Environ. Sci. Technol. 1994, 28,692-700
Transformatlon of Carbon Tetrachloride by Pyrite in Aqueous Solution Michelle R. Kriegman-King+ and M. Reinhard'
Department of Civil Engineering, Stanford University, Stanford, California 94305-4020 The reactivity of C c 4 with pyrite was investigated by measuring the CC14 transformation rates and products under aerobic and anaerobic conditions. Under all reaction conditions, >90% of the CC14 was transformed within 1236 days in the presence of 1.2-1.4 m2/L pyrite at 25 "C. A zero-order dependence on the C C 4 concentration supports a surface-controlled reaction mechanism wherein the rate of reaction depends on the absorbed CC14 concentration. In aerobic systems >70% of the CC14was transformed to C02 whereas in a fresh-ground/anaerobic pyrite system approximately 50% of the CC14 was transformed to CHC13. X-ray photoelectron spectroscopy of the reacted pyrite showed that an FeOOH coating formed on the surface under aerobic conditions. Under all reaction conditions, the pyrite surface was depleted of ferrous iron.
CCl,
cs2-
do,
Figure 1. Known abiotic transformation products for CCI, formed under aerobic and anaerobic condltions (after refs 17 and 18). Pathway 4 occurs via reaction of the trichloromethyl radical with O2 (16).
Exp/Aer), 02-exposed pyrite reacted anaerobically (02Exp/An), 02-exposed pyrite reacted in the presence of sulfide (Oz-Exp/HS-), fresh-ground pyrite reacted anaerobically (Fresh/An), and acid-treated pyrite reacted anaerobically (Acid/An). These conditions (except Acid/ An) were chosen to simulate different geochemical sceIntroduction narios. Although pyrite is formed in anaerobic environIron sulfide materials are ubiquitous in sulfate-reducing ments, pyrite is often exposed to aerobic conditions upon environments (e.g., refs 1-5). The iron sulfide minerals weathering (e.g., ref 19). Under aerobic conditions, an pyrite and marcasite were shown to reduce CC14to CHC13, iron oxide coating will develop on the pyrite surface and but most of the C C 4products were not accounted for, and will inhibit the reactivity with 02 (20) and presumably the reaction mechanism was unknown (6). Disulfide other oxidants, such as Cc4. Additionally, 02 and its groups on the pyrite surface (S22-) have been the proposed reaction products may compete directly with CCld for electron donor in the oxidation of pyrite by different reaction sites. The extent of competition and inhibition oxidants (7-14). CC4 is proposed to accept an electron by 0 2 and the effect on the C C 4product distribution was from the S22- r* (anti-bonding) orbital. The electron is investigated. Oxygen-exposedpyrite may be re-introduced transferred from the ir* orbital on Sz2- to a u* (antiinto an anaerobic or sulfide-rich environment since sulfide bonding) orbital in CC4,which is the symmetry allowed is often present in plumes from hazardous waste sites and according to the frontier molecular orbital theory (14).To landfills (21). Sulfide may be able to regenerate the pyrite determine if the reaction is energetically favorable, the surface through reductive dissolution of the iron oxide energy of the lowest unoccupied molecular orbital (ELUMO) coating (22-24). Acid-treated pyrite was studied because acid treatment is commonly used in pyrite dissolution and of the oxidant has to be less than or within 6 eV of the energy of the highest occupied molecular orbital (EHOMO) oxidation research to remove high strain areas induced by grinding (25) and to obtain a reproducible surface (9-1 1, of the electron donor (14). For the case of C C 4and pyrite, 26,27). ELUMO= -0.28 eV (15) and EHOMO = -3.9 eV (14), Oxidation rates of pyrite by oxidants such as Fe3+, 02, respectively, indicating that the reaction is energetically favorable. and H202 have been measured under different reaction conditions (27-29). In these studies, the rates of pyrite The purpose of this study was to assess the reactivity oxidation were measured indirectly by either the disapof C C 4with pyrite (FeSz), specifically to (1)evaluate the pearance of the oxidant or the appearance of sulfate. This kinetics of the reaction of C C 4 with pyrite, (2) study the indirect method yields accurate results if the stoichiometry ability of CC14to oxidize pyrite in the presence and absence of the reaction under consideration is known. Assuming of 02 and HS-, (3) measure the effect of pretreating the that pyrite-S is oxidized to SO42- while the oxidation state FeS2 surface with 0 2 and acid, (4) determine the ccl4 of iron is unchanged, the oxidation of pyrite by 02 and transformation products, (5) investigate the effect of Fe3+ can be described by the overall reactions, eqs 1and reaction conditions on the product distribution, and (6) 2 , respectively (29). monitor the aqueous and surface oxidation products of pyrite. Since 0 2 is known to oxidize Sz2-groups (10,141 and to react with the trichloromethyl radical intermediate + H,0(1) Fe2+(aq)+ 2S04'-(aq) + FeS,(s) + 502(aq) 7 (161, both the rates and the products of C c 4 transfor2H+(aq) (1) mation are expected to be influenced by the presence of 0 2 . CC14transformation products that have been detected FeS2(s)+ 14Fe3+(aq)+ 8H20(l) in previous studies are summarized in Figure 1 (17, 18). The pyrite treatments and reaction conditions studied 15Fe2+(aq)+ 2SO;-(aq) + 16H+(aq) (2) were as follows: 02-exposed pyrite reacted aerobically ( 0 2 If the appearance of sod2-or the disappearance of Fe3+ * Corresponding author. is used to monitor the rate of pyrite oxidation and eqs 1 t Present address: Erler & Kalinowski, Inc., 1730 South Amphlett or 2 apply, then the pyrite oxidation rate equals Blvd., Suite 320, San Mateo, CA 94402.
-
-+
692
Envlron. Sci. Technol.. Vol. 28, No. 4, 1994
0013-936X/94/0928-0692$04.50/0
0 1994 American Chemical Society
One in 10 analyses of pyrite showed the presence of 1 atomic % Cu. Substrates, standards, and reagents were the same as those previously used (17) with the following exceptions: When using S04z-to monitor the pyrite oxidation rate, it anhydrous sulfate (Na2S04, >99 72 ) and sulfite (NazSOs, is assumed that intermediates do not accumulate appre>99.5%) were stored in a desiccator; 0.1 N thiosulfate ciably. The only aqueous sulfur intermediate detected in standard solution (Fluka Chemical Co., Buchs, Switzerthe oxidation of pyrite at circumneutral pH was S ~ 0 3 ~ - , land) was stored in the anaerobic glovebox. but its accumulation was negligible (IO). Filling and Sealing Ampules. Transformation studThe stoichiometry for the oxidation of pyrite by ccb ies in the presence of pyrite were conducted in 10-mL is unknown, but by analogy is assumed to be flame-sealed glass ampules that were acid-washed, ovendried, and placed in the anaerobic glovebox to outgas for FeS,(s) 14CC14(aq)+ 8H20(l) 2 days ( 1 7 ) . Pyrite (0.2 f 0.003 g) was weighed into the ampules. For all conditions, except the aerobic system, Fe2+(aq)+ 2SO:-(aq) + 14C13C'(aq) + a 1mM NaCl solution was deoxygenated with Nz for 1 h 14Cl-(aq) + 16H+(aq) (4) and placed in the glovebox. The NaCl solution was then sparged with the glovebox atmosphere for a minimum of where 14 mol of C C 4 react with 1 mol of pyrite to form 30 min. Sulfide solutions were made as per the method sulfate and the intermediate Cl3C'. Under anaerobic described in Kriegman-King and Reinhard (I 7). Ampules conditions with C c 4 as the only oxidant, pyrite43 may were filled with approximately 13.5 mL of either the NaCl not be fully oxidized to S042-; partially oxidized sulfur or sulfide solution that was filtered through a sterile 0.2compounds such as polysulfides or thiosulfate may be pm nylon filter (Nalgene Corp., Rochester, NY). Once relatively stable. This equation also assumes that C c 4 is the ampules were filled, they were spiked and sealed while reduced only to C13C*;whereas for formation of CHC13, taking precautions to maintain anaerobic conditions ( I 7, CO, or HCOOH, two electrons are required, resulting in 30). For the high C C 4concentration experiments (0.1-1 a stoichiometric coefficient of 7 rather than 14: thereby mM), a methanolic spike was used rather than an aqueous doubling the pyrite oxidation rate. By monitoring the spike. Sealed ampules were weighed and placed in the disappearance of CC4, we can use a stoichiometric dark in a 24.7 "C constant-temperature water bath ( f O . l coefficient of 14 to provide a low estimate of the pyrite "C) for the duration of the experiment. oxidation rate, thus enabling us to compare the reactivity For ampules to be reacted aerobically, the 1mM NaCl of pyrite with C c 4 to other oxidants. solution was sparged with laboratory air for 45 min to strip any residual CHC13 from the Milli-Q water. PyriteExperimental Methods filled ampules were removed from the glovebox and filled with approximately 13.5mL of a 1mM NaCl solution that Transformation Studies, For all CCl, transformation was also filtered. Ampules were spiked with C C 4 and rate and product studies, 1pM C C 4was reacted in aqueous immediately flame-sealed. systems containing 1.2-1.4 m2/L pretreated pyrite at pH For all conditions, ampules were shaken manually once 6.5 and 25 "C, except the experiments conducted with sulfide, which were at pH 7.75. Since a pH buffer was not daily for the first 10 days of the reaction and then every used because of the potential confounding effects on C c 4 other day until the experiment was complete. At each reactivity, experiments were conducted in a 1 mM NaCl sampling time, two ampules for each experimental conionic medium. Controls were established by reacting C c 4 dition were removed from the constant-temperature water in homogeneous solutions of NaCl, or HS-. To bath, centrifuged at 4 "C and 2800g for 20 min, and stored study the pyrite oxidation products due to both pyrite at 4 "C for amaximum of 3 h until preparation for analysis. pretreatments and reaction with ccl4, large particles of Chemical Analyses. cc/4and CCl4Product Analysis. pretreated pyrite (0.2 g) were reacted with and without Extraction procedures and chemical analyses for CC4, 0.1-1 mM CC4. CHC13, CS2, CO, and 14C-radiolabeledproducts (nonvolMinerals, Pretreatments, and Reagents. Pyrite from atiles, volatiles, and COz) were the same as those described Zacatecas, Mexico (Ward's ScientificEstablishment, Inc., in Kriegman-King and Reinhard (17). An ion chromatoRochester, NY), was ground with a ceramic mortar and graph (Dionex Series 4000i) equipped with a conductivity pestle, sieved to 75-300 pm, sonicated in deoxygenated detector and a Dionex AS-4A separator column was used Milli-Q water, and air-dried; all in a 90% Nz/lO% H2to measure formate (5 mM borate eluent, 2.0 mL/min flow atmosphere glovebox (Coy Products, Ann Arbor, MI). rate). The detection limit for formate was approximately Batches of sonicated pyrite (20 g) were washed twice for 1pM. Formate was identified by matching retention times 2 rnin in 20 mL of 1N HC1, rinsed 10 times with 40-mL and was assumed to equal the aqueous nonvolatile fraction aliquots of deoxygenated Milli-Q water, and dried in the quantified from [l4C1CC14 studies. glovebox atmosphere. Additional batches of sonicated Adsorbed formate and COZ were obtained using a pyrite was oxidizedby exposure to air for 3 days and placed presumptive method. The supernatant was removed from back in the glovebox. the ampules using a syringe, 0.4 mL of 1M HzSO4 or 1M Single-point BET specific surface areas of fresh-ground, NaOH was added to the ampules, and the ampules were acid-treated, and 02-exposed pyrite determined using a allowed to sit for 10 min. A total of 1 mL of water was Micrometrics Flowsorb I1 2300 system with 30% N2/70% added to the ampules and mixed with the slurry. After He gas were 0.10, 0.11, and 0.083 m2/g,respectively. The letting the pyrite settle for 10 min, a l-mL aliquot of the chemical composition of the near-surface of fresh-cleaved aqueous phase was placed into a scintillation vial, purged pyrite was measured using X-ray photoelectron spectroswith Nz for 15 min, and counted on a Packard Tricarb copy (XPS). The S:Fe ratio was determined to be 2.1. Model 4530 liquid scintillation counter. The strong acidic d[FeS,I = -1d[SO:-l = - 1d[Fe3+1 (3) rate= - dt 2 dt 14 dt
+
-
Environ. Sci. Technol., Vol. 28, No. 4, 1994
693
and basic conditions presumably desorbed both Conand formate. The pHpzc of pyrite has been reported to be 1.2 (311. By purging the system with Nz, the COZwas stripped from solution at acid pH leaving the "adsorbed" formate in solution, whereas the basic solution retained both the "adsorbed" COz and formate. The adsorbed volatile fraction could not be measured with this method. Aqueous Pyrite Oxidation Products. For analysis of so?, S Z O ~and ~ - , S O S ~centrifuged , ampules were cracked open in the anaerobic glovebox to prevent the oxidation of SO? and S Z O ~ ~A- 0.5-mL . aliquot of supernatant was placed in a 10-mL volumetric flask and diluted to 10 mL with deoxygenated Milli-Qwater. A portion of the diluted sample was then transferred to a 2.9-mL glass vial to fill the vial with no headspace and sealed with a PTFE/silicone septum-lined cap. The sample vials were stored in the glovebox up to 6 h and were removed from the glovebox immediately before analysis. Standards at three calibration levels were made in the glovebox with deoxygenated Milli-Q water and treated identically to the unknowns. Samples were withdrawn from the vials with a 2-mL gastight syringe that had been filled with Nz and analyzed on a Dionex Series 4000i ion chromatograph equipped with a conductivity detector and a Dionex AS-5 separator column (4.5 mM NazC03/2.0 mM NaOH eluent, 1.5 mL/ min flow rate). Surface Pyrite Oxidation Products. Ampules were cracked open in the glovebox, and the supernatant was removed. Using tweezers and a microbiological loop, a large particle of pyrite was removed from the ampule, dipped in deoxygenated Milli-Q water, and adhered to an XPS sample holder using double-stick tape or silver paint. Samples were allowed to dry overnight in a desiccator in the glovebox. Within the glovebox, the desiccator was transferred to a glovebag and sealed with low-oxygen permeable tape (Coy Products, Ann Arbor, MI). The sample holder was transferred into the XPS instrument by sealing the glovebag opening around the transfer chamber of the instrument and flushing the glovebag with nitrogen. The desiccator was opened and the sample holder was placed in the transfer chamber. XPS analysis was conducted using a Surface Science S-Probe equipped with a monochromatic A1-Ka x-ray source. A spot-size of 150 X 800 wm was used with a pass energy of 150 eV for broad scans and 50 eV for narrow scans. Normalized concentrations were calculated for the peak of interest by dividing the peak area by a sensitivity factor comprising the Scofield cross-section (32) and the relative kinetic energy of the photoelectrons (12). Results and Discussion Kinetics of the Reaction of CC14 with Pyrite. In order to establish the reaction kinetics for the disappearance of C C 4with 1.2 mZ/L pyrite, the data were fit to both a first- and zero-order reaction model. Rate constants for the first- and zero-order models were obtained from the slopes of semilogarithmic plots of [CC141/ [CC1410versus time and linear plots of [CCl~l/[CCl~lo versus time, respectively. An example of the results of the kinetic model fits for the OS-Exp/Aer and Acid/An experiments are depicted in Figure 2. The data show that the zero-order kinetic model describes the experimental data much more accurately than the first-order model with respect to both the overall fit of the data and the predicted y-intercept. 604
Environ. Sci. Technol., Vol. 28, No. 4, 1994
h
..
02-Exp/Aer Data 02-Exp/Aer Zeroth-Order Fit: 02-Exp/Aer
--. First-Order Fit:
1'4 1.2
-
A AcidAnData
--. First-Order Fit: AcidAn
- Zeroth-Order Fit:
0
10
30
20
AcidAn
40
50
Time (day) Figure 2. First- and zero-order kinetic model fits for data from 1 ~ L M CCI, In O,-Exp/Aer and Acld/An at 25 OC. First-order models are represented as dashed lines; zero-order models are represented as solid lines.
Table 1. Comparison between First- and Zero-Order Fits of CCl4 Transformation Data with 1.2-1.4 m2/L pyrite Reacted under Aerobic and Anerobic Conditions at 25 OC
pyrite conditions 02-ExpIAer Oz-Exp/HS02-ExpIAn Fresh/An Acid/An
first-order model slope (d-9 R2 0.081 0.085 0.24 0.21 0.27
zero-order model slope (d-1) R2
0.73 0.70 0.66 0.49 0.80
0.025 0.031 0.057 0.056 0.082
0.93 0.87 0.85 0.65 0.96
Table 2. Zero-Order Rate Constants for CCl4 Transformation with 1.2-1.4 m2/L Pyrite Reached under Aerobic and Anaerobic Conditions at 25 OC
pyrite conditions
slope (d-1)
R2
(mol m-2 d-9
kL,
95% confidence interval
02-ExpIAer 02-ExplHS02-Exp/An Fresh/An AcidIAn
0.025 0.031 0.057 0.056 0.082
0.93 0.87 0.85 0.65 0.96
0.021 0.026 0.047 0.039 0.053
0.017-0.026 0.020-0.032 0.035-0.049 0.022-0.056 0.046-0.060
The slopes and coefficients of determination (R2) for the first- and zero-order models for all experimental conditions are summarized in Table 1. The R2improves significantly when the data are modeled with zero-order kinetics. Zeroorder rate constants and 95 % confidence intervals for the disappearance of CC14 (k;,) are shown in Table 2. The rate constants were normalized by the pyrite surface concentration, assuming the reaction was first-order with respect to the surface concentration, SC (29,33),according to the equation d[CTETl = -k;,,4[SCl dt
(5)
where SC is the product of the specific surface area of the solid (rnz/g) and the solids loading (g/L). A zero-order dependence on oxidant concentration is expected when a heterogeneous reaction is surface chemically controlled rather than diffusion controlled (9). In other words, the rate-limiting step of the reaction is dependent on the concentration of CCl, at the pyrite surface rather than the C c 4 concentration in solution.
Zero-order reaction kinetics are expected when surface sites are saturated with CC4. When the surface sites are no longer saturated with CC14 (e.g., at low C c 4 concentrations), one would expect the reaction order to become first order with respect to the C C l ~concentration in solution. This transition from zero- to first-order kinetics was not observed in our experimental systems, suggesting that it occurs below the detection limit of the gas chromatograph (0.03 pM). At 0.03 pM CC14 in solution and 1.2 m2/L pyrite, the ratio of CC14molecules to pyrite surface is calculated to be 15 000 molecules of CC14/nm2 of pyrite. Sincethe size of aCC4 molecule is approximately 0.4 nm2 (34),it is not surprising that the transition from zero- to first-order kinetics was not observed in these systems. In systems containing a higher SC, this kinetic transition may presumably be evaluated. The reaction of CC4 with pyrite further supports a heterogeneous reaction mechanism when the reaction rate in pyrite systems is compared with the reaction rate in similar homogeneous systems. In Acid/An, >90% of 1 pM C C 4 was transformed in 12 days at 25 "C, whereas half-lives in homogeneous solution are 1400 days with 1 mM HS-at 25 "C and 105 days with 0.1 mM Fe2+(aq)at 50 "C (34). Assuming an activation energy of 60-120 kJ/ mol (17) for the reaction with Fe2+(aq),the half-life of CC14 with 0.1 mM Fe2+(aq)at 25 "C ranges from 700 to 4500 days. The aqueous Fe2+and HS- concentrations in suspensions of pyrite in deoxygenated Milli-Qwater have recently been measured to be approximately lo4 M, in a 1:2 ratio, respectively (L. Ronngren and S. Sjoberg, University of Umeb, Sweden, personal communication, 1993), suggesting that the solubility product of pyrite is higher than previouslyreported. The CC4 transformation rates that were measured in the Fez+ and HS- aqueous systems were thus at concentrations that would be expected in the pyrite systems. The rate data thus support a surface-controlled reaction mechanism for the transformation of C c 4 by pyrite because (1) zero-order kinetics were observed and (2) the reaction with pyrite was much faster than in homogeneous solutions. Over longer time periods however, the reaction may become diffusion controlled when an iron oxide coating forms on the pyrite and the oxidant and products have to diffuse through the oxide coating (9,20). Effect of Pyrite Pretreatment on CCl4 Transformation Rate. The data in Table 2 show that CC14 reacts the fastest with the acid-treated pyrite (Acid/An), although Oz-Exp/An is not statistically slower. As expected, the slowest transformation rate was observed under aerobic conditions (OZ-Exp/Aer). However,the rate constant was only 2.5 times slower than Acid/An. The large error associated with Fresh/An precludes detailed comparison with other rate constants. Increases in surface energy induced by grinding (25)may be responsible for the scatter in Fresh/An. This heterogeneity appears to have been removed during pretreatment by 0 2 or acid. The rate constants are compared to rates of pyrite oxidation by 02,Fe3+, and HzOz in Table 3. In order to compare pyrite oxidation rates by C C 4 to rates by other oxidants, it is assumed that CC4 oxidizes pyrite4 to S042-. Using eq 4, the pyrite oxidation rate is 1/14 the C C 4 disappearance rate. When the pyrite oxidation rate is calculated from the disappearance rate of CC4,competing reactions by other oxidants such as 0 2 are not accounted for. Although solution conditions and pretreatments
Table 3. Comparison of Pyrite Oxidation Rate by CClr with Literature Rates of Oxidation by 02, Feat, and HzOz at Room Temperature
pretreatment this work
02-exposure 02-exposure mildHClwash Moses and boiling HCl wash Herman (26) boiling HCI wash boiling HCl wash McKibben and mild HNO, wash Barnes (24) mild HNOs wash mild HNOs wash Wiersma and none Rimstidt (25)
pH
oxidant
oxidation rate (nmol m-2 8-1)
6.5 CC4,02 6.5 cc4
15'** 450
6.0 6.0 6.0 2.0 2.0 2.0 2.0
10 1 0.5
6.5
CC4 Fe(II1)
02 02, Fe(II1)
7w
Fe(1II)c
5
0zc
500
HzOf Fe(III), 02
2x108
10
a Assumes a stoichiometry of 14 mol of CC4 reduced to ClsC- for 1mol of pyrite oxidized to SO4". These numbers represent a lower
limit for the pyrite oxidation rate. b Represents pyrite oxidation rate due to CC4, not the total rate due to both 02 and CC4. Did not observe zero-order dependence for oxidants. Used 0.3 mM oxidant concentration to calculate rate.
differ, it is clear that C C 4 reacts with pyrite as fast or faster than O2 and Fe3+ (Table 3). In environmental situations where C C 4 will likely be present with 02 and Fe3+, pyrite oxidation by C C 4 will not 'be necessarily inhibited or outcompeted by 0 2 and Fe3+. Hydrogen peroxide reacts with pyrite orders of magnitude faster than CC4, 0 2 , or Fe3+. However, if an oxide coating develops with time, the reactivity of the pyrite surface toward cc4, or any oxidant, will eventually become diffusion controlled (20). The rate data from 02-Exp/HS- show that treatment of an oxidized pyrite surface with HS-does not restore the reactivity of pyrite. Rather, sulfide appears to inhibit the transformation of C c 4 by pyrite even relative to Oz-Exp/ An. At pH 7.75,85%of the sulfide is present as HS- and more than 100 pM is present as HzS. Since reaction sites on pyrite are saturated with CC4 when [CC141= 1 kM and [HzS] is 100 times more concentrated than CC4, it is conceivable that H2S blocks cc14 reaction sites. Characterization of the pyrite surface chemistry is necessary to understand the interaction of sulfide species with the pyrite surface. CC14 Transformation Products. As shown in Table 4, the C c 4 product distribution varies greatly depending on the reaction conditions even though k0cc4only varies by a factor of 2.5. Under aerobic conditions (02-Exp/ Aer), the major product was COz (60-70%, including adsorbed Cod. Adding the C02 formed by hydrolysis of CS2 (pathway 3, Figure 1; refs 17 and 351, COZaccounts for 70-80% of the ccl4 transformed. In contrast, the Fresh/An system forms approximately 50 % CHCl3 and ultimately only 10-20% Con. Interestingly, some CS2 was formed in all systems, suggesting that CC4 or its reactive intermediates must react with Sz2-sites on pyrite, even in the presence of 02. The volatile fraction measured from scintillation counting agreed with the volatiles measured by gas chromatography (CC4, CHCS, and CSZ),indicating that CO (pathway 1,Figure 1) and CzC4 (pathway6, Figure 1) are not formed in these systems. A loss in total radioactivity from solution as a function of time suggests that a fraction of C c 4 or some of its transformation products are adsorbed to pyrite. Assuming the method to measure adsorbed C02 and HCOOH is valid, it is likely that the missing fraction is a compound that is volatile Envlron. Sci. Technol., Vol. 28, No. 4, 1994 695
Table 4. CCl4 Product Distribution from Reaction with Pyrite under Aerobic and Anaerobic Conditions at 25 O C
condition (time)“
CHClS ( % )
Oz-Exp/Aer (42 d)
5-6
0-1
formate* (%)
CSZ (%) 11-15
52-59
2
Oz-Exp/HS- (31 d) 02-Exp/An (20 d)
0-10 0-1
21-22 28-30
NMe 0-3
NM 26-30
NM 7-9
Fresh/An (13 d) AcidiAn (13 d)
1 6-10
48 20-21
2
10
19-20
17
5 4
mass balanced
adsorbedC(%
(%)
84-87
10
(2 % NV, 8% COz) NM
NM 82-84
12
(7% NV, 5% COz) 78 78
12f
9
(2 % NV, 7 % C02) Reaction time in days is in the parentheses. Formate was not directly measured in these experiments. The formate concentration was assumed to equal to nonvolatile concentration measured using 14C analysis. Adsorbed amount does not account for volatile compounds adsorbed. Mass balance of aqueous volatile compounds was obtained in all cases. Total radioactivity in solution + adsorbed nonvolatile and C02 fractions are equal to the mass balance within 5 % . The missing fraction is likely to be adsorbed volatiles. e NM = not measured. Breakdown of adsorbed products not measured.
02-Exposed Pyrite Reacted Aerobically (Os-ExplAer). Using the product data as a function of time, the rate constants for the formation of the products illustrated in Figure 1can be estimated. Because the disappearance of C c 4 was zero order, the appearance of products is also assumed to be zero order except when secondary products are formed that require more complexrate laws. The rate constants for the appearance of CHC13 and HCOOH (kocHclsand ~ O N V , respectively) were calculated assuming azero-order rate law (Table 5). Under aerobic conditions, COZcan form via pathways 3 (17)or 4 (16). If any COz is formed by pathway 3, the appearance of CO;! cannot be modeled by a zero-order equation. Assuming COz is formed only by the CSz pathway (31, eqs 6a and 6b can be used to solve for [CSzI and [COzl as a function of time
CC14
HCOOH
s
h
1.0 31
w
. u,
0.8
-
e
0.6
-
0.4
-
0.2
-
-
R o
n
e
*
.C
52
3
u
*- ,
0.0 0
7
5
i
8 I
I
10
15
20
Time (day) Figure 3. Disappearanceof 100 pM CCi4and measured appearance of formate with 1.2 m2/L pyrite reacted anaerobically at 25 ‘C.
when desorbed and is not detected with our experimental methods. In an experiment conducted anaerobically with 100 pM CCl4 and 1.2 m2/L pyrite, the appearance of formate was monitored. As shown in Figure 3, after the disappearance of 95% of the CC4, approximately 4-6 pM formate was detected, accounting for 4-6% of the initial C c 4 concentration. For comparison, in anaerobic experiments using 14C-labeledCC4, approximately 5% of the ccl4 is transformed to anonvolatile product (Table 4). The direct measurement of formate by ion chromatography and its agreement with the fraction of nonvolatile products measured by scintillation counting agrees with the above assumption that the nonvolatile compound is formate.
where kOcszis a zero-order rate constant for the appearance of CSz, and k’co, is a first-order rate constant for the formation of COz. Using SYSTAT (SYSTAT, Inc., Evanston, IL), the CSz data were fit with the equation (7)
to solve for the rate constants, kocsz and k‘coz (Table 5). These constants were then substituted into the equation
Table 5. Rate Constants for Disappearance of CCl4 and Appearance of Intermediates and Products from Reaction with Pyrite under Aerobic and Anaerobic Conditions at 25 O C
rate constanta k&, (mol m-2 d-l) k! (mol m-2 d-9 k& (mol m-2 d-l) k& (Lm-2 d-9 kboa (Lm-2 d-l) kEHCI,(mol m-2 d-l) kiv (mol m-2 d-l)
air-exposed pyrite reacted aerobically no intermediateb intermediateC (R2,dj = 0.85) (Rzadj = 0.78)d 0.021
0.021 0.020
0.00092 0.00040
- - cs2 - coz.
0.053
0.023
0.012 0.012 0.00092 0.00040
0.12 0.012 0.0027
ko = zero-order rate constant; k’ = first-order rate constant. For complete definition of symbols, see text. CC4 Int R2,dj accounts for the number of fitting parameters.
6g6
Environ. Sci. Technoi., Vol. 28, No. 4, 1994
0.053
0.022
0.0078 0.040
acid-treated pyrite reacted anaerobically no intermediate* intermediatec (RZadj = 0.85) (R2,dj = 0.85)
7.7 0.12 0.012 0.0027
- CSz
COz. CCl,
1 . 7
""D"
h
s
cs2
""D"
1.0
cs2
-t
E
u
0.8
e
.-*
0.6
$
0.4
ua
0.2
c 8
-'-'. Unknown
Int.
"
,' A A
*'
0.0 0
10
20
30
40
0
50
10
- -
for the formation of COZ (the integrated form of eq 6b) to graphically verify the fit of the data. As shown in Figure 4,the dashed line predicts much less COZproduction than was measured, suggesting that C02 is also formed via pathway 4. However, the curve for the appearance of CS2 also does not fit the data well (R2,dj = 0.78). There is a time lag before [CSd starts to increase, suggesting the formation of a relatively stable intermediate in the path to form CSZ. If it is hypothesized that a stable intermediate is formed, then the appearance of CSZcan be modeled using the equations
-d[CS21- k'cs,[Intermedl dt
- k'co,[CS21
40
50
Time (day)
Time (day) Flgure 4. Disappearance of CCI4in Oz-Exp/Aerat 25 OC. Appearance of the products, CSz and C02, with model results assuming the only path to form C02 is CCI4 CS2 COz. The rest of the COP may be formlng via reactlon of CCIB' with 02.
d[Intermedl dt = ko,- k'cs,[Intermedl
30
20
Figure5. Disappearance of CCi4inOn-Exp/Aer at 25 OC. Appearance of the products, CS2 and COP, with model results assuming the only path to form COPfrom CCI4 is CCI4 Intermed. CS2 Con. A mass balance is obtained with the inclusion of the intermediate.
-
- -
I .L
s
h
1.o 0.8
0.6 0.4 0.2
0.0
5
0
10
15
Time (day)
(8) (9)
where k o is ~ the zero-order rate constant for the formation of the intermediate. In eqs 8 and 9, the rate constant for the appearance of CSZ(k'cs,) is now assumed to be first order. Using the solution to eq 8, which is of the same form as eq 7, eq. 9 can be solved to give the CS2 concentration
The curve-fitting results obtained from eq 10 are shown in Table 5 and Figure 5. There is an improvement in the CSZfit (Rzadj = 0.85) and the COOdata are also fit quite well. The curve predicts formation of 5-10% more COZ than was measured in solution, which might correspond to the 8 % COz adsorbed (Table 4). The appearance of the hypothetical intermediate is also included in Figure 5. The model predicts that the intermediate attains a steadystate concentration of approximately 15 % which agrees with the missing mass balance (Table 4). Acid-Treated Pyrite Reacted Anaerobically (AcidlAn). A similar fitting analysis was conducted on the results from AcidIAn. In this case, no significant difference in the CS2fit was observed if the appearance of a hypothetical
Figure 6. Disappearanceof CCI4 In Acld/An at 25 'C. Appearance of the products, CS2 and COP, with model results assuming the only CS2 --t CO2. path to form COZ is CCi4
-
intermediate was included. As shown in Table 5, the rate constant for the disappearance of the intermediate (k'cs2) is very large, indicating that the intermediate is shortlived. The rate constant k01 is thus approximately equal to the zero-order rate constant for the appearance of CS2 (kocs2). In Figure 6 the predicted COZ concentrations deviate significantly from the measured ones. Although the overpredicted COZconcentrations again likely correspond to adsorbed COZ(Table 41, the measured COZdata do not have a lag; COz appears to form independently of CSz hydrolysis through a yet unidentified pathway. Pyrite Oxidation Products. Aqueous Sulfur Oxidation Products. The presence and the absence of C C 4 did not affect the aqueous sulfate concentrations (data not shown). In an experiment in which pyrite was reacted anaerobically with 100 r M CC4, the consistent formation of 2-4 pM 5 ~ 0 3 was ~ - observed. The control, reacted in the absence of CC4,contained only a sporadic appearance of S ~ 0 3 at ~ -
