Experimental studies of the interaction

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0016-7037/97 $17.00 + 00

PIT S0016-7037(97)00117-8

Experimental studies of the interaction of aqueous metal cations with mineral substrates: Lead, cadmium, and copper with perthitic feldspar, muscovite, and biotite

Morac L FARQUHAR,' DAVID J VAUGHAN,' COLIN R Hucues,! JoHN M CHARNOCK,!” and KATHARINE E R ENGLAND!

‘Department of Earth Sciences, University of Manchester, Manchester M13 9PL, United Kingdom

*CLRC Daresbury Laboratory, Warrington, Cheshire WA4 4AD, United Kingdom (Received November 6, 1996; accepted in revised form March 25, 1997) Abstract—The interactions between each of the metal ions copper(II), cadmium(II), and lead (II) in

aqueous solution and the surfaces of the minerals muscovite, biotite, and perthitic feldspar have been

studied using batch sorption experiments and X-ray photoelectron (XPS) and X-ray absorption (XAS)

spectroscopies The purpose of the work has been to establish the extent of removal from solution of

these metal ions by interaction with the mineral surfaces and the mechanisms involved (whether ion

exchange, precipitation, or adsorption)

The experiments utilised both powdered samples and flat polished or cleaved surfaces of natural

minerals which were characterised compositionally and structurally by electron probe microanalysis and

X-ray diffraction Reaction solutions contained Cu(II), Cd(II), or Pb(IL) in a background electrolyte

of NaNO; (0.1 or 0.01 M) at a pH of 5.4 + 0.2 with a wide range of initial concentrations The extent

of uptake of each of the metals by a particular mineral substrate was measured by analysis of the liquid

portion separated by centrifugation after a controlled period of exposure (solution analyses were per-

formed by atomic absorption spectroscopy or by inductively coupled plasma mass spectroscopy as

appropriate) The data obtained were used to plot sorption isotherms for the various metal ions and

substrates showing the dependence of uptake on initial concentration of metal in solution and background

electrolyte concentration Studies of the reacted mineral surfaces using XPS were used to identify the

chemical speciation and bonding environment of Cu, Cd, and Pb present at the surface and to assess

surface coverage in planar surface samples and whether significant infiltration of the metal cation into

the mineral surface had occurred Paralle! studies using XAS (in particular Extended X-ray Absorption

Fine Structure Spectroscopy-EXAFS ) were employed to determine the local environments of Cu, Cd,

and Pb at the surface of powdered mineral samples Planar surfaces of muscovite, biotite, and perthitic

feldspar exposed to Cu(II) and of muscovite exposed to Cd(II) were also studied using Reflection

Extended X-ray Absorption Fine Structure (REFLEXAFS ) spectroscopy

The results show that muscovite, biotite, and perthitic feldspar have surfaces that can strongly interact

with (and remove) particularly Cu(II) and Pb(II) in such mildly acidic aqueous solutions Biotite also

strongly interacts with Cd(II) in solution Generally biotite provides the most reactive mineral surface

followed by muscovite, with perthitic feldspar being the least reactive Of the possible mechanisms for

removal of metals under the conditions of these experiments, precipitation was not observed, but three

other mechanisms (inner-sphere complexation, outer-sphere complexation, and ion exchange) were all

observed The inner-sphere complexation and ion exchange mechanisms often take place in combination:

however, Pb(II) forms no outer-sphere complexes on the surfaces of these minerals, and Cd(II) forms

only outer-sphere complexes with perthitic feldspar and muscovite Cu(II) in solution promotes the

release of K* from the surface region of muscovite and, particularly, biotite, and subsequent uptake of

Cu(II) by ion exchange

These findings have important environmental implications and show the significant role that major

rock-forming aluminosilicates may play in the geochemical cycling of Cu(II), Cd(1I), and Pb(II)

1 INTRODUCTION The mechanisms by which metals may be removed from

aqueous solution by interaction with minerals include precip-

itation, cation exchange, and adsorption including both in-

ner- and outer-sphere complexation The purpose of this

work has been to establish the roles of adsorption, ion ex-

change, and precipitation as mechanisms for the removal

of Cd(II), Cu(If), and Pb(II) from aqueous solution by

interaction with the surfaces of perthitic feldspar, muscovite,

and biotite and to acquire information about the fundamental

mechanisms in each case Feldspars and micas are abundant

rock-forming minerals and, thus, are commonly found in

rocks, sediments, and soils Pb(II), Cd(II), and Cu(II) are

3051

widely occurring pollutants associated with heavy industry, base metal mining and processing operations worldwide, Cd(11) is also found (sometimes reaching dangerous levels

in terms of ecotoxicity ) in landfill sites containing significant amounts of waste derived from batteries

When exposed to aqueous fluids, mineral surfaces undergo

a variety of reactions which may include hydration, dissolution, precipitation, leaching, and the sorption of protons, hydroxyl ions, and many other inorganic as well as organic atoms and molecules Most of these reactions are little under- stood in terms of controls and mechanisms at the atomic level Reactions leading to the removal of metals from solution include cation exchange and surface sorption reactions

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at dangling oxygens or bridging oxygens The most im-

portant active surface sites are those that are potentially

charged; Sverjensky (1994) calculated the pHppzc (pH at the

point of zero charge) of low albite to be 5.2 and of microcline

to be 6.1 Blum and Lasaga (1991) state that, for feldspars,

{ >(AL, Si)-OH," |} groups at the surface are the dominant

charged surface species In acid solution, H* exchanges with

cation sites at the feldspar/solution interface to a depth of

several tens of Angstrom units Wollast and Chou (1992)

report the depth of the cation exchange layer in albite to be

21 A; Schweda (1989) reports 37 A for microcline

Perthitic feldspar contains intimately intergrown albite

and microcline The main difference between these mineral

structures is the slight distortion of the lattice required to

accommodate the smaller Na* cation in albite In acid solu-

tion, both feldspars undergo rapid exchange of cations for

protons to form a hydrogen-feldspar surface several unit cells

thick This implies that the surface structure and composition

of albite and microcline at the mineral-solution interface are

similar and are consistent with the similar dissolution rates

of albite and microcline in acid solution In addition, for both

naturally weathered and experimentally dissolved feldspars,

etch pits are observed Screw dislocations provide a continu-

ous source of steps on the surface; these and other disloca-

tions introduce strain into the surface, thus potentially in-

creasing rates of dissolution (Blum and Lasaga, 1987) and,

presumably, the rates of other reactions

Phyllosilicates and feldspars undergo similar surface reac-

tions in aqueous solution The pHppzc for muscovite has

been calculated as 6.6 by Sverjensky (1994) Phyllosilicates

possess three types of surface hydroxide groups associated

with ruptured bonds along the crystal edge: aluminol, silanol,

and Lewis acid groups Sorption may occur on edge hydrox-

yls or charged sites The basal plane, however, is made up

of siloxane rings, where oxygen bonding is fully satisfied

by silicon (as silanol Si,O0 units) Recently, the proton asso-

ciation constants (K) for several important surface reactions

involving various (hydr) oxides have been calculated (Hiem-

stra et al., 1989) The log K values that were calculated for

some surface species are as follows: Al,-O 12.3, Al-OH 10.0,

$1-O 11.9, Si-OH -1.9, and Si,-O -16.9 This means that,

except for extreme conditions of pH, there is effectively no

protonation of silanol and the group can be considered as

inert Thus the basal plane is considered hydrophobic unless

there is some permanent structural charge, as is the case in

both biotite and muscovite In this case, as in many clay

minerals, protons or cations will sorb to the ditrigonal cavity

to balance that charge Although reactions in aqueous solu-

tion occur preferentially at edge sites, Johnsson et al (1992)

have shown that the basal surface of muscovite is also highly

reactive, particularly with respect to water adsorption and

subsequent dissolution and precipitation Turpault and Tro-

tignon (1994) have shown that the basal plane of biotite is

also highly reactive

Protons exchange readily with interlayer cations at the

edges of phyllosilicate crystals; many cations also exchange

at these sites Dissolution in acidic solution occurs primarily

at edge octahedral layer sites It appears that the presence

of shallow step edges, structural defects, and impurities on

the basal plane can promote dissolution and, presumably,

other reactions

Previous studies of the sorption of metal ions in solution

onto aluminosilicate mineral surfaces have tended to concen- trate on clay minerals, batch experiments being employed

to elucidate likely reactions and mechanisms Farrah and Pickering (1976) found that in alkaline solution, kaolinite suspensions act as nucleation centres for hydroxy-bridged

Cu species The adsorption of Cu, Cd, and Pb at the kaolinite/water interface was studied by Schindler et al (1987)

as a function of both pH and ionic strength They found that the extent of adsorption increased with increasing pH and decreasing ionic strength and explained both effects by using

a model that assumes two kinds of binding sites

In the present study, batch experiments were conducted

to determine to what extent Pb(II), Cd(II), and Cu(II) participate in sorption reactions on the perthitic feldspar and muscovite surfaces Powders and planar mineral samples which had been exposed to metal-electrolyte solutions were examined by X-ray Photoelectron Spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS) These techniques were used to probe the coordination environments of these elements at the mineral surface and, thereby, determine whether the sorbed metals were inner- or outer-sphere complexes or whether precipitates had formed at the mineral surface, The basal plane of the micas were studied as this plane dominates the surface area of these minerals; the aim was to determine what reaction, if any, occurs between this plane and aqueous Cd(II), Cu(II), and Pb(II) cations Specifically, XPS was used to identify the chemical speci-

ation of Cd and Pb present at the surface of the minerals

XPS was also used to assess the surface coverage of Cd, Cu,

or Pb ions on planar surface samples and whether significant infiltration of Pb, Cd, and Cu ions into the mineral surface

had occurred XPS has previously been used to investigate metal sorption from solution at the mineral surface For ex-

ample, Koppelman and Dillard (1977) studied the adsorption

of Ni and Cu onto kaolinite, chlorite, and illite From this study they concluded that Ni(II) is probably bound as the aqua ion while Cu(II) may be adsorbed as Cu(OH)* on these mineral substrates Other examples of the use of XPS

in the study of sorbed metal ions on clay minerals are dis- cussed in a review by Dillard (1988)

XAS was also employed to determine the local environment surrounding Pb, Cd, and Cu present at the surfaces of powdered muscovite and perthitic feldspar Copper present

at the planar surfaces of muscovite (001), biotite (001), and perthitic feldspar were studied by Reflection Extended X-ray Absorption Fine Structure Spectroscopy (REFLEXAFS), as was Cd on the (001) surface of muscovite Information ob- tainable by these methods includes the number, types, and bond distances of near-neighbour atoms surrounding the absorber element Recently, XAS has become a useful tool for the study of sorbed metals at the mineral/water interface For example, Chisholm-Brause et al (1990a) used XAS to study adsorption of Pb(II) on y-Al,O; They concluded that their data provide the first direct evidence that Pb(II) sorbed onto y-Al,O,; from aqueous solution under the conditions they employed is truly chemically bound as an inner-sphere complex They also demonstrated that Pb is not present as

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a precipitate or coprecipitate and does not diffuse into the

solid Many other studies have employed XAS to examine

the surface speciation of sorbed metal ions on metal oxide

and aluminosilicate surfaces These include the work of

Chisholm-Brause et al (1990b, 1990c) who studied the sur-

face complexes of Co(II) formed on g-AlO; at different

sorption densities and Co(I1) complexes on kaolinite and

rutile; O’Day et al (1990, 1994) studied Co(II) sorption

complexes on kaolinite and quartz surfaces, and Manceau et

al (1992) studied the sorption and speciation of various

heavy metal ions on hydrous iron and manganese oxides

2 EXPERIMENTAL METHODS

2.1 Materials

Two kinds of muscovite specimen were used in this study, one

for powder samples and the other for large flat surfaces to be used

in REFLEXAFS and XPS studies The powdered perthitic feldspar

and muscovite samples were both size separated, using gravity set-

tling in deionised water The <2 ym size fraction was used in

this study Thin sheets (<0.25 mm) of muscovite and biotite were

mounted on glass slides (approximately 3 cm x 5 cm for RE-

FLEXAFS and 1.5 cm x 1.5 cm for XPS) For both muscovite and

biotite, the (001) basal cleavage plane was the exposed flat surface

For the planar surface of perthitic feldspar, the sample was cut,

mounted on a glass slide, and polished with a series of diamond

pastes

X-ray powder diffraction (XRD) analysis of the perthitic feldspar

sample confirmed the presence of microcline and albite No impuri-

ties were found in the muscovite and biotite samples by XRD The

samples were also analysed by Electron Probe Microanalysis using

a Cameca Camebax instrument operating at 15 kV and 15 nA The

average stoichiometries are listed in Table 1 In the perthitic feldspar

sample, microcline and albite were present with concentrations of

approximately 75% and 25%, respectively Single point BET surface

area analysis gave a surface area for the <2 ym size fraction of

muscovite of 39.8 m’g~' and 18.7 m’g~' for the <2 ym size fraction

of perthitic feldspar Repeated standard measurements gave an accu-

racy of + 5% for this BET method Further information on mineral

characterisation is given in Farquhar (1996)

2.2 Batch Experiments

For the batch experiments approximately 0.100 g of the <2 mm

size fraction of perthitic feldspar or muscovite were mixed with 20

mL of reaction solution in plastic centrifuge tubes These tubes were

sealed and tumbled end-over-end for approximately 36 h It had

previously been established that there was no loss of Cd(1I), Cu(II),

or Pb(II) to the reaction vessels; it was also determined that the systems had achieved equilibrium within 36 h by comparing similar experiments carried out over 36 and 150 h Reacticn solutions contained Cd(II), Cu(II), or Pb(II) in a background electrolyte of NaNO; (0.1 or 0.01 M) at an initial pH of 5.4 + 0.2 The background electrolyte NaNO; was chosen, as neither Na* nor NO,” form strong complexes with other ligands present in solution Mildly acidic conditions were chosen to provide significant cation scrption but minimise aluminosilicate dissolution Solid and liquid portions were sub- sequently separated by centrifuging Liquid portions were analysed

by Atomic Absorption Spectroscopy (AAS) or Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) for Cd, Cu, or Pb, within 6

h All experiments were performed in duplicate (Precision, 2s, for AAS better than 1.5%, ICP-MS better than 3%)

2.3 X-ray Photoelectron Spectroscopy The model compounds Cd(NO;);, CdSO¿, Pb(metal), PbO;, PbO, and Pb(NO;); were obtained from Aldrich Chemical Company (purity >99.99%) Cd(OH); and Pb(OH); were prepared by reaction of their respective nitrates with excess NaOH The metal nitrates and hydroxides were studied using XPS so that comparison with the spectra from experimental runs could be made, and any precipitates

of these compounds on the mineral surfaces identified The other compounds were studied to provide reference binding energy values Powdered samples of <2 mm perthitic feldspar and muscovite were exposed to either Cu(II) or Pb( II) electrolyte (0.1 M NaNO;) solutions for 24 h, at pH 5.4 + 0.2, in batch sorption reactions, as described above Metal cation concentrations were high enough to produce saturation of the mineral surface (Cu(II) ~ 50 mgL~' and Pb(IT) ~ 80 mẹL `" ) Planar surfaces of perthitic feldspar, muscovite, and biotite were suspended in Cd(II), Cu(II), or Pb(II) electrolyte (0.1 M NaNO) solutions for 24 h, at pH 5.2—5.6 Metal cation concentrations were again high enough to produce saturation

of the mineral surface (Cd(II) ~ 50 mgL~', Cu(H) ~ 50 mgL~" and Pb(II) ~ 80 mgL.~') After exposure, samples were washed in

a jet of deionised water for approximately 20 s before air drying All XPS spectra were collected ona KRATOS XSAM 800 surface analysis instrument, using a Mg Ka (1253.6 eV, FWHM 0.7 eV) X-ray gun operating at 20 mA emission and 15 kV The binding energies of gold, silver, and copper (Au 4f3, 84.00 eV; Ag 3ds,> 368.27 eV; Cu 2p3;2 932.66 eV) were used to calibrate the energy scale of the instrument In this work, all spectra were calibrated by using the adventitious carbon Is peak at 284.6 eV (B.E.) Powders were introduced to the spectrometer fixed onto the sample holder with double-sided adhesive tape Planar surface samples were wrapped in aluminum foil, leaving only a few square millimetres exposed; this was found to help reduce charging problems experi- enced when studying these insulators

Depth profile analyses for Cd, Cu, or Pb were carried out on the planar surface samples by sputtering the surface with argon ions

Table 1 Electron Probe Microanalysis data for perthitic feldspar, powdered muscovite (musc P), planar surface muscovite (muse 3) and biotite expressed as the number of cations in the mineral formula; not accounting for the presence of structurally bound water The number

of ions for muscovite and biotite was calculated on the basis of 22 oxygens, for microcline and albite on the basis of 8 oxygens The number

of points analysed for each mineral was as follows: feldspar (microcline) 15, feldspar (albite) 5, muse (P) 16, musc (S) 16 and biotite 16

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(for periods of 10s to 10 min) followed by acquisition of detailed

spectra for the elements of interest To prevent the creation of etch

pits, the ion beam was rastered, thus ensuring that the area etched

was much larger than the area from which the spectra were acquired

The etch rate of these samples was considered to be similar to

that for SiO,, measured as 200 A/h under identical experimental

conditions in previous work

The XPS spectra of the planar surface samples exposed to metal-

containing solutions were used to calculate the atomic surface com-

positions of those samples (for approximately the topmost four to

five atomic layers) Surface compositions were also obtained for

unreacted mineral samples These were achieved by creating a win-

dow around the peaks, stripping the background from the spectra,

using a Shirley background subtraction routine and measuring peak

areas for all the elements present (KRATOS software) Peak areas

were then divided by Atomic Sensitivity Factors, derived by Wagner

et al (1981), to give the concentration of each element at the surface

in atomic percent

2.4 X-ray Absorption Spectroscopy

Freeze-dried samples of powdered <2 jm muscovite and perthitic

feldspar were exposed to either Cd(II), Cu(II), or Pb(ID) electrolyte

(0.1M NaNO) solutions, pH 5.4 + 0.2, in batch sorption reactions

Metal cation concentrations were high enough to produce saturation

of the mineral surface (Cd(II) ~ 50 mgh.~!, Cu(II) ~ 50 mgL7'

and Pb(II) ~ 56 mgL~') Solid and liquid fractions were separated

by centrifuging Traces of excess metal electrolyte solutions after

exposure were removed from the mineral by adding a small volume

of water (~ 20 cm’) to the solid portion and immediately centrifug-

ing Metal solution exposed samples were then either kept as a slurry,

freeze-dried, or the slurry placed into plastic tubes and frozen in

liquid nitrogen

Freeze-dried samples were homogenised using an agate mortar

and pestle and a thin film of the powder placed in an aluminium

sample holder using sellotape windows Slurries were placed in

aluminium sample holders using mylar windows Frozen samples

were removed from the plastic tubing to an aluminium sample holder

which was then placed into an evacuated cryostat chamber kept cool

with liquid nitrogen

Flat surface samples of muscovite, biotite, or perthitic feldspar

were suspended in Cu(II) electrolyte (0.1 M NaNO;) solutions for

24 hr, pH 5.4 + 0.2 Muscovite samples A and B were suspended

in solutions containing ~ 2 mgL.~' Cu(II); samples C and D were

suspended in solutions containing ~ 50 mgL~! Cu(II) Perthitic

feldspar and biotite samples were suspended in solutions containing

~ 60 mgL~' Cu(II) Muscovite samples B and D were air dried,

and samples A and C were washed in a jet of deionised water for

30 s before air drying All other samples were washed with deionised

water as above and air dried Planar surface samples of muscovite

were suspended in Cd(II) electrolyte (0.1 M NaNO; ) solutions for

24 hr Metal cation concentrations (Cd ~ 50 mgL~') were again

high enough to produce saturation of the mineral surface Samples

E and F were prepared at pH values 6.5 and 7, respectively All

samples were washed with deionised water as above and air dried

Solutions of the metal nitrates (~50 mgL~' Cd(II), Cu(II), or

Pb(II) in 0.1 M NaNO) were also studied to determine the metal

environment in aqueous solution

XAS data (Cu K-edges, Cd K-edges, and Pb Ly-edges) were

collected on Stations 7.1, 8.1, 9.2, and 9.3 at the Daresbury Synchro-

tron Radiation Source, operating at 2 GeV with an average current

of 150 mA In order to minimise harmonic contamination of the

beam, monochromators were detuned to reject 50% of the signal

EXAFS data were collected in fluorescence mode and between six

and sixteen scans were collected for each sample For REFLEXAFS

(Station 9.3) a reflectivity scan was run to determine the critical

angle, and the angle of incidence was set to approximately half this

angle, typically 80-120 millidegrees Full details of the experimental

set-up for REFLEXAFS can be found elsewhere (Greaves, 1991)

2.4.1 Data analysis

Data were processed using the Daresbury programs EXCALIB,

EXBACK, and EXCURV92 (Binsted et al., 1991) In EXCALIB,

the EXAFS signal from each element of the detector was examined individually and signals with spurious features or Bragg peaks (from the crystalline micas) discarded The remaining signals were summed and spectra from each experimental] run then summed The program EXBACK was used to subtract a smooth background from the spectra

EXCURV92 uses exact curved wave theory (Lee and Pendry, 1975; Gurman et al., 1984) In EXCURV92 the background sub- tracted EXAFS modulations are converted from a function of energy, x(£), to a function of wave vector k, x(k) x(k) was weighted by

a factor of k* to enhance EXAFS amplitudes at high & values The

Cd, Cu, Pb, O, and Al/Si phaseshifts were calculated by ab initio methods in the program using Hedin-Lundqvist potentials (Hedin and Lundqvist, 1969) To provide a radial distribution function,

a Fourier transform was performed on the experimental EXAFS spectrum In the radial distribution function, peaks represent shells of backscatterers surrounding the central absorber atom A theoretical model spectrum of the environment surrounding the central absorber atom is then created and compared with the experimental spectrum The best fit was obtained by adding shells of backscatterers, thereby defining a coordination number, N;, and by iterating the distances, R,, and the Debye-Waller factors 207 Different values of N,; and types of backscatterer were tested to find the best fit The number

of independent parameters iterated was two for each shell; namely

R and 2c’, and one additional parameter E;, the Fermi energy

On addition of each shell, the Joyner statistical test (Joyner et al., 1987) was used to assess the fit improvement; only shells that gave

an improvement at the 1% confidence level were accepted Brown and Parks (1989) state that assuming that systematic errors have been minimised, errors of +0.02 A for bond distances and 15-20% for coordination numbers for the first shell can be expected Bond lengths for shells further away from the central absorber, >2 A, will not exceed this error by much, +£0.05 A, while coordination numbers

at these distances are not particularly reliable

The fits from the REFLEXAFS data may be affected by the polarisation of the X-ray beam Synchrotron radiation is horizontally polarised, and although the station was operating using a pre-monochro- mator mirror which affects this polarisation, the incident beam would still be approximately 85% horizontally polarised The effect of this would be to enhance the contribution to the signal of bonds in the plane of the polarisation, i.e., parallel to the surface, and to reduce the contribution of bonds perpendicular to the surface Thus, a Jahn- Teller distorted octahedral CuO, site, in which the two longer bonds are perpendicular to the surface would give an EXAFS spectrum dominated by contributions from the four shorter bonds in the surface, and the best fit would be with four, rather than with six, oxygen atoms In view of this it is not possible to determine accurately the coordination number from REFLEXAFS data alone

3 RESULTS AND DISCUSSION 3.1 Batch Experiments

The uptake of metal onto a mineral substrate was assumed

to be the initial metal concentration in solution minus the final concentration after mineral exposure The experimental data in Fig 1 show the variation in uptake with initia] metal concentration on an equal surface area basis

As Na* does not hydrolyse significantly or form strong complexes with other ligands in aqueous solution, it forms mainly outer-sphere complexes at the aluminosilicate/ water interface Competition by Na* from the background electrolyte indicates that a cation is probably taking part in outer- sphere complexation Interactions that show no dependancy

on Na* concentration and, hence, indicate no outer-sphere complexation are Pb(I1) with muscovite and perthitic feldspar and Cu(II) with muscovite The interaction of Cu(II with perthitic feldspar and Cd(II) with muscovite shows some dependency on Na™ concentration indicating some

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5 E 15003 h

b

=

Fig 1 Plots for muscovite (musc) and perthitic feldspar (feld) exposed to Pb(I1), Cd(1I), and Cu(II)-containing

electrolyte solutions (0.01 and 0.1 M NaNO) showing: (a) initial Pb(II) conc vs uptake per m’ for muscovite; (b)

initial Cd(II) conc vs uptake per m? for muscovite; (c) initial Cu(II) conc vs uptake per m? for muscovite; (d)

initial Pb(IE) conc vs uptake per m? for perthitic feldspar; (e) initial Cd(II) conc vs uptake per m? for perthitic

feldspar; and (f) initial Cu(II) conc vs uptake per m? for perthitic feldspar

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3056 M L Farquhar et al

4f

=

Binding Energy (eV) Fig 2 Lead 4f X-ray photoelectron spectra of (a) Pb(OH).; (b)

a muscovite planar surface exposed to Pb(II); (c) a perthitic feldspar

planar surface exposed to Pb(II); and (d) a biotite planar surface

exposed to Pb(II)

outer-sphere complexation Cd (II) interaction with perthitic

feldspar shows complete dependency on Na* concentration

indicating a high degree of outer-sphere complexation in this

system

The step in the upper curves which show uptake of Cd(II)

and Cu(II) from the 0.01 M solutions onto perthitic feldspar

suggests that, as the sorption density increases, sorption be-

comes more energetically favourable, followed by less ener-

getically favourable conditions at higher sorption densities

Possible explanations for this may be the filling of one type

of sorption site followed by the filling of another, or that at

some critical sorption density, co-operative forces exist that

make sorption energetically favourable again

Sorption densities for Pb(II), Cd (II), and Cu(II) interac-

tion with muscovite are higher in all cases, indicating that

muscovite has a larger number of available sorption sites

per unit surface area compared with perthitic feldspar From

the respective sorption densities, it is clear that under the

conditions of our experiments, Pb sorption is slightly more

favourable than Cu sorption which is, in turn, more favour-

able than Cd sorption on muscovite For perthitic feldspar,

sorption of Pb is more favourable than Cu which is, in turn,

more favourable than Cd

3.2 X-ray Photoelectron Spectroscopy

Representative XPS spectra obtained are shown in Figs

2 and 3 Results in the form of binding energies for electrons

in specific orbitals are summarised in Table 2 The errors in

binding energies are approximately +0.1 eV

3d,

E

—Ÿ———“Í

D

2

@

3

=

he

Binding Energy (eV)

Fig 3 Cadmium 3d X-ray photoelectron spectra of (a) Cd(OH);; (b) Cd(NO;);; (c) a muscovite planar surface exposed to Cd(II); (d) a perthitic feldspar planar surface exposed to Cd(II); and (e)

a biotite planar surface exposed to Cd(II)

Although the binding energies of the Pb model compounds studied in our work do not match exactly those reported by Pederson (1982), the general trends are the same Studies referenced in Koppelman and Dillard (1977) indicate that

the binding energies of metal hydroxides adsorbed on metal

oxide and clay mineral substrates are the same as those

Table 2 Electron binding energies (B.E in eV) from XPS of Pb and

Cd compounds or mineral surfaces exposed to solutions containing these metals Also shown are values published by Pederson (1982) Powder samples (P) and planar surface samples (S) have been used as substrates

4f;; B.E 4f; B.E 3d; B.E Sample This Study Pederson This Study

Smec + Pb (P) 139.3 Musc + Pb (P) 139.4 Feld + Pb (S) 139.5 Musc + Pb (S) 139.8 Biotite + Pb (S) 139.9

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of the pure metal hydroxides For the <2 um powdered

muscovite, planar surface muscovite, perthitic feldspar, and

biotite exposed to Pb(II) electrolyte solutions, the XPS spec-

tra are very similar (see Fig 2) Comparison of the Pb

4f;,2 binding energies of the samples with those of lead oxide,

nitrate, and hydroxide, reveal that the sample binding ener-

gies are higher This suggests that Pb is bound to highly

electron withdrawing groups and also suggests that oxide,

nitrate, and hydroxide compounds are not, themselves, pres-

ent in the reacted samples The data also confirm that Pb is

in the Pb** state The Pb 4f,, binding energies of the samples

are also higher than the binding energy reported by Pederson

(1982) for PbSiO;, which is 138.45 eV

For planar surface perthitic feldspar and muscovite ex-

posed to Cd(II) electrolyte solutions, the spectra are very

similar (see Fig 3) The binding energies for the Cd 3d;,;

peak in these samples are 405.7 and 405.8 eV, respectively,

and once errors are taken into account, these values can be

considered identical There are also low energy and intensity

peaks in these spectra which are nitrate peaks The biotite

sample has a higher binding energy for the Cd 3d5, peak of

406.4 eV No single reference could be found to compare

the binding energies with those of the model compounds

However, comparison with other metal compounds suggests

that the binding energies of the model compounds are cor-

rect Comparison of the Cd 3ds, binding energies of the

samples with those of cadmium hydroxide and cadmium

nitrate show that none of the samples exhibit the characteris-

A

0 200 400 600 800 (secs)

a

~~

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2

~~

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0 200 400 600 800 (secs)

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Etch Time Fig 4 X-ray photoelectron depth profiles of (a) Pb 4f7,2 depth

profile of planar perthitic feldspar exposed to Pb(II) electrolyte

solution; (b) Cd 3ds, depth profile of planar muscovite exposed to

CdD electrolyte solution; and (c) Cu 2p3,2 depth profile of planar

biotite exposed to Cu(II) electrolyte solution

Table 3 Information on surface composition obtained from X-ray Photoelectron Spectroscopy Analysis (in atomic percent) of muscovite (musc), perthitic feldspar (feld) and biotite after exposure to Cd(ID, Cu(ID or Pb(I])-containing electrolyte solutions Also included are the estimated penetration depths of the metals into the mineral surfaces as determined by X-ray Photoelectron Spectroscopy Depth Profile Analysis

Sample Element Percent Penetration Depth (A)

Note: * residual sign

* 25% highest signal

* signal never reduces below 40%

tics of the hydroxide The muscovite and perthitic feldspar

samples, however, have identical binding energies to cadmium nitrate The binding energy of the Cd on the biotite

sample surface is higher than those of cadmium hydroxide,

nitrate, and sulphate, which suggests that the Cd is bound

to highly withdrawing electron groups All of the samples have Cd 3ds, binding energies characteristic of Cd?* For all mineral samples exposed to Cu(II) electrolyte solutions, there is strong evidence of partial photoreduction of

Cu?* to Cu'*, as seen by the loss of the shake up satellite

peaks; this has been noted many times before (e.g., Frost et al., 1972; Klein et al., 1984) Therefore, direct comparison

of binding energies with model compounds cannot be made

A common feature seen in all depth profiles is an initial increase in Pb, Cu, or Cd concentration as a function of

depth in the first few scans This is probably due to carbon contamination on the surface which is then sputtered away

After the initial concentration increase, the metal signal de- creases to a residual value with sputter time; for example,

see the Pb 4f depth profile for the perthitic feldspar which

is shown in Fig 4 There are two exceptions to this pattern, biotite exposed to Cu(IT) and muscovite exposed to Cd(II); these are also shown in Fig 4 In the biotite sample, the Cu signal drops by ~ 40% after 24 min then rises again With the muscovite sample, the Cd signal reduces to almost zero after 75 s then rises and falls; this could be due to the high signal to noise ratio exhibited by the Cd samples The residual Pb, Cu, or Cd signal could arise from mar.y causes; for example, that some metal has been knocked in by sputtering, redeposition of sputtered material, ion-gun, or X-ray gun

misalignment

Depth profile and elemental surface analysis data are in Table 3 Depth profiling of the Pb(II)-exposed planar surface muscovite showed that, at an etch depth of ~3 A, only

a faint residual Pb 4f,, signal is observed; this suggests that Pb(IL) does not penetrate the (001 ) basal plane significantly

On planar surface perthitic feldspar the Pb signal disappears

after etching to a depth of ~17 A, indicating that some

penetration of Pb(IL) into the perthitic feldspar structure

Trang 8

-100 T r T T T -100 T T T T T † -100 T T T T

Fig 5 Determined by X-ray photoelectron spectroscopy the difference in surface compositions of planar surface

(a) muscovite (musc), (b) biotite (bio), and (c) perthitic feldspar (feld) after exposure to Pb(II), Cu(ID), or Cd (II)

electrolyte solutions as a percentage of unexposed surface composition

may have occurred The planar surface biotite sample shows

the greatest penetration by Pb(II), to a depth of ~67 A In

the depth profile analysis of Cd(II)-exposed planar surface

muscovite and perthitic feldspar samples, no significant pen-

etration by Cd(II) beneath the surface is indicated The

Cd(II) -exposed biotite sample has a more persistent Cd sig-

nal, which reduces to a residual level at ~11 A, suggesting

some penetration into the structure Depth profiling of

Cu(II)-exposed planar surface muscovite showed that, at an

etch depth of ~ 47 A, one quarter of the Cu 2p;, signal

remains; this suggests that Cu(II) does penetrate the musco-

vite structure significantly The depth profile analysis of pla-

nar surface biotite suggests that Cu(II) penetrates even fur-

ther into the structure, in this case the Cu signal never re-

duces to less than 40% of the highest value The planar

surface perthitic feldspar sample has a residual Cu signal at

~19 A, a similar depth to that found for perthitic feldspar

exposed to Pb(II); again, this indicates some penetration of

Cu(II) into the perthitic feldspar surface

Elemental surface analysis of the planar muscovite and

perthitic feldspar before exposure to the metal-bearing solu-

tions agrees fairly well with the data obtained by Electron

Probe Microanalysis The biotite sample, however, shows

significant differences between the surface analysis obtained

by XPS and the bulk composition From the planar surface

samples exposed to Cd(II), Cu(II), and Pb(II) in solution,

it is clear that muscovite preferentially takes up Cu which

is present at 2.1%, whereas the surface concentrations of Cd

and Pb are 0.3 and 0.2%, respectively Biotite preferentially

takes up Cu at 1.1% whereas the surface concentrations of

Cd and Pb are 0.8 and 0.7%, respectively Perthitic feldspar

preferentially takes up Pb at 1.4% with surface concentra-

tions at 0.4% for Cu and Cd Biotite appears to be less

selective in metal sorption than either muscovite or perthitic

feldspar

It was expected that the surface compositions of the miner-

als would change on exposure to the metal electrolyte solu-

tions due to dissolution and surface reactions (see Fig 5)

In all samples, O, Si, and Al are relatively little affected by

exposure to metal electrolyte solutions Iron in biotite is

affected more than O, Si, and Al at + 35% of the unexposed

biotite surface composition The surface concentration of K

is depleted in all samples, as seen in greater detail in Fig

6 In the perthitic feldspar samples exposed to Cd(Il), Cu(II), and Pb(II) in solution, K is depleted to ~80% of its former value This was expected because, in acid solution, feldspars undergo a rapid exchange of cations The presence

of CdD), Cu(II), or Pb(II) in the electrolyte solution does

not appear to affect the release of K from the perthitic feld-

spar structure Potassium is not released in as large quantities from the muscovite samples exposed to Cd(II), Cu(II), and Pb(II) solutions as from the perthitic feldspar samples; however, in all samples K is depleted When in contact with Pb(II) solution, there is a depletion of ~20%, whereas with Cd(II) it is ~35% and with Cu(II]) ~45% The presence

of Cu(II) in the electrolyte solution does appear to cause an

increase in the release of K from the muscovite structure Biotite shows a similar, but more marked, pattern of behav- iour compared with muscovite, with K depletion levels of

~30, ~20, and ~90% for Pb(1II), Cd(II), and Cu(IT) samples, respectively In this case, it is clear that the presence

of Cu(II) in the electrolyte solution greatly promotes the release of K from the biotite structure

0

a

»

a

Metal Fig 6 Plot of the differences in surface potassium concentration determined by X-ray photoelectron spectroscopy of planar surface muscovite (musc), biotite (bio), and perthitic feldspar (feld) after exposure to Pb(II), Cu(II), or Cd(II) electrolyte solutions (as a percentage of potassium concentration of unexposed surface)

Trang 9

The evidence described above suggests that ion exchange

of Pb(II) for K* is a dominant mechanism in the removal

of Pb(IL) from aqueous solution by biotite and perthitic

feldspar Since there is little penetration by Pb(II) of the

muscovite surface, it is unlikely that Pb(II) is participating

in ion exchange with K~ (this may be due to the increased

stability of the dioctahedral muscovite over the trioctahedral

biotite) However, it does seem likely that, given the high

binding energy, Pb(II) is chemically bound to the muscovite

surface

The apparent low affinity of Cd(II) for perthitic feldspar

and muscovite surfaces combined with its lack of penetration

into either structure, suggest that Cd(II) only weakly inter-

acts with these surfaces (forms outer-sphere complexes)

The increased penetration and higher electron binding energy

for the Cd 3ds, XPS peaks suggest that Cd(II) may take

part in K* ion exchange or surface complexation when inter-

acting with biotite

The evidence suggests that ion exchange of Cu(II) for

K* is an important mechanism for Cu(II) uptake by these

micas This conclusion 1s supported by the work of Ilton et

al (1992) who found that the reaction of biotite and phlogo-

pite with an acidic solution of CuSO, at 25°C and | atm

produced submicroscopic inclusions of Cu These Cu inclu-

sions occur in the interlayer region of the mica and were

associated with Cu-enriched, expanded (hydrated ) interlay-

ers and with K depletion Although Cu is found at a higher

concentration on the muscovite surface, it appears to pene-

trate further into the biotite structure This could be due to

the relative stabilities of the two micas in solution; biotite

is a trioctahedral mica and, as such, releases K more readily

than the dioctahedral muscovite Also, the initial oxidation

of Fe in biotite is thought to promote the release of K to

solution by reducing the layer charge (see IIton et al., 1992,

and references therein) Copper also penetrates the perthitic

feldspar structure, and elemental surface analysis shows that

there is considerable depletion of K in the surface layers of

the sample This indicates that Cu(I1) ion exchange for K*

is a dominant mechanism for the removal of Cu(II) from

aqueous solution by perthitic feldspar

3.3 X-ray Absorption Spectroscopy

Figure 7 illustrates the EXAFS spectra and associated

Fourier transforms from the powdered perthitic feldspar and

muscovite samples exposed to Pb(II), Cd(H), or Cu(IE)-

containing solutions, it also contains the EXAFS spectra and

Fourier transforms from Pb(IE) and Cd(I1) nitrate solutions

The EXAFS spectra and Fourier transforms of planar musco-

vite, perthitic feldspar, and biotite exposed to Cu(II)-con-

taining solutions are presented in Fig 8 Summaries of the

structural parameters derived from fitting of the EXAFS data

for powdered and planar surface samples are provided in

Tables 4 and 5, respectively No differences were observed

between in situ and ex situ powder samples; this was ex-

pected as the fitting of only one backscatterer shell was

statistically justified for data from the powder experiments

In general, the EXAFS experimental data were noisy for

Pb leading to poor structural information Coordination num-

bers for Ly,-edges are not particularly reliable, even for the

4

0-1 WOO

Cc

1.6 4 1.2 4

F

0.4 4

1.6 + 1.2 4

Na

R (A)

12 4

16 4

12 4

44

4 4

K (A”)

Fig 7 Lead L-edge, Cadmium K-edge, and Copper K-edge EX- AFS spectra and associated Fourier transforms for powdered samples

of (a) perthitic feldspar exposed to Pb(II); (b) muscovite exposed

to Pb(II); (c) Pb(II) in aqueous solution; (d) perthitic feldspar exposed to Cd(II); (e) muscovite exposed to Cd(II): (f) Cd( IT)

in aqueous solution; (g) perthitic feldspar exposed to Cu(II); and (h) muscovite exposed to Cu(II)

Trang 10

1 A from the central absorber for powdered <2 ym perthitic feldspar (feld)

2 08 B given Also given is comparable information for Cd(II) and Pb(I) in

ten

30

1D

16 4

12 J

G

8 4

44

Đín

4

K (A’) Fig 8 Copper K-edge REFLEXAFS spectra and associated Fou-

rier transforms for planar: (a) muscovite exposed to Cu(II); (b)

biotite exposed to Cu(II); and (c) perthitic feldspar exposed to

Cu(II) Also experimental (solid line) and best fit theoretical

(dashed line) REFLEXAFS Cu K-edge spectra and associated Fou-

rier transforms for planar muscovite exposed to Cu(II)

first backscatterer shell Therefore, the noninteger values of

N do not provide an accurate indication of the coordination

number For <2 ym perthitic feldspar and muscovite miner-

als exposed to Pb(II) electrolyte solutions, low oxygen coor-

dination numbers of 2 and 2.3 gave the best fit For both

fits, a Pb-O bond length of 2.28 A was calculated with mod-

erately high Debye-Waller factors indicating some deviation

in the Pb-O bond distances from the average The short bond

lengths indicate a high degree of covalency in the Pb-O bonds The high Debye-Waller factors and intermediate coordination numbers indicate that Pb is not in a unique bonding environment In many Pb and O containing compounds, there is a wide range of Pb-O bond distances The low coordination numbers calculated for these samples may only re- flect shorter bond distances because longer ones may be sufficiently out of phase with one another for their EXAFS contributions to cancel However, many lead silicates display Pb-O bond lengths in the range 2.27~—2.42 A with oxygen coordination numbers between 2 and 4 (Boucher and Peacor, 1968; von Katsuo, 1980; Rouse et al., 1988)

None of the samples exhibit bond length and coordination environments characteristic of lead nitrate (Ohba et al., 1985) No structural information exists for simple lead hydroxide; however, information for lead hydroxide chloride and lead hydroxide oxide shows that none of the samples exhibit hydroxide characteristics (Venetopoulos and Rentze-

Table 5 Atoms, coordination numbers (NV), and bond distances (R) in successive shells around the central Cu absorber for planar surface biotite, perthitic feldspar (feld) and muscovite (musc) samples exposed to Cu(II)-containing electrolyte solutions; Debye-Wal- ler factors (207) are also given

Sample Absorber Shell Atom N (A) (A’)

3 AILSi 4 3.13 0.008

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