Available online at www.sciencedirect.com

Ceramics International 40 (20

ree

S

, Kynvir

d foine

Boron is known to be an essential element for living plants,

applied in practice to remove boron from groundwater,because cost-effective reactive materials are yet to be found.

for PRBs. Materials in PRBs must have appropriate reactivity,

only anionic species but also heavy metals [19,20]. In thesereactions, the immobilization mechanism is mainly based on

co-precipitation with Mg(OH)2. In other words, calcinedMgO-rich phases are destroyed during the immobilizationand transformed into hydroxides. The calcination conditions

0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.http://dx.doi.org/10.1016/j.ceramint.2013.07.056

nCorresponding author. Tel./fax: +81 92 802 3338.E-mail address: keikos@mine.kyushu-u.ac.jp (K. Sasaki).animals and humans [1]. However, excess intake of boronsometimes causes problems such as nervous system disorders[2] and infertility [3]. Because of its potential toxicity, themaximum contaminant levels of boron are regulated by theWorld Health Organization at 10 mg L1 for industrial dis-charges and 1 mg L1 for drinking water [4]. Boron issometimes discharged from mining sites, and from electronic,semi-conductor and glass factories, potentially resulting ingroundwater contamination [5]. Permeable reactive barriers(PRBs) are used in in situ groundwater remediation for theremoval of contaminants [68]. However, PRBs have not been

permeability, availability and cost. Until now, immobilizationof borate on inorganic materials such as activated carbon [10],activated alumina [11], clay minerals [1214] and y ash, [15]has been investigated. In these approaches, low capacities andinconsistent quality are some of the drawbacks. Of thematerials tested, the highest reactivity was observed withmagnesium oxide [16,17].Magnesium oxide is a common material, which can be

obtained by calcination of the natural minerals, magnesite(Mg2CO3) and/or hydromagnesite (Mg2(CO3)(OH)2) [18].Magnesium oxide has been used for immobilization of notsolutions. Here we examine the effect of calcination temperature on sorption of borate for MgO-rich phases produced by calcination of magnesiteat 8731373 K. Calcination at or above 1273 K produced a single magnesium oxide phase, whereas basic magnesium carbonates(mMgCO3 nMg(OH)2 xH2O) formed in association with magnesium oxide at or below 1073 K. Calcination temperature directly affected theefciency of decarbonation of magnesium carbonate, and the solubility and basicity of magnesium oxide in the resultant calcined products. Thesefactors (along with the boron concentration) essentially control the immobilization mechanism of borate on the calcined MgO-rich phases. Aftersorption of borate on products calcined at lower temperatures, different types of basic magnesium carbonates were formed that are less effective atimmobilizing borate. At low borate concentrations, under saturation for magnesium borate hydrate (Mg7B4O13 7H2O), co-precipitation of boratewith Mg(OH)2 predominates. However, as magnesium borate hydrate becomes supersaturated, both precipitation of Mg7B4O13 7H2O andco-precipitation with Mg(OH)2 contribute signicantly to borate immobilization. Calcination temperature is a key practical factor affecting theborate sorption efciency by changing the immobilization mechanism.& 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Borate; Calcination temperature; Magnesium oxide; Magnesium borate hydrate; Basicity

1. Introduction Advanced materials, like boron-selective resins and mem-branes, have been intensively developed [9], but are unsuitableMagnesia (MgO), which can be obtained by calcination of natural magnesite, is one of the most effective known sorbents for borate in aqueousEffect of calcination temperatuof MgO-rich phas

Keiko Sasakia,n,aDepartment of Earth Resource Engineering

bFukuoka Prefecture Institute of Health and E

Received 29 June 2013; received in reviseAvailable onl

AbstractCERAMICSINTERNATIONAL

14) 16511660

for magnesite on interactions with boric acid

ayo Moriyamab

ushu University, Fukuoka 819-0395, Japanonmental Sciences, Fukuoka 818-0135, Japan

rm 11 July 2013; accepted 11 July 201319 July 2013

www.elsevier.com/locate/ceramint

s Inaffect the reactivity in applications. For example, Birchal et al.have reported that the higher calcination temperature ofmagnesite showed higher conversion levels of hydration[21]. This suggests the basicity of magnesium oxide isinuenced by the calcination temperature. The immobilizationmechanism of borate with magnesium oxide involves ligand-promoted dissolution of magnesium oxide [22]. While chelat-ing reagents inhibit MgO hydration [23], borate is a weakligand so hydration happens in the presence of borate.Additionally, precipitates which form and include the targetspecies need to be considered, because precipitation is also avery effective sink for target species. In the present work, theeffect of the calcination temperature used to produce magne-sium oxide from magnesite on the removal of borate isdiscussed, and is related to the borate removal mechanism.

2. Experimental

MgO-rich phases were produced by heating magnesiumcarbonate (special grade, Sigma-Aldrich, St. Louis, MO, US)using an electric furnace TME 2200 (EYELA, Tokyo, Japan)from room temperature to the reaction temperature (873, 1073,1273 or 1373 K) at 15 1C/min, holding at temperature for 1 hunder static air, and then cooling down naturally to roomtemperature. The calcined products were stored in a lowhumidity storage case V-30 (Shinei, Osaka, Japan) prior touse, and characterization and borate sorption experiments wereperformed within a week. Under these conditions, decarbona-tion of magnesium carbonate primarily produces magnesiumoxide. Thus the products are termed: MgO873, MgO1073,MgO1273, and MgO1373, respectively.The products calcined at different temperatures were char-

acterized by X-ray diffraction (XRD, Multi Flex, Rigaku,Akishima, Japan) using Cu K radiation at 20 mA and 40 kVfor crystal phase identication and evaluation of lattice strain() and crystal size (). The and values for each calcinedproduct were calculated by the HalderWagner method [24],as shown in

bn

dn 1

e bdn2

2

2; 1

where is the peak width at half height of a diffraction peak(corrected for instrument broadening using a Si standard), dn is2 sin / and n is cos /. The values of and can beobtained from plots of n/dn versus (/dn)2 [25].Measurements of specic surface areas were performed

using a gas sorption analyzer (Autosorb-1, Yuasa, Osaka,Japan) and the seven-point BrunauerEmmettTeller (BET)method. The average specic gravity of each calcined productwas determined using an Ostwald type pycnometer.Temperature programmed desorption curves for carbon

dioxide (CO2-TPD) were also acquired to evaluate basicityand the numbers of basic sites of the calcined products. Themeasurements were performed using a gas absorption mea-surement instrument BEL SORP (Bel Japan Inc., Toyonaka,

K. Sasaki, S. Moriyama / Ceramic1652Japan), with carbon dioxide as a probe gas. After pre-treatmentby heating at 973 K for 50 min, CO2-TPD curves weremeasured from room temperature to 1023 K. Peak deconvolu-tions were performed using a Chem Master ver. 1.2.0.2 (BelJapan Inc.).Scanning electron microscopic (SEM) images were obtained

using a VE-9800 SEM (KEYENCE, Osaka, Japan) with a15 kV acceleration voltage; transmission electron microscopic(TEM) images were obtained with an FEI TECNAI-20 (JEOL,Tokyo, Japan) TEM.Borate sorption experiments were carried out with an initial

H3BO3 (Wako, special grade) concentration of 6.08 mM at pH9.070.1, as adjusted with 0.2 M sodium hydroxide. 0.12 g ofcalcined product was added to 40 mL of the above boratesolution in a plastic bottle. Each bottle was tightly sealed witha plastic cap, laid down and horizontally shaken on a rotaryshaker (Takasaki Kagaku, Co. ltd, Kawaguchi, Japan) at100 rpm and 298 K until equilibrium was achieved. Atintervals, 1.0 mL of supernatant was sampled from each bottle,and ltered; the concentrations of boron and magnesium ionswithin the solution were evaluated by inductively coupledplasma atomic emission spectrometry (ICP-AES, VISTA-MPX, Seiko Instruments, Chiba, Japan). To obtain sorptionisotherms of borate on the calcined, MgO-rich phase products,additional batch tests were conducted using 0.10 g of sorbentsin 1.0067.90 mM borate solutions at an initial pH of9.070.1.Solid residues after borate sorption equilibrium tests were

collected and freeze-dried. XRD, SEM and TEM measure-ments were carried out as mentioned previously, and the boronto magnesium molar ratio was calculated by determination ofresidual dissolved concentrations using ICP-AES.

3. Results and discussion

Fig. 1 shows the XRD patterns for the products calcined at8731373 K. The XRD patterns for products calcined at1273 K or more showed the formation of a well crystallized,single phase of magnesium oxide (JCDPS 45-946). Withdecrease in calcination temperature, less crystalline magnesiumoxide was formed. After calcination at 1073 K, small amountsof Mg2CO3(OH)2 0.5H2O were found (JCPDS 37-454), alongwith less crystalline magnesium oxide as the predominantphase. At 873 K, uncalcined magnesium carbonate(MgCO3 5H2O, JCPDS 35-680) also remained, as well asless crystalline magnesium oxide and Mg2CO3(OH)2 0.5H2O.Thus, decarbonation of magnesium carbonate is incompleteafter calcination at temperatures of 1073 K or less [26].The HalderWagner [24] plots based on the results in Fig. 1

are depicted in Fig. 2. The correlation coefcient was morethan 0.95 in all cases. The slope was much steeper forMgO873 than for the other samples (Fig. 2(a)), so the plotsfor MgO1073, MgO1273 and MgO1373 were presented withexpanded y-axes in Fig. 2(b). Higher calcination temperatureswere found to produce larger crystal sizes () and smallerlattice strains (); calcination temperatures above 873 Ksignicantly changed the structural characteristics of the

ternational 40 (2014) 16511660magnesium oxide. The and values for the magnesiumoxide phase are summarized in Table 1.

120x10-6

100

80

60

r2 0.956

r2 0.985

0.04

0.02

0.00

(* /d

* )2

25x10-320151050*/ (d*)2

s InFig. 3 shows TEM images of the calcined products, againindicating increase in crystal size with increase in calcinationtemperature. Aggregation of hexagonal and rectangular plateswas more clearly observed at higher calcination temperatures.Compared with the crystal sizes estimated from powder XRD

Inte

nsity

70605040302010

2 kc

ps

2 kc

ps

1 kc

ps

200

cps

220

111

112

2

21

MgO1373

MgO1273

MgO1073

MgO873

200

320

111

020

130

230

Diffraction angle, 2 [Cu K] / degree

Fig. 1. XRD patterns of products after calcination of MgCO3 for 1 h at 873, 1073,1273, or 1373 K. Symbols: , MgO (JCPDS 45-946); , Mg2CO3(OH)2 0.5H2O(JCPDS 37-454); , MgCO3 5H2O (JCPDS 35-680).

K. Sasaki, S. Moriyama / Ceramic(Table 1), direct observation showed much smaller sizes(1060 nm). This difference has sometimes been observedwith other powders, because of size distribution [27]. Theelectron diffraction patterns also conrmed that higher crystal-linity was obtained at higher calcination temperature.As shown in Table 1, the specic surface areas of the

calcined products decreased with increase in calcinationtemperature above 1073 K. The average specic gravity ofthe calcined product was 2.29 g/cm3 for MgO873, andincreased with increasing calcination temperature to approach3.58 g/cm3, the theoretical value for MgO [28], for MgO1373.The products calcined at lower temperatures are characterizedby imperfect decarbonation, and magnesium carbonates havelower densities [29], e.g. 1.85 g/cm3 for MgCO3 3H2O.Changes in specic surface area reected both decarbonationand sintering. The decarbonation process results in an increasein specic surface area, but the additional sintering process,which occurs more in the samples sintered at 10731273 K,decreases the specic surface area. Both decarbonation andsintering are enhanced with increasing calcination temperature.The CO2-TPD curves were collected, converted into sorbed

mass of carbon dioxide per unit surface area, and deconvolutedinto three components, as shown in Fig. 4. The sorbed mass ofcarbon dioxide per specic surface area decreased withdecrease in calcination temperature. The basicity around370 1C has been previously shown to increase with increasingcalcination temperature, which affects hydration [21,23].0.12

0.10

0.08

0.06

(* /d

* )2

MgO1373 MgO1273 MgO1073 MgO873

r2 0.956

ternational 40 (2014) 16511660 1653Fig. 5 shows examples of batch sorption, to demonstrate theeffect of calcination temperature on changes in boron andmagnesium concentrations in solutions and on pH, for aninitial boron concentration of 6.08 mM. The equilibrium boronconcentrations were in the order: MgO1373MgO1273oMgO1073oMgO873 (Fig. 5(a)). This is clearly related to thefact that there is less magnesium oxide in MgO873 than in theother sorbents (as shown in Fig. 1). Soon after contact ofmagnesium oxide with boric acid, magnesium ions werereleased (Fig. 5(b)). As reported in the previous work [17],the larger amounts of magnesium ions released from MgO873were derived from the less crystalline magnesium oxide withinthis sorbent, not from carbonates, because the carbonates are

40

20

025x10-320151050

r2 0.995

*/ (d*)2

Fig. 2. Estimation of crystal size () and lattice strain () by HalderWagnerplots for MgO873, MgO1073, MgO1273, and MgO1373. Correlation coef-cients are 0.956, 0.956, 0.985 and 0.995 for MgO873, MgO1073, MgO1273,and MgO1373, respectively.

()

c gr

s InTable 1BET specic surface area, specic gravity, crystal size () and lattice strainHalderWagner plots in Fig. 2.

BET specic surface area (m2/g) Speci

MgO 873 49.8 2.29MgO1073 79.0 3.14MgO1273 29.1 3.44MgO1373 12.7 3.58

K. Sasaki, S. Moriyama / Ceramic1654less soluble than magnesium oxide. However, the quantity ofmagnesium ions released from MgO873 cannot be explainedby the solubility only, because the concentration is approxi-mately twice that of a background sample (i.e., when thesorbent was immersed in water rather than boron-containingsolution).This suggests that some additional mechanism exists. Also,

more than 1 mM of magnesium ions was initially releasedfrom the other calcined products. This phenomenon hasnot been observed during sorption of uoride on these calcinedproducts [17], and is considered to be derived from reactionmechanisms of magnesium oxide with borate that differ fromthose with uoride. Both immobilizations are based on

50 nm

50 nm

Fig. 3. TEM images of (a) MgO873, (b) MgOfor calcined products at different temperatures. and were estimated by

avity (g/cm3) Crystal size, (nm) Lattice strain,

61 0.0063275 0.0056298 0.0022414 0.0026

ternational 40 (2014) 16511660co-precipitation with Mg(OH)2, which is a hydration productof MgO. During reaction of magnesium oxide with boric acid,Mg2+ ions are complexed with H3BO3 to form [MgB(OH)4]

+

(K101.34101.63 at 25 1C, [30]), leading to ligand-promoted dissolution of magnesium oxide. Separately, it hasbeen observed that larger initial boron concentrations inducedrelease of greater amounts of magnesium ions from the samemagnesium oxide sorbents [22]. Ligand-promoted dissolutionof magnesium oxide is not observed with uoride.The equilibrium pH was clearly higher for samples calcined

at higher temperatures (Fig. 5(c)); this would facilitate theprecipitation of Mg(OH)2, resulting in lower equilibriummagnesium ion concentrations (Fig. 5(b)). It has been

50 nm

50 nm

1073, (c) MgO1273, and (d) MgO1373.

6

5

4 (m

M)

6

5

4

3

2

1

0

B c

once

ntra

tion

(mM

)

120100806040200Time (h)

MgO1373 MgO1273 MgO1073 MgO873

s InDes

orpt

ion

of C

O2

500040003000200010000Time / sec

100

V m

-2

(0.6753 mmol/m2) 1073K

(1.252 mmol/m2) 1273K

(1.558 mmol/m2) 1373K

12.2

2%

34.1

5%

54.5

4%

9.19

% 49.2

2%

42.0

4%

3.12

% 30.4

4%

65.7

2%

K. Sasaki, S. Moriyama / Ceramicsuggested that there is a competition between water andH3BO3 to access the surfaces of magnesium oxide [31]. Therate of sorption of borate was highest for MgO1073, followedby MgO1273 and MgO1373 (Fig. 5(a)). This was probablybecause of faster hydration of the higher temperature-calcinedproducts (which have greater basicities); this hydration theninterferes with access of H3BO3 to MgO surfaces.The effects of calcination temperature on the sorption

isotherms of borate on calcined products are depicted inFig. 6. The numbers on the gure indicate the initial boronconcentrations in mM. There is a trend that greater sorptiondensities were obtained with products calcined at highertemperatures, although the sorption density of borate is verysimilar for MgO1273 and MgO1373 at all the equilibriumconcentration (Ce) values studied (Fig. 6(a)). When Ce is lessthan 2 mM (Fig. 6(b)), the performance of MgO1073 is alsosimilar to that of the samples calcined at higher temperatures.MgO873 includes non-negligible amounts of Mg2CO3(OH)2and MgCO3, which do not meaningfully contribute to sorptionof borate [23], as shown in Fig. 1. Sorption isotherms of borateincreased sharply at Ce30 mM, indicating that there areadditional sorption mechanisms which depend on the concen-tration of borate.The sorption density of borate is not obviously inuenced

by the specic surface areas of the calcined products, because

700600500400300Temperature / K

Fig. 4. CO2-TPD curves and their deconvolution for products calcined atdifferent temperatures. The numbers in brackets indicate total basicitiesexpressed as mass of carbon dioxide sorbed per unit surface area. Percentagesindicate the relative areas of each peak after deconvolution.7

ternational 40 (2014) 16511660 1655of destructive sorption [23], but by the basicity per unit surfacearea (Fig. 4). This trend was also observed for the sorption ofuoride, that is, a larger basicity per unit surface area was

3

2

1

0

Mg

conc

entra

tion

120100806040200Time (h)

11.5

11.0

10.5

10.0

9.5

9.0

8.5

pH

120100806040200Time (h)

Fig. 5. Changes of (a) boron concentration, (b) magnesium concentration, and(c) pH with time during sorption of 6.08 mM boron at 298 K on 0.12 g ofproducts calcined at different temperatures.

XRD patterns for solid residues from MgO-rich phasescalcined at 8731373 K, after sorption equilibrium tests in6.08 mM and 67.9 mM borate solutions, are shown in Fig. 7(a) and (b). The solid residues include brucite (Mg(OH)2,JCPDS 441482) as the dominant phase in most cases. At aninitial concentration of 6.08 mM, poorly crystallized hydro-magnesite (Mg5(CO3)4(OH)2 4H2O, JCPDS 25-513) was alsoformed, along with Mg(OH)2, for all calcination temperatures.

s International 40 (2014) 1651166012

10

8

6

(mm

ol/g

)

MgO1373 MgO1273 MgO1073 MgO873

K. Sasaki, S. Moriyama / Ceramic1656observed at higher calcination temperature [17]. However, thereis a difference between the effects of calcination temperature onsorption densities of uoride and borate. Products calcined atlower temperatures tended to show larger sorption densities ofuoride at higher Ce of uoride, as a result of the formation ofMg(OH)2xFx [17]. In the case of uoride, sorption isothermscould be tted to Freundlich-type isotherm equations, withoutany regions where the gradient of the curve increased. How-ever, for sorption isotherms of borate, products calcined athigher temperatures consistently showed larger sorption den-sities, independent of borate concentrations; regions of increas-ing gradient were also seen at Ce30 mM (Fig. 6(a)).

4

2

0

Q

6050403020100Ce (mM)

2.0

1.5

1.0

0.5

0.0

Q (m

mol

/g)

1086420Ce (mM)

Fig. 6. Sorption isotherms of boron on magnesium oxide calcined at differenttemperatures. The region of Ceo10 mM in (a) is expanded in (b).The numbers (mM) in both gures indicate the initial boron concentrations.

Inte

nsity

70605040302010

MgO1373

MgO1273

MgO1073

MgO873

200

100

001

211 421 02

2

322

143

0.5

kcps

101

220

102

110

201

123

110

100 020

102

011

332

103

230

111

212 221

B/Mg 0.04

B/Mg 0.04

B/Mg 0.03

B/Mg 0.02

Inte

nsity

2 kc

ps

2 kc

ps1

kcps

MgO1373

MgO1273

MgO1073

MgO873020

Diffraction angle, 2 [Cu K ]/ degree70605040302010

2 kc

ps

100 10

4

110 011

020

102 321

023

113 21

1 31

0 2 02

040

1 13

145

125

300

204

Diffraction angle, 2 [Cu K ]/ degree

Fig. 7. XRD patterns of solid residues after sorption of (a) 6.08 mMand (b) 67.9 mM B on MgO873, MgO1073, MgO1273, and MgO1373.Symbols: , MgO; , Mg(OH)2 (JCPDS 44-1482); , Mg5(CO3)4(OH)2 4H2O(JCPDS 25-513); , Mg7(CO3)5(OH)4 24H2O (JCPDS 47-1880); ,

MgB3O3(OH)2 5H2O (JCPDS 7-595); , Mg7B4O13 7H2O (magnesiumborate hydrate) (JCPDS 019-0754).

The newly produced Mg5(CO3)4(OH)2 4H2O is a solidsolution of MgCO3 and Mg(OH)2, and can also be expressedas 4MgCO3 Mg(OH)2 4H2O. At higher calcination tempera-tures, the relative intensities of residual MgO were larger, andthe Mg(OH)2 formed was more crystalline, but the crystallinityof the Mg5(CO3)4(OH)2 4H2O that formed was not affected

very much (Fig. 7(a)). With MgO873, less crystalline Mg(OH)2 and more crystalline Mg5(CO3)4(OH)24H2O wereformed. The basic magnesium carbonates are expected to formby reaction of released Mg2+ ions from poorly crystallinemagnesium oxide with carbon dioxide derived from airwhen the pH is not high enough to precipitate Mg(OH)2.

K. Sasaki, S. Moriyama / Ceramics International 40 (2014) 16511660 1657Fig. 8. SEM images of solid residues after sorption of (a)(d) 6.08 mM and (e)(h)and MgO1373 ((d), (h)). The scale bars indicate 5 m.67.9 mM boron on: MgO873 ((a), (e)); MgO1073 ((b), (f)); MgO1273 ((c), (g))

The equilibrium pH after sorption of borate is higher forsorbents which were calcined at higher temperatures (Fig. 5(c)); this would facilitate the precipitation of Mg(OH)2. Basedon this mechanism for reaction of borate with magnesiumoxide, greater basicity was obtained at higher calcinationtemperatures in Fig. 4. This then meant that ligand-promoteddissolution of MgO was more feasible, and led to moreeffective immobilization of borate in Mg(OH)2 phase. Incontrast, for samples calcined at lower temperatures, thecalcined products are less basic, and thus ligand-promoteddissolution of MgO is less feasible. Therefore, the fate of freeMg2+ ions derived from simple dissolution of lower crystal-linity MgO, is to be precipitated as basic magnesium carbo-nates, as shown in Fig. 7(a), because the solution becomessuper-saturated with respect to Mg5(CO3)4(OH)2 4H2O. Thisprecipitation is expected to contribute negligibly to immobili-zation of boron, because the magnesium carbonate complexdoes not involve borate. The boron to magnesium molar ratioin the solid residues ranged from 0.02 to 0.04 (Fig. 7(a)),where the smaller values were obtained with the lowercalcination temperatures.Meanwhile, after sorption of 67.9 mM borate on MgO1073,

MgO1273, and MgO1373, peaks assigned to magnesium boratehydrate (Mg7B4O13 7H2O, JCPDS 019-0754) were observed,

along with Mg(OH)2, in the solid residues (Fig. 7(b)). Themagnesium borate hydrate is not an oxide, but instead is basicmagnesium borate triborate, expressed as Mg7(OH)7(B(OH)4)[B3O6(OH)3], which is precipitated through the complex[MgB(OH)4]

+, produced as an intermediate during ligand-promoted dissolution of MgO. For MgO873, kurnakovite(MgB3O3(OH)55H2O, JCPDS 7-596) was formed, as well asMg(OH)2 and Mg5(CO3)4(OH)2 4H2O. Kurnakovite is a solidsolution of Mg(OH)2 and B3O3(OH)3, expressed as Mg(OH)2B3O3(OH)3 5H2O. The boron to magnesium molar ratioin the solid residues, at this higher boron concentration, rangedfrom 0.24 to 0.43, where the smaller values were obtained forsorbents calcined at lower temperatures. In the presence of highboron concentrations, both precipitation involving boron speciesand co-precipitation with Mg(OH)2 contribute to tri-dimensionalimmobilization of B. In Fig. 6(a), precipitation involving boronspecies is responsible for the increase in gradient of the sorptionisotherms when Ce is greater than 30 mM. With the increasedinitial boron concentration, a lower equilibrium pH wasobserved; this then interferes with the precipitation of Mg(OH)2. The suppression of magnesium hydroxide precipitationalso suggests that H3BO3 interferes with the access of watermolecules to MgO surfaces. Therefore, magnesium ions, in thepresence of high boron concentrations, are incorporated into

K. Sasaki, S. Moriyama / Ceramics International 40 (2014) 165116601658200 nm

100 nmFig. 9. TEM images of the solid residues after sorption of 6.08 mM boro200 nm

200 nm n on (a) MgO873, (b) MgO1073, (c) MgO1273, and (d) MgO1373.

[5] R.O. Nable, G.S. Banuelos, J.G. Paul, Boron toxicity, Plant and Soil 193(1997) 181198.

Groundwater Remediation Using Permeable Reactive Barriers: Applica-

223 (2008) 3137.

s Inmagnesium borate hydrate precipitates; for example, Gf0 is- 8449.59 kJ/mol for MgB3O3(OH)5 5H2O [32].SEM images of solid residues after sorption of borate

showed that the surface morphologies had clearly changed(Fig. 8). The images of the calcined products were similar,independent of calcination temperature, and showed rod-shaped crystals, 56 m in diameter. After sorption of6.08 mM borate, the surface morphologies of the solid residueschanged into honeycomb structures, probably from partialdissolution of MgO and formation of bulky Mg(OH)2 (Fig. 8(a)(d), Fig. S1). In the solid residues after sorption of67.9 mM borate, segregation of each particle seems to haveprogressed, and regular honeycomb structure is hard to see(Fig. 8(e)(h)); this is probably because of the formation ofadditional Mg7B4O13 phase (Fig. 7(b)).TEM images of the solid residues after sorption of 6.08 mM

borate were totally different from those before sorption ofborate (Fig. 9). As can be seen in the XRD results (Fig. 7(a)and (b)), as boron concentration increases, peaks from hk0planes become narrower, while those from hkl planes arebroadened. This occurs because immobilized H3BO3 interfereswith stacking along the c-axis direction of Mg(OH)2. Accord-ing to Fig. 6(b), sorption densities are similar for MgO1073,MgO1273 and MgO1373 at the initial boron concentration of6.08 mM, and are around double that of MgO873. The TEMimages show developed plate-like crystals, consistent with theXRD results shown in Fig. 7(a). The crystal size of the platesin solid residues is affected by the calcination temperature usedfor the sorbent: the largest plates occur for MgO1273 andMgO1373, whilst much smaller crystals occur for MgO873. Itcan be seen that sorption of borate did not interfere with theexpansion and aking of the plates in Mg(OH)2.

4. Conclusions

MgO-rich phases were formed by calcination of magnesiumcarbonate at 873, 1073, 1273 or 1373 K for 1 h. Highercalcination temperatures led to higher crystallinity of MgO, andgreater efciency in removal of borate. The efciency of removalof borate was reasonably well correlated with the basicity per unitsurface area of magnesium oxide in each product, as obtainedfrom CO2-TPD analysis, and did not depend upon specicsurface area. The greater basicity with higher calcination tem-perature promotes processes in which a molecular form of boricacid accesses the surface of magnesium oxide. The nal pHvalues after sorption of borate always increased, and weresufcient to lead to super saturation with respect to Mg(OH)2.It is considered that the borate removal process can be dividedinto co-precipitation with Mg(OH)2 and precipitation ofMg7B4O13 7H2O and MgB3O3(OH)5 5H2O. At high borateconcentrations, the additional mechanism of precipitation ofboron-containing species such as Mg7B4O13 7H2O and MgB3O3(OH)5 5H2O contributes to immobilization of boron. In thisconcentration range, the sorption isotherms do not show Freun-dlich behaviorinstead the gradient of the sorption density curve

K. Sasaki, S. Moriyama / Ceramicincreases with increasing concentration. Calcination temperaturesdirectly affect the efciency of decarbonation of magnesium[12] S. Karahan, M. Yurdako, Y. Seki, K. Yurdako, Removal of boron fromaqueous solution by clays and modied clays, Journal of Colloid andInterface Science 293 (2006) 3642.

[13] R. Keren, F.T. Bingham, Boron in water, soils, and plants, Advances inSoil Sciences 1 (1985) 229276.

[14] S. Goldberg, H.S. Froster, S.M. Lesch, E.L. Heick, Inuence of aniontions to Adionuclides, Trace Metals, and Nutrients, Academic Press, SanDiego, CA, 2003.

[9] J. Wolska, M. Bryjak, Methods for boron removal from aqueoussolutionsa review, Desalination 310 (2013) 1824.

[10] Z.C. Celik, B.Z. Can, M.M. Kocakerim, Boron removal from aqueoussolutions byactivatedcarbon impregnated with salicylic acid, Journal ofHazardous Materials 152 (2008) 415422.

[11] W. Bouguerra, A. Mnif, B. Hamrouni, M. Dhahbi, Boron removal byadsorption onto activated alumina and by reverse osmosis, Desalination[6] D.W. Blowes, C.J. Ptacek, S.G. Bernner, C.W.T. Mcrae, R.W. Puls,Treatment of dissolved metals and nutrients using permeable reactivebarriers, Journal of Contaminant Hydrology 45 (2000) 123137.

[7] M.M. Scherer, S. Richter, R.L. Valentine, P.J.J. Alvarez, Chemistry andmicrobiology of permeable reactive barriers for in situ groundwater cleanup, Environmental Science and Technology 30 (2000) 363411.

[8] D.L. Naftz, S.J. Morrison, C.C. Fuller, J.A. Davis, Handbook ofcarbonate and the basicity of the magnesium oxide containedwithin the calcined products; these factors, as well as boronconcentration, signicantly inuence the immobilization mechan-ism of borate on the calcined MgO-rich phases.

Acknowledgment

Financial support was provided to KS by JSPS FundingProgram for Next-Generation World Leading Researchers(NEXT Program) GR078. TEM observation was conductedin high voltage electron microscopy (HVEM), KyushuUniversity.

Appendix A. Supporting information

Supplementary data associated with this article can be foundin the online version at http://dx.doi.org/10.1016/j.ceramint.2013.07.056.

References

[1] T. Sutton, U. Baumann, J. Hayes, N.C. Collins, B.-J. Shi, T. Schnurb,A. Hay, G. Mayo, M. Pallotta, M. Tester, P. Langridge, Boron-toxicitytolerance in barley arising from efux transporter amplication, Science318 (5855) (2007) 14461449.

[2] R. Liu, J. Wilmalasea, R.L. Bowen, C.S. Atwood, Luteining hormonereceptor mediates neuronal pregnenolone production via up-regulation ofsteroidogenic acute regulatory protein expression, Journal of Neurochem-istry 100 (2007) 13291339.

[3] U. Sayli, Y. Tekdemysr, H.E. Cubuk, S. Avci, E. Tccar, H.A. Elhan,A. Uz, The course of the special peroneal nerve: an anatomical cadaverstudy, Foot and Ankle Surgery 4 (1998) 69 (69).

[4] WHO, Environmental Health Criteria Monograph, IPCS, Geneva204.

ternational 40 (2014) 16511660 1659competition on boron adsorption by clays and soils, Soil Science 161(1996) 99103.

[15] N. ztrk, D. Kavak, Adsorption of boron from aqueous solutions using yash: Batch and column studies, Journal of Hazardous Materials 127 (2005)8188.

[16] M.M. De La Fuente Garca-Soto, E.M. Camacho, Boron removal bymeans of adsorption with magnesium oxide, Separation and PuricationTechnology 48 (2006) 3644.

[17] K. Sasaki, N. Fukumoto, S. Moriyama, T. Hirajima, Sorption character-istics of uoride on to magnesium oxide-rich phases calcined at differenttemperature, Journal of Hazardous Materials 191 (2011) 240248.

[18] R.Y. Zhang, J.G. Lion, Signicance of magnesite paragenesis in ultrahigh-pressure metamorphic rocks, American Mineralogist 79 (1994) 397400.

[19] M.A. Garca, J.M. Chimenos, A.I. Fernndez, L. Miralles, M. Segarra,F. Espiell, Low-grade MgO used to stabilize heavy metals in highlycontaminated soils, Chemosphere 56 (2004) 481491.

[20] S. Zhang, F. Cheng, Z. Tao, F. Gao, J. Chen, Removal of nickel ionsfrom wastewater by Mg(OH)2/MgO nanostructures embedded in Al2O3membranes, Journal of Alloys and Compounds 426 (2006) 281285.

[21] V.S.S. Birchal, S.D.F. Rocha, V.S.T. Ciminelli, The effect of magnesitecalcination conditions on magnesia hydration, Minerals Engineering 13(2000) 16291633.

[22] K. Sasaki, X. Qiu, S. Moriyama, C. Tokoro, K. Ideta, J. Miyawaki,Characteristic sorption of H3BO3/B(OH)4

on magnesium oxide. MaterialsTransactions, http://dx.doi.org10.2320/matertrans.M-M2013814, in press.

[23] L.F. Amaral, I.R. Oliveira, P. Bonadia, R. Saloma~o, V.C. Pandolfelli,Chelants to inhibit magnesia (MgO) hydration, Ceramics International 37(2011) 15371542.

[24] N.C. Halder, N.C.J. Wagner, Separation of particle size and lattice strain inintegral breadth measurements, Acta Crystallographica 20 (1966) 312313.

[25] A. Korchef, Y. Champion, N. Njah, X-ray diffraction analysis ofaluminum containing Al8Fe2Si processed by equal channel angularpressing, Journal od Alloys and Compounds 427 (2007) 176182.

[26] Y. Sawada, J. Yamaguchi, O. Sakurai, K. Uematsu, N. Mizutani,M. Kato, Thermogravimetric study on the decomposition of hydromag-nesite 4 MgCO3 Mg(OH)2 4 H2O, Thermochimica Acta 33 (1979)127140.

[27] C.E. Sjgren, C. Johansson, A. Nvestad, K. Briley-Sb, A.K. Fahlvik,Crystal size and properties of superparamagnetic iron oxide (SPIO)particles, Magnetic Resonance Imaging 15 (1997) 5567.

[28] M.S. Mel'gunov, V.B. Fenelonov, E.A. Mel'gunova, A.F. Bedilo,K.J. Klabunde, Textural changes during topochemical decomposition ofnanocrystalline Mg(OH)2 to MgO, Journal of Physical Chemistry B 107(2003) 24272434.

[29] H.N. Ramesh, S.D. Venkataraja Mohan, Compaction characteristics ofalkalis treated expansive and non-expansive soil contaminated with acids,in: Proceedings of the Indian Geotechnical Conference, Kochi, India,2011, pp. 733736.

[30] R.L. Bassett, A critical evaluation of the thermodynamic data for boronions, ion pairs, complexes, and polyanions in aqueous solution at298.15 K and 1 bar, Geochimica Cosmochimica Acta 44 (1980)11511160.

[31] J.L. Anchel, A.C. Hess, H2O dissociation at low-coordinated sites on(MgO)n Clusters, n4, 8, Journal of Physical Chemistry 100 (1966)1831718321.

[32] J. Li, B. Li, S. Gao, Calculation of thermodynamic properties of hydratedborates by group contribution method, Physics and Chemistry of Minerals27 (2000) 342346.

K. Sasaki, S. Moriyama / Ceramics International 40 (2014) 165116601660

Effect of calcination temperature for magnesite on interaction of MgO-rich phases with boric acidIntroductionExperimentalResults and discussionConclusionsAcknowledgmentSupporting informationReferences