Chemical Geology 352 (2013) 134–142

Contents lists available at SciVerse ScienceDirect

Chemical Geology

j ourna l homepage: www.e lsev ie r .com/ locate /chemgeo

Comparative effect of chrysotile leaching in nitric, sulfuric and oxalicacids at room temperature

Marisa Rozalen ⁎, F. Javier HuertasInstituto Andaluz de Ciencias de la Tierra (IACT), CSIC-University of Granada, Avda. de las Palmeras 4, 18100 Armilla, Granada, Spain

⁎ Corresponding author. Tel.: +34 958 230000x1901E-mail address: marisarozalen@ugr.es (M. Rozalen).

0009-2541/$ – see front matter © 2013 Published by Elhttp://dx.doi.org/10.1016/j.chemgeo.2013.06.004

a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 November 2012Received in revised form 3 June 2013Accepted 4 June 2013Available online 10 June 2013

Editor: J. Fein

Keywords:ChrysotileAcid leachingOxalateGlushinskiteDissolution mechanism

The acid leaching of chrysotile was investigated in different acid media with the aim of quantifying andobtaining insights into the dissolution mechanism. Chrysotile was leached in batch reactors for 1 to30 days at 25 °C and pH 1 in aqueous solutions of nitric and sulfuric acid and different concentrations ofoxalic acid (50, 100 and 200 mmol L−1). The combined analysis of solutions and solids by XRD and FTIRshows different effects: nitric acid induces a strong dissolution after 30 days, lowering the crystallinity ofthe sample and initiating the transformation of the chrysotile into an amorphous siliceous material. In thecase of sulfuric acid, the dissolution is so intense that it is able to destroy the brucitic sheet of chrysotile, lead-ing to an amorphous silica byproduct. Finally oxalic acid is also able to induce amorphization of chrysotile andthe precipitation of glushinskite (MgC2O4·2H2O). As the concentration of oxalic increases from 50 to200 mmol L−1 the amorphization process becomes faster. Finally, the relative effectiveness of acid attackto chrysotile is oxalic acid (9 days) > sulfuric acid (30 days) > nitric acid.

© 2013 Published by Elsevier B.V.

1. Introduction

Despite the fact that the use of asbestos has been banned in theEuropean Union since 2005 there exist many countries, such as Russia,India and China that mine and commercialize chrysotile. Moreover theintensive use of asbestos in the past, due to its properties as an insulatorand its mechanical resistance, has led to a growing concern with itspresence in the environment. To prevent adverse effects on humansand biota, it is crucial to identify and understand the degradation pro-cesses which are vitally important for the development of decontami-nation strategies, not only for abandoned mines but also to treatasbestos containing materials (ACMs). Moreover, the mechanism ofchrysotile dissolution is also of critical importance for practical aspectsof carbon capture and storage (e.g. O'Connor et al., 2002; Park et al.,2003; Park and Fan, 2004). For example, a mixture of weak acids andchemical additives enhanced the dissolution of ground serpentine atlow-energy costs while preventing the precipitation of Fe(III) on thesurface of the Mg-bearing mineral particles (Park et al., 2003).

SinceHargreaves andTaylor (1946) reported that iffibrous chrysotileis treated with dilute acids, the magnesium can be completely removed,many other studies have focused on leaching with mineral acids (Nagyand Bates, 1952; Fripiat and Mendelovici, 1968; Johan et al., 1974;Allen and Smith, 1975; Papirer et al., 1976). In particular, Pacco et al.(1976) found, by Fourier-transform infrared spectroscopy (FTIR) and

20.

sevier B.V.

nuclear magnetic resonance (NMR) spectroscopy, that the acid attackproduces an ordered fibrous silica gel, retaining the fibrous morphologyof the original mineral.

Subsequently, several authors reported the effect of organic acids,such as oxalic (Thomassin et al., 1977) and pyruvic acids (Goni et al.,1979), for leaching in lixiviating magnesium from the chrysotilestructure. The complexation capacity of these acids favors the inter-action between the hydroxyl groups in the Mg(OH)2 sheet and thehydrogen ions liberated from the acids, thereby eliminating themag-nesium hydroxide sheet. Moreover, the effectiveness with which theMg(OH)2 sheet is broken in acid media has been investigated usingother materials, such as titanium chloride (Cozak et al., 1983).Cosak found that titanium is able to cause the morphological trans-formation of a rolled fiber structure into an open or unrolled amor-phous one.

Environmentally friendly policies have led to a growing interestin the transformation of asbestos-containing materials into non-hazardous phases, as displayed by recent literature. Most of thesestudies transformed chrysotile into an amorphous phase by leachingchrysotile with hydrochloric acid together with a range of thermal ormechanical treatments (e.g. Keane et al., 1999; Wypych et al., 2005;Wang et al., 2006; Liu et al., 2007; Gualtieri et al., 2008; Silva et al.,2011). Other authors (Mirick, 1991; Mirick and Forrister, 1993;Sugama et al., 1998) demonstrated and patented a method that in-corporated fluoride ions into the acidic media, which significantlypromotes the rate of chemical decomposition of asbestos. Presum-ably the attack by acids on the chrysotile causes hydrolysis of theouter brucitic sheet in the structure, while fluoride ions favorably

135M. Rozalen, F.J. Huertas / Chemical Geology 352 (2013) 134–142

react with Si in the inner siliceous sheets, thereby destroying the fi-brous nature of the chrysotile.

However, the danger of hydrofluoric acid and the use of high tem-peratures to enhance the transformation reaction increase the cost ofthe recycling process. As an alternative some other authors have triedto use organic additives to enhance the transformation at ambient tem-perature. One of the most often used is oxalic acid, because it is presentin many soils and lichens secrete it as a metabolite. Favero-Longo et al.(2005) showed that the action of some lichen species (Crustose andFoliose) actively helps the weathering of serpentinite outcrops. Theseshow a natural deactivation (transformation into a non-hazardous ma-terial) due to selectiveMg-depletion of those chrysotilefibers in contactwith lichens. Different techniques (X-ray photoelectron spectroscopyand scanning electron microscopy) have shown a depletion of magne-sium on the surface but no X-ray diffraction data are available to dem-onstrate whether the initial material turns into an amorphousmaterial.

Finally, and despite the vast number of studies dedicated to chryso-tile acid leaching, there are many questions about the transformationmechanism that remain unsolved. For instance, is it a question of pH?Is themechanism same for all mineral acids? Do they have the same ef-fect? Canwe improve the transformation rate by increasing the concen-tration of oxalic acid? The aim of this study is therefore to investigatethe mechanism of chrysotile transformation using three different buff-ered acid media (HNO3, H2SO4 and oxalic acid) adjusted at pH 1, inorder to see if all of them are able to turn chrysotile into an amorphousmaterial and obtain some information about themechanismof this pro-cess. The information obtained in this studywill help to develop and im-prove remediation techniques for asbestos-containingmaterials (ACM).

2. Materials and methods

2.1. Characterization of the mineral sample

All experiments were done using chrysotile collected from MinaLaurel, an old asbestos mine in the Ojén ultramafic massif (Málaga,Spain). The starting material was studied by X-ray diffraction (XRD)and corresponds to chrysotile, without any accompanying minerals.Scanning electron microscopy (SEM) images show mainly fiber bun-dles, made up of acicular curved structures with a wide variety oflength:diameter ratios (Fig. 1). The length of the fibers varies fromthose that are easily observed with the eye (2–3 mm) to a large quan-tity of very fine material (length b 2 μm). The composition of individ-ual fibers was obtained by transmission and analytical electronmicroscopy (TEM–AEM). The average composition obtained fromseveral tens of fibers was used to calculate the chrysotile structuralformula, giving Ca0.02(Al0.06Fe0.10Mg2.75)(Si2)O5(OH)4. The corre-sponding Mg/Si atomic ratio is 1.38.

50 µm

Fig. 1. SEM image of the micromorphology of the chrysotile used as starting material.

2.2. Experimental setup

Acid leaching experiments were carried out at room temperaturein batch reactors using a solid/solution ratio of 2 g L−1. Five sets ofexperiments were performed using inorganic buffered solutions atpH 1, namely HNO3 (0.1 mol L−1) and H2SO4 (0.05 mol L−1) and or-ganic buffered solutions with 50 mmol L−1, 100 mmol L−1 and200 mmol L−1 of oxalic acid (series Ox50, Ox100, Ox200) at pH 1(Table 1).

For every set, 6 different batch reactors were prepared, placing0.08 g of chrysotile directly in acid-cleaned PP bottles and adding40 mL of buffer solution. The reaction vessels were shaken everyday to prevent the reaction being controlled by diffusion. After theappropriate reaction time for each batch (1, 2, 5, 9, 19 or 30 days),the residual solids were centrifuged for 15 min at 5000 rpm and thesupernatant filtered through a 0.45 μm filter into a previouslyacid-cleaned polyethylene bottle for solution analysis.

The solutionswere analyzed for pH and total dissolved Si, Mg and ox-alate. The pH of the solutions wasmeasured at room temperature with aCRISON micropH 2000 pH-meter, using a standard Crison pH 52–03dehydrated membrane electrode, standardized against pH 4.01 and7.00 Crison buffer solutions at room temperature. The reported accuracywas ±0.02 pH units. The Si concentration in the samples was deter-mined by colorimetry, using the molybdate blue method (Grasshoff etal., 1983)with a Visible/UV spectrophotometer at 825 nm. The detectionlimit is 5 ppb for Si and the associated error is 5%. The Mg concentrationin every solution was determined by ion chromatography using aMetrohm 883 Basic IC Plus Ion Chromatograph with a MetrosepC3-250 column. The eluent was prepared with 3.5 mmol L−1 HNO3.The detection limit is 1 ppbwith 3% associated error. Finally, the concen-tration of oxalate was measured using a Metrohm 883 Basic IC Plus IonChromatograph, with a with a Metrosep A Supp 4–250 column andchemical suppression. The eluent was prepared with 1.7 mmol L−1

NaHCO3 and 1.8 mmol L−1 NaCO3. A sodium oxalate solution was usedas a standard. The detection limit is 0.9 ppm and the associated error is5%.

Once the experiments were completed, the solid samples werewashed with distilled water, centrifuged three times and finallydried at 40 °C in capped bottles to avoid dispersion of the fibers,and stored to proceed with analysis by X-ray diffractions (XRD) andFTIR.

Powder XRD was used to determine changes in mineralogy beforeand after leaching. Back-loaded powder mounts were run on aPANalytical X'Pert Pro diffractometer with an X'Celerator detector,operated at 45 kV and 40 mA, with Ni filtered CuKα radiation. Pat-terns were recorded from 4 to 70 2θ degrees. Minerals were identifiedusing PANalytical X'Pert HighScore Plus v.2.2a and the ICDD PDF-2database.

FTIR was used to determine changes in crystal chemistry beforeand after leaching. FTIR spectra were recorded in absorbance modein the 4000–400 cm−1 range using a Perkin Elmer Spectrum Onespectrometer with a spectral resolution of 4 cm−1 from the averageof 100 spectra. The sample was prepared in KBr pressed pellets by

Table 1Stability constants of oxalic acid (H2Ox) and aqueous Mg2+-oxalate complexes calcu-lated at 25 °C.

Reaction Constant Reference

H2Ox = HOx− + H+ pK1 = 1.27 a

HOx− = Ox2− + H+ pK2 = 4.26 a

Mg2+ + Ox2− = Mg(Ox)aq logK1 = 2.10 b

Mg2+ + 2Ox2− = Mg(Ox)22− logβ2 = 6.67 b

a EQ3/6 database (Wolery, 1992).b Prapaipong et al. (1999)

5000

0

0

0

0

0

9000

0

0 10 20 30 40 50 60

1600

1600

1600

1600

4000

Position [o 2Theta] (Copper (Cu))In

ten

sity

a

b

c

d

e

f

g

Crx

Crx

Crx

Crx Crx CrxCrx

Fig. 2. XRDpatterns of: a) initial chrysotile and nitric acid leached samples after: b) 1 day;c) 2 days; d) 5 days; e) 9 days; f) 19 days; and g) 30 days of reaction.

y = -50.545x + 1887 R = 0.95318

0

500

1000

1500

2000

2500

0 10 20 30 40

(200

) p

eak

area

(a.

u.)

Time (d)

HNO3

H2SO4

Ox 50

Ox 100

Ox 200

Fig. 3. Changes in the area of the (200) reflection of nictric, sulfuric and oxalic acidsleached chrysotile at different reaction times.

136 M. Rozalen, F.J. Huertas / Chemical Geology 352 (2013) 134–142

diluting 1 mg of sample in 100 mg of dried KBr. The pellets wereheated overnight at 110 °C before analysis. Grams/32 (ThermoFisher Scientific Inc., 2011) program was used to plot and analyzethe spectra. In the spectra the absorbance was normalized againstthe Si\O stretching vibration band at 1080 cm−1 in order to comparechanges in the intensity of the bands.

2.3. Saturation calculations

Ideally, the chrysotile used in this study will be dissolved accordingto the following hydrolysis reaction:

Ca0:02 Al0:06Fe0:10 Mg2:75ð Þ Si2ð ÞO5 OHð Þ4 þ 6:02Hþ→→0:02Ca2þ þ 2:75 Mg2þ þ 0:1Fe3þ þ 2SiO2 þ 0:06Al3þ þ 5:01H2O:

ð1Þ

The equilibrium constant for this reaction was estimated from theequations and parameters reported by Vieillard (2000) to be logKeq = 25.29.

The saturation state of the solution with respect to the solidphases can be calculated in terms of the free energy of the reaction,ΔGr:

ΔGr ¼ RT lnIAPKeq

!ð2Þ

where R is the gas constant, T the absolute temperature and IAP andKeq, respectively, stand for the ion activity product and the equilibri-um constant for the dissolution reaction. Aqueous activities andchemical affinities have been determined using the EQ3NR geochem-ical code (Wolery, 1992). The IAP was calculated from the pH, Si andMg concentrations measured in every batch experiment. Ca, Fe and Alconcentrations were estimated from the Si values (stoichiometric re-lease). The influence of variations of minor cations (Al, Fe and Ca)concentrations on ΔGr was tested in EQ3NR using stoichiometric con-centrations as well as values 10 times lower and higher than stoichio-metric values. Changing the concentration of minor cations by twoorders of magnitude does not affect ΔGr in the second decimal figure.The above program was also used to model the capacity of oxalate toform aqueous Mg species. The aqueous Mg–oxalate complexes areimplemented in the Lawrence Livermore National Laboratory ther-modynamic database (data 0.cmp in EQ3/6 package) using the reac-tions of aqueous Mg and oxalate at 25 °C (Sillen and Martell, 1971;Wolery, 1992) (Table 1).

3. Results and discussion

3.1. Nitric acid series

The changes in color and morphology of the sample, relative to thestarting material, were observed as a function of time. Initially, thechrysotile fibers are flexible and lie parallel, giving columnar growth,strong and easily separable by hand pressure. The fibers have a silkyluster, and a medium-gray color. After treatment with HNO3, thesolid samples turned a lighter color with time.

Fig. 2 shows the XRD patterns for HNO3 leached chrysotile as afunction of time. The characteristic reflections of the initial sampleare observed in all cases. No new peaks or significant changes in thechrysotile patterns are observed after the acid treatment. However,an increase of the base line intensity and attenuation of the intensitiesof the chrysotile-related lines as time progresses suggest a process ofdissolution.

In order to evaluate the progress of the dissolution process, thearea of the (200) reflection (7.3 Å) is plotted versus elapsed time(Fig. 3). These data can be fitted with a straight line showing that

the area measured at 30 days has decreased by a factor of 4 with re-spect to the area measured after the first day of the experiment.

The results obtained by XRD are also supported by FTIR studies,since the major chemical groups in chrysotile were also identifiedin the spectra of the HNO3 leached samples (Fig. 4). In theOH-stretching region (Fig. 4a), from 4000 to 3200 cm−1, a decreasein the intensity of the bands confirm the process of dissolution.Moreover, the increase of the band at 3428 cm−1 is associated withwater physisorbed on to Si\OH groups (Suquet, 1989). For the first9 days of reaction the lattice region (1500–500 cm−1) of the chrys-otile spectrum (Fig. 4b), showed no significant changes but the in-tensity of a little shoulder at 1250 cm−1 starts to increase. Thisshoulder was observed by Wypych et al. (2005) after heating chrys-otile at 100 °C and corresponds to the Si\O\Si stretching vibrationfound in the two adjacent inverted SiO4 tetrahedra. Moreover anew weak band appears at 802 cm−1, associated with the distorted

3690 cm-1

3428 cm-1

3645 cm-1

Wavenumber (cm-1)

Ab

sorb

ance

32003400360038004000 12001400 6008001000 450

a

b

c

d

e

f

b

c

d

e

f

a

604

558

802

962

1080

1250

Fig. 4. FTIR spectra of: a) initial chrysotile and samples leached with HNO3 after: b) 1 day; c) 2 days; d) 9 days; e) 19 days and f) 30 days of reaction.

137M. Rozalen, F.J. Huertas / Chemical Geology 352 (2013) 134–142

ring structure of the tetrahedral SiO4 (Depege et al., 1996; Costa etal., 1997). After 19 days, changes to the initial chrysotile spectrumare clear. The band at 960 cm−1 decreases significantly togetherwith increase of the shoulder at 1250 cm−1. Subsequent changes inthe spectra after 30 days, including the shoulder at 1250 cm−1 to-gether with the bands at 960 and 802 cm−1, suggest the beginningof the transformation of chrysotile into an amorphous material.

In solution, chrysotile is dissolved in acid media according to theprevious hydrolysis reaction. The measured Mg/Si ratio (Table 2) showsthat the dissolution is highly non-stoichiometric and the magnesiumsheet of chrysotile dissolvesmore than 10 times faster than silica. Dissolu-tion occurs under far from equilibrium conditions, as demonstrated bythe ΔGr values ranging from −52.45 and −40.99 kcal mol−1. Further-more, the solution is undersaturated with respect to the Mg phasesbut close to saturation for the Si phases. In particular, for amorphoussilica, ΔGr reaches−0.21 kcal mol−1, which suggests that the amor-phous silica identified in the leached samples could be a byproduct ofincongruent dissolution as well as a precipitate of silica, as suggestedby the FTIR results.

Consequently, chrysotile dissolution in the presence of nitric acidproceeds via a series of steps involving Si and Mg. The hydrolysis re-action of Mg\O\Si bonds proceeds much faster than the Si\O\Sihydrolysis reaction. The electrophilic attack of a hydronium ion onthe bridge oxygen of the Si\O\Mg group produces Si\OH and a hy-drated Mg2+ species. The surface of the mineral becomes depleted ofMg, leaving behind a leached layer of silica, which is the rate-limiting

Table 2Experimental conditions and measured concentrations of released cations to the solu-tions for HNO3 series.

Name Time (days) Input pH Output pH Si (ppm) Mg (ppm) Mg/Si

Crx_HNO3_1 1 1.17 – – – –

Crx_HNO3_2 2 1.18 1.08 4.1 4.8 1Crx_HNO3_3 5 1.17 1.05 5.9 92 18Crx_HNO3_4 9 1.15 1.07 15.7 127 9Crx_HNO3_5 19 1.16 1.18 29.4 320 13Crx_HNO3_6 30 1.15 1.26 38.4 406 12

step, as generally accepted for phyllosilicates (Hume and Rimstidt,1992; Brantley and Chen, 1995; Lasaga, 1995; Nagy, 1995; Jurinskiand Rimstidt, 2001; Bibi et al., 2011).

3.2. Sulfuric acid series

In the case of the sulfuric acid series of experiments (Crx_H2SO4),changes in color and morphology are visible after the experiments.The initial gray color of the chrysotile becomes lighter with reactiontime and is completely bleached after 20–30 days of reaction. TheXRD results show the characteristic sharp reflections of the initialsample for samples reacted for less than 19 days (Fig. 5). Neverthe-less, the crystallinity of the sample is reduced. After 30 days of reac-tion, major structural changes are clear and only a broad humpcentered at approximately 3.5 Å confirms that the byproduct canbe identified as amorphous silica (Fig. 5g). Other authors have alsoidentified this amorphous material by XRD but using very aggressiveconditions. Wypych et al. (2005) heated chrysotile at 100 °C for15 min. Wang et al. (2006) reacted crushed chrysotile from Chinain 3 mol L−1 HCl at 100 °C for 1.5 h and obtained a light white mate-rial with a XRD pattern similar to ours (Fig. 5g) corresponding to anano-fibriform silica. Suquet (1989) also found a non-crystallinema-terial after grinding and leaching chrysotile with HCl 6 mol L−1 at90 °C. Our results confirm that an amorphous siliceous material isobtained after room temperature treatment of chrysotile withH2SO4 for 30 days.

Visible changes are also observed by FTIR. In the OH-stretchingregion (4000–3200 cm−1), the characteristic bands at 3642 and3686 cm−1, attributed to the inner and outer vibrational modes ofthe chrysotile MgO\H stretching (Viti and Mellini, 1997) disappearcompletely after 30 days reaction time (Fig. 6f). Moreover, the in-crease of the band intensity at 3248 cm−1confirms that the siliceousbyproduct is hydrated. In the lattice region (1500–500 cm−1),changes are observed only after 5 days reaction, as observed withnitric acid. After 9 days of reaction, a new Si\OH stretching peak at802 cm−1 (Fig. 6d) indicates that the O\H bond length is elongatedas a result of hydrogen bonding and the distance between H+ andthe active oxygen in the Si\O tetrahedron is asymmetric (Wen,

Position [°2Theta] (Copper (Cu))

10 20 30 40 50 60

0

0

1600

0

1600

0

1600

0

900

0

400

0

400

5000

Inte

nsi

ty

0

b

a

g

f

e

d

c

Crx

CrxCrxCrxCrx

Crx

Crx

CrxCrx

SiO2 (am)

Fig. 5. XRD patterns of: a) initial chrysotile and sulfuric acid leached samples afterb) 1 day; c) 2 days; d) 5 days; e) 9 days; f) 19 days; and g) 30 days of reaction.

138 M. Rozalen, F.J. Huertas / Chemical Geology 352 (2013) 134–142

1988). The shift of the band at 1080 cm−1 to 1086 cm−1, togetherwith the shoulder at 1215 cm−1, are associated with the vibrationsof n(Si\O\Si) in amorphous silica.

After 30 days reaction, the intensity of the band at 952 cm−1 pro-gressively decreases (Fig. 6). This band is assigned to the terminalSi\O\Mg deformation (Fonseca et al., 2001) and confirms the deple-tion ofMg in the chrysotile structure. This is supported by the extinctionof bands at 604 and 558 cm−1, corresponding to the vibrations ofMg\O (out of plane bending mode of Mg-octahedra) and δ(OH). The

3690 cm-1

3428 cm-1

3645 cm-1

Wavenum

Ab

sorb

ance

32003400360038004000

a

b

c d

e

f

Fig. 6. FTIR spectra of: a) initial chrysotile and samples leached with sulfuric a

solution analysis, with measured pH and concentrations of silica andmagnesium, is reported in Table 3. No significant changes in pH are ob-served in any of the experiments. As can be observed in Fig. 7a, silica israpidly released into solution within the first 19 days, at a rate approx-imately proportional to the elapsed time. With longer reaction times,the Si release tends to stabilize as the solution becomesmore saturated.Comparing with the HNO3 leaching experiments, the silica concentra-tion is 1.4 times higherwhen chrysotile is leachedwithH2SO4. As occurswith nitric acid, the reaction is highly non-stoichiometric and Mg2+ isreleased to the solution preferentially, being up to than one order ofmagnitude higher than Si after 30 days reaction. Comparing the releaseof Mg2+ in the presence of HNO3 or H2SO4, the stronger effect of thesecond acid is clear (Fig. 7b) since the amount of magnesium dissolvedafter 30 days is 1.3 times higher for sulfuric acid compared with nitricacid.

The presence of sulfate anions in the system may scavenge Mg2+

from solution, promoting the dissolution of the brucite sheet and en-hancing the effect of sulfuric acid compared to nitric acid. Sugama etal. (1998) reported the precipitation of kieserite (MgSO4·H2O) in asolution of H2SO4 and HF acids. However this phase has not beendetected either by XRD or FTIR in our samples. Alternatively, an aque-ous MgSO4 complex may form. A speciation test using the programVisual-MINTEQ (Gustafsson, 2010) shows that at a pH of 1.3 around30% of magnesium is present as MgSO4(aq). In addition, the fact thatSi and Mg are released around 1.4 times faster in the presence ofH2SO4 compared with HNO3 gives experimental evidence of the pro-moted dissolution of a chrysotile by SO4

2-ions, being capable ofcompletely dissolving the brucitic sheet and transforming the initialchrysotile into an amorphous siliceous material. Moreover, the for-mation of aqueous sulfate complexes of Al and Fe can also contributecatalyze the dissolution of chrysotile structure, as observed byFlaathen et al. (2010) for basaltic glass.

3.3. Oxalic acid series

The effect of oxalic acid leaching on the chrysotile was studied atthe same pH as used in the experiments with the inorganic acids.

ber (cm-1)

12001400 6008001000 450

b

c

d

e

f

a

604

558

962

1080

1250802

cid after b) 1 day; c) 2 days; d) 9 days; e) 19 days; f) 30 days of reaction.

Table 3Experimental conditions and measured concentrations of released cations to the solu-tions for H2SO4 series.

Name Time (days) Input pH Output pH Si (ppm) Mg (ppm) Mg/Si

Crx_H2SO4_1 1 1.34 – – – –

Crx_H2SO4_2 2 1.34 1.32 9.68 8.8 1Crx_H2SO4_3 5 1.33 1.28 16.6 167 12Crx_H2SO4_4 9 1.33 1.32 39.2 312 9Crx_H2SO4_5 19 1.33 1.48 53.0 483 11Crx_H2SO4_6 30 1.33 1.53 52.6 540 12

Position [°2Theta] (Copper (Cu)) 10 20 30 40 50 60

0

0

1600

0

900

0

1600

0

3600

0

10000

0

10000

5000

Inte

nsi

ty

0

b

a

e

d

c

g

f

Crx

CrxCrxCrxCrx

Crx

Crx

Gl

GlGlGl

GlGl

Fig. 8. XRD patterns of: a) initial chrysotile and samples leached with oxalic acid (seriesCtl_Ox50) after b) 1 day; c) 2 days; d) 5 days; e) 9 days; f) 19 days; and g) 30 days ofreaction.

139M. Rozalen, F.J. Huertas / Chemical Geology 352 (2013) 134–142

The effect of oxalate concentration (50, 100 and 200 mmol L−1) isalso assessed. Morphological changes are clearly visible after only5 days. The samples evolve to a light-gray color and turn towhite-yellow after 10 days reaction for the series Cr_Ox50 andCr_Ox100. In the case of the Cr_Ox200 series, discoloration is visibleafter only 2 days.

The XRD results show, for the lowest oxalate concentration(Crx_Ox50), the appearance of new peaks after 5 days of reaction.The analysis of the diffraction pattern confirms the formation ofglushinskite (Mg2C2O4·2H2O) (Fig. 8d), often found on the weath-ered surface of lichen-encrusted serpentinites (e.g., Wilson andBayliss, 1987), where it is formed by reaction between Mg-bearingminerals and oxalic acid secreted by the lichens. After 9 days(Fig. 8e), the intensity of the chrysotile peaks decreases significantlyand after 19 days the chrysotile reflections are entirely absent,suggesting the complete depletion of magnesium from the bruciticsheet. By increasing the concentration of oxalic acid from 50 to200 mmol L−1, the glushinskite precipitates faster, even within2 days reaction time. The chrysotile peaks disappear completelyafter 9 days. However, the fate of the silica is unsolved. The use ofFTIR spectroscopy can help us to clarify this question.

In the OH stretching region, a progressive change in the spectra isobserved (Fig. 9). The band associated with water vibration increases

a

c

Fig. 7. Evolution of the a) Si and b) Mg concentration released to the solution versus time

and also shifts from 3428 cm−1 (corresponding to chrysotile) to3379 cm−1 (characteristic of glushinskite and visible only after 5 daysreaction). Moreover, the main chrysotile bands disappear from this fre-quency range (3690–3645 cm−1) after 9 days reaction. In the lattice re-gion (1500–500 cm−1), the spectra confirm the formation ofglushinskite after only 5 days for the series Cr_Ox50 (Fig. 9d). After

d

b

for nitric and sulfuric acid series. Evolution of c) Si and d) Mg for oxalic acid series.

3690 cm-1

3428 cm-1

3645 cm-1

Wavenumber (cm )

Ab

sorb

ance

3200

-1

3400360038004000 12001400 6008001000 450

a

b

c

e

f

g

b

c d

e

g

a

f

d

604558

962

1080

1664

1641

1373

1326

802

830

693

3379

Fig. 9. FTIR spectra of: a) initial chrysotile and samples leached with oxalic acid (series Ctl_Ox50) after b) 1 day; c) 2 days; d) 5 days; e) 9 days; f) 19 days; and g) 30 days of reaction.

Table 4Experimental conditions and measured concentrations of released cations to the solu-tions for the three series of oxalic acid leaching experiments.

Name Time(days)

InputpH

OutputpH

Si(ppm)

Mg(ppm)

Mg/Si Oxalate(mmol L−1)

Crx_Ox50_1 1 1.59 – – – – –

Crx_Ox50_2 2 1.58 1.60 5.62 20.1 4 38.2Crx_Ox50_3 5 1.58 1.66 10.5 199 22 36.8Crx_Ox50_4 9 1.58 1.79 16.1 250 18 38.8Crx_Ox50_5 19 1.59 1.96 22.9 262 13 30.9Crx_Ox50_6 30 1.6 2.03 28.9 259 10 31.4Crx_Ox100_1 2 1.38 1.32 1.07 18.2 20 89Crx_Ox100_2 5 1.37 1.36 5.87 237 47 88Crx_Ox100_3 6 1.38 1.38 11.8 239 23 88Crx_Ox100_4 9 1.37 1.40 17.3 442 30 83Crx_Ox100_5 19 1.37 1.48 26.4 384 17 82Crx_Ox100_6 30 1.37 1.50 30.7 305 11 81Crx_Ox200_1 2 1.19 1.11 0.06 14.4 261 191Crx_Ox200_2 5 1.19 1.10 2.69 361 155 188Crx_Ox200_3 6 1.19 1.13 14.7 386 30 190Crx_Ox200_4 9 1.20 1.12 18.2 376 24 181Crx_Ox200_5 19 1.18 1.17 28.7 310 12 183Crx_Ox200_6 30 1.18 1.19 32.9 502 18 186

140 M. Rozalen, F.J. Huertas / Chemical Geology 352 (2013) 134–142

9 days, the presence of a doublet structure at 1664/1641 cm−1 isassigned to the antisymmetric stretching vibration of the oxalategroup of glushinskite (Monje and Baran, 2005). The correspondingsymmetric stretching vibrations of the oxalate group appear as anotherdoublet at 1373/1326 cm−1.

Besides the formation of glushinskite, an amorphization of thesample, due to the depletion of magnesium from the chrysotile struc-ture. This is confirmed by the shifting of the band at 962 cm−1 to950 cm−1 and the appearance of a new band at 802 cm−1 associatedwith amorphous silica and visible only after 9 days reaction. Com-plete dissolution of the brucitic sheet is observed after 30 days be-cause the bands associated with δ(OH) and Mg\O vibrations (603and 558 cm−1) disappear.

By increasing the concentration of oxalic acid (series Cr_Ox100and Cr_Ox200), the process of transformation is faster. Glushinskiteis identified by FTIR after 5 days for the series Cr_Ox100 and afteronly 2 days for the series Cr_Ox200. With 200 mmol L−1 oxalicacid, the FTIR results confirm the complete dissolution of the bruciticsheet (disappearance of bands associated with Mg\O bonds at700–500 cm−1; Fig. 9e–g) after 9 days. In conclusion, the FTIR aswell as XRD results suggest that oxalic acid liberates Mg2+ from thechrysotile structure and enhances the dissolution by precipitation ofglushinskite.

Finally, the concentrations of dissolved magnesium and silica,and the measured pH are reported in Table 4. The values of pH arevery stable during the experiment for Cr_Ox100 and Cr_Ox200 se-ries. However a drift from 1.6 to 2.03, after 20 days reaction, is ob-served for the lowest oxalate concentration (series Cr_Ox50). Asshown in Fig. 7, the extent of silica released into solution increaseswith elapsed time for the series Cr_Ox50. As the oxalic acid concen-tration is increased, from 100 to 200 mmol L−1, the amount of silicareleased into solution is slightly higher, since the dissolution processis more intense. After 30 days, the amount of silica released for theseries Cr_Ox200 is 1.14 times higher than for Cr_Ox50. The mea-sured Mg/Si ratios (Table 4) show a non-stoichiometric dissolution,with a preferential release of Mg to the solution, compared withsilica.

3.4. Comparing the effect of nitric, sulfuric and oxalic acids

The results of this study allow us to conclude that in acid solutionsthe proton attack on the chrysotile surface causes hydrolysis of theouter Mg(OH)2 sheets. The depletion of magnesium is more intense,depending on the strength of aqueous Mg2+ complexes or solidphases formed with the corresponding anion of the acid. Consequent-ly, for mineral acids, the attack using H2SO4 solutions is more effectivethan HNO3 solutions, due to the formation of the aqueous MgSO4

complex. After 30 days at room temperature, chrysotile is completelydissolved in sulfuric acid, but only partially in nitric acid. Teir et al.(2007) performed acid dissolution experiments in very aggressiveacidic conditions (1–4 mol L−1, 20–70 °C) for short time (1–2 h),

141M. Rozalen, F.J. Huertas / Chemical Geology 352 (2013) 134–142

that lead to a rapid release of Mg and Si, in concentrations muchhigher than those obtained in this study. However, the trend of theacid effectiveness is consistent with ours. For example, they foundthat sulfuric acid yielded the highest magnesium concentration,followed by hydrochloric, nitric, formic and acetic acid. For nitricacid, the transformation into an amorphous by-product at room tem-perature is also possible, as described by Rozalen et al. (2012), after70 days reaction at pH 1 in HNO3 solution.

In oxalic acid solutions, a new phase, glushinskite, is formed.The formation of this solid phase is more efficient than the forma-tion of soluble complexes (as in case of sulfuric acid) for removal ofMg2+ from the chrysotile structure. Using a concentration of200 mmol L−1, the destruction of chrysotile is complete afteronly 9 days. The products of the reaction are amorphous silicaand magnesium oxalate (glushinskite). In nature, the formation ofglushinskite, as a mineral, has been attributed to the reaction ofoxalic acid produced by the fungal mycobiont of the lichenLecanora altra growing on a Mg-rich substrate of serpentinites(Wilson et al., 1980; Wilson and Bayliss, 1987). The use of lichensable to excrete oxalic acid could help to develop remediation strat-egies for contaminated sites.

Other organic ligands, as citrate or salicylate, have shown to beeffective in catalyzing dissolution of others silicates (e.g. Ramos etal., 2011). Their aqueous solutions should be tested as suitablesolvents to be used for extracting magnesium in serpentinites andasbestos mine-tailings, and subsequent CO2 sequestration by pre-cipitation of magnesium carbonates. Moreover this process willdestroy the morphology of chrysotile helping in remediation pro-cesses concerning natural outcrops or abandoned mines. Morestudies are necessary to investigate the resulting siliceous materialand its resistance and confirm its harmlessness by in vitro and vivotests.

4. Conclusions

In the present study, the effect of nitric, sulfuric and oxalicacids on the degradation of chrysotile has been investigated at25 °C and pH 1, combining XRD and FTIR techniques and solutionanalysis. The results from these experiments support the followingconclusions:

1) Nitric acid partially dissolves chrysotile after 30 days. XRD resultsconfirm the degradation of chrysotile, as a lower crystallinity isobserved. Proton attack on the chrysotile surface leads to a partialdepletion of magnesium in the brucitic sheets and the formationof an amorphous silica precipitate (confirmed by FTIR).

2) Sulfuric acid is able to dissolve the entire brucitic layer,transforming the initial chrysotile into an amorphous siliceousby-product after 30 days reaction, as confirmed by XRD and FTIRresults. The presence of sulfate anions in the systemmay scavengeMg2+ from the solutions, forming an aqueous complex, MgSO4,which promotes dissolution of the brucite sheet and enhancesthe effect of sulfuric acid compared to nitric acid.

3) Oxalic acid is able to dissolve the entire brucitic layer faster thansulfuric acid. Moreover increasing the concentration from 50 to200 mmol L−1, the transformation process is completed faster(reducing the time from 19 days to 9 days). The precipitation ofglushinskite (MgC2O4·2H2O) was identified by XRD and FTIR.The precipitation of this solid phase is more efficient than the for-mation of soluble complexes (as in case of sulfuric acid) for the re-moval of Mg2+ from the chrysotile structure. Consistently, theformation of a siliceous by-product and the destruction of the ini-tial asbestos containing material, is also faster.

4) The relative effectiveness of the destruction of the chrysotile is:oxalic > sulfuric > nitric acids. This sequence is associated with

the capacity of the anion to sequester Mg2+ ions by formation ofsoluble complexes or precipitates.

Acknowledgements

Financial support was obtained from project P07-RNM-02772(Junta de Andalucía); MR benefited from a JAE-Doc contract fromCSIC with contribution of FEDER funds. The authors thank CarlosJové Colón for the helpful advice to include oxalate complexes inthe LLNL database for EQ3NR and Eduardo Flores for laboratorywork. FJH, amdg.

References

Allen, M.P., Smith, R.W., 1975. Dissolution of asbestos minerals in acid and buffered saltsolutions. 3rd conf. Intern. Phys. Chem. Mineral. Asbestos. Univ. Laval, Quebec(Sect. 4–16).

Bibi, I., Singh, B., Silvester, E., 2011. Dissolution of illite in saline-acidic solutions at25 °C. Geochimica et Cosmochimica Acta 75, 3237–3249.

Brantley, S.L., Chen, Y., 1995. Chemical weathering rates of pyroxenes and amphiboles.In: White, A.F., Brantley, S.L. (Eds.), Chemical Weathering Rates of Silicate Minerals,vol. 31. Mineralogical Society of America, pp. 119–172.

Costa, T.M.H., Gallas, M.R., Benvenutti, E.V., Da Jornada, J.A.H., 1997. Infrared andthermogravimetric study of high pressure consolidation in alkoxide silica gel powders.Journal of Non-Crystalline Solids 220, 195–201.

Cozak, D., Barbeau, C., Gauvin, F., Barry, J.P., DeBlois, C., DeWolf, R., Kimmerle, F., 1983.The reaction of chrysotile asbestos with titanium(III) chloride. Characterization ofthe reaction products. Canadian Journal of Chemistry 61, 2753–2760.

Depege, C.C., El Metoui, F.C., Forano, C., De Roy, A., Dupuis, J., Besse, J.P., 1996. Polymeri-zation of silicates in layered double hydroxides. Chemistry of Materials 8, 952–960.

Favero-Longo, S.E., Turci, F., Tomatis, M., Castelli, D., Bonfante, P., Hochella, M.F.,Piervittori, R., Fubini, B., 2005. Chrysotile asbestos is progressively converted intoa non-fibrous amorphous material by the chelating action of lichen metabolites.Journal of Environmental Monitoring 7 (8), 764–766.

Flaathen, T.K., Gislason, S.R., Oelkers, E.H., 2010. The effect of aqueous sulphate on ba-saltic glass dissolution rates. Chemical Geology 277, 345–354.

Fonseca, M.G., Oliveira, A.S., Airoldi, C., 2001. Silylating agents grafted onto silica derivedfrom leached chrysotile. Journal of Colloid and Interface Science 240, 533–538.

Fripiat, J.J., Mendelovici, E., 1968. Derives organiques des silicates. L. Le derive methyledu chrysotile. Bulletin de la Société Chimique de France 2, 483–492.

Goni, J., Thomassin, J.H., Jaurand, M.C., Touray, J.C., 1979. Photoelectron spectroscopyanalysis of asbestos dissolution in acidic media of biological interest. Physics andChemistry of the Earth 11, 807–817.

Grasshoff, K., Ehrhardt, M., Kremling, K., 1983. Methods of Seawater Analysis, Seconded. Verlag Chemie.

Gualtieri, A.F., Lassinantti Gualtieri, M., Tonelli, M., 2008. In situ ESEM study of the ther-mal decomposition of chrysotile asbestos in view of safe recycling of the transforma-tion product. Journal of Hazardous Materials 156, 260–266.

Gustafsson, J.P., 2010. Visual MINTEQ 3.0 for Windows XP/7.Hargreaves, A., Taylor, W.H., 1946. An X-ray examination of decomposition products of

chrysotile (asbestos) and serpentine. Mineralogical Magazine 27, 204–216.Hume, L.A., Rimstidt, J.D., 1992. The biodurability of chrysotile asbestos. American Min-

eralogist 77, 1125–1128.Johan, Z., Goni, J., Sarcia, C., Bonnaud, G., Bignon, J., 1974. Influnce de certains acides

organiques sur la stabilite du chrysotile. Advances in Organic Geochemistry. Tehcnip,Paris, pp. 883–903.

Jurinski, J.B., Rimstidt, J.D., 2001. Biodurability of talc. AmericanMineralogist 86, 392–399.Keane, M., Stephens, J., Zhong, B., Miller, W., Ong, T., Wallace, W., 1999. A study of the

effect of chrysotile fiber surface composition on genotoxicity in vitro. Journal ofToxicology and Environmental Health. Part A 57, 529–541.

Lasaga, A.C., 1995. Fundamental approaches to describing mineral dissolution and pre-cipitation rates. In: White, A.F., Brantley, S.L. (Eds.), Chemical Weathering Rates ofSilicate Minerals. , 31. Mineralogical Society of America, pp. 23–86.

Liu, K., Feng, Q., Yang, Y., Zhang, G., Ou, L., Lu, Y., 2007. Preparation and characterizationof amorphous silica nanowires from natural chrysotile. Journal of Non-CrystallineSolids 1534–1539.

Mirick, W., 1991. Method for treating asbestos. US Patent 5,041,277.Mirick, W., Forrister, W. B., 1993. Method for treating asbestos. U.S. Patent 5, 264,655.Monje, P., Baran, E., 2005. Evidence of formation of glushinskite as a biomineral in a

Cactaceae species. Phytochemistry 66, 611–614.Nagy, K.L., 1995. Dissolution and precipitation kinetics of sheet silicates. In: White, A.F.,

Brantley, S.L. (Eds.), Chemical Weathering Rates of Silicate Minerals. , 31. MineralogicalSociety of America, pp. 173–233.

Nagy, B., Bates, T.F., 1952. Stability of chrysotile asbestos. American Mineralogist 37,1055–1058.

O'Connor, W.K., Dahlin, D.C., Nilsen, D.N., Gerdemann, S.J., Rush, G.E., Penner, L.R.,Walters, R.P., Turner, P.C., 2002. Continuing studies on direct aqueous mineral car-bonation for CO2 sequestration. The Proceedings of the 27th International Techni-cal Conference on Coal Utilization & Fuel Systems, pp. 819–830.

Pacco, F., Van Grangh, L., Fripiat, J.J., 1976. Etude par spectroscopie infrarouge et reso-nance magnetiqnetique nuclearire de la distribution homogene des groupessilanols d'un gel silice fibreux. Bulletin de la Société de France 7–8, 1022–1026.

142 M. Rozalen, F.J. Huertas / Chemical Geology 352 (2013) 134–142

Papirer, E., Dovergne, G., Leroy, P., 1976. Modifications physico-chimiques du chrysotilepar attaque chimiqirer, menagee. Bulletin de la Société Chimique de France 5–6,651–657.

Park, A.-H., Fan, L.-S., 2004. CO2 mineral sequestration: physically activated dissolutionof serpentine and pH swing process. Chemical Engineering Science 59, 5241–5247.

Park, A.-H., Jadhav, R., Fan, L.-S., 2003. CO2 mineral sequestration: chemically enhancedaqueous carbonation of serpentine. Canadian Journal of Chemical Engineering 81(3–4), 885–890.

Prapaipong, P., Shock, E.L., Koretsky, C.M., 1999. Metal-organic complexes in geochem-ical processes: temperature dependence of the standard thermodynamic proper-ties of aqueous complexes between metal cations and dicarboxylate ligands.Geochimica et Cosmochimica Acta 63, 2547–2577.

Ramos, M.E., Cappelli, C., Rozalen, M., Fiore, S., Huertas, F.J., 2011. Effect of lactate, gly-cine, and citrate on the kinetics of montmorillonite dissolution. American Mineral-ogist 96, 768–780.

Rozalen, M., Ramos, M.E., Hernandez-Corbalan, M.D., Gervilla, F., Huertas, F.J., Oxalatepromoted dissolution of chrysotile. Macla 16, 2012,156–157.

Sillen, L.G., Martell, A.E., 1971. Stability constants of metal–ion complexes. Special Pub-lication No. 25.The Chemical Society, London.

Silva, S.P., Wander, A.P., Bisatto, R., Galland, G.B., 2011. Preparation and characteriza-tion of chrysotile for use as nanofiller in polyolefins. Nanotechnology 22, 6.

Sugama, T., Sabatini, R., Petrakis, L., 1998. Decomposition of chrysotile asbestos byfluorosulfonic acid. Industrial and Engineering Chemistry Research 37, 79–88.

Suquet, H., 1989. Effects of day grinding and leaching on the crystal structure of chrys-otile. Clays and Clay Minerals 37, 439–445.

Teir, S., Revitzer, H., Eloneva, S., Fogelholm, C., Zevenhoven, R., 2007. Dissolution of nat-ural serpentinite in mineral and organic acids. International Journal of Mineral Pro-cessing 83, 36–46.

Thermo Fisher Scientific Inc., 2011. GRAMS/32 software v.9.1. Waltham, MA (USA).Thomassin, J.H., Goni, J., Baillif, P., Touray, J.C., Jaurand, M.C., 1977. An XPS study of the

dissolution kinetics of chrysotile in 0.1 N oxalic acid at different temperatures.Physics and Chemistry of Minerals 1, 385–398.

Vieillard, P., 2000. A new method for the prediction of Gibbs free energies of formationof hydrated clay minerals based on the electronegativity scale. Clays and Clay Min-erals 48, 459–473.

Viti, C., Mellini, M., 1997. Contrasting chemical compositions in associated lizardite andchrysotile in veins from Elba, Italy. European Journal of Mineralogy 9, 585–596.

Wang, L., Lu, A., Wang, C., Zheng, X., Zhao, D., 2006. Nano-fibriform production of silicafrom natural chrysotile. Journal of Colloid and Interface Science 295, 436–439.

Wen, L., 1988. The Infrared Spectroscopy of Minerals. Chongqing University Press 89.Wilson, M.J., Bayliss, P., 1987. Mineral nomenclature: glushinskite. Mineralogical Mag-

azine 51, 327–328.Wilson, M.J., Jones, D., Russel, J.D., 1980. Glushinskite a naturally occurring magnesium

oxalate. Mineralogical Magazine 43, 837–840.Wolery, T.J., 1992. EQ3NR, a computer program for geochemical aqueous speciation-

solubility calculations: theoretical manual, user's guide and related documentation(version 7). UCRL-MA-110662 III.Laurence Livermore National Laboratory.

Wypych, F., Adad, L.B., Mattoso, N., Marangon, A.A.S., Schreiner, W.H., 2005. Synthesisand characterization of disordered layered silica obtained by selective leaching ofoctahedral sheets from chrysotile and phlogopite structures. Journal of Colloidand Interface Science 283, 107–112.