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Column leaching of unretorted and retorted oil shales and claystone from the rundle deposit: water leaching
H"at. R e s Vol, 24. No. 2, pp. 131-141. 1990 Printed in Great Britain. All rights reserved
0043-1354~90 $3.0{) + 0.00 Copyright ,c 1990 Pergamon Press plc
COLUMN LEACHING OF UNRETORTED A N D RETORTED OIL SHALES A N D CLAYSTONE FROM THE RUNDLE DEPOSIT: WATER LEACHING DAVID R. JONES, BERNARDM. CHAPMANand ROBERTF. JUNG CSIRO Division of Coal Technology, P.O. Box 136, North Ryde, NSW 2113, Australia (First received July 1988: accepted in revised form June 1989) Abstract--The chemical compositions were determined for leachates from both small and large columns containing a number of solid waste components likely to be present in the dumps of a future shale oil industry on the site of the Rundle oil shale deposit. Samples of unretorted and retorted shales and interburden claystone were studied. These materials were leached both individually and in admixture under unsaturated flow conditions with distilled water. Long term trends in leachate composition were obtained by maintaining each of the columns for at least one year. A number of components in the first pore volume of leachate from some of the columns were found to exceed Australian drinking and irrigation water criteria. These were Mn, NO~, Mo, As, Se and B. Thiosulphate was present in high concentrations in the leachate from retorted shale. Almost 50% of the Cu and Ni leached from the Kerosene Ck shale was present in the form of metallo-organic complexes. The composition of leachate in the first pore volume from both the small and large columns was found to be the same, indicating that the results of small scale column tests may provide a good model for the leaching of the actual waste dumps to be constructed at Rundle. Key words--column leaching, Rundle, oil shale, retorted shale, claystone, trace elements, solubility, metallo-organic complexes
INTRODUCTION Extensive research has been done in the United States on the composition of aqueous leachates generated from Green River raw and retorted oil shales. However, the resultant data are almost certainly not directly applicable to the Australian context since the extent of leaching of potentially harmful elements and compounds, both before and after retorting, is likely to be very dependent on the way in which they are bound in the mineral and organic matrices of particular shale types. The work to be reported here is based on the Rundle deposit which is located 40 km north of Gladstone on the central Queensland coast of Australia. This reserve contains four major and two minor seams of oil shale which are separated by layers which are low in organic content. Samples from two of the major seams (called the Kerosene Ck and Lower Ramsay Crossing seams) were used for this study. The mineralogy of the Rundle shales is dominated by smectite clays with montmorillonite and illite being the dominant forms (Coshell and Loughnan, 1986). In contrast, carbonate minerals such as calcite and dolomite are the major inorganic components of Green River shale (Baughman, 1978). Most of the published work on water-related environmental aspects of the Rundle Project has focused on the leaching potential of shale from the Kerosene Ck seam which has been retorted in a Lurgi pilot
plant, It has been found by us (Jones and Chapman, 1983) and other workers (Bell et al., 1982, 1986: Batley, 1983) that the initial leachate produced from columns of this material contains high concentrations of Na, Ca, Mg, SO4 and C1. The elements Mo, Se and As have been identified as being worthy of note (Jones and Chapman, 1983; Bell et al., 1982). Detailed batch leaching tests (spanning a pH range of 2-9.5) have shown that As occurs in concentrations above detection limit in the leachate only at pH values greater than 7.5 and that, depending on the extent of the decrease in pH from the starting value, one or more of Mn, Zn, Cu, Fe and A1 are also mobilized (Jones, 1990). The concentrations of As, Cd, Cu, Ni, Zn and Mn from columns containing retorted shale produced by the Fischer assay technique have been found to be higher than the United States Environmental Protection Agency (U.S.EPA) recommended limits for drinking water (Barley, 1983). However, shale from several different seams will ultimately be processed in the retorts to be constructed at Rundle and the waste dumps will contain mixtures of low grade shale, interlayer claystone and retorted shale. Thus data are required both on the potential for seam to seam variation in leachability of trace elements and on the effects of mixing the materials which are likely to be incorporated into the waste heaps. The study reported here was designed to begin to address these issues. Samples of unretorted
131
DAVID R. JONES et al.
132
(raw) shale (from the K e r o s e n e Ck a n d Lower R a m s a y Crossing seams) a n d retorted shale (from the Kerosene Ck seam) a n d i n t e r b u r d e n claystone (from the Lower R a m s a y C r o s s i n g seam) have been investigated b o t h individually a n d in admixture. Both small a n d large columns were used for the leaching studies in order to investigate the relationship between sample size a n d leachate quality. This i n f o r m a t i o n is essential for o b t a i n i n g a n indication o f h o w applicable the conclusions from the l a b o r a t o r y leaching studies m i g h t be to a real waste d u m p . In c o n t r a s t to m a n y o t h e r g r o u p s (Stollenwerk a n d Runnells, 1981; Batley, 1983) we have used u n s a t u r ated flow c o n d i t i o n s to o b t a i n leachate since in real surface d u m p s liquid flow is mainly u n s a t u r a t e d with a n aerobic or a n a e r o b i c gaseous phase in c o n t a c t with the liquid. Long term trends in leachate c o m p o s i t i o n were o b t a i n e d by m a i n t a i n i n g each of the columns for a period o f at least 1 year. Since the current Stage i plan for the R u n d l e shale oil project calls for o p e n - c u t m i n i n g o f the Kerosene Ck seam to provide feed stock for a 25,000 t o n n e / d a y r e t o r t t r a i n ( M o o r e a n d T h o m p s o n , 1984), the extensive d a t a presented here on the leaching o f raw a n d retorted shales from this seam should be o f m u c h interest. EXPERIMENTAL
Columns The small columns were commercially available Wrigh trM borosilicate glass chromatography columns, 0.6 m long and 44ram internal diameter. One end was sealed with a polyamide membrane (20 # m pore size) and associated bed support. The large columns were fabricated from acrylic tubing of I04 mm internal diameter and a maximum of 1.7 m packed length was allowed. One end was sealed with an acrylic bed support and leachate collector assembly. Two 0.2mm polyethylene gauze screens (offset by 45 °) were at~xed to the bed support to prevent particulate matter from being washed out of the column. Column packing and leaching The leaching behaviour of columns containing samples of Lower Ramsay Crossing seam raw shale, Kerosene Ck seam raw and retorted shales and 1:1 : 1 mixtures of the Kerosene Ck raw and retorted shales and Ramsay Crossing claystone was investigated. The samples of raw and retorted shale and claystone used in this work were supplied by Esso Australia Ltd, the operator of the Esso Exploration and Production Australia Inc./Southern Pacific Petroleum NL/Central Pacific Minerals NL joint venture. The shale samples from the Kerosene Ck and Lower Ramsay crossing seams had been crushed to < 4 mm prior to delivery in plastic-lined 44 gallon drums. The claystone from the latter seam was supplied wet and had to be dried at 60°C for 3 days before it could be crushed to <4.8 ram. A I : 1: I mixture of claystone and Kerosene Ck raw and retorted shales was used to provide a simulation of a real waste dump. At the time these experiments were set up little information was available on the ratio of interburden to overburden to high grade shale to be expected in a commercial operation at Rundle. The small columns were packed by pouring in small amounts of shale material and compacting this by vigorously tapping the sides of the column. A 1.5 cm layer of
Table 1. Columns studied Column code Sample type Ramsay Crossing raw shale Kerosene Ck raw shale Kerosene Ck retorted shale Mixture (Kerosene Ck raw and retorted shale. Ramsay Crossing clayston¢)
Small Large RC -R LR S LS M LM
acid-leached glass chips was then placed on the top of each column to ensure an even application of leach solution across the shale. The large columns were packed in a similar manner. The small columns were flooded initially by capping and inverting them and pumping distilled water from the bottom to the top. Once the water front reached the opposite end, pumping was stopped and the columns restored to their original orientation. They were then allowed to drain for 2 days. By this means flow was maintained in one direction and all of the column material wetted. When draining was complete, the amount of water retained by the column was calculated by weight differences, This volume was defined as the operational pore volume--abbreviated to pore volume (PV) for the rest of this paper--and is the appropriate volume to use for the unsaturated flow conditions employed here. The large columns could not be flooded in the same manner as above and hence had to be wetted by downward percolation of distilled water. Once the liquid fronts had reached the bottoms of the columns, the inputs were stopped and the columns allowed to drain. The pore volumes were then determined in the same way as for the small columns. The code designations of the columns which were set up are listed in Table I and the weight of material in each column, together with the associated pore volume, are summarized in Table 2. The rate of application of distilled water to the columns (8 and 48 ml h- ~ for the small and large columns, respectively) was equivalent to 33 m of rainfall per year whereas the annual average rainfall at Rundle is approx. 0.9m (Rankin¢ and Hill, 1978). This value was chosen since it enabled the leaching to be carried out on an accelerated time scale yet was below the rate at which flooding of the columns would occur. An intermittent irrigation regime was employed with a cycle which consisted of five days on and two days off. All columns were maintained at 34 + 2°C in a constant temperature room for the duration of the leaching tests. This temperature was used since (a) the average daily summer temperatures at Rundle range between 22 and 30°C (Rankine and Hill, 1980) and (b) retorted shale dumps have been observed to retain their heat for several years (Rankine and Hill, 1980}.
Sampling and monitoring Column leach solutions were accumulated initially daily, then weekly and finally monthly in acid-leached polyethylene or polypropylene containers. Conductivity and air-equilibrated pH measurements were made immediately on aliquots obtained from the collector vessels. An aliquot
Code R S M RC RA LR LS LM
Table 2. Column packing parameters Sample weight Density Pore volume (g) (kg m -3) (ml) 940 1002 266 783 834 509 957 1020 433 922 983 285 861 918 207 15,800 1120 4200 12,800 913 7300 14,800 1060 5800
Column leaching of oi! shale of each of the leachate solutions was filtered through a 0.1 # m porosity membrane in order to differentiate particulate and dissolved components. The total and filtered samples were preserved by acidification with nitric acid to ensure a final pH of less than 1.5. Non-acidified aliquots were set aside for anion and total organic carbon (TOC) analysis. All samples for analysis were stored in acid-leached polyethylene vials.
133
nanomolar (abbreviated to mM, g M and nM, respectively) units. Conductivity and pH values appear as a function of leach volume in the top panel of each diagram. The initial conductivity is very high for all of the columns. This is due to the high concentrations of Na, Mg, Ca, CI and SO4. The behaviour of Ca is particularly interesting since typically its concentration falls rapidly to a plateau value of 12-16 mM after the first pore volume and then declines sharply to a new steady state value after five pore volumes. Saturation index calculations carried out using the chemical equilibrium program MINEQL (Westall et al., 1976) indicate that the Ca concentration in the plateau region is being controlled by the solubility of CaSO4. The behaviour of Mg parallels this trend at lower concentration values. These results are in agreement with the published work of Bell et al. (1986). In order to facilitate further analysis of the results the data will be discussed in terms of two groups: (1) pH, HCO3 and (2) Cu, Ni, Zn, Mn, Cd and TOC.
Instrumentation and analvtwal methods Measurements of pH were made at 25 + 0.5C in the laboratory with a Radiometer PHM 64 pH meter. Major cations and A1, Fe, Mn and Zn were determined by flame atomic absorption spectrophotometry (AAS). A Labtest Model UV25 inductively coupled plasma emission spectrometer was used for the analysis of Sr, Mo and B in the column leachate solutions. Trace concentrations of Cu, Ni, As, Se and Cd were measured by graphite furnace AAS. C1, SO4, NO3 and thiosulphate concentrations in the column leach solutions were obtained by using a Dionex 10 ion chromatograph. Solution concentrations of organic carbon (TOC) were determined with a Beckman 915B TOC analyser. Samples collected for TOC measurement were stored at 2°C prior to analysis. Bicarbonate concentrations were measured by potentiometric titration.
RESULTS AND DISCUSSION
pH, HCO~
Since the leaching profiles for the large (LR, LS and LM) columns were essentially identical to those observed for the small (R, S and M) columns the data from only the former group are shown here (Figs 1-3). For Ramsay Crossing shale a small column, only, was studied (Fig. 4). All concentration values, with the exception of those for organic carbon, are expressed in millimolar, micromolar and
The pH in the columns is controlled by the HCO.~/CO2 buffer system. In all of the raw shale and mixture columns the pH values oscillate within the range 8.2-8.8 and the HCO3 concentrations decline steadily to reach plateau values at high pore volumes. However the retorted shale columns behave in a significantly different manner. For column LS (see Fig. 2) there is a decline in pH from 7.9 to 7.6 over 12OOO
\ pH
-.~..
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8000 CONDY
pH
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2
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~ 1~ 17 21 25 29 PORE VOLUMES
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Fig. t. Water leaching of the large column (LR) containing Kerosene Ck raw shale (CONDY = conductivity).
L34
DAVIDR. JONESet J
1
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Fig. 2. Concentrations of elements in leachate from retorted Kerosene Ck shale packed into a large column (LS). The small downward-pointing arrow on the copper line indicates that the detection limit for this element has been reached.
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POREVOLUMES
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Fig. 3. Water leaching of a 1: 1: 1 mixture of Kerosene Ck raw and retorted shales and Lower Ramsay Crossing claystone(column LM). Downward-pointingarrows indicate that the detection Zimitfor analysis has been reached.
Column leaching Of oil shale i
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0
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I. S 25 &5 65 PORE VOLL~ES
i5 ,d5 , 5,25
Fig. 4. Levels of components in the leachate from a small column (RC) packed with Ramsay Crossing shale. the first three pore volumes (PV). This value is maintained up to 9 PV and then begins to steadily rise until it reaches 8.4. The change in HCO3 concentrations follow the pH trends. Thus in the case of the LS column the HCO3 concentration at 5PV is 0.2mM whereas by 22PV it has risen to 2 m M . Similar pH and HCO~ profiles were exhibited by column S. The reason for the behaviour of the pH and HCO3 values in the retorted shale columns is not readily apparent. Reductions in pH from I 1 to 7.6 over the first pore volume of leachate have been reported for field lysimeters containing retorted shale from Anvil Point. Colo. (Garland et al., 1979). However, these workers considered that uptake of atmospheric CO., by the retorted shale was unlikely to be the cause. Instead, the oxidation of the reduced S species, thiosulphate, to sulphuric acid by microbial or chemical means was advanced as the probable explanation. Evidence for the presence of substantial amounts of thiosulphate in the retorted shale used for the current work is provided by the significant concentrations of this ion that were found in the first PV of leachate (see later). Whilst oxidation of this species may account for the initial drop in pH, the subsequent rise [the field lysimeter data from Garland et al. (1979) indicate a similar trend] is still unaccounted for. Previous workers have examined the leaching of Kerosene Ck retorted shale under both aerobic and
anaerobic conditions (Bell et al., 1982) and found that the pH in an aerobic column decreased to ~ 7.5 after 6 PV whereas it increased from 8.5 to ~ 8.8 in another column which was maintained under a nitrogen atmosphere. The following explanation for the behaviour of the retorted shale columns studied in this work can be advanced on the basis of these results. For several pore volumes after the start of leaching, the whole column was probably aerobic owing to occluded air. During this period oxidation of thiosulphate led to a decrease in pH. Ultimately, however, the bottom section of the column became anaerobic and at this point the pH of the leachate began to rise until the system achieved a new steady state. Cu, Ni, Zn, Mn, Cd and TOC
The data presented in Figs 1-5 represent the total concentrations (dissolved plus particulate) of these metals in the ieachates, The results for Cd are not plotted since in all cases they were below detection limit (3 nM). Filtered samples were also collected to enable the dissolved fraction to be measured. The concentrations of these metals in the first and subsequent pore volumes of the retorted shale columns were either on or below the instrumental detection limits. However the raw shale column (R, RC, LR) leachates contained readily detectable levels. There was very little ( < 2%) particulate Ni or Zn in leachate solutions from the columns. However, in the first half
136
DAVIDR. JON~ et at. I
I
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---
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----LRI
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a
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16
20
~
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PORE VOLUMES
Fig. 5. Concentrations of arsenic as a function of pore volume in leachate from selected columns. of a pore volume up to 30% of the Cu for the R and M columns and up to 90% for the S column was in a form that would not pass through a 0.1/am filter membrane. Total organic carbon (TOC) is included with this group of metals since the complexing ability of natural organic ligands for Cu, Ni, Zn and Mn is well known (Mantoura et al., 1978; Shoikovitz and Copland, 1981). Particularly strong complexes are formed with Cu. Circumstantial evidence for Cu-organic association in this case is provided by the strong linear relationship between Cu and organic carbon for leachate from the LR column ([Cu]nM = 40.1 [TOC]mM - 4.9; correlation coefficient = 0.994). When LR leachate which had been filtered through a membrane of 0.1 g m pore size was passed through a column of Biorad SM2 macro-reticular resin, 50-60% of the Cu and Ni were removed from solution. This result strongly implies that the metal bound to the resin was present in the column leachate in the form of relatively hydrophobic metallo-organic complexes. The majority of the remaining Cu and Ni is probably associated with carbonate (Stiff, 1971; Leenhecr et al., 1981; Gruber and Tripathi, 1983) in ion pairs. No evidence was found for the existence of Mn--organic complexes. The data presented here for the concentrations of Cu, Ni, Zn, Cd and Mn in the retorted shale leachates are similar to those published by Bell et al. (1982) for leachate from Lurgi retorted Kerosene Ck shale. All values are just above or below instrumental detection limits. However, results obtained by Batley (1983) for Cu, Zn and Cd in the first pore volume of leachate obtained from freshly retorted (by the Fischer assay procedure) Rundle shale were orders of magnitude higher. The reported values for these elements were
120, 933 and 87/JM, respectively. Moreover, the concentrations in leachate from raw shale (27, 23 and 5.3/aM, respectively) were also much higher than those recorded in the present work. The origin of these enormous differences is at present unknown but it could have been the particular sample which was used for the Fischer assay experiments. A further possibility is that the product of this simple laboratory method for determining oil yield could display markedly different leaching behaviour to that of the pilot Lurgi process. Workers in the United States have shown that different retorting methods can yield products with quite different leaching characteristics (Stollenwerk and Runnells, 1981). It may also be significant that the retorted shale used by us and by Bell et al. (1982) had been stored in sealed drums for 1-2 years prior to the leaching tests. The reasons for the above discrepancies in elemental concentrations should be established since the levels detected by us and Bell et al. (1983) are well below Australian Water Resources Council (AWRC) and World Health Organization (WHO) guidelines (Garman, 1983; World Health Organization, 1971) whereas those quoted by Batley are orders of magnitude higher. Other components o f column leach solutions
The components discussed above were measured in all columns. However there are a number of species which were (a) not measured in all columns or (b) were not detected in all columns. The elements Mo and B fall into the former category whereas As, Se, NO~- and thiosulphate fall into the latter. Of the components which were detected in the leachate solutions these are the ones which are likely to have the greatest environmental impact since they are present in significant concentrations at ambient pH.
Column leaching of oil shale
137
(D)
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~R
--Ls \
--" ---LM --RC
\ A/'X ISel
-'
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(B)
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----LM ~
[Mol pH 15
LS
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60
PORE VOLUMES
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lag
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Fig. 6. Concentration profiles for the leaching of thiosulphate (S:O~-) (A), molybdenum (B), boron (C), selenium (D) and nitrate (E) from selected columns. Arsenic. The concentrations of As in leachate from columns containing retorted shale fell rapidly from a maximum value in the first half of a pore volume to a plateau value of ~ 0 . 4 ~ M at 2 pore volumes (Fig. 5). The concentrations decreased only very slightly thereafter. An exception to this behaviour was provided by the S and LS columns, in which the concentrations rose to 0.7 and 0.5 gM, respectively after 10 PV. Measurable concentrations of As were also detected in the leachate from the LR column. However an initial high concentration pulse was not observed and throughout the life of the column the concentration fluctuated around 0.15 g M. Since there was no significant difference in concentration between filtered and unfiltered samples for the five columns appearing in Fig. 5 all of the As is in solution. The pH dependence of the leaching of As has already been discussed previously (Jones, 1990) and so this will not be repeated here. However, it should be noted that the pH of aged retorted shale (such as in the case of the sample used for this work) is invariably lower than that of fresh material owing
to adsorption by the solid of atmospheric CO:. Thus if freshly retorted material had been leached, substantially higher levels of As may have been observed in the first pore volume. Thiosulphate. Thiosuiphate is not found in raw shale or claystone but is produced in the reducing environment which occurs during retorting. Since the thiosulphate anion is highly soluble it is found as a major component in the first pore volume of leachate from retorted shale [Fig. 6(A)]. It is an environmentally important product since its oxidation leads to the production of sulphuric acid. Previous reference has been made to the role which this process may play in the reduction of pH in field lysimeters containing retorted shale (Garland et al., 1979). Concentrations of up to 2 mM were found in leachates from the retorted shale columns. However, an unexpected result was that no thiosulphate was detected in leachate from the columns containing the mixture of solid wastes (i.e. M and LM). There are two possible explanations for this phenomenon. Firstly, the thiosulphate may have been adsorbed by
138
DAVIDR. JONESet al.
the raw shale or claystone. Secondly, the unretorted materials may have contained bacteria which were capable of rapidly oxidizing thiosulphate. At present there is insufficient evidence to decide between these possibilities. Molybdenum and boron. The concentration of Mo in the first quarter pore volume of the retorted shale is about 25 times higher than that observed for the raw shale [Fig. 6(B)]. However the exponential decrease in concentration over the first 2 PV in leachate from the former column indicates that the concentration of Mo is principally controlled by the dissolution of readily soluble salts present on the surface of the retorted shale particles. The essentially constant concentration of ~ 1 laM for up to 4 PV in raw shale leacbate suggests that slow dissolution of a less soluble form is the controlling process. Previous workers, using a chemical equilibrium computer model, have shown that under the pH conditions prevailing in the columns the MoO~- ion should be the predominant form in solution, with its concentration being controlled by the solubility of powellite, CaMoO4 (Stoilenwerk and Runnells, 1981; Gruber and Tripathi, 1983). Unlike its effect on Mo concentration, retorting of the shale does not lead to a dramatic increase in the concentration of B in the first fraction of a pore volume [Fig. 6(C)]. Instead, the concentration in retorted shale leachate is consistently twice that observed for raw shale up to 25 PV. Although data points for the LM column are available for only 5 PV it is quite clear that the levels of B are equal to, or exceed, those observed for the retorted shale. This result is not in accord with a simple mixing ratio. However, it is known that B tends to be associated with clay minerals (Stoilenwerk and Runnells, 1981) and if these materials are the reservoir for this element in Rundle shale, then the above observations can be accounted for. Selenium. The data for this element are plotted in Fig. 6(D). Although the concentrations were below the instrumental detection limit (0.002gM) in leachates from the retorted shale columns, high concentrations were observed in the first pore volume of the columns which contained raw shale. This latter result was not expected since Se is usually found as a component of very insoluble sulphide minerals; for example, pyrite (Fischer et al., 1978). However, water soluble Se salts could have been liberated as a result of oxidation of the pyrite in the shale. Since a very large reservoir of soluble sulphate is present in the shale then this anion would tend to occupy the anion exchange and adsorption sites which, in its absence, would have immobilized the trace levels of oxidized Se compounds. Selenate, especially, should be mobile under these conditions whereas any selenite produced would probably be tightly bound by the iron oxides and hydroxides and smectite clays present in the shales (Geering et aL, 1968; Ahlrichs and Hossner, 1987). Solution speciation studies on the column
leachate showed that all of the Se was indeed present as selenate. Nitrate. The occurrence of very high concentrations of NO3 in the first pore volume from columns containing Kerosene Ck raw shale was unexpected [Fig. 6(E)]. None was detected in leachate from Kerosene Ck retorted shale (presumably the NO~ was destroyed during retorting) or Slot Cut raw shale (RC). The origin of the NO3 is unknown but it is most unlikely to have arisen from fertilizer input. Local farmers do have grazing rights but the land is not cultivated. Oxidation of the organic matter within the shale seam could lead to the production of NO, (Leenheer et al., 1981; Ronen et al., 1983). However, if this explanation is correct then the absence of NO3 in leachates from the Ramsay Crossing shale needs to be accounted for. Effect o f column size on leachate composition
The large columns contained, on average, 16.2 times more material than the small columns and the cross-sectional areas were in the ratio of 6:1. In order to ensure that both sets of columns received the same "rainfall equivalents", the ratio of application rates was also 6:1. However, the packed length of the large columns was almost three times longer than their smaller counterparts and so the question arises of how to best compare the leaching behaviour of the two types of columns. The most fundamental way of achieving this is to compare the leachate compositions on a pore volume basis. From the point of view of environmental impact the first pore volume is of the most interest since the concentrations of the majority of the potentially harmful trace elements are at their highest in this initial volume of leachate. Consequently, the greatest impact on receiving surface and groundwaters is likely to be made under these conditions. Accordingly, the overall volume-weighted concentrations of species present in leachate collected over the first pore volume for the small and large columns are presented in Tables 4--6. There is excellent correspondence between the concentrations calculated for most components (in the case of A1, B, Mo and Sr, data are available only for the large columns). It would thus appear that the leachate composition over the first pore volume is independent of the amount of material leached. Although the raw and retorted shale columns contain very similar amounts of Na, K and Mg (data from Tables 2 and 3), comparison between the first pore volume concentrations of these components in Tables 4 and 5 reveals that there has been a major reduction in their leachability following retorting. This result was unexpected since these elements are not lost to any significant extent during this process (Dale and Fardy, 1984). Moreover, the surface area of the retorted shale is approximately twice that of
Column leaching of oil shale Table 3. Composition of shale samples (retool kg-~) * Analyte
Kerosene Creek shale
Retorted Kerosene Creek shale
226 (15) 240 (15) 280 (20)
330 (20) 360 (20) 440 (30)
Na K Mg
• Figures in parentheses represent two standard deviations for replicate determinations. The concentrations of these elements, were determined by AAS analysis of solutions produced by HF/HNO3 digestions of the solid samples
the raw shale (Bell et al., 1986) and so any dissolved salts on the surface of the particles should have been more accessible to the leaching medium. It has been well documented that decomposition or transformation of the mineral matrix may occur as oil shales are heated to high temperatures (Campbell and Burnham, 1978; Ward-Smith et al., 1978; Patterson, 1986; Quezada and Patterson. 1986) and that the rate and extent of these processes is markedly dependent on the temperature attained and the retort technology that is used (Ward-Smith et al., 1978). The retorted shale supplied for this work was produced in a Lurgi pilot plant in which temperatures probably ranged from ~ 500C in the retort up to ~ 700°C in the lift pipe section where the residual carbon is combusted. Since a portion of the combusted shale is recycled as a heat exchange medium, some of the material may have been exposed to elevated temperatures for a considerable time. Published work on high temperature reactions of Green River shale (Ward-Smith et al., 1978) indicates that a temperature of 700°C would not be high enough to account for the immobilization of Na, K and Mg by the formation of the metamorphic minerals augite and melilite. However, the kinetics of these processes are likely to be very dependent on the specific mineralogy of the shale which is retorted and so this possibility cannot, as yet. be ruled out for Rundle shale. Table 4. Composition of first pore volume raw shale columns Analyte
Small
Large
Na mM K Mg Ca Sr
207 1.97 38.7 16.04 ND*
169 1.55 31.7 17.12 0.08
Mn p M Cu Ni Zn A1 Mo B Se As Cd nM
92.8 0.57 0.72 0.55 ND ND ND 2.15 <0.03 <3
84.3 0.35 0.41 0.69 3.0 0.88 88.5 2.27 0.18 <3
CI mM NO~ SOl HCO3
93.7 43 84.7 10.0
85.8 40 58.8 8.77
SzOi -
< 0.02
< 0.02
TOC m g l ~
190
170
*ND = not determined.
139
Table 5. Composition of first pore volume retorted shale columns Analyte
Small
Large
Na mM K Mg Ca Sr
11.9 0.62 1.56 22.2 ND*
11.6 0.56 1.41 25.6 0.11
Mn ,aM Cu Ni Zn AI Mo B Se As Cd nM
<1 0.008 < 0.03 0.17 ND ND ND <0.01 0.91 <3
<1 0.017 < 0.03 <0.08 26.4 7.4 141 <0.02 0.84 <3
C1 mM NOf
18.6 < 0.02
22.1 < 0.02
]8.8
19.8
sot HCO3 S,O~TOC mg I - ~
0.5 1.33 8.0
0.5 1.95 8.3
*ND = not determined.
Effects o f mixing o f shale wastes In a real waste dump reject raw shale, retorted shale and interburden will be mixed together. Accordingly, columns containing a 1 : 1 : 1 mixture of these materials were also prepared (note--the raw shale was of a much higher grade than would normally be placed in the waste dump). The data for the concentrations of components in the first pore volume of leachate from the large columns (Tables 4--6) can be used to evaluate the effect of mixing. For Ni, Mo, As, Se, NO3 and TOC the concentration values follow approximately the mixing ratio. However, for Cu the reduction is 4-fold greater than the mixing factor and for Mn it is 2-fold. The reduction for Mn is very significant since this element has been identified as exceeding water quality guidelines in the leachate from several of the columns containing raw shale. Table 6. Composition of first pore volume mixed shale columns Analyte
Small
Large
Na mM K Mg Ca
93.0 1.57 16.8 15.3
109 1.48 23.4 18.5
Mn /~M Cu Ni Zn B Se As Cd nM
10.7 0.03 0.19 0.2 ND* 0.44 0.45 <3
14.5 0.03 0.14 0.4 133 0.72 1.0 <3
42 8.7
66 8.7
52.4
55.4
1.80 < 0.02 29
1.97 < 0.02 55
C1- mM NO~SOlHCO 3 S,O~TOC mg I- '
*ND = not determined.
140
DAVIDR. JONESet al.
Table 7. Australiandrinkingand irrigationwatercriteria(,uM units) Element Drinking water* Irrigation watert Mn 9 9 Cu 23.6 3 Ni -3.4 Zn 76.5 30.6 Mo -0.1 B 93 69.3 Se 0.13 0.25 As 0.67 1.33 NO; 710 -*Garman (1983). ";'Derived working levels for water used continuouslyon soils (Hart. 1974).
Rundte Project Group of ESSO Australia Ltd, for providing the oil shale samples and associated background information. Funding for this project was partly provided by the National Energy Research Development and Demonstration Program (NERDDP) and ESSO Australia Ltd. Robyn Shapland, Nadia Shargholi, Michael Randle, Jaswant Jiwan, Joe Borg, Donna Harkess and Mary-Ann Stanley assisted with the analyses of the hundreds of water samples generated throughout the project. John Eames from the CSIRO Division of Exploration Geoscience supervised the ICP analyses. The authors would like to thank the Queensland State Government Analytical Laboratories for doing many of the initial organic carbon analyses.
CONCLUSIONS
REFERENCES
Comparison of Australian drinking and irrigation water quality criteria (Garman, 1983; Hart, 1974) with the concentration values obtained for the first pore volume of leachate from the columns (see Tables 4-6) show that a number of components exceed the recommended levels. These are Mn, NO3, Se and B for the Kerosene Ck raw and mixed shale columns (R, LR, M, LM) and Mo, B and As for the mixed and retorted shale columns (M, LM, S, LS). The Mn, Mo, NO3 and Se levels are particularly noteworthy since their respective water quality limits are exceeded by at least one order of magnitude. The Se levels are also high in the initial leachate from the column containing Ramsay Crossing raw shale [see Fig. 6(D)]. Thiosulphate was the major trace component in leachate from columns containing retorted shale alone. Concentrations of up to 2 mM were measured in the first two pore volumes. However, this anion was not detected in leachate from the columns which contained a mixture of retorted shale with raw shale and claystone. Concentrations of the heavy metals Cu, Ni, Zn and Cd did not exceed the water quality guidelines in the leachate from any of the columns. Approximately 50% of the Cu and Ni leached from the Kerosene Ck shale was found to be complexed by water soluble organic compounds. The concentrations of components in the first pore volume of leachate from both the small and large columns were very similar. The large columns contained an average of 16 times more material than their smaller counterparts and this provides a reasonable basis for concluding that the composition of the first pore volume is independent of the amount of material which is leached. Thus the results of small scale column leaching tests might reasonably be extrapolated to the leachate which would be generated within the waste dumps to be constructed at Rundle. However. in order to obtain an indication of the likely concentrations of components in the leachate, a mixture of solid wastes with a composition which is more representative of the proposed commercial operation would have to be used. An essential requirement would be that the retorted shale sample is produced by a pilot plant which closely simulates the actual retorting conditions that will be employed.
Ahlrichs J. S. and Hossner L. R. (1987) Selenate and selenite mobility in overburden by saturated flow. J. envir. Qual. 16, 95-98. Batley G. E. (1983) Composition of oil shale leachates and process waters. In Proceedings of the First Australian Workshop on Oil Shale, pp. 179-182. Lucas Heights, NSW. Baughman G. L. (Ed.) (1978) Synthetic Fuels Data Handbook, 2nd. edition, pp. 13-24. Cameron Engineers Inc., Denver, Colo. Bell P. R. F., Geronimos G. L., Doleson K. and Greenfield P. F. (1982) Characteristics of leachates from Rundle shale processed in a large pilot retort. Envir. Technol. Lett. 3, 433 ~40. Bell P. R. F., Krol A. A. and Greenfield P. F. (1986) Factors controlling the leaching of major and minor constituents from processed Rundle oil shale. Wat. Res. 20, 741-750. Campbell J. H. and Burnham A. K. (1978) Oil shale retorting: kinetics of the decomposition of carbonate minerals and subsequent reaction of CO2 with char. In l lth Oil Shale Symposium Proceedings, pp. 242-259, Colorado School of Mines Press, Golden, Colo. Coshell L. and Loughnan F. C. (1986) Mineralogy of the Rundle oil shale deposit. In Proceedings o f the Third Australian Workshop on Oil Shale, pp. 35-39. Lucas Heights, NSW. Dale L. S. and Fardy J. J. (1984) Trace element partitioning during the restoring of some Australian oil shales. Envir. Sci. Technol. 18, 887-889. Fischer R.. Zemann J. and Leutwein F. (1978) Selenium. In Handbook of Geochemistry (Edited by Wedepohl K. H.), Vol. 2 (part 3), pp. 34-G-1 to 34-L-2. Springer, New York. Garland T. R., Wildung R. E. and Harbert H. P. (1979) Influence of irrigation and weathering reactions on the composition of percolate from retorted oil shale in field lysimeters. In 12th Oil Shale Symposium Proceedings, pp. 52-57. Colorado School of Mines Press, Golden, Colo. Garman D. E. J. (1983) Water quality issues in Australia. In Water 2000: Consultants Report No. 7 prepared for Department of Resources and Energy. Australian Government Publishing Service, Canberra, ACT. Geering H. R., Cary E. E., Jones L. H. P. and Allaway W. H. (1968) Solubility and redox criteria for the possible forms of selenium in soils. Soil Sci. Soc. Am. Proe. 32, 35-40. Gruber J. and Tripathi V. S. (1983) Potential equilibrium concentrations of nickel and molybdenum in ground water as indicators for the necessary isolation of radioactive waste from INTOR. Final Report for the INTOR/NET Study. Contract 084/81-I I/FU-D/NET. Hart B. T. (1974) A compilation of Australian water quality criteria. Australian Water Resources Council technical paper No. 7.
Acknowledgements--The authors would like to thank the
Column leaching of oil shale Jones D. R. (1990) The influence of pH on the leaching of trace elements from Rundle oil shale. J. envir. Qual. In press. Jones D. R. and Chapman B. M. (1983) An investigation of the leaching behaviour of solid wastes from Rundle oil shale. In Proceedings o f the First Australian Workshop on Oil Shale, pp. 183-186. Lucas Heights, NSW. Leenheer J. A., Stuber H. A. and Noyes T. 1. (1981) Chemical and physical interactions of an in-situ oil shale process water with a surface soil. In 14th Oil Shale Symposium Proceedings, pp. 357-375. Colorado School of Mines Press, Golden, Colo. Mantoura R. F. C., Dickson A. and Riley J. P. (1978) The complexation of metals with humic materials in natural waters. Estuar. coast, mar. Sci. 6~ 387--408. Moore A. J. and Thompson B. J. (1984) Status of technical development of the Rundle oil shale project. In Proceedings o f the Second Australian Workshop on Oil Shale, pp. 9-14. Brisbane, Queensland. Patterson J. H. (1986) Comparative mineralogy and effects on processing of Queensland tertiary oil shales. Investigation Report EC/IR017, CSIRO Division of Energy Chemistry, Australia. Quezada R. A. and Patterson J. H. (1986) Mossbauer spectroscopy studies on retorting of Australian oil shales. In Proceedings o f the Third Australian Workshop on Oil Shale, pp. 79-83. Lucas Heights, NSW. Rankine and Hill Consulting Engineers (1978) Rundle Oil Shale Proposal--Environmental Stud), Report, Vol. 1, pp. 2.35-2.37. Sydney, NSW.
141
Rankine and Hill Consulting Engineers (1980) Rundle Oil Shale Project Environmental Impact Statement, Supplementary Report, Sydney, NSW, Appendix l l, Laboratory Leachate Analysis. Ronen D., Kanfi Y. and Magaritz M. (1983) Sources of nitrates in groundwater of the coastal plain of Israel-evolution of ideas. War. Res. 17, 1499-1503. Sholkovitz E. R. and Copland D. (1981) The coagulation, solubility and adsorption properties of Fe, Mn, Cu, Ni, Cd, Co and humic acids in a river water. Geochim. cosmochim. Acta 45, 181-189. Stiff M. J. (1971) The chemical states of copper in polluted fresh water and a scheme of analysis to differentiate them. War. Res. 5, 585-599. Stollenwerk K. G. and Runnells D, D. (1981) Composition of leachate from surface-retorted and unretorted Colorado oil shale. Envir. Sci. Technol. 15, 1340-1346. Ward-Smith J., Robb W. A. and Young N. B. (1978) High temperature reactions of oil shale minerals and their benefit to oil shale processing in place. In llth Oil Shale Symposium Proceedings, pp. 100--112. Colorado School of Mines Press, Golden, Colo. Westall J. C., Zachery J. L. and Morel F. M. M. (1976) MINEQL: A computer program for the calculation of chemical equilibrium composition of aqueous systems. Tech. Note 18. Water Qual. Lab. Mass. Inst. of Technol., Cambridge. WHO ( 1971) International Standards .for Drinking Water, 3rd edition. World Health Organization, Geneva.
0043-1354~90 $3.0{) + 0.00 Copyright ,c 1990 Pergamon Press plc
COLUMN LEACHING OF UNRETORTED A N D RETORTED OIL SHALES A N D CLAYSTONE FROM THE RUNDLE DEPOSIT: WATER LEACHING DAVID R. JONES, BERNARDM. CHAPMANand ROBERTF. JUNG CSIRO Division of Coal Technology, P.O. Box 136, North Ryde, NSW 2113, Australia (First received July 1988: accepted in revised form June 1989) Abstract--The chemical compositions were determined for leachates from both small and large columns containing a number of solid waste components likely to be present in the dumps of a future shale oil industry on the site of the Rundle oil shale deposit. Samples of unretorted and retorted shales and interburden claystone were studied. These materials were leached both individually and in admixture under unsaturated flow conditions with distilled water. Long term trends in leachate composition were obtained by maintaining each of the columns for at least one year. A number of components in the first pore volume of leachate from some of the columns were found to exceed Australian drinking and irrigation water criteria. These were Mn, NO~, Mo, As, Se and B. Thiosulphate was present in high concentrations in the leachate from retorted shale. Almost 50% of the Cu and Ni leached from the Kerosene Ck shale was present in the form of metallo-organic complexes. The composition of leachate in the first pore volume from both the small and large columns was found to be the same, indicating that the results of small scale column tests may provide a good model for the leaching of the actual waste dumps to be constructed at Rundle. Key words--column leaching, Rundle, oil shale, retorted shale, claystone, trace elements, solubility, metallo-organic complexes
INTRODUCTION Extensive research has been done in the United States on the composition of aqueous leachates generated from Green River raw and retorted oil shales. However, the resultant data are almost certainly not directly applicable to the Australian context since the extent of leaching of potentially harmful elements and compounds, both before and after retorting, is likely to be very dependent on the way in which they are bound in the mineral and organic matrices of particular shale types. The work to be reported here is based on the Rundle deposit which is located 40 km north of Gladstone on the central Queensland coast of Australia. This reserve contains four major and two minor seams of oil shale which are separated by layers which are low in organic content. Samples from two of the major seams (called the Kerosene Ck and Lower Ramsay Crossing seams) were used for this study. The mineralogy of the Rundle shales is dominated by smectite clays with montmorillonite and illite being the dominant forms (Coshell and Loughnan, 1986). In contrast, carbonate minerals such as calcite and dolomite are the major inorganic components of Green River shale (Baughman, 1978). Most of the published work on water-related environmental aspects of the Rundle Project has focused on the leaching potential of shale from the Kerosene Ck seam which has been retorted in a Lurgi pilot
plant, It has been found by us (Jones and Chapman, 1983) and other workers (Bell et al., 1982, 1986: Batley, 1983) that the initial leachate produced from columns of this material contains high concentrations of Na, Ca, Mg, SO4 and C1. The elements Mo, Se and As have been identified as being worthy of note (Jones and Chapman, 1983; Bell et al., 1982). Detailed batch leaching tests (spanning a pH range of 2-9.5) have shown that As occurs in concentrations above detection limit in the leachate only at pH values greater than 7.5 and that, depending on the extent of the decrease in pH from the starting value, one or more of Mn, Zn, Cu, Fe and A1 are also mobilized (Jones, 1990). The concentrations of As, Cd, Cu, Ni, Zn and Mn from columns containing retorted shale produced by the Fischer assay technique have been found to be higher than the United States Environmental Protection Agency (U.S.EPA) recommended limits for drinking water (Barley, 1983). However, shale from several different seams will ultimately be processed in the retorts to be constructed at Rundle and the waste dumps will contain mixtures of low grade shale, interlayer claystone and retorted shale. Thus data are required both on the potential for seam to seam variation in leachability of trace elements and on the effects of mixing the materials which are likely to be incorporated into the waste heaps. The study reported here was designed to begin to address these issues. Samples of unretorted
131
DAVID R. JONES et al.
132
(raw) shale (from the K e r o s e n e Ck a n d Lower R a m s a y Crossing seams) a n d retorted shale (from the Kerosene Ck seam) a n d i n t e r b u r d e n claystone (from the Lower R a m s a y C r o s s i n g seam) have been investigated b o t h individually a n d in admixture. Both small a n d large columns were used for the leaching studies in order to investigate the relationship between sample size a n d leachate quality. This i n f o r m a t i o n is essential for o b t a i n i n g a n indication o f h o w applicable the conclusions from the l a b o r a t o r y leaching studies m i g h t be to a real waste d u m p . In c o n t r a s t to m a n y o t h e r g r o u p s (Stollenwerk a n d Runnells, 1981; Batley, 1983) we have used u n s a t u r ated flow c o n d i t i o n s to o b t a i n leachate since in real surface d u m p s liquid flow is mainly u n s a t u r a t e d with a n aerobic or a n a e r o b i c gaseous phase in c o n t a c t with the liquid. Long term trends in leachate c o m p o s i t i o n were o b t a i n e d by m a i n t a i n i n g each of the columns for a period o f at least 1 year. Since the current Stage i plan for the R u n d l e shale oil project calls for o p e n - c u t m i n i n g o f the Kerosene Ck seam to provide feed stock for a 25,000 t o n n e / d a y r e t o r t t r a i n ( M o o r e a n d T h o m p s o n , 1984), the extensive d a t a presented here on the leaching o f raw a n d retorted shales from this seam should be o f m u c h interest. EXPERIMENTAL
Columns The small columns were commercially available Wrigh trM borosilicate glass chromatography columns, 0.6 m long and 44ram internal diameter. One end was sealed with a polyamide membrane (20 # m pore size) and associated bed support. The large columns were fabricated from acrylic tubing of I04 mm internal diameter and a maximum of 1.7 m packed length was allowed. One end was sealed with an acrylic bed support and leachate collector assembly. Two 0.2mm polyethylene gauze screens (offset by 45 °) were at~xed to the bed support to prevent particulate matter from being washed out of the column. Column packing and leaching The leaching behaviour of columns containing samples of Lower Ramsay Crossing seam raw shale, Kerosene Ck seam raw and retorted shales and 1:1 : 1 mixtures of the Kerosene Ck raw and retorted shales and Ramsay Crossing claystone was investigated. The samples of raw and retorted shale and claystone used in this work were supplied by Esso Australia Ltd, the operator of the Esso Exploration and Production Australia Inc./Southern Pacific Petroleum NL/Central Pacific Minerals NL joint venture. The shale samples from the Kerosene Ck and Lower Ramsay crossing seams had been crushed to < 4 mm prior to delivery in plastic-lined 44 gallon drums. The claystone from the latter seam was supplied wet and had to be dried at 60°C for 3 days before it could be crushed to <4.8 ram. A I : 1: I mixture of claystone and Kerosene Ck raw and retorted shales was used to provide a simulation of a real waste dump. At the time these experiments were set up little information was available on the ratio of interburden to overburden to high grade shale to be expected in a commercial operation at Rundle. The small columns were packed by pouring in small amounts of shale material and compacting this by vigorously tapping the sides of the column. A 1.5 cm layer of
Table 1. Columns studied Column code Sample type Ramsay Crossing raw shale Kerosene Ck raw shale Kerosene Ck retorted shale Mixture (Kerosene Ck raw and retorted shale. Ramsay Crossing clayston¢)
Small Large RC -R LR S LS M LM
acid-leached glass chips was then placed on the top of each column to ensure an even application of leach solution across the shale. The large columns were packed in a similar manner. The small columns were flooded initially by capping and inverting them and pumping distilled water from the bottom to the top. Once the water front reached the opposite end, pumping was stopped and the columns restored to their original orientation. They were then allowed to drain for 2 days. By this means flow was maintained in one direction and all of the column material wetted. When draining was complete, the amount of water retained by the column was calculated by weight differences, This volume was defined as the operational pore volume--abbreviated to pore volume (PV) for the rest of this paper--and is the appropriate volume to use for the unsaturated flow conditions employed here. The large columns could not be flooded in the same manner as above and hence had to be wetted by downward percolation of distilled water. Once the liquid fronts had reached the bottoms of the columns, the inputs were stopped and the columns allowed to drain. The pore volumes were then determined in the same way as for the small columns. The code designations of the columns which were set up are listed in Table I and the weight of material in each column, together with the associated pore volume, are summarized in Table 2. The rate of application of distilled water to the columns (8 and 48 ml h- ~ for the small and large columns, respectively) was equivalent to 33 m of rainfall per year whereas the annual average rainfall at Rundle is approx. 0.9m (Rankin¢ and Hill, 1978). This value was chosen since it enabled the leaching to be carried out on an accelerated time scale yet was below the rate at which flooding of the columns would occur. An intermittent irrigation regime was employed with a cycle which consisted of five days on and two days off. All columns were maintained at 34 + 2°C in a constant temperature room for the duration of the leaching tests. This temperature was used since (a) the average daily summer temperatures at Rundle range between 22 and 30°C (Rankine and Hill, 1980) and (b) retorted shale dumps have been observed to retain their heat for several years (Rankine and Hill, 1980}.
Sampling and monitoring Column leach solutions were accumulated initially daily, then weekly and finally monthly in acid-leached polyethylene or polypropylene containers. Conductivity and air-equilibrated pH measurements were made immediately on aliquots obtained from the collector vessels. An aliquot
Code R S M RC RA LR LS LM
Table 2. Column packing parameters Sample weight Density Pore volume (g) (kg m -3) (ml) 940 1002 266 783 834 509 957 1020 433 922 983 285 861 918 207 15,800 1120 4200 12,800 913 7300 14,800 1060 5800
Column leaching of oi! shale of each of the leachate solutions was filtered through a 0.1 # m porosity membrane in order to differentiate particulate and dissolved components. The total and filtered samples were preserved by acidification with nitric acid to ensure a final pH of less than 1.5. Non-acidified aliquots were set aside for anion and total organic carbon (TOC) analysis. All samples for analysis were stored in acid-leached polyethylene vials.
133
nanomolar (abbreviated to mM, g M and nM, respectively) units. Conductivity and pH values appear as a function of leach volume in the top panel of each diagram. The initial conductivity is very high for all of the columns. This is due to the high concentrations of Na, Mg, Ca, CI and SO4. The behaviour of Ca is particularly interesting since typically its concentration falls rapidly to a plateau value of 12-16 mM after the first pore volume and then declines sharply to a new steady state value after five pore volumes. Saturation index calculations carried out using the chemical equilibrium program MINEQL (Westall et al., 1976) indicate that the Ca concentration in the plateau region is being controlled by the solubility of CaSO4. The behaviour of Mg parallels this trend at lower concentration values. These results are in agreement with the published work of Bell et al. (1986). In order to facilitate further analysis of the results the data will be discussed in terms of two groups: (1) pH, HCO3 and (2) Cu, Ni, Zn, Mn, Cd and TOC.
Instrumentation and analvtwal methods Measurements of pH were made at 25 + 0.5C in the laboratory with a Radiometer PHM 64 pH meter. Major cations and A1, Fe, Mn and Zn were determined by flame atomic absorption spectrophotometry (AAS). A Labtest Model UV25 inductively coupled plasma emission spectrometer was used for the analysis of Sr, Mo and B in the column leachate solutions. Trace concentrations of Cu, Ni, As, Se and Cd were measured by graphite furnace AAS. C1, SO4, NO3 and thiosulphate concentrations in the column leach solutions were obtained by using a Dionex 10 ion chromatograph. Solution concentrations of organic carbon (TOC) were determined with a Beckman 915B TOC analyser. Samples collected for TOC measurement were stored at 2°C prior to analysis. Bicarbonate concentrations were measured by potentiometric titration.
RESULTS AND DISCUSSION
pH, HCO~
Since the leaching profiles for the large (LR, LS and LM) columns were essentially identical to those observed for the small (R, S and M) columns the data from only the former group are shown here (Figs 1-3). For Ramsay Crossing shale a small column, only, was studied (Fig. 4). All concentration values, with the exception of those for organic carbon, are expressed in millimolar, micromolar and
The pH in the columns is controlled by the HCO.~/CO2 buffer system. In all of the raw shale and mixture columns the pH values oscillate within the range 8.2-8.8 and the HCO3 concentrations decline steadily to reach plateau values at high pore volumes. However the retorted shale columns behave in a significantly different manner. For column LS (see Fig. 2) there is a decline in pH from 7.9 to 7.6 over 12OOO
\ pH
-.~..
g'4
8000 CONDY
pH
Condy - /,000'uS cm"~
$'2
I
t
I
!
r
t
L
--
0
J
9"O
Co
s~ [Col 12 IHC031 IF /' mM ~
Hg HCO3
"x,-:--
~ I
t
I
1"5 ~Cu
Ni
ICul }~ [Nil 1.00
1"0 |Zn|
~Zn
O'S/JH
nH 200
t.nl ISr]
"/
/\
~M
\
'-.
ISOtj /..o mH
2-0
-% I
6"0 lMgl
---
Hn TOC
~ S r
\
[TOe1
~VL 0
I
1
I
2
I
3
I
~. !,
I
I
I
I
~ 1~ 17 21 25 29 PORE VOLUMES
I--°--~
'-'-
33 37
d
Fig. t. Water leaching of the large column (LR) containing Kerosene Ck raw shale (CONDY = conductivity).
L34
DAVIDR. JONESet J
1
[
i
i
1
al.
i
1
I
pH
"~
a, t
8-2 .i ~
6OOO
pH
H
CONOY
7,8
-~2000
"~'\
7"6 2e
16 ~
! (Mgl
Ro112 Is~l e Jlld 4-
[I"EO}] m.
1
w
~Cu ---TOC 'Sr
ICul
Mn.lpld Ni 430nil
nM
~rl /JH
fmCl milL
0
1
2
3
k
9
13 1"/ 21
PORE VOLUMES
5
Fig. 2. Concentrations of elements in leachate from retorted Kerosene Ck shale packed into a large column (LS). The small downward-pointing arrow on the copper line indicates that the detection limit for this element has been reached.
i
1
I
I
1 "
I
I
I
I
~1200
h ' l i f pH \ 8"4 ' ~ . . . . . . '
"'\,/
I
I
pH
CONOY
Condy
8000
#S cm"1
8.2
t.O00 "'--' . . . . . .
•
[% oJ [SOt,] , 1
"\
"~'..
raM2( 1
I
't
I
"
"
i
r
i
|
0
~'IHC03I
2mM
50t Ng - - - Hco~.
200
Cu
Ni
[Nil nM
100
nH ¢
,
I
;
i
120
Nn
11'00
TO(: Sr I
0
t
2
3
4. S
9
I1
POREVOLUMES
I
80aglL [Srl I
I
17 21 25 Z9 33
Fig. 3. Water leaching of a 1: 1: 1 mixture of Kerosene Ck raw and retorted shales and Lower Ramsay Crossing claystone(column LM). Downward-pointingarrows indicate that the detection Zimitfor analysis has been reached.
Column leaching Of oil shale i
\
1
,
w
i
I
1
i
135 I
"\
i
i
1/ooo coNoY 80oo ,usc~1 4ooo
pH Candy
8.6 PHs.6 8.2 I
120 lMgl 80 I
t
1
t
"-"T ""-I" ~r
,
-
• .
Ca
s0/. - Hg • HCO3
"X
mM
-
[~l I mM ~01
i
[CnUM] I 1200
,
,
I
,
t
i
~0 10 ~
Cu Ni
\
0
t'lNil IZn] ,tim
Zn
800 ~,00 0 IMn]60 I
r
I
i
0
Mn
--- TOC
ll~l ~'° mglL20 f 1
0
i
-\
2
~
I
J'~
"'r"
-"T"
I. S 25 &5 65 PORE VOLL~ES
i5 ,d5 , 5,25
Fig. 4. Levels of components in the leachate from a small column (RC) packed with Ramsay Crossing shale. the first three pore volumes (PV). This value is maintained up to 9 PV and then begins to steadily rise until it reaches 8.4. The change in HCO3 concentrations follow the pH trends. Thus in the case of the LS column the HCO3 concentration at 5PV is 0.2mM whereas by 22PV it has risen to 2 m M . Similar pH and HCO~ profiles were exhibited by column S. The reason for the behaviour of the pH and HCO3 values in the retorted shale columns is not readily apparent. Reductions in pH from I 1 to 7.6 over the first pore volume of leachate have been reported for field lysimeters containing retorted shale from Anvil Point. Colo. (Garland et al., 1979). However, these workers considered that uptake of atmospheric CO., by the retorted shale was unlikely to be the cause. Instead, the oxidation of the reduced S species, thiosulphate, to sulphuric acid by microbial or chemical means was advanced as the probable explanation. Evidence for the presence of substantial amounts of thiosulphate in the retorted shale used for the current work is provided by the significant concentrations of this ion that were found in the first PV of leachate (see later). Whilst oxidation of this species may account for the initial drop in pH, the subsequent rise [the field lysimeter data from Garland et al. (1979) indicate a similar trend] is still unaccounted for. Previous workers have examined the leaching of Kerosene Ck retorted shale under both aerobic and
anaerobic conditions (Bell et al., 1982) and found that the pH in an aerobic column decreased to ~ 7.5 after 6 PV whereas it increased from 8.5 to ~ 8.8 in another column which was maintained under a nitrogen atmosphere. The following explanation for the behaviour of the retorted shale columns studied in this work can be advanced on the basis of these results. For several pore volumes after the start of leaching, the whole column was probably aerobic owing to occluded air. During this period oxidation of thiosulphate led to a decrease in pH. Ultimately, however, the bottom section of the column became anaerobic and at this point the pH of the leachate began to rise until the system achieved a new steady state. Cu, Ni, Zn, Mn, Cd and TOC
The data presented in Figs 1-5 represent the total concentrations (dissolved plus particulate) of these metals in the ieachates, The results for Cd are not plotted since in all cases they were below detection limit (3 nM). Filtered samples were also collected to enable the dissolved fraction to be measured. The concentrations of these metals in the first and subsequent pore volumes of the retorted shale columns were either on or below the instrumental detection limits. However the raw shale column (R, RC, LR) leachates contained readily detectable levels. There was very little ( < 2%) particulate Ni or Zn in leachate solutions from the columns. However, in the first half
136
DAVIDR. JON~ et at. I
I
I
~S wS ~LSI
2"0
---
LMI
----LRI
1-S
#M 1.0
i*
0-5
!
0
1
l
3
t,
a
12
16
20
~
18 32
PORE VOLUMES
Fig. 5. Concentrations of arsenic as a function of pore volume in leachate from selected columns. of a pore volume up to 30% of the Cu for the R and M columns and up to 90% for the S column was in a form that would not pass through a 0.1/am filter membrane. Total organic carbon (TOC) is included with this group of metals since the complexing ability of natural organic ligands for Cu, Ni, Zn and Mn is well known (Mantoura et al., 1978; Shoikovitz and Copland, 1981). Particularly strong complexes are formed with Cu. Circumstantial evidence for Cu-organic association in this case is provided by the strong linear relationship between Cu and organic carbon for leachate from the LR column ([Cu]nM = 40.1 [TOC]mM - 4.9; correlation coefficient = 0.994). When LR leachate which had been filtered through a membrane of 0.1 g m pore size was passed through a column of Biorad SM2 macro-reticular resin, 50-60% of the Cu and Ni were removed from solution. This result strongly implies that the metal bound to the resin was present in the column leachate in the form of relatively hydrophobic metallo-organic complexes. The majority of the remaining Cu and Ni is probably associated with carbonate (Stiff, 1971; Leenhecr et al., 1981; Gruber and Tripathi, 1983) in ion pairs. No evidence was found for the existence of Mn--organic complexes. The data presented here for the concentrations of Cu, Ni, Zn, Cd and Mn in the retorted shale leachates are similar to those published by Bell et al. (1982) for leachate from Lurgi retorted Kerosene Ck shale. All values are just above or below instrumental detection limits. However, results obtained by Batley (1983) for Cu, Zn and Cd in the first pore volume of leachate obtained from freshly retorted (by the Fischer assay procedure) Rundle shale were orders of magnitude higher. The reported values for these elements were
120, 933 and 87/JM, respectively. Moreover, the concentrations in leachate from raw shale (27, 23 and 5.3/aM, respectively) were also much higher than those recorded in the present work. The origin of these enormous differences is at present unknown but it could have been the particular sample which was used for the Fischer assay experiments. A further possibility is that the product of this simple laboratory method for determining oil yield could display markedly different leaching behaviour to that of the pilot Lurgi process. Workers in the United States have shown that different retorting methods can yield products with quite different leaching characteristics (Stollenwerk and Runnells, 1981). It may also be significant that the retorted shale used by us and by Bell et al. (1982) had been stored in sealed drums for 1-2 years prior to the leaching tests. The reasons for the above discrepancies in elemental concentrations should be established since the levels detected by us and Bell et al. (1983) are well below Australian Water Resources Council (AWRC) and World Health Organization (WHO) guidelines (Garman, 1983; World Health Organization, 1971) whereas those quoted by Batley are orders of magnitude higher. Other components o f column leach solutions
The components discussed above were measured in all columns. However there are a number of species which were (a) not measured in all columns or (b) were not detected in all columns. The elements Mo and B fall into the former category whereas As, Se, NO~- and thiosulphate fall into the latter. Of the components which were detected in the leachate solutions these are the ones which are likely to have the greatest environmental impact since they are present in significant concentrations at ambient pH.
Column leaching of oil shale
137
(D)
'r I/I
IM
~R
--Ls \
--" ---LM --RC
\ A/'X ISel
-'
•
2-
(B)
~LR
0
1
----LM ~
[Mol pH 15
LS
1(]
!
60
PORE VOLUMES
r
lag
i'.,,
I
i
i
- - - LM
I
tR
tel
3
1
PORE VOLt,II4ES
--'-LN ~ LS
I.O
I Hill mM
iiii 120 ~M 11o
10
t,o
I lo
L 20
i
1
30
t.o
PORE VOLUMES
',,)., \ 1
2
PORE VOLUMES
Fig. 6. Concentration profiles for the leaching of thiosulphate (S:O~-) (A), molybdenum (B), boron (C), selenium (D) and nitrate (E) from selected columns. Arsenic. The concentrations of As in leachate from columns containing retorted shale fell rapidly from a maximum value in the first half of a pore volume to a plateau value of ~ 0 . 4 ~ M at 2 pore volumes (Fig. 5). The concentrations decreased only very slightly thereafter. An exception to this behaviour was provided by the S and LS columns, in which the concentrations rose to 0.7 and 0.5 gM, respectively after 10 PV. Measurable concentrations of As were also detected in the leachate from the LR column. However an initial high concentration pulse was not observed and throughout the life of the column the concentration fluctuated around 0.15 g M. Since there was no significant difference in concentration between filtered and unfiltered samples for the five columns appearing in Fig. 5 all of the As is in solution. The pH dependence of the leaching of As has already been discussed previously (Jones, 1990) and so this will not be repeated here. However, it should be noted that the pH of aged retorted shale (such as in the case of the sample used for this work) is invariably lower than that of fresh material owing
to adsorption by the solid of atmospheric CO:. Thus if freshly retorted material had been leached, substantially higher levels of As may have been observed in the first pore volume. Thiosulphate. Thiosuiphate is not found in raw shale or claystone but is produced in the reducing environment which occurs during retorting. Since the thiosulphate anion is highly soluble it is found as a major component in the first pore volume of leachate from retorted shale [Fig. 6(A)]. It is an environmentally important product since its oxidation leads to the production of sulphuric acid. Previous reference has been made to the role which this process may play in the reduction of pH in field lysimeters containing retorted shale (Garland et al., 1979). Concentrations of up to 2 mM were found in leachates from the retorted shale columns. However, an unexpected result was that no thiosulphate was detected in leachate from the columns containing the mixture of solid wastes (i.e. M and LM). There are two possible explanations for this phenomenon. Firstly, the thiosulphate may have been adsorbed by
138
DAVIDR. JONESet al.
the raw shale or claystone. Secondly, the unretorted materials may have contained bacteria which were capable of rapidly oxidizing thiosulphate. At present there is insufficient evidence to decide between these possibilities. Molybdenum and boron. The concentration of Mo in the first quarter pore volume of the retorted shale is about 25 times higher than that observed for the raw shale [Fig. 6(B)]. However the exponential decrease in concentration over the first 2 PV in leachate from the former column indicates that the concentration of Mo is principally controlled by the dissolution of readily soluble salts present on the surface of the retorted shale particles. The essentially constant concentration of ~ 1 laM for up to 4 PV in raw shale leacbate suggests that slow dissolution of a less soluble form is the controlling process. Previous workers, using a chemical equilibrium computer model, have shown that under the pH conditions prevailing in the columns the MoO~- ion should be the predominant form in solution, with its concentration being controlled by the solubility of powellite, CaMoO4 (Stoilenwerk and Runnells, 1981; Gruber and Tripathi, 1983). Unlike its effect on Mo concentration, retorting of the shale does not lead to a dramatic increase in the concentration of B in the first fraction of a pore volume [Fig. 6(C)]. Instead, the concentration in retorted shale leachate is consistently twice that observed for raw shale up to 25 PV. Although data points for the LM column are available for only 5 PV it is quite clear that the levels of B are equal to, or exceed, those observed for the retorted shale. This result is not in accord with a simple mixing ratio. However, it is known that B tends to be associated with clay minerals (Stoilenwerk and Runnells, 1981) and if these materials are the reservoir for this element in Rundle shale, then the above observations can be accounted for. Selenium. The data for this element are plotted in Fig. 6(D). Although the concentrations were below the instrumental detection limit (0.002gM) in leachates from the retorted shale columns, high concentrations were observed in the first pore volume of the columns which contained raw shale. This latter result was not expected since Se is usually found as a component of very insoluble sulphide minerals; for example, pyrite (Fischer et al., 1978). However, water soluble Se salts could have been liberated as a result of oxidation of the pyrite in the shale. Since a very large reservoir of soluble sulphate is present in the shale then this anion would tend to occupy the anion exchange and adsorption sites which, in its absence, would have immobilized the trace levels of oxidized Se compounds. Selenate, especially, should be mobile under these conditions whereas any selenite produced would probably be tightly bound by the iron oxides and hydroxides and smectite clays present in the shales (Geering et aL, 1968; Ahlrichs and Hossner, 1987). Solution speciation studies on the column
leachate showed that all of the Se was indeed present as selenate. Nitrate. The occurrence of very high concentrations of NO3 in the first pore volume from columns containing Kerosene Ck raw shale was unexpected [Fig. 6(E)]. None was detected in leachate from Kerosene Ck retorted shale (presumably the NO~ was destroyed during retorting) or Slot Cut raw shale (RC). The origin of the NO3 is unknown but it is most unlikely to have arisen from fertilizer input. Local farmers do have grazing rights but the land is not cultivated. Oxidation of the organic matter within the shale seam could lead to the production of NO, (Leenheer et al., 1981; Ronen et al., 1983). However, if this explanation is correct then the absence of NO3 in leachates from the Ramsay Crossing shale needs to be accounted for. Effect o f column size on leachate composition
The large columns contained, on average, 16.2 times more material than the small columns and the cross-sectional areas were in the ratio of 6:1. In order to ensure that both sets of columns received the same "rainfall equivalents", the ratio of application rates was also 6:1. However, the packed length of the large columns was almost three times longer than their smaller counterparts and so the question arises of how to best compare the leaching behaviour of the two types of columns. The most fundamental way of achieving this is to compare the leachate compositions on a pore volume basis. From the point of view of environmental impact the first pore volume is of the most interest since the concentrations of the majority of the potentially harmful trace elements are at their highest in this initial volume of leachate. Consequently, the greatest impact on receiving surface and groundwaters is likely to be made under these conditions. Accordingly, the overall volume-weighted concentrations of species present in leachate collected over the first pore volume for the small and large columns are presented in Tables 4--6. There is excellent correspondence between the concentrations calculated for most components (in the case of A1, B, Mo and Sr, data are available only for the large columns). It would thus appear that the leachate composition over the first pore volume is independent of the amount of material leached. Although the raw and retorted shale columns contain very similar amounts of Na, K and Mg (data from Tables 2 and 3), comparison between the first pore volume concentrations of these components in Tables 4 and 5 reveals that there has been a major reduction in their leachability following retorting. This result was unexpected since these elements are not lost to any significant extent during this process (Dale and Fardy, 1984). Moreover, the surface area of the retorted shale is approximately twice that of
Column leaching of oil shale Table 3. Composition of shale samples (retool kg-~) * Analyte
Kerosene Creek shale
Retorted Kerosene Creek shale
226 (15) 240 (15) 280 (20)
330 (20) 360 (20) 440 (30)
Na K Mg
• Figures in parentheses represent two standard deviations for replicate determinations. The concentrations of these elements, were determined by AAS analysis of solutions produced by HF/HNO3 digestions of the solid samples
the raw shale (Bell et al., 1986) and so any dissolved salts on the surface of the particles should have been more accessible to the leaching medium. It has been well documented that decomposition or transformation of the mineral matrix may occur as oil shales are heated to high temperatures (Campbell and Burnham, 1978; Ward-Smith et al., 1978; Patterson, 1986; Quezada and Patterson. 1986) and that the rate and extent of these processes is markedly dependent on the temperature attained and the retort technology that is used (Ward-Smith et al., 1978). The retorted shale supplied for this work was produced in a Lurgi pilot plant in which temperatures probably ranged from ~ 500C in the retort up to ~ 700°C in the lift pipe section where the residual carbon is combusted. Since a portion of the combusted shale is recycled as a heat exchange medium, some of the material may have been exposed to elevated temperatures for a considerable time. Published work on high temperature reactions of Green River shale (Ward-Smith et al., 1978) indicates that a temperature of 700°C would not be high enough to account for the immobilization of Na, K and Mg by the formation of the metamorphic minerals augite and melilite. However, the kinetics of these processes are likely to be very dependent on the specific mineralogy of the shale which is retorted and so this possibility cannot, as yet. be ruled out for Rundle shale. Table 4. Composition of first pore volume raw shale columns Analyte
Small
Large
Na mM K Mg Ca Sr
207 1.97 38.7 16.04 ND*
169 1.55 31.7 17.12 0.08
Mn p M Cu Ni Zn A1 Mo B Se As Cd nM
92.8 0.57 0.72 0.55 ND ND ND 2.15 <0.03 <3
84.3 0.35 0.41 0.69 3.0 0.88 88.5 2.27 0.18 <3
CI mM NO~ SOl HCO3
93.7 43 84.7 10.0
85.8 40 58.8 8.77
SzOi -
< 0.02
< 0.02
TOC m g l ~
190
170
*ND = not determined.
139
Table 5. Composition of first pore volume retorted shale columns Analyte
Small
Large
Na mM K Mg Ca Sr
11.9 0.62 1.56 22.2 ND*
11.6 0.56 1.41 25.6 0.11
Mn ,aM Cu Ni Zn AI Mo B Se As Cd nM
<1 0.008 < 0.03 0.17 ND ND ND <0.01 0.91 <3
<1 0.017 < 0.03 <0.08 26.4 7.4 141 <0.02 0.84 <3
C1 mM NOf
18.6 < 0.02
22.1 < 0.02
]8.8
19.8
sot HCO3 S,O~TOC mg I - ~
0.5 1.33 8.0
0.5 1.95 8.3
*ND = not determined.
Effects o f mixing o f shale wastes In a real waste dump reject raw shale, retorted shale and interburden will be mixed together. Accordingly, columns containing a 1 : 1 : 1 mixture of these materials were also prepared (note--the raw shale was of a much higher grade than would normally be placed in the waste dump). The data for the concentrations of components in the first pore volume of leachate from the large columns (Tables 4--6) can be used to evaluate the effect of mixing. For Ni, Mo, As, Se, NO3 and TOC the concentration values follow approximately the mixing ratio. However, for Cu the reduction is 4-fold greater than the mixing factor and for Mn it is 2-fold. The reduction for Mn is very significant since this element has been identified as exceeding water quality guidelines in the leachate from several of the columns containing raw shale. Table 6. Composition of first pore volume mixed shale columns Analyte
Small
Large
Na mM K Mg Ca
93.0 1.57 16.8 15.3
109 1.48 23.4 18.5
Mn /~M Cu Ni Zn B Se As Cd nM
10.7 0.03 0.19 0.2 ND* 0.44 0.45 <3
14.5 0.03 0.14 0.4 133 0.72 1.0 <3
42 8.7
66 8.7
52.4
55.4
1.80 < 0.02 29
1.97 < 0.02 55
C1- mM NO~SOlHCO 3 S,O~TOC mg I- '
*ND = not determined.
140
DAVIDR. JONESet al.
Table 7. Australiandrinkingand irrigationwatercriteria(,uM units) Element Drinking water* Irrigation watert Mn 9 9 Cu 23.6 3 Ni -3.4 Zn 76.5 30.6 Mo -0.1 B 93 69.3 Se 0.13 0.25 As 0.67 1.33 NO; 710 -*Garman (1983). ";'Derived working levels for water used continuouslyon soils (Hart. 1974).
Rundte Project Group of ESSO Australia Ltd, for providing the oil shale samples and associated background information. Funding for this project was partly provided by the National Energy Research Development and Demonstration Program (NERDDP) and ESSO Australia Ltd. Robyn Shapland, Nadia Shargholi, Michael Randle, Jaswant Jiwan, Joe Borg, Donna Harkess and Mary-Ann Stanley assisted with the analyses of the hundreds of water samples generated throughout the project. John Eames from the CSIRO Division of Exploration Geoscience supervised the ICP analyses. The authors would like to thank the Queensland State Government Analytical Laboratories for doing many of the initial organic carbon analyses.
CONCLUSIONS
REFERENCES
Comparison of Australian drinking and irrigation water quality criteria (Garman, 1983; Hart, 1974) with the concentration values obtained for the first pore volume of leachate from the columns (see Tables 4-6) show that a number of components exceed the recommended levels. These are Mn, NO3, Se and B for the Kerosene Ck raw and mixed shale columns (R, LR, M, LM) and Mo, B and As for the mixed and retorted shale columns (M, LM, S, LS). The Mn, Mo, NO3 and Se levels are particularly noteworthy since their respective water quality limits are exceeded by at least one order of magnitude. The Se levels are also high in the initial leachate from the column containing Ramsay Crossing raw shale [see Fig. 6(D)]. Thiosulphate was the major trace component in leachate from columns containing retorted shale alone. Concentrations of up to 2 mM were measured in the first two pore volumes. However, this anion was not detected in leachate from the columns which contained a mixture of retorted shale with raw shale and claystone. Concentrations of the heavy metals Cu, Ni, Zn and Cd did not exceed the water quality guidelines in the leachate from any of the columns. Approximately 50% of the Cu and Ni leached from the Kerosene Ck shale was found to be complexed by water soluble organic compounds. The concentrations of components in the first pore volume of leachate from both the small and large columns were very similar. The large columns contained an average of 16 times more material than their smaller counterparts and this provides a reasonable basis for concluding that the composition of the first pore volume is independent of the amount of material which is leached. Thus the results of small scale column leaching tests might reasonably be extrapolated to the leachate which would be generated within the waste dumps to be constructed at Rundle. However. in order to obtain an indication of the likely concentrations of components in the leachate, a mixture of solid wastes with a composition which is more representative of the proposed commercial operation would have to be used. An essential requirement would be that the retorted shale sample is produced by a pilot plant which closely simulates the actual retorting conditions that will be employed.
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Acknowledgements--The authors would like to thank the
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