieve desalination and the removal of toxic trace contaminants such as heavy metals and boron. We report a series of robust, selective, and tunable adsorptive membranes that feature porous aromatic framework nanoparticles embedded within ion exchange polymers and demonstrate their use in an efficient, one-step separation strategy termed ion-capture electrodialysis. This process uses electrodialysis configurations with adsorptive membranes to simultaneously desalinate complex water sources and capture diverse target solutes with negligible capture of competing ions. Our methods are applicable to the development of efficient and selective multifunctional separations that use adsorptive membranes.

Separation of Boron from Geothermal Waters with Membrane System

This study presents the separation and recovery of boron from geothermal waters with a polymeric membrane system and suggests a transport mechanism. The optimum relative parameters of the transport were examined. The recovery value of boron was 60.46% by using polymeric membrane system from prepared aquatic solution to the acceptor phase. The membrane's capacity and selectivity of the transport process were examined. Kinetics values were calculated for each transport parameter. The optimum kinetic values were 1.4785 × 10^-6 (s^-1), 7.3273 × 10^-8 (m/s), 13.5691 × 10^-8 (mol/m².s), 5.8174 × 10^-12 (m²/s) for constant rate, permeability coefficient, flux, and diffusion coefficient, respectively. Boron was transported selectively and successfully from geothermal waters in the presence of other metal cations with 59.85% recovery value. This study indicates the application of real samples in polymeric membrane systems, which are very practical, economic, and easy to use for large-scale applications. The chemical and physical properties of polymer inclusion membranes (PIMs) offer the opportunity to be specially designed for specific applications.

Chelating Fabrics Prepared by an Organic Solvent-Free Process for Boron Removal from Water

A chelating fabric was prepared by graft polymerization of glycidyl methacrylate (GMA) onto a nonwoven fabric, followed by attachment reaction of N-methyl-D-glucamine (NMDG) using an organic solvent-free process. The graft polymerization was performed by immersing the gamma-ray pre-irradiated fabric into the GMA emulsion, while the attachment reaction was carried out by immersing the grafted fabric in the NMDG aqueous solution. The chelating capacity of the chelating fabric prepared by reaction in the NMDG aqueous solution without any additives reached 1.74 mmol/g, which further increased to above 2.0 mmol/g when surfactant and acid catalyst were added in the solution. The boron chelation of the chelating fabric was evaluated in a batch mode. Fourier transform infrared spectrophotometer (FTIR) was used to characterize the fabrics. The chelating fabric can quickly chelate boron from water to form a boron ester, and a high boron chelating ability close to 18.3 mg/g was achieved in the concentrated boron solution. The chelated boron can be eluted completely by HCl solution. The regeneration and stability of the chelating fabric were tested by 10 cycles of the chelation-elution operations. Considering the organic solvent-free preparation process and the high boron chelating performance, the chelating fabric is promising for the boron removal from water.

Assessment of different nanofiltration and reverse osmosis membranes for simultaneous removal of arsenic and boron from spent geothermal water

One of the factors that determine agricultural crops' yield is the quality of water used during irrigation. In this study, we assessed the usability of spent geothermal water for agricultural irrigation after membrane treatment. Preliminary membrane tests were conducted on a laboratory-scale set up followed by mini-pilot scale tests in a geothermal heating center. In part I, three commercially available membranes (XLE BWRO, NF90, and Osmonics CK- NF) were tested using a cross-flow flat-sheet membrane testing unit (Sepa CF II, GE-Osmonics) under constant applied pressure of 20 bar. In part II, different spiral wound membranes (TR-NE90-NF, TR-BE-BW, and BW30) other than the ones used in laboratory tests were employed for the mini-pilot scale studies in a continuous mode. Water recovery and applied pressure were maintained constant at 60% and 12 bar, respectively. Performances of the membranes were assessed in terms of the permeate flux, boron and arsenic removals. In laboratory tests, the permeate fluxes were measured as 94.3, 87.9, and 64.3 L m^-2 h^-1 for XLE BWRO, CK-NF and NF90 membranes, respectively. The arsenic removals were found as 99.0%, 87.5% and 83.6% while the boron removals were 56.8%, 54.2%, and 26.1% for XLE BWRO, NF90 and CK-NF membranes, respectively. In field tests, permeate fluxes were 49.9, 26.8 and 24.0 L m^-2 h^-1 for TR-NE90-NF, BW30-RO and TR-BE-BW membranes, respectively. Boron removals were calculated as 49.9%, 44.1% and 40.7% for TR-BE-BW, TR-NE90-NF and BW30-RO membranes, respectively. Removal efficiencies of arsenic in mini-pilot scale membrane tests were all over 90%. Quality of the permeate water produced was suitable for irrigation in terms of the electrical conductivity (EC) and the total dissolved solids (TDS) for all tested membranes with respect to guidelines set by the Turkish Ministry of Environment and Urbanisation (TMEU). However, XLE BWRO, CK-NF and NF90 membranes failed to meet the required limits for irrigation in terms of boron and arsenic concentrations in the product water. The permeate streams of TR-BE-BW, TR-NE90-NF and BW30-RO membranes complied with the irrigation water standards in terms of EC, TDS and arsenic concentration while boron concentration remained above the allowable limit.

Removal of boron from water by GO/ZIF-67 hybrid material adsorption

With the development of the boron industry, boron pollution is getting more and more serious, and excessive boron will harm human health. In this paper, graphene oxide was used as a template to prepare ZIF-67, and GO/ZIF-67 was successfully prepared. GO/ZIF-67 was used for the first time to remove boron from water. SEM, XRD, and other characterization methods were used to confirm the structure. The adsorption kinetics, adsorption isotherm, adsorption thermodynamics, and adsorption mechanism of boron by GO/ZIF-67 were studied in this paper. The adsorption capacity of GO/ZIF-67 for boron is up to 66.65 mg·g^-1 at 25 °C, and adsorption process reaches equilibrium in 400 min. Adsorption kinetics indicates that the adsorption process conforms to the pseudo-first-order kinetic model, and adsorption thermodynamics indicates that the adsorption process is a spontaneous endothermic process controlled by entropy change. The adsorption capacity of boron by GO/ZIF-67 does not decrease significantly after four cycles. The adsorption of boron by GO/ZIF-67 has both chemical and physical adsorption. From Zeta potential and adsorption kinetics, it can be seen that there is physical adsorption during the adsorption process and boron mainly has positive charge on the surface of GO/ZIF-67 and graphene oxide hydroxyl bonding. Based on the adsorption thermodynamics and XPS, it is known that there is chemisorption during the adsorption process, and mainly the combination of boron and cobalt sites.

Removal of boron and silicon by a modified resin and their competitive adsorption mechanisms

Boron and silicon are essential trace elements for living organisms. However, these are undesirable in excess amounts owing to the toxic effects of boron on plants, animals, and humans, and the silica scale formation by silicon in water treatment processes. Herein, a new diol-type adsorbent (T-resin) was synthesized by grafting tiron (disodium 4,5-dihydroxy-1,3-benzenedisulfonate) onto an ion-exchange resin (grafting amount is 1.2 mmol/g dry) to separate boron and silicon from a solution. The effects of pH, initial concentration, and coexisting anions, particularly, the effect of the coexistence of silicate ion on the adsorption of boron, were investigated. T-resin showed good adsorption properties for both boron and silicon in a wide pH range (pH 2-10). The adsorption of boron and silicon was effectively described by the Langmuir isotherm, and the maximum adsorption capacities of boron and silicon were 21.25 mg/g and 8.36 mg/g, respectively. In a competitive adsorption system, boron and silicon were simultaneously adsorbed on the T-resin, but the adsorption rate of boron was faster than silicon. However, silicon could replace the boron adsorbed on the resin, indicating that the adsorption of silicon was more stable than boron. ¹¹B and ²⁹Si solid state NMR data confirmed the different adsorption mechanisms of the two elements. Boron was adsorbed via two types of complexes, a triangular complex of [LB(OH)], as well as 1:1 tetrahedral complex of [LB(OH)₂] and 1:2 tetrahedral complex of [BL₂], whereas silicon was only adsorbed via a 1:3 octahedral complex of [SiL₃]. Graphical abstract A new diol-type absorbent was synthesized by grafting tiron onto an ion-exchange resin to separate boron and silicon from a solution. Boron and silicon competitively adsorbed on the T-resin, and silicon could replace the boron adsorbed on the resin. ¹¹B and ²⁹Si solid state NMR data confirmed the different adsorption mechanisms of the two elements. Boron was adsorbed via two types of complexes, a triangular complex of [LB(OH)], as well as 1:1 tetrahedral complex of [LB(OH)₂] and 1: 2 tetrahedral complex of [BL₂], whereas silicon was only adsorbed via a 1:3 octahedral complex of [SiL₃].

Electrocoagulation of tetrafluoroborate (BF4-) and the derived boron and fluorine using aluminum electrodes

Tetrafluoroborate anion (BF₄^-) is found in the streams of flue-gas desulfurization and borosilicate glasses etching which deteriorates water quality through slow hydrolysis into boric acid and fluoride. Decomposition and electrocoagulation (EC) of BF₄^- were studied using metallic aluminum as the sacrificial electrode. The dissolved Al(III) from the anode could efficiently decompose BF₄^- in forms of fluoroaluminate complexes, and the derived boric acid and fluoride ion were removed by sweep flocculation. Major variables were investigated to optimize EC, including the reaction pH, initial concentration of BF₄^-, current density and electrolyte type. The mechanism of EC process was elucidated with the kinetics of consecutive reactions. Experimental results suggested that the removal of BF₄^- and total fluoride were less influenced by pH, and that of total boron reached a maximum at pH 8 which favored the surface complexation between borate species and EC precipitates. Under the conditions: [BF₄^-]₀ = 9.3 mM, [NaCl] = 10 mM, pH = 8.0, current density = 5 mA/cm², 98.3% of BF₄^- was decomposed and the removal of total fluoride and boron attained 98.2% and 74.1%, respectively within 3 h. EC using the Al electrode outperformed the conventional chemical coagulation and reduced the levels of BF₄, B(OH)₃ and F^- in aqueous solution synergically.

Preparation of Superhydrophilic Adsorbents with 3DOM Structure by Water-Soluble Colloidal Crystal Templates for Boron Removal from Natural Seawater

Three-dimensionally ordered macroporous cross-linked poly(glycidyl methacrylate) (3DOM) was constructed by water-soluble colloidal crystal templates and further functionalized with N-methyl-d-glucamine (NMDG) to prepare superhydrophilic adsorbents for boron removal from natural seawater. 3DOM adsorbents possess features of interconnected macropore structure, ultrathin pore wall, and superhydrophilicity, making efficient adsorption possible. The effect of cross-linking degree on the adsorption capacity toward boron was investigated. The NMDG-modified 3DOM adsorbent with rich vicinal diol functional groups showed superhydrophilicity and outstanding performance of adsorption. Significantly, its adsorption effect in boron removal from natural seawater indicated that the concentration of boron in natural seawater could decline to 0.16 from 4.24 mg·L^-1 when the adsorbent dosage was 1 g·L^-1, whereas the boron rejection reached 96.2%. After 10 regeneration-adsorption cycles, the adsorption capacity of 3DOM adsorbent remained over 85% of the initial value and the ordered structure was hardly changed. Additionally, 3DOM adsorbent could be directly and quickly separated from the seawater by a filter mesh of 16 mesh number. Research shows that the 3DOM adsorbent exhibits an adsorption performance for practical applications in boron removal from natural seawater.

Synthesis of novel magnetic nano-sorbent functionalized with N-methyl-D-glucamine by click chemistry and removal of boron with magnetic separation method

Click chemistry refers to a group of reactions that are fast, simple to use, easy to purify, versatile, regiospecific, and give high product yields. Therefore, a novel, efficient magnetic nano-sorbent based on N-methyl-D-glucamine attached to magnetic nanoparticles was prepared using click coupling method. Its boron sorption capacity was compared with N-methyl-D-glucamine direct attached nano-sorbent. The characterization of the magnetic sorbents was investigated by several techniques such as X-ray diffraction, scanning electron microscope, transmission electron microscope, dynamic light scattering, thermogravimetric analysis, Fourier transform infrared spectrophotometer, and vibrating sample magnetometer. The boron sorption capacity of sorbents was compared by studying various essential factors influencing the sorption, like sorbate concentration, sorbents dosage, pH of the solution, and contact time. Langmuir and Freundlich and Dubinin-Radushkevich adsorption isotherms models were applied. Percent removal and sorption capacities efficiencies of sorbents obtained with direct and click coupling are found to be 49.5%, 98.7% and 6.68 mg/g, 13.44 mg/g respectively. Both sorbents have been found to be compatible with Langmuir isotherm, and the boron sorption kinetics conforms to the pseudo second order kinetics. The reusability study of sorbents was carried out five times for boron sorption and desorption.

Simultaneous immobilization of borate, arsenate, and silicate from geothermal water derived from mining activity by co-precipitation with hydroxyapatite

The treatment of the geothermal water discharged through mining activity is a critical issue because the rate of discharge is 12,000 m³ per day and the discharge contains high concentrations of borate (>20 mg/L) and arsenate (ca. 0.4 mg/L) as well as silicate and carbonate. The simultaneous reduction of borate and arsenate concentrations to acceptable levels was successfully performed by co-precipitation with hydroxyapatite (HAp). Although the coexisting high concentrations of carbonate act as a disturbing element, the co-precipitation equilibrium of borate was shifted to lower values by adjusting the P/Ca molar ratio, and the removal rate of borate was accelerated by using Al³⁺ additives, resulting in the efficient reduction of borate within 1 h. The initially immobilized boron in HAp is in the tetragonal form, which probably occupies the hydroxyl sites in HAp, gradually transforming into the trigonal form in the solid state, as interpreted by ¹H NMR and ¹¹B-NMR. The coexisting silicate was also immobilized in an ellestadite form, as confirmed by ²⁹Si-NMR measurements. Arsenate and silicate were immobilized before borate in geothermal water. A dissolution assay of borate in the solid residues after co-precipitation with HAp verified the acceptable stability of borate, which is independent of the amount of added Al³⁺.

Study of the Equilibrium, Kinetics, and Thermodynamics of Boron Removal from Waters with Commercial Magnesium Oxide

In the present work, the equilibrium, thermodynamics, and kinetics of boron removal from aqueous solutions by the adsorption on commercial magnesium oxide powder were studied in a batch reactor. The adsorption efficiency of boron removal increases with temperature from 25°C to 50°C. The experimental results were fitted to the Langmuir, Freundlich, and Dubinin–Radushkevich (DR) adsorption isotherm models. The Freundlich model provided the best fitting, and the maximum monolayer adsorption capacity of MgO was 36.11 mg·g−1. In addition, experimental kinetic data interpretations were attempted for the pseudo-first-order kinetic model and pseudo-second-order kinetic model. The results show that the pseudo-second-order kinetic model provides the best fit. Such result suggests that the adsorption process seems to occur in two stages due to the two straight slopes obtained through the application of the pseudo-first-order kinetic model, which is confirmed by the adjustment of the results to the pseudo-second-order model. The calculated activation energy (Ea) was 45.5 kJ·mol−1, and the values calculated for ∆G°, ∆H°, and ∆S° were −4.16 kJ·mol−1, 21.7 kJ·mol−1, and 87.3 kJ·mol−1, respectively. These values confirm the spontaneous and endothermic nature of the adsorption process and indicated that the disorder increased at the solid-liquid interface. The results indicate that the controlling step of boron adsorption process on MgO is of a physical nature.

Enhancing boron rejection in FO using alkaline draw solutions

This study provides a novel method to enhance boron removal in a forward osmosis (FO) process. It utilizes the reverse solute diffusion (RSD) of ions from alkaline draw solutions (DSs) and the concentration polarization of the hydroxyl ions to create a highly alkaline environment near the membrane active surface. The results show that boron rejection can be significantly enhanced by increasing the pH of NaCl DS to 12.5 in the active-layer-facing-feed-solution (AL-FS) orientation. The effect of RSD enhanced boron rejection was further promoted in the presence of concentration polarization (e.g., in the active-layer-facing-draw-solution (AL-DS) orientation). The current study opens a new dimension for controlling contaminant removal by FO using tailored DS chemistry, where the RSD-induced localized water chemistry change is taken advantage in contrast to the conventional method of chemical dosing to the bulk feed water.