The mechanism of Zn2+ adsorption on calcite

Geochimica

el Cosmochimica

ACNI Vol. 52, pp. 228 I-2291

C016-7037/88/$3.lXl

CapyriatQ 1988 F’ergamon Press plc.Printed in U.S.A.

+ .oO

The mechanism of Zn2+ adsorption on calcite J. M. ZACHARA’, J. A.

KUTRICK~

and J. B. HARSH~

‘Environmental Sciences Department, Pacific Northwest Laboratory, Richland, WA 99352, U.S.A. 2Department of Agronomy and Soils, Washington State University, Pullman, WA 99 163, U.S.A. (Received December 15, 1987; accepted in revisedform June 20, 1988) Abstract-The adsorption of Znzf on calcite (CaCOsr.3 was investigated from aqueous solutions in equilibrium with CaCOXlj and undersaturated with respect to Zns(OHk(COs),., . Zinc adsorption occurred via exchange with Ca*+ in a surface-adsorbed layer on calcite. The validity of this exchange reaction was supported by adsorption isotherm and constant concentration experiments, where Ca& was varied by systematically changing the pH and C02(pj.Greater adsorption of ZnZ+ occurred at higher pH and CO,,, levels, where Ca2+ activities were lowest. Sites available for Zn2+ sorption were less than 10% of Ca2+ sites on the calcite surface. Surface exchange of Zn2+ did not affect the solubility of calcite. Zinc sorption was apparently independent of surface charge, which suggested that the surface complex had covalent character. Desorption and isotopic exchange experiments indicated that the surface complex remained hydrated and labile as Zn2* was rapidly exchangeable with Ca 2+. Careful analysis of the adsorption data showed that Zn2+ and ZnOH+ were the sorbing species. The exchange reaction was generalized as a power exchange function: K = 0.62 =

’

I

Zinc adsorption on calcite was compared to and was consistent with that of Co’+, but Zn2+ was more strongly sorbed. INTRODUCTION

the surface (MCBRIDE, 1979, 1980), but data to the contrary also exists (DAVIS et al., 1987). The adsorption of Mn2+ and Cd2+ at higher surface loadings displays no maxima and apparently exceeds lattice concentrations of Ca2+ or CO:-, implying a continuum between adsorption and precipitation (MCBRIDE, 1979, 1980; DAVIS et al., 1987). It is not known if Me surface exchange is influenced by CaCO,,r surface charge. Calcium and C@- are potential determining ions for the CaCOr(,, surface (PARKS, 1975), and FOXALLet al. (1979) determined the pzc at pCa = 4.4. However, little agreement exists on the pH (pm) (DOUGLASand WALKER, 1950; S~MASUNDARAN and AGAR, 1967; FOXALLet al., 1979; SIFFERTand FIMBEL,1984). This confusion derives from the presence of multiple surface hydrolysis reactions and the fact that H+ is not potential determining. The pH (pm) has been measured to range from pH 8.0-9.5 in solutions saturated with CaCOs(,, and atmospheric C02w. A slow time-dependent sorption reaction is evident for some divalent metals on calcite. The observed rate is dependent on experimental conditions (i.e., CaCOX,, surface area, electrolyte composition) (L~RENS. 1978: MCBRIDE. 1979. 1980: KORNICKERet al.. 1985: FRANKLINand’MoRSE, 1983; DAVIS er al., 1987) and appears to differ between metals. The phenomenon has been ascribed to 1) diffusion into or within a hydrated calcium carbonate layer on the calcite surface (DAVIS et al., 1987), 2) dehydration of the adsorbed metal and formation of discrete MeCOs bonds on the calcite surface that may or may not have lateral continuity (MORSE, 1986; FRANKLIN and MORSE, 1983), and 3) CaCOs recrystallization leading to incorporation of the adsorbed metal as a solid solution in the uppermost layers ofthe crystal (LORENS, 1978, 1981; DAVISet al.. 1987).

DIVALENT METALLIC CATIONS forming rhombohedral carbonates (Cd, Mn, Co, Zn, Fe, and Mg) sorb on calcite which is ubiquitous in nature. Sorption can influence metal ion availability and mass transport in porous media, and record the geochemical conditions under which calcite was formed. Of the metals forming rhombohedral carbonates, Zn’+ is particularly important because it is essential for biological activity at trace levels but is a common environmental contaminant at higher concentrations. The mechanism of Zn2+ adsorption on CaC03(,, remains largely undefined and is the focus of this work. The adsorption of Zn2+ on calcite has been noted by numerous investigators. In studies of Zn2+ coprecipitation with CaCOr,,, adsorbed Zn’* had to be removed from the CaCOH,, surface to give an unbiased measure of the solid phase distribution coefficient (CROCKE?T and WINCHESTER,1966). The importance of zinc adsorption on CaCOr has often been underestimated by using crystallites of low surface area and presenting sorption data on a mass basis without regard to surface area and potentially reactive surface sites (PICKERING, 1983; BRUMMERet al., 1983; KITANO et al.. 1976). Zinc is apparently adsorbed to calcite surface sites that are important in crystal growth because Zn*+ inhibits calcite precipitation on seed crystals WRENS, 1978). Zinc can also assist and/or nucleate calcite precip itation from super-saturated solutions (GLASNERand WEISS, 1980). A thermodynamic study suggests that adsorbed Zn*’ on calcite remains hydrated, unlike its behavior on magnesite and dolomite (JURINAKand BAUER, 1956). The interaction of other metals with CaCOu,, has been more extensively studied. The metallic cations Cd2+, Mn*+, Co*+, and Cu*+ sorb on calcite viu a combination of adsorption and precipitation reactions that have not been completely decoupled (MCBRIDE,1979, 1980; KORNICKERet al., 1985; FRANKLIN and MORSE, 1982, 1983; DAVISet al., 1987). At initial Me concentrations below where precipitation of stable carbonate or basic carbonate solids occur, there is a rapid initial adsorption reaction whose magnitude reflects the CaCOwd surface area (MCBRIDE,1979,198O; FRANKLINand MORSE, 1983) and the presence of other surface reactive solutes (i.e. Ca2’, Mg*+;KORNICKERet al., 1985; FRANKLIN and MORSE, 1983). Metal adsorption is postulated by some to occur by exchange with Ca*+ at

Zinc adsorption on CaC03(,) was investigated in this work to clarify the surface reaction mechanism and provide a quantitative measure of its intensity. A calcite exhibiting slow recrystallization rates was used to allow analysis of surface reactions that are unbiased by incorporation into the bulk solid. Sorption experiments ranging in time, pH, and partial pressure of COzu were used to determine the influence of aqueous Zn2+ speciation, Ca2” concentration, and CaCOs(,, surface charge on the Zn2+ surface reaction. Desorption was measured to determine if surface-bound Zn2+ readily responds to changes in aqueous Zn2+ and Ca2’ activities. Iso2281

2282

J. M. Zachara, J. A. Kittrick and J. B. Harsh

topic exchange of adsorbed Zr?’ was used to probe the lability of the St&ace complex and determine if surface bonding changes with surface coverage. The experimental data was analyzed in terms of a surface exchange reaction and compared to the sorption of Co’+, which has an identical ionic radius. EXPERKMENTAL

PROCEDURES

A crystallized reagent-grade calcite (CA-Fisher Scientific Lot745658) exhibiting a stable adsorption surface was used for most adsomtion exneriments. The calcite was >99.99% CaCQ%bv chemical am&is, and-x-ray diffraction showed the presence of minor aragonite. Surface analysis of CaCOso by X-my photoelectron spectroscopy (ZACHARA et al., 1988), which received pre-equilibration in 0.1 M NaClO,, showed only minor surface contamination by Cl and Si. Average edge lengths of the CaCOH,, rhoms was estimated from electron microscopy at 7.0 pm. No aragonite was observed by electron microscopy. The surface area as measured by multipoint Krypton gas adsorption was 0.252 m2/g. A second Fisher calcite cB, (Lot-854606; 0,356 m2/g) exhibited fast recrystallization rates in time course experiments with Zn and is only briefly discussed. Another reagent-grade calcite with a similar surface area (BakerUltrex Lot 502141) and CaCOsl,, prepared and aged according to REDDYand NANCOLLAS( 197 1) reacted with Zn comparably to CA in selected adsorption experiments.

Ten equilibrium CaC03(,,-CaC0,,~, suspensions, ranging - _ in pH over 2.5 units with an approximate~ionic~strength of 0.1 M, w&e made at each partial pressure of COzo ( 1O-2.46,1O-‘.*, 1O-4.46atm). The geochemical code MINTEQ (FILMY et al., 1984) was used to calculate CaCOs(,) dissolution at the desired pH and COau,, with a fixed ionic strength of 0.1 M. These calculations were used to determine the approximate amounts of Ca(C104)2(aql, NaHCOsr,,, Na2C0s(,,, and NaCIOy,, needed for each solution to achieve equilibrium with CaCOsta) at the desired pH. After mixing the reagents and CaCOso the solutions were bubbled with an N&O2 gas mixture or air, and small amounts of 0.1 M HC 1O4 or NaOH were added to stabilize the final PH. The CaCO~~~~~~~~ suspensions were stored under the appropriate atmosp~ of CO,. Inorganic carbon analysis of these solutions was used to precisely identify the COzo level in each glovebox. The saturation indices with respect to CaCO%(,,(log IAP/&) ranged from - -0.24 to 0. Time-course experiments The requisite volume of CaCOjus, in equilibrium with CaCO,(,) at pH 8.4 and CO,,., = 1O-3.46atm (0.22~rrrn filtered) was added to a 500-mL jacketed%yrex flask and‘the solution mass recorded to 0.00 1 g. New CaCOs,,, was added to yield 2.52 or 6.3 1 m2/L, and its mass recorded to 0.001 g. The suspension was stirred gently for 8 hr with a caged Teflon stir bar, and allowed to sit quiescent for 48 hr. The suspension was then stirred again for 8 hr in contact with the atmosphere. It was reweighed to 0.001 g to determine the final solution mass, connected to a constant temperature water bath (25“C), and spiked with 10m3,lo-’ or IO-’ M ZnCl, (Aldrich Gold Label) with a 65Zn radiolabel to yield initial concentrations of lo-$, 10e6, or 10s7 M Zn. Initial solution activity of *‘Zn was approximately 20,000 dpm/g. The spike mass was recorded to 0.0001 g. Ten-mL aliquots of the stirred suspension were removed with a plastic pipette at each sampling time and placed in a lo-mL plastic syringe connected to a 0.22~pm Swinnex filter. The first 4 mL of the suspension were rapidly fdtered and discarded to saturate the filter; the remainder was filtered into a plastic tube. Previous testing had shown that this filter, unlike others, had a low affinity for Zn*+ in CaCO,,,r/NaClO,. Two l.OO-mL aliquots were then removed to tared, acidified scintillation vials (10 aL of Ultrex HCl) for counting, while a 2.5mL aliquot was removed for ICAP analysis. Each time-

course experiment was performed in duplicate. Initial testing had shown that counting precision and accuracy were greatly improved if sample aliquots varied by co.01 mL and their mass was carefully recorded to 0.0001 g. Adsorptionisotherms Isotherms were measured at each partial pressure of COzrnlat pH values that were near, above, or below the pH of minimum soiubility (i.e., DH of 8.34 at CO,,., = lo-‘.& atm). The solids concentration &as typically 6.31 m’/t”caco,,, (25 g/L CA). One isotherm was repeated at 2.52 m*/L C’aC(& (10 g/L CA)to determine the influence of solids concentration. The initial con~ntmtion range of Zn was lo-’ M to lo+ M. ~~ipi~tion occurred at the integrate and higher ends of this inanition range for many ~mbinations of pH and C02(,,. Zinc precipitation was clearly indicated by a decrease in suspension pH, a saturation index of approximately 1 for hydrozinc&e, and a sharp, positive increase in the slope of the adsorption isotherm (ZACHARAet al., 1988). Only concentrations when: precipitation of discrete Zn solids did not occur will be discussed herein. At each desired initial concentration, 20 mL of the appropriate CaCOH.,, solution in equilibrium with CaCOH,, (0.22 Brn filtered) was added to three 3%mL polycarbonate centrifuge tubes and the solution mass recorded to 0.0001 g. Fresh CaCOsa, [0.50 g] was added to two of the three tubes and the added sorbent mass recorded to 0.0001 g. The settled suspensions were allowed to equilibrate for 48 hr in contact with a water-saturated atmosphere of appropriate Co,,, to stabilize the CaCO~&aCO~W, interface. The CaCOw,, blank was carried at each concentration to identify container adsorption and hom~eneous precipi~tion. Each tube was then spiked with approximately 0.20 mL of a ZnC&)- %r standard, at pH of 4.5. .Standatd molarity of the spiking solution was determined gravimetrically and by ICAP analysis. The addition of ZnC12(Wr65Znspike, had no effect on the pH of the CaCOIu,&aCOpu, suspension because of its high buffering capacity. Suspension pH changed, however, when precipitation occurred. Initial solution counts ranged from 20,000-10,000 dpm/g of solution. The mass of the spike was recorded to 0.0001 g, yielding a precise measure of the added cold and hot Zn. The tubes were shaken for 24 hr in contact with the appropriate atmosphere and centrifuged at 25,000 rcf for 30 minutes to separate the phases. Identical results were obtained when presaturated, 0.22.rrn Millipore GS filters were used for phase separation. Isotherms at COzw = lo-“* atm were sampled after 4 hr, 24 hr, and 7 days ofequilibmtion. Replicate I-mL aliquots (weighed to 0.0001 g) and a 2.5-mL aliquot were removed by pipette. The aliquots were acidified with 10 pL of ultrex HCl for ScintilIation counting of 65Zn and ICAP analysis of Ca, Zn, and Na, respectively. A Ross combination electrode was used to measure the final pH in the settled suspension in contact with the appropriate COau atmosphere. The adsorbed mass was calculated from the final aqueous concentration. A correction was applied to account for that adsorbed to the tube, typically 3% to 5% of the total counts. pH adsorptionedges Adsorption “edges” were measured on CA with Zn2+ at a constant initial concentration of either lo-* or lo-’ M, and at a constant activity of Zr?+ @Zn2c = 7.5). The Zn concentrations needed to yield nZn*’ = 7.5 were Rm~c~at~ usina the ~~lib~urn code MINTEQ and tbe complexation consent in Table 1. These concentrations ranged from 8.8 X l5-* M to 5.75 X 10m6M. Eight different equilibrium CaCOu,) solutions, varying in pH, were used at each COzco,level. These solutions had pH values and Ca concentrations spanning the various estimated pH, and pC% of CaCOx8,. The pre-equilibration, spiking, equilibration, phase separation, and analytical procedures were identical to those employed for the isotherms. Desorption Desorption was measured by independently changing the equilibrium Zn2+ and CL?* concentration.

2283

Zinc adsorption on calcite Table 1.

Equilibrium Constants used for Speciation and Solubility Calculations

Reaction

H+ + CO:- = HCO;

w:oc:

:=0

10.3

(a)

6.35 (a)

H+ + HCO; = H2CO;(,q)

1.26 (a)

Nat + CO:- = N&O; Nat + HCO; = NaHCO:( 4)

-0.22 (a)

caz+ + co:- = CaCO;(,I)

3.15 (a)

l

1.00 (a)

Ca'+ + HCO; = CaHCO: Ca*+ + HP0 = CaOH+ + H+

-12.6

The spike mass was recorded to 0.00 1 g and the suspensions stirred for 24 hr. A IO-mL aliquot of the settled suspension was removed and weighed. Subsamples were removed, filtered, and acidified ( IO-NL Ultrex HCl) for scintillation ( 10W6M) and ICAP (IO-’ M) analysis of the starting Zn and Ca concentrations. The remaining solution was spiked with carrier-free 65Zn and then mixed, and triplicate lOOpL aliquots removed, weighed, and acidified for counting. The hot spiking solution was added to the vessel, yielding an initial 6’Zn activity of approximately 50,000 dpm/g solution that was precisely known through mass accounting. Sampling of the stirred suspensions began immediately. Ten-mL aliquots of the stirred suspension were removed and treated as described in the time-course experiments. Sampling and filtration were completed within 20 set of removal, Each isotopic exchange experiment was performed in duplicate.

(a)

RESULTS zn=+ + HP0 = ZnOH+ + H+

-8.96 (b)

Zn2+ + 2H20 = Z"(OH);(,g) + 2H+

-16.9

(b)

Z"'+ + 3H20 = Zn(OH); + 3H+

-28.4

(b)

Z"*+ + 4H20 = Zn(OH):- + 4H+

-41.4

(b)

Z"*+ + HCO; = ZnHCO+ %a3 zn2+ + co:- = ZrlCOS( q)

(d)

l(P) =

Zn(OH), ?(COJ,, ,(.) + 2H+ '( ) (ba) (c) (d) (e)

(c)

3.9

8.48 (a)

Cal+ + CO:- = CaCOS( ) , Zn*+ + 1.6 Hz0 + 0.4CO

2.1

-9.18 (e)

B 11 et al 1981 t&s and M;smer 1976 Zirino and Yamamoto 1972 Bilinski et al. 1976 Schindler et al. 1969

Adsorption reversibility induced by a change in the equilibrium Zn2+ concentration was determined by performing a limited threeconcentration isotherm at pH 8.2 and 7.65, with CO,, = 10-2.46 atm. After a 24-hr equilibration period the experiment was sampled as described previously. The equilibrium solution was then removed, the occluded volume determined by mass, and new CaCOs( ) added to the CaC03(a). The replacement CaCOx,, contained no Z$&. The suspensions were mixed and immediately resampled. Electrolyte replacement reduced the aqueous Zn2’ concentration by approximately 90%. The tubes were shaken and sampled after 22, 46, 118, and 166 hr. A pH-edge experiment was performed with an initial Zn concentration of 10V6M Zn using six CaCOXW, solutions, with CO2(,, = lo-“,” atm. Two replicates and a blank were run at each pH. After determining the equilibrium Zn concentration, the equilibrium solutions were removed and the occluded volume determined by mass. New CaCOI(,, solutions were added with the pH approximately 0.7 units lower then the original electrolytes. Before addition these solutions were spiked to contain, as closely as possible, the same Zn concentrations as were present in the original equilibrium solutions. Thus, total moles of Zn and counts of “Zn were conserved in each tube. The tubes were shaken and sampled after 24 and 48 hr. Final pH was measured in the settled suspension alter each sampling.

Sorption with time The amount of Zn2+ sorption over time depended on the initial Zn concentration, the surface area of CaCOX;,, and the CaCOJcr, sample CA or cB (Fig. 1). Ionic strength was of little consequence, as shown by the comparative sorption behavior of 10e6 M Zn at ionic strengths of 0.1 M and 0.03 M. The greatest amount of sorption (on a mole/m’ basis) occurred rapidly within 12 hr of Zn addition. After this rapid uptake, sorption either ceased ( 10e6, lo-’ M) or continued ( 10m5M) at a rate that was approximated by a first order kinetic relation. The timedependent incorporation of 10m5M Zn on both CA and cB was obviously not precipitation as a discrete, pure Zn solid because aqueous concentrations fell below those maintained by Zn5(OH)6(CO~)2~,J,(Fig. I), the most probable solubility controlling solid (ZACHARA et al., 1988; S~HINDLER et al., 1969). While formation of a mixed precipitate on the calcite surface, e.g. (Zn, Cal_X)C03~S~ is possible and has been observed for Mn*+ on calcite in seawater (FRANKLIN and MORSE, 1983), the data in Fig. 1 are insufficient to imply coprecipitation. Discrete metallic precipitates on calcite have been reported for Cd, Cu, and Mn (MCBRIDE, 1980; FRANKLIN and MORSE, 1982, 1983). An in depth analysis of zinc surface precipitation on CaC03(S, is given elsewhere (ZACHARA et al., 1988).

Isotopic exchange Isotopic exchange experiments were performed in jacketed 2% mL Pyrex reaction flasks at 25°C with CaCOs(,, at pH 8.4, COzcsj = 10-3.46atm, and pCa = 3.08. The pre-equilibration procedure described for the time-course experiments was replicated with CaCO%,, (CA)at 6.3 1 m’/L. After pre-equiIibmtion the suspensions were spiked with cold (lo-’ M) or hot (IO-’ M) standards to yield initial concentrations of 10m5and 10m6M Zn. The use of a radiolabel was required for the 10T6M concentration because the equlibrium concentration after adsorption was below the ICAP detection limit (0.02 &mL).

FIG. I. Time-course equilibrations of Zn2’ on CAin CaCOXW(,, with I = 0.1 and CaCO 3(aq)in distilled deionized Hz0 (I - 0.03). Starting and final aqueous solutions in equilibrium with CaCOXs,.

J. M. Zachara, J. A. Kittrick and J. B. Harsh

2284

The time dependent sorption noted for 10m5M Zn on CA and for all concentrations on cB (shown only for 10e5 M) was similar to that observed for Mn2+, Co”, and Cd2+ at lower concentrations (LORENS, 1978; FRANKLIN and MORSE, 1983; KORNICKERet al., 1985; DAVIS et al., 1987), and was consistent with recrystallization (LORENS 1978, 198 1; MOZETO et al., 1984; DAVIS et al., 1987). Recrystallization occurs as smaller crystals dissolve at the expense of the growth of larger crystals under conditions when there is no net precipitation (LORENS, 1981). It is driven by surface energy and can yield a surface coprecipitate on larger crystals when adsorbed ions are incorporated into new surface layers of the crystal (DAVIS et al., 1987). MOZETOet al. ( 1984) identified a two-step sorption process on CaCOs(,) using “C as a tracer. The first step was rapid and dominated by adsorption or surface exchange of 13C. The second step occurred over time as surface layers recrystallized. Recrystallization was rapid for unaged laboratory calcite precipitates and dominated 13Cremoval by rearrangement of multiple surface layers. Adsorption was evident and more important for aged and well crystallized calcites where a more stable interface underwent slow recrystallization. The sorption of Zn with time (Fig. 1) was similar to the results of MOZETO et al. (1984). Recrystallization (step 2) dominates Zn sorption on cB, while adsorption is more important on CA at 10m6and lo-’ M Zn. Subsequent results and discussion in this paper focus on CA because of its slower recrystallization rate. Recrystallization may be inhibited by Mg2+ (MILLER and SASTRI, 1973; MOZETO et al., 1984; DAVIS et al., 1987) or encouraged by Zn2+ at certain concentrations (GLASNERand WEISS, 1980). The greater decrease in [ZnTls with time in the 10m5M Zn experiments on CA compared to that for lower concentrations paralleled the findings of GLASNERand WEISS (1980). They grew larger CaC03(s) crystals in the presence of increasing Zn concentrations, and suggested that Zn increases the recrystallization rate of CaCOs(,,. The time dependent removal of Zn is expected to increase with recrystallization rate as Zn is incorporated into surface layers of the crystal, and new reaction sites are generated on the CaC03(,, surface. Isotherms Zinc adsorption isotherms in equilibrium suspensions of CaC03(,, (CA) were highly nonlinear at all combinations of pH and COzcg),but yielded mostly linear plots on a log-log basis (Fig. 2a,b). Some reduction in slope occurred at the highest adsorption densities. The data were well described over much of the concentration range by the Freundlich, Langmuir-Freundlich (KINNIBURGH, 1986), and two-part Langmuir equations. Nonlinearity over the entire adsorbate concentration range and adherence to the Freundlich isotherm attests to the non-uniformity or heterogeneity of sorption sites (SPOSITO, 1980; BENJAMINand LECKIE, 1981) on the calcite surface. The log-log isotherms plots displayed similar slopes (Freundlich N value - .55-.60) over a wide range in pH and COzcsI.Over 1 week, the slow sorption component exerted an observable but minor influence on the aqueous and bound concentrations (Fig. 2a). This change was comparable

to that noted

in the time-course

experiments

with

FIG. 2. Adsorption isotherms of Zn2+ on CA in CaC03(,, with I = 0. I. Starting and final solutions in equilibrium with CaC03(,, . a) 1O-“.46atm with sampling times and CaC03(,) as noted. b) CO2(,, = CO&) = 1O-2.46atm with sampling after 24 hr. Desorption induced by electrolyte change shown with solid symbols; sampling times as noted.

CA (Fig. 1). Sorption was observed to increase with time, with the only significant effect noted at higher pH. The solids concentration (m2/L) was not a major influence on the placement of the isotherm (Fig. 2a), but possibly influenced the slope. The isotherms were influenced by pH and COzo, with sorption always increasing with pH (Fig. 2a,b). Similar behavior was also noted for two isotherms at COZ(~)= 10-4.00 atm (these isotherms are not shown). The trend was toward increasing sorption as aqueous Ca2+ decreases and CC)- increases. Similar Zn2+ binding at pH 8.9/C02(,, = 1O-3.46atm (Fig. 2a) and pH 8.2/C02(,, = 10-2.46atm (Fig. 2b) suggested that pH may only be a secondary variable by influencing Ca2+ and CO:- activities through CaC03(s) solubility. The highest adsorption density achieved was approximately low6 moles Zn/m’ of CaC03(,, . Almost all linear and log-log isotherms displayed a reduction in slope at a surface density of 10-6.5- 10-6,3 mole Zn/m2, which represents an empirical measure of the adsorption maxima (shown more clearly in Fig. 9). Two-part Langmuir adsorption plots of the data yielded a similar extrapolated adsorption maxima.

pH edges With a single initial concentration of Zn2+, fractional adsorption increased with the pH of the equilibrium calcite

Zinc adsorption on calcite 100 -

co21.1 = 1o-3’B atm [10e’M]6.31

$ % < 8

2285

[lo-%] m’/L CaCOJ

60

r-----i---

[lo-‘Ml251

I ,_____-_&-__.i

m’/L

40

01

I_ 70

I

7.5

,

I

6.0

1

I

6.5

90

I

I

. Adrorptlon 24hr 0Derorptlo” 24h, 6Desorptmn 46h,

I

9.5

PH

FIG. 3. Adsorption edges of Zn2+ on CA in CaC09(,, varying in pH with COzcs,= 1O-“.46 atm. Effectsof surface area and Zn concentration are shown. Starting and final solutions in equilibrium with

FIG. 5. Zn2+ desorption from 6.31 m2/L CA with 10m6M initial Zn and C02u, = 10-3.46atm. Desorption induced by replacing the equilibrium solution with new CaCOX,, that was approximately 0.7 pH units lower and contained the same [Zn], . All starting and final solutions in equilibrium with CaCOXIj.

CaC03cs).

suspension (Fig. 3), in direct contrast to the sorption behavior of Cd’+ noted by bAvIS et al. (1987). The resulting pH edge resembled metal cation adsorption on oxides and results from similar phenomena: changing adsorbate speciation and activities of the potential determining ions (in this case, Ca*+ and CO:-). The pH edge was not as steep as oxides and levels off between pH 8.5 and 9.0. Fractional adsorption increased withsurfaceareaanddecreasingadsorbateconcentration (Fig. 3). This sorption behavior was more consistent with Zn*+ mass action/exchange on surface sites than precipitation. Fractional adsorption was expected to increase with increasing initial Zn concentration if precipitation was the dominant sorption mechanism. The shift in the adsorption edges did

not reflect changes in Zn concentration or CaCOr(,) surface area in a linear way. The positioning of the adsorption edges for a given Zn concentration depended on the partial pressure of C02(,, (Fig. 4a,b). Increasing the COrc,, at a given pH increased Zn*+ adsorption, and the edges moved to a lower pH. This adsorption increase followed a decrease in Ca” activities maintained by the solubility of CaCOr(,,. Identical behavior was noted for both Zn concentrations although the 10e7 M edges were steeper in slope, especially at lower pH values. The constant activity edges at pZn2+ = 7.5 (Fig. 4b) closely followed the lo-’ M data, until the initial Zn concentration was increased to 10e6.j M and above to account for hydrolysis and carbonate/bicarbonate complexation. While the percent adsorption decreased below that of the constant concentration edges, total adsorption (mol/m2) was greater. Desorption The extent of desorption depended on how the surface complex was perturbed at equilibrium. If the equilibrium Zn2+ concentration was reduced by electrolyte/CaC03(asj replacement at constant pH and Ca*+ concentration, Zn*+ desorption was limited and new aqueous concentrations fell above the isotherms (Fig. 2b). Observance of desorption was further complicated by the slow sorption process, which led to a steady increase in bound Zn2+ with time. In direct contrast, complete reversibility was observed when Zn*+ desorption was induced by lowering the pH value, and therefore increasing the Ca’+ concentration of the CaCOX,, electrolyte (Fig. 5). Desorption was complete within the time frame of adsorption. The surface reaction of Zn*+ therefore appeared reversible to Ca*+, which increases in concentration as the pH of the CaCOS(asjis reduced. Isotopic exchange After a 24-hr equilibration period, adsorbed Zn*+ was fully exchangeable with 65Zn in 6 hr (Fig. 6). Most of the surface-

FIG. 4. Adsorption edges of Zn2+ on 6.31 m2/L CA in CaCO varying in pH and COzu,. a) 10m6M initial Zn concentration. b) 1:’ M initial concentration and 1O-7.5M initial activity. Starting and final solutions in equilibrium with CaCOr(,,.

bound Zn*+ was instantaneously exchanged, with greater than 9090 and 75% exchangeable within 30 set for 10m6M and 10e5 M concentrations, respectively. The difference in exchange behavior between the concentrations was consistent

J. M. Zachara, J. A. Kittrick and J. B. Harsh

2286

from these data alone, but it is implied that multiple Zn surface complexes exist with varying binding intensities or different surface environments. The initial exchange (up to 0.25 hr for low6 M and 2 hr for 10e5 M) was approximately linear and slopes from these plots yield a uniform rate of 6.48 (kO.63) X 10-s mole me2 hr-‘. The nearly identical exchange rates suggested that the nature of the surface species, whether a coprecipitate or surface complex, was the same when calcite was treated with lo-’ and 10m6M Zn. The rapid rate of exchange, however, and combined with the ease in desorbability of the 10V6M surface species (Fig. 5) indicated that the surface species was an ionic complex rather than a co-precipitate. DISCUSSION The zinc adsorption, desorption, and isotopic exchange data are consistent with a reversible surface exchange or complexation reaction involving Ca2+: ,o

_________

________

_____

.._.

0 *______-cr

--...

__ _ _ - - - p_--____-_..-.

FIG. 6. Isotopic exchange of 6sZn on 6.31 m*/L CA with COr(,, = 1O-3.46 atm and pH - 8.3. Fraction exchanged(F,) = (l-F&,&/ [ 1 - (C+&)] where C, and C, are equilibrium and initial concentrations, respectively. Open and closed symbols are replicate exneriments: a) low6M initial Zn, b) lo-’ M initial Zn.

with the Zn distribution between the aqueous and solid phases; 95% of the Zn2+ was adsorbed in the 10e6 M concentration as compared to approximately 45% in the 10e5 concentration. This apparent lability of the surface complex paralleled the reversibility noted in response to Ca2+ activity (Fig. 5). Analogous to Zn, approximately 50% of the estimated surface Ca” on CaCO,(,) was rapidly exchangeable at pCa = 3.22, and equilibrium was attained within 20 hr (MILLER and SASTRI, 1973, 1974). MOZETO et al. (1984) observed that adsorbed 13C (as HCO; or CO:-) on calcite was fully reversible to a change in the isotopic ratio of the aqueous phase. The data were plotted according to the McKay equation (MCKAY, 1938; KYLE et al., 1975) for homogeneous (single rate process) exchange (Fig. 6a,b): log(l-Fe,)=-R

(

a+b ab

t 1

(1)

where F,, is the fraction exchanged (defined in Fig. 6), R is the rate of exchange (both radioactive and nonradioactive) of Zn2+ between the solid and liquid phases, and a and b are equilibrium Zn2+ concentrations for the solid and solution phases in mole/L. A plot of log (1 - F,,) on the ordinate, and t on the abscissa will be linear with slope equal to -R[(a + b)/ab] if a single rate process exists. The overall isotopic exchange was not homogeneous (Fig. 6), suggesting the simultaneous occurrence of two or more rate processes (ATKINSONet al., 1971). A mechanistic interpretation of this heterogeneous exchange was not possible

where -Ca$ and -Z$ are the adsorbed cations. The formal charge and specific nature of the surface complex (i.e. -C$$ and -Zn$ is not known. KENT et al. (1988) presumed that adsorbed divalent ions occupy exposed lattice positions and ascribed a single charge to these complexes on calcite (i.e. -Me&). For conceptual convenience and in the absence of definitive information, the complex is viewed, herein, as the divalent ion bound to an anionic surface site that may be in the crystalline lattice or in a surface layer on calcite. The experimental data (Figs. 2, 3, and 4) were subjected to speciation calculations using the geochemical code MINTEQ to yield single ion activities. All adsorption data included in the following analysis were undersaturated with respect to Zn5(QH)&Q&, . Zinc adsorption also had little influence on solution pH, and CaC03(,, equilibrium was maintained at all sorption densities below the occurrence of Zn precipitation. The speciated isotherm data are plotted on Fig. 7 according to a log-linearization of the power exchange function (LANGMUIR, 198 1): log [(Zn’+).,/(Ca’+),]

= log K., + n log [ZnX/CaX]

(3)

where ( ) denotes activities, Zti and Cti are surface mole fractions, and K is the exchange constant of the reaction. Since the calcite exhibited well-developed rhombohedral morphology, surface mole fractions were calculated assuming that all crystal faces exhibited a Ca site density equal to that of the lOi4 cleavage plane (8.31 X 10m6mole/m’; MOLLER and SASTRI, 1974). That is, ZnX = (mole Zn/m’ calcite)/ 8.31 X 10m6and Cti = 1 - ZnX. It is therefore assumed that the reactive site density is governed by exposed lattice sites rather than a disordered hydrated CaC03 surface layer. While a more accurate value could be achieved with 45Ca isotopic exchange, this site density, adopted by others, serves as a convenient reference point. The isotherm data from Fig. 2 and others not shown with 10-4.0 atm converge on a single function (Fig. 7) co*(g) =

2287

Zinc adsorption on calcite

(5)

FIG.7. Powerexchangeanalysis of Zn*+and Co*’(Mg’) adsorption on calcite. Figure contains speciated adsorption data from eight Zn adsorption isotherms on CA varying in pH and COa,. Co*+data from KORNICKER et al. (1985); speciated using constants from COSOVICet al. (1982).

when only the constants summarized in Table 1 are used. In spite of the hypothesized importance of Zn(CO&& at higher carbonate concentrations ( 10e3- 1O-’ M; GLASNERand WEISS, 1980), convergence of the isotherm data is not achieved when this complex, with a stability constant estimated by MATTIGOD and SPOSITO( 1977), is included in the speciation calculations. Solubility data (ZACHARA et al., 1988) also support the minimal contribution of this complex. The data also converged when ZnOH+ was allowed as a reactive species, yielding the regression equation log {(Zn2+ + ZnOH’)/(Ca2’)j = 1.685 log {ZnX/C&Y) - .209 (r* = .946). This relationship was statistically indistinguishable at the 95% confidence limit from the regression equation in Fig. 7. Variability in the isotherm data near the experimentally observed sorption maximum and uncertainty in the formation constants for Zn-carbonate/bicarbonate complexes precludes confirmation of ZnOH+ as a sorbing species from the exchange isotherm data alone. Zinc is preferred over Ca” and this preference increases with decreasing aqueous Zn2+ activities and surface coverage (Fig. 7). Similar behavior and power exchange constants are noted for Ca2+-Cd*+ exchange on montmorillonite at low aqueous Cd concentrations (LANGMUIR, 1981). Zinc adsorption tends toward nonpreference exchange behavior near and above log [ZnX/CaX], - - 1.O.Non-preference exchange is denoted by K,, = 1 and n = 1 (Eqn. 3; see LANGMUIR, 198 1, for example); it occurs when the solid surface exhibits equal exchange selectivity for the individual exchanging ions. The speciated adsorption data from the pH edge experiments were used to determine if Zn’+-Ca2+ surface exchange exhibited 1:1 stoichiometry. A modified exchange equation was derived: log [nz,/(Zn2’)]

= log [(Kex * CaX. nT)J - log (Ca2’)

(4)

where nzn is adsorbed Zn (mole/m2), nT is the total number of surface sites (mole/m2), and other terms are as previously described. This equation follows from the mass action expression

when the rational activity coefficient ratio (f&‘fca) is fixed at unity. Surface sites are assumed to remain constant with pH. A similar approach has been applied to PO:- sorption on CaC03(EJ(ISHIKAWAand ICHIKUNJ, 198 1; HOUSE and DONALDSON,1986). Plotting Eqn. (4) [log {nZn/(Zn2’)} vs. log (Ca2’)] for the constant concentration pH edge experiments yielded plots with two discrete linear segments with an increase in slope occurring at the pH where hydrolysis becomes significant (not shown). The two linear segments were statistically different in slope. When a modified form of Eqn. (4) is used incorporating Zn2+ and ZnOH+ as the sorbing species via their activity sum [(Zn*‘) + (ZnOH’)]: log {n&(Zn2’)

+ (ZnOH+)l} = log [(K,,*CaX*m)]

- log (Ca*‘)

(6)

a linear relationship is obtained over the whole range of data with high correlation coefficients and a common intercept (Fig. 8). K., in Eqn. (6) represents an average exchange constant for both reactive species, Zn2+ and ZnOH+. It is concluded that both these species adsorb on the calcite surface. The adsorption of these species shows a 1: 1 dependency on (Ca2’) at lo-’ M Zn that decreases to approximately 0.83 at 10m6M (Fig. 8). This difference in stoichiometry is significant at the 95% confidence limit for data measured at COzu = 10-3.46atm. While binding constants for Zn2+ and ZnOH+ differ from one another on iron oxides (DAVIS and LECKIE, 1978) and phyllosilicates (HARMSEN, 1979), our data suggest that these species exhibit similar selectivities for the calcite surface. The linear relationships observed in Fig. 8 indicate that Kav is relatively constant over a range in nz,,, Ca2+, and pH. This observation suggests that K&Zn2+ is similar to &ZnOH+, because K,,” will vary with (Zn2’)/(ZnOH’) if the two species exhibit different selectivities for the surface (HARMSEN,1979). The constant Zn*+ activity experiments yield curvilinear plots (Fig. 8) that intersect the constant concentration edges

* 0

T

[l o-‘Ml

B (lo-“)

0

5 ’ t

c

c?

3 z-0 3

coa., = 10-2.’ atm .

[l o-‘M]

l

[lo-'M]

c

No-"1

coagl = 1o-&Oam

. [lo-w -1

]

. [lO-‘M]

FIG. 8. Evaluation of exchange stoichiometry on 6.31 m*/L CA. Figure includes data from constant concentration and constant activity pH edges at COzw = 10-2.46,10-3.46,10-“w atm. Calcium concentrations were analytically determined and speziated with MINTEQ to yield activities. (Zn + ZnOH) represents the sum of the two activities, i.e., [(Zn”) + (ZnOH+)].

2288

J. M. Zachara, J. A. Kittrick and J. B. Harsh Table 2.

ComparativeData on Metallic Cations Forming on RhombohedralCarbonates

log K b' (Ca*+-Me2+)a K "'CO$(.) Ca2+

0

-8.48

Cd*+

0

-11.3e.

Zn2+

0.26

-10.8

co*+

0.26

C.

d.

-9.84

. ” K "*"c03(#q)

" K UecO:(lq) 3.15

(K IK )d. C'COS(.) "SCOS(S) 0

1.00

4.0e.

2.a3e.

3.9

2.1

2.32

2.46

1.89

1.36

a'+-Me") - difference in ionic radius. - solubilityproduct. "SCOJ(S) K . - formation constant. K Ye"COs(lq) u+#q)' For the solid solution reaction (McIntire (1963)): 1) CaC03(S) + Me?:,) t" MeCOS(,) + Ca*Tlq)

e.

2) K=K /K C'COS(.) "SCOS(S) Davis et al. (1987).

where similar initial concentrations of Zn were used. The constant activity data actually define a series of lines that include and roughly parallel the constant concentration experiments, and that decrease in slope with increasing initial Zn concentration, It is clear that the initial concentration rather than the activity governs Zn adsorption, and that the surface complex is stronger than Zn(OHh(,,, which predominates above pH 8.5. The lower dependency of Zn*+ adsorption on (Ca*+) at increasing Zn concentrations reflects a decreasing selectivity of Zn for the surface, or possibly binding to surface sites that are not saturated with Ca*+. Significantly, there is no apparent influence of surface charge on Zn adsorption. The plots in Fig. 8 do not show inflection near the various reported pCa(,, (4.40; FOXALL et al., 1979; - 3.26; SOMASUNDARAN and AGAR, 1967). This behavior indicates the predominance of chemical over electrostatic forces in the surface complex (BALISTRIERI and CHAO, 1987) and contrasts with that of cationic and anionic surfactants, whose adsorption on calcite is presumably electrostatically driven (SOMASUNDAFUNand AGAR, 1967). Becauseof the similarionic radiiof Co*+ and Zn*+ (Table

2), the adsorption data of KORNICKER et al. (1985) were reevaluated according to procedures presented in this paper to determine if their adsorption behavior on calcite is comparable. At similar pH values, C02cp), and (Ca”), Zn*+ is adsorbed more strongly than Co*+ (Fig. 9). Cobalt, however, exhibits nearly identical isotherm behavior and displays a similar sorption maximum (- 1O-6.25mole/m*), in spite of the use of a different calcite sorbent. The speciated adsorption data (calculated with stability constants in COSOWC et al., 1982) also conformed to the power exchange model (Fig. 7). Applying the same site density used for Zn*+ calculations, Co*+ exhibits nearly ideal exchange; paralleling, but more selective than, the non-preference isotherm (Fig. 7). The enhanced selectivity of Zn over Co shows that in addition to cation size the strength of the surface complex is influenced by factors related to electron configuration, such as crystal field effects and covalency. Data (Table 2) are in-

sufficient to suggest whether the relative strength of the surface complex parallels that of the aqueous hydrated carbonate complexes or the solid anhydrous carbonate. The ratio of the solubility products (Kc~coJ&~o,) is used to estimate the thermodynamic equilibrium constant for solid solution formation or distribution into the solid phase (MCINTIRE1963; SPOSITO, 1981; DAVISet al., 1987). This ratio for Zn (lti’*) is higher than conditional exchange constants calculated for Zn*+ exchange on CaCOSw from the isotherm data: log

K

=

c

log

(ca*+)ma (Zn*+)[Cti]

= - 1.0-2.17

with nT = 8.3 1 X 10e6 mole/m*. The solubility product ratio may, therefore, not be a relevant predictor of sorption when surface exchange rather than coprecipitation is the suspected sorption mechanism. In contrastto Zn, DAVISet al. (1987) calculated an exchange coefficient for Cd ( lO’,‘s)that exceeded the solubility ratio ( 102.83,Table 2) when coprecipitation occurred. The solubiity product ratio for Zn, however, is greater than that of Co, which therefore approximates the trend in adsorption. Zinc is also preferred over Co*’ by zeolites, where

FIG. 9. Adsorption isotherms of Zn*+ and Co*+on CaCO,,,,.

Zinc adsorption on calcite both ion size and electron configuration influence exchange (MAES and CREMERS, 1975). The recrystallization rates of the CaCOr(,, are a major factor controlling whether adsorption or coprecipitation dominates metal sorption. DAVISet al. (1987), like MOZETOet al. (1984), observed a multimechanistic sorption process for Cd’+ on calcite. The rapid Cd’+ adsorption reaction (step l), however, was surpassed by coprecipitation of Cd’+ with CaCOs(,, (step 2), as recrystallization of the calcite progressed slowly. Time variant sorption of Zn*+ that was similar to Cd*+ was observed in this work using cB. It is surmised, but was not proven, that coprecipitation also dominated Zn*+ sorption on cB. Adsorption (step 1) via Ca*+-Me*+ exchange was most important for Zn*+ on CA and presumably for Co*+ (KORNICKER et al., 1985) because the recrystallization rates of these calcite sorbents was slower than cB and the CaC03(sJ used by DAVIS et al. (1987). The smaller ionic radii of Zn2+ and Co*+ and their higher solvation energies relative to Cd*+ and Ca*+ may also influence the propensity to form solid solutions in calcite surface layers. Limited coprecipitation of Zn with CaC03(JJ has been observed between 25°C and 250°C (CROCKET and WINCHESTER,1966; TSUSUEand HOLLAND, 1966). ZnCOsc,, and CoCOJ(,, exhibit limited miscibility in CaCOscs, at high temperatures and pressures (GOLDSMITHand NORTHROP, 1965; REEDER, 1983) as compared to CdCOscE,(CHANG and BRICE, 197 1; BORODINet al., 1979). The high temperature solid state solubility data, while not readily extrapolatable to lower temperatures and pressures, suggest that Zn and Co solid solutions with CaCOs(,, may be less likely to form and less stable at room temperature and pressure than those with Cd. Kinetic considerations are also important. ZnCOx,,, unlike CdC03(JJ, is slow to form under low temperature and pressure conditions (SCHINDLERet al., 1969), because a high metal ion solvation energy inhibits precipitation of the anhydrous carbonate. This desolvation barrier may promote enhanced stability of a hydrated adsorbed ion, or precipitation of hydrozincite (Zn5(OH)6(C03)2(rJ which is not miscible with as surface and solution concentrations increase. CaCO3b,, Metal ion sorption on calcite appears to follow the affinity series Cd > Mn > Zn > Co (MCBRIDE 1979, 1980; KORNICKER et al., 1985; DAVIS et al., 1987; this paper). This series parallels a decrease in the dehydrated metal ion radii and percent MeC03(,, miscibility in CaC03(,, at elevated temperatures, and an increase in the solubility of the anhydrous carbonate (K~cco~,,J. Regardless of the relative importance of adsorption versus solid solution formation, the stability of the surface species appears related, in part, to the properties of the anhydrous, ionic solids.

2289

surface charging was observed, suggesting a surface complex stabilized by chemical forces. On CA where recrystallization is not rapid, experimental evidence suggests that the exchange reaction occurs in a partially or fully hydrated adsorbed layer on the calcite surface rather than through strict lattice replacement. The identification of ZnOH+ as a sorbing species discounts strict lattice replacement because the ionic size of ZnOH+ exceeds that of Ca*+. Less than one to one Zn*+ to Ca*+ exchange stoichiometry at certain Zn concentrations suggests binding to sites that are not fully occupied with Ca*+, as might be expected in an adsorbed layer with hydrated anionic sites. The importance of an adsorbed layer is widely proposed as a determinant in the surface reactivity of sparingly soluble salts (BROWNand CHOW, 1983), precipitation/dissolution of CaCOscl, (AMANKONAHand SOMASUNDARAN,1985; CASSFORD et al., 1983; PLUMMER et al., 1978; LIPPMAN, 1973; MUCCI and MORSE, 1985) and ion adsorption/solid solution formation on calcite (LAHANNand SIEBERT,1982; DAVIS et al., 1987). The atomic depth, structure, ion density, and surface chemical properties of these adsorbed layers have eluded rigorous quantification. The hydration status of adsorbed Zn*+ is unclear, but the lability of the surface complex as shown by rapid rates of isotopic exchange and reversibility to Ca$,) indicates that the adsorbed Zn ions are not completely desolvated. However, selectivity differences between Zn and Co and lack of surface charge influence suggest that covalent bonding is a factor in the surface complex, and therefore, that partial dehydration of the metal ion must have occurred. Adsorption measurements showed that the site population accessible to Zn*+ and Co*+ represents less than 10% of the Ca*+ lattice sites that are estimated to reside on a crystalline monolayer. This site population is relatively constant with pH. Whether these sites are anionic groups in a hydrated CaC03 layer as proposed by DAVISet al. (1987) is not known and cannot be resolved with these data. Reaction sites have also been suggested to reside on kinks and steps located on corners, faces, and edges of crystals (PLJJMMERand WIGLEY, 1976; BERNERand MORSE, 1974), where Ca*+ and CO:- are only partially coordinated by lattice atoms. These same reaction sites must also control CaC03(.) precipitation and dissolution because adsorbed Zn and other ions (e.g., PO4 and Mg) markedly inhibit the process (MORSE, 1983; REDDY, 1977; SJ~BERG, 1978). Conditional equilibrium constants for the Zn exchange reaction decrease with surface loading, suggesting site heterogeneity and preferential ion siting. An empirical power exchange law was found to describe the rapid adsorption data over a wide range of solution conditions:

CONCLUSIONS Rapid zinc adsorption on calcite appears to occur via exchange of Zn*+ and ZnOH+ with surface-bound Ca”. The effects of pH and COrcp)are significant only in their influence on 1) Ca*+ activity through the solubility relationship with CaCO3(+ and 2) Zn hydrolysis and aqueous complexation. These results conflict with recent findings on Cd sorption by calcite (DAVIS et al., 1987), but agree with the sorption behavior of Co*+ (KORNICKER et al., 1985). No influence of

=

Oh2

=

(Zn2+

+

&OH+)

zd 1

1.69

(Ca)*+ K

I[

cg

.

An estimated site density based on a crystalline monolayer for the CaCOsu, was assumed. It is not known whether this function will apply to other calcites differing in surface microtopography, which is thought to control the amount and surface energy of adsorption sites.

2290

J. M. Zachara, J. A. K.&tick and J. B. Harsh

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