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Chemical oxo-precipitation (COP) is an enhanced precipitation method for boron removal with the conversion of boric acid to perborate anions. When using barium-based precipitant, the boron can be effectively precipitated as barium perborates (BaPBs). The phase transformation of BaPBs from amorphous (A-BaPB, Ba(B(OH) 3 OOH) 2) to crystalline (C–BaPB, BaB 2 (OO) 2 (OH) 4) form is crucial for effective boron removal. However, scaling up this phase transformation of BaPBs is hindered by poor diffusion. This study aims to promote the growth of C–BaPB through seed-induced crystal growth, eliminating the need for phase transformation. By examining the relationship between crystal growth rate and supersaturation, surface spiral growth was identified as the rate-limiting step of the growth of micron-sized seeds near pH pzc. To enable continuous crystal growth, granular seeds of C–BaPB were prepared and employed as the medium for fluidized-bed crystallization (FBC). The system reached steady state 3 hydraulic retention times, achieving 90% boron removal. The effect of surface loading, ionic strength, and dosages on steady-state crystal growth rate was studied, revealing a shift of the rate-limiting step in FBC to diffusion. Lastly, the system that constituted of two FBCs in-series for sequential crystallization of A-BaPB and C–BaPB was demonstrated. The integrated system provided 97.8% of boron removal from synthetic wastewater containing 500 mg-B/L, with 92.3% of boron crystallized on the granular seeds of BaPBs. [Display omitted] • Seed-induced crystal growth of C–BaPB enabled rapid boron removal. • Spiral surface growth was the rate-limiting step near pH pzc. • Continuous crystal growth of granular C–BaPB seeds removed 90% boron in FBC. • Two FBCs in-series eliminated boron from 500 to 8 mg-B/L, >92% on granular seeds. [ABSTRACT FROM AUTHOR]

The electrocoagulation (EC) using metallic Ni foam as electrodes was studied for the removal of boron from solution. The electrolytic parameters were pH (4–12), current density (0.6–2.5 mA cm −2 ), and initial concentration of boron (10–100 mg L −1 ). Experimental results revealed that removal efficiency was maximized at pH 8–9, and decreased as the pH increased beyond that range. At particular onset potentials (0.5–0.8 V vs. Hg/HgO), the micro-granular nickel oxide that was created on the surface of the nickel metal substrate depended on pH, as determined by cyclic voltammetry. Most of the crystallites of the precipitates comprised a mixed phase of β-Ni(OH) 2 , a theophrastite phase, and NiOOH, as revealed by XRD and SEM analyses. A current density of 1.25 mA cm −2 was effective in the EC of boron, and increasing the concentration of boric acid from 10 to 100 mg L −1 did not greatly impair removal efficiency. A kinetic investigation revealed that the reaction followed a pseudo-second order rate model. The optimal conditions under which 99.2% of boron was removed from treated wastewater with 10 mg L −1 -B, leaving less than 0.1 mg L −1 -B in the electrolyte, were pH 8 and 1.25 mA cm −2 for 120 min. [ABSTRACT FROM AUTHOR]

[Display omitted] • 85% of boron was crystallized as barium perborate granules within FBC. • Effluent boron levels were close to equilibrium solubility of barium perborate. • The sludge production rate increased drastically with supersaturation. • Algorithm based on supersaturation was developed to predict the performance of FBC. • The treatment cost of Ba-COP in FBC is 30% lower than in batch stirred-tank. The development of treatment process for boron removal is urgent to control the anthropogenic emission of boron to freshwater. Barium-based chemical oxo-precipitation (Ba-COP) is an effective method to treat boron-rich streams in ambient condition by the precipitation of barium perborate (BaPB), but it is limited to batch operation and mass production of sludge. This study aims to circumvent these drawbacks by performing Ba-COP in fluidized-bed crystallization system (Ba-COP@FBC) to immobilize BaPB on the surface of the fluidized granules continuously. When Ba-COP@FBC was operated at pH 10, 85% of total removal (TR) and 83% of crystallization ratio (CR) were attained from an input boron level of 600 mg-B/L, indicating that the homogeneous nucleation could be suppressed. Despite that both TR and CR varied greatly upon the dosage of H 2 O 2 and Ba, initial concentration and reflux ratio, the influence of these parameters can be summarized by supersaturation. TR was related to the supersaturation at the effluent (S E), which was fixed at 1.15 ± 0.04; the sludge production rate increased drastically with the supersaturation at the mixing zone (S M), which in turn reflects on CR. Accordingly, an algorithm based on the supersaturation ratio was developed to predict the performance of Ba-COP@FBC based on the initial concentration, hydraulic condition, dosages and ionic strength. The economic analysis reveals that Ba-COP@FBC is more competitive than the Ba-COP conducted in batch stirred-tank reactor in terms of solid disposal cost due to the difference of water content between granule and sludge. [ABSTRACT FROM AUTHOR]

The anthropogenic emission of boron to river has become a serious problem that deteriorates the water quality and endangers the ecosystem. Although boron is a micronutrient, it is toxic to plants, animals and humans upon exposure. In this review, we first present the sources of the boron-containing streams and their composition, and then summarize the recent progress of boron removal methods based on adsorption and coagulation systematically. The boron-spiked streams are produced from coal-fired and geothermal power plants, the manufacturing and the activities of oil/gas excavation and mining. The adsorbents for boron removal are classified into the ones functionalized by chelating groups, the ones on the basis of clays or metal oxide. Three subgroups reside in the coagulation approach: electrocoagulation, chemical precipitation and chemical oxo-precipitation. The hybrid technology that combines membrane process and adsorption/coagulation was covered as well. To provide a comprehensive view of each method, we addressed the reaction mechanism, specified the strength and weakness and summarized the progress in the past 5 years. Ultimately, the prospective for future research and the possible improvement on applicability and recyclability were proposed. ga1 • The sources and compositions of boron-containing wastewaters were summarized. • The adsorption materials are chelating group-functionalized, clay-based and metal oxide-based. • The coagulation methods were reviewed for the first time, including electrocoagulation and precipitation. • The majority of the reference is within the past five years. [ABSTRACT FROM AUTHOR]

Tetrafluoroborate anion (BF 4 −) 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 BF 4 − were studied using metallic aluminum as the sacrificial electrode. The dissolved Al(III) from the anode could efficiently decompose BF 4 − 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 BF 4 −, 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 BF 4 − 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: [BF 4 −] 0 = 9.3 mM, [NaCl] = 10 mM, pH = 8.0, current density = 5 mA/cm2, 98.3% of BF 4 − 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 BF 4 , B(OH) 3 and F− in aqueous solution synergically. Graphical abstract Image 1 Highlights • Efficiency of BF 4 − decomposition attained more than 90% in EC regardless of pH. • 95% and 80% of the derived fluoride and boron were removed at pH 8. • The electrode consumption and operation cost increased with current density. • Kinetics of BF 4 − decomposition was evaluated using a consecutive reaction model. [ABSTRACT FROM AUTHOR]

Magnetically separable spinel ferrites were created in an electrocoagulation (EC) process for removing boron from aqueous solution. Coprecipitates of NiFe 2 O 4 , CoFe 2 O 4 and CuFe 2 O 4 were obtained using sacrificial iron anodes (EC-Fe) in an electrolyte that contained transition metal salts (Ni, Co, Cu). The use of nickel chloride (NiCl 2 ) as the supporting electrolyte yielded the highest boron removal since the maximum adsorption capacity of the resulting sludge was 28.9 mg-B/g. An EC that used iron and nickel as anodes (EC-Fe/Ni) in NaCl electrolyte was then employed to form nickel ferrite by electrochemical dissolution of ferrous (Fe(II)) and nickel (Ni(II)) ions, providing comparable removal efficiency but minimizing the residual level of Ni(II) in the treated water. The saturation magnetization of the precipitate that was produced in the EC-Fe/Ni system was 50.3 emu/g which exceeded that in the EC-Fe system with nickel chloride – 21.8 emu/g, indicating its outstanding magnetic separability. EC-Fe/Ni was optimized to remove 95% of boron from solution in 60 min with an initial boron concentration of 10 ppm at pH 8 and a current density of 3.75 mA/cm 2 . [ABSTRACT FROM AUTHOR]

Chemical oxo-precipitation (COP) is a facile method to remove boron by the precipitation of perborate salts with the aid of hydrogen peroxide. Despite that the COP using barium precipitant (Ba–COP) has been advanced to provide excellent boron removal, the production of hazardous sludge limits the implementation. This study aims to improve the COP using calcium precipitant (Ca–COP) to provide boron removal comparable to COP without the production of hazardous waste. In addition to the precipitation of calcium perborate, the formation of calcium carbonate and calcium peroxide played decisive roles in the Ca–COP. The instant precipitation of calcium perborates (CaPBs) at pH 11 could eliminate boron from 1000 to 30 mg-B/L in 15 min. The prominent uptake of atmospheric CO 2 induced the precipitation of calcium carbonate and consumed calcium ion, which resulted in the increment in boron level due to the incongruent dissolution of CaPBs. The formation of calcium carbonate and calcium perborate provided an additional route to remove boron through sorption. Accordingly, a multi-stage Ca–COP process was developed to remove precipitated CaPBs for preventing the incongruent dissolution and to dose calcium precipitant multiple times to induce the sorptive removal. The proposed process could eliminate boron from 1000 to 5 mg-B/L, which is comparable to the state-of-art Ba–COP process with 45% of its cost. [Display omitted] • Dissolution of calcium perborate was triggered by the calcium depletion by CaCO 3. • Formation of CaCO 3 and CaO 2 provided a sorptive route to remove boron. • Multi-step Ca–COP process attained 99.5% of boron removal from 1 g-B/L solution. • Ca–COP process was 45% cheaper than the Ba–COP process by avoiding hazardous sludge. [ABSTRACT FROM AUTHOR]

Chemical oxo-precipitation (COP) is a facile method to remove boron by the precipitation of perborate salts with the aid of hydrogen peroxide. Despite that the COP using barium precipitant (Ba–COP) has been advanced to provide excellent boron removal, the production of hazardous sludge limits the implementation. This study aims to improve the COP using calcium precipitant (Ca–COP) to provide boron removal comparable to COP without the production of hazardous waste. In addition to the precipitation of calcium perborate, the formation of calcium carbonate and calcium peroxide played decisive roles in the Ca–COP. The instant precipitation of calcium perborates (CaPBs) at pH 11 could eliminate boron from 1000 to 30 mg-B/L in 15 min. The prominent uptake of atmospheric CO 2 induced the precipitation of calcium carbonate and consumed calcium ion, which resulted in the increment in boron level due to the incongruent dissolution of CaPBs. The formation of calcium carbonate and calcium perborate provided an additional route to remove boron through sorption. Accordingly, a multi-stage Ca–COP process was developed to remove precipitated CaPBs for preventing the incongruent dissolution and to dose calcium precipitant multiple times to induce the sorptive removal. The proposed process could eliminate boron from 1000 to 5 mg-B/L, which is comparable to the state-of-art Ba–COP process with 45% of its cost. [Display omitted] • Dissolution of calcium perborate was triggered by the calcium depletion by CaCO 3. • Formation of CaCO 3 and CaO 2 provided a sorptive route to remove boron. • Multi-step Ca–COP process attained 99.5% of boron removal from 1 g-B/L solution. • Ca–COP process was 45% cheaper than the Ba–COP process by avoiding hazardous sludge. [ABSTRACT FROM AUTHOR]

Chemical oxo-precipitation (COP), which combines treatment with an oxidant and precipitation using metal salts, was developed for treating boron-containing water under milder conditions (room temperature, pH 10) than those of conventional coagulation processes. The concentration of boron compounds was 1000 mg-B L-1. They included boric acid (H3BO3) and perborate (NaBO3). Precipitation using calcium chloride eliminated 80% of the boron from the perborate solution, but was unable to treat boric acid. COP uses hydrogen peroxide (H2O2) to pretreat boric acid, substantially increasing the removal of boron from boric acid solution by chemical precipitation from less than 5% to 80%. Furthermore, of alkaline earth metals, barium ions are the most efficient precipitant, and can increase the 80% boron removal to 98.5% at [H2O2]/[B] and [Ba]/[B] molar ratios of 2 and 1, respectively. The residual boron in the end water of COP contained 15 ppm-B: this value cannot be achieved using conventional coagulation processes. [ABSTRACT FROM AUTHOR]