Ind. Eng. Chem. Res. 2007, 46, 4577-4583
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Laboratory Study of Boron Removal by Mg/Al Double-Layered Hydroxides Jia-Qian Jiang,*,† Yonglan Xu,† Kieran Quill,‡ John Simon,‡ and Keith Shettle‡ School of Engineering (C5), UniVersity of Surrey, Guildford, Surrey GU2 7XH, and Borax Europe Ltd., 1A Guildford Business Park, Guildford, Surrey GU2 8XG
This paper is concerned with the preparation and use of Mg/Al double-layered hydroxides for boron removal from waste liquor. The structure of the DLHs synthesized is proposed to be Mg2AlNa1.4(OH)7.57Cl0.03(NO3)0.8‚ x(H2O) which does not contain any carbonate anions. For treating model waters with various starting boron concentrations (5-500 mg/L), the maximum boron percentage removal was >80% for DLH-60 and >90% for DLH-450. The boron removal capacity is 5.4-17.3 mg of B/g for DLH-450 and 1.2-13 mg of B/g for DLH-60, respectively. The raw water’s pH does not affect the boron removal performance. After six cumulative regenerations, boron percentage removal with regenerated DLHs decreased to about 40%. Overall, DLH-450 has a greater boron removal capacity than DLH-60 for both freshly prepared and regenerated materials. For the treatment of industrial effluent ([B]0 ) 17 mg/L), 86.6% boron removal was achieved at the dose of 36 g/L for DLH-60 and 93.5% at the dose of 16 g/L for DLH-450. Arsenic can be completely removed by both DLHs. The main mechanism of boron removal with DLH-60 is proposed to be anion exchange while that with DLH-450 is adsorption. For DLH-450, both Langmuir and Freundlich isotherm models fit well for the experimental results. Introduction Boron is a naturally occurring element bound to oxygen in the form of borates, which are extensively used in the manufacture of glass wool, ceramics, borosilicate glass, flame retardants, detergents, wood preservatives, antifreeze, and micronutrient fertilizers. High boron doses cause reproductive and developmental effects in several species (rats, mice, and rabbits).1,2 Based on the animal reproductive effects, various environmental regulation organizations have set up standards or guidelines to regulate the boron concentration in drinking water. In the revised European Community Drinking Water Directive,3 boron concentration should be less than 1.0 mg/L. The recent updated World Health Organization (WHO) guidelines4 for drinking water quality retain the recommended guideline value of boron at 0.5 mg/L. Recently, the U.S. Environmental Protection Agency (USEPA) published the second version of the Contaminant Candidate List5 where the boron is included and its concentration is recommended not to exceed to 1 mg/L. There are a range of technologies that could be used for removing boron/borate from wastewaters, such as electrocoagulation,6,7 which has recently attracted a lot of interest, chemical precipitation,8 ion exchange,9 and reverse osmosis.10 All these techniques consist of advantages and disadvantages and the selection of these processes should be based on the treatment efficiency and operating cost. Nevertheless, the exploration of alternative techniques to treat borate-containing wastewaters has re-drawn interest with the implementation of more stringent wastewater discharge regulations and drinking water standards. Double-layered hydroxides (DLHs) or hydrotalcites are a class of synthetic anionic clays, and their structure can be represented by the general formula M(II)1-xM(III)x(OH)2(An-)x/n‚mH2O in which both divalent (M(II)) and trivalent cations (M(III)) give * To whom correspondence should be addressed. Fax: +44 1483 450984. E-mail:
[email protected]. † University of Surrey. ‡ Borax Europe Ltd.
positively charged sheets. The positive charge is balanced by intercalation of anions (An-) in the hydrated interlayer regions. The interlayer anion together with the stoichiometric coefficient (x) may be varied over a wide range, giving a range of isostructural materials. DLHs have been used for adsorption of various anions, such as F-, Br-, NO3-, and HPO42-. One formula of DLHs is Mg6Al2(OH)16CO3‚4H2O,11 which sorbs anions in solution and returns to the DLH structure. However, the results showed that, after the first heating and rehydration cycle, the formula of DLH becomes disordered, with its sorption capacity reduced by 50%. Obviously, DLH with carbonate anions cannot be reversibly exchanged and this limits its use. Zhang and Reardon12 have investigated the removal of B, Cr, Mo, and Se oxyanions from high pH waters by two DLHs, hydrocalumite (Ca4Al2(OH)12(OH)2‚6H2O) and ettringite (Ca6Al2(OH)12(SO4)3‚26H2O). The study shows that hydrocalumite and ettringite are capable of reducing the concentrations of borate, chromate, molybdate, and selenate from solutions. Hydrocalumite in particular can reduce the oxyanion concentration levels to below drinking water standards. However, the most important limitation in using these calcium aluminates to control oxyanion levels in water is pH. High pH conditions must be maintained in treatment of the environment because both hydrocalumite and ettringite are unstable at low pH. Gabrisova et al.13 indicate that pH values greater than 10.7 are required to stabilize ettringite and greater than 11.6 to stabilize hydrocalumite. In the present work, we aim to prepare a different formula of DLH materials to solve the limitations of using Mg/Alcarbonate-DLH, hydrocalumite, and ettringite as detailed above, and then to explore the use of DLHs for borate treatment systematically, to propose boron removal mechanisms and to assess the regeneration efficiencies. Materials and Experimental Methods Preparation of Mg-Al DLH Compounds. Magnesium and aluminum were selected as basic metals to prepare Mg-Al DLHs. One hundred forty milliliters of a mixed solution containing 0.2 mol of Mg(NO3)2‚6H2O (Fisher, UK) and 0.1
10.1021/ie0703639 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/26/2007
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Table 1. Quality Characteristics of the Industry Effluenta parameter (mg/L) a
B 17.0
Al3+ 1.39 ×
10-2
As3+ 0.97
Ca2+ 110.4
T. Fe ions 3.65 ×
10-2
Mg2+
Na +
SO42-
Cl-
PO43 -
NO3-
18.0
74.8
316.7
111.7
1.2
8.4
Effluent: pH ) 7.9; conductivity ) 880 µS/cm.
mol of AlCl3‚6H2O(Fisher, UK) (the molar ratio of Mg to Al was 2:1) was slowly added into 300 mL of a 2.0 M NaOH solution by a peristaltic pump for 1 h and under vigorous mechanical stirring. During this process, the reaction temperature was controlled at 45 ( 3 °C. After reaction, the thick slurry was aged at 85 ( 3 °C for 2 h. The solid products were separated by centrifugation (3500 rpm for 5 min) and washed six times with deionized water. Finally, the DLH products were dried at 60 °C for 24 h and 450 °C for 2 h, respectively. And then they were crushed to powders named as DLH-60 and DLH-450. Characterization of DLHs Products. X-ray diffraction (XRD) patterns were obtained for randomly orientated samples using a Siemens D5000 diffraction system and the data was processed using Siemens Diffrac Plus processing software. The composition of DLHs was determined by mass balance protocol. The volume of supernatant of synthesis slurry was accurately measured and recorded. Concentrations of cations and anions in the supernatant were analyzed by an inductively coupled plasma atomic emission spectrophotometer (ICP-AES) and an ion chromatograph (IC, Metrohm Ltd.), respectively. Then the residual mass of each ion in the supernatant was calculated. And then, based on the mass difference between ion dosed in the preparation and ion residuals in the supernatant, the composition of the DLHs can be estimated using the following equation. The mass of each ion can be converted to moles which can be used to estimate the formula of the DLH product.
mass of the ith ion in the DLH product (g) ) dose in the preparation residual mass in the supernatant ) dose in the preparation (g) 0.334 (L)*[the i th ion] (mg/L)/1000 (mg/g) * 0.334 L ) the total volume of each supernatant Surface areas of DLHs products were measured with a Micrometrics Gemini Apparatus. The samples were outgassed overnight under nitrogen. Surface areas were determined using the Brunauer-Emmett-Teller (BET) equation using nitrogen as the adsorbate. Densities of DLHs products were measured. A volumetric cylinder with 10 mL capacity was weighed and the mass of the cylinder was recorded. Then 10 mL of HT samples was filled in the cylinder and weighed. The mass difference between two measurements is calculated, which is divided by 10 mL, resulting in the density of the DLHs sample. The particle size of DLHs was observed by a high-resolution scanning electron microscope (SEM) (Philips FEI FEG XL30). Model Water. In this work, the model water was made by mixing a given amount of boric acid with 1 L of 0.26 g/L (4.44 mM) sodium chloride (prepared using deionized water with GR grade NaCl) to make the boron concentration in a range from 5 to 500 mg/L. Five and 1 M sodium hydroxide solutions were used to adjust the initial pH value. Industry Effluent. The quality characteristics of an industry effluent sample can be seen in Table 1. Adsorption Kinetics. Two hundred milligrams of DLH was weighed and added into 25 mL of 10 mg/L boron model water
(the initial pH was adjusted to 7 by 1 M NaOH) with vigorously shaking. For different time intervals (30 min to 4 h), water samples were withdrawn and the boron concentration was measured. Experimental Setup for Boron Removal. Various doses of DLH-60 and DLH-450 were added into 50 mL screw-top plastic tubes filled with 25 mL of model water with boron concentrations ranging from 5 to 500 mg/L. The DLH was mixed with model water in the tubes via a shaker with a shaking speed of 300 rpm for 4 h and then centrifuged. Finally, the supernatant of the solution was collected for the analysis of concentrations of boron and other elements and the data were used for obtaining boron reduction isotherms and for the estimation of removal mechanisms. Effect of pH on the Boron Removal Efficiency. Initial pH of the boron model water ([B]0 ) 10 mg/L) was adjusted to 4-11 by 5 M NaOH or 1 M HCl. Twenty-five milliliters of such water was mixed with 750 mg of DLH. After 4 h of vigorous shaking, water samples were withdrawn and boron concentrations were measured. Adsorption Isotherm. Twenty-five milliliters of boron model water samples ([B]0 ) 0-70 mg/L) were mixed with 200 mg of DLH. After 4 h of vigorous shaking at room temperature (25 °C), boron concentrations of the treated samples were measured. Overall Processes of Regeneration. Since the DLHs regenerated with 5 M NaNO3 resulted in a relatively better boron removal in comparison with that regenerated by either NaCl or Na2SO4, 5 M NaNO3 was selected as the reagent for subsequent regeneration tests. The regenerated DLHs were used for the boron removal tests following the same procedure as detailed above. Figure 1 shows the overall processes of adsorption and regeneration. Twenty-five milliliters of 100 mg/L boron model water with 0.7 g of DLH-60 or 0.3 g of DLH-450 were mixed with a shaking speed of 300 rpm for 4 h and then centrifuged. The supernatant was withdrawn and concentrations of Cl-, NO3-, and B were analyzed and recorded. The solid phase was washed by distilled water two times. And then 25 mL of 5 M NaNO3 was added into the tubes and mixed completely with shaking for 4 h. The mixture was then centrifuged and the supernatant was analyzed for the measurement of anions concentration. The solid phase was washed by deionized water several times and then heated at either 60 °C for 24 h or 450 °C for 2 h. Same as previously stated, after first regeneration, the gained DLH materials were named as DLH-60-1 or DLH-450-1, respectively, and for each subsequent regeneration, the regenerated DLHs were named as DLH-60-2 or DLH-450-2, etc. Mechanisms of Boron Removal with DLHs. The boron removal mechanisms by DLHs were carried out using boron model water with various concentrations. After 4 h of shaking and centrifugation, the supernatant was collected for the analysis of anions and cations by ICP and IC. The changes in concentrations of boron, nitrate, and chloride were used to interpret the possible mechanisms. Results and Discussion XRD Analysis of DLHs. For all DLHs, a hydrotalcite structure is confirmed by the XRD analysis. However, a standard
Ind. Eng. Chem. Res., Vol. 46, No. 13, 2007 4579
Figure 1. Regeneration process for DLH-60 (DLH-450).
Figure 2. XRD analysis of DLH-60.
hydrotalcite consists of CO32- anion but DLH prepared in this study consists of NO3- and Cl- anions. In addition to this, after regeneration, the structure of the DLH-60 changes; DLH-60-1 consists of bayerite, a type of Al(OH)3, while the original DLH60 does not. However, this does not change the surface area significantly as shown later. Figure 2 presents an example of the XRD results for DLH-60. Composition of DLH Products. Based on the mass balance results shown in Table 2, the formula of DLHs prepared for this study can be proposed as
Mg2AlNa1.4(OH)7.57Cl0.03(NO3)0.8‚x(H2O) where the OH content was established based on the results of mass balance and electronic balance. Density of DLHs. The density of DLH-60 is 1.09 g/cm3 and DLH-450 is 0.90 g/cm3.
Surface Areas of DLHs. Table 3 displays surface areas of DLHs. It can be seen that the surface area of DLH-60 was much smaller than that of DLH-450, while boron removal efficiency of both DLHs was not significantly different (15% in difference). Then, we can speculate that the boron removal mechanisms with DLH-60 and DLH-450 are very different and this will be discussed in a later section. Particle Size of DLH-60 and DLH-450. Figure 3 shows two pictures taken by a high-resolution scanning electron microscope (SEM). It can be seen that both DLHs have an average size less than 100 nm in diameter. Effect of DLH Dose on Boron Removal. Boron removal performance with DLH-60 and DLH-450 was studied at pH 7 via model waters with various boron starting concentrations, ranging from 5 to 500 mg/L. For both DLHs, boron percentage removal increased with increasing doses until a maximum value was reached (>80% for the DLH-60 and >90% for DLH-450).
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Figure 3. SEM image of two DLHs: Left: DLH-60; right: DLH-450.
Figure 4. Boron removal vs DLHs’ dose. [B]0 )10 and 500 mg/L; T ) 25 °C; pH0 ) 7. Table 2. Composition of DLH content in DLH ion
dose in the preparation (g)
residual in the supernatant (g)
(g)
(mol)
Mg2+ Al3+ Na+ ClNO3-
4.80 2.70 13.80 10.65 24.80
0.00 0.03 10.60 6.42 19.82
4.80 2.67 3.20 0.10a 4.98
0.20 0.10 0.14 0.003 0.08
a Obtained by XRF measurement results. Most of the chloride was washed away during the washing procedure.
Table 3. Surface Areas of DLHs samples
surface area (m2/g)
boron % removal
DLH-60 DLH-450
31.30 130.18
84 97
Table 4. Langmuir and Freundlich Constants for DLH-450 Langmuira
Freundlichb
[B]0 (mg/L)
q0 (mg/g)
b (L/mg)
RL
R2
Kf
n
R2
100 250 500
16.1 17.0 17.3
0.26 0.11 0.06
0.04 0.04 0.03
0.9226 0.9226 0.9602
2.805 4.864 4.710
1.873 3.986 4.255
0.8958 0.9657 0.9225
a In Langmuir equation, the constant q signifies the adsorption capacity 0 (mg/g) and b is related to the energy of adsorption (L/mg). b In Freundlich equation, Kf and n are adsorption isotherm constants, being indicative of the capacity and intensity of adsorption.
Figure 4 shows an example of the comparative boron removal by DLH-60 and DLH-450 for model waters with two starting boron concentrations. It can be seen that, at given dose ranges of the DLH, boron percentage removal with DLH-450 was 50% higher than that with DLH-60.
pH-Dependent Experiments. Twenty-five milliliters of 10 and 100 mg/L boron model water was adjusted to its initial pH to between 3 and 11 by either 5 M NaOH or 1 M HCl and then mixed with 0.75 g of HT-60 and 0.3 g of HT-450, respectively. After 4 h of vigorous shaking, water samples were filtrated, and boron concentrations and filtrate pHs were determined. Filtrate pHt remained constant around 8.6 for DLH-60 and 10.5 for DLH-450, indicating that DLH has a high buffering capacity. Adsorption Isotherm. Adsorption study was conducted using DLH-450 and the isotherms were developed based on the results achieved. Table 4 is illustrated by plotting various isotherms results with Langmuir and Freundlich equations. The fitness of using Langmuir equation to describe the adsorption process can be assessed by a term “RL”, which is a dimensionless constant and is defined as RL ) 1/(1 + bC0) (where b is a constant in the Langmuir equation and C0 is boron equilibrium concentration). The parameter RL indicates the shape of the isotherm accordingly: RL > 1, unfavorable; RL ) 1, linear; 0 < RL < 1, favorable; and RL ) 0, irreversible. Similarly, the fitness of using the Freundlich equation to describe the adsorption can be assessed by the constant, n. If 1 < n < 10, the Freundlich equation is adequate for use. It can seen from Table 4 that both Langmuir and Freundlich equations fit the adsorption results and they can be used to propose the adsorption capacity of DLH-450 on the boron removal. The adsorption capacity of DLH-450 ranges from 16.1 to 17.3 mg of B/g of adsorbent for three starting boron concentrations studied. Boron Removal Mechanisms with DLHs. The DLHs have a permanent positive charge originating from the isomorphic substitution of bivalent (Mg2+) by trivalent (Al3+) ions, and amphoteric charges resulting from surface hydroxyl groups (broken edges), which develop a variable charge through protonation-deprotonation reactions.14 The net positive charge on the DLHs is balanced by anions close to the planar surface and interlayer in the DLH. The removal of boron/borate can occur by anion exchange with both the intercalated and surface anionic charge of the DLH. Besides exchanging ions, which is mainly an electrostatic process, DLHs have surface groups that can establish chemical bonds with boron/borate molecules. Mass variations of boron, Mg, Al, and NO3- in the supernatant can be seen in Table 5. Al almost did not release into the solution, indicating that the main structure of DLH-60 (AlOHx)n remained unchanged. However, concentrations of Mg2+ and NO3- in the supernatant increased while the mass of borate decreased after 4 h of mixing. The amount of NO3- released was greater or similar to the amount of boron removed for the
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Figure 5. XRD analysis of DLH-450.
Table 5. Mass Difference in the Model Water before and after Treatment with DLH-60a mass of B model in the ∆B ∆Mg water model water (removed) (increased) sample (mmol) (mmol) (mmol) 1 2 3 4 5 6 a
0.01 0.02 0.12 0.23 0.58 1.16
0.01 0.02 0.11 0.21 0.49 0.91
0.18 0.17 0.19 0.23 0.30 0.38
∆Al (increased) (mmol)
∆NO3(increased) (mmol)
9.25 × 10-5 6.48 × 10-5 9.25 × 10-5 9.25 × 10-5 1.11 × 10-4 1.67 × 10-4
0.45 0.40 0.42 0.46 0.53 0.60
DLH-60 dose ) 50 g/L; pH0 ) 7.
Table 6. DLH-450 for Boron Removal from 100 mg/L Boron Model Water, Dose ) 28 g/L NO3-
[B] pH0
(mg/L)
(mmol)
model water 7.00 100.00 0.23 DLH-450 10.60 4.02 9.30 × 10-3 removed or 0.22 released ion
(mg/L)
(mmol)
0 3.22
0 1.30 × 10-3 1.30 × 10-3
B% removal 0 95.65
samples with relative low boron starting mass/concentrations (samples 1-5, 5 to 100 mg/L) which is adequate to exchange with borate in the interlayer of DLH-60. For higher boron starting concentration (sample 6, [B]0 ) 500 mg/L), the released NO3- was less than the amount of boron removed, and then the released Mg ions contributed to the borate removal via the proposed Mg-B precipitation. Nevertheless, anion exchange seems to be the main mechanism of boron removal by DLH60 with the evidence of the mass difference of samples 1-5 shown in Table 5. Results shown in Table 5 also raise the concern of whether the DLH-60 is stable or not for long-term use for water and wastewater treatment. Under most study conditions and for the model water, nitrate release is higher than that required for the exchange of boron, and excess nitrate release indicates that, after one use, the DLH-60 has to be regenerated with nitrate salts to repossess an anion exchange capacity. The subsequent regeneration study has confirmed this (see later section). However, the
Mg releasing for each use is not significant; the maximum amount of Mg releasing is about 4% of the original Mg content for a dose of DLH-60 (e.g., 50 g/L). Therefore, DLH-60 could be stable for a long-term use. Table 6 shows the boron removal with DLH-450 and the released amount of NO3- during the treatment. Only a few NO3-’s were released from DLH-450, and then the high boron percentage removal (95.65%) with DLH-450 cannot be attributed to the anion exchange, but most possibly by a physicochemical adsorption due to its relatively high surface area (see Table 3) and this is confirmed by the adsorption isothermal data presented in Table 4. A few NO3-’s releasing from DLH-450 in the treatment (Table 6) can be judged that DLHs calcined at 450 °C lose their layered structure and become oxides; thus, the majority of anions (NO3-) release during the calcination process. This is evident by XRD spectra of DLH450, where the intensity of DLHs structure decreased significantly with increasing in the basal spacing and oxide structures appear (Figure 5). The proposed boron removal mechanisms are presented in Figure 6 which includes anion exchange for DLH-60 and borate sorption via ligand complexion, mainly for DLH-450. OH ligands exist in all edges of the DLH and they are ready to complex with borate and thus remove boron from solutions. The similar complexes between OH ligands and borate or boric acid were proposed previously when hydroxycaroxylic acids were tested to complex and precipitate boric acid.15 A desorption test was carried out to validate the above adsorption mechanism by DLH-450. DLH-450 (0.3 g) was mixed with 25 mL of model water ([B]0 ) 100 mg/L) for shaking at 300 rpm for 4 h, and the mixture was separated by centrifugation. The resulting supernatant was collected and concentrations of boron and other components were analyzed. The separated DLH-450 was washed and then mixed with 25 mL of 5 M Na2SO4, under conditions of shaking at 300 rpm for 4 h. The mixture with desorpted DLH-450 was then separated and concentrations of boron and others in the supernatant were analyzed. The mass balance was made based on the boron removed in adsorption and boron released in
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Table 7. Quality Characteristics of the Industry Effluent Treated by DLHsa parameter
boron (mg/L)
As3+ (mg/L)
Ca2+ (mg/L)
Mg2+ (mg/L)
Na+ (mg/L)
SO42(mg/L)
PO43(mg/L)
NO3(mg/L)
pH
B%
industry effluent DLH-60 DLH-450
17.0 2.3 1.1
0.97 90% with DLH-450). The boron removal capacity for DLH450 is 5.4-17.3 mg of B/g and that for DLH-60 is 1.2-13 mg of B/g, respectively. The raw water’s pH does not affect the boron removal performance with DLHs. For the treatment of industrial effluent ([B]0 ) 17 mg/L), boron removal increased with DLH doses: 86.6% by the dose of 36 g/L for DLH-60 and 93.5% by the dose of 16 g/L for DLH-450. Arsenic can be completely removed by both DLHs.
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Comparison with DLH-60, less Al, Mg, Na, Cl-, and NO3were released and more Ca and Fe were removed with DLH-450. The main mechanism of boron removal with DLH-60 is to be anion exchange while that with DLH-450 is adsorption. For DLH-450, both Langmuir and Freundlich isotherm models fit well with the experimental results. Boron removal performance with DLHs decreased gradually after each step of regeneration. After six cumulative regenerations, boron percentage removal with regenerated DLHs decreased to about 40%. Overall, DLH-450 has a greater boron removal capacity than DLH-60 for both freshly prepared and regenerated materials. Acknowledgment The authors are grateful for the financial support from the Borax Europe Ltd. The views of this paper are not necessary representing that of the Borax Europe Ltd. Literature Cited (1) Guidelines for Drinking-Water Quality 2nd ed.; Addendum to Vol. 1. Recommendations of Boron, pp 4-6, and Addendum to Vol. 2, Boron, pp 15-29; World Health Organisation: Geneva, 1998. (2) European Food Standards Agency. Opinion Of The Scientific Panel on Dietetic Products, Nutrition and Allergies on a Request from the Commission Related to the Tolerable Upper Intake LeVel of Boron (sodium borate and boric acid); EFSA, J. 80. 2004. (3) Council of European Communities Directive (CECD) 98/83. On the Quality of Water Intended for Human Consumption; EC Official Journal, L330/41, Brussels, 1998. (4) Guidelines for Drinking-Water Quality. 3rd ed., Vol. 1, Recommendations; World Health Organisation: Geneva, 2004.
(5) U.S. Environmental Protection Agency, The Drinking Water Contaminant Candidate List 2. Fact Sheet EPA 815-F-05-001; EPA: Washington DC, 2005. (6) Jiang, J. Q.; Xu, Y.; Simon, J.; Quill, K.; Shettle, K. Removal of Boron (B) from Waste Liquors. Water Sci. Technol. 2006, 53 (11), 73. (7) Jiang, J. Q.; Xu, Y.; Simon, J.; Quill, K.; Shettle, K. Mechanism of Boron Removal with Electrocoagulation. EnViron. Chem. 2006, 3, 350. (8) Choi, W.-W.; Chen, K. Y. Evaluation of Boron Removal by Adsorption on Solids. EnViron. Sci. Technol. 1979, 13, 189. (9) Badruk, M.; Kabay, N.; Demircioglu, M.; Mordogan, H.; Ipekoglu, U. Removal of Boron from Wastewater of Geothermal Power Plant by Selective Ion-Exchange Resins. II. Column sorption-elution studies. Sep. Sci. Technol. 1999, 34, 2981. (10) Masahide, T.; Yoshinari, F.; Tsuyoshi, N.; Masaru, K. Boron Removal in RO Seawater Desalination. Desalination 2004, 167, 419. (11) Parker, L. M.; Milestone, N. B.; Newman, R. H. The Use of Hydrotalcite as an Anion Absorbent. Ind. Eng. Chem. Res. 1995, 34, 1196. (12) Zhang, M.; Reardon, E. J. Removal of B, Cr, Mo, and Se from Wastewater by Incorporation into Hydrocalumite and Ettringite. EnViron. Sci. Technol. 2003, 37, 2947. (13) Gabrisova, A.; Havlica, J.; Sahu, S. Stability of Calcium Sulphoaluminate Hydrates in Water Solutions with Various pH Values. Cement Concrete Res. 1991, 21, 1023. (14) Delgado, R. R.; Vidaurreb, M. A.; de Pauli, C. P.; Ulibarria, M. A.; Avena, M. J. Surface-Charging Behavior of Zn-Cr Layered Double Hydroxide. J. Colloid Interface Sci. 2004, 280 (2), 431. (15) Farmer, J. B. The Complexation of Borate from Solution by Hydroxycarboxylic Acids; Technical Report No. TR-74-7; Borax Technical Ltd.: London, 1974.
ReceiVed for reView March 9, 2007 ReVised manuscript receiVed April 9, 2007 Accepted April 21, 2007 IE0703639
