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 removalfrom 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 boronconcentrations (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 forDLH-60, respectively. The raw water’s pH does not affect the boron removal performance. After six cumulativeregenerations, boron percentage removal with regenerated DLHs decreased to about 40%. Overall, DLH-450has a greater boron removal capacity than DLH-60 for both freshly prepared and regenerated materials. Forthe treatment of industrial effluent ([B]0 ) 17 mg/L), 86.6% boron removal was achieved at the dose of 36g/L for DLH-60 and 93.5% at the dose of 16 g/L for DLH-450. Arsenic can be completely removed by bothDLHs. The main mechanism of boron removal with DLH-60 is proposed to be anion exchange while thatwith DLH-450 is adsorption. For DLH-450, both Langmuir and Freundlich isotherm models fit well for theexperimental results.

Introduction

Boron is a naturally occurring element bound to oxygen inthe form of borates, which are extensively used in themanufacture of glass wool, ceramics, borosilicate glass, flameretardants, detergents, wood preservatives, antifreeze, and mi-cronutrient fertilizers.

High boron doses cause reproductive and developmentaleffects in several species (rats, mice, and rabbits).1,2 Based onthe animal reproductive effects, various environmental regulationorganizations have set up standards or guidelines to regulatethe boron concentration in drinking water. In the revisedEuropean Community Drinking Water Directive,3 boron con-centration should be less than 1.0 mg/L. The recent updatedWorld Health Organization (WHO) guidelines4 for drinkingwater quality retain the recommended guideline value of boronat 0.5 mg/L. Recently, the U.S. Environmental ProtectionAgency (USEPA) published the second version of the Con-taminant Candidate List5 where the boron is included and itsconcentration is recommended not to exceed to 1 mg/L.

There are a range of technologies that could be used forremoving boron/borate from wastewaters, such as electroco-agulation,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 disadvantagesand the selection of these processes should be based on thetreatment efficiency and operating cost. Nevertheless, theexploration of alternative techniques to treat borate-containingwastewaters has re-drawn interest with the implementation ofmore stringent wastewater discharge regulations and drinkingwater standards.

Double-layered hydroxides (DLHs) or hydrotalcites are a classof synthetic anionic clays, and their structure can be representedby the general formula M(II)1-xM(III) x(OH)2(An-)x/n‚mH2O inwhich both divalent (M(II)) and trivalent cations (M(III)) give

positively charged sheets. The positive charge is balanced byintercalation 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 ofisostructural materials.

DLHs have been used for adsorption of various anions, suchas F-, Br-, NO3

-, and HPO42-. One formula of DLHs is Mg6-

Al2(OH)16CO3‚4H2O,11 which sorbs anions in solution andreturns to the DLH structure. However, the results showed that,after the first heating and rehydration cycle, the formula of DLHbecomes disordered, with its sorption capacity reduced by 50%.Obviously, DLH with carbonate anions cannot be reversiblyexchanged 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 andettringite are capable of reducing the concentrations of borate,chromate, molybdate, and selenate from solutions. Hydrocalu-mite in particular can reduce the oxyanion concentration levelsto below drinking water standards. However, the most importantlimitation in using these calcium aluminates to control oxyanionlevels in water is pH. High pH conditions must be maintainedin treatment of the environment because both hydrocalumiteand ettringite are unstable at low pH. Gabrisova et al.13 indicatethat pH values greater than 10.7 are required to stabilizeettringite and greater than 11.6 to stabilize hydrocalumite.

In the present work, we aim to prepare a different formulaof DLH materials to solve the limitations of using Mg/Al-carbonate-DLH, hydrocalumite, and ettringite as detailed above,and then to explore the use of DLHs for borate treatmentsystematically, to propose boron removal mechanisms and toassess the regeneration efficiencies.

Materials and Experimental Methods

Preparation of Mg-Al DLH Compounds. Magnesium andaluminum were selected as basic metals to prepare Mg-AlDLHs. One hundred forty milliliters of a mixed solutioncontaining 0.2 mol of Mg(NO3)2‚6H2O (Fisher, UK) and 0.1

* To whom correspondence should be addressed. Fax:+44 1483450984. E-mail: j.jiang@surrey.ac.uk.

† University of Surrey.‡ Borax Europe Ltd.

4577Ind. Eng. Chem. Res.2007,46, 4577-4583

10.1021/ie0703639 CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 05/26/2007

mol of AlCl3‚6H2O(Fisher, UK) (the molar ratio of Mg to Alwas 2:1) was slowly added into 300 mL of a 2.0 M NaOHsolution by a peristaltic pump for 1 h and under vigorousmechanical stirring. During this process, the reaction temperaturewas controlled at 45( 3 °C. After reaction, the thick slurrywas aged at 85( 3 °C for 2 h. The solid products were separatedby centrifugation (3500 rpm for 5 min) and washed six timeswith deionized water. Finally, the DLH products were dried at60 °C for 24 h and 450°C for 2 h, respectively. And then theywere crushed to powders named as DLH-60 and DLH-450.

Characterization of DLHs Products. X-ray diffraction(XRD) patterns were obtained for randomly orientated samplesusing a Siemens D5000 diffraction system and the data wasprocessed using Siemens Diffrac Plus processing software. Thecomposition of DLHs was determined by mass balance protocol.The volume of supernatant of synthesis slurry was accuratelymeasured and recorded. Concentrations of cations and anionsin the supernatant were analyzed by an inductively coupledplasma atomic emission spectrophotometer (ICP-AES) and anion chromatograph (IC, Metrohm Ltd.), respectively. Then theresidual mass of each ion in the supernatant was calculated.And then, based on the mass difference between ion dosed inthe preparation and ion residuals in the supernatant, thecomposition of the DLHs can be estimated using the followingequation. The mass of each ion can be converted to moles whichcan be used to estimate the formula of the DLH product.

Surface areas of DLHs products were measured with aMicrometrics Gemini Apparatus. The samples were outgassedovernight under nitrogen. Surface areas were determined usingthe Brunauer-Emmett-Teller (BET) equation using nitrogenas the adsorbate.

Densities of DLHs products were measured. A volumetriccylinder with 10 mL capacity was weighed and the mass of thecylinder was recorded. Then 10 mL of HT samples was filledin the cylinder and weighed. The mass difference between twomeasurements 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-resolutionscanning electron microscope (SEM) (Philips FEI FEG XL30).

Model Water. In this work, the model water was made bymixing a given amount of boric acid with 1 L of 0.26 g/L (4.44mM) sodium chloride (prepared using deionized water with GRgrade NaCl) to make the boron concentration in a range from5 to 500 mg/L. Five and 1 M sodium hydroxide solutions wereused to adjust the initial pH value.

Industry Effluent. The quality characteristics of an industryeffluent sample can be seen in Table 1.

Adsorption Kinetics. Two hundred milligrams of DLH wasweighed and added into 25 mL of 10 mg/L boron model water

(the initial pH was adjusted to 7 by 1 M NaOH) with vigorouslyshaking. For different time intervals (30 min to 4 h), water sam-ples were withdrawn and the boron concentration was measured.

Experimental Setup for Boron Removal.Various doses ofDLH-60 and DLH-450 were added into 50 mL screw-top plastictubes filled with 25 mL of model water with boron concentra-tions ranging from 5 to 500 mg/L. The DLH was mixed withmodel water in the tubes via a shaker with a shaking speed of300 rpm for 4 h and then centrifuged. Finally, the supernatantof the solution was collected for the analysis of concentrationsof boron and other elements and the data were used for obtainingboron reduction isotherms and for the estimation of removalmechanisms.

Effect of pH on the Boron Removal Efficiency. Initial pHof the boron model water ([B]0 ) 10 mg/L) was adjusted to4-11 by 5 M NaOH or 1 M HCl. Twenty-five milliliters ofsuch water was mixed with 750 mg of DLH. After 4 h ofvigorous shaking, water samples were withdrawn and boronconcentrations were measured.

Adsorption Isotherm. Twenty-five milliliters of boron modelwater samples ([B]0 ) 0-70 mg/L) were mixed with 200 mgof DLH. After 4 h of vigorous shaking at room temperature(25 °C), boron concentrations of the treated samples weremeasured.

Overall Processes of Regeneration.Since the DLHs regen-erated with 5 M NaNO3 resulted in a relatively better boronremoval in comparison with that regenerated by either NaCl orNa2SO4, 5 M NaNO3 was selected as the reagent for subsequentregeneration tests. The regenerated DLHs were used for theboron removal tests following the same procedure as detailedabove.

Figure 1 shows the overall processes of adsorption andregeneration. Twenty-five milliliters of 100 mg/L boron modelwater with 0.7 g of DLH-60 or 0.3 g of DLH-450 were mixedwith 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 waswashed by distilled water two times. And then 25 mL of 5 MNaNO3 was added into the tubes and mixed completely withshaking for 4 h. The mixture was then centrifuged and the super-natant was analyzed for the measurement of anions concentra-tion. The solid phase was washed by deionized water severaltimes and then heated at either 60°C for 24 h or 450°C for 2h. Same as previously stated, after first regeneration, the gainedDLH materials were named as DLH-60-1 or DLH-450-1,respectively, and for each subsequent regeneration, the regener-ated DLHs were named as DLH-60-2 or DLH-450-2, etc.

Mechanisms of Boron Removal with DLHs.The boronremoval mechanisms by DLHs were carried out using boronmodel water with various concentrations. After 4 h of shakingand centrifugation, the supernatant was collected for the analysisof anions and cations by ICP and IC. The changes in concentra-tions of boron, nitrate, and chloride were used to interpret thepossible mechanisms.

Results and Discussion

XRD Analysis of DLHs. For all DLHs, a hydrotalcitestructure is confirmed by the XRD analysis. However, a standard

Table 1. Quality Characteristics of the Industry Effluenta

parameter B Al3+ As3+ Ca2+ T. Fe ions Mg2+ Na+ SO42- Cl- PO4

3 - NO3-

(mg/L) 17.0 1.39× 10-2 0.97 110.4 3.65× 10-2 18.0 74.8 316.7 111.7 1.2 8.4

a Effluent: pH ) 7.9; conductivity) 880 µS/cm.

mass of theith ion in the DLH product (g)

) dose in the preparation-residual mass in the supernatant

) dose in the preparation (g)-0.334 (L)*[thei th ion] (mg/L)/1000 (mg/g)

* 0.334 L ) the total volume of each supernatant

4578 Ind. Eng. Chem. Res., Vol. 46, No. 13, 2007

hydrotalcite consists of CO32- anion but DLH prepared in thisstudy consists of NO3- and Cl- anions. In addition to this, afterregeneration, the structure of the DLH-60 changes; DLH-60-1consists of bayerite, a type of Al(OH)3, while the original DLH-60 does not. However, this does not change the surface areasignificantly as shown later. Figure 2 presents an example ofthe XRD results for DLH-60.

Composition of DLH Products. Based on the mass balanceresults shown in Table 2, the formula of DLHs prepared forthis study can be proposed as

where the OH content was established based on the results ofmass balance and electronic balance.

Density of DLHs. The density of DLH-60 is 1.09 g/cm3 andDLH-450 is 0.90 g/cm3.

Surface Areas of DLHs.Table 3 displays surface areas ofDLHs. It can be seen that the surface area of DLH-60 was muchsmaller than that of DLH-450, while boron removal efficiencyof both DLHs was not significantly different (15% in difference).Then, we can speculate that the boron removal mechanisms withDLH-60 and DLH-450 are very different and this will bediscussed in a later section.

Particle Size of DLH-60 and DLH-450. Figure 3 shows twopictures taken by a high-resolution scanning electron microscope(SEM). It can be seen that both DLHs have an average sizeless than 100 nm in diameter.

Effect of DLH Dose on Boron Removal. Boron removalperformance with DLH-60 and DLH-450 was studied at pH 7via model waters with various boron starting concentrations,ranging from 5 to 500 mg/L. For both DLHs, boron percentageremoval increased with increasing doses until a maximum valuewas reached (>80% for the DLH-60 and>90% for DLH-450).

Figure 1. Regeneration process for DLH-60 (DLH-450).

Figure 2. XRD analysis of DLH-60.

Mg2AlNa1.4(OH)7.57Cl0.03(NO3)0.8‚x(H2O)

Ind. Eng. Chem. Res., Vol. 46, No. 13, 20074579

Figure 4 shows an example of the comparative boron removalby DLH-60 and DLH-450 for model waters with two startingboron concentrations. It can be seen that, at given dose rangesof the DLH, boron percentage removal with DLH-450 was 50%higher than that with DLH-60.

pH-Dependent Experiments.Twenty-five milliliters of 10and 100 mg/L boron model water was adjusted to its initial pHto between 3 and 11 by either 5 M NaOH or 1 M HCl and thenmixed 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 10.5for DLH-450, indicating that DLH has a high buffering capacity.

Adsorption Isotherm. Adsorption study was conducted usingDLH-450 and the isotherms were developed based on the resultsachieved. Table 4 is illustrated by plotting various isothermsresults with Langmuir and Freundlich equations. The fitness ofusing Langmuir equation to describe the adsorption process canbe assessed by a term “RL”, which is a dimensionless constantand is defined asRL ) 1/(1 + bC0) (whereb is a constant inthe Langmuir equation andC0 is boron equilibrium concentra-tion). The parameterRL indicates the shape of the isothermaccordingly: RL > 1, unfavorable;RL ) 1, linear; 0< RL < 1,favorable; andRL ) 0, irreversible. Similarly, the fitness of usingthe Freundlich equation to describe the adsorption can beassessed by the constant,n. If 1 < n < 10, the Freundlichequation is adequate for use.

It can seen from Table 4 that both Langmuir and Freundlichequations fit the adsorption results and they can be used topropose the adsorption capacity of DLH-450 on the boronremoval. The adsorption capacity of DLH-450 ranges from 16.1to 17.3 mg of B/g of adsorbent for three starting boronconcentrations studied.

Boron Removal Mechanisms with DLHs.The DLHs havea permanent positive charge originating from the isomorphicsubstitution of bivalent (Mg2+) by trivalent (Al3+) ions, andamphoteric charges resulting from surface hydroxyl groups(broken edges), which develop a variable charge throughprotonation-deprotonation reactions.14

The net positive charge on the DLHs is balanced by anionsclose to the planar surface and interlayer in the DLH. Theremoval of boron/borate can occur by anion exchange with boththe intercalated and surface anionic charge of the DLH. Besidesexchanging ions, which is mainly an electrostatic process, DLHshave surface groups that can establish chemical bonds withboron/borate molecules.

Mass variations of boron, Mg, Al, and NO3- in the super-

natant can be seen in Table 5. Al almost did not release intothe solution, indicating that the main structure of DLH-60 (Al-OHx)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- releasedwas greater or similar to the amount of boron removed for the

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

iondose in the

preparation (g)residual in thesupernatant (g) (g) (mol)

Mg2+ 4.80 0.00 4.80 0.20Al3+ 2.70 0.03 2.67 0.10Na+ 13.80 10.60 3.20 0.14Cl- 10.65 6.42 0.10a 0.003NO3

- 24.80 19.82 4.98 0.08

a Obtained by XRF measurement results. Most of the chloride waswashed away during the washing procedure.

Table 3. Surface Areas of DLHs

samplessurface

area (m2/g)boron

% removal

DLH-60 31.30 84DLH-450 130.18 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 16.1 0.26 0.04 0.9226 2.805 1.873 0.8958250 17.0 0.11 0.04 0.9226 4.864 3.986 0.9657500 17.3 0.06 0.03 0.9602 4.710 4.255 0.9225

a In Langmuir equation, the constantq0 signifies the adsorption capacity(mg/g) andb is related to the energy of adsorption (L/mg).b In Freundlichequation,Kf andn are adsorption isotherm constants, being indicative ofthe capacity and intensity of adsorption.

4580 Ind. Eng. Chem. Res., Vol. 46, No. 13, 2007

samples with relative low boron starting mass/concentrations(samples 1-5, 5 to 100 mg/L) which is adequate to exchangewith borate in the interlayer of DLH-60. For higher boronstarting concentration (sample 6, [B]0 ) 500 mg/L), the releasedNO3

- was less than the amount of boron removed, and thenthe released Mg ions contributed to the borate removal via theproposed Mg-B precipitation. Nevertheless, anion exchangeseems to be the main mechanism of boron removal by DLH-60 with the evidence of the mass difference of samples 1-5shown in Table 5.

Results shown in Table 5 also raise the concern of whetherthe DLH-60 is stable or not for long-term use for water andwastewater treatment. Under most study conditions and for themodel water, nitrate release is higher than that required for theexchange of boron, and excess nitrate release indicates that, afterone use, the DLH-60 has to be regenerated with nitrate salts torepossess an anion exchange capacity. The subsequent regenera-tion study has confirmed this (see later section). However, the

Mg releasing for each use is not significant; the maximumamount of Mg releasing is about 4% of the original Mg contentfor a dose of DLH-60 (e.g., 50 g/L). Therefore, DLH-60 couldbe stable for a long-term use.

Table 6 shows the boron removal with DLH-450 and thereleased amount of NO3- during the treatment. Only a fewNO3

-’s were released from DLH-450, and then the high boronpercentage removal (95.65%) with DLH-450 cannot be at-tributed to the anion exchange, but most possibly by aphysicochemical adsorption due to its relatively high surfacearea (see Table 3) and this is confirmed by the adsorptionisothermal data presented in Table 4. A few NO3

-’s releasingfrom DLH-450 in the treatment (Table 6) can be judged thatDLHs calcined at 450°C lose their layered structure and becomeoxides; thus, the majority of anions (NO3

-) release during thecalcination process. This is evident by XRD spectra of DLH-450, where the intensity of DLHs structure decreased signifi-cantly with increasing in the basal spacing and oxide structuresappear (Figure 5).

The proposed boron removal mechanisms are presented inFigure 6 which includes anion exchange for DLH-60 and boratesorption via ligand complexion, mainly for DLH-450. OHligands exist in all edges of the DLH and they are ready tocomplex with borate and thus remove boron from solutions.The similar complexes between OH ligands and borate or boricacid were proposed previously when hydroxycaroxylic acidswere tested to complex and precipitate boric acid.15

A desorption test was carried out to validate the aboveadsorption mechanism by DLH-450. DLH-450 (0.3 g) wasmixed with 25 mL of model water ([B]0 ) 100 mg/L) forshaking at 300 rpm for 4 h, and the mixture was separated bycentrifugation. The resulting supernatant was collected andconcentrations of boron and other components were analyzed.The separated DLH-450 was washed and then mixed with 25mL of 5 M Na2SO4, under conditions of shaking at 300 rpmfor 4 h. The mixture with desorpted DLH-450 was thenseparated and concentrations of boron and others in thesupernatant were analyzed. The mass balance was made basedon the boron removed in adsorption and boron released in

Figure 5. XRD analysis of DLH-450.

Table 5. Mass Difference in the Model Water before and afterTreatment with DLH-60 a

modelwater

sample

mass of Bin the

model water(mmol)

∆B(removed)

(mmol)

∆Mg(increased)

(mmol)

∆Al(increased)

(mmol)

∆NO3-

(increased)(mmol)

1 0.01 0.01 0.18 9.25× 10-5 0.452 0.02 0.02 0.17 6.48× 10-5 0.403 0.12 0.11 0.19 9.25× 10-5 0.424 0.23 0.21 0.23 9.25× 10-5 0.465 0.58 0.49 0.30 1.11× 10-4 0.536 1.16 0.91 0.38 1.67× 10-4 0.60

a DLH-60 dose) 50 g/L; pH0 ) 7.

Table 6. DLH-450 for Boron Removal from 100 mg/L Boron ModelWater, Dose) 28 g/L

[B] NO3-

pH0 (mg/L) (mmol) (mg/L) (mmol)B %

removal

model water 7.00 100.00 0.23 0 0 0DLH-450 10.60 4.02 9.30× 10-3 3.22 1.30× 10-3

removed orreleased ion

0.22 1.30× 10-3 95.65

Ind. Eng. Chem. Res., Vol. 46, No. 13, 20074581

desorption, which shows that 96% boron can be adsorbed (orremoved) in the adsorption and 90% of boron binding withDLH-450 can be released to Na2SO4 solution in the desorption.

Preliminary Trials of the Regeneration of DLH-60.NaNO3, NaCl, and Na2SO4 were selected as regenerationsolutions. The selection of a regenerated solution was based onboth the high regeneration efficiency and a high boron removalefficiency of the regenerated DLH. Then, a 5M NaNO3 wasselected as the regeneration reagent for further studies.

Regeneration Results.Figure 7 compares the boron removalcapacities with raw and six regenerated DLHs. Raw materialswere denoted as DLH-60 or DLH-450 while regenerated DLHsamples were denoted as DLH-60-1-6 or DLH-450-1-6,respectively. For cumulative six regenerations, the efficiencyof the regenerations was in a range between 45 and 65%. Table6 shows obviously that, for the raw and each stage-regeneratedDLHs, DLH-450 possesses a greater boron removal capacity.

Generally, after regeneration, the boron removal performanceof DLH-60 and DLH-450 decreased gradually. After sixcumulative regenerations, boron percentage removal with theregenerated DLHs decreased to about 40%.

It is worth noting that the boron removal with new DLH-450 is extremely high, 97%, but after the third regeneration,this decreased to 54%. In comparison, after the fourth regenera-tion, the DLH-60 still can achieve about 69% removal of boron.Another feature of using the DLHs is that, after variousregenerations, the high buffer capacity of DLHs was hardly

changed, and the pH remained constant in the treated waters,around pH 8.2 for DLH-60 and pH 10 for DLH-450.

Industry Effluent Results. Boron removal increased withDLH doses; as high as 86.6% and 93.5% of boron can beremoved at a dose of 36 g/L for DLH-60 and 16 g/L for DLH-450. Table 7 shows that arsenic can be removed completely byboth DLHs. A comparison with DLH-60, less Mg, Na, andNO3

- were released and more Ca was removed with DLH-450.However, the final pH of water treated by DLH-450 increasedto about 11.

A much larger release of NO3- was observed in treatingindustrial effluent with DLH-60 in comparison with the treat-ment of model water (Table 5): 1.6 mmol (equivalent to 4200mg/L, Table 7) for the industrial effluent vs 0.6 mmol for themodel water. This is because, in industrial effluent, not onlyboron but also arsenic, sulfate, and phosphate were removedvia the anion exchange mechanism with nitrate. As statedpreviously, DLH-60 has to be regenerated with nitrate salts torepossess an anion exchange capacity and this will not affectthe long-term use of DLH-60.

Table 7 also shows that DLH-450 slightly reduced NO3-

concentration in the industrial effluent (from 8.4 to 7.1 mg/L).This suggests that the oxide structure of DLH-450 could bereverting back to a double-layer hydrotalcite structure, due tothe adsorption of NO3-. Nevertheless, results show that this doesnot affect the overall boron removal performance with DLH-450 for the batch study conditions. As the adsorbed nitrate inthe saturated DLH-450 will release in the regeneration underthe same calcination process at 450°C, it appears that thestability of the regenerated DLH-450 will not be affected.

Conclusions

The following collusions can be made through this study:The XRD results confirm that all prepared or regenerated

DLH materials used for this study have a similar structure tothe conventional hydrotalcites but without carbonate anions. Thestructure of the DLHs synthesized is proposed to be

The density of DLH-60 is 1.09 g/mL and DLH-450 is 0.90g/mL, respectively. The surface area of DLH-60 is 31.3 m2/gand that of HT-450 is 130.18 m2/g.

For treating model waters with various starting boronconcentrations (5-500 mg/ L), the boron percentage removalincreased with increasing doses of both DLH-60 and DLH-450until a maximum value was reached (>80% with DLH-60 and>90% with DLH-450). The boron removal capacity for DLH-450 is 5.4-17.3 mg of B/g and that for DLH-60 is 1.2-13 mgof B/g, respectively. The raw water’s pH does not affect theboron 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 doseof 36 g/L for DLH-60 and 93.5% by the dose of 16 g/L forDLH-450. Arsenic can be completely removed by both DLHs.

Table 7. Quality Characteristics of the Industry Effluent Treated by DLHsa

parameterboron(mg/L)

As3+

(mg/L)Ca2+

(mg/L)Mg2+

(mg/L)Na+

(mg/L)SO4

2-

(mg/L)PO4

3-

(mg/L)NO3

-

(mg/L) pH B%

industry effluent 17.0 0.97 110.4 18.0 74.8 316.7 1.2 8.4 7.9DLH-60 2.3 <1.97× 10-2 110.7 206.8 441.5 0.36 0 4.2× 103 8.1 86.6DLH-450 1.1 <1.97× 10-2 59.2 148.8 235.8 0.29 0 7.1 10.9 93.5

a Note: DLH-60 dose: 36 g/L; DLH-450 dose: 16 g/L.

Figure 6. DLHs structure and boron removal schemes.

Figure 7. Boron removal capacity with DLH before and after regeneration.

Mg2AlNa1.4(OH)7.57Cl0.03(NO3)0.8‚x(H2O)

4582 Ind. Eng. Chem. Res., Vol. 46, No. 13, 2007

Comparison with DLH-60, less Al, Mg, Na, Cl-, and NO3-

were released and more Ca and Fe were removed withDLH-450.

The main mechanism of boron removal with DLH-60 is tobe anion exchange while that with DLH-450 is adsorption. ForDLH-450, both Langmuir and Freundlich isotherm models fitwell with the experimental results.

Boron removal performance with DLHs decreased graduallyafter each step of regeneration. After six cumulative regenera-tions, boron percentage removal with regenerated DLHs de-creased to about 40%. Overall, DLH-450 has a greater boronremoval capacity than DLH-60 for both freshly prepared andregenerated materials.

Acknowledgment

The authors are grateful for the financial support from theBorax Europe Ltd. The views of this paper are not necessaryrepresenting 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 Panelon Dietetic Products, Nutrition and Allergies on a Request from theCommission Related to the Tolerable Upper Intake LeVel of Boron (sodiumborate and boric acid); EFSA, J. 80. 2004.

(3) Council of European Communities Directive (CECD) 98/83.On theQuality of Water Intended for Human Consumption; EC Official Journal,L330/41, Brussels, 1998.

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ReceiVed for reView March 9, 2007ReVised manuscript receiVed April 9, 2007

AcceptedApril 21, 2007

IE0703639

Ind. Eng. Chem. Res., Vol. 46, No. 13, 20074583