Introduction

During the past year I have received numerous requests for copies of various ABA testwork procedures, particularly those developed for use in the Canadian metalliferous mining industry. While MEND Reports and Documents (e.g. Coastech, 1989, 1991; Day, 1991; Lawrence, 1995; Lawrence & Wang, 1996) are a primary source of hard-copy versions of many of the procedures, and there are numerous other sources (e.g. Steffen, Robertson and Kirsten, 1992), there appears to be no single-source electronic compilation. There has also been much interest in the peroxide method of siderite compensation in the Sobek Procedure developed by the Pennsylvania-West Virginia Overburden Task Force.

The following procedures have been culled from a number of sources, which are referenced, and represent testing methods that have seen commercial use. A degree of editing has been used to simplify electronic publishing, and to present methods in a similar format.

It is current practice to calculate acid generation potential for materials in Canadian metalliferous mines from sulphide sulphur content. This is usually accepted as the difference between total sulphur by Leco furnace or other method, and sulphate sulphur determined chemically. For a further discussion of this see Day, 1991 and Lawrence, 1997. On the other hand, acid generation potential for overburden material in Appalachian coal mines is normally calculated from total sulphur by Leco furnace.

For a discussion of the mineralogical and chemical characteristics of ABA testwork procedures, see the Acid Base Accounting (ABA) page at this site.

I am indebted to my colleagues Sohan Basra of CESL, Keith Brady of Pennsylvania DEP, Charles Bucknam of Newmont, Tom Durkin of South Dakota DENR, Kim Lapakko of Minnesota DNR, Rick Lawrence of UBC, Jeff Skousen of WVU, Rik Vos of BCRI and Carl Weatherell of MEND for assisting me in various stages of this compilation. I would also like to thank Patricia Keen and Tim O'Hearn of BC Research Inc., Vancouver, for the photographs accompanying the description of the BCRI Initial Method.

 

BC RESEARCH INC. INITIAL TEST (CHEMICAL) PROCEDURE FOR EVALUATING ACID PRODUCTION POTENTIAL OF ORE AND WASTE ROCK

Sample

The sample must be taken in such a manner that it is representative of the type of mineralization being examined. A composite consisting of split drill core or randomly selected grab samples should be satisfactory. The number of samples to be examined will depend on the variability of the mineralization and must be left to the discretion of the geologist.

The bulk sample is cone crushed to minus 10 mesh. A representative 250 g portion is split out, dried and pulverized to around 60% minus 400 mesh for assay, the titration test and, if necessary, the confirmations test.

Assay

The pulverized sample is assayed in duplicate for total* sulfur in a Leco furnace or by wet chemical methods. The acid production potential of the sample, expressed as kg of sulfuric acid** per tonne of sample, is calculated on the basis of the total* sulfur assay.

Titration Test

Duplicate l0 g portions of the pulverized sample are suspended in 100 ml of distilled water and stirred for approximately 15 minutes. The natural pH of the sample is recorded. The sample is then titrated to pH 3.5 with 1.0 N sulfuric acid using on automatic titrator. The test is continued until less than 0.1 ml of acid is added over a 4 hour period. The total volume of acid added is recorded and converted to kg per tonne of sample.

For a 10 g sample, the acid consumption is; given by:

ml 1.0 N H2SO4 x 4.9 kg/tonne

Interpretation

If the acid consumption value (in kg of acid** per tonne of sample) exceeds the acid-producing potential (kg per tonne), the sample will not be a source of acid mine drainage and no additional work is necessary. If the acid consumption is less than the acid production potential or the difference is marginal, the possibility of acid mine water production exists and the confirmation test is conducted. A pH of 3.5 is chosen for titration, as above this value, the acid-generating bacterium Thiobacillus ferrooxidans is not active.

Editor's Notes

*It is current practice to use sulphide sulphur, not total sulphur.

** It is current practice to express acid and neutralizing potential in kg/tonne CaCO3, not H2SO4.

Original Reference

Duncan, D.W. and Bruynesteyn, A. (1979), Determination of Acid Production Potential of Waste Materials, Met. Soc. AIME, paper A79-29, 10p.

Photograph 1: Bank of Automatic Titrators for BCRI Initial NP Determination

Photograph 2: Pair of Automatic Titrators for BCRI Initial NP Determination

Photographs Courtesy of BC Research Inc., Vancouver, BC

BC RESEARCH CONFIRMATION TEST

Objectives

Principles of Test

The acid potential derived by assuming total oxidation of sulphur (sulphides) in static prediction testing may not, under field conditions, be realized. To determine the degree to which the sulphur content of a sample might be oxidized, and to assess if this amount of acid is sufficient to overcome the neutralizing capacity of the sample, a biological oxidation test can be carried out. The test is usually only carried out if the sample is shown to be potentially acid producing in static testing.

A pre-acidified pulp containing the finely ground test sample is inoculated with an active culture of sulphide-oxidizing bacteria such as T. ferrooxidans and agitated under ideal conditions for bacterial oxidation. The pulp pH is monitored until stable indicating the end of oxidation. An equivalent weight to the original sample is then added in two increments after 24 and 48 hours and the pH is measured 24 hours after each addition. If the pH is above 3.5 at these times, the sample is classified as a non-acid producer, since the pH is out of the range considered essential for the growth and oxidative activity of the bacteria. If the pH remains below 3.5, a potential for acid generation is indicated.

Equipment

  1. 250 ml Erlenmeyer flasks, preferably with a baffled base to facilitate oxygen mass transfer during agitation
  2. Temperature controlled gyratory or reciprocating shaker/incubator equipped with clamps for Erlenmeyer flasks (provision for CO2 enrichment of the air is desirable)
  3. pH meter equipped with a combination pH electrode
  4. Pipette, 5 ml

Reagents

  1. Sulphuric acid, 6 N or 12 N
  2. Distilled or deionized water
  3. Reagent grade nutrient salts, typically (NH4)2SO4, KCl, K2HPO4, MgSO4.7H2O, and Ca(NO3)2
  4. Bacterial culture containing T. ferrooxidans [Cultures should be selected based on their known ability to be able to oxidize ores, waste rock or tailings of similar mineralogy to the test sample. See Section 5.9 for further discussion].

Procedure

  1. Crush and pulverize the sample to pass a 400 mesh (Tyler) screen.
  2. Prepare bacterial cultures for use as inocula using standard laboratory techniques. The cultures should have been previously grown on and adapted to ore or waste rock containing pyrite with a sulphur content at least as high as the test sample. Whole pulp inocula are preferred to cells recovered from solution only. If solids free inocula containing very low soluble metal concentrations are required, these should be prepared by differential centrifugation techniques.
  3. In duplicate, weigh out 15-30 g (low weight for high sulphur contents) of sample into an Erlenmeyer flask with 70 ml of a nutrient medium containing (typically) 3 g/l (NH4)2SO4, 0.1 g/l KCl, 0.5 g/l K 2HPO4, 0.5 g/l MgSO4.7H2O, and 0.01 g/l Ca(NO3)2.
  4. Place flask on shaker and periodically add sulphuric acid (6 or 12 N) as required to bring pH to a stable value between pH 2.5 and 2.8. Do not proceed until pH is stable.
  5. Inoculate flasks with 5 ml of an active T. ferrooxidans culture. Record weight of flask, cap flask with a cotton or foam plug, and place on shaker at 35 deg C.
  6. Monitor flask regularly for pH. Before each measurement, add distilled water to bring flask to original weight to allow for evaporation. In some cases, the sampling of flasks for a soluble species (e.g. Fe, Cu. Zn) might assist in determining progress of the oxidation process.
  7. When oxidative activity has ceased, as evidenced by a stable pH (or metal concentration), add half the weight of sample originally used and continue shaking for 24 hours.
  8. Record the pH and if above 3.5 terminate the test. If not, again add half the weight of sample and agitate for up to an additional 72 hours and record the final pH.

Typically, about 3 to 4 weeks following inoculation are required to complete this test.

Interpretation of Results

A decreasing pH following inoculation of the test pulp with bacteria indicates the biochemical oxidation of sulphides contained in the sample.

Once a stable pH or soluble metal concentration has been achieved it is assumed that all sulphide available for oxidation has reacted and the maximum of acidity has been generated. By adding more sample equal to the original weight after reaction and observing the pH change, the ability of the sample to generate acid in excess of its neutralizing capacity can be assessed. Specifically, the sample is confirmed to be an acid producer if the pH remains below 3.5. Above this pH, biochemical oxidation is considered to be unlikely, and the sample is classified as a non-acid producer.

Reporting of Results

The results of the test should be tabulated to provide the following information:

Sample description, test duration (days after inoculation), initial pH, pH after biological oxidation, pH after 1st increment of sample addition, pH after 2nd increment of sample addition (final pH), confirmation of acid production potential (yes/no)

Advantages of Test

Disadvantages of Test

Original Reference

Bruynesteyn, A. and Hackl, R.P. (1984), Evaluation of acid production potential of mining waste materials. Minerals and the Environment, 4(1), 5-8.

COASTECH MODIFIED BIOLOGICAL OXIDATION TEST

Some of the disadvantages of the BC Research Confirmation Test can be reduced by adopting the following modifications to the standard procedure. The modified test has been used routinely for a number of years.

The initial sample weight is selected on the basis of the sulphur content as follows (basis: 50 ml nutrient solution):

Table 1: Sample Weight for Various Sample Sulphur Contents, Coastech Modified Biological Oxidation Test (from Lawrence, R.W. and Sadeghnobari, A., 1986b)

% SULPHUR

SAMPLE WEIGHT, g

<4

10.0

4-8

7.5

8-15

5.0

>15

3.0

At the end of the test, following the full addition of extra sample, if the pH is still below 3.5, sodium hydroxide solution (3 to 6 N) is added to the pulp, stoichiometrically equivalent to the acid added initially to bring the pH into the biochemical oxidation range. The final pH is recorded after 1 hour. This procedure removes the bias towards an acid classification.

Original Reference

Lawrence, R.W. and Sadeghnobari, A. (1986), In-House Development of a Modified Biological Confirmation Test for AMD Prediction, Coastech Research.

see also: Lawrence, R.W., Poling, G.W. and Marchant, P.B.(1989), Investigation of predictive techniques for acid mine drainage. Report on DSS Contract No. 23440-7-9178/01-SQ, Energy Mines and Resources, Canada, MEND Report 1.16.1(a).

LAPAKKO PROCEDURE FOR THE DETERMINATION OF NEUTRALIZING POTENTIAL OF METAL MINE WASTE

The method used is a modification of the BC Research Initial Test. The major difference between the two methods is the use of a pH 6.0 endpoint instead of pH 3.5.

Sample Preparation

Grind or pulverize sample to 70% passing 325 mesh – i.e. 70% of particles should have a diameter of less than 44 µm.

Titration Test

Prepare a stirred mixture of 10 g sample in 100 ml distilled water.

Using an automatic titrator (Type 45 AR Chemtrix pH controllers with Cole Parmer electrodes were used for the original work), titrate with 1.0 N sulphuric acid until a pH of 6.0 is reached. The test is complete when less than 0.1 ml acid is added over a 4 hour period.

Notes (Kim Lapakko)

The proposed alternative method of NP determination is the same as the B.C. Research initial test, except a titration endpoint of 6.0 was used in the present study rather than 3.5. pH 6 was selected since it is a commonly applied water quality standard. Therefore, the neutralization potential available above this pH represents the amount of acid that a mine waste could neutralize while maintaining drainage pH in a range that meets water quality standards.

It should be noted that this method is intended the NP present rather than the NP available for reaction in the field. Consequently, the value determined for waste rock will exceed that which is practically available in the field. This is the natural consequence of not running the test on field size particles, which would consider the mode of occurrence of neutralizing minerals. It's just a bit difficult to squeeze some of that field-sized material into these beakers. Nonetheless, for future testing I would consider using a larger particle size for waste rock. In our lab we have made the additional modification of using 2 g rather than 10 g in the test.

I must mention that for some solids, the procedure can be quite slow...as Rick Lawrence has mentioned numerous times (e.g. Lawrence & Wang, 1996). This is particularly the case for elevated carbonate contents and cases where the carbonate occurs with magnesium or magnesium and iron mixtures.

Original Reference

Lapakko, K. (1994), Evaluation of neutralization potential determinations for metal mine waste and a proposed alternative, Proc. International Land Reclamation and Mine Drainage Conference, Pittsburgh, USBM SP 06A-94, p129-137.

NEUTRALIZATION POTENTIAL - EPA (SOBEK) METHOD

Principles

The amount of neutralizing bases, including carbonates, present in overburden materials is found by treating a sample with a known excess of standardized hydrochloric acid. The sample and acid are heated to insure that the reaction between the acid and the neutralizers goes to completion.

The calcium carbonate equivalent of the sample is obtained by determining the amount of unconsumed acid by titration with standardized sodium hydroxide.

Comments

A fizz rating of the neutralization potential is made for each sample to insure the addition of sufficient acid to react all the calcium carbonate present.

During digestion, do not boil samples. If boiling occurs, discard sample and rerun. Before titrating with acid, fill burette with acid and drain completely. Before titrating with base, fill burette with base and drain completely to assure that free titrant is being added to the sample.

Chemicals

  1. Carbon dioxide-free water: Heat distilled water just to boiling in a beaker. Allow to cool slightly and pour into a container equipped with ascarite tube. Cool to room temperature before using.
  2. Hydrochloric acid (HCl) solution, 0.1 N, certified grade (Fisher So-A-54 or equivalent).
  3. Sodium hydroxide (NaOH), approximately 0.5 N: Dissolve 20.0 g of NaOH pellets in carbon dioxide-free water and dilute to 1 liter. Protect from CO2 in the air with ascarite tube. Standardize solution by placing 50 ml of certified 0.1 N HCl in a beaker and titrating with the prepared 0.5 N NaOH until a pH of 7.00 is obtained. Calculate the Normality of the NaOH using the following equation:

N2 = (N1V1) / V2, where

V1 = Volume of HCl used.

N1 = Normality of HCl used.

V2 = Volume of NaOH used.

N2 = Calculated Normality of NaOH

  1. Sodium hydroxide (NaOH) approximately 0.1 N: Dilute 200 ml of 0.5 N NaOH with carbon dioxide-free water to a volume of 1 liter. Protect from CO2 in air with ascarite tube. Standardize solution by placing 20 ml of certified 0.1 N HCl in a beaker and titrating with the prepared 0.1 N NaOH until a pH of 7.00 is obtained. Calculate the Normality of the NaOH..
  2. Hydrochloric acid (HCl), approximately 0.5 N: Dilute 42 ml of concentrated HCl to a volume of 1 liter with distilled water. Standardize solution by placing 20 ml of the known Normality NaOH prepared in a beaker and titrating with prepared HCl until a pH of 7.00 is obtained.

    Calculate the Normality of the HCl using the following equation:

    N1 = (N2V2) / V1, where

V2 = Volume of NaOH used.

N2 = Normality of NaOH used.

V1 = Volume of HCl used.

N1 = Calculated Normality of HCl.

  1. Hydrochloric acid (HCl), approximately 0.1 N: Dilute 200 ml of 0.5 N HCl to a volume of 1 liter with distilled water. Standardize solution as before, but use 20 ml of the known Normality NaOH
  2. Hydrochloric acid (HCl), 1 part acid to 3 parts water: Dilute 250 ml of concentrated HCl with 750 ml of distilled water.

Materials

  1. Flasks, Erlenmeyer, 250 ml
  2. Burst, 100 ml (one required for each acid and one for each base).
  3. Hot plate, steam bath can be substituted.
  4. pH meter (Corning Model 12 or equivalent) equipped with combination electrode.
  5. Balance, can be read to 0.01 g.

Procedure

  1. Place approximately 0.5 g of sample (less than 60 mesh) on a piece of aluminum foil.
  2. Add one or two drops of 1:3 HCl to the sample. The presence of CaCO3 is indicated by a bubbling or audible "fizz."
  3. Rate the bubbling or "fizz" in step 2 as indicated in Table 2.
  4. Weigh 2.00 g of sample (less than 60 mesh) into a 250 ml Erlenmeyer flask.
  5. Carefully add HCl indicated by Table 2 into the flask containing sample.
  6. Heat nearly to boiling, swirling flask every 5 minutes, until reaction is complete. NOTE: reaction is complete when no gas evolution is visible and particles settle evenly over the bottom of the flask.

Table 2: Volume and Normality of Hydrochloric Acid Used for Each Fizz Rating (from Sobek et al., 1978)

FIZZ RATING

HCl (ml)

HCl (NORMALITY)

None

20

0.1

Slight

40

0.1

Moderate

40

0.5

Strong

80

0.5
  1. Add distilled water to make a total volume of 125 ml.
  2. Boil contents of flask for one minute and cool to slightly above room temperature. Cover tightly and cool to room temperature. CAUTION: Do not place rubber stopper in hot flask as it may implode upon cooling.
  3. Titrate using 0.1 N NaOH or 0.5 N NaOH (concentration exactly known), to pH 7.00 using an electrometric pH meter and burette. The concentration of NaOH used in the titration should correspond to the concentration of the HCl used in step 5. NOTE: Titrate with NaOH until a constant reading of pH 7.0 remains for at least 30 seconds.
  4. If less than 3 ml of the NaOH is required to obtain a pH of 7.0, it is likely that the HCl added was not sufficient to neutralize all of the base present in the 2.00 g of sample. A duplicate sample should be run using the next higher volume or concentration of acid as indicated in Table 2.
  5. Run a blank for each volume or normality using steps 5, 7, 8, and 9

Calculations

1. Constant (C) = (ml acid in blank) / (ml base in blank).

2. ml acid consumed = (ml acid added) - (ml base added x C).

3. Tons CaCO3 equivalent / thousand tons of material = (ml of acid consumed) x (25.0) x (N of add).

Original Reference

Sobek, A., Schuller, Freeman, W.J. and Smith, R. (1978), Field and Laboratory Methods Applicable to Overburdens and Minesoil, (West Virginia Univ., Morgantown College of Agriculture and Forestry): EPA report no. EPA-600/2-78-054 p.47-50.

STATIC NET ACID GENERATION (NAG) PROCEDURE

Sample Preparation

Drill core and bulk rock samples should be crushed to nominal 4 mm and a sub sample pulverized to approximately 200 Mesh (<75 µm). Tailing and process residue samples can be tested 'as received'.

Reagents

Reagent 1: H202 - BDH 'Analar' Analytical Reagent 30% w/v (100 V), or equivalent, diluted 1:1 with deionized H20 to 15%. (Refer to Note 1).

Reagent 2: NaOH -0.50 M Standardized Solution.

Reagent 3: NaOH - 0.10 M Standardized Solution.

Method

  1. Add 250 ml of Reagent 1(15 % H2O2) to 2.5 g of pulverized sample in a 500 ml wide mouth conical flask, or equivalent. Cover with a watch glass, and place in a fume-hood or well ventilated area (refer to note 2). The H2O2 should be at room temperature before commencing test.
  2. Allow sample to react until ‘boiling’ or effervescing ceases. Heat sample on hot plate and gently boil until effervescence stops or for a minimum of 2 hours. Do not allow sample to boil dry - add deionized water if necessary.
  3. Allow solution to cool to room temperature then record final pH (NAG pH).
  4. Rinse the sample that has adhered to the sides of the flask down into the solution with deionized water. Add deionized water to give a final volume of 250 ml.
  5. Titrate solution to pH 4.5 while stirring with the appropriate NaOH concentration based on final NAG solution pH as follows:

Table 3: NaOH Concentration Required for Titration, Static Net Acid Generation (NAG) Procedure (from Miller, S., Robertson, A. and Donahue, T., 1997)

NAG SOLUTION pH

REAGENT

NaOH CONCENTRATION

>2

3

0.10 M

<2

2

0.50 M

4. Calculation

Net Acid Generation

NAG = 49 x V x M/W

where:

NAG = net acid generation (kg H2SO4/tonne)

V = volume of base NaOH titrated (ml)

M = molarity of base NaOH (moles/l)

W = weight of sample reacted (g)

NOTE: If NAG value exceeds 25 kg H2SO4 per tonne, repeat using a 1.00 g samples.

Notes and Precautions

  1. 1. The pH of the H2O2 used in the NAG test should be checked to ensure it is between pH 4 and 7. If the pH is less than 4 then add dilute NaOH (use a solution made up by adding 1 g NaOH to 100 ml deionized H2O) until the pH is greater than 4 (aim for a pH between 4 and 6). The pH is adjusted to greater than pH 4 to ensure that the phosphoric acid, used to stabilize H2O2 in some brands, is neutralized. The pH of the 15 % H2O2 should always be checked to ensure that any stabilizing acid is neutralized, otherwise, false positive results may be obtained.
  2. The NAG reaction can be vigorous and sample solutions can 'boil' at temperatures of up to 120 0C. Great care must be taken to place samples in a well ventilated area or fume cupboard.
  3. Caution should be taken in the interpretation of NAG test results for coal reject samples and other materials which may contain a high content of organic material (such as potential acid sulphate soils, dredge sediments and other lake or marine sediments). All organic material must be completely oxidized otherwise acid NAG results can occur which are unrelated to sulphides. Several aliquots of H2O2 reagent may be added to the sample to breakdown any organic acidity.
  4. Samples with positive NAPP value, high sulphur content and high ANC must be carefully evaluated.

Reference

Miller, S., Robertson, A. and Donahue, T. (1997), Advances in Acid Drainage Prediction using the Net Acid Generation (NAG) Test, Proc. 4th International Conference on Acid Rock Drainage, Vancouver, BC, 0533-549.

see also: Lawrence, R.W., Jaffe, S. and Broughton, L.M. (1988), In-House Development of the Net Acid Production Test Method, Coastech Research.

 

THE MODIFIED ACID BASE ACCOUNTING PROCEDURE FOR NEUTRALIZATION POTENTIAL (LAWRENCE)

 

  1. Add a few drops of 25% HCl to 1 to 2 g of pulverized sample on a watch glass or piece of aluminum foil. Observe the degree of reaction and assign a fizz rating as "none, slight, moderate, or strong fizz".
  2. Weigh approximately 2.00 g of pulverized sample into a 250 ml conical flask and add approximately 90 ml of distilled water.
  3. At the beginning of the test (time = 0), add a volume of certified or standardized 1.0 N HCl according to the fizz rating as follows:

    Table 4: Volume of HCl Added for Various Fizz Ratings, The Modified Acid Base Accounting Procedure for Neutralization Potential (Lawrence) (from Lawrence and Wang, 1997)

    FIZZ RATING

    VOLUME OF 1.0 N HCl (ml)

    AT TIME = 0 hour

    AT TIME = 2 hour

    None

    1.0

    1.0

    Slight

    2.0

    1.0

    Moderate

    2.0

    2.0

    Strong

    3.0

    2.0
  4. Place the flask on a shaking apparatus such a reciprocating shaker, maintained at room temperature. After approximately 2 hours, add the second acid quantity as indicated in the above table.
  5. After approximately 22 hours, check the pH of the pulp. If it is greater than 2.5, add a measured volume of 1.0 N HCl to bring the pH into the range 2.0 to 2.5. If the pH is less than 2.0, too much acid was added in steps 2 and 3. In this case, repeat the test adding a reduced volume of HCl.
  6. After 24 hours, terminate the test and add distilled water to the flask to bring volume to approximately 125 ml. Measure and record the pH, making sure it is in the required range of 2.0 to 2.5.
  7. Titrate the contents of the flask to a pH of 8.3 using certified or standardized 0.5 N or 0.1 N NaOH.
  8. Calculate the NP of the sample as follows:

    Modified NP (kg CaCO3/t)

    = [(N x vol (ml) of HCl) – (N x vol (ml) NaOH) x 50] / [weight of sample (g)]

The acid generating potential of the sample should be calculated on the basis of the sulphide-sulphur content (AP = S= x 31.25). Sulphide-sulphur is typically determined as the difference between total sulphur and sulphate-sulphur, although analysis of other sulphur species such as elemental sulphur and barite-sulphur is sometimes justified. Caution should be exercised for certain samples in interpreting sulphate-sulphur analyses as this form can be either inert (e.g. gypsum) or essentially stored products of acid drainage that could become mobilized if conditions within a waste change. Reference to the MEND Report 1.16.1c is suggested (Norecol Environmental Consultants, 1991) for a discussion of sulphur species.

Reference

Lawrence, R.W. and Wang, Y. (1997), Determination of Neutralization Potential in the Prediction of Acid Rock Drainage, Proc. 4th International Conference on Acid Rock Drainage, Vancouver, BC, p449-464.

see also: Coastech Research (1991), Acid Rock Drainage Prediction Manual, MEND Project Report 1.16.1b, MEND, Ottawa, Ontario.

PEROXIDE SIDERITE CORRECTION FOR SOBEK METHOD

(Skousen et al.)

The following discussion and method is taken from Skousen et al. (1997) and is reproduced here with the kind permission of the authors.

 Discussion

 Meek (1981) suggested that the NP of certain rock units is overestimated when siderite (FeCO3) is present. Siderite, along with calcite and dolomite, is a common carbonate mineral in the overburden associated with Appalachian coal beds (Cecil et al., 1985; Morrison et al., 1990b). Even though natural weathering of siderite is a slow process, siderite will react if exposed to aqueous and acidic conditions used in the laboratory to determine NP (Doolittle et al., 1992). When present in an overburden sample, siderite reaction with acid contributes to the apparent alkaline-producing potential of the rock (Meek, 1981; Morrison et al., 1990a; Wiram, 1992). Continued weathering of siderite, however, produces a neutral (Meek, 1981; Shelton et al., 1984) to slightly acid solution as indicated in the following reaction (Cravotta, 1991; Doolittle et al., 1992; Frisbee and Hossner, 1995; Postma, 1983):

FeCO3 + 0.25 O2 + 2.5 H2O = Fe(OH)3 + H2CO3 [1]

Where H2CO3 = H2O + CO2 (aq)

In the laboratory determination of NP in ABA, many reaction steps during the titration can affect the pH. The first step in the reaction of siderite with excess hydrochloric acid (HCl) is:

FeCO3 + 2HCl = Fe2+ + 2Cl- + H2O + CO2 [2]

Because the solution is acidic, the CO2 is exsolved as a gas. The ferrous iron (Fe2+) produced by reaction 2 is also unstable and will slowly oxidize to ferric iron (Fe3+) and consume additional HCl:

Fe2+ + 0.25O2 + HCl = Fe3+ + Cl- + 0.5H2O [3]

The ferric iron (Fe3+) produced will consume base ions upon titration with sodium hydroxide (NaOH) and precipitate as ferric hydroxide:

Fe3+ + 3NaOH = Fe(OH)3 + 3Na+                 [4]

The overall reaction is essentially reaction 1 with the addition of sodium (Na+) and chloride (Cl-):

FeCO3 + 0.25O2 + 3HCl + 3NaOH = Fe(OH)3 + 3Na+ + 3Cl- + 1.5H2O + CO2 (g) [5]

Reaction 5, which represents the major reactants and products for the NP digestion and titration, shows that 3 mol of acidity (HCl) and 3 mol of base (NaOH) are consumed, and that CO2 is exsolved. As a result, the overall reaction yields a zero NP for siderite (no net acidity or alkalinity). Because the standard NP procedure as outlined by Sobek et al. (1978) does not allow sufficient time for ferrous iron oxidation and subsequent precipitation of ferric hydroxide, the procedure accounts for only the initial reaction, resulting in 3 mol of alkalinity (Eq. [2]-[3]). Therefore, erroneously high NP values can be generated with samples containing high amounts of siderite. Such an analytical oversight can lead to incorrect post-mining water quality predictions, result in premature mine closure, and produce costly, long-term reclamation liabilities for mining companies (Wiram, 1992).

Meek (1981) was the first to suggest adding a small quantity of 30% hydrogen peroxide (H2O2) to the filtrate of an HCl-digested siderite overburden sample to oxidize ferrous iron to ferric iron before back-titration. Because the resulting ferric iron is precipitated as Fe(OH)3 upon titration, the solution yields a more accurate NP value. The addition of H2O2 after the initial titration results in the formation of additional Fe(OH)3 due to enhanced oxidation at higher pH values.

Method

The method, as developed by a Pennsylvania-West Virginia Overburden Task Force (Leavitt et al., 1995), is as follows (for triplicate samples and a blank):

  1. Place 2 g sample in each of three beakers with a fourth beaker having no sample.
  2. Add an amount and strength of HCl to each beaker based on the sample fizz rating, and adjust volume to 100 ml.
  3. Add boiling chips to beakers and cover with watch glass.
  4. Boil gently for 5 min., then allow to cool.
  5. Gravity filter beakers contents using Whatman No. 40 (0.45 µm) filter.
  6. Add 5 ml of 30% H2O2 to the filtrate.
  7. Boil for a further 5 min. (boiling chips and watch glasses), then cool.
  8. Hand titrate, as per Sobek Method, with standard NaOH to achieve and hold an endpoint pH of 7.0 for 30 sec.

Original Reference

Skousen, J., Renton, J., Brown, H., Evans, P., Leavitt, B., Brady, K., Cohen, L. and Ziemkiewicz, P. (1997), Neutralization Potential of Overburden Samples containing Siderite, Journal of Environmental Quality, v26, n3, p673-681.

 

PASTE pH

Editor's Note: The following procedure for Paste pH is that currently recommended by the British Columbia Ministry of Employment and Investment, Energy and Minerals Division (Price, 1997). It is based on the method of Sobek et al. (1978), with modifications by Page et al. (1982) and others. Modifications to the method of Sobek are shown in italics.

 

Care must be taken to insure electrode life and accurate pH measurements:

1. Electrode should not remain in the sample longer than necessary for a reading, especially if more alkaline than pH 9.0 or more acidic than pH 2.0.

2. Electrode should be washed with a jet of distilled water from a wash bottle after every measurement (sample or buffer solution).

3. Electrode should be dipped in dilute (0.1 N) hydrochloric acid for a few seconds and washed with distilled water to remove any calcium carbonate film which may form, especially from alkaline samples.

4. Drying out of the electrode should be avoided.

5. Electrode is cleaned and suspended in distilled water (which is protected from evaporation) for storage.

6. Place pH meter in standby position when electrode is not in a solution.

The pH meter and electrode should be standardized with buffers differing by 3 or 4 pH units, such as 7.0 and 4.0, before beginning a series of measurements. After every tenth measurement, recheck the standardization with both buffers. Care should be taken not to contaminate one buffer with the other buffer or with the test solution. Never return used standard buffers to their stock bottles.

Chemicals (from Sobek et al. 1978)

1. Standard buffer solutions, pH 4.00 and pH 7.00.

2. Distilled water (H2O).

Materials (from Sobek et al.. 1978)

1. pH meter equipped with combination electrode.

2. Paper cups, 30 ni capacity.

3. Plastic cups.

4. Stirring rod.

5. Wash bottle containing distilled water.

6. Balance, can be read to 0.1 g.

Procedure (1:1 Solid:Solution Ratio)

1. Turn on pH meter, adjust temperature setting and "zero" pH meter per instruction manual.

2. Place pH 4.0 and pH 7.0 standard buffers in two plastic cups (one buffer in each cup).

Note: Never return used buffers to stock bottles.

3. Place electrode in the pH 7.0 buffer.

4. Adjust pH meter to read 7.0.

5. Remove electrode from buffer solution and wash with a jet of distilled water from a wash bottle.

6. Place electrode in the pH 4.0 buffer and check the pH reading.

Note: If pH meter varies more than ±0.1 pH units from 4.0, something is wrong with the pH meter, electrode or buffers.

Note: the following adapted from Page et al. (1982).

7. Weight or measure 20g of air-dry test material and 20 ml of distilled water.

8. Mix thoroughly for 5 sec, preferably with a portable mechanical stirrer.

9. Let stand for l0 min.

10. Insert electrode(s) into the container, and stir the supernatant by swirling the electrodes slightly. Protect the electrodes taking care not to contact sealed particles, moving the electrode carefully about to insure removal of water film around the electrode. Electrodes are easily scratched.

11. When reading remains constant, record pH and remove electrode from the supernatant Carefully wash electrode with distilled water. If all pH measurements are completed, the electrode should be stored in a beaker of distilled water. Note: After every 10 samples, check meter calibration with standard buffers.

References

Price, W.A. (1997), DRAFT Guidelines and Recommended Methods for the Prediction of Metal Leaching and Acid Rock Drainage at Minesites in British Columbia, British Columbia Ministry of Employment and Investment, Energy and Minerals Division, Smithers, BC, (April), 143p.

Sobek, A.A., Schuller, W.A., Freeman, J.R. and Smith, R.M. (1978), Field and Laboratory Methods Applicable to Overburden and Minesoils, Report EPA-600/2-78-054, U.S. National Technical Information Service Report PB-280 495.

 

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