Sorption mechanism of organic compounds in karst: SOM or HSACM dependent?

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Groundwater Quality: Natural and Enhanced Restoration of Groundwater Pollution (Proceedings ofthe Groundwater Quality 2001 Conference held al Sheffield. UK. June 2001). IAHS I'ubl. no. 275. 2002.

1 Groundwater Quality: Natural and Enhanced Restoration of Groundwater Pollution (Proceedings ofthe Groundwater Quality 2001 Conference held al Sheffield. UK. June 2001). IAHS I'ubl. no Sorption mechanism of organic compounds in karst: SOM or HSACM dependent? A. O. ODUTOLA & K. P. WALSH Environmental Engineering Research Group, School of Civil Engineering, The Queen's University of Belfast, Belfast BT7 INN, Northern Ireland Abstract Modelling the fate and transport of contaminants in groundwater requires an in-depth understanding of sorption processes in aquifer material. Sorption isotherms for chlorinated solvents and aromatic organic compounds have been developed for sands and clays. However, limited information exists regarding these compounds in limestone and other karst rock types. Research has shown that the sorption of organic contaminants is principally influenced by the presence of soil organic matter. High-surface area carbonaceous material (HSACM) has also been found to significantly affect sorption of contaminants. Limestone samples from various origins were characterized in terms of specific surface area, clay mineral, organic matter nature and content. Batch sorption experiments were carried out and isotherms were developed over a concentration range of 0.001% to 1% aqueous solubility of chloroform and dichloromethane. Analysis showed the fraction of organic carbon in these samples is less than 0.1%. However sorption was found to be greater than theoretically calculated. Key words chloroform; dichloromethane; karst aquifer; kinetics; limestone; sorption INTRODUCTION Karst systems are important sources of drinking water supply in Europe. Approximately 35% of the surface water supply in Europe comes from karst. In the UK and the Republic of Ireland, a high percentage of major aquifers are situated in highly fissured and karst rocks amounting to a significant proportion of the countries' hydrogeology. Groundwater provides 25% of the Republic of Ireland's public water supply and karstified rocks provide 5% the total water supply. In the UK, 30% of public water supply is from groundwater and 20% of the total water supply is from karstified rocks (European Commission, 1995/ Due to industrial and agricultural activities, karst systems are vulnerable to chemical pollution and require protection. Karst can be defined as a terrain with hydrology and landforms that have developed from rocks with high solubility. This results in a landscape with welldeveloped secondary porosity such as caves, stream sinks, springs and underground conduits and channels (Ford & Williams, 1993). Karst aquifers can be classified into hypersensitive to slightly-sensitive karst aquifers based on a combination of storage, flow and recharge parameters (Adams & Smart, 1995). Most research into karst aquifers has been on hypersensitive aquifers whose hydrogeology differs significantly from those of less sensitive karst aquifers. Due to the significant change in hydrogeology in karst aquifers it is important to understand the fate and transport of contaminants in the different classes of karst.

182 A. O. Odutola & K. P. Walsh The widespread use of chlorinated hydrocarbons as degreasers and solvents has resulted in their presence in the subsurface.

2 182 A. O. Odutola & K. P. Walsh The widespread use of chlorinated hydrocarbons as degreasers and solvents has resulted in their presence in the subsurface. Sorption and desorption processes are important in the fate and transport of contaminants in groundwater. Sorption of chlorinated hydrocarbons onto aquifer material is controlled primarily by the partitioning of a contaminant to the organic carbon (Brusseau & Rao, 1989). Partitioning of contaminants to sediment is typically linear when the fraction of organic carbon (f oc ) is greater than 0.1%. In aquifer materials with f oc below 0.1%, sorption is typically controlled by other factors such as mineralogy and surface area (Ball & Roberts, 1991). Researchers have hypothesized that nonlinear sorption may exist due to physical and chemical variations in the sorption sites (Weber & Huang, 1996). For example Xia & Pignatello (2001) attribute nonlinearity to physical variations within the structure of the organic matter, i.e. condensed or amorphous. The nonlinearity is kinetically caused by the presence of dual mode sorption kinetics (Xia & Pignatello, 2001). Recent work has also shown that chemical variations in the nature of organic matter in sedimentary rocks may cause similar dual-mode kinetics and hence nonlinear sorption isotherms (Kleineidam et al, 1999). Commonly linear isotherms based on organic carbon content are used for the sorption of chloroform (CF) and dichloromethane (DCM) in aquifers. The aim of this paper is to investigate if linear sorption kinetics for CF and DCM would govern their transport in karstified limestone. MATERIAL AND METHODS Karstified limestone was pulverized to a grain size of <0.3 mm and pre-treated by drying in an oven at 103 C for at least 8 h and then stored in a desiccator (as Pavlostathis & Mathavan, 1992). Limestone characterization Carbonate content (CO3 2 ) was determined by mass difference of pre- and post-acid treatment as Kleineidam et al (1999). The acid treatment was 10 g of pulverized limestone sample dissolved in 50% hydrogen chloride (HC1) solution. The residue was washed and centrifuged prior to weighing. The residue was also used to determine the f oc by mass difference after baking the non-carbonate residue at 850 C (Kleineidam et al, 1999). Intra-particle porosity, bulk density and surface area were determined by mercury intrusion using Fission Instruments Mercury Porosimeter 2000WS. Mineralogy analysis was by X-ray fluorescence (XRF). Solute Aqueous solutions of compounds from stock methanol solutions were used. Final solutions for equilibrium experiments were at % aqueous solubility of chloroform and dichloromethane. Sodium azide was used at 200 mg l" 1 (Deitsch et al, 2000; Rugner et al, 1999) as a biocide during the sorption experiment. CF and DCM were analysed by gas chromatography (GC) using a Hewlett Packard 5890 with Electron Capture Detector; BP624 column (cyanopropylphenyl polysiloxane) 10 m x 0.53 mm x 3 pm; 51.5 ml min" 1 total flow rate; 3:1 split ratio; detector 260 C, injector 240 C, oven isothermal 25 C; CF retention time 0.49 min and DCM retention time 1.1 min. Sorption experiment Sorption experiments were carried out to determine the sorption rates of CF and DCM in limestone. 7 g of pulverized pre-treated limestone aquifer material was added to 7 ml of solute solution in a 33 ml borosilicate amber

3 Sorption mechanism of organic compounds: SOMor HSACM dependent 183 glass bottle with screw caps with Mininert valve and Tefion -faced silicon septa (solid to liquid volume ratio of 1:3). All experiment bottles were prepared in triplicate. The bottles were mixed on a shaker at 23 C. Samples were analysed after a period of 72 h. Each analysis data point was performed in triplicate. Equilibrium data analysis Two main types of sorption isotherm models have been established in the literature: the linear and nonlinear models (Farrell & Reinhard, 1994). The nonlinear Freundlich isotherm model was chosen in this study as it is commonly used in samples with a low fraction of organic carbon. The model is mathematically described as (Karapanagioti et al., 2000): q e = K fr C e N where q e = mass of contaminant sorbed per dry unit weight of solid [ ig kg" 1 ]; Kf r = Freundlich sotption constant [jig kg"'] [1 fig" 1 ], C e = concentration of the contaminant in solution [mg l" 1 ], N = Freundlich exponent. Where sorption is principally controlled by organic carbon, the isotherm is linear and may be expressed as (Karapanagioti et ai, 2000): K ll = q e /C e (2) where K,i = sorption distribution coefficient [1 kg" 1 ]. Determination of Kd for nonlinear isotherms was therefore obtained by combining equations (1) and (2), giving (Karapanagioti et ah, 2000): (1) Kd-KftCf 1 (3) RESULTS AND DISCUSSION Limestone characteristics The samples contain ayoc of less than the 0.1%, which is below the threshold for organic carbon dominated sorption or partitioning. Further characterization of the limestone materials showed that the sample contained about 95% inorganic carbon or carbonate (Table 1). The XRF results (Table 2) and X-ray diffraction (XRD) results (not shown) indicate that clay minerals were present at <1% in the clay size fraction of the samples. Therefore the influence of clay minerals on sorption is unlikely to be significant. The remaining composition of the samples was predominantly aluminium and iron oxides. Table 1 Physical characteristics of limestone. Inorganic Fraction of Intra-particle Surface area carbon (%) organic carbon (-) porosity (%) (nrg" 1 ) Bulk density Average pore (g cm'") radius (um) Range of pore radius (um) Table 2 XRF analysis of limestone samples (LOI = loss on ij ignition). Sample A Fe MgO CaO Na 2 0 K 2 0 Ti0 2 MnO P LOI Total Reef Oolitic

4 184 A. O. Odutola & K. P. Walsh Examination of thin sections of the limestone samples under high magnification showed transformation of dolomite to calcite crystals. Opaque matter was also observed under transmitted and cross-polar light (Fig. 1) mainly between ooids or calcite crystals. The opaque matter was considered to be organic matter that is kerogen rich or of wood origin as observed in other limestone samples by Kleineidam et al. (1999). Organic matter opaque under transmitter and cross-polar light. Mg 10x Fig. 1 Photo of thin section of reef limestone under transmitted light showing organic matter. Equilibrium batch Equilibrium isotherms were determined from the batch sorption experiments to be nonlinear. Although linear isotherms are typically used to model sorption of chloroform and dichloromethane, the low organic carbon content of the samples indicated that nonlinear isotherms would most likely apply. The isotherms were modelled by the Freundlich kinetic equation (R~ > 0.90 for CF and DCM). A Freundlich exponent of 0.7 in CF, and in DCM were observed (Figs 2 and 3). Freundlich exponents of less than 1 are confirmation of nonlinear sorption. A closer examination of the chloroform isotherm shows an.y-type curve. Xia & Pignatello (2001) observed a similar curve shape for CF sorption onto high-organic soils. They attributed the kinetics to dual mode sorption caused by the combined condensed and amorphous nature of the organic matter. Therefore sorption observed in the limestone samples is thought to be due to one or more chemical or physical properties causing dual-mode kinetics reef limestone cd [tic limestone (a) & (n -, 3.5 <> reef limestone oolitic limestone O 0 0 (b) Log solute concentration (ug/l) Log solute concentration (ugl) Fig. 2 Sorption isotherms of chloroform (a), and dichloromethane (b), in limestone.

5 Sorption mechanism of organic compounds: SOMor HSACM dependent 185 The relationship between organic carbon and the extent of soiption in the limestone samples was investigated further. Based on the surface area data shown in Table 1 thef oc was predicted using equation (4) (Brasseau & Rao, 1989), which applies in aquifer material with / o c less than 0.1. The f oc was used to theoretically predict K CI (equation (5)). Organic carbon normalized soiption as determined from the batch experiments was 2-3 orders of magnitude greater than theoretically calculated (equations (3)-(5)). / oc = SA/200 x/o 8 4 (4) where SA = surface area [m 2 g" 1 ], K ow = octanol-water partitioning coefficient [1 mg" 1 ]. Kd can then be derived from the organic carbon content normalized distribution coefficient (K oc ) (Karapanagioti et al., 2000): K d = K oc xf oc (5) Grathwohl (1990), Kleineidam et al. (1999) and Karapanagioti et al. (2000) also measured greater sorption in sedimentary rock samples with lower organic carbon than theoretically expected. This was attributed to the nature of the organic carbon. Younger organic matter, such as humic substances and pollen, were demonstrated to have soiption capacities two orders of magnitude lower than matured organic mattersuch as coal. A mixture of organic carbon ages may result in an intermediate sorption capacity. Alternatively, the organic matter may be high-surface area carbonaceous material (HSACM) as seen by Chiou et al. (2000), who also observed higher sorption capacities than predicted. It may be inferred that the limestone samples analysed and presented here, have matured and/or high surface area organic carbon present giving rise to significantly higher sorption than theoretically predicted. FIELD IMPLICATION In contaminant transport modelling field retardation factors (Rf) are derived using equations (5) and (6). The Rf obtained is dependent on properties of the aquifermaterial and the contaminant. Table 3 shows Rf values predicted from aquifer and contaminant properties as well as retardation factors from the experimental data. Field retardation factors can be estimated using (Bedient et al., 1994): R f =l+(pxkd)/r\ (6) where p = bulk density and r\ = porosity. Table 3 Comparison of predicted and experimental R f values for chloroform in a range of limestone samples. Parameters F oc Ri predicted for bulk sample Rf predicted crush sample R /^experimental f experimental bulk crush sample sample

186 A. O. Odutola & K. P. Walsh The variation in the nature of the organic matter is not accounted for in theoretical values therefore a significantly lower retardation is predicted.

6 186 A. O. Odutola & K. P. Walsh The variation in the nature of the organic matter is not accounted for in theoretical values therefore a significantly lower retardation is predicted. The batch experiment results indicate that literature and f oc calculated retardation would significantly over estimate the plume distance travelled by chlorinated solvents in limestone aquifers. The over estimation was present in three distinctly different types of limestone materials. Further investigation of a wider range of limestone would be beneficial in clarifying the extent of influence of HSACM on sorption in limestone. Acknowledgements The authors would like to acknowledge Ms Sussy Johnston, Mr Pat McBride and Mr Rory Doherty for their technical support. Also, the School of Civil Engineering for funding this project. REFERENCES Adams, B. & Smart, P. (1995) National report for the United Kingdom. In: COST Aclion 65- Hydrogeological Aspects of Groundwater Protection in Karstic Areas. Final report, EUR16547EN. European Commission. Ball, W. P. & Roberts, P. V. (1991) Long-term sorption of halogenated organic chemicals by aquifer material. 2. Equilibrium. Environ. Sci. Technol. 25, Bedient, P. B., Rafai, H. S. & Newell, C. j. (1994) Ground Water Contamination Transport and Remediation. Prentice Hall, New Jersey, USA. Brusseau, M. L. & Rao, P. S. C. (1989) Sorption nonideality during organic contaminant transport in porous media. Critical Review Environ. Sci. Technol. 19, Chiou, C. T., Kile, D. E., Rutherford, D. W., Sheng, G. & Boyd, S. A. (2000) Sorption of selected organic compounds from water to a peat soil and its humic-acid and humin fraction: potential sources of sorption non-linearity. Environ. Sci. Technol. 34, Deitsch J. J Smith, J. A., Cuver, T. B., Brown, R. A. & Riddle, S. A. (2000) Distributed-rate model analysis of 1,2- dichlorobenzene batch sorption and desorption rates for five natural sorbents. Environ. Sci. Technol. 34, European Commission (1995) COST Action 65 Hydrogeological Aspects of Groundwater Protection in Karstic Areas: Final report. EUR16547EN. European Commission. Farrell, J. & Reinhard, M. (1994) Desorption of halogenated organics from model solids, sediments and soil under unsaturated conditions. 1. Isotherms. Environ. Sci. Technol. 28, Ford, D. & Williams, P. (1993) Karst Geomophology and Hydrology. Chapman and Hall, London, UK. Grathwohl, P. (1990) Influence of organic matter from soils and sediments fom various origins on the sorption of some chlorinated aliphatic hydrocarbons: implications on K x correlations. Environ. Sci. Technol. 24( 11), Karapanagioti, H. K.. Kleineidam, S., Sabatini, D. A., Grathwohl, P. & Ligouis, B. (2000) Impact of heterogeous organic matter on phenanthrene sorption: equilibrium and kinetic studies with aquifer material. Environ. Sci. Technol. 34, Kleineidam, S., Rugner, H., Ligouis, B. & Grathwohl, P. (1999) Organic matter facies and equilibrium sorption of phenanthrene. Environ. Sci. Technol. 33, Pavlostathis, S. G. & Mathavan, G. N. (1992) Desorption kinetics of selected volatile organic compounds from field contaminated soils. Environ. Sci. Technol. 26, Rugner, H., Kleineidam, S. & Grathwohl, P. (1999) Long term sorption kinetics of phenanthrene in aquifer materials. Environ. Sci. Technol. 33, Weber Jr, W. J. & Huang, W. (1996) A distributed reactivity model for sorption by soils and sediments. 4. lntraparticle heterogeneity and phase-distribution relationships under non-equilibrium conditions. Environ. Sci. Technol. 30, Xia, G. & Pignatello, J. J. (2001) Detailed sorption isotherms for polar and apolar compounds in a high-organic soil. Environ. Sci. Technol. 35,

Chapter 1. Introduction

Chapter 1. Introduction Introduction 1 Introduction Scope Numerous organic chemicals are introduced into the environment by natural (e.g. forest fires, volcanic activity, biological processes) and human activities (e.g. industrial