Reclaiming Boron as Calcium Perborate Pellets from Synthetic

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Reclaiming Boron as Calcium Perborate Pellets from Synthetic Wastewater by Integrating Chemical Oxo-Precipitation within a Fluidized-Bed Crystallizer Xuantung Vu,† Jui-Yen Lin,† Yu-Jen Shih,*,‡ and Yao-Hui Huang*,†,§ †

Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan Institute of Environmental Engineering, National Sun Yat-sen University, Kaohsiung 804, Taiwan § Sustainable Environment Research Center, National Cheng Kung University, Tainan 701, Taiwan ‡

S Supporting Information *

ABSTRACT: Chemical oxo-precipitation (COP) is a modified precipitation process in which hydrogen peroxide is used to transform boric acid to perborate anions, which are precipitated with calcium salt under ambient conditions. To minimize the production of sludge, chemical oxo-precipitation was performed in a fluidized-bed reactor to reclaim boron as unseeded calcium perborate pellets. Several major experimental parameters, including effluent pH, calcium dosage, and surface loading that affected the degree of supersaturation and the efficiency of boron removal, were tested. A crystallization ratio of around 60% was attained under the following conditions: initial boron concentration = 1000 ppm, molar ratios of [Ca]/[B] = 0.6 and [H2O2]/[B] = 2, effluent pH = 10.6, bed height = 80 cm, and hydraulic retention time = 18 min. On the basis of the characterization of XRD, SEM, and Raman spectroscopy, the granules recovered were amorphous calcium perborates Ca(B(OH)3OOH)2 and CaB(OH)3OOB(OH)3. KEYWORDS: Boron removal, Fluidized-bed crystallizer, Homogeneous nucleation, Perborate, Hydrogen peroxide

■

sorption under high temperature (60 °C).3 Generally, the conventional precipitation required heating at high pH (60−90 °C, pH 11.2−13) to recover boron as (Ca2B2O5·H2O), and the resulted removals of boron are averagely low.9 Chemical oxo-precipitation (COP) is an emerging method for effectively treating boron-containing wastewater at room temperature with the assistance of hydrogen peroxide, which converts boric acid into perborate ions and thus considerably improves the precipitation of boron by alkaline earth metals at room temperature.8 The peroxolysis of boron dissociates as several perborate species at pH 8−11 and the prominent species include B(OH)3OOH−, B(OH)2(OO)2B(OH)22−, B(OH)2(OOH)2−, and B(OH)3OOB(OH)32− (Figure 1). Calcium and barium salts have been proven to be effective in precipitating peroxoborates.14 However, the excessive dose of barium salt employed could be a great challenge after treatment of boron since Taiwan Environmental Protection Agency regulated the effluent standard of barium ion in industrial streams to be 2 mg/L. In this work, calcium chloride was chosen as the precipitant to avoid the concerns of barium toxicity in aqueous solution.15

INTRODUCTION Boron has been applied in many industries, being used in moderators in nuclear reactors, borosilicate glass, fertilizers, ceramics, and the manufacture of thin film transistor liquid crystal display (TFT-LCD) and semiconductors.1 The World Health Organization (WHO) established a guideline value of boron in drinking water of 2.4 mg/L.2 The Environmental Protection Administration of Taiwan (EPA) regulates a stringent boron standard of 1 mg/L for industrial streams to minimize the impact of boron on environments. Numerous methods have been developed to remove boron from aqueous solutions, including ion exchange, adsorption, reverse osmosis, electrocoagulation and precipitation.3 Ion exchange resins show high selectivity in the removal of boron. However, the cost and complicated regeneration of resins limit its implementation.4 Several adsorbents made of mineral clays and industrial wastes were studied to reduce the cost in removing boron,5,6 but they suffered from limited capacity. Reverse osmosis is ineffective since boron exists as molecular form (B(OH)3) at neutral pH which penetrates through the membrane.7 Electrocoagulation using sacrificial anodes requires no chemicals but creates large amounts of sludge.8 Precipitation adopts lime (Ca(OH)2) with coprecipitants such as sulfuric acid and phosphorous acid to produce mineral-like precipitate (hydroxyapatite, HAp, Ca10(PO4)6(OH)2) to remove boron via © XXXX American Chemical Society

Received: October 29, 2017 Revised: January 10, 2018 Published: February 27, 2018 A

DOI: 10.1021/acssuschemeng.7b03951 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

criteria of boron removal (BR%), and crystallization ratio (CR %).

■

MATERIALS AND METHODS

Chemicals. All reagents were of analytical grade and used without further purification. The synthetic wastewater was prepared by boric acid (B(OH)3, Showa). Hydrogen peroxide (H2O2, 35 wt %, Merck) was used to transform boric acid or borate ion to perborate ions; calcium chloride (CaCl2, Showa) was used as the precipitant. All solutions were prepared with a deionized water that was purified using a laboratory-grade RO-ultrapure water system. Reaction Apparatus. The batch tests of COP were carried out in a jar test apparatus. The FBHC experiments were conducted in a fluidized-bed reactor (FBR) as shown in Figure 2. FBR was made of

Figure 2. Apparatus of fluidized-bed reactor for FBHC. Figure 1. (a) Reactions between boric acid, hydrogen peroxide, perborates, and protons, and their stability constants.10−13 (b) Speciation of boric acid/hydrogen peroxide system (boron concentration = 1000 ppm = 92.6 mM, total peroxo concentration = 185.2 mM).

transparent glass and can be divided into lower and upper part. The diameter and length of the lower part were 3.5 and 110 cm and those of the upper part were 7 and 30 cm, respectively. The total volume of the FBR was 1500 mL. There are two inlets at the bottom of FBR, one for B/H2O2 solution and the other for Ca. The boron solution was mixed with H2O2 in a pretreatment tank before pumping into FBR to prevent the decomposition of H2O2. The input flow rates of B/H2O2 stream (QB) and Ca stream (QCa) were the same. A recirculation stream (Qr) was controlled to enable the fluidization of granules. Experimental Procedure. The jar test was preliminarily performed to assess the operative conditions of COP. A volume of 500 mL of boron solution in 2000 ppm-B were mixed with H2O2 to yield a desired molar ratio of [H2O2]/[B]. The pH of the solutions was adjusted immediately to 10.5 and mixed for 20 min for the complete speciation of perborate species.2 Afterward, 500 mL of solution that contains a known concentration of CaCl2 (based on the molar ratio of [Ca]/[B]) were added into the mixture to yield an initial boron concentration of 1000 ppm, and start the chemical precipitation. The reaction pH was monitored and controlled at desirable value. The reaction proceeded for 7 h, and 5 mL of the solution was withdrawn at specific intervals and filtered through a 0.45 μm PVDF syringe filter (CHROMAFIL) for analysis. Before initiating FBHC, a preceding step was required to produce granular calcium perborate as homogeneous seeds. Initially, fine nuclei were formed under specific pH value. At a low superficial velocity, the fine particles were unable to be carried to the effluent, favoring the collisions with each other and the growth of particles (0.3−0.5 mm), which eventually became fluidizable granules. Treatment of boroncontaining solution via FBHC was carried out within the reactor which has been filled with a desirable bed height of granules. The boron solution and H2O2 were pumped into the pretreatment tank with certain flow rate and well-mixed for 20 min before flowed into the reactor. The up flow rate (Qup) was maintained at 410 mL/min by

Fluidized bed crystallization (FBC) is a good alternative to conventional coagulation in wastewater treatment. By controlling hydraulic conditions and degree of supersaturation, FBC recovers sparingly soluble species on the surface of existing seeds, normally quartz sands.16 FBC technology has been commercially applied in treatment of streams containing fluoride,17 hardness,18 and phosphate,19 and the reclaimed pellets normally require no further dewatering.20 FBC without seeds yielded homogeneous granules (denoted as fluidized-bed homogeneous crystallizer, FBHC) and has been used to recover various species from wastewaters, such as calcium carbonate,21 calcium oxalate,22 ferric phosphate,23 and lead carbonate.24 Furthermore, the removal efficiency of FBHC is higher than that of FBC since the activation energy of nucleation on homogeneous surfaces is lower than on heterogeneous surfaces.25 To our best knowledge, the fluidized-bed crystallization has not yet been applied to continuously remove boron from aqueous solutions by a mechanism of precipitation/crystal growth. This study aims to combine FBHC process with COP in order to recover aqueous boron as calcium perborate pellets and diminish the production of sludge. The experimental parameters investigated herein included the effluent pH, molar ratio of Ca/B, height of bed and surface loading based on the B


Research Article

ACS Sustainable Chemistry & Engineering adjusting the recirculation flow rate (Qr), equivalent to a superficial velocity of 25.6 m/h in the lower part of FBR. A typical run for aquatic measurements lasted at least 9 hydraulic retention time (HRT (min) = V (mL)/Qt (mL/min)) and samples were taken at effluent afterward. The efficacy of FBHC was evaluated through boron removal (BR %) and crystallization ratio (CR %). BR is the measure of the total removal of boron by analyzing the residual boron in the filtrates ([B]s). However, the removed boron could be attributed to either fines formation or immobilization onto fluidized particles. By digesting the samples in effluents with nitric acid, the portion of fines could be estimated by subtracting the residual boron from the digested boron concentrations ([B]t). The removed boron by crystallization onto the fluidized granules which referred to CR could be therefore calculated. BR% =

CR% =

[B]in Q B − [B]s Q t [B]in Q B [B]in Q B − [B]t Q t [B]in Q B

above 12, the boron removal drops sharply. In alkaline solution (pH > 11), the hydroxyl ion competes the Lewis acid sites of boron with hydrogen peroxide and thus borate anion (B(OH)4−) dominates the species of boron. Therefore, the dissolution of metal perborate improves at pH higher than 11.26 On the other hand, the solution was exposed to open air. The carbon dioxide absorption in aqueous solution was the main source of carbonate ion that would precipitate as calcium carbonate (CaCO3). The interference of CO2 in COP process has been verified in previous studies.8,9,14 The boron concentration reduced from 1000 to 150 ppm in the first 20 min and to 87 ppm in the following 3 h as shown in Figure 4. In the authors’ early work, the phase transformation of

× 100 (1) × 100 (2)

where QB (mL/min) is the input flow rate of B/H2O2 solution; Qt represents the sum of flow rates of B/H2O2 and Ca feedings. The concentrations of boron and calcium were analyzed using an inductively coupled plasma-optical emission spectrometer (ICP-OES, ULTIMA 2000, HORIDA). To determine the concentration of soluble peroxo species, the filtrate was titrated using 0.01 M permanganate (KMnO4, Showa) in 100 mL of 3% H2SO4 solution. The precipitates collected from batch test and granules recovered by FBHC were rinsed several times and then dried at 60 °C for 1 day. The crystalline phases of the collected samples were verified by an Xray diffraction analyzer (Rigaku, RX III) with Cu Kα radiation (40 kV, 30 mA). Raman microscopy (DRX, Thermo Scientific) was used to detect the functional groups on the surface of solid samples with a 780 nm laser. The morphology of the calcium perborate pellets was examined using a scanning electron microscope (SEM, Hitachi SU8010). The sieving technique was adopted to determine the particles size distribution of the granules.

Figure 4. Concentration of residual boron and the consumption ratios of [oxo]/[B] and [Ca]/[B] in the solution as functions of time (initial [B] = 1000 ppm, [Ca]/[B] =1, [H2O2]/[B] = 2, pH = 10).

amorphous Ba(B(OH) 3 OOH) 2 to crystalline BaB(OH)2(OO)2B(OH)2 was critical in further reducing boron to below 10 ppm.9 Obviously, however, it is no longer true when barium chloride was replaced by calcium chloride as the precipitant since the steep decline of boron was absent in the whole experimental period after the first 20 min precipitation reaction. The tendency of boron concentration change could specify that the transition of B(OH)3OOH) 22− to B(OH)2(OO)2B(OH)22− of calcium salts was never essential. To determine the composition of precipitates, mass balances on boron, peroxide, and calcium were performed, where [B], [Ca], and [oxo] herein refer to molar concentrations in the filtrates through a 0.45 μm filter. The molar consumption ratio of peroxide species to boron (Δ[oxo]/Δ[B]) is defined as eq 3. Δ[oxo]/Δ[B] refers to the mole of peroxide species transformed from aqueous to solid phases per unit mole of boron removed. Therefore, it is an indicator of molar ratio of peroxide to boron in the precipitates. According to the stoichiometry, the peroxo/boron ratios of B(OH)OOB(OH) 3 2− , B(OH)3OOH− and B(OH)2(OO)2B(OH)22−, B(OH)2(OOH)2− are 0.5, 1, 1 and 2, respectively. Likewise, eq 4 defines the molar consumption ratio of calcium ions to boron (Δ[Ca]/Δ[B]), which means the possible ratio of calcium to boron in the precipitates.

■

RESULTS AND DISCUSSION COP in Batch System. Our previous research revealed that pH significantly affected the solubility of barium perborates due to the speciation of peroxoborate anions (B(OH)3OOH−, B(OH)2(OO)2B(OH)22−, and B(OH)2(OOH)2−).26 Starting with synthetic wastewater containing 1000 ppm-B, COP was conducted with a molar ratio of [Ca]/[B] of 1, [H2O2]/[B] of 2, and varied reaction pH from 7 to 11. As presented in Figure 3, the removal of boron increases from 12% to 96.6% as pH is elevated from 7 to 11. The efficiency of COP is maximized at ∼pH 10.5, at which the species of peroxoborate (Figure 1b) can be efficiently precipitated with calcium salt. As pH increases

Δ[oxo]/Δ[B] =

Δ[Ca]/Δ[B] = Figure 3. Boron removal in batch system as a function of pH (initial [B] = 1000 ppm, [Ca]/[B] =1, [H2O2]/[B] = 2).

[H 2O2 ]0 − [oxo] [B]0 − [B] [Ca]0 − [Ca] [B]0 − [B]

(3)

(4)

and C


Research Article


The effect of pH on the performance of FBHC using BR and CR was investigated according to the results of the batch system. The concentration of boron in the inlet tank was 1000 ppm to yield an initial B of 500 ppm after mixing with the feeding of calcium chloride at the bottom of the reactor (the flow rates of boron and precipitant were the same). As displayed in Figure 5a, both BR and CR increase with pH. The

[B] = [B(OH)3 ] + [B(OH)4 − ] + [B(OH)3 OOH−] + [B(OH)2 (OOH)2− ] + 2[B(OH)3 OOH(OH)32 − ] + 2[B(OH)2 (OO)2 B(OH)2 2 − ] (5) [oxo] = [H 2O2 ] + [HO2−] + [B(OH)3 OOH−] + 2[B(OH)2 (OOH)2− ] + [B(OH)3 OOH(OH)32 − ] + 2[B(OH)2 (OO)2 B(OH)2 2 − ]

(6)

Initially (in 20 min), the consumption ratios Δ[oxo]/Δ[B] and Δ[Ca]/Δ[B] were 0.71 and 0.65, respectively. It reveals that the precipitates could be a mixture of CaB(OH)3OOB(OH)3 and Ca(B(OH3)OOH)2 (the presence of CaB(OH)2(OO)2B(OH)2 is excluded based on Raman microscopy in the following discussion). The boron level and Δ[oxo]/Δ[B] reached the steady values at 100 ppm-B and 0.75, respectively, in 120 min, indicating that the chemical equilibrium of calcium perborate precipitation has been attained. On the other hand, an increase in Δ[Ca]/Δ[B] implies that calcium ions are continuously consumed. Precipitation of calcium hydroxide (Ca(OH)2) and calcium carbonate (CaCO3) which may be responsible for the consumption of calcium ions could improve the removal of boron via coprecipitation and adsorption.27−29 On the basis of the solubility products of CaCO3 (10−8.39) and Ca(OH)2 (10−5.19), the hydrolysis of calcium ion and the equilibrium carbonate concentration in an open system (pCO2 = 10−3.5 atm), the solubility of CaCO3 and Ca(OH)2 in dilute electrolyte solution at pH 10 are estimated to be 10−6.71 M and 100.81 M, respectively. It argues that CaCO3 forms more likely in COP process. Integration of COP and FBHC for Boron Removal. The boron removal via COP mechanism was conducted in a FBR which has been filled with calcium perborate pellets (in a specific bed height). The Ergun equation was applied to estimate the minimum fluidization velocity (Vmf) beforehand.30

Vmf =

Figure 5. (a) Effect of effluent pH on the boron removal (BR%) and crystallization ratio (CR%). (b) The concentrations of dissolved oxygen (DO) and peroxo species as a function of effluent pH (initial [B] = 1000 ppm, [Ca]/[B] = 0.6, [H2O2]/[B] = 2, bed height = 80 cm, HRT = 18 min).

maximum BR and CR are 58.3% and 56.3%, respectively, at pH 10.5, and CR then drops rapidly to 30% when pH further increases to 11. The deviation between BR and CR was caused by the shifted degree of supersaturation. The degree of supersaturation governs the behavior of precipitation: a high degree of supersaturation leads to the formation of small nuclei. In a solution whose degree of supersaturation lies in metastable zone, the growth of existing grains is favored but instantaneous precipitation is suppressed.31 When performing FBHC system at pH > 10.5, the degree of supersaturation reached the labile zone, under which calcium perborate formed as small fines rather than grew on the existing surface of the particles. On the contrary, CR is close to BR in the range pH 8.5−10.5, implying that FBHC proceeds under suitable supersaturation and the boron was removed in a form of immobilized crystal on the fluidized particles. The idea that metastable zone is beneficial to crystalline growth onto the existing surfaces has been proposed. The metastable zones of struvite,32 calcium phosphates,33 and calcite34 have been determined in relevant to the rates of crystal growth. As the degree of supersaturation was high and the solute concentration increased to the region of homogeneous nucleation, the fine particulates formed.35,36 Costodes37 designed multiple feed points for FBC reactor that distributed supersaturation evenly, minimizing the formation of fines and improving the crystallization efficacy. Figure 5b plots the concentrations of dissolved oxygen (DO) and soluble peroxides against effluent pH. The DO levels that were higher than the saturated value (∼8 mg-O2 L−1) in the

Remf μf Dρf

(7)

Remf denotes the Reynolds number at minimal fluidization, which is derived from a rheological term, expressed by Archimedes number (Ar), Ar =

D3ρf (ρs − ρf )g μf 2

(8)

and Ar =

150(1 − ε) 1.75 × Remf + × Remf 2 2 3 3 Φε Φε

(9)

where Φ and D refer to sphericity and mean diameter (cm) of the particles; ε represents the bed void fraction which is typically 0.5 when particles begin to fluidize;24 ρs and ρf are the specific gravities of solid and fluid (g cm−3); g is the gravitational acceleration (980 cm s−2); and μf is the dynamic viscosity of fluid (0.00894 g cm−1 s−1 at 25 °C for water). ρs of calcium perborate was 2.604 g cm−3 from a pycnometer method, and the mean dimeter of seed particles was about 0.05 cm. Thus, Vmf was estimated to be 12.1 m h−1. In other words, the superficial velocity in the FBR was initiated at least twice the value of Vmf to ensure the fluidization. D


Research Article


increases with increasing [Ca] and reaches its highest plateau when the molar ratio [Ca]/[B] exceeds 0.7. On the other hand, CR is close to BR at [Ca]/[B] < 0.7, suggesting that the calcium perborate was recovered as fluidized granules. At the mole ratio of [Ca]/[B] > 0.7, CR deviates markedly from BR. The influence of calcium ions (as a precipitant) on crystallization in an FBHC is similar to that of protons (pH). On the basis of the proposed stoichiometry of calcium perborate: Ca(B(OH)3OOH)2 and CaB(OH)3OO(OH)3, the mole ratio [Ca]/[B] is 0.5 (eqs 12 and 13). As the dose of calcium chloride increases, the excess calcium ion attributes to high degree of supersaturation, and calcium perborates then favors the formation of fines.

selected pH range were inversely proportional to the concentration of aqueous peroxo-content. A remarkable increase in DO at a specific reaction time has been proposed to be an indicator of an enhancement of boron removal that was attributed to the transformation of amorphous barium perborate to well-crystallized one.8 In this work, the DO level increased with elevated effluent pH and reached its highest value at pH 10.5, where both BR and CR were maximized, and the aqueous peroxide approached its minimum. Restated, when calcium perborate reached its highest recovery rate at pH 10.5, the peroxide species were redistributed so that H 2 O 2 predominated among peroxides (including H2O2, HO2−, and perborate anions). The decomposition of H2O2 is therefore attributable to an increase in dissolved oxygen concentration with increasing pH due to the presence of HO2− ion (eqs 10 and 11).38 H 2O2 ↔ HO2− + H+

pK a = 11.51

H 2O2 + HO2− → O2 + H 2O + OH−

Ca(B(OH)3 OOH)2 (s) = Ca 2 + + 2B(OH)3 OOH− (12a) 2+

− 2

(10)

K sp1 = {Ca }{B(OH)3 OOH }

(11)

CaB(OH)3 OO(OH)3 (s) = Ca 2 + + B(OH)3 OO(OH)32 −

(12b)

(13a)

The mole ratio of [Ca]/[B] could affect the boron recovery in the fluidized-bed system. Figure 6a shows that the BR

2+

2−

K sp2 = {Ca }{B(OH)3 OO(OH)3 }

(13b)

The bed height in a fluidized-bed reactor influences the hydrodynamics.16 In this study, it represents the static bed height of the calcium perborates granules, i.e., the bed height when Qup = 0. The height of the crystal bed determines the reactive surface area for adsorption and heterogeneous nucleation.39 Figure 6b demonstrates clearly that BR is slightly improved from 38% to 46% as the bed height increases from 40 to 100 cm, while CR sharply increases with the bed height above 50 cm. Restated, when the bed height was less than 50 cm, the supersaturation was ineffectively reduced by less external surfaces which resulted in formation of primary nuclei. The surface loading (L, kg m−2 h−1) refers to the mass of boron in wastewater that is treated by the fluidized-bed reactor per unit cross-section area per unit time, which is critical for scaling up a laboratory-scale reactor to a pilot-scale reactor. The input flow rate controls the surface loading and affects the hydraulic retention time (HRT) as well. As displayed in Figure 6c, BR and CR are around 50% when a surface loading is lower than 2 kg m−2 h−1; CR declines to below 10% as L increases beyond 3 kg m−2 h−1. Since varying the surface loading by controlling the flow rate directly changed the HRT, shortening the HRT (from 1 h to 18 min for an L of 3.3 to 0.8 kg m−2 h−1) and reducing the frequency of collision of the initially formed small nuclei.26 Meanwhile, a high up flow rate also caused the easy draining out of small particles from the fluidized-bed reactor before they could agglomerate to form larger ones. The BR and CR operated with the absence of heterogeneous seed have been proved to be averagely higher than those using CaCO3 particles as seeds in FBC (provided in the Supporting Information). In other words, a fluidized bed prepared using the homogeneous material of calcium perborates granules could moderately enhance the recovery of COP products. Characterization of Calcium Perborate Pellets. Figure 7a shows the XRD patterns of precipitates that were collected from batch system at different pH values. The noisy signals from the precipitates collected at pH 8−9 reflect the poorly crystallized structure of calcium perborate, while some diffraction peaks that are attributed to calcite (CaCO3) and CaO2 phases appeared in ones obtained at pH 10−12. No obvious characteristic peaks are found to indicate the

Figure 6. Effects of (a) molar ratio of [Ca]/[B], (b) bed height (HRT = 18 min), and (c) surface loading (HRT from 18 to 64 min) on the efficiency of COP in a FBR reactor (initial [B] = 1000 ppm, [H2O2]/ [B] = 2, effluent pH 10). E


Research Article


Figure 7b presents the Raman spectra of calcium perborate precipitates to verify the functional groups on the surface. The band at 1551 cm−1 is attributed by O−O stretching of gaseous oxygen.40 The presence of symmetric stretching vibration of carbonate at 1084 cm−1 proves the formation of aforementioned CaCO3.41 The bands at 884 and 745 cm−1 are contributed by the peroxo group vsym(B−O(OO)) in perborate and metaborate vsym(B−O(OH)), respectively.42 The absence of bands of dinuclear ring-shaped perborate (−B(OO)2B−), which should appear at 984, 934, and 711 cm−1,43 indicates that CaB(OH)2(OO)2B(OH)2 is not a major phase in the calcium perborate. Since Δ[oxo]/Δ[B] has been found to approach a steady value of 0.75 (Figure 4), the calcium perborates could have a stoichiometric ratio of peroxide to boron ranged from 0.5 to 1. CaB(OH)3OOB(OH)3 and Ca(B(OH3)OOH)2 with molar ratios of peroxide to boron of 0.5 and 1, respectively, are therefore proposed to be the major constituents in the COP precipitates. Thus, the chemical reactions that were involved in the precipitation of perborate and the removal of boron in the COP process using CaCl2 as a precipitant exposed to the atmosphere are proposed as follows. 2B(OH)4 − + 2H 2O2 + Ca 2 + ↔ Ca(B(OH)3 OOH)2 (s) + 2H 2O

(14)

2B(OH)4 − + H 2O2 + Ca 2 +

Figure 7. (a) XRD patterns and (b) Raman spectra of calcium perborate precipitates.

crystallinity of the proposed precipitated compounds, such as CaB(OH)3OOB(OH)3 and Ca(B(OH3)OOH)2. Restated, the calcium perborate shall be amorphous, unlike BaB(OH)2(OO)2B(OH)2 which formed from slow phase transformation of Ba(B(OH3)OOH)2 using barium salt as a precipitant.26 The formation of CaCO3 was attributed to the precipitation of Ca with the absorbed CO2 in alkaline solution, consuming calcium salts and thereby reducing BR in the aforementioned discussion. As pH was elevated to 12, Ca(OH)2 precipitated and then was converted to CaO2· 8H2O at the presence of hydrogen peroxide.38

↔ CaB(OH)3 OOB(OH)3 (s) + 2H 2O

(15)

Ca 2 + + CO2 + 2OH− ↔ CaCO3(s) + H 2O

(16)

Ca 2 + + 2OH− + H 2O2 ↔ CaO2 (s) + 2H 2O

(17)

Figure 8 shows the morphology of the calcium perborate granules produced in FBHC for various duration of operation (24−300 h) under following conditions: initial [B] = 1000 ppm, [Ca]/[B] = 0.6, [H2O2]/[B] = 2, effluent pH 10, and HRT = 18 min. The pellet size grew up with increasing time, and the interior microtextures of crystallite under higher magnification differed obviously. At early stage, the pellets were

Figure 8. SEM observation of micromorphology of calcium perborate (upper) and the corresponding appearance of pellets (lower) collected at different duration of fluidized bed operation: (a) 24 h, (b) 100 h, (c) 200 h, and (d) 300 h (lower picture of part d shows the view of the transverse section). F


Research Article


Figure 9. Particle size distribution of FBC pellets.

instance, the authors were trying to convert the perborate to trimethyl borate (TMB), a precursor of sodium borohydride in the Schlesinger process; in such a case, boron has to be leached out as boric acid using strong acid in methanol. The low water content of granulated perborate substantially improved the efficiency of esterification with methanol and the final TMB yield. Moreover, considering the discharge limits of boron in water body (2.5 mg L−1 by WHO and 1 mg L−1 by Taiwan EPA), COP and FBHC must be integrated with the ternary processes which aimed to treat low level boron-containing water, such as adsorption, ion exchange, electrocoagulation, and membrane, to ultimately meet the regulations, even though an above 95% of boron removal was attained via the COP mechanism. Therefore, in the present work, COP and FBHC shall serve as an operation unit prior to the existing methods that are capable of lowering the boron level to regulation standard. Finally, the effects of coexisting species on COP performance are still unclear. Anions like fluoride, iodine, phosphate, and sulfate, cations such as zinc, copper, ammonia, and hardness and even organic compounds are reported to present in real boron-containing wastewater.1,49−51 The influence of those components on COP performance ought to be verified to implement this technology in the field widely.

Table 1 compares the precipitation of boron from solution using calcium salts. An elevated temperature and hydrothermal condition are required for the conventional precipitation. Lime (Ca(OH)2) was as the common precipitant to promote the formation of calcium borate (CaB2O5·H2O) in alkaline solution, along with some coprecipitation of mineral phases using sulfuric acid and phosphorous acid that could enhance the boron removal. COP is so powerful in removing boron through the peroxolysis and precipitation of boron in early works. Nevertheless, a large amount of water-rich sludge created by precipitation of boron needed further dewatering. This study has demonstrated a new approach to remove aqueous boron continuously by recovering granules of calcium perborate that contained little water content. In a fluidized-bed reactor, the amount of COP sludge could be minimized, simplifying the dewatering and improving the recoverability of byproduct (i.e., a pure calcium perborate) after treatment of boron solution. The removal of boron in a form of granulated products using FBHC is beneficial to further reuse of metal perborate. For

This study demonstrated the removal of boron from synthetic wastewater by integrating chemical oxo-precipitation (COP) with fluidized-bed homogeneous crystallizer (FBHC). The recovery of calcium perborate pellets resulted from peroxolysis of boric acid and homogeneous crystallization in a fluidized-bed reactor. The jar-test experiments revealed that batch COP was capable of removing 96.6% of boron (initial 1000 ppm-B) from solution by using CaCl2 at around pH 11, yielding a residual boron level lower than 50 ppm-B in the final solution. When conducting COP continuously in a fluidized-bed reactor, the crystallization ratio (CR) representing the ability to reclaim boron as particles in the FBHC system was related to the degree of supersaturation, which was strongly affected by effluent pH, calcium dosage, and surface loading. Characterization of the products by XRD, SEM, and Raman spectroscopy suggested that the calcium perborate particles were poorly crystallized and incorporated CaCO3 and CaO2 at high pH (>11). Ultimately, 60% of boron was recovered as unseeded calcium perborate granules in FBHC under the optimal

aggregated with needle-like nuclei; when keeping FBC operation, these fines became irregularly shaped. The crystallite of calcium perborate comprised mainly the plate-like grains after 300 h. Furthermore, the transverse plane of pellets proves that the product is free from heterogeneous seed material (Figure 8d). As suggested by the particle size distribution in Figure 9, the FBHC pellets of calcium perborate after 300 h of operation had a mass-median-diameter (D50) of approximately 1 mm.

■

CONCLUSIONS

Table 1. Comparison of Precipitation Methods Using Calcium Precipitants for Boron Removal initial boron concentration (ppm)

conditions

boron removal and products

ref

batch; room temperature; pH = 12; 20 min batch; 130 °C; 14 h

>95%; sludge of HApa, Ca(OH)2, CaCO3

44

99%; sludge of Ca2B2O5·H2O and HAp

45

batch; 150 °C; 2 h batch; 130 °C by microwave; pH = 13; 10 min batch; 90 °C; pH = 11.2; 2 h

99.2%; sludge of Ca2B2O5·H2O and CaF2 99%; sludge of Ca2B2O5·H2O and HApa

46 47

93%; sludge of Ca2B2O5·H2O and CaSO4

29

750 1000

batch; 60 °C; pH = 12.4; 8 h batch; room temperature; pH = 11

1000

[H2O2]/[B] = 2 [CaCl2] = 1

FBHC; room temperature; pH = 11; HRT = 18 min

87%; sludge of Ca2B2O5·H2O 96.6%; sludge of Ca(B(OH)3OOH)2 and CaB(OH)3OOB(OH)3 ∼60%; granules of Ca(B(OH)3OOH)2 and CaB(OH)3OOB(OH)3

48 14, this work this work

500 500 (as BF4 ) 500 700

a

reactants and dosages [Ca(OH)2]/[B] = 60 [(NH4)2HPO4]/[B] = 25 [Ca(OH)2]/[B] = 17.5 [H3PO4]/[B] = 6.5 [Ca(OH)2]/[B] = 10 [Ca(OH)2]/[B] = 19 [H3PO4]/[B] = 11 [Ca(OH)2]/[B] = 10 [H2SO4]/[B] = 8 [Ca(OH)2]/[B] = 2 [H2O2]/[B] = 2 [CaCl2] = 1

17.5

HAp represents hydroxyapatite (Ca10(PO4)6(OH)2). G


Research Article


peroxide: the role of dioxaborirane as the catalytic intermediate for a wide range of substrates. Org. Biomol. Chem. 2013, 11 (2), 309−17. (11) Durrant, M. C.; Davies, D. M.; Deary, M. E. Dioxaborirane: a highly reactive peroxide that is the likely intermediate in borate catalysed electrophilic reactions of hydrogen peroxide in alkaline aqueous solution. Org. Biomol. Chem. 2011, 9 (20), 7249−54. (12) Sadovskii, Y. S.; Solomoichenko, T.; Prokop’eva, T.; Piskunova, Z. P.; Razumova, N.; Panchenko, B.; Popov, A. Reactivity of the H2O2/B(OH)3/HO− system in the decomposition of 4-nitrophenyl esters of diethylphosphonic and diethylphosphoric acids. Theor. Exp. Chem. 2012, 48 (3), 163−171. (13) Chernyshov, B. N.; Vaseva, A. Y.; Brovkina, O. V. Study of peroxoborate complexes in aqueous and aqueous-acetone solution of sodium peroxoborate by the B-11 NMR method. Russ. J. Inorg. Chem. 1991, 36 (2), 460−463. (14) Shih, Y.-J.; Liu, C.-H.; Lan, W.-C.; Huang, Y.-H. A novel chemical oxo-precipitation (COP) process for efficient remediation of boron wastewater at room temperature. Chemosphere 2014, 111, 232− 237. (15) Bhoelan, B.; Stevering, C.; van der Boog, A.; Van der Heyden, M. Barium toxicity and the role of the potassium inward rectifier current. Clin. Toxicol. 2014, 52 (6), 584−593. (16) Bello, M. M.; Abdul Raman, A. A.; Purushothaman, M. Applications of fluidized bed reactors in wastewater treatment − A review of the major design and operational parameters. J. Cleaner Prod. 2017, 141, 1492−1514. (17) Jiang, K.; Zhou, K.; Yang, Y.; Du, H. A pilot-scale study of cryolite precipitation from high fluoride-containing wastewater in a reaction-separation integrated reactor. J. Environ. Sci. 2013, 25 (7), 1331−1337. (18) van Schagen, K.; Rietveld, L.; Babuška, R.; Baars, E. Control of the fluidised bed in the pellet softening process. Chem. Eng. Sci. 2008, 63 (5), 1390−1400. (19) Rahaman, M. S.; Mavinic, D. S.; Meikleham, A.; Ellis, N. Modeling phosphorus removal and recovery from anaerobic digester supernatant through struvite crystallization in a fluidized bed reactor. Water Res. 2014, 51, 1−10. (20) Garcia-Segura, S.; Bellotindos, L. M.; Huang, Y.-H.; Brillas, E.; Lu, M.-C. Fluidized-bed Fenton process as alternative wastewater treatment technologyA review. J. Taiwan Inst. Chem. Eng. 2016, 67, 211−225. (21) Mahasti, N. N. N.; Shih, Y.-J.; Vu, X.-T.; Huang, Y. H. Removal of calcium hardness from solution by fluidized-bed homogeneous crystallization (FBHC) process. J. Taiwan Inst. Chem. Eng. 2017, 78, 378−385. (22) Guevara, H. P. R.; Ballesteros, F. C.; Vilando, A. C.; de Luna, M. D. G.; Lu, M.-C. Recovery of oxalate from bauxite wastewater using fluidized-bed homogeneous granulation process. J. Cleaner Prod. 2017, 154, 130−138. (23) Priambodo, R.; Tan, Y.-L.; Shih, Y.-J.; Huang, Y.-H. Fluidizedbed crystallization of iron phosphate from solution containing phosphorus. J. Taiwan Inst. Chem. Eng. 2017, 80, 247−254. (24) Chen, C.-S.; Shih, Y.-J.; Huang, Y.-H. Remediation of lead (Pb(II)) wastewater through recovery of lead carbonate in a fluidizedbed homogeneous crystallization (FBHC) system. Chem. Eng. J. 2015, 279, 120−128. (25) Su, C.-C.; Dulfo, L. D.; Dalida, M. L. P.; Lu, M.-C. Magnesium phosphate crystallization in a fluidized-bed reactor: Effects of pH, Mg:P molar ratio and seed. Sep. Purif. Technol. 2014, 125, 90−96. (26) Lin, J.-Y.; Song, Y.-J.; Shih, Y.-J.; Huang, Y.-H. Solubility products of sparingly soluble barium perborates in aqueous solution that contains B(OH)3 and H2O2 at 25°C. J. Colloid Interface Sci. 2017, 505, 703−710. (27) Demey, H.; Vincent, T.; Ruiz, M.; Sastre, A.; Guibal, E. Development of a new chitosan/Ni(OH)2-based sorbent for boron removal. Chem. Eng. J. 2014, 244, 576−586. (28) Goldberg, S. Reactions of boron with soils. Plant Soil 1997, 193 (1), 35−48.

conditions of initial [B] = 1000 ppm, molar ratios of [Ca]/[B] = 0.6 and [H2O2]/[B] = 2, effluent pH = 10.6, bed height = 80 cm, and HRT = 18 min.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03951. COP of boron using CaCO3 as seeds and the particle growth after 4 h of fluidization reaction (Fig. S1) and efficiency of total boron removal (TR) and crystallization ratio (CR) as a function flow rate and molar ratio of Ca to B within a FBC system that used CaCO3 as seeds (Fig. S2) (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*Phone: +886 2757575 ext. 62636. Fax: +886 6 2344496. Email: [email protected]. *Phone: +8869973155271. E-mail: [email protected]. ORCID

Yao-Hui Huang: 0000-0002-7488-5759 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors would like to thank the Ministry of Science and Technology, Taiwan for financially supporting this research under Contract No. MOST 106-2622-E-006-003-CC2.

■

REFERENCES

(1) Isa, M. H.; Ezechi, E. H.; Ahmed, Z.; Magram, S. F.; Kutty, S. R. M. Boron removal by electrocoagulation and recovery. Water Res. 2014, 51, 113−123. (2) WHO. Chemical aspects. In Guidelines for Drinking-Water Quality, 4th ed.; World Health Organization: Geneva Switzerland, 2011; Chapter 8, pp 155−201. (3) Tagliabue, M.; Reverberi, A. P.; Bagatin, R. Boron removal from water: needs, challenges and perspectives. J. Cleaner Prod. 2014, 77, 56−64. (4) Arias, M. F. C.; Bru, L. V. i; Rico, D. P.; Galvañ, P. V. Comparison of ion exchange resins used in reduction of boron in desalinated water for human consumption. Desalination 2011, 278 (1− 3), 244−249. (5) Guan, Z.; Lv, J.; Bai, P.; Guo, X. Boron removal from aqueous solutions by adsorptionA review. Desalination 2016, 383, 29−37. (6) Karahan, S.; Yurdakoç, M.; Seki, Y.; Yurdakoç, K. Removal of boron from aqueous solution by clays and modified clays. J. Colloid Interface Sci. 2006, 293 (1), 36−42. (7) Tang, Y. P.; Luo, L.; Thong, Z.; Chung, T. S. Recent advances in membrane materials and technologies for boron removal. J. Membr. Sci. 2017, 541, 434−446. (8) Lin, J.-Y.; Shih, Y.-J.; Chen, P.-Y.; Huang, Y.-H. Precipitation recovery of boron from aqueous solution by chemical oxoprecipitation at room temperature. Appl. Energy 2016, 164, 1052− 1058. (9) Lin, J.-Y.; Shih, Y.-J.; Hsieh, T.-Y.; Huang, Y.-H. Role of phase transformation of barium perborates in the effective removal of boron from aqueous solution via chemical oxo-precipitation. RSC Adv. 2016, 6 (68), 63206−6321. (10) Deary, M. E.; Durrant, M. C.; Davies, D. M. A kinetic and theoretical study of the borate catalysed reactions of hydrogen H


Research Article

ACS Sustainable Chemistry & Engineering (29) Remy, P.; Muhr, H.; Plasari, E.; Ouerdiane, I. Removal of boron from wastewater by precipitation of a sparingly soluble salt. Environ. Prog. 2005, 24 (1), 105−110. (30) Pérez, N. P.; Pedroso, D. T.; Machin, E. B.; Antunes, J. S.; Ramos, R. A. V.; Silveira, J. L. Fluid dynamic study of mixtures of sugarcane bagasse and sand particles: Minimum fluidization velocity. Biomass Bioenergy 2017, 107, 135−149. (31) Karpinski, P. H.; Wey, J. S. Precipitation processes. In Handbook of Industrial Crystallization ,2nd ed.; Myerson, A. S., Ed. ButterworthHeinemann: Woburn, MA, 2002; pp 141−160. (32) Bhuiyan, M. I. H.; Mavinic, D. S.; Beckie, R. D. Nucleation and growth kinetics of struvite in a fluidized bed reactor. J. Cryst. Growth 2008, 310 (6), 1187−1194. (33) Kim, E.-H.; Yim, S.-B.; Jung, H.-C.; Lee, E.-J. Hydroxyapatite crystallization from a highly concentrated phosphate solution using powdered converter slag as a seed material. J. Hazard. Mater. 2006, 136 (3), 690−697. (34) Tai, C. Y.; Chien, W. C.; Chen, C. Y. Crystal growth kinetics of calcite in a dense fluidized-bed crystallizer. AIChE J. 1999, 45 (8), 1605−1614. (35) Aldaco, R.; Irabien, A.; Luis, P. Fluidized bed reactor for fluoride removal. Chem. Eng. J. 2005, 107 (1), 113−117. (36) Mokone, T.; Van Hille, R.; Lewis, A. Metal sulphides from wastewater: Assessing the impact of supersaturation control strategies. Water Res. 2012, 46 (7), 2088−2100. (37) Costodes, V. T.; Lewis, A. E. Reactive crystallization of nickel hydroxy-carbonate in fluidized-bed reactor: fines production and column design. Chem. Eng. Sci. 2006, 61 (5), 1377−1385. (38) Bouniol, P.; Lapuerta-Cochet, S. The solubility constant of calcium peroxide octahydrate in relation to temperature; its influence on radiolysis in cement-based materials. J. Nucl. Mater. 2012, 420 (1), 16−22. (39) Andrade, M. V. F.; Sakamoto, I. K.; Corbi, J. J.; Silva, E. L.; Varesche, M. B. A. Effects of hydraulic retention time, co-substrate and nitrogen source on laundry wastewater anionic surfactant degradation in fluidized bed reactors. Bioresour. Technol. 2017, 224, 246−254. (40) Walrafen, G. E. Surface Raman scattering from effervescent magnetic peroxyborates. J. Chem. Phys. 1982, 77 (8), 3840. (41) Pasierb, P.; Komornicki, S.; Rokita, M.; Rȩkas, M. Structural properties of Li2CO3−BaCO3 system derived from IR and Raman spectroscopy. J. Mol. Struct. 2001, 596 (1−3), 151−156. (42) Flanagan, J.; Griffith, W. P.; Powell, R. D.; West, A. P. Vibrational spectra of alkali metal peroxoborates. Spectrochim. Acta, Part A 1989, 45 (9), 951−955. (43) Flanagan, J.; Griffith, W. P.; Powell, R. D.; West, A. P. Nature of peroxoborate species in aqueous solution: a study by boron-11 nuclear magnetic resonance and Raman spectroscopy. J. Chem. Soc., Dalton Trans. 1989, 9, 1651−1655. (44) Yoshikawa, E.; Sasaki, A.; Endo, M. Removal of boron from wastewater by the hydroxyapatite formation reaction using acceleration effect of ammonia. J. Hazard. Mater. 2012, 237−238, 277−82. (45) Itakura, T.; Sasai, R.; Itoh, H. Precipitation recovery of boron from wastewater by hydrothermal mineralization. Water Res. 2005, 39 (12), 2543−2548. (46) Itakura, T.; Sasai, R.; Itoh, H. In situ solid/liquid separation effect for high-yield recovery of boron and fluorine from aqueous media containing borate or fluoroborate ions. Bull. Chem. Soc. Jpn. 2007, 80 (10), 2014−2018. (47) Tsai, H. C.; Lo, S. L. Boron removal and recovery from concentrated wastewater using a microwave hydrothermal method. J. Hazard. Mater. 2011, 186 (2), 1431−1437. (48) Irawan, C.; Kuo, Y.-L.; Liu, J. Treatment of boron-containing optoelectronic wastewater by precipitation process. Desalination 2011, 280 (1), 146−151. (49) Kabay, N.; Yılmaz, I.; Yamac, S.; Samatya, S.; Yuksel, M.; Yuksel, U.; Arda, M.; Sağlam, M.; Iwanaga, T.; Hirowatari, K. Removal and recovery of boron from geothermal wastewater by selective ion exchange resins. I. Laboratory tests. React. Funct. Polym. 2004, 60, 163−170.

(50) Itakura, T.; Sasai, R.; Itoh, H. A novel recovery method for treating wastewater containing fluoride and fluoroboric acid. Bull. Chem. Soc. Jpn. 2006, 79 (8), 1303−1307. (51) Kentjono, L.; Liu, J.; Chang, W.; Irawan, C. Removal of boron and iodine from optoelectronic wastewater using Mg−Al(NO3) layered double hydroxide. Desalination 2010, 262 (1), 280−283.

I


Reclaiming Boron as Calcium Perborate Pellets from Synthetic

Recommend Documents