Published in J. Environ. Qual. 32:2373-2378 (2003).
© ASA, CSSA, SSSA
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TECHNICAL REPORTS
Waste Management
Effect of Peroxide on Neutralization-Potential Values of Siderite and Other Carbonate Minerals
J. L. Jambor*,a,
J. E. Dutrizacb,
M. Raudseppa and
L. A. Groata
a Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada
b Canada Centre for Mineral and Energy Technology (CANMET), 555 Booth Street, Ottawa, Ontario K1A 0G1, Canada
* Corresponding author (JLJambor{at}aol.com).
Received for publication July 22, 2002.
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ABSTRACT
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To assess quantitatively the effect of peroxide addition to standard static tests of the neutralization potential (NP) of mine wastes, 10 specimens of carbonate minerals, including five of siderite (FeCO3) and two of rhodochrosite (MnCO3), were analyzed by electron microprobe. The compositions of the siderite span a range from 60 to 86 mol % Fe. Tests of NP for the siderite diluted with 80% (w/w) kaolinite gave values of 647 to 737 kg CaCO3 equivalent per Mg for determinations by the standard Sobek method. However, if it is assumed that the ferrous carbonate component of the mineral does not contribute to NP in field situations because oxidation of Fe(II) to Fe(III) and the subsequent hydrolysis of Fe(III) leads to the release of an equivalent amount of acid, then the calculated NP for the samples ranges from 110 to 390 kg CaCO3 equivalent per Mg. Two different methods involving the addition of peroxide to the test solutions were successful in bringing the measured NP values closer to the theoretical ones. By contrast, the tests with rhodochrosite indicated the Mn(II) to be stable. For long-term environmental planning, especially for wastes from metalliferous sulfide-poor deposits in which gradual dissolution of silicate and aluminosilicate minerals may be involved in attenuating the acidity, consideration in the overall NP budget needs to be given to the ferrous iron content of those minerals. The presence of Fe2+–bearing minerals, especially carbonates, in tested mine-waste materials may lead to overestimated Sobek NP values, thus increasing the risk of poor-quality drainage and the need for costly remediation.
Abbreviations: NP, neutralization potential
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INTRODUCTION
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THE ASSESSMENT of potential acid drainage is crucial in the management of large-scale disturbances of soils or subsurface materials, especially if those materials contain significant amounts of sulfide-bearing minerals. Assessment by predicting acid drainage is commonly done by obtaining an acid–base accounting, typically a measure of the total alkalinity and total acidity that is yielded by each sample. The potential acidity is most commonly determined by measuring either the total sulfur content or the sulfur present as sulfides. In either case, all of the sulfur is assumed to be present as pyrite that will oxidize and thereby generate acidity. The alkalinity, by contrast, is released from the nonsulfide minerals. Some minerals, such as quartz, are inert for practical purposes, and the amount of base contributed by most of the common rock-forming silicates and aluminosilicates is relatively low (Jambor et al., 2000, 2002). The simple carbonate minerals, however, are readily soluble in weak acids and yield substantial alkalinity.
In acid–base accounting, the alkalinity of a sample is referred to as its neutralization potential (NP). To quantify NP for environmental purposes, the standard procedure is to relate the measured values to the potential neutralization capacity of calcite (CaCO3), whose molar weight is nominally 100. The standard unit for expressing the NP is kilograms of CaCO3 equivalent per Mg of material (or variations of this standard, such as U.S. tons per 1000 U.S. tons). Further details concerning the derivation, determination, and interpretation of NP are given in numerous publications, such as those by Sobek et al. (1978), Mine Environment Neutral Drainage (1991), Skousen et al. (1997), Lawrence and Wang (1997), and White et al. (1999).
The most widely used method to assess NP is that of Sobek et al. (1978). In the Sobek method, a small portion of the sample is tested for its "fizz" rating by the addition of a few drops of 25% HCl. The resulting fizz, denoted by none, slight, moderate, or strong, governs the volume and strength of HCl to be added to a 2-g test sample, that is, no fizz, 20 mL at 0.1 M; slight fizz, 40 mL at 0.1 M; moderate fizz, 40 mL at 0.5 M; strong fizz, 80 mL at 0.5 M. The appropriate amount of acid is added to the sample, and the contents are heated to almost boiling until the bubbling from the solids ceases. The solution is then diluted with water to 100 mL, boiled for 1 min, and then cooled and titrated with 0.1 M NaOH to a pH 7.0 end point.
The fizz rating, although subjective, is important because the results determine the volume and strength of the HCl that is added to the sample. The effervescence is the evolution of gas from carbonate dissolution and does not include the reaction of silicates and aluminosilicates. However, siderite [FeCO3], as well as the Fe2+ component of other carbonate minerals such as ferroan dolomite [Ca(Mg,Fe,Mn)(CO3)2] or ankerite [Ca(Fe,Mg,Mn)(CO3)2], will not produce as much NP as nonferrous minerals because the neutralization is offset by the subsequent oxidation of Fe2+ and hydrolysis to yield Fe(OH)3. In the Sobek test, however, there is insufficient time for the oxidation and hydrolysis to occur, or to react to completion. The consequence is that the presence of ferroan carbonates will lead to an overestimation of NP because, even though the Fe2+CO3 component contributes to the NP of the Sobek test, the net NP in field settings is zero. Siderite and ankerite are commonly present as gangue minerals in association with metalliferous occurrences that span a diverse geological spectrum, such as massive sulfide, gold-vein, and polymetallic-vein deposits, and siderite is common in association with some coal seams (Palache et al., 1951; Renton, 1982). Thus, the failure either to recognize the presence of ferroan carbonates or to compensate for their effects in measured Sobek NP values has important implications in predicting acid drainage for mining plans and mine closure.
The difficulty of estimating NP in the presence of ferroan carbonates was discussed by Skousen et al. (1997), who tested several siderite-bearing samples and compared the results that were obtained after employing a variety of sample-testing methods. The samples used by Skousen et al. (1997) consisted of overburden materials whose mineralogy was determined semiquantitatively by bulk-sample X-ray diffractometry. We have extended the work of Skousen et al. (1997) and Frisbee and Hossner (1995) by performing standard Sobek tests and additional tests with peroxide for 10 monomineralic specimens of carbonate minerals, including five siderite samples, and all samples were characterized by electron microprobe analyses. The additional compositional quantification has allowed comparisons of the measured versus predicted NP values both before and after addition of peroxide in the NP tests. Peroxide accelerates the oxidation of Fe(II), thus overcoming the aforementioned problem concerning the incomplete hydrolysis, to Fe(OH)3, that is encountered in the Sobek test. A principal objective of this study was the assessment of whether peroxide addition to minerals containing ferrous iron brought their NP values close to the theoretical values. Here we compare the results from the two methods of NP estimation.
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MATERIALS AND METHODS
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The carbonate minerals were obtained from collections at the University of British Columbia. Samples were culled from museum-grade display specimens and from monomineralic and near-monomineralic teaching specimens (Table 1), and a small portion of each was examined by scanning electron microscopy and energy-dispersion analysis to estimate the composition. Several samples were rejected after these analyses because they were near end-member calcite or dolomite, or the composition differed little from those of other selected minerals. The 10 samples chosen for further study were crushed by hand to pass screening at 0.25 mm (-60 mesh), and the screened material was then subsampled for X-ray diffractograms and polished sections for electron-probe microanalysis (EPMA). The EPMA analyses were obtained with a Cameca (Courbevoie, France) probe in the Department of Earth and Ocean Sciences at the University of British Columbia. The CO3 contents of the minerals were calculated from the cation contents, and the formulas were normalized assuming carbonate stoichiometry (Table 2).
Initial Sobek tests and those using peroxide addition (the SobPer method of Skousen et al., 1997) were performed by autotitration at B.C. Research Inc., Vancouver, BC, Canada. All subsequent tests were done at Canada Centre for Mineral and Energy Technology (CANMET) by manual titration. Kaolinite used as a diluent in some of the tests is from Macon, GA, and is the same material for which previous tests gave NP = 0 (Jambor et al., 2000). Although no longer in its original container, the kaolinite is part of a set of clay standards at UBC and is believed to be the American Petroleum Institute standard for which X-ray data are given by Molloy and Kerr (1961). In the tests involving kaolinite, the 2-g sample consisted of 1.6 g kaolinite and 0.4 g of the carbonate mineral. In all standard Sobek tests, the NP of a sample, in kilograms of CaCO3 equivalent per Mg of material, is calculated as:
 | [1] |
where a is the molarity of HCl, b is the molarity of NaOH, c is the sample weight in grams, x is the volume (mL) of added HCl, and y is volume (mL) of NaOH added to achieve pH 7 (Mine Environment Neutral Drainage, 1991).
Two different protocols involving peroxide addition were used. In the SobPer method of Skousen et al. (1997), the standard Sobek procedure was followed and included titration to pH 7, addition of 5 mL of 30% H2O2, boiling the solution for 1 min, cooling, and titration again to pH 7. A second cycle of peroxide treatment is employed if the solution appears blackish or greenish or if the pH decreases. The total amount of NaOH from all of the titrations is used in calculating the NP. The second peroxide test, which for convenience is referred to as the Mine Environment Neutral Drainage (MEND) test, follows the Sobek procedure to the point at which the NaOH titration begins. At this point the leachate is filtered, 5 mL of 30% H2O2 are added, the solution is boiled again, cooled, and titrated to pH 7. In the text that follows, all NP values are in kilograms of CaCO3 equivalent per Mg of the mineral or material that was tested.
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RESULTS AND DISCUSSION
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Carbonate Minerals Other Than Siderite
Table 3 provides a comparison of the results that were obtained by using the various test methods, both standard and with peroxide additions. The two dolomite samples were low in Fe, and all test methods, including the standard Sobek test, gave results that, although not precisely matching the calculated theoretical NP, are at least of the correct order of magnitude. Not shown, however, are the initial SobPer results, which for all 10 samples gave NP values of less than 100. The reason for this discrepancy is not known, but it was thought that the use of undiluted monomineralic samples may have been overwhelmingly carbonate-rich for the test system. For example, even with the standard Sobek test it was found necessary to increase the acid addition from 80 mL of 0.5 M HCl to 120 mL of 0.5 M HCl; the Sobek NP values in Table 3 are for the higher volume of acid. Therefore, subsequent tests were conducted with samples consisting of 80% (w/w) kaolinite and 20% (w/w) of the carbonate mineral.
With all of the test methods the smithsonite sample gave highly erratic NP values, all of which are substantially lower than the theoretical NP (Table 3). The erratic results are related to the formation, during the NaOH titration, of a voluminous greenish precipitate that gradually turned reddish brown. The compact, cohesive, waxy-appearing precipitate that was collected from the Sobek test of undiluted smithsonite was found by X-ray diffractometry to consist of halite (NaCl), simonkolleite [Zn5(OH)8Cl2·H2O], and a compound with a hydrotalcite-type structure, possibly corresponding to [Zn6Fe3+2(CO3)(OH)16·4H2O]. The presence of the hydrotalcite-type phase indicates that the acidification reaction did not go to completion.
The two rhodochrosite samples tested are low in Fe [maximum 0.19% (w/w) FeO]. Both produced a small amount of orange precipitate in the Sobek test, thus indicating that hydrolysis of the Fe had occurred. Rhodochrosite no. 4 gave NP values consistently lower than the theoretical value, but comparison of the Sobek versus SobPer results suggests that Mn(II) is not readily susceptible to oxidation; unlike Fe(II), there may not be a need for a compensatory NP adjustment for the presence of Mn(II).
Siderite
The five specimens of siderite span a substantial range in composition and solid-solution Fe (Table 3). The nonperoxide tests for siderite gave substantially higher NP values than the theoretical ones calculated by assuming that the Fe2+ component of siderite does not contribute to the NP because the subsequent hydrolysis of Fe(III) creates acidity. The reasoning behind the disallowance of Fe2+ can be seen in the following equations that are given by Skousen et al. (1997) to describe the reaction of siderite with the excess of HCl in the Sobek test:
 | [2] |
Additional HCl is consumed as the ferrous ion slowly oxidizes to ferric ion:
 | [3] |
Upon titration with sodium hydroxide:
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Reaction [4] as given by Skousen et al. (1997) can be written as:
 | [5] |
 | [6] |
where the NaOH represents the base added during the back-titration to culminate at pH 7. The net effect is that two moles of HCl are consumed in Eq. [2], and another mole in Eq. [3], whereas 3 mol NaOH are used in Eq. [6], resulting in zero NP. In typical acid-drainage conditions the Fe(II) is oxidized to Fe(III), but in the short duration of the standard Sobek test little of Eq. [3] is completed, thus accounting not only for the high NP test values of siderite (Table 3), but also for the observed variation. The addition of peroxide accelerates the oxidation of Fe(II) and overcomes the difficulties of the short-duration Sobek test.
The results in Table 3 indicate that both the SobPer and the Mine Environment Neutral Drainage tests were effective, as is summarized in Table 4. If the results for all of the carbonate minerals, not just siderite, are considered, SobPer arguably appears to be the slightly superior of the two methods. As was noted by Skousen et al. (1997), the addition of peroxide after the initial titration, rather than before it, results in a higher solution pH in the SobPer method, thereby promoting the precipitation of Fe(OH)3. An operational advantage of the SobPer method is that it does not require a filtration stage; however, a disadvantage is that the peroxide may oxidize sulfides that are in the residue (Jennings et al., 2000).
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Table 4. Summary of the results for the SobPer and Mine Environment Neutral Drainage (MEND) tests of siderite, where the term  is the difference between the calculated neutralization potential (NP) and the measured value.
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Silicates and Aluminosilicates
Although the principal focus was on the effects of Fe(II) on the NP that is derived from carbonate-mineral dissolution, it is common in metalliferous mineral deposits to have Fe2+–bearing gangue minerals other than carbonates. The silicates and aluminosilicates in most metalliferous deposits are Mg-dominant, but some degree of Mg–Fe solid solution is the norm, and in rare cases Fe2+ is dominant. However, little attention has been paid to this aspect of the interpretation of the Sobek NP results from static tests, possibly because significant dissolution of the noncarbonate minerals is kinetically hindered and does not occur until acidic conditions have already been established. Nonetheless, the contribution of noncarbonate NP is of practical importance insofar as NP from bulk assemblages may be involved or invoked in the amelioration of acid drainage in long-term scenarios (Ritchie, 1994; Strömberg and Banwart, 1994; Kwong and Ferguson, 1997; Lawrence and Scheske, 1997; Paktunc, 1999; Hutt and Morin, 2000). Moreover, whether the reaction of silicates and aluminosilicates alone can be effective in the attenuation of acidity within the microdomains of some low-sulfide [<0.2% (w/w) S] wastes is currently moot; however, the presence of a Fe2+ component in the dissolution minerals needs to be considered as a potentially significant variable if site-to-site comparisons of the acid-neutralizing assemblages in such settings are to be made.
To illustrate the principle that Fe2+ in noncarbonate minerals merits attention, two specimens of fayalite (Fe-dominant olivine) and one of grunerite (amphibole) were tested by the standard Sobek method and by peroxide methods (Table 5). In all of the tests the addition of peroxide was found to substantially reduce the NP, with the highest reduction occurring for the most Fe-rich samples.
Different volumes and molarities of HCl also affect NP measurements (Fig. 1
, Table 5). Aside from possible variations in results of the fizz test, a coincident effect is a more vigorous attack on the noncarbonate minerals while the pH is low. Thus, the effect on most minerals (but not all; see below) is to increase falsely their NP contribution. In the tests listed in Table 5, it was observed that the two fayalite samples dissolved completely when the acid addition was 80 mL and 0.5 M.
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Fig. 1. Effect of acidity on the neutralization potential (NP) of fayalite as determined by the Sobek and SobPer methods.
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Effect of Kaolinite Additions
Among the options considered as a means of decreasing the proportion of carbonate minerals in the test samples were the addition of quartz as a diluent, or simply a decrease in sample weight. The latter was rejected because of the desire to follow rigorously the Sobek test protocol, which stipulates that a 2-g sample be used. Quartz was not accepted because the samples are crushed only to less than 0.25 mm (<60 mesh), which is considered here to be a relatively coarse particle size, and there was concern about homogeneity because of the small mass of each blended sample that was to be used in the tests. Kaolinite was chosen because of its superior mixing properties, its stabile and relatively simple and stoichiometric composition when well crystalline, and because previous Sobek tests of the material available had consistently given NP values of 0 ± 1 (Jambor et al., 2000).
In view of the pronounced effects that were observed to ensue from variations at the acidification stage of the Sobek test (Table 5), similar tests were performed with kaolinite. The results show a consistent trend in which the NP values become more negative as the level of acidification is increased (Table 6). The largest negative NP (-22.5) occurred with 80 mL of 0.5 M HCl, which is the same volume and concentration used in the tests of the carbonate minerals (Table 3). However, as only 80% of the mass of each test sample consisted of kaolinite, the largest effect, even assuming no buffering by carbonate minerals, would be -18 units of NP. Although the effect of strong acidification in this case is to decrease the NP, for most samples the effect of over-acidification is to increase the NP because of the increased dissolution of the silicate–aluminosilicate assemblage; this trend, which has been well-demonstrated in several studies (Lawrence and Wang, 1997; White et al., 1999; Jambor and Dutrizac, 2002), emphasizes the importance of the fizz test in determining the volume and molarity of acid that is to be added to a test sample.
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CONCLUSIONS
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Overestimation of NP values as a consequence of the presence of ferroan carbonate minerals can have a significant impact, and may result in inappropriate design of mine-waste storage and mine closure. Design failure because of overestimation of NP could result in poor-quality drainage, long-term financial liability, and the need for costly remediation in the ensuing period. Tests of well-characterized minerals have allowed comparison of theoretical and measured NP values. Peroxide addition to oxidize Fe(II) more completely is effective in decreasing the contribution to NP that is related to the presence of Fe2+–bearing carbonates. Skousen et al. (1997) compared the results from peroxide and nonperoxide NP tests for 31 samples of overburden and suggested that the peroxide method should be used by laboratories conducting NP tests.
Overestimation of NP applies not only to ferroan carbonates and to the prediction stage of acid drainage; the same hydrolysis reaction that is the underlying basis for the overestimation also applies to the more slowly dissolving Fe2+–bearing aluminosilicates that may be invoked or involved in attenuating acidity over the long term. Thus, it is noteworthy that tests of Fe2+–dominant olivine and amphibole showed the same effects as tests of Fe2+–bearing carbonate minerals.
At many mines the principal source of NP is not a single carbonate mineral of end-member composition; rather, solid–solution compositional variation is common, as is the presence of more than one carbonate-mineral species. Detailed mineralogical studies may be necessary to establish the variations, but to monitor them in proposed or accumulating wastes can be time-consuming and expensive. However, knowledge about such variations is especially important for wastes that, in the acid–base account, yield net NP values of less than about 30 kg CaCO3 equivalent per Mg. The environmental assessment of material with such marginal net NP values can be strongly influenced by the presence of even small amounts of Fe2+ minerals, and a judicious strategy in environmental assessments or monitoring is suggested to be the adoption or periodic inclusion of the peroxide-modified Sobek tests for NP. Of particular interest to the community involved in the study of mine wastes would be detailed comparisons and interpretations of the results obtained from run-of-mine materials tested by both the standard Sobek and the peroxide methods.
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ACKNOWLEDGMENTS
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We thank D.J. Hardy of Canada Centre for Mineral and Energy Technology (CANMET) for assistance, and T. O'Hearne and T.-L. Delaney of B.C. Research Inc. for their cooperation in conducting the NP tests. Technical assistance at UBC was kindly provided by B. Pemberton. Comments by Associate Editor Michael Ebinger and two anonymous referees are appreciated and helped to improve the final version of the manuscript.
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ABSTRACT
INTRODUCTION
