Groundwater Quality: Natural and Enhanced Restoration of Groundwater Pollution (Proceedings o f tile G r o u n d w a t e r Quality 2 0 0 1 Conference held at Sheffield. U K . June 2 0 0 1 ). 1AI1S Publ. no. 2 7 5 . 2 0 0 2 .
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Leachate attenuation characteristics of colliery spoils
R. J. F R E E W O O D , J. C. CRIPPS & C. C. SMITH Department of Civil and Structural Engineering. Sheffield SI 3JD, UK e-mail: i.c,
[email protected]
University of Sheffield, Mappin Street,
Abstract This paper describes the results of a laboratory-based test prog ramme investigating the leachate attenuation characteristics of a range of UK colliery spoils for use in landfill liner systems. The majority of the tests were carried out using a synthetic landfill leachate, and were compared with a limited number of tests using a natural leachate. A study of the influence of various factors on attenuation was performed using the batch test method on different spoils. The variables studied included equilibrium pH, leachate composition, temperature, liquid to solid ratio, and sorbent composition. The results indicate that both inorganic and organic solutes may be significantly attenuated. A range of values for solute partition coefficients was obtained which was influenced by the properties of both the spoils and test conditions. Key words colliery spoil; leachate attenuation; mixed layer clay; organic carbon
INTRODUCTION Colliery spoils have been used in the construction of a significant number of landfill liners due to the large redundant volumes of little economic use. Colliery spoil contains a number of constituents that may be able to interact with landfill leachate components and reduce their impact on groundwater quality. Previous investigations into the leachate attenuation capacity of spoil are limited, and due to its inherent variability, spoils from different sites may possess differing attenuation characteristics. This variability should be incoiporated within risk assessment models, for example LandSim (Golder, 1996), when assessing the potential impacts from landfill leachates on groundwater quality for proposed sites using colliery spoil as an attenuating material within the liner system. Therefore, the attenuation characteristics of a diverse range of colliery spoils were tested using various test methods and test conditions in order to investigate the potential variability in the field more accurately. This paper presents selected batch test attenuation data from a more extensive programme of laboratory and field scale leaching and attenuation tests carried out on various colliery spoils (Freewood, 2001). Colliery spoils from five different sites within the U K were studied. The spoils were coarse spoils labelled B P 1 - B P 3 from the Bryn Pica landfill site, near Aberdare, south Wales; a fresh spoil (WS1) and relatively weathered spoils (WF1 and WF2) from the Selby Colliery, south Yorkshire, obtained from the Welbeck Land Reclamation Scheme, near Wakefield; a spoil labelled CHI from the Clifton Hall Landfill site, Greater Manchester; a spoil (PW1) from the Abernant Colliery, within the south Wales anthracite coal field; and lagoon spoils (PE1 and PE2) from the Peckfield Landfill site, near Garforth, west Yorkshire. The major components (i.e. mixed layer clay, carbonate and organic carbon) expected to be functional in leachate attenuation for spoils investigated during the
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Fig. 1 Colliery spoil composition showing organic carbon, mixed layer clay and carbonate content.
present study are compared with those from other studies (Taylor, 1984; Collins, 1976) in Fig. 1. Clay minerals, especially mixed layer clays, are the main spoil components that are active in cation exchange, due to their charged platy surfaces with high surface areas. The ability of the liner material to maintain high pH values, even when permeated with acidic leachates, is important to hold the solution pH in the optimum range to reduce the solubility of metals bound to spoil surfaces. The spoil carbonate content is the most important acidity buffering mineral since it has the ability to buffer the p H above 6.0. Most sorption of hydrophobic organic pollutants takes place onto organic carbon, and organic carbon often displays a high affinity for heavy metals. Figure 1 indicates that spoils tested in the present study show a similar range of functional components to those observed by other researchers from various locations across the UK. These compositional plots also show the variability in composition of spoils within sites due to various factors including the coal seam worked, localized weathering and the type of spoil deposited. Significant variation in spoil composition also arises because fine discards from the coal washing processes were often codisposed with coarse discards (Taylor, 1984).
METHODS Both natural and synthetic landfill leachates were used in the batch tests in the present study. A range of single-solute (i.e. only one solute present) and multisolute (i.e. various solutes present) synthetic leachates were used that contained key contaminants of environmental concern within the approximate concentration range observed by other researchers in landfill leachates (e.g. Christensen et ah, 1994). Ammonium and
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phenol were regularly used in the synthetic leachates because these compounds are often observed within landfill leachates in elevated concentration and have low drinking water concentration limits. Furthermore, the sorption of these two contam inants was expected to be dependant on the main functional groups of the colliery spoils discussed previously, i.e. clay minerals in cation exchange reactions for attenu ation of ammonium, and organic carbon in hydrophobic sorption processes for phenol. Similar solute concentrations were used throughout the tests with a total ion con centration in the multisolute (MSC) synthetic leachate of approximately 500 m g 1"' of SO4 and CI, 150 m g f ' o f Ca, Mg, Mn, Na, K, Mn and N H , and 30 m g f ' o f Ni, Zn, Cu, Cr, phenol, jP-cresol and o-cresol. A natural landfill leachate was obtained from ALcontrol Laboratories Ltd, and was used in both unspiked and spiked form, in which solutes used to produce the synthetic leachates were added to increase the solute con centration to similar levels as in the synthetic leachate. The unspiked natural leachate pH was 7.9 and contained 165 m g f ' o f ammonium. Sodium azide (NaN3) was added to selected leachates containing organic solutes to study the effects of biodégradation inhibition. Solute concentrations for metals were determined by ICP-AES, anions and ammonium by flow injection ion chromatography, and organic solutes by HPLC. 4
The batch tests used a 1:10 solid to solution ratio with spoils sieved below 2 mm (ASTM, D4646), which were constantly agitated at room temperature for 7 days, this being greater than the required chemical equilibration time of approximately 5 days observed in multisolute batch tests. The batch test method allows the rapid deter mination of sorption parameters and leaching characteristics, and was therefore used in the present study to investigate various parameters concerning attenuation of solutes. These parameters were the influence of sample composition, initial solute concen tration, competing solutes, system temperature, liquid to solid ratio, and equilibrium pH, on attenuation characteristics.
RESULTS AND DISCUSSION The results from the attenuation tests were converted to an apparent partition coefficient K (1 kg" ), which represents the partitioning between liquid and solid phases given. Therefore, the partition coefficient includes the mass of solute removed from solution by both sorption and precipitation processes. Despite the limitations of quoting a single K value to characterize the attenuation processes, K values provide a useful baseline comparator of attenuation capacities. A summary table showing K values for various attenuation tests is presented in Table 1. The suggested LandSim values are also shown in Table 1 for comparison. 1
The colliery spoils studied showed differing leachate attenuation potentials depending on the spoil composition, in particular the organic carbon and mixed layer clay content. In general, spoils with a high organic carbon content (e.g. PE1 and BP2 spoil) displayed the highest potential for sorption of phenol in the leachate. Spoils with a high mixed layer clay content (e.g. PE2 and BP1 spoil) displayed the highest potential for attenuation of ammonium. The order of phenol soiption for the spoils from the single-solute tests comparing K values was: PE1 > BP2 > PE2 > WS1 > WF1 > PW1 > BP1 > C H I > WF2 > BP3 and for ammonium sorption was:
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PE2 > BP3 > PW1 > BP1 > BP2 > W S 1 > WF1 > W F 2 > C H I > PE1 However, the order for ammonium sorption onto the spoil was less correlated to the spoil mixed layer clay content than phenol sorption onto organic carbon, as illustrated in Fig. 1. This was attributed to other spoil compositional characteristics significantly influencing ammonium sorption, such as particle size and other clay minerals such as kaolinite that were also observed within the spoils. 1
Table 1 Summary table of K values (1 kg" ) obtained from attenuation tests for various solutes. Test
Material
NH
4
Mn Ni
Single-solute (SS) BP1 SS BP2 SS BP3 SS CHI SS PE1 SS PE2 SS PW1 SS WF1 SS WF2 SS WS1 Natural leachate (NL) WF1 NL (spiked NL+NaN ) WF1 NL (spiked NL no NaN ) WF1 pH3 (MSC) WF1 pH5 (MSC) WF1 pH7 (MSC) WF1 pH3 (MSC) PE1 pH5 (MSC) PE1 pH7 (MSC) PE1 pH3 (MSC) PE2 pH5 (MSC) PE2 pH7 (MSC) PE2 10°C (SS) WF1 20°C (SS) WF1 50°C (SS) WF1 10°C (SS) WF1 20°C (SS) WF1 50°C (SS) WF1 10°C(MSC) WF1 20°C (MSC) WF1 50°C (MSC) WF1 10°C (MSC) BP1 20°C (MSC) BP1 50°C (MSC) BP1 Sol'msolid ratio 2.5 (1 kg" ) WF1 (MSC leachate) 10 (1kg"') WF1 100 (1kg" ) WF1 500 (1 kg" ) WF1
4.2 3.9 4.7 2.6 2.3 5.6 4.3 3.3 3.1 3.8 1.6 1.7 1.9 1.2 0.8 1.2 0.7 1.1 0.6 1 1.2 3.5 3.6 2.9 2.4 1.0 0.9 3.5 1.4 0.8 1.2 1.9 4.0 19.3
- 101 3 - 287 - 94 - 14 - 45 - 10.5 - 12 - 10.4 0.7 0.3 0.7 4 1.4 13 0.7 0.3 1.6 11 1.2 4 2.9 7 8.7 62 - 10.7 - 12 - 17.3 2.1 4.0 2.3 4.4 4.5 63.7 3.4 8.0 3.9 10.0 11.5 64.7 2.2 6.8 3.9 13.9 3.4 21.4 2.7 29.9
LandSim range*
1-50
3- 2100 800
3
3
1
1
1
Cu
Zn -
11.6 3.1 4.5 0.5 37 1300 0.5 980 4 9 2400 0.3 3.6 16.9 45.7 1600
Cr
2.9 14.5 178 1849 32000 72000 880 no 44000 65000 25 40 400 2600 15500 25000 - 1-4000
phenol o-cresol /j-cresol 8.1 22.1 5.2 7.0 69.2 13 9.2 8.2 6.7 8.4 bdl bdl bdl 3.4 4.8 5.9 12.1 6.9 27 3 5.7 6.5 3.6 5.6 6.1 3.5 4.8 6.3 107 176 226 124 197 248 10 25 32 9.8 19 26 24 8.3 18 4.1 2.9 4.8 3.2 4.6 5.7 3.7 6.1 6.0 2.9 3.9 1.9 2.1 3.3 4.3 2.9 5.4 7.0 7.1 6.9 10.8 5.0 10.4 9.5 7.6 8.3 11.5 22.8 15.2 14.6 _ _
* LandSim range from LandSim risk assessment model v. 1.01 (Golder, 1996). NL-spiked: natural leachate spiked with various key solutes; SS: single solute synthetic leachate; MS: multisolute combined organic and inorganic leachate; - indicates component not included or measured.
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The tests with the natural leachate matrix showed that Ni and N H K values for spiked tests were similar to those in the unspiked tests. The K values obtained for Cr, Cu, and Zn were close to the instrument detection limit for both unspiked and spiked tests due to precipitation and are therefore not shown in Table 1. Organic solutes all showed increased sorption without azide additions, attributed to microbial degradation of these organic compounds. The K values for the inorganic and organic solutes within the natural leachate spiked tests with azide additions were slightly above the K values obtained for the M S C leachate tests. This suggests that the influence of the natural leachate matrix (e.g. presence of humic substances) has not reduced the sotption of organic or inorganic solutes onto the spoil. Further testing would be required to test the statistical viability of this conclusion. 4
The sorption of solutes most affected by temperature were Ni and N H within the multisolute leachate. At 60°C, Ni sorption showed increases in distribution coefficients of factors of 10 or more compared with those at 10°C for both spoils used. The reverse was observed for N H sorption where distribution coefficients obtained at 60°C were factors of 10 or less than at 10°C. However, the single-solute tests for Ni and N H did not show significant temperature dependant sorption, implying that competitive sorption is temperature dependent with Ni being preferentially sorbed at higher temp eratures. This may be due to expansion of the clay mineral lattice allowing increased sorption of Ni, which may occur at the elevated temperatures observed within landfills (e.g. 80-90°C during initial composting phase of landfill; W M P 2 6 B , 1995). Mn also showed temperature dependent sorption, and was a factor of 4 greater at 60°C than at 10°C. The sorption of organic solutes was affected by temperature less than inorganic sorption with increases of factors of 2 observed at 60°C compared with values at 10°C. 4
4
4
The solute most influenced by the solutiomsolid ratio was Ni, showing an increasing K with increasing solutiomsolid ratio with a factor of 5 variation between the 2.5:1 and 500:1 solutiomsolid ratios. Similar results were observed in the single solute Ni test with increased sorption values. For phenol and ammonium the distribution coefficients increase significantly above a ratio of 100:1. However, M n sorption is relatively unaffected by the solutiomsolid ratio. Similar results were observed by Voice et al. (1983) who attributed the cause of this particle concentration effect to dissolved particles (e.g. colloids) originating from the solid particles, which bind with solutes and hold them in solution. The dissolved fraction originating from the spoil may be interacting with the solutes, in particular Ni, which retains them in solution producing decreased distribution coefficients. It should be noted that the solid:solution ratio may appear to influence sorption if significant precipitation of solute occurs which could be significant for Zn in the tests. This is due to the K calculation incorporating solid mass since precipitation remains relatively independent of solid added, and the solid particles may only increase precipitation slightly by functioning as nucleation sites for precipitation with the majority of precipitation being a function of solution p H only. The solution p H may also be influenced by the mass of solid added depending on the p H buffering capacity ofthe spoil. The K values for N H obtained in the present study lie at the lower end of the LandSim range. The attenuation of Mn onto colliery spoils was less p H dependent than for heavy metals with the range obtained in the present study also being at the lower end of the LandSim range. The large range of K values obtained during the present study 4
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for the heavy metals highlights the importance of pH on attenuation, with the optimum pH range for Cr attenuation being above pH 5.0 due to precipitation as oxides and hydroxides. Cu attenuation is also p H dependent due to precipitation above approxi mately pH 5.0. Zn attenuation was also pH dependent with a high range of K values observed, and Zn attenuation at the high K range was significantly greater than suggested in the LandSim model. Ni K values were also at the lower end of the LandSim range. If all the test parameters used were the same as the system that was being modelled using K, then the K values determined in the present study may be applicable. However, other factors including the solutes leached from the sorbent material during the test may significantly affect the sorption process. This highlights the problems associated with using K based models, such as LandSim, for modelling multicomponent soiption in complex systems such as occur during the migration of landfill leachate of varying composition through a landfill liner. A large uniform distribution range for K (i.e. equal probability of all values in the range occurring) may be used in these models to overcome the problem of accurately modelling these complex effects. However, this approach is liable to produce large uncertainties in contaminant breakthrough predictions.
CONCLUSIONS The colliery spoils studied showed differing leachate attenuation potentials depending on the spoil composition, in particular organic carbon and mixed layer clay content. Therefore, spoils are likely to attenuate landfill leachates to differing extents if they are used within the landfill liner system. A deterministic consideration of the results indicates that, in general, spoils with a high organic carbon content will display the highest potential for sorption of hydrophobic organic contaminants in the leachate. Spoils with a high mixed layer clay content will have the highest potential for attenuation of ammonium. Spoils with a high carbonate content will be able to buffer the leachate pH most effectively, particularly during the acidogenic-acetogenic phases of the landfill, and therefore allow attenuation of heavy metals via precipitation processes to occur. However, modelling attenuation within a colliery spoil landfill liner during the risk assessment phase for liner design may be problematic without a large number of tests using various spoil samples and test conditions to determine the range of attenuation values due to the inherent variability of colliery spoils.
REFERENCES Christensen, T. II., Kjeldsen, P., Albrechestsen, H.-J., Heron, G., Nielsen, P. II., Bjerg, P. L. & Holm, P. E. (1994) Attenuation of landfill leachate pollutants in aquifers. Cril. Rev. Environ. Sci. Technol. 24(2), 119-202. Collins, R. J. (1976) A method for measuring the mineralogica! variation of spoils from British collieries. Clay Minerals 11,31-49. Freewood, R. J. (2001) Landfill leachate attenuation characteristics of colliery spoil. PhD Thesis, University of Sheffield. UK. Colder (1996) LandSim Manual. Report to the DoE CWM 094/96 under contract EPG 1/7/04 CL0172. Taylor, R. K. (1984) Composition and Engineering Properties of British Co/lierv Discards. Mining Department, NCB, UK. Voice, T. C , Rice, P. & Weber, W. .1. (1983) Effect of solids concentration on the sorptive partitioning of hydrophobic pollutants in aquatic systems. Environ. Sci. Technol. 17(9), 513-518. WMP26B (1995) Landfill Design, Construction and Operational Practice. Waste Management Paper 26B. DoE Publication, UK.
