Environ. Sci. Technol. lQ83, 17, 513-518
Effect of Solids Concentration on the Sorptive Partitioning of Hydrophobic Pollutants in Aquatic Systems Thomas C. Voice"
Department of Civil Engineering and Great Lakes Research Division, The University of Michigan, Ann Arbor, Michigan 48109 Clifford P. Rice
Great Lakes Research Division, The University of Michigan, Ann Arbor, Michigan 48109 Walter J. Weber, Jr.
Department of Civil Engineering, The University of Michigan, Ann Arbor, Michigan 48109 Partition coefficients are commonly used to quantify the distribution of organic pollutants between the aqueous and particulate phases of natural aquatic systems. The magnitude of the partition coefficient for a specific pollutant in a particular system has been shown to be related to the octanol-water partition coefficient of the pollutant and the organic carbon content of the solid phase onto which sorption occurs. The present work demonstrates that a third variable is important-the concentration of the sorbing solid phase in the system. Laboratory partitioning experiments were performed over a range of solids concentrations by using three different Lake Michigan sediment samples as sorbents and four hydrophobic priority pollutants (chlorobenzene, naphthalene, 2,5,2'-trichlorobiphenyl, and 2,4,5,2',4',5'-hexachlorobiphenyl) as sorbates. The results indicate a significant increase in partitioning as solids concentration decreases. A predictive relationship is developed, and potential mechanistic explanations are explored.
Introduction Although it is presently beyond our ability to produce rigorous a priori assessments of the environmental behavior of specific organic pollutants, significant progress toward this goal has been made in recent years. Several investigators (1-6) have developed or improved mathematical models designed to quantify the accumulation or movement of chemical species in both idealized and actual environmental systems. Central to all such modeling efforts are the segmentation of the system of interest into reasonably homogeneous compartments (e.g., air, water, sediments, biota) and a description of pertinent intercompartmental transfer reactions (e.g., volatilization, sorption, biological accumulation, and depuration) and transformation reactions (e.g., hydrolysis, photolysis, biodegradation). These are usually accomplished by constructing a system of coupled differential equations that describe the parametric character of such reactions and the rates at which they occur within and among each of the compartments. It is generally acknowledged that the fundamental limitations to state-of-the-art predictability of trace organic contaminant behavior in the environment lie not in the area of macrosystem model development but rather in the accurate mathematical description of the associated physical, chemical, and biological transport and transformation reactions. The present work focuses on one such intercompartmental transfer reaction-the partitioning of pollutants between water and solid particles suspended in solution. *To whom correspondence should be addressed at Versar InGPO Box 1549, Springfield, VA 22151. 0013-936X/83/0917-0513$01.50/0
In aquatic systems, this partitioning is one of the most crucial reactions because it influences the further transfer of the compound to the biotic compartment and ultimately to humans. The study was concerned with organic compounds classed as hydrophobic (arbitrarily defined herein as compounds with an octanol-water partition coefficient greater than 100) as these pollutants are likely to sorb extensively and their environmental behavior is thus likely to be strongly dependent upon sorption phenomena.
Background The theory and experimental evidence relating to the sorption of hydrophobic compounds by sediments and suspended solids has been reviewed in a previous publication (7) and will not be repeated here. It can be concluded generally that the extent of such reactions is proportional to the octanol-water partition coefficient of the solute and the organic carbon content of the sorbent. Karickhoff et al. (8) developed the predictive relationship log KO, log KO,- 0.21
(1)
where KO, is the organic carbon normalized partition coefficient
KO, = Kp/foc
(2)
KO,is the octanol-water partition coefficient, and f , is the fraction of the solids, by weight, comprised of organic carbon. Estimates for Kp,the mass-normalized partition coefficient that is commonly used, were reported to be within a factor of 2-3 of measured values. While the concept of linear partitioning is assumed to be valid only at low solute concentrations (9, the use of partition coefficients has remained largely unchallenged because most organic pollutants of concern exist in the environment at concentrations far below their solubility. Recently, however, the concept of linear partitioning has come into question, although not as a result of solute concentration induced nonlinearities. On the basis of the work of Connolly (9) with Kepone and a review of the literature covering studies involving a number of organic and inorganic solutes and sorbents including sediments, soils, clays, and digested sewage sludge, O'Connor and Connolly (10) reported that linear partition coefficients are inversely dependent upon the concentration of solids in the system. Partition coefficients were observed to increase as much as an order of magnitude for every order of magnitude decrease in solids concentration. In light of these reports, we explored and found additional evidence of a dependence of partitioning on solids concentration. The data of Rice and Sikka (11)on DDT uptake by algae was reevaluated to produce partition coefficients (as opposed to percent uptake) and plotted as
0 1983 American Chemical Society
Environ. Sci. Technol., Vol. 17, No. 9, 1983 513
Tab12 I. Experimental Materials organic
log KO,
solids
c, %
chlorobenzene (MCB)
2.8
2.9
naphthalene (NAP)
3.4
offshore Grand Haven (OGH) nearshore Grand Haven (NGH) Benton Harbor (BH)
solutes
0.0
20.0
40.0
60.0
80.0
100.0
120.0
CELL DENSITY (pGG/Ll
Flgure 1. Sorption of DDT by SIX species of algae: effect of cell density (after Rice and Sikka (77)).
a function of cell density in Figure 1. The observed exponential dependence is similar to the nearly log-linear results of O'Connor and Connolly (10). Weber et al. (12), noting that sorption isotherms are frequently measured by using variable concentrations of sorbent, found a similar effect for Aroclor 1254 and a wide range of solids typical of freshwater aquatic systems. I t was further noted by these authors that this behavior is exhibited by any isotherm data obtained by the variable solids technique and that when fitted by the Freundlich model qe = KFCe1In
(3)
(qe = solid phase concentration, C, = liquid phase concentration, KFand l / n are characteristic constants) yield an exponent l / n greater than 1. The common usage of this experimental technique and the large number of studies reporting Freundlich exponents greater than 1 suggest that the dependence of partitioning on solids concentration may have been an important but unrecognized effect in countless sorption studies.
Objectives This study was initiated to quantitatively assess the effect of solids concentration on the partitioning of hydrophobic compounds. The study was oriented toward pollutant behavior in the Laurentian Great Lakes. Four solutes of concern in this system and three sediments from Lake Michigan were selected for study. Table I lists these materials along with the octanol-water partition coefficients of the solutes and the organic carbon contents of the sorbents. This range of solute and sorbent properties was selected to allow the development of a model similar to that of Karickhoff et al. (8)but incorporating the solids effect. Finally, this study attempts to conduct the necessary experimental tests at levels as close as possible to those found in the Great Lakes to avoid errors resulting from extrapolation of results outside the development data set. Analytical objectives were thus defined as 1 ng/L solute concentration and 1 mg/L solids concentration. Methods Sorption isotherms were conducted on various combinations of solids and solutes. All solutes were purchased as 14C-labeledcompounds, allowing quantification well below the 1ng/L goal, using liquid scintillation counting. Octanol-water partition coefficients were determined by a modification of the technique described by Karickhoff and Brown (14). The solids were collected from the near-surface sediments of eastern Lake Michigan with a dredge sampler and were washed, dried, and sieved to remove particles larger 514
Environ. Sci. Technol., Vol. 17, No. 9, 1983
2,5,2'-trichlorobiphenyl 5.4 (TCB) 2,4,5,2' ,4', 5' 6.7a hexachlorobiphenyl (HCB) a As reported in Chiou e t al. ( 1 3 ) .
3.4 3.8
than 60 pm. Karickhoff et al. (8) found that larger sediment particles generally contain little organic carbon, have little sorptive capacity, and can be treated essentially as a diluent which serves to reduce the overall organic carbon content of a bulk sample, as compared to that of the fines alone. The organic carbon contents shown in Table I were measured by the persulfate oxidation technique. Solids concentrations as low as 10 mg/L could be adequately recovered from solution, thus defining a practical lower limit. An upper limit of 400 mg/L was selected on the basis of acceptable counting efficiency when measuring the amount of sorbed compound. Each isotherm was conducted according to the following procedure. A constant mass of sediment was weighed into each of seven 50-mL Corex or Pyrex centrifuge bottles. Different volumes of a stock solution of concentrated solute in acetone were added to each bottle by using a microliter syringe. This quantity was varied so as to produce a range of aqueous equilibrium concentrations as low as a few nanograms per liter and within the range of concentrations where adsorption is linear, in all cases below 1pg/L. After the solvent had evaporated under a fume hood, 50 mL of distilled water was added to each, and the bottle was sealed and placed on a reciprocating shaker. An eighth bottle, containing sediment but no solute, was used in each set as a blank. After 24 h of equilibration time, the bottles were removed and centrifuged at 2000 rpm (RCF = 879 g) for 2 h. Each bottle was then carefully transferred to a secure clamp to prevent disturbing the solids, and the water was slowly removed by pipet. The water and a solvent rinse of the pipet were transferred to a serum bottle containing 20 mL of petroleum ether. The bottle was sealed with a Teflon liner and an aluminum crimp-cap and was placed on the shaker for 2 h. After this time the mixture was placed in a separatory funnel and allowed to stand for approximately 2 h to achieve phase separation. The water (lower phase) was discarded, and the solvent was added to scintillation vials containing aqueous counting scintillant (ACS). Alternatively, the water phase from the centrifuge bottle was at times counted directly, although the limit of detection was found to be somewhat higher. The solids and few milliliters of water left in the centrifuge bottles were rinsed with distilled water into a filter apparatus containing a Gelman type AE glass fiber filter. The retained solids and filter were added to a scintillation vial containing ACS. Finally, the centrifuge tube was rinsed twice with petroleum ether to extract all solute that may have been adsorbed to the walls of the vessel. This solvent was also added to vials for counting. The scintillation vials from the three phases-water, solids, and centrifuge tubes-were counted on a Beckman Model LS-7500 scintillation counter. The total number
3.. ..
c
6
-% ' .
H
u2..
HCB
b.bs.
H
+
NGH
HCB + OGH TCB + BH
4
i
TCB
i OGH
*\
70.0
Flgure 2. Sorption of 2,4,5,2',4',5'-hexachiorobiphenyl by Lake Michigan sediment (OGH).
'a
NGH
TCE
llAP + CGH
MCB +
OGH
"
;'I
-
Q
0
"
n
L?, Q
" %O
2.00
3.00
4.00
Table 11. Effect of Solids Concentration for Various Solute/Sorbent Combinations no. corr of isointercoeff therms ceptb solute sorbent slopea
Ui' 00
Z
1.00
Figure 4. Dependence of partition coefficients on solids concentration for four solutes and three Lake Michigan sediments.
.'
u
9.M
LOG SOLIOS CONCENTRRTION (MG/Ll
51 H LL W
8
,
.
20.0
.
.
40.0
,
.
60.0
,
,
80.0
,
100.0
120.0
140.0
160.0
180.0
HCB
200.0
SOLIDS CONCENTRATION IMG/L)
Figure 3. Dependence of the partition coefficient on solids concentration for 2,4,5,2',4',5'-hexachlorobiphenyl and Lake Michigan sediment (OGH).
of counts from each phase (which may have been contained in several vials) was converted to mass via the specific activity of the compound. As all three phases which contain significant quantities of compound were quantified, a complete mass balance was possible. Recoveries, based on a direct spike of stock solution into ACS, were found to range from 80% to 100% It is suspected that losses can be attributed to volatilization and incomplete recovery from the solid phase. Both concentrations used to determine K , were measured by using this technique, whereas methods frequently described in the literature measure one concentration and calculate the other by difference. Control studies determined that the quantity of solute lost in evaporation, extraction, and filtration operations was below the limits of detection. Additional studies using an equilibration time of 4 h indicated no changes in partition coefficients. This suggests that equilibrium is in fact reached and is consistent with the rapid kinetics observed by Weber et al. (12) and others for hydrophobic compounds. Centrifugation at 4000 and 5000 rpm was attempted by using the Corex bottles, and no changes in partition coefficients were observed. Attempts to use stainless steel centrifuge tubes capable of withstahding much higher speeds were unsuccessful, as nearly all of the solute added was found adsorbed to the container walls.
Results and Discussion Figure 2 shows the results of a single isotherm experiment conducted at constant solids concentration and is typical of the results obtained throughout the study. Reasonable linearity over the range of concentration can be observed, and the near zero intercept of a linear regression of the data supports the validity of a linear partitioning model. The partition coefficient can be determined from the slope of the linear fit, with appropriate
TCB NAP MCB
OGH NGH BH OGH NGH BH OGH BH OGH BH
-0.59 -0.39 -0.15 -0.39 -0.56 -0.14 -0.57 - 0.48 -0.86 -0.70
5.29 5.09 5.06 3.82 4.22 3.84 3.19 3.17 3.02 3.07
0.926 0.937 0.934 0.936 0.945 0.924 0.887 0.935 0.882 0.910
6 3 4 4 8 5 7 6 6 6
d(1og K,)/d(log S), where S is in milligrams per liter. Intercept at 1.0 mg/L.
a
b
correction for the units used. (In this case, K p = 1000 X slope, as the units of C, and qe are in nanograms per liter and nanograms per gram, respectively.) Partition coefficients found at various solids concentrations using the same solute (2,4,5,2',4',5'-hexachlorobiphenyl)and sediment (Offshore Grand Haven) combination are shown in Figure 3. Figure 4 shows partition coefficients developed at various solids concentrations for the ten solute/sediment combinations investigated in this study. On the basis of the apparent exponential relationship observed in Figure 3, the results were plotted by using log coordinates, and a reasonable degree of linearity was found. In all cases the partition coefficient increased significantly as the concentration of solids in the system was decreased. Similar slopes were found for all combinations. Table I1 shows the relevant parameters for a linear regression of these data. The average value of the slope is -0.47,indicating approximately an order of magnitude increase in the partition coefficient for every 2 orders of magnitude decrease in solids concentration. The expected dependence of partitioning on octanol-water partition coefficients and on organic carbon contents can also be seen in Figure 4. In all cases the absolute magnitude of the best-fit lines increases as these two parameters increase. The data shown in Figure 4 were analyzed by a leastsquares multiple linear regression procedure. Log K was selected as the dependent variable while log KO,,fog S (solids concentration in milligrams per liter), and log f,, Environ. Sci. Technol., Vol. 17, No. 9, 1983 515
were selected as independent variables. This choice was based on the previously cited reports of a log-linear relationship between K,,.and each of the independent variables, and on the desire to develop a predictive equation for the partition coefficient. The resulting relationship given in eq 4 was found to explain 94.4% of the total variancb. log K p = 0.748 log KO,- 0.648 log S - 0.131 log f,,, + 0.364 (4) While a relationship such as eq 4 appears to accurately quantify the dependence of partitioning on solids concentration, it offers little insight into the reasons for this behavior. Any investigation into possible theoretical explanations immediately confronts the unlikely possibility that the observed effect appears to violate the laws of thermodynamic equilibria. Under thermodynamically ideal conditions, the ratios of concentrations between two immiscible phases should be constant and independent of the mass or volume of each phase. Departure from ideality, due to factors such as high concentrations or a reaction within one phase, produces a dependence of the partition coefficient on the solute concentration in either phase (see Prausnitz (15)). The solids effect reported herein shows that, at any given concentration in one phase, the equilibrium concentration in the other phase depends upon the mass of the solid phase. The obvious question to be addressed in attempting to resolve this anomaly is whether the experimental system is at equilibrium or not. The consistency in partition coefficients when 4-h equilibration times were used suggests that the differences are too great to be explained on this basis alone. Furthermore, departure from equilibrium should increase as the solid-phase concentration increases even at constant solids concentration. No such nonlinearities were observed. A clue to a plausible explanation for the solids dependence phenomenon can be found in the work reported by O'Connor and Connolly (IO). In their search of the literature, these investigators uncovered reports of this effect for several classes of solutes, including hydrophobic organic compounds, heavy metals, stable isotopes, and calcium, and for several types of sorbents, including clays, sediments, soils, and sewage sludges. The diversity of solute/sorbent combinations that exhibit this behavior tends to rule out any explanation based on specific chemical interactions and suggests a nonspecific, perhaps physical, explanation. It is proposed that the observed change in partitioning behavior due to solids concentration can be attributed to a transfer of sorbing, or solute-binding, material from the solid phase to the liquid phase during the course of the partitioning experiment. This material, whether dissolved, macromolecular, or microparticulate in nature, is not removed from the liquid phase during the separation procedure and is capable of stabilizing the compound of interest in solution. The amount of material contributed to the liquid phase is most likely proportional to the amount of solid phase present, and thus, the capacity of the liquid phase to accommodate solute depends upon the concentration of solids in the system. The overall effect can be viewed either as a two-phase system, where the properties of one phase (liquid) vary with the mass of the other (solids), thus resolving the perceived thermodynamic inconsistency, or as a three-phase system consisting of water, solids, and a third phase that is not separated from the water but possesses a higher capacity for the solute than the water itself. Given the demonstrated dependence of partitioning on the organic content of the sorbent, it is 516
Environ. Sci. Technol., Vol. 17, No. 9, 1983
6
1
2
3
4
5
ninn
I
2
3
4
5 8189
,?IO1
2
,!I$
3
4
5 878s 1x103
SOLIDS CONCENTRRTION (nC/LI
Flgure 5. Residual turbidity in the aqueous phase following solid/liquid separation by centrifugation.
Q
0
Q
O
0
Q
Q
, 2 I
3
1 5 6789 ,!IO1
I
2
3
4
s
8188 012!
6 i n 9
1!L
SOLIDS CONCENTRATION lflG/Ll
Flgure 6. Residual total organic carbon (TOC) in the aqueous phase following solld/liquid separation by centrifugation.
likely that the material transferred between phases is at least partially organic in nature. To investigate this explanation, several experiments were performed to elucidate the nature of this third phase. In each experiment, a different mass of solids, but no solute, was added to each of several centrifuge tubes and was treated exactly as in the isotherm procedure. The liquid phase was analyzed after separation for total organic carbon (TOC), turibidity, UV absorbance at 254 nm, and fluorescence (excitation at 365 nm, emission at 415 nm). Figure 5 shows the turbidity remaining in solution at different concentrations of solids. A significant trend of increased turbidity with increased solids concentration can be observed. Also shown is the result of an experiment in which the solid/liquid separation was accomplished by centrifugation at 27100g. While lower absolute turbidity was found, a slight increase with increased solids concentration can still be observed. This suggests that the change in liquid phase may be due to the presence of microparticles. While high-speed centrifugation does provide better separation, this technique cannot be used in isotherm studies of the type reported here due to the problems noted previously with the stainless steel centrifuge tubes required at such high speeds. Further, the slight dependence of turbidity on solids concentration even after high-speed centrifugation suggests that some of the residual particles are extremely small and resistant to separation. Residual TOC was measured by using a low-level TOC analyzer (Dohrmann DC-80). To ensure representative sampling and to avoid problems with the analysis due to the presence the microparticles, the instrument was used with an automatic sampling device. In this configuration, the sample is pumped into the reaction vessel with a peristaltic pump as opposed to the manual method using syringe injection. Replicate samples from a single isotherm bottle were reasonably reproducible, and no problems
attributable to the microparticles such as long reaction times or system contamination were encountered. The results for one sediment (OGH), shown in Figure 6, indicate a trend toward increased TOC with increased solids, but considerable scatter can be observed. Similar data were obtained for the other two sediments. The scatter might be attributed to analytical difficulties in working at such low levels. Given the more consistent data in the turbidity measurements, however, and the reproducibility of TOC measurements from individual isotherm bottles, it appears that there may also be a fluctuating contribution of dissolved organic material from the solids. The data obtained from both UV absorption and fluorescence measurements did not indicate any clear trends. The very low values of TOC found in the liquid phase are also consistent with the microparticle theory. A system with, for example, 100 mg/L of solids represents only a few milligrams per liter of TOC in the solid phase. If a small fraction of this organic material is transferred to the liquid phase, the resultant increase in TOC is likely to be minimal. This material is likely to have a partition coefficient similar to the bulk solids (on an organic carbon basis), and therefore, the amount of solute carried to the liquid phase may be quite significant for even moderately hydrophobic compounds. Given that the effect of solids concentration on partitioning can be quantified and a plausible explanation put forth, the relevant question that arises is as follows: How does the phenomenon affect partitioning in actual aquatic systems? The distribution of organic carbon between separable solids and nonseparable microparticles in laboratory studies is presumably dependent upon the conditions of the partitioning experiment, including the handling of the solids, the type of agitation used in equilibration, and the method by which the two phases are separated. The relationship developed here (eq 4) is therefore an estimation of the partition coefficient to be expected in a laboratory experiment conducted under conditions similar to those used in this study. This distribution of organic carbon, and therefore the distribution of solute, is likely to be different in laboratory studies that are performed differently. More importantly, little is known about this type of distribution in actual aquatic systems. It is logical to expect, however, that a high concentration of solids will correspond to a high concentration of microparticles. Thus, a similar solids effect in natural systems is expected, but the magnitude of the effect is unknown. Unfortunately, these results indicate that rigorous a priori predictions of sediment-water partitioning are further from realization than previously assumed. A t present there appears to be no practical method to evaluate the distribution of organic carbon between separable solids and nonseparable microparticles. Centrifugation techniques make a distinction at some point (combination of particle size and density), but measurements of the water phase do not produce the desired information. TOC measurements, for example, are generally not sensitive enough and cannot distinguish between microparticles, macromolecules, and dissolved compounds. Light-scattering techniques cannot distinguish between organic and inorganic particles. I t is possible that some combination of techniques or new methodologies will evolve, but no simple solutions are readily apparent. Conclusions
The findings presented herein suggest that solid-liquid partitioning in aquatic systems is more complex than previously assumed and that laboratory studies of partitioning may not accurately reflect partitioning phenomena
occurring in the environment. Specifically, this study has shown the following: Partition coefficients developed in laboratory isotherm studies are inversely related to the concentration of sorbing solids used in the experimental systems, at least over the range of solids concentration reported here. The solids effect can be quantified, and a reasonable estimate of the partition coefficient can be produced on the basis of the organic carbon content of the solid, the octanol-water partition coefficient of the solute, and the concentration of solids. The solids effect appears to result from the presence of microparticles contributed by the solids and not removed from suspension in the separation procedure. No simple means exists by which the presence of these microparticles can be quantified, and thus, extrapolation of the results to other systems can only be inferred. On the basis of these results, the following precautions are in order relative to the use of partitioning relationships developed by laboratory measurements: Partition coefficients developed at one concentration of solids are not necessarily appropriate at other solids concentrations. Partition coefficients produced in studies using different techniques are not necessarily comparable. The solids effect is likely to occur in the environment although the extent of the effect relative to that observed in the laboratory is unknown. Laboratory partition coefficients are not, therefore, directly applicable to real systems. Acknowledgments
We thank Dr. Brian Eadie of GLERL for his valuable contributions and express our particular appreciation to Karen Husby-Copeland and Sandra Lopez for their diligence in the experimental portion of this work. Registry NO.MCB, 108-90-7; NAP, 91-20-3;TCB, 37680-65-2; HCB, 35065-27-1.
Literature Cited Baughman, G. L.; Lassiter, R. ASTM S T P 657, Philadelphia, 1978, 35. MacKay, D.; Paterson, S. Environ. Sci. Technol. 1981,15, 1006. Smith, J. H.; Mabey, W. R.; Bohonos, N.; Holt, B. R.; Lee, S. S.; Chou, T.-W.; Bomberger, D. C.; Mill, T. U.S. Environmental Protection Agency, 1978; EPA-60017-78-074, Davidson, J. M.; Rao, P. S.C.; Ou, L. T.; Wheeler, W. B.; Rothwell, D. F. U S . Environmental Protection Agency, 1980; EPA-60012-80-124. Weber, W. J., Jr.; Pirbazari, M.; Uchrin, C. G.; Voice, T. C. “Sorption of Polychlorinated Biphenyls on Suspended Solids and Their Distribution and Differential Accumulation in Rivers, Harbors and Lakes”; Michigan Sea Grant, 1980. Eadie, B. J.; McCormick, M. J.; Rice, C.; LeVon, P.; Simmons, M. National Oceanic and Atmospheric Administration, 1981; ERL GLERL-37. Voice, T. C.; Weber, W. J., Jr. Water Res., in press. Karickhoff, S. W.; Brown, D. S.; Scott, T. A. Water Res. 1979, 13, 241. Connolly, J. P. Ph.D. Thesis, University of Texas at Austin, 1980. O’Connor, D. J.; Connolly, J. P. Water Res. 1980,14,1517. Rice, C. P.; Sikka, H. C. J.Agric. Food Chem. 1973,21,148. Weber, W. J., Jr.; Voice, T. C.; Pirbazari, M.; Hunt, G.; Ulanoff, D. Water Res., in press. Chiou, C. T.; Freed, V. H.; Schmedding, D. W.; Kohnert, R. L. Environ. Sci. Technol. 1977, 11, 475. Karickhoff, S.; Brown, D. U.S. Environmental Protection Agency, 1980; EPA-60014-79-032, Environ. Sci. Technol., Vol. 17, No. 9, 1983
517
Environ. Sci. Technol. 1983, 17, 518-522
(15) Prausnitz, J. “Molecular Thermodynamics of Fluid Phase Equilibria”;Prentice-Hall: Englewood Cliffs, NJ, 1973; pp 340-344.
Received for review July 9,1982. Revised manuscript received
February 22, 1983. Accepted April 18, 1983. This work was supported in part by the Cooperative Research Program between the Great Lakes Environmental Research Laboratory (GLERL) of the National Oceanic and Atmospheric Administration, Department of Commerce, and The University of Michigan.
Weddellite on Limestone in the Venice Environment Marco Del Monte Istituto di Geologia, Universitl di Bologna, 40126 Bologna, Italy
Crlstlna Sabbloni”
.
Istituto Fisbat, CNR, 40126 Bologna, Italy
This is the first report of a large deposit of weddellite (CaC2O4-2H20), the formation of which is still in progress. Mineralogical and physical characteristics of this mineral were studied and the origins of its formation established. The presence of weddellite is rarely reported in natural outcrops, has controversial origins, and has always been found in very small quantities. Thus, in natural environments, the stable form appears to be calcium oxalate monohydrate (whewellite) of which there is evidence of wider diffusion. In our case, weddellite is the main component of the alteration affecting every marble surface on the Isle of Torcello in the Venetian Lagoon. Ita origin must be related to the presence of numerous Cyanophites,whose role is to provide the system with oxalic acid, which in the presence of a carbonatic substratum precipitates in the form of calcium oxalate dihydrate. The enormous proliferation of these perforating “algae”,as of other endemic species, is connected to characteristics that are most peculiar to the Venetian Lagoon.
Introduction The interaction between atmosphere (hydrosphere and biosphere) and stone surfaces is a phenomenon known as weathering. An aspect both important and typical of weathering is the action of highly polluted urban atmospheres on stone used in the construction of buildings, monuments, and wall facings. One of the most investigated effects of weathering is the sulfation of calcium carbonates, noticeable on carbonatic surfaces such as limestone and marble in the form of gypsum crusts. This process of transformation depends on the chemical and physical characteristics of the stones, as well as on environmental parameters such as exposure, geometry of surfaces, and nature and composition of the local atmosphere (pollutant concentration, rain composition, etc.). The specific role played by airborne carbonaceous particles in sulfation processes taking place on carbonatic surfaces and the action of washout in relation to the different features of marble alteration have been investigated in previous papers (1-3). Observing different types of atmospheres (urban, suburban, rural, coastal, lagoon, and marine), we were able to find interesting analogies between the processes occurring in polluted and clean areas. In northern and central Italy, no solution of continuity seems to exist between the two. The object of this study is to discuss a particular kind of alteration observed in the Venetian Lagoon, an area of interesting transitional character, lying as it does between marsh and sea. From the study of facades of the wellknown Cathedral of Santa Maria della Assunta and the Church of Santa Fosca on the Isle of Torcello, the presence 518
Envlron. Sci. Technol., Vol. 17, No. 9, 1983
of thick layers of weddellite (CaCzO4-2H20) has been found on the carbonatic surfaces. The presence of this oxalate is important for these reasons: (a) With the exclusion of the vegetable kingdom and human pathology, reports on the occurrence of this mineral are very rare and controversial. In fact, weddellite is a mineral commonly found in vegetable cells and in kidney stones, while its presence is rarely reported in natural environments, and many authors hypothesize that the formation of this mineral occurs after sampling, during storage. (b) This occurrence differs from the already existing, but scarce, information in literature for the percentage of weddellite found in the samples analyzed (80-90%), the large areas covered by this mineral, and the thickness of the layer observed (1-7 mm). ( c ) This type of alteration was never recorded before on marble monuments or on carbonatic surfaces in general. It is well-known that the most common mineral found in weathering processes is gypsum; dolomite is rare ( 4 ) and whewellite even rarer (5).
Previous Works The two calcium oxalates, weddellite (CaC204-2H20) and whewellite (CaC204-HzO),have been mentioned in scientific literature since the 17th century. The Italians Malpighi (6) and Buonanni (7) studied them. Philipsborn (8) has extensively reviewed the works concerning this subject. Both oxalates have a wide diffusion in the vegetable world and in the cells of many higher and lower plants. Many findings are reported in the animal world, for instance, in the Malpighi vessels, in the moult of insects, in cocoons, inside bees’ intestines (€9,and in the gizzard plates of the deepwater gastropod (Scaphander cylindrellus) (9). Further examples are found in honey and in the urine of many mammals (in particular, herbivorous mammals). Furthermore, weddellite and whewellite are also the main components of kidney and bladder stones (10-13). In a natural nonbiological environment whewellite appears to be more stable than weddellite and, consequently, is more widespread. It has been found in coal mines, uranium mines (14), calcareous rocks, calcareous geodes, clay, septarian limestone (15,16), and weathering crusts of travertine (Coliseum) and marble monuments (Constantine’s Arch) in Rome (5). Reports concerning weddellite are controversial and scarce. The first concerns sediments dredged from the bottom of the Central Weddell Sea (Antarctica) at a depth of 4500-5000 m ( 1 7 ) . However, Arrhenius (18)points out how subsequent attempts to find weddellite in antarctic sediments have failed. He suggests the possibility that the weddellite reported by Bannister and Hey (17) might have been the result of alteration occurring during the storage of samples, due to the interaction of decomposing proteic
0013-936X/83/0917-05 18$01.50/0
63 1983 American Chemical Society
