This may be the author’s version of a work that was submitted/acceptedfor publication in the following source:

Theiss, Frederick, Ayoko, Godwin, & Frost, Ray(2013)Removal of boron species by layered double hydroxides: A review.Journal of Colloid and Interface Science, 402(1), pp. 114-121.

This file was downloaded from: https://eprints.qut.edu.au/59715/

c© Consult author(s) regarding copyright matters

This work is covered by copyright. Unless the document is being made available under aCreative Commons Licence, you must assume that re-use is limited to personal use andthat permission from the copyright owner must be obtained for all other uses. If the docu-ment is available under a Creative Commons License (or other specified license) then referto the Licence for details of permitted re-use. It is a condition of access that users recog-nise and abide by the legal requirements associated with these rights. If you believe thatthis work infringes copyright please provide details by email to qut.copyright@qut.edu.au

License: Creative Commons: Attribution-Noncommercial-No DerivativeWorks 2.5

Notice: Please note that this document may not be the Version of Record(i.e. published version) of the work. Author manuscript versions (as Sub-mitted for peer review or as Accepted for publication after peer review) canbe identified by an absence of publisher branding and/or typeset appear-ance. If there is any doubt, please refer to the published source.

https://doi.org/10.1016/j.jcis.2013.03.051

1

Removal of Boron Species by Layered Double Hydroxides: A

Review

Frederick L. Theiss, Godwin A. Ayoko, and Ray L. Frost

School of Chemistry, Physics and Mechanical Engineering, Science and Engineering

Faculty, Queensland University of Technology, Brisbane Queensland 4001, Australia.

Author to whom correspondence should be addressed (r.frost@qut.edu.au)

2

Abstract

Boron, which is an essential element for plants, is toxic to humans and animals at high

concentrations. Layered double hydroxides (LDHs) and thermally activated LDHs have

shown good uptake of a range of boron species in laboratory scale experiments when

compared to current available methods, which are for the most part ineffective or

prohibitively expensive. LDHs were able to remove anions from water by anion exchange,

the reformation (or memory) effect and direct precipitation. The main mechanism of boron

uptake appeared to be anion exchange, which was confirmed by powder X-ray diffraction

(XRD) measurements. Solution pH appeared to have little effect on boron sorption while

thermal activation did not always significantly improve boron uptake. In addition,

perpetration of numerous LDHs with varying boron anions in the interlayer region by direct

co-precipitation and anion exchange have been reported by a number of groups. The

composition and orientation of the interlayer boron ions could be identified with reasonable

certainty by applying a number of characterisation techniques including: powder XRD,

nuclear magnetic resonance spectroscopy (NMR), X-ray photoelectron spectroscopy (XPS)

and infrared (IR) spectroscopy. There is still considerable scope for future research on the

application of LDHs for the removal of boron contaminants.

Keywords: Layered double hydroxide, hydrotalcite, boron, boric acid, borate, water

purification

3

Introduction

Boron is one of the many elements that can enter water supplies from both natural sources

and human activity. Though boron is an essential element for plants (possibly also in animals

and humans) it is only beneficial in extremely small quantities. An excessive intake of boron

may lead to health complications including focal seizure disorders, irritability and

gastrointestinal disturbances [1, 2]. As a result, the World Health Organisation, (WHO) has

set a guideline value of 2.4 mg/L for boron in drinking water [2]. Boron also occurs naturally

in seawater as well as in many groundwater sources, particularly in areas associated with

geothermal activity [3]. Numerous industrial processes including the manufacture of flame

retardants, electronics, glass, antiseptics, pharmaceuticals, cosmetics, detergents and soaps

require various boron compounds and may produce effluents containing levels of boron

above acceptable WHO guideline values [1, 2, 4].

Current methods for the removal of boron from water are for the most part either ineffective

when compared to the required guideline values or prohibitively expensive in terms of energy

or cost [1, 2]. As a result effective methods for the removal of boron species from water will

undoubtedly have many applications. Layered double hydroxides (LDHs) have received

considerable attention in recent years because of their ability to remove a wide range of

anions from aqueous solutions. Some oxyanions of interest that have been studied include:

arsenate (AsO43-) [5-8], selenite (SeO4

2-) [8-11], molybdate (MoO42-) [6-9, 12-14] and

chromate (CrO42-) [8, 9, 12, 13]. LDHs which are also known as anionic clays or hydrotalcite

like materials are a group of clay minerals with a similar structure to the mineral brucite

(Mg(OH)2), which consists of divalent magnesium ions occupying octahedral positions

4

surrounded by OH groups in a complex layered structure. LDHs have a similar structure in

which a number of divalent cations are substituted with trivalent cations resulting in a layered

structure with a net positive charge that is balanced by exchangeable anions intercalated into

the interlayer regions. LDHs can be represented by the general formula:

[M1-x2+Mx

3+(OH)2]x+[An-]x/n·mH2O (1)

In which M2+ and M3+ are the divalent and trivalent layer cations respectively, 0.2 < x < 0.33

and An- is the exchangeable anion (often carbonate, chloride or nitrate) [15-17]. A large

number of cations have been successfully incorporated into LDH layers including:

magnesium [18-24] and aluminium [25], lithium [26], chromium [27, 28], Iron [17, 29-36],

calcium [37], zinc [38-41], gallium [42-45], nickel [31, 40, 46-48], indium [43], cobalt [31],

manganese [49] and copper [48]. Because of their structure, LDHs are the only clay minerals

that exhibit natural anion exchange properties. LDHs are also naturally abundant, readily

synthesised at industrial scale, inexpensive and stable at high temperature (up to 500-600°C)

and pH conditions. These make LDHs promising candidates for numerous water purification

applications, including the removal of boron contaminants. LDHs are able to remove anions

from water by anion exchange [7, 19, 50], the reformation (or memory) effect [7, 15, 19] and

direct precipitation [9]. All three of these mechanisms have been investigated for the removal

of various boron species from both synthetic and real world wastewaters.

Mg/Al Hydrotalcite like LDHs for the Removal of Boron from Synthetic Wastewaters

Mg/Al hydrotalcite like LDHs are by far the most commonly studied LDHs. Consequently,

these materials have been investigated as sorbents for the removal of boron from aqueous

solutions. Parker, Milestone and Newman [51] reported on the exchange and adsorption of

5

several oxyanions including tetrahydroxyborate (B(OH)4-). Parker, Milestone and Newman

were also able to produce a similar order of anion affinities to that originally reported by

Miyata [50, 52]. The anion affinity from greatest to least affinity was in the order:

SO42- > F- > HPO4

2- > Cl- > B(OH)4- > NO3

-

Ferreira et al. [53] reported on the removal of boric acid/borate from water using Mg/Al and

Mg/Fe LDHs prepared by laboratory scale co-precipitation at variable pH. Chemical analysis

confirmed that the Mg:Al ratio of the material was 2.1 and powder X-ray diffraction (XRD)

showed the characteristic d(003) and d(006) peaks typical of a LDH type material. Boron

sorption was independent of pH and maximum boron uptake did not change significantly

when the pH was adjusted using sodium hydroxide or nitric acid. This pH independence of

boron uptake was attributed to the high buffering capacity of the LDH [53]. A maximum

boron sorption capacity of 92% occurred at an adsorbent dose of 2.5 g/L. The data showed a

good fit for the Langmuir adsorption isotherm model with a correlation coefficient (r2) value

of 0.999 indicating that boron sorption was a monolayer process. Ferreira et al. calculated the

maximum boron removal capacity of 14.0mg/g using the Langmuir model [53].

Ay et al. [3] investigated the removal of boron from water using a Mg/Al LDH prepared with

nitrate as the interlayer anion and a sample of the LDH that had been thermally activated at

400°C for 4 hours under a nitrogen atmosphere. The d(003) interlayer spacing, obtained from

powder XRD was 8.6Å which was sufficient to accommodate nitrate anions in a tilted

position. Boron sorption was carried using 50mL aliquots of 250ppm boron stock solution

prepared from boric acid (H3BO3) and (NH4)2B4O7·4H2O. All sorption experiments were

6

carried out at 30°C and without any pH adjustment to reduce interference caused by

competing anions. A maximum boron sorption greater than 95% was observed with 0.6 g of

both the LDH and the thermally activated LDH adsorbent, indicating that there was only a

slight improvement by using thermally activated LDHs [3]. This is surprising as thermal

activation usually greatly increases the anion uptake capacity of LDHs. Powder XRD showed

that the interlayer space increased from 3.8 to 4.8Å to accommodate borate ions indicating

that the mechanism of boron uptake for the uncalcined LDH was anion exchange [3].

Figure 1: Structures of some boron species believed to exist in aqueous solutions. Based on a

Figure from Simon & Smith [3, 54].

Jiang et al. [55] investigated the removal of boron from aqueous solutions using Mg/Al

LDHs with and without thermal activation. The LDHs used for boron removal were prepared

from magnesium nitrate hexahydrate (Mg(NO3)2·6H2O) and aluminium chloride hexahydrate

(AlCl3·6H2O) resulting in a LDH that most likely contained a mixture of chloride and nitrate

anions in the interlayer region. A portion of the LDH was thermally activated at 450°C for 2

7

hours. Samples of the LDH sorbents were treated with 25mL of boron solution ranging in

concentration from 5 to 500mg/L [55]. Jiang et al. confirmed that altering the solution pH had

no significant effect on boron uptake. The pH independence was again attributed to the high

buffering capacity of the LDHs. Adsorption isotherms were investigated by fitting the

experimental results of the thermally activated LDH to both the Langmuir and Freundlich

adsorption isotherm models. Both models showed good correlation with the experimental

data suggesting that adsorption process was complex (possibly a combination of monolayer

and multilayer adsorption) [55]. The maximum reported boron removal was >80% for the

Mg/Al LDH and >90% for the thermally activated Mg/Al LDH suggesting that thermal

activation may indeed have an impact on boron sorption.

Finally, the possibility of regenerating the LDH sorbents was investigated. A 5M solution of

sodium nitrate (NaNO3) was chosen for regenerating the LDHs as it was expected to result in

the highest boron uptake capacity. The LDHs were tested for 6 regeneration cycles each (as

described in Figure 2). Boron removal decreased with the increasing number of regenerations

with the Mg/Al LDH still removing 69% of boron present. The thermally activated LDH

removed only around 54% of boron present after three regenerations indicating that a

significant decomposition of the adsorbent must have occurred during the repeated

regeneration cycles [55].

8

Figure 2: Graphic representation of the process used by Jiang et al. to test the regeneration of a Mg/Al LDH adsorbent [55].

Mg/Al Hydrotalcite like LDHs for the Removal of Tetrafluoroborate from Synthetic

Wastewaters

Yoshioka et al. [56] investigated the removal of tetrafluoroborate (BF4-) using a thermally

activated LDH. The 4:1 Mg/Al LDH was prepared by co-precipitation and thermally

activated at 500°C for one hour. To more accurately simulate a real world water sample a

1mM solution of NaBF4 was prepared and allowed to stand for approximately one month.

The resulting solution contained tetrafluoroborate (0.47mM), Fluoride (F-, 2.10mM) and

Boric acid (H3BO3, 0.53M) [56]. BF4- removal was calculated from the residual concentration

that remained after anion sorption and was measured by ion chromatography. A maximum

tetrafluoroborate removal of 93% was observed after 24 hours indicating that 4:1 Mg/Al

LDHs may be suitable adsorbents for treating wastewaters contaminated with

9

tetrafluoroborate [56], however, partial dissolution of the LDH adsorbent occurred during

tetrafluoroborate sorption resulting in a decrease of the Mg:Al ratio to 3.6:1 [56].

Tetrafluoroborate is thermodynamically unstable in the pH range at which the adsorption was

carried out (pH 10-11) and hydrolyses in water to give boric acid, and fluoride ions [56].

Yoshioka et al. proposed that tetrafluoroborate remained in solution because the hydrolysis

does not equilibrate within 24 hours, however, removal of fluoride appeared to occur faster

than boron removal suggesting that at least partial hydrolysis did indeed occur [56]. Yoshioka

et al. did not thoroughly characterise the LDH adsorbent after reformation and anion uptake.

A powder XRD pattern of the LDH after reformation was obtained but not compared with

that of the original LDH. As a result it is impossible to determine if the LDH did indeed

contain tetrafluoroborate anions or simply a mixture of fluoride and borates. Characterisation

of the LDH residue by techniques such as powder XRD, FTIR and 11B MAS nuclear

magnetic resonance spectroscopy (NMR) must be carried out before the mechanism of

tetrafluoroborate can be properly understood.

Mechanically Prepared Mg/Al Hydrotalcite like LDH for the Removal of Boron from

Water

Ay, Zümreoglu-Karan and Mafra [57] have investigated the potential of preparing a Mg/Al

LDH by mechanical grinding with a mortar and pestle. The Mg/Al LDH was synthesised by

manually grinding sodium hydroxide pellets (1.940g), powdered magnesium nitrate (4.00g)

and aluminium nitrate (2.93g) into a fine paste. The paste was then washed using deionised

10

water (20mL) and dried under vacuum before characterisation by various techniques. A

second Mg/Al LDH was prepared by a conventional co-precipitation for comparison [57].

The anion exchange capacity of the mechanically prepared LDH was tested by attempting to

remove tetraborate from an aqueous solution. Boron uptake of 52% and 85% were obtained

for the mechanically prepared ad co-precipitated LDHs respectively. The percentage anion

uptake capacity was calculated based on maximum theoretical exchange of available

interlayer nitrate for tetraborate. The presence of boron in the LDH sample was confirmed by

Fourier transform infrared spectroscopy (FTIR) with characteristic bands at 1450 and

1360cm-1, which were attributed to the ν3 BO3 and B-OH in-plane bending modes. The ν4

BO4 stretching mode occurred at 1150 cm-1and bands corresponding to the ν1and ν2 trigonal

modes of borate were observed at 935 and 800 cm-1 [3].

Mg/Fe Type LDHs for the Removal of Boron from Water

Ferreira et al. [53] compared the boron uptake capacity of a Mg/Al LDH with that of a Mg/Fe

LDH (Table 1). A Mg/Fe LDH with a molar ratio of 2:1 was prepared by a co-precipitation

using a method similar to the one previously described for the Mg/Al LDH [53]. Boron

uptake by the Mg/Fe LDH was significantly lower than that of the Mg/Al LDH in all cases.

Boron sorption increased linearly with adsorbent dose with a maximum boron uptake of

around 33% compared to 92% for the Mg/Al LDH with an adsorbent dose of 2.25 g/L of

adsorbent at pH of 9.0. It was proposed that the Mg/Fe LDH only remove boron through

surface adsorption (rather than anion exchange), which may account for the comparatively

low boron uptake [53].

11

Table 1: Comparison of the Mg/Al and Fe/Al LDHs prepared by Ferreira et al. [53].

LDH Adsorbent M2+: M3+ Ratio Interlayer

Spacing (Å) Maximum Boron

Uptake (%) Mechanism of Boron Uptake

Mg/Al 2:1 8.89 92 Anion exchange Mg/Fe 2:1 7.85 33 Surface adsorption

Hydrocalumite (a Ca/Al LDH) and Ettringite for the Removal of Boron from Water

Zhang and Reardon [9] investigated the potential of two layered materials based on calcium,

hydrocalumite, (Ca4Al2(OH)12(OH)2·6H2O) and ettringite (Ca6Al2(OH)12(SO4)3·2H2O) for

removal of a range of oxyanions including tetrahydroxyborate (B(OH)4-) from aqueous

solutions. The ultimate aim was to determine if these materials would be suitable for the

treatment of fly ash leachates in alkaline environments [9]. Hydrocalumite, as described by

the above formula is a calcium substituted LDH structurally similar to hydrotalcite. Ettringite

is a similar but pillared material, which exhibits different anion sorption behaviour [9]. The

relative affinity of hydrocalumite and ettringite for a range of oxyanions including

tetrahydroxyborate was tested using a single stock solution containing oxyanions of boron

(B(OH)4-) , chromium (CrO4

2-), molybdenum (MoO42-) and selenium (SeO4

2-) with 10ppm

of each oxyanion. The oxyanions were incorporated by direct co-precipitation of either

hydrocalumite or ettringite. Hydrocalumite was precipitated by slowly combining portlandite

(Ca(OH)2), 7.0mmol), monocalcium aluminate (CaAl2O4, 0.7mmol) and the oxyanion

solution (40mL). Ettringite was prepared using a similar method by combining a Ca(OH)2

solution (7.0mmol) with a second solution (40mL) containing Al2(SO4)3 (0.02M) and the

same oxyanions. In both cases CaOH2 was added in excess to ensure the desired aluminium

content of the products was achieved [9].

12

The order of anion selectivity for hydrocalumite and ettringite were found to differ

significantly. Hydrocalumite showed lowest affinity for boron in the presence of other anions,

however, hydrocalumite was still able to reduce most anion concentrations to below their

guideline values. In contrast ettringite was less effective with lower anion removal observed

but showed a preference for B(OH)4- over the other anions. The overall order of affinity of

ettringite was: B(OH)4- > SeO4

2- > CrO42- > MoO4

2- [9]. The difference in adsorption

behaviour between hydrocalumite and ettringite was attributed to the structural differences of

the two materials. The interlayer spacing of hydrocalumite, like other LDH materials can be

expanded to accommodate large anions. In ettringite the interlayer expansion required to

accommodate larger anions is limited because of layer pillaring. The preference for borate

was attributed to its similar size to sulphate, the anion it displaced during anion exchange [9].

The buffering capacity of the boric acid/borate reaction is well known. In acidic conditions

the neutral boric acid is not easily adsorbed by LDH materials predominates. The equilibrium

can be easily shifted by increasing the solution pH, however, the solubility of hydrocalumite

and ettringite increases significantly under basic conditions limiting the effectiveness of these

materials as boron sorbents [9].

Mg/Al LDHs for the Removal of Boron from a Real Industrial Waste: Optoelectronic

Wastewater

Kentjono et al. [4] investigated the removal of boron from real world optoelectronic

wastewater, which is generated during the production of thin film liquid crystal displays [4].

Optoelectronic wastewater is difficult to treat due to the high concentrations of boron and

iodine which limits the effectiveness of biological treatment processes (due to the

13

antimicrobial properties of iodine) [4]. LDHs were proposed as suitable adsorbents because

of their ability to simultaneously remove anionic boron and iodine species from the

wastewater. A real world waste water sample was collected from a manufacturing facility in

Tainan, Taiwan. The sample was analysed and the overall concentrations of boron and iodine

were 753.01 mg/L and 973.98 mg/L respectively. Other minor constituents of interest

included carbon (16.06 mg/L), zinc (7.69 mg/L) and chloride (14.58 mg/L) [4]. A Mg/Al

hydrotalcite like LDH with a molar ratio of 2:1 was prepared by co-precipitation method.

Elemental analysis was used to determine the empirical formula of the Mg/Al LDH which

was: Mg0.66Al0.34(OH)2(NO3)0.34·0.52H2O [4].

Boron and iodine uptake was carried out using the LDH without thermal activation and as a

result the anion uptake most likely occurred through exchange of interlayer nitrate ions from

the LDH or the formation of chemical bonds with the outer surface. Next, the effect of pH on

boron removal was investigated. The pH of the wastewater was adjusted by addition of

sodium hydroxide and nitric acid. The solution pH was found to affect boron uptake between

pH 8.0-11.2. Maximum boron uptake occurred at pH 9 (37.90mg/g), which is in good

agreement with other studies. The experimental data exhibited good correlation when fitted to

the Langmuir adsorption model, suggesting that boron sorption was a monolayer adsorption

process [4]. The effect of temperature on boron removal was investigated by carrying out the

adsorption at temperatures ranging from 22 to 50°C at a pH of 9.0. Temperature had little

effect on boron uptake, which decreased slightly with increasing temperature indicating that

boron sorption was a spontaneous and exothermic process [4].

14

Zeta potential measurements of the LDH were taken before and after treatment of the

wastewater at pH 9.0. The zeta potential of the LDH became negative after treatment with the

wastewater; indicating that some boron may be removed by surface complex formation [4].

From the available evidence Kentjono et al. concluded that the synthetic LDH was an

effective sorbent for the removal of boron from industrial optoelectronic wastewater. Boron

uptake appeared to occur through a combination of anion exchange and surface adsorption,

and the effect of the other ions in the wastewater could be dismissed because of their

comparatively low concentration [4].

Mg/Al LDHs for the Removal of Boron from Geothermal Waters: Applications for

Geothermal Energy

Abundant geothermal resources are available in many geologically active regions of the

world. Unfortunately, geothermal and ground waters typically contain high levels of boron

and other inorganic species [58]. Thermal stability of the adsorbent is an important factor that

must be considered because of the high temperatures of geothermal waters. LDHs have good

thermal stability when compared to organic anion exchange materials making them

promising candidates for this application. Wajima [58] investigated Mg/Al hydrotalcite like

LDHs and thermally activated Mg/Al hydrotalcite like LDHs for reducing the boron

concentration of geothermal waters in Japan.

Unlike most groups Wajima used a commercially available hydrotalcite (KW-1000, Kyowa

Kagaku Kogyo Co. Ltd) instead of synthesising the material. Geothermal water was collected

from the Sumikawa geothermal plant in the Hachimantai volcanic region in northeast Japan

[58]. The Mg/Al hydrotalcite was unable to reduce the boron concentration of the geothermal

15

water to below the target value of 10mg/L, however, arsenic and silica concentrations were

decreased to below their target values. Next, samples of the LDH were thermally activated at

500°C for three hours [58]. The thermally activated LDH was able to reduce the boron

concentration below the target value after six hours. The effect of temptature on boron uptake

was also investigated at various temperatures ranging from 25 to 80°C. Temperature

appeared to have no significant effect on boron uptake at any of the temperatures investigated

[58].

LDHs Prepared with Borate in the Interlayer

A number of authors have reported on the preparation and characterisation of LDH materials

containing boron species intercalated into the interlayer regions [20-24]. Key publications

have reported on the characterisation of LDHs by powerful techniques including solid state

nuclear magnetic resonance spectroscopy (NMR) and X-ray photoelectron spectroscopy

(XPS), techniques which have not been yet used to study residues recovered after boron

sorption. None of the articles described in this section have quantified boron uptake,

however, the materials described in this section could be considered the products of optimum

boron uptake and may provide important information about the physical and chemical

properties of the final LDH product and the mechanisms of boron uptake.

Mg/Al and Zn/Al Type LDHs Synthesised with Interlayer Triborate Anions

Bhattacharyya and Hall [20] prepared Mg/Al and Zn/Al LDHs with triborate (of the type

B3O3(OH)4-) in its interlayer. The LDHs were prepared by dissolving boric acid (20.868g,

0.3375mol) and sodium hydroxide (25.544g, 0.6375mol) in deionised water (200mL). The

16

solution was transferred to a three necked round bottom flask equipped with a reflux

condenser, thermometer and magnetic stirrer [20]. A second solution containing magnesium

nitrate hexahydrate (38.46g, 0.15mol) and aluminium nitrate nonahydrate (28.13g, 0.075mol)

dissolved in deionised water (200mL) was added dropwise to the first solution over the

course of one hour. The reaction was carried out under a nitrogen atmosphere to minimise

intercalation of carbonate [20]. The LDH slurry was aged at 70-80°C for approximately 15

hours before the solid LDH was recovered by vacuum filtration and dried overnight under

vacuum at 70°C [20]. The Zn/Al LDH was prepared using a similar method to the one

described for the Mg/Al LDH, in which that magnesium nitrate hexahydrate was replaced by

zinc nitrate hexahydrate. Twice as much deionised water was also used to reduce the

formation of a zinc oxide phase which formed under the conditions used to prepare the

Mg/Al LDH [20].

XRD analysis showed a typical LDH pattern with a d spacing of 10.8Å which corresponded

to an interlayer spacing of 6.0Å. This interlayer spacing was sufficient to accommodate the

triborate anion in its preferred orientation (Figure 3) [20]. 11B MAS NMR spectra consisted

of a singlet and a complex second-order quadrupolar broadened pattern. Peaks at 19.6 and

2.9ppm were assigned to trigonal and tetrahedral boron respectively. The relative intensity of

the two boron peaks was between 2-3, which was consistent with intercalation of the

triborate, which contains two trigonal and one tetrahedral boron atom [20].

17

Figure 3: Schematic representation of triborate intercalated into a LDH structure as proposed

by Bhattacharyya and Hall [20]. Based on a figure by Bhattacharyya and Hall [20].

Shi et al. [22] investigated borate intercalated LDHs as polymer additives, particularly for

their flame and smoke suppressant properties [23]. A carbonate LDH precursor was prepared

with a Mg/Al ratio of 2.0. Carbonate in the interlayer of the LDH was exchanged with borate

by dispersing the carbonate LDH (22% by weight) in distilled water (10mL) followed by the

addition of boric acid (H3BO3) solution (3.2M) at 100°C. The mixture was allowed to stir at

this temperature for 2 hours before the solids were filtered, washed and dried in an oven at

70°C. The formula of the LDH determined by elemental analysis was:

Mg0.65Al0.33(OH)2(B3O5)0.35·0.65H2O [23].

Powder XRD patterns of the LDH obtained before and after borate exchange were compared.

Boron exchange caused the basal spacing of the LDH increased by 0.31nm to 1.07nm and the

18

gallery height of the boron intercalated LDH was 0.59nm. This spacing was consistent with a

LDH structure containing triborate (B3O5·yH2O) anions intercalated into the interlayer region

[23]. IR spectroscopy confirmed presence of boron species in the Mg/Al LDH. Bands

between 1500 and 1250cm-1 were assigned to ν3 BO3- stretching modes, while bands between

1200 and 800 were assigned to ν3 BO4- stretching modes. IR spectroscopy could not be used

to confirm the absence of carbonate because significant carbonate bands (such as the ν3 CO32-

) stretching modes overlap with the ν3 BO3- stretching modes [23]. Consequently, elemental

analysis was required to confirm the absence of significant amounts of carbonate. The boron

to carbon ratio of the LDH was reported as 28:1 [23]. The high affinity LDHs exhibit for

carbonate is well known. It is therefore somewhat surprising that carbonate can be exchanged

for triborate so readily.

The 11B MAS NMR spectrum consisted of two distinct features. A singlet observed at

1.6ppm attributed to tetrahedrally coordinated Boron and the pattern at approximately

12.1ppm was assigned to trigonal boron. These observations were consistent with the

triborate as an interlayer anion [23]. It was not possible to determine the ratio of the trigonal

and tetrahedral boron present in the sample from the NMR spectrum due to considerable

overlap of the peaks. The BO4 units form a hexagonal B3O3 ring with alternating boron and

oxygen atoms and two BO3 units. It was proposed that the boron rings were orientated

perpendicular to the cation layers of the LDH. The interlayer spacing for this configuration

was consistent with the value obtained by powder XRD [23].

Mg/Al Type LDHs Synthesised with Interlayer Tetraborate Anions

19

Li et al. [21] reported on the synthesis and characterisation of a Mg/Al LDH with tetraborate

(B4O5(OH)42-) anions in the interlayer region. The LDH was synthesised by the hydrothermal

method which involved the preparation of a solution containing magnesium nitrate

hexahydrate (3.85g) and aluminium nitrate nonahydrate (2.81g) dissolved in deionised water

(40mL). Precipitation of LDH occurred with slow addition of sodium hydroxide solution

(2M) [23]. The LDH was recovered by filtration and washed resulting in either a LDH slurry

or a wet LDH filter “cake”. The LDHs were re-dispersed into a hot solution containing

Na2B4O5(OH)4.8H2O (20mL, 0.5M). The pH of the slurry was adjusted to pH 9 before

hydrothermal treatment in a Teflon-lined autoclave at 100°C for 20 hours [23]. Finally, the

LDH was recovered by filtration, washed and air dried [23].

Powder XRD demonstrated that drying the LDH to a slurry before tetraborate exchange

resulted in a more crystalline end produce than using a LDH filter “cake”. It was proposed

that the LDH layers were more open in the freshly precipitated slurry allowing for a greater

exchange of nitrate for tetraborate anions, and forming a product with a more uniform

structure [21]. The XRD pattern presented for the wet “cake” method also appears to contain

a number of additional peaks when compared to that of the product obtained from the slurry

method. Phase analysis was not carried out so the presence of additional crystalline phases in

the wet “cake” sample cannot be ruled out. The d(003) spacing of the LDH was 11.0Å, which

corresponded to an interlayer spacing of approximately 4.8Å [21]. It was proposed that the

tetraborate anions adopt a configuration in which the tetrahedral BO4 linkages are

perpendicular to the cation layers of the LDH [21].

20

The 11B MAS NMR spectra of the LDH (prepared by the slurry method) were obtained

before and after calcination of the LDH at 300°C for three hours. Samples obtained before

calcination consisted of two bands at 2.9 and 14.0ppm, which were assigned to tetrahedrally

and trigonally coordinated boron respectively [21]. Deconvolution of the 11B MAS NMR

spectrum gave a 1:1 ratio of tetrahedrally to trigonally coordinated boron, which is consistent

with the structure of the tetraborate (B4O5(OH)42-) anion [21]. After calcination at 300°C the

tetrahedral boron band shifted to 2.6ppm (was 2.9ppm) and the ratio of tetrahedraly to

trigonaly coordinated boron changed to 0.85:1. This change was assigned to the conversion

of approximately 8% of the tetrahedral boron to trigonal boron [21]. The 27Al MAS NMR

spectrum of the LDH was also recorded before and after calcinations of the LDH at 300°C.

Only a single band assigned to octahedral coordinated aluminium was observed in both

spectra at 9.2ppm [21].

Mg/Al LDHs Synthesised with Interlayer Boron Species by a pH Controlled Method

An interesting and potentially useful feature of boron oxyanions is the wide range of anionic

species that exist at different pH values. Control of pH during LDH synthesis presents the

possibility of selective control of the interlayer boron oxyanions in the final LDH product.

Bechara et al. [22] investigated this by precipitating a number of Mg/Al type LDHs at

different pH values. A solution containing magnesium nitrate hexahydrate and aluminium

nitrate nonahydrate (in a Mg:Al ratio of 1.9:1) was prepared and slowly added to a second

solution containing excess boric acid. The addition of sodium hydroxide was used to maintain

the pH of the reaction mixture. The mixture was aged for 15 hours before the LDH was

separated by filtration, washed and dried at 80°C under vacuum. Portions of the LDHs were

calcined for 4 hours at 400°C to remove excess water [22]. The boron content of all LDHs

21

was significantly lower than the theoretical maximum predicted (Table 2). The lower than

expected boron uptake was attributed to competition from other boron species [22]. The LDH

prepared at pH 7.5 exhibited low crystalinity with a d(003) spacing of 8.07Å, which was

consistent with intercalation of either monoborate (B(OH)4-) or triborate (B3O3(OH)4

-) anions

[22].

Maximum boron uptake was predicted to occur between pH 8.3 and 9.0 as the B3O3(OH)4-

anion predominates in this pH region. The LDH containing the largest amount of boron was

indeed prepared at pH 9.0 which consisted of only 5.3 wt% boron [22]. The powder XRD

pattern of this material appeared to consist of two LDH phases with d(003) spacings of 8.07

and 10.78Å . Bechara et al. proposed that the predominant phase with the smaller d(003)

spacing due to intercalation of monoborate (B(OH)4-) anions [22]. The 11B MAS NMR

spectrum of the LDH prepared at pH 9.0 contains two peaks at 1.6 and 13.6ppm, which were

assigned to tetrahedrally and trigonally coordinated boron respectively. Bechara et al.

proposed that the peak corresponding to trigonally coordinated boron which usually occurs

around 17ppm was shifted due to electrostatic interactions with aluminium ions in the cation

layers [22].

The third LDH prepared at pH 11.0 was highly crystalline and the first sample to contain only

one type of anion in the interlayer, however, the d(003) spacing was not reported [22]. The

elemental composition of the surface of the LDHs was determined by XPS and the results

compared to those of the bulk of the samples as measured by atomic absorption spectroscopy.

The LDH prepared at pH 11.0 was the only sample which exhibited the same distribution of

magnesium and aluminium ions on the surface as in the bulk of the material [22]. Bechara et

22

al. concluded that it was not possible to selectively intercalate boron species into a Mg/Al

LDH by pH control alone. Both samples prepared below pH 11.0 contained a mixture of

interlayer boron anions and the maximum theoretical uptake of boron was never achieved

[22].

Table 2: Comparison of Mg/Al LDHs prepared at different pH values. Data from Bechara et

al. [22].

pH used to Prepare Sample

Mg:Al Molar Ratio

d(003)

Spacing % Boron

Proposed Interlayer Anions

7.5 0.9 8.07 4.7 B(OH)4-, B3O3(OH)4

- 9.0 1.2 8.07, 10.78 5.3 B(OH)4

-, B3O3(OH)4-

11.0 1.4 --- 4.2 B(OH)4-

Comparison of Some Mg/Al LDHs with Interlayer Boron Species Prepared by Direct

Co-precipitation and Anion Exchange

Ay et al. [24] synthesised a number of Mg/Al LDH type materials with boron species in the

interlayer using different conditions and methods. A Mg/Al LDH containing nitrate anions in

the interlayer was prepared by slow addition of sodium hydroxide (300mL, 0.74M) to 200mL

of a solution containing magnesium nitrate hexahydrate (22.0g, 86.0mmol) and aluminium

nitrate nonahydrate (16.1g,4.30mmol) at 90°C for approximately 4 hours. The resulting slurry

was aged for three weeks before being filtered washed and dried [24]. LDHs containing

adipate or terephtalate as interlayer anions were prepared using a similar co-precipitation

method [24]. Additional LDHs were then prepared by exchanging the existing interlayer

anions (nitrate, adipate or terephtalate) with boron species by treatment with tetraborate and

boric acid solutions [24].

23

A number of LDHs containing boron species were successfully prepared by anion exchange.

The LDH prepared by exchange of nitrate for borate appeared to contain a LDH phase with a

gallery height of 4.78Å and a boehmite (γ-AlOOH) phase. The gallery spacing suggested that

trigonal triborate anions occupy the interlayer region in a tilted geometry. The sample of

nitrate LDH treated with boric acid also contained boehmite (γ-AlOOH) in addition to the

LDH phase with a basal spacing of 9.46Å [24]. The larger interlayer spacing was attributed to

the presence of tetraborate anions adopting a tilting or flat orientation [24]. 11B MAS NMR

spectroscopy identified polyborate ions at all pH values investigated even those in which

monoborate ions (such as B(OH)4-, pH > 8) are expected to predominate. Ay et al. concluded

that some boron polymerisation may occur due to the different conditions present in the

interlayer compared to the solution. It was proposed that electrostatic interactions and

differences in the interlayer pH contribute to this effect [24]. Adipate and terephtalate were

used to increase the interlayer spacing before boron exchange. Adipate was readily

exchanged for borate in both and tetraborate and boric acid solutions. In contrast, terephtalate

was only exchanged for borate when the LDH was treated with tetraborate solution. Pre-

swelling of the LDHs with either adipate or terephtalate appeared to have little effect on

overall boron uptake [24].

Finally, LDHs with interlayer boron were prepared by direct co-precipitation with tetraborate

and boric acid solutions [24]. A LDH was co-precipitated by dropwise addition of a solution

containing sodium hydroxide (60mL, 1.8M) and ammonium tetraborate (100mL, 0.30M) to a

second solution containing aluminium nitrate nonahydrate (6.05g, 16.1mmol) magnesium

nitrate hexahydrate (8.26g, 32.2mmol) in degassed deionised water (100mL). The resulting

24

mixture was refluxed for approximately 5 hours then aged at room temperature for 5 days

before filtering, washing and drying under vacuumed at 40°C [24]. Another LDH was

prepared using a similar method in which only the composition of the initial solutions used to

prepare the LDHs were changed. A solution containing sodium hydroxide (38mL, 6.6M) and

boric acid (250mL, 0.6M) was added dropwise to a solution containing aluminium nitrate

nonahydrate (6.56g, 17.5mmol) magnesium nitrate hexahydrate (8.97g, 35.0mmol) dissolved

in deionised water (50mL) [24].

The LDHs prepared by direct co-precipitation in the presence of tetraborate or boric acid

were characterised by powder XRD. The sample prepared in the presence of tetraborate

contained only amorphous material that could not be characterised by powder XRD [24]. The

LDH prepared in the presence of boric acid contained a number of phases including a LDH,

boehmite (γ-AlOOH) and aluminium hydroxide (Al(OH)3) which are common impurities that

may result from co-precipitation syntheses [24]. The 11B MAS NMR spectra could not be

used to identify the interlayer boron species present in the samples because of the intense

probe background signal which dominated the spectrum [24]. The 27Al MAS NMR spectra

obtained from the LDHs with interlayer boron all consisted of a single peak at around

9.67ppm assigned to octahedrally coordinated aluminium. Ay et al. indicated that this was

strong evidence that boron intercalation did not significantly affect the structure of the cation

layers in the Mg/Al LDHs [24].

Conclusions

25

Layered double hydroxides (LDHs) have shown promise as adsorbents for the removal of

various boron species from both synthetic and real world wastewaters under laboratory scale

conditions. Solution pH did not appear to have a significant effect on boron removal by ion

exchange or reformation. The pH independence of boron sorption was general attributed to

the high pH buffering capacity of LDHs. The main mechanism of boron removal appeared to

be anion exchange. Powder XRD revealed a changes in the d(003) spacings which were

sufficient to incorporate anionic boron species. This provides strong evidence for anion

exchange/intercalation over simple adsorption of the anions to the outer surface of the LDH

particles. Surprisingly, the reformation of thermally activated LDHs did not appear to

significantly improve boron sorption in all studies reviewed in this article. This also provides

additional evidence to support the conclusion that anion exchange is the mechanism of boron

uptake. The main remaining difficulty that must be overcome is the high pH required to form

anionic boron species in aqueous solution that can be adsorbed by LDHs.

A number of LDHs with various boron anions in the interlayer region have been prepared by

direct co-precipitation and anion exchange. Direct precipitation of the LDH in the presence of

boron species did not always result in the intercalation of the expected interlayer anions. The

LDHs were characterised by techniques including powder XRD, 11B MAS NMR, 27Al MAS

NMR, XPS and IR spectroscopy. These techniques were able to provide significant

information about the LDHs. Of particular interest was the composition and orientation of the

interlayer boron ions which could be identified with reasonable certainty. Applying these

techniques to characterise the LDH residues collected after boron sorption experiments will

help provide further information on the mechanisms of boron uptake by LDHs and thermally

activated LDHs. Considerable research is still required before pilot or commercial scale

operations can be established. There are currently few studies concerning desorption of

26

boron, regeneration of the LDH adsorbent, use of a flow system (rather than small scale batch

processes), the application of natural a commercially available LDHs or other non Mg/Al

LDHs. There has also been limited research into the removal of boron from real world

wastewaters and the effect of competing anions they may contain. There is still considerable

scope for future research. Experiments concerning the removal of boron species from

aqueous solutions by LDHs will undoubtedly continue to be reported in the literature for

years to come.

Acknowledgements

The financial and infra-structure support of the Discipline of Nanotechnology and Molecular

Science and the School of Chemistry, Physics and Mechanical Engineering of the Faculty of

Science and Engineering, Queensland University of Technology is gratefully acknowledged.

27

References

1. Fawell, J.K., Boron in Drinking-water, Background document for devlopment of WHO Guidelines for Drinking-water Quality, 2009, World Health Organization: Geneva, Switzerland.

2. Guidelines for Drinking-water Quality. 4th ed. 2011, WHO Press, World Health Organization: Geneva, Switzerland.

3. Ay, A.N., B. Zümreoglu-Karan, and A. Temel, Boron removal by hydrotalcite-like, carbonate-free Mg–Al–NO3-LDH and a rationale on the mechanism. Microporous and Mesoporous Materials, 2007. 98(1–3): p. 1-5.

4. Kentjono, L., et al., Removal of boron and iodine from optoelectronic wastewater using Mg–Al (NO3) layered double hydroxide. Desalination, 2010. 262(1–3): p. 280-283.

5. Frost, R.L. and K.L. Erickson, Near-infrared spectroscopy of stitchtite, iowaite, desautelsite and arsenate exchanged takovite and hydrotalcite. Spectrochim. Acta, Part A, 2005. 61A(1-2): p. 51-56.

6. Palmer Sara, J., et al., Thermally activated seawater neutralised red mud used for the removal of arsenate, vanadate and molybdate from aqueous solutions. J Colloid Interface Sci, 2010. 342(1): p. 147-54.

7. Palmer Sara, J., A. Soisonard, and L. Frost Ray, Determination of the mechanism(s) for the inclusion of arsenate, vanadate, or molybdate anions into hydrotalcites with variable cationic ratio. J Colloid Interface Sci, 2009. 329(2): p. 404-9.

8. Goh, K.-H., T.-T. Lim, and Z. Dong, Application of layered double hydroxides for removal of oxyanions: A review. Water Res., 2008. 42(6–7): p. 1343-1368.

9. Zhang, M. and E.J. Reardon, Removal of B, Cr, Mo, and Se from Wastewater by Incorporation into Hydrocalumite and Ettringite. Environmental Science & Technology, 2003. 37(13): p. 2947-2952.

10. Liu, R., R.L. Frost, and W.N. Martens, Absorption of the selenite anion from aqueous solutions by thermally activated layered double hydroxide. Water Res., 2009. 43(5): p. 1323-1329.

11. You, Y., G.F. Vance, and H. Zhao, Selenium adsorption on Mg-Al and Zn-Al layered double hydroxides. Applied clay science, 2001. 20(1-2): p. 13-25.

12. Frost, R.L., et al., Thermal decomposition of hydrotalcite with chromate, molybdate or sulphate in the interlayer. Thermochim. Acta, 2005. 429(2): p. 179-187.

13. Frost, R.L., et al., Raman spectroscopy of hydrotalcites with sulphate, molybdate and chromate in the interlayer. J. Raman Spectrosc., 2005. 36(10): p. 925-931.

28

14. Frost, R.L., S.J. Palmer, and T.M. Nguyen, Thermal decomposition of hydrotalcire with molybdate and vandate anions in the interlayer J. Therm. Anal. Calorim., 2008. 92(3): p. 879-886.

15. Rives, V., Characterisation of layered double hydroxides and their decomposition products Materials Chemistry and Physics, 2002. 75: p. 19-25.

16. Frost, R.L., S.J. Palmer, and H.J. Spratt, Thermal decomposition of hydrotalcites with variable cationic ratios. J. Therm. Anal. Calorim., 2009. 95(1): p. 123-129.

17. Zhao, Y., R.L. Frost, and W.N. Martens, Synthesis and characterization of gallium oxide nanostructures via a soft-chemistry route. Journal of Physical Chemistry C, 2007. 111(44): p. 16290-16299.

18. Duan, X. and D.G. Evans. Structure and Bonding: Layered Double Hydroxides. 2006 v. 119]; Available from: http://books.google.com.au/books?id=qujowYGyNagC.

19. Rives, V., Layered Double Hydroxides: Present and Future2001, New York: Nova Science Pub Inc

20. Bhattacharyya, A. and D.B. Hall, New triborate-pillared hydrotalcites. Inorganic Chemistry, 1992. 31(18): p. 3869-3870.

21. Li, L., et al., Synthesis and Characterization of Tetraborate Pillared Hydrotalcite. Chem. Mater., 1996. 8(1): p. 204-208.

22. Bechara, R., et al., Synthesis and Characterization of Boron Hydrotalcite-Like Compounds as Catalyst for Gas-Phase Transposition of Cyclohexanone-Oxime. Catalysis Letters, 2002. 82(1-2): p. 59-67.

23. Shi, L., et al., Synthesis, Flame-Retardant and Smoke-Suppressant Properties of a Borate-Intercalated Layered Double Hydroxide. Clays and Clay Minerals, 2005. 53(3): p. 294-300.

24. Ay, A.N., et al., Layered double hydroxides with interlayer borate anions: A critical evaluation of synthesis methodology and pH-independent orientations in nano-galleries. Applied clay science, 2011. 51(3): p. 308-316.

25. Olfs, H.W., et al., Comparison of different synthesis routes for Mg-Al layered double hydroxides (LDH): Characterization of the structural phases and anion exchange properties. Applied clay science, 2009. 43(3-4): p. 459-464.

26. Zhang, T., et al., Synthesis of Li–Al Layered Double Hydroxides (LDHs) for Efficient Fluoride Removal. Ind. Eng. Chem. Res., 2012.

27. Labajos, F.M. and V. Rives, Thermal Evolution of Chromium(III) Ions in Hydrotalcite-like Compounds. Inorganic Chemistry, 1996. 35(18): p. 5313-5318.

28. Bouzaid, J. and R.L. Frost, Thermal decomposition of stichtite. J. Therm. Anal. Calorim., 2007. 89(1): p. 133-135.

29

29. Chitrakar, R., et al., Fe-Al layered double hydroxides in bromate reduction: Synthesis and reactivity. J. Colloid Interface Sci., 2011. 354(2): p. 798-803.

30. Palmer, S.J., R.L. Frost, and H.J. Spratt, Synthesis and Raman spectroscopic study of Mg/Al,Fe hydrotalcites with variable cationic ratios. J. Raman Spectrosc., 2009. 40(9): p. 1138-1143.

31. Bakon, K.H., S.J. Palmer, and R.L. Frost, Thermal analysis of synthetic reevesite and cobalt substituted reevesite (Ni,Co)6Fe2(OH)16(CO3) · 4H2O. J. Therm. Anal. Calorim., 2010. 100(1): p. 125-131.

32. Frost, R., et al., Thermal decomposition of the syenthetic hydrotalcite woodallite J. Therm. Anal. Calorim., 2006. 86(2): p. 437-441.

33. Frost, R.L., et al., Thermal decomposition of the syenthetic hydrotalcite iowaite. J. Therm. Anal. Calorim., 2006 86: p. 437-441.

34. Frost, R.L. and K.L. Erickson, Thermal decomposition of natural iowaite. J. Therm. Anal. Calorim., 2004. 78(2): p. 367-373.

35. Triantafyllidis, K.S., et al., Iron-modified hydrotalcite-like materials as highly efficient phosphate sorbents. journal of colloid and interface science, 2010. 342(2): p. 427-436.

36. Chitrakar, R., et al., Synthesis and bromate reduction of sulfate intercalated Fe(II)–Al(III) layered double hydroxides. Separation and Purification Technology, 2011. 80(3): p. 652-657.

37. Chrysochoou, M. and D. Dermatas, Evaluation of ettringite and hydrocalumite formation for heavy metal immobilization: Literature review and experimental study. J. Hazard. Mater., 2006. 136(1): p. 20-33.

38. Pode, R., et al., Sorption of phosphates and thiocyanates on isomorphic substituted Mg/Zn-Al-type hydrotalcites. J. Serb. Chem. Soc., 2008. 73(8-9): p. 835-843.

39. Frost, R.L., W.N. Martens, and K.L. Erickson, Thermal decomposition of the hydrotalcite. J. Therm. Anal. Calorim., 2005. 82: p. 603-608.

40. Lin, Y.-H., et al., Thermogravimetric analysis of hydrotalcites based on the takovite formula NixZn6-xAl2(OH)16(CO3)·4H2O. J. Therm. Anal. Calorim., 2005. 81(1): p. 83-89.

41. Das, D.P., J. Das, and K. Parida, Physicochemical characterization and adsorption behavior of calcined Zn/Al hydrotalcite-like compound (HTlc) towards removal of fluoride from aqueous solution. Journal of Colloid and Interface Science, 2002. 261: p. 213-220.

42. Aramendı ́a, M.a.A., et al., Synthesis, Characterization, and1H and71Ga MAS NMR Spectroscopy of a Novel Mg/Ga Double Layered Hydroxide. J. Solid State Chem., 1997. 131(1): p. 78-83.

30

43. María A. Aramendía, et al., Comparative Study of Mg/M(III) (M=Al, Ga, In) Layered Double Hydroxides Obtained by Coprecipitation and the Sol–Gel Method. J. Solid State Chem., 2002. 168(1): p. 156-161.

44. Grand, L.-M., S.J. Palmer, and R.L. Frost, Synthesis and thermal stability of hydrotalcites containing gallium. J. Therm. Anal. Calorim., 2009.

45. Grand, L.-M., S.J. Palmer, and R.L. Frost, Synthesis and thermal stability of hydrotalcites based upon gallium. J. Therm. Anal. Calorim., 2010. 101(1): p. 195-198.

46. Benito, P., et al., Microwave-assisted reconstruction of Ni,Al hydrotalcite-like compounds. J. Solid State Chem., 2008. 181(5): p. 987-996.

47. Frost, R.L., S.J. Palmer, and L.-M. Grand, Synthesis and thermal analysis of indium-based hydrotalcites of formula Mg6In2(CO3)(OH)16.4H2O. J. Therm. Anal. Calorim., 2010. 101(3): p. 859-863.

48. Rives, V., Layered double hydroxides with the hydrotalcite-type structure containing Cu2+, Ni2+ and Al3+. The Journal of the Royal Society of Chemistry 1999. 10: p. 489-495.

49. Grand, L.-M., S.J. Palmer, and R.L. Frost, Synthesis and thermal stability of hydrotalcites containing manganese. J. Therm. Anal. Calorim., 2009.

50. Miyata, S., Anion Exchange Properties of Hydrotalcite-like Compounds Clays and Clay Minerals, 1983. 31(4): p. 305-311.

51. Parker, L.M., N.B. Milestone, and R.H. Newman, The Use of Hydrotalcite as an Anion Absorbent. Ind. Eng. Chem. Res., 1995. 34(4): p. 1196-1202.

52. Miyata, S., Physico-Chemical Properties of Synthetic Hhydrotalcites in Relation to Composition. Clays and Clay Minerals, 1980. 28(1): p. 50-56.

53. Ferreira, O.P., et al., Evaluation of boron removal from water by hydrotalcite-like compounds. Chemosphere, 2006. 62(1): p. 80-88.

54. Simon, J.M. and R.A. Smith, Borate raw materials. Glass Technol., 2000. 41(6): p. 169-173.

55. Jiang, J.-Q., et al., Laboratory Study of Boron Removal by Mg/Al Double-Layered Hydroxides. Ind. Eng. Chem. Res., 2007. 46(13): p. 4577-4583.

56. Yoshioka, T., et al., Removal of tetrafluoroborate ion from aqueous solution using magnesium–aluminum oxide produced by the thermal decomposition of a hydrotalcite-like compound. Chemosphere, 2007. 69(5): p. 832-835.

57. Ay, A.N., B. Zumreoglu-Karan, and L. Mafra, A simple mechanochemical route to layered double hydroxides: synthesis of hydrotalcite-like Mg-Al-NO3-LDH by manual grinding in a mortar. Journal of Inorganic and General Chemistry 2009(635): p. 1470-1475.

31

58. Wajima, T., Removal of boron from geothermal water using hydrotalcite. Toxicological & Environmental Chemistry, 2010. 92(5): p. 879-884.

32

Tables

Table 1: Comparison of the Mg/Al and Fe/Al LDHs prepared by Ferreira et al. [53].

LDH Adsorbent M2+: M3+ Ratio Interlayer

Spacing (Å) Maximum Boron

Uptake (%) Mechanism of Boron Uptake

Mg/Al 2:1 8.89 92 Anion exchange Mg/Fe 2:1 7.85 33 Surface adsorption

33

Table 2: Comparison of Mg/Al LDHs prepared at different pH values. Data from Bechara et

al. [22].

pH used to Prepare Sample

Mg:Al Molar Ratio

d(003)

Spacing % Boron

Proposed Interlayer Anions

7.5 0.9 8.07 4.7 B(OH)4-, B3O3(OH)4

- 9.0 1.2 8.07, 10.78 5.3 B(OH)4

-, B3O3(OH)4-

11.0 1.4 --- 4.2 B(OH)4-

34

Figures

Figure 1: Structures of some boron species believed to exist in aqueous solutions. Based

on a Figure from Ay et al. [3].

35

Figure 2: Graphic representation of the process used by Jiang et al. to test the regeneration of a Mg/Al LDH adsorbent [55].

36

Figure 3: Schematic representation of triborate intercalated into a LDH structure as

proposed by Bhattacharyya and Hall [20]. Based on a figure by Bhattacharyya and Hall

[20].

37

List of Figures:

Figure 1: Structures of some boron species believed to exist in aqueous solutions.

Figure 2: Graphic representation of the process to test the regeneration of a Mg/Al LDH

adsorbent.

Figure 3: Schematic representation of triborate intercalated into a LDH structure as

proposed by Bhattacharyya and Hall