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Geochimica et Cosmochimica Acta 119 (2013) 149–163

Cu(II) incorporation to schwertmannite:Effect on stability and reactivity

under AMD conditions

Juan Antelo a,⇑, Sarah Fiol b, Dora Gondar b, Claudio Perez b, Rocıo Lopez b,Florencio Arce b

a Department of Soil Science and Agricultural Chemistry, University of Santiago de Compostela, 15782 Santiago de Compostela, Spainb Department of Physical Chemistry, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain

Received 26 November 2012; accepted in revised form 21 May 2013; available online 31 May 2013

Abstract

The formation, transformation and surface chemistry of iron oxides is geologically important in surface waters polluted byacid mine drainage (AMD). The geochemical behaviour of these oxides controls the availability and mobility of contaminantsin such highly polluted systems. The low pH values, together with the presence of large amounts of sulphate, favour the for-mation of schwertmannite, which is a metastable iron oxide that is transformed to goethite under oxic conditions. However, incopper mining environments, co-precipitation of iron and copper ions is expected and therefore the surface chemistry of theiron oxides present in these systems may be different.

Several schwertmannite samples were prepared in the presence of high concentrations of sulphate and different concentra-tions of copper to simulate copper-rich mining environments. Long-term transformation experiments were conducted to studythe mineral oxide stability and variations in surface chemistry produced over a period of 15 months. Schwertmannite-like par-ticles were initially formed, but sulphate release and the dissolution and re-precipitation of iron and copper ions occurredunder more acidic conditions throughout the experiment, which led to the formation of goethite-like particles. The capacityfor arsenate adsorption was highest in the initial schwertmannite-like particles, and arsenate mobility increased in the presenceof more stable iron mineral phases that were formed throughout the transformation experiment.� 2013 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

Mining activities are a major worldwide source of con-tamination in soils, sediments and aquatic systems. Amongother elements, arsenic is a common constituent of watersaffected by acid mine drainage (AMD) and can reach veryhigh concentrations that would produce severe contamina-tion of surface waters and groundwater (Smedley andKinniburgh, 2002). The high concentrations of arsenicfound in mining environments are mainly due to arsenopy-rite oxidation. The behaviour of trace elements in areas af-

0016-7037/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.gca.2013.05.029

⇑ Corresponding author.E-mail address: juan.antelo@usc.es (J. Antelo).

fected by AMD depends on various factors, such as oremineralogy and weathering conditions; however, the pre-cipitation of secondary minerals is particularly importantsince this leads to the incorporation of large amounts of ele-ments via either surface adsorption or co-precipitation(Martınez and McBride, 2001; Lee et al., 2002).

It is well known that the iron oxy(hydr)oxidfes schwert-mannite, jarosite and goethite, which are formed in AMDconditions, contribute to the removal of trace elements(Schroth and Parnell, 2005; Acero et al., 2006; Jonssonet al., 2006; Burgos et al., 2012) and of oxyanions such asarsenate, phosphate and chromate (Carlson et al., 2002;Antelo et al., 2005; Regenspurg and Peiffer, 2005; Moha-patra et al., 2006; Villalobos and Perez-Gallegos, 2008).Weathering of iron sulphide minerals leads to the formation

150 J. Antelo et al. / Geochimica et Cosmochimica Acta 119 (2013) 149–163

of large amounts of secondary iron precipitates when Fe(II)is oxidized to Fe(III), in a process that may be mediated bybacterial activity following exposure of the minerals to theatmosphere (Kawano and Tomita, 2001; Jonsson et al.,2005; Perez-Lopez et al., 2011).

Schwertmannite has only recently been identified in min-ing environments and has not yet been as well characterizedas some other iron minerals. The composition of this poorlycrystalline Fe(III)-oxyhydroxysulphate is variable and isrepresented by the ideal formula Fe8O8(OH)8�2x(SO4)x,where x ranges between 1 and 1.75. It is commonly foundin sulphate-rich environments, such as water streams andlakes affected by AMD, and in acid sulphate soil systems,in which it precipitates at between pH 3 and 4 (Bighamet al., 1990, 1996; Regenspurg et al., 2004; Acero et al.,2006; Burton et al., 2006, 2007; Collins et al., 2010). Be-cause of its metastable nature, schwertmannite can undergophase transformation to goethite or to other more crystal-line minerals within weeks or months, depending on thepH and oxidizing conditions of the medium (Bighamet al., 1996; Acero et al., 2006; Kumpulainen et al., 2008).Variations in the composition of the aqueous phase, mainlyin total Fe, SO4 and H+, have been reported to occur dur-ing the transformation process (Bigham et al., 1996; Jons-son et al., 2005; Acero et al., 2006).

The importance of schwertmannite lies in its ability toadsorb the metal ions present in AMD onto its surface.This contributes to lowering the aqueous concentration ofsuch ions (Jonsson et al., 2005) and eventually to the reten-tion of some of these trace metals during the transforma-tion of schwertmannite to other mineral phases (Aceroet al., 2006). It is a priori expected that the more crystallinephases will have a lower capacity to retain trace elementseither in their structure or on their surface. Therefore, itis important to be able to accurately predict the reactivityof schwertmannite to some target species and how this reac-tivity may be affected by the medium to long-term mineraltransformations that are likely to occur in such environ-ments. Several transformation studies have been developedto identify the mineral phase that determines the mobilityand fate of contaminants in different natural environments,mainly AMD and nearby surface waters (Bigham et al.,1996; Regenspurg et al., 2004; Jonsson et al., 2005). Factorssuch as pH, temperature and the presence of organic mat-ter, silica, Fe(II) and other ionic species contribute to mod-ifying the rate of transformation from schwertmannite tothe more crystalline forms (Bigham et al., 1996; Aceroet al., 2006; Knorr and Blodau, 2007; Kumpulainen et al.,2008; Jones et al., 2009). The most favourable conditionsfor the transformation lead to the appearance of goethitein less than 100 days, while much longer times (up to years)are required under the most unfavourable conditions.

Very few studies have investigated the incorporation oftrace elements into the secondary iron precipitates formedin AMD systems. Regenspurg and Peiffer (2005) studiedthe incorporation of oxyanions in schwertmannite by ana-lyzing synthetic samples that were prepared in solutionscontaining variable amounts of chromate and arsenate.These authors concluded that both anions stabilize theschwertmannite structure and therefore hinder its dissolu-

tion and transformation to goethite, as a consequence ofthe stronger binding affinity between arsenate/chromateand Fe(III) surface groups than that observed between sul-phate and Fe(III) surface groups.

The presence of metal impurities in natural iron oxides isnot unusual, since metal cations can co-precipitate withFe(III) ions during the formation of these iron oxidesand/or can be adsorbed on the mineral surface (Martınezand McBride, 2001; Zachara et al., 2001). The influencethat metal cations have on the formation and surface prop-erties of crystalline iron oxides such as goethite (FeOOH)and hematite (Fe2O3) is well-known. The substitution ofFe(III) by Al(III) is particularly important in the crystallinestructure of iron mineral oxides present in soil systems, as itaffects the adsorption of nutrients and contaminants as wellas the phase transformation of these mineral oxides (Dom-inik et al., 2002; Hansel et al., 2011; Bazilevskaya et al.,2012). Efforts have also been made to improve the adsorp-tion characteristics of iron oxides through the incorpora-tion of other metal ions in the crystalline structure(Zhang et al., 2003; Krehula and Music, 2007). Thus, par-tial substitution of Fe(III) by Cu(II), Ni(II) or Co(II) ingoethite leads to an increase in the surface area of ironoxide and an increase in arsenate adsorption (Mohapatraet al., 2006). So far, no information was found in the liter-ature on the effect that metal substitution has on schwert-mannite stability and reactivity.

The aim of the present study was to analyze the influ-ence that the incorporation of copper has on the stabilityand adsorption behaviour of schwertmannite. Copper is ametal that is often found in areas affected by AMD andits presence in aquatic systems could affect the flora andfauna. The inclusion of copper ions in the crystalline struc-ture during the precipitation of schwertmannite may havean important effect on the surface properties of this min-eral. In addition to the formation and transformation inAMD conditions of the different schwertmannite samples,the reactivity of the transformation products was also stud-ied. These reactivity experiments involved analysis of thearsenate adsorption in pure and Cu-doped schwertmannitesamples, as well as in the intermediate and final productsobtained during the transformation of several preparations.Finally, the main objective of the present study was to im-prove our understanding of the processes that control themobility of trace elements in AMD systems.

2. MATERIALS AND METHODS

2.1. Schwertmannite synthesis

Pure synthetic schwertmannite (Sch) was obtained bythe procedure described by Bigham et al. (1996). Briefly,10.8 g of FeCl3�6H2O (�40 mM Fe(III)) and 3 g of Na2SO4

(�10 mM SO4) were added to 2 L of preheated (60 �C) dis-tilled water. The orange/red precipitate thus obtained washeated again at 60 �C for 12 more minutes and then cooledat room temperature. The obtained precipitate was thenplaced in cellulose membranes and dialysed against dou-ble-distilled water for 33 days. The final product wasfreeze-dried to obtain a dry powder. The same procedure

J. Antelo et al. / Geochimica et Cosmochimica Acta 119 (2013) 149–163 151

was used to prepare three Cu-doped schwertmannite sam-ples, with different amounts of copper incorporated duringthe precipitation of Fe(III): 1 (SchCu1), 5 (SchCu5) and 10(SchCu10) mM Cu(NO3)2, which correspond to Cu:Fe ra-tios of approximately 0.03, 0.15 and 0.30, respectively.The lowest ratio compares to that usually found in AMD(Kimball et al., 2009; Asta et al., 2010) and the higher Cuconcentrations were used in order to ensure that the Cu:Feratio after coprecipitation remains in the range of mine sed-iments (Rodrıguez-Jorda et al., 2012).

Powder X-ray diffraction (XRD) measurements weremade with a Phillips PW1710 diffractometer between 10�and 70� 2h, with a step size of 0.02� and a counting time of6 s per step. Scanning electron microscopy (SEM) was per-formed with a Zeiss UltraPlus Analytical FESEM micro-scope. The composition of schwertmannite samples wasdetermined after digestion of 0.05 g of the oxide in 50 mLof 6 M HCl for 3 h. Fe and Cu contents were measured byinductively coupled plasma optical emission spectroscopy(ICP-OES, Perkin–Elmer Optima 3300DV) and the sulphatecontent by a turbidimetric method (Clesceri et al., 1998).

2.2. Transformation experiment

All schwertmannite preparations, with and without cop-per, were suspended in double-distilled CO2-free water to afinal concentration of 1 g L�1. Two subsamples of final vol-ume 2 L were separated from each original sample to per-form the transformation experiment at pH 3, which iscloser to the natural pH at which schwertmannite appearsin AMD environments, and at pH 5, in order to studythe influence of pH on the transformation process. Thetransformation experiment was conducted during a 15-month period in which the polycarbonate flasks with thesuspensions were continuously and homogeneously shakenin a thermostated room at 20 ± 2 �C and the temperaturewas periodically registered. The pH of the suspensionswas monitored daily (with a pre-calibrated radiometerGK2401C electrode connected to a Crison 2001 micro-pHmeter) and readjusted, where necessary, to the initialpH value by addition of 0.1 M HNO3 or KOH solutions.These additions of acid and base along with the release ofSO4, produced a variation of the ionic strength during thetransformation experiment which ranged between 6�10�4

and 9�10�3 M at pH 3. At pH 5 there was an initial releaseof sulphate to the solution that led to a higher ionic strengthvalue, 1�10�3 M, and at the end of the experiment it reacheda value of 3�10�3 M. During the first 2 months, aliquots(6 mL) of the suspensions were extracted every other day,then every 15 days between the 3rd and 6th months, andthen once a month from the 7th month to the end of theexperiment. The aliquots were then filtered through0.45 lm Millipore membrane filters and the concentrationsof Fe, Cu, and SO4 in the filtrate were measured by thesame methods used to characterise the schwertmannitepreparations (i.e. ICP-OES and turbidimetry).

The solid phase in the suspension was sampled after 45,94, 152, 297 and 410 days. At each time, 150 mL aliquotsof each suspension were carefully washed with distilled waterand centrifuged for 10 min at 6000 rpm, to remove the ionic

species present in solution, and then the solid was re-sus-pensed in 150 mL double-distilled water. The conductivityof the supernatant solution was measured to ensure thatthese ionic species were not present. After three washing cy-cles, the suspension was freeze-dried in order to obtain solidsamples for mineralogical analysis and reactivity studies. Todetermine the changes in the composition of the schwertman-nite preparations, the samples were digested with 6 M HCland analyzed by XRD and SEM, as previously indicated.

2.3. Adsorption experiments

The solid samples extracted at different times in the trans-formation experiments were used to study arsenate adsorp-tion. For this purpose, batch experiments were conducted, atpH 4.5, with 0.5 g L�1 suspensions in 20 mL of 0.1 MKNO3. Arsenate (KH2AsO4) was added to the suspensionsat four different concentrations (400, 800, 1200, and1600 lM), and the pH was adjusted to the desired value byaddition of 0.1 M HNO3 or KOH solutions. The pH was se-lected to allow the comparison of arsenate adsorption on thetransformation products with that reported for other ironoxides (Antelo et al. 2005, 2012). Also, this low pH is withinthe pH range that can be found in surface waters affected byAMD. Preliminary kinetic experiments with the initial andintermediate products showed that a contact time of 72 hwas required to achieve equilibrium, as previously reportedfor schwertmannite by Burton et al. (2009) and Antelo et al.(2012). Shorter equilibration times were sufficient for mostof the final products obtained in the transformation experi-ments. During the equilibration period, the samples werecontinuously shaken; the pH was periodically measuredand, where necessary, readjusted. Special care was takento avoid the presence of CO2, by maintaining the suspen-sions in N2 atmosphere. After the equilibration period, thesamples were filtered through 0.45 lm Millipore membranefilters, and a colorimetric method was used to determine theconcentration of arsenate in solution by UV–visible spec-troscopy (Lenoble et al., 2003). The amount of adsorbedarsenate was determined as the difference between the totalamount added and the final amount remaining in solution.Although the content of sulphate is minimal in most of thetransformation products analyzed, the concentration of sul-phate in solution was also measured after the adsorptionexperiments, by a turbidimetric method (Clesceri et al.,1998). The arsenate adsorption on the initial schwertman-nite preparations (with and without copper) was also ana-lyzed, as a control, following the same procedure.

All chemicals were of Merck p.a. quality and the waterused in the experiments was double-distilled and CO2 free.Polycarbonate flasks were used to avoid contamination ofthe samples with silicate, and the temperature was main-tained at 25 ± 1 �C in all adsorption experiments.

3. RESULTS AND DISCUSSION

3.1. Characterization of the schwertmannite samples

The nature of the products obtained during the synthesisof schwertmannite with varying concentrations of Cu(II)

152 J. Antelo et al. / Geochimica et Cosmochimica Acta 119 (2013) 149–163

was analyzed by XRD. The X-ray diffractograms (Fig. S1in Supplementary data) confirmed that in all cases the syn-thesized solid was schwertmannite, with the characteristicfive peaks of the oxyhydroxysulphate (d-spacing values of0.33, 0.25, 0.19, 0.17 and 0.15 nm). The addition of differentconcentrations of copper to the initial solution used for theFe(III) precipitation scarcely affected the XRD pattern ofthe preparations, since no clear differences were observedin the diffractograms. Moreover, no signs of other iron orcopper crystalline phases were found. Nevertheless, thephysical appearance of the schwertmannite samples variedbetween yellowish brown, in the absence of Cu(II), to red-dish brown, when Cu(II) was co-precipitated with Fe(III).

SEM micrographs of the four preparations (Sch;SchCu1; SchCu5; SchCu10) were also very similar and nosignificant differences were observed between the samplesobtained in the absence or presence of Cu(II). Analysis ofthe SEM images of the schwertmannite particles (Fig. S2in Supplementary data) revealed a very heterogeneouscloud-like morphology of variable size and rough poroussurface. There was no signal in the micrographs of the“pin-cushion” or “hedgehog” aggregates of schwertmannitetypically found in field studies (Acero et al., 2006; Burgoset al., 2012). Natural schwertmannites are normally formedafter oxidation of Fe(II) mediated by bacterial activity, andthe synthetic analogues used in the present study were ob-tained by precipitation of Fe(III) at high temperature, fol-lowed by an ageing process. Although the pin-cushionmorphology is observed in some synthetic samples((Regenspurg et al.,2004; Paikaray et al., 2011), it is not al-ways detectable, and higher resolution micrographs shouldbe collected for this purpose (Bigham and Nordstrom,2000; Jonsson et al., 2005).

As previously indicated, schwertmannite is described bythe general formula Fe8O8(OH)8�2x(SO4)x, with x rangingbetween 1 and 1.75. After complete digestion of the syn-thetic schwertmannite (Sch) the amounts of Fe(III) andSO4 measured in solution were 8.4 and 1.03 mmol g�1,respectively. This composition leads to the chemical for-mula Fe8O8(OH)5.94(SO4)1.03, showing that the syntheticpreparation is representative of a schwertmannite samplewith a relatively low sulphate content. This suggests thatthe sulphate is mainly present as inner-sphere complexesand that the presence of outer-sphere complexes is not rel-evant (Fernandez-Martinez et al., 2010).

The composition and chemical formula of the threeschwertmannite samples prepared in the presence of Cu(II)are shown in Table 1. The concentrations of sulphate mea-sured after complete digestion of the SchCu1, SchCu5 and

Table 1Chemical analysis of the schwertmannite preparations.

Sample Cu added (mmol/L) Composition of the solid

% Fe (w/w) % SO

Sch 0 46.97 9.90SchCu1 1 44.64 11.14SchCu5 5 46.87 11.81SchCu10 10 47.99 11.42

SchCu10 samples were 1.16, 1.23 and 1.19 mmol g�1,respectively. The sulphate content of all three samples iswithin the range found for schwertmannite. As expectedfrom previous studies carried out with other crystalline ironoxides, such as goethite and hematite, the Cu(II) ions arenot completely incorporated into the schwertmannite dur-ing the co-precipitation process under the experimentalconditions used in the present study. Although the ionic ra-dius of Cu(II) (r = 73 pm) is similar to that ofFe(III)(r = 64 pm) and is compatible with substitution ofthe octahedral sites present in iron oxides crystals, substitu-tion of Fe(III) by divalent cations is not as favourable assubstitution by trivalent cations. While one third of theFe(III) ions present in the crystalline structure of ironoxy(hydr)oxides can be substituted by Al(III) (0.33 molFe

molAl�1), the maximum substitution by Cu(II) is

0.05–0.06 molFe molCu�1 (Cornell and Schwertmann,

1996). Although the concentration of copper in the schwert-mannite samples SchCu1, SchCu5 and SchCu10 appearedrather low (0.024, 0.035 and 0.110 mmol g�1, respectively),it has been shown for otheriron oxides that the presence ofsmall amounts of divalent cations in the crystal structure(<1%) may have an important effect on the reactivity andother chemical properties of these oxides (Mohapatraet al., 2006; Mustafa et al., 2009, 2010).

3.2. Ageing experiment

3.2.1. Release of Fe and Cu

It is well known that schwertmannite is transformed to amore stable mineral, such as goethite, according to the fol-lowing reaction:

Fe8O8ðOHÞ8�2xðSO4ÞxðsÞ þ 2xH2O

! 8FeOOHðsÞ þ xSO4 þ 2xHþ ð1Þ

This ageing process may be accelerated or delayeddepending on the pH of the system and the presence ofother ions (in solution or in the mineral structure). In thepresent study, the stability of the four synthetic schwert-mannites, prepared in the absence and in the presence ofCu(II) ions, was analyzed at two different pH values, 3and 5. The amount of Fe(III) measured in solution duringthe transformation of pure and Cu-doped samples at pH3 varied, as shown in Fig. 1a. Maximum concentrationsof Fe(III) of �40–50 lmol g�1 were measured in solutionwithin the first 200 days of the experiment, and thereafterthe amount of Fe(III) decreased to a constant value of�10 lmol g�1. The amounts of Fe(III) in solution were

phase Chemical formula

4 (w/w) % Cu (w/w)

0 Fe8O8(OH)5.94(SO4)1.03

0.15 Fe8O8(OH)5.68(SO4)1.16Cu0.02

0.22 Fe8O8(OH)5.54(SO4)1.23Cu0.04

0.70 Fe8O8(OH)5.62(SO4)1.19Cu0.11

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 100 200 300 400 500

[Fe(

III)

] rel

ease

d (μ

mol

g-1

)

Time (days)

0

10

20

30

40

50

60

0 100 200 300 400 500

[Fe(

III)

] rel

ease

d (μ

mol

g-1

)

Time (days)

(a) (b)

Fig. 1. Iron released from pure and Cu-doped schwertmannite samples during the ageing process at (a) pH 3 and (b) pH 5. (}) Sch and (d)SchCu10.

J. Antelo et al. / Geochimica et Cosmochimica Acta 119 (2013) 149–163 153

always very small compared with the initial amount in theoriginal solids (�8 mmol g�1). Although monitoring Fe(III)does not provide direct information about the ageing andstability of schwertmannite, the transformation from thisoxyhydroxysulphate mineral to goethite supposedly in-volves dissolution of schwermannite accompanied by simul-taneous re-precipitation of Fe(III) ions in solution to formthe new mineral phase. The appearance of a maximum inthe amount of Fe(III) measured in solution may indicatethat the dissolution of schwertmannite occurs faster thanthe re-precipitation of goethite. On the other hand, theamount of Fe(III) released at pH 5 (Fig. 1b), althoughdetectable for most of the measurements, was below�0.2 lmol g�1. As also observed in Fig. 1, the presence ofCu(II) did not appear to have a clear effect on the rate ofdissolution or re-precipitation of Fe(III).

The concentrations of Fe(III) in solution after 200 daysare similar to those reported by Regenspurg et al. (2004),who found that Fe release was only significant at verylow pH. It appears that the transformation process investi-gated in the present study needs longer to reach equilibriumthan the system studied by Regenspurg et al. (2004). Sincethe same method of synthesis was used in both studiesand the experimental conditions in the ageing experimentwere similar, the same behaviour was expected. However,the differences may be due to the small differences in the

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0 100 200 300 400 500

[Cu(

II)]

rele

ased

(µm

ol g

-1)

Time (days)

0.0

0.1

0.2

0 100 200 300[Cu(

II)]

rele

ased

(µm

ol g

-1)

Time (days)

Fig. 2. Copper released from SchCu10 sample during the trans-formation experiment at pH 3 (d) and pH 5 (}). Inset: Detailedplot of data at pH 5.

amount of sulphate present in the samples, which may alterthe reactivity and stability of the mineral surface. The sam-ple prepared by Regenspurg et al. (2004) may representsamples with an intermediate content of sulphate(1.30 mmol g�1).

The concentration of Cu(II) released from the solid wasalso measured throughout the transformation experiment.The variation in Cu(II) in solution for the samples withthe highest initial concentration of copper, SchCu10, atboth pH values is shown in Fig. 2. The sample aged atpH 3 exhibited more variations than that aged at pH 5,although in both cases nearly all the Cu released had disap-peared from solution by the end of the experiment. Theseresults are consistent with those observed for the Fe(III)in Fig. 1, with a first stage in which dissolution is the mainprocess and a second stage in which re-precipitation be-comes more important. The amount of Cu released by sam-ple SchCu10 at pH 3 reached a maximum of �6.3% of theinitial amount within �40–45 days, and from this momentdecreased until almost no copper was detected in solution.The concentration of Cu(II) did not vary significantly in thesupernatant of samples aged at pH 5, which indicates thegreater stability of the solid under the less acidic conditions.At this pH, Cu(II) concentrations of more than0.15 lmol L�1 (close to the detection limit for this elementin the ICP-OES measurements) were not detected at anytime during the transformation period.

The information obtained from the ageing experiment isnot sufficient to clarify whether Cu(II) ions were incorpo-rated into the crystalline structure of the newly formed goe-thite or were adsorbed onto its surface. However, Ponthieuet al. (2006) showed that Cu adsorption onto goethite isnegligible at pH < 4, and that at pH 5 only 10% was ad-sorbed for a Cu/mineral ratio of 0.013 mmol g�1 and 20%for a ratio of 0.262 mmol g�1. Therefore, in the presentstudy, only incorporation of Cu(II) to the crystalline struc-ture may have occurred during the transformation process.Spectroscopic analysis by X-ray absorption spectroscopy orX-ray photoelectron spectroscopy of the intermediate andfinal products would help shed light on this aspect, sincethe use of both techniques would provide informationabout the mineral surface properties, the crystalline struc-ture, and the structural location and forms of cationsand/or anions.

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

0 100 200 300 400 500

[SO

4]re

leas

ed(m

mol

g-1

)

Time (days)

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

0 100 200 300 400 500

[SO

4]re

leas

ed (m

mol

g-1

)

Time (days)

(a) (b)

Fig. 3. Amount of sulphate released by pure and doped samples during ageing at (a) pH 3 and (b) pH 5. (}) Sch and (d) SchCu10. Thedashed and dotted lines correspond to the total amount of sulphate in the initial preparations Sch and SchCu10, respectively (1.03 and1.19 mmol g�1).

R² = 0.3984

R² = 0.9081

0

200

400

600

800

1000

1200

0 20 40 60

[SO

4]re

leas

ed(μ

M)

[Fe(III)]released (μM)

t = 190 d

t = 410 d

t = 164 dt = 2 d

Fig. 4. Correlation between SO4 and Fe(III) ions measured insolution during the transformation experiment at pH 3 for sampleSch. Diamonds represent the correlation in the first 164 days, andsquares represent the correlation after 190 days.

154 J. Antelo et al. / Geochimica et Cosmochimica Acta 119 (2013) 149–163

3.2.2. Release of SO4

The release of sulphate from the schwertmannite struc-ture to the solution during the transformation experimentwas confirmed. This release appeared to occur in two stepsand to be strongly pH dependent. For precipitates obtainedat pH 3, the average rate of sulphate release (RSO4), whichcan be defined as the slope of the plot of [SO4] released overtime, was very slow, 1.1 ± 0.2 lmol L�1 d�1, within the first64 days (Fig. 3a). However, the rate increased more sharplyfrom this moment until the end of the experiment(RSO4 = 3.0 ± 0.3 lmol L�1 d�1). Only the rate observedduring the first stage is consistent with that observed byRegenspurg et al. (2004) at the same pH, 1.1 lmol L�1 d�1.However, there was a large difference in the amount of sul-phate measured in solution at the end of the transformationexperiment. Although Regenspurg et al. (2004) reportedthat 0.33 mmol g�1 of sulphate was released after approxi-mately 300 days (i.e. 27% of the total sulphate,1.30 mmol g�1), we measured a sulphate release of1.03 mmol g�1 for Sch after 297 days (i.e. 100 % of the totalinitial sulphate).

A different pattern was observed during the transforma-tion at pH 5 (Fig. 3b). Although no correction was madefor the initial loss of sulphate due to exchange with thehydroxide ions used to adjust the pH, in this case, more sul-phate was released from the solid at the initial stages of theprocess, and then remained essentially constant after200 days. The release rate within a 64 day-period wasRSO4 = 2.3 ± 0.3 lmol L�1 d�1 and thereafter decreased to0.6 ± 0.2 lmol L�1 d�1. At this pH, the amount of sulphatereleased at the end of the experiment was approximately50% of the total sulphate. Clearly, the pH not only affectsthe rate at which sulphate is released from the solid to thesolution, but also affects the final amount of sulphate re-leased. At intermediate pH values, the transformation fromschwertmannite to goethite is slower and does not reachcompletion, whereas at lower pH the transformation rateis higher (with all of the sulphate being released) and leadsto the formation of goethite-like particles. These results areconsistent with those obtained by Kumpulainen et al.(2008), who found that the slowest transformation processoccurred at pH 6 and the fastest at lower pH. Nevertheless,no single effect of this parameter has been reported, whichindicates that different factors, such as the origin of the

schwertmannite precipitates, the method of synthesis usedand the presence of Fe(II) or other inorganic species insolution, are also important in these ageing processes(Schwertmann and Carlson, 2005; Jonsson et al., 2005;Knorr and Blodau, 2007; Jones et al., 2009).

In addition to the general trend followed by the sulphaterelease, it was found that the release rate, RSO4, was inver-sely proportional to the amount of copper in the schwert-mannite preparation. At pH 3, RSO4 varied between 1.3and 0.8 lmol L�1 d�1 for SchCu1 and SchCu10, respec-tively, during the first stage of the transformation process,whereas the respective values were 3.3 and 2.6 lmol L�1 -d�1 for the same samples at the second stage. This indicatesthat the presence of copper ions in the crystal structure in-creases the stability of the schwertmannite. Another sign ofthe greater stability is that, unlike in the Sch sample, not allthe sulphate was released from the Cu-doped samples at theend of the ageing experiment.

There was also a correlation between the amounts ofSO4 and Fe(III) in solution during the transformation pro-cess at pH 3. For sample Sch, two different types of behav-iour can be observed in Fig. 4: (i) in the period from 0 to�164 days, the concentration of Fe increased, while theSO4 remained almost constant or increased slowly, which

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may indicate that dissolution of the Fe(III) from schwert-mannite is the dominant process and (ii) the period from190 days until the end of the experiment was characterizedby a rapid decrease in the Fe(III) concentration coincidentwith a rapid increase in the SO4 concentration, indicatingthat the dominant process is re-precipitation of the Fe(III)as goethite. The presence of copper mainly affects theamount of SO4 in solution, whereas the release of Fe(III)is only slightly affected. There was no correlation betweenSO4 and Fe(III) in the transformation experiment carriedout at pH 5, probably because of the lower rate of transfor-mation from schwertmannite to goethite at the higher pH.

3.2.3. Analysis of the transformation products

3.2.3.1. XRD. Comparison of the XRD patterns obtainedfor pure (Sch) and Cu-doped (SchCu10) samples collectedat different stages of the transformation process at pH 3and pH 5 are shown in Fig. 5. It can be concluded thatthe transformation rate is affected by the pH maintainedin the schwertmannite suspensions during the transforma-tion experiment, as well as by the incorporation of Cu(II)ions during the initial synthesis. In all cases, the initial dif-fractogram shows the characteristic five peaks of schwert-mannite and, depending on these two factors, thediffractograms of the samples collected from 45 days tothe end of the experiment will present more peaks thatcould be assigned to goethite. During the ageing at pH 3(Fig. 5a), typical goethite peaks are detected in both pure(Sch) and doped (SchCu10) samples, although perfect defi-

15 25 35 45 55 65

Initial

410 days

47 days94 days

297 days

152 daysSch

SchC

u10

Goe

thit

e (a)

15 25 35 45 55 65

º2θ Cu Kα

Initial

410 days

47 days

94 days

297 days

152 days

Fig. 5. X-ray diffractograms of goethite (reference material), and pure (S(b) pH 5.

nition of such peaks is reached at different times, dependingon whether Cu(II) is present or absent. The transformationclearly occurs faster in the absence of Cu(II) (i.e. for pureschwertmannite). At pH 3, for sample Sch, goethite peaksappeared within 47 days and the intensity of these peakscontinued to increase over time until the transformationprocess appeared to be complete, after 152–297 days. How-ever, poorly crystalline mineral oxides, such as schwertman-nite, may be difficult to detect by XRD in mixtures withmore crystalline forms, such as goethite, and therefore itis difficult to confirm that all the schwertmannite presentin the suspension has been transformed to goethite-likeforms. Nevertheless, according to the measurements ofthe sulphate concentration in solution (Fig. 3), release ofall of the sulphate was reached after 300 days(�1 mmol SO4 g�1), which is also indicative of the disap-pearance or total transformation of the schwertmannite-like particles in the suspension.

Interestingly, the incorporation of Cu(II) ions into theschwertmannite structure, in sample SchCu10 (Fig. 5) andalso in samples SchCu1 and SchCu5 (Fig. S7 in Supplemen-tary data), affected the metastability of schwertmannite par-ticles, and the transformation rate decreased. Goethitepeaks appeared in the diffractograms of SchCu10 after94 days, and their intensity increased up until the period be-tween 297 and 410 days. For this sample, the transforma-tion to goethite appeared to be completed at the end ofthe experiment. However, re-analysis of the sulphate re-leased from the mineral showed that the concentration of

15 25 35 45 55 65

º2θ Cu Kα

Initial

410 days

47 days

94 days

297 days152 days

15 25 35 45 55 65

Initial

410 days

47 days

94 days

297 days

152 days

Goe

thit

eSc

hSc

hCu1

0

(b)

ch) and Cu-doped (SchCu10) schwertmannite aged at (a) pH 3 and

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sulphate in solution after 410 days was �0.85 mmolSO4 g�1, which is close to 75% of the initial sulphate con-tent for this sample, indicating that not all the schwertman-nite-like particles were transformed, or perhaps that somesulphate ions are adsorbed onto the newly formed goe-thite-like particles (process that may be favoured in theexperimental conditions used). The results confirm thatthe presence of copper ions increased the stability of schw-ertmannite particles under very acidic conditions. The mainaspects that remain to be clarified in the present study arethe stabilization mechanism and the role of copper in thisprocess. Unfortunately, a more detailed and advanced spec-troscopic study must be carried out to be able to discern themain stabilization process.

On the other hand, analysis of XRD patterns corre-sponding to the ageing process at pH 5 (Fig. 5b) revealsthat the pure sample (Sch) underwent phase transformationtowards a more crystalline, goethite-like structure after94 days, but that the transformation process was not com-pleted within the experimental period. The X-ray diffractro-gram for the sample collected after 410 days is similar tothat obtained for the same sample aged at pH 3 for 94 days,indicating that the stability of the schwertmannite particlesis much lower at the more acidic pH. This is consistent withthe sulphate concentration measured in solution (Fig. 3)

45

94 days 15

297 days Fi

Initial

Fig. 6. SEM micrographs showing the evolution of pure schwer

that reached �70% at pH 5, while at pH 3 almost all thesulphate ions were released.

At pH 5, the doped sample, SchCu10, apparently re-mained unaltered within the study period and no signalscorresponding to goethite peaks were observed. The sameresult was also found for the other Cu-doped samples(Fig. S7, Supplementary data). Again, this finding can beinterpreted as Cu(II) ions hindering the phase transforma-tion. On the other hand, comparison of the XRD patternsfrom samples collected at pH 3 and pH 5 shows the effectthat pH has on the transformation rate, which was fasterat the lower pH, as in the case of pure schwertmannite. Big-ham et al. (1996) indicated that schwertmannite is metasta-ble with respect to goethite over a range of pH 2–6, andKumpulainen et al. (2008) showed that transformationfrom schwertmannite to goethite occurs faster at pH 2and pH 8 than at pH 6.

3.2.3.2. SEM. SEM analysis of the samples collected duringthe ageing experiment at pH 3 (Figs. 6 and 7) revealed thatthe evolution of the mineral phase differs when Cu(II) ionsare present in the crystalline structure. For the pure sample,the evidence of the first crystalline structures in the SEMmicrographs appeared within 94 days (Fig. 6). Develop-ment of acicular needles, (which are typical of goethite) of

days

2 days

nal

tmannite, Sch, during the transformation process at pH 3.

45 days

94 days 152 days

297 days Final

Initial

Fig. 7. SEM micrographs showing the evolution of the Cu-doped schwertmannite sample, SchCu10, during the transformation process at pH3. The circles highlight the presence of individual goethite crystals in the transformation products obtained at 94 and 152 days.

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average length and width of 190 and 20 nm, respectively,was observed between 94 and 152 days (intermediate prod-ucts). The goethite crystals are similar in size to those de-scribed by Bigham et al. (1996), although these authorsonly measured the size of the particles obtained at theend of the ageing experiment (543 days). In the presentstudy, the goethite crystals were smaller at the end of thetransformation process (297 days) than at other stages ofthe process, reaching an average length of 50 nm and aver-age width of 13 nm. The main difference between bothexperiments is the higher initial pH used by Bigham et al.(1996), between 3.90 and 3.95, which undoubtedly had animportant effect on how the transformation process oc-curred and also on the final result.

The observed decrease in the crystal size indicates thatthe mineral phase developed into a more pure crystallinestructure. This may be associated with the transition froma microrod particle size (or multidomainic) goethite to ananorod particle size (or monodomainic) goethite. Thecrystal lattice is slowly rearranged during the ageing of min-eral oxides, which leads to the most thermodynamicallyfavourable crystalline structure. Therefore, at the end ofthe ageing process, no imperfections should be present in

the goethite nanoparticles formed, and an ideal distributionof crystalline faces is expected, with predominant 110 and/or 100 faces (Pbma space group) consisting of long chainsof iron octahedral groups terminated at the top end by001 and/or 012 faces (Weidler et al., 1998; Sone et al.,2005).

As observed above with XRD, evolution of the Cu-doped schwertmannite (SchCu10) differed from that ofthe pure sample. During the transformation experiment,SEM micrographs (Fig. 7) did not reveal any significantchanges in morphology until 297 days, except that someindividual acicular goethite crystals were detected after 94and 152 days. Nevertheless, the presence of goethite wasrather scarce in the samples collected before 297 days andmade only a small contribution to XRD patterns(Fig. 5a), which indicates that transformation from schw-ertmannite to goethite occurs very slowly in the presenceof Cu(II). As in the case of the pure samples, the goethiteparticles decreased in size between the intermediate and fi-nal stages, which may be attributed to the more crystallinenature of the final products. In samples collected between94 and 152 days, the goethite particles were of averagelength and width of 300 nm and 24 nm, respectively, while

pH 5pH 3

Fig. 8. Comparison of the SEM micrographs of the final products obtained during the transformation experiment with pure schwertmanniteaged at pH 3 and 5.

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in the samples obtained at the end of the transformationexperiment (297–410 days), the goethite crystals were ofaverage length 80 nm and average width 17 nm. In compar-ison with particles obtained from the pure sample, the goe-thite crystals in both the intermediate and finaltransformation products of the Cu-doped sample weregreatly elongated in the c-axis direction and only a slight in-crease is observed in the b-axis direction. This is consistentwith the information found in the literature, since the pres-ence of metal cations, such as Cu(II), Mn(II), or Ni(II),during the formation of iron oxy(hydr)oxide favoured thedevelopment and growth of goethite crystals (Cornell andSchwertmann, 1996; Krehula and Music, 2007; Mustafaet al., 2010).

Comparison can also be made between final transforma-tion products of samples without copper at both pH values(pH 3 and 5). In both samples, small acicular crystals,which are indicative of the presence of goethite, appearedat the end of the ageing experiment, although some amor-phous schwertmannite agglomerates were still present atpH 5 (Fig. 8). These SEM images confirm that at pH 3the transformation from schwertmannite to goethite wascompleted within 410 days, whereas the process appearedto take longer at pH 5. Goethite crystals formed at pH 5are intermediate between the nanorod and microrod parti-cle size (average length 97 nm and average width 13 nm).Moreover, the final goethite nanoparticles showed strongaggregation at pH 3, while individual goethite crystals wereobserved at pH 5.

3.3. Reactivity: arsenate adsorption on pure and Cu-doped

samples

3.3.1. Reactivity of the initial schwertmannite samples

The effect of copper co-precipitation during the synthe-sis of schwertmannite was studied by comparing the arse-nate retention capacity of the synthesized samples.Although the adsorption experiments were only carriedout at pH 4.5, a reduction in the amount of arsenate ad-sorbed is expected at higher pH. Arsenate adsorption ontopure and Cu-doped samples showed that the doped samplespresent higher affinity for this anion than the pure sample(Fig. 9) and the same behaviour would be expected at otherpH values. The amount adsorbed increased by �30%, whenCu was present in the structure at the lower AsO4 concen-

tration, and by �20% at higher concentrations. Moreover,there were no significant differences in arsenate retention onthe different Cu-doped samples. Mohapatra et al. (2006) re-ported similar results for Cu-doped goethite. These authorsfound that 35% more arsenate was adsorbed on goethitecontaining 0.5% Cu than that adsorbed on pure goethiteand also that much higher loadings of Cu did not signifi-cantly change the amount of arsenate retained. Although0.5% Cu is required in goethite, only 0.15% Cu is requiredin schwertmannite to achieve a similar increase in theamount of AsO4 adsorbed. This confirms that substitutionof Fe(III) by Cu(II) in the crystalline structure changes thereactivity as well as the stability of the schwertmannite.Changes in surface properties, such as the specific surfacearea, surface charge and point of zero charge (PZC), whichaffect the reactivity of mineral oxides, are expected with theincorporation of divalent cations in the mineral.

On the other hand, the sulphate content differed in thefour samples (Table 1) and was highest in the Cu-dopedschwertmannites. As indicated in the literature, arsenateadsorption on schwertmannite is controlled by two pro-cesses: surface complexation with the hydroxyl groupsand anion-exchange with the sulphate groups (Fukushiet al., 2003; Burton et al., 2009; Antelo et al., 2012). Sincethe Cu-doped samples have a higher content of sulphate,arsenate adsorption may be expected to be higher in thesesamples. However, analysis of the sulphate released duringthe adsorption experiment revealed that the concentrationof sulphate in solution did not vary significantly betweenthe different samples, indicating that almost all the sulphatepresent in all the samples has been exchanged with arsenate,and therefore differences in arsenate adsorption should notbe attributed to the sulphate content.

A more detailed surface chemical analysis with X-rayabsorption spectroscopy, XANES and EXAFS, should becarried out in future studies to clarify which surface com-plexes are formed, the importance of the anion-exchangereactions and the influence of the presence of Cu(II) inthe crystalline structure of schwertmannite.

3.3.2. Reactivity of the intermediate and final products

Arsenate adsorption isotherms of the solids collectedduring the ageing experiment were obtained at pH 4.5 toexamine how the reactivity of the samples changed as theyunderwent structural transformation. Only the samples

0

10

20

30

40

50

60

70

80

90

100

Sch SchCu1 SchCu5 SchCu10

[AsO

4]ad

sorb

ed(%

)

Fig. 9. Arsenate adsorption on pure (Sch) and Cu-doped samples(SchCu1, SchCu5 and SchCu10) at pH 4.5 and different initialconcentrations of arsenate: (h) 800 lM; ( ) 1200 lM; and (j)1600 lM.

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corresponding to the original preparation SchCu10 wereanalyzed in detail (Fig. 10). Significant differences betweenthe adsorption on initial (schwertmannite) and final (goe-thite-like) solids were observed, with maximum arsenateadsorption of 1600 and 600 lmol g�1, respectively. More-over, arsenate adsorption on the intermediate products(94–152 days) was also lower than on the initial schwert-mannite sample and a decrease in the maximum adsorptionwas observed throughout the transformation experiment.These results are consistent with the previous observationsfrom XRD and SEM analysis of the solid samples, whichshowed that development from schwertmannite to a morecrystalline structure (goethite-like) occurred at the lowerpH during the ageing experiment. As expected, the increasein the crystallinity produced during the ageing experimentled to a decrease in the adsorption capacity of the solids.

Although there were signs of the presence of goethite-like particles in the intermediate period (94–152 days), theamounts were rather small and schwertmannite particlesmay be the dominant reactive surface present in the system.As previously indicated, the presence of sulphate ions in themineral structure of schwertmannite is known to contribute

0

200

400

600

800

1000

1200

1400

1600

1800

0 500 1000 1500

[AsO

4]ad

sorb

ed(µ

mol

g-1

)

[AsO4]solution (μM )

Initial

45 d94 d

152 d

297 d

410 d

Fig. 10. Arsenate adsorption isotherms at pH 4.5 and I = 0.1 MKNO3 for the samples collected during the ageing process ofSchCu10 at pH 3.

to the adsorption of oxyanions through an anion-exchangemechanism. Release of sulphate occurred during the trans-formation experiments, and the intermediate products (94–152 days) contained less sulphate than the correspondingpredecessors (0–45 days), which resulted in lower anion-ex-change between arsenate and sulphate and consequently inan overall lower degree of arsenate adsorption. The differ-ence in the maximum adsorption capacity of the intermedi-ate and final products indicates the transition fromschwertmannite-like behaviour to goethite-like behaviour.

Adsorption on the final products should be similar tothat of goethite if complete transformation is produced,and differences can be attributed to the presence of Cu(II)in the new mineral phase formed. However, Antelo et al.(2005) reported that the maximum adsorption capacity ofsynthetic goethite for arsenate at pH 4.5 was�140 lmol g�1, which is approximately 4 times lower thanthat observed for the final product analyzed in the presentstudy (�600 lmol g�1). Nevertheless, the goethite-like solidformed at the final stages of the experiment was of nanorodparticle size, according to the measurements made from theSEM micrographs, and the goethite synthesized by Anteloet al. (2005) was of microrod particle size. Therefore, thisdifference indicates that the final product is not comparablewith pure goethite or that schwertmannite-like particlespersist at the end of the ageing period.

The effect that the transformation and the presence ofCu(II) ions has on the reactivity was analyzed throughthe study of arsenate adsorption, at two different loadings(400 and 1200 lM), onto the products obtained from theageing experiment with samples Sch and SchCu10(Fig. 11). The results indicate that less arsenate was retainedon samples aged at pH 3 as the transformation proceeded(Fig. 11a). At this pH, and for both arsenate loadings,the amount of arsenate adsorbed on sample Sch decreasedby �40% within the time period studied. As in the case ofCu-doped samples, the transformation from schwertman-nite to goethite was responsible for the observed decreasein arsenate adsorption. Crystals of goethite were identifiedafter 94 days and represented the dominant mineral phaseafter 152 days. There were no differences in the amountsof arsenate adsorbed on the final products, indicating thatthe mineral phase did not develop any further after297 days. Interestingly, adsorption of arsenate on the solidcollected at 152 days, in which goethite particles predomi-nated, was 20–30% higher than the amount adsorbed onthe solid collected during the final period. Even though goe-thite dominated in the intermediate samples, the presence ofschwertmannite or other amorphous iron oxides is stillimportant. Moreover, SEM micrographs showed that thegoethite phase changed throughout the transformationexperiment, and a transition from microrod to nanorodgoethite particles occurred in the period between 152 and297 days.

A similar trend was observed for the SchCu10 sample,with the main difference being that the amount of arsenateadsorbed appeared to decrease more slowly in this case,which indicates that Cu(II) contributes to increase thestability of the initial schwertmannite sample. For theCu-doped sample, no clear decrease was observed until

0

20

40

60

80

100

120

0 100 200 300 400 500

[AsO

4]ad

sorb

ed(%

)

Time (days)

0

20

40

60

80

100

120

0 100 200 300 400 500

[AsO

4]ad

sorb

ed(%

)

Time (days)

(a) (b)

Fig. 11. Comparison of arsenate adsorption on products transformed at (a) pH = 3 and (b) at pH 5 for samples SchCu10 (closed symbols)and Sch (open symbols). [AsO4] = 400 lM (circles) and 1200 lM (squares). The lines indicate adsorption of AsO4 onto synthetic goethite(dashed line), 400 lM AsO4 and (dotted line) 1200 lM AsO4. Data for goethite were obtained from Antelo et al. (2005).

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some point between 150 and 300 days, because of theslower transformation process. Only a 10% reduction inthe arsenate adsorption was observed for an initial concen-tration of 1200 lM during the initial period up to 150 days(intermediate products), and during the final period,adsorption of arsenate decreased from �50% to �20%.As explained above, the schwertmannite-like minerals pre-dominated in the initial and intermediate products (0–152 days), whereas goethite-like particles predominated inthe final products (297–410 days). The crystallographicand morphological analysis did not reveal any significantdifference between the Cu-doped samples. However, thereactivity study revealed small differences in the adsorptioncapacity of the intermediate and final products, showingthat variable Cu(II) substitution has minor effects on thestability of the initial precipitate. As stated earlier, morepowerful spectroscopic techniques would be more helpfulin the search of the transformation mechanism and to dis-cern the arsenate adsorption mechanism. The combinationof molecular-scale information with the findings of the pres-ent study would improve the understanding of the mobilityof arsenate in AMD systems.

The reactivity of samples aged at pH 5 (Fig. 11b) basi-cally remained unaltered throughout the entire experiment.Only a small decrease (less than 10%) in the amount ofAsO4 adsorbed was observed after 300 days for sampleSch. A similar result was observed for the products derivedfrom the SchCu10 sample, and the main difference was thatin the final products (297 and 410 days), arsenate adsorp-tion was slightly higher than in the solids that did not con-tain copper. The samples with and without copper, aged atthe higher pH, were more stable and did not appear to suf-fer important alterations in either their crystalline structureand morphology or their reactivity. The higher stability wasalso reflected in the XRD analysis, which showed the pres-ence of goethite peaks in those samples without Cu(II),whereas no trace of a more crystalline phase was found inthe products containing copper.

3.4. Environmental implications

Extractive mining of Cu, Pb, Zn and Ag produces largeamounts of rock waste and mine tailings, and more than

95% of the extracted materials are converted into solidmine waste (Nordstrom, 2011). The action of the atmo-sphere or microorganisms on the iron sulphide-materialsgenerates AMD, since oxidation of pyrite, chalcopyriteor pyrrhotite will occur and will produce acidity and therelease of large amounts of metal to the surface waters.During the formation of schwertmannite in sulphate-richsystems affected by AMD, other metal cations present insolution can be incorporated into the crystalline structureby Fe(III) substitution or surface adsorption. The presenceof such metal cations, which are normally found in highconcentrations in AMD waters (Nordstrom, 2011), maygreatly affect the mobility and availability of contaminantsin the environment.

It is well known that schwertmannite is metastable andtends to undergo transformation to a more stable phase.However, it can be considered as a temporary scavengerfor metals or other contaminants present in AMD and willplay an important role in metal cycling. Schwertmanniteshows high affinity for heavy metal cations, and for arse-nate, molybdate and other oxyanions (Jonsson et al.,2006; Burton et al., 2009; Antelo et al., 2012). Therefore,increasing the stability of this iron oxyhydroxysulphatenot only decreases the mobility of the species adsorbed ontothe mineral, but also favours remediation of sites contami-nated by AMD and decreases the environmental risk forrivers and streams in the area.

There is little available information on the effects thatco-precipitation of potential contaminants present inAMD have on the formation, stability and reactivity ofschwertmannite. Transformation from schwertmannite togoethite or other crystalline phases, a process that normallyrequires more than 100 days, will lead to fewer reactive sitesbeing available and will lead to a decrease in the uptake ofthe contaminants present in AMD system and remobiliza-tion of the adsorbed species. As shown in the present study,the incorporation of Cu(II) ions into the crystalline struc-ture of schwertmannite will affect the stability of thisFe(III)-oxyhydroxysulphate mineral and delay its transfor-mation to a more crystalline phase like goethite. Increasingthe stability of schwertmannite and the reactivity of theproducts obtained during the subsequent transformationprocess will decrease the mobility of oxyanions (arsenate,

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molybdate, chromate. . .) present in solution and will reducea source of contamination.

The initial products obtained during the ageing experi-ment can remove oxyanions from solution by two mecha-nisms: surface complexation with the hydroxyl groups andanion-exchange with the sulphate groups. In natural systems,co-precipitation into the mineral structure may be anothermechanism for the removal of oxyanions, but will lead tohigher concentrations of sulphate in solution. At low pH,the arsenate retention capacity of schwertmannite changesas the transformation proceeds, and the products collectedat the intermediate and final stages of the process are lessreactive. Intermediate products are constituted by schwert-mannite-like particles with lower sulphate content, and thepresence of goethite particles becomes important, whereasgoethite nanorods predominate in the final products. The in-crease in pH and the presence/co-precipitation of Cu(II) ionsdelays the release of sulphate from schwertmannite, allowingthe oxyanion-sulphate exchange to occur for longer periodsand hindering the formation of goethite. The decrease inarsenate retention by transformation products was lowerfor the samples with the highest Cu(II) concentration, so thatthe presence of copper not only slowed down the phase trans-formation but also contributed to decreasing the mobility ofarsenate. Similar effects would be expected if other divalentcations, which may be present in the AMD systems, wereincorporated, although more information is required to con-firm that these cations would also produce an increase onschwertmannite stability.

At relatively low pH (but higher than the normal pH ofAMD waters, i.e. pH 2–4), the stability of schwertmanniteclearly increased and the presence of copper in schwertman-nite prevented the formation of goethite during the studyperiod. Therefore, the presence of schwertmannite in watersof intermediate pH (5–6) may determine the mobility ofarsenate and other oxyanions on relatively long time scales(from months to years). The higher stability may make theschwertmannite-like particles found in AMD or syntheticanalogues good materials for remediation of contaminatedsurface waters, including those affected by AMD or byother human activities that may introduce large amountsof anions to the water. Nevertheless, natural secondaryFe(III) minerals present in AMD systems may behave dif-ferently from their synthetic analogues. Future studiesshould analyse the behaviour of the natural secondary min-erals formed in copper mine environments or precipitatedfrom AMD waters containing large amounts of Fe and Cu.

4. CONCLUSIONS

The results of the present study demonstrate that Cu(II)incorporation into the mineral structure of schwertmanniteaffects the stability of this mineral and its transformation togoethite. These changes will have an important effect on themobility of arsenate when schwertmannite is present in thesystem. Based on the experimental study carried out the fol-lowing aspects could be summarized:

� The pH clearly affects the transformation process ofschwertmannite particles. The ageing process at pH 3

leads to the formation of goethite nanorods(�300 days), through an intermediate stage where goe-thite microrods are dominant (�100 days). On the otherhand, at pH 5 this transformation process requires long-er time and leads to a mixture of goethite crystals, withintermediate particle size, and amorphous schwertman-nite agglomerates.� Incorporation of Cu(II) ions into the schwertmannite

structure increases the stability of this iron oxyhydroxy-sulphate by decreasing its transformation rate towardsgoethite. According to the XRD, SEM and reactivityinformation, at pH 3 the schwertmannite-like particlesare dominant until �300 days with minimum presenceof goethite crystals, whereas goethite-like particles pre-dominate in the time range between 300 and 400 days.Again, the transformation process is affected by pH,since schwertmannite apparently remains unaltered atpH 5 in the time frame of the study.� Cu(II) co-precipitation during schwertmannite forma-

tion also affects the arsenate retention capacity, whichresulted approximately 25% higher for Cu-doped sam-ples than for the pure schwertmannite. Moreover, thetransformation process also affects the mobility of arse-nate, since at the more acidic pH the adsorptiondecreases in pure and Cu-doped samples due to thetransformation to goethite-like particles.

ACKNOWLEDGMENTS

This work was financially supported by the Ministerio de Edu-cacion y Ciencia under the research Project CTM2008-03455. Theauthors would like to thank Pilar Bermejo of the Analytical Chem-istry Department of the University of Santiago de Compostela(USC) for the ICP-OES measurements. We also acknowledge theassistance of Ana Cabrerizo during the SEM micrographs collec-tion and the collaboration of Silvia Quinteiro with the schwertman-nite preparation. Finally, we would like to thank the associateeditor and the three anonymous reviewers for their insightful andconstructive comments.

APPENDIX A. SUPPLEMENTARY DATA

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.gca.2013.05.029.

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Associate editor: Christopher John Daughney