**3. Acid mine drainage**

Acid Mine Drainage (AMD) is the problem of acid drainage, traditionally referred in Australia and North America as Acid Rock Drainage (ARD). It seems to be a significant environmental impact of mining activities especially in opencast mines. It may damage long after the operation has ended because process and reaction have taken time. Runoff passing through the sourcing area can then give rise to severe threat. Moreover, AMD potentially dissolves and leaches out some toxic metals from the heap, mining waste dump and even natural soil and rock prior to contamination of surface water and groundwater. AMD is usually generated by the oxidation of sulfides in mining wastes; consequently, water supply from the area would be sulfide-rich drainage with acidic leaching property that may lead to mobilization of metals. Sulfides bound up in the waste rocks and tailings usually have various forms. Mineral sulfides are crystalline substances that contain sulfur combined with metal or semi-metal without oxygen. The most general form is "pyrite" (FeS2), moreover, other forms also include Fe1-xSx, Fe3S4. FeS, CuFeS4, ZnS, PbS, HgS, CoAsS etc. After these sulfide minerals are exposed to the air and water, the sulfide ions are oxidized into soluble sulfates as well as toxic metal ions and hydrogen ions may in turn be released into the environment. Initial factors for acid generation are: 1) sulfide minerals in the solid wastes (e.g., rocks and tailings); 2) water or a humid atmosphere; 3) an oxidant (usually oxygen in the form of O2). Therefore, processes of acid generation and metal release would be taken place together during the formation of AMD which are closely related to oxidation of pyrite and precipitation of Fe hydroxides. There are four common chemical reactions represent AMD formed from pyrite. The first equation shows that an important oxidant of pyrite is oxygen. Ferrous iron is released and sulfur is oxidized and changed to sulfate. This equation shown 2 moles of acidity generated from each mole of pyrite. The second equation is the conversion of ferrous iron to ferric iron. It consumes one mole of acidity. The third equation is a hydrolysis reaction which splits the water molecule; consequently, moles of acidity are generated as by-product. The fourth reaction is the oxidation of additional pyrite by ferric iron. The ferric irons generated in reaction steps 1 and 2 are cycle and propagation of the overall reaction. They take place very rapidly and continue until either the ferric iron or pyrite are depleted. In this reaction, iron is the main oxidizing agent instead of oxygen.

$$\begin{array}{ccccccccc}2\text{FeS}\_2 & + & 7\text{O}\_2 & + & 2H\_2\text{O} & \rightarrow & 2Fe^{2+} & + & 2SO\_4^{2-} & +4H^+\\ \text{pyruvate} & & \text{oxygen} & \text{water} & \text{fermions iron} & \text{sulfate} & \text{acidity}\end{array} \tag{1}$$

$$\begin{array}{ccccc} 4\text{Fe}^{2+} & + & O\_2 & + & 4H^+ & \rightarrow & 4Fe^{3+} & + & 2H\_2O \\ \text{fermous iron} & & \text{oxygen acidity} & \text{fercic iron} & & & \text{water} \end{array} \tag{2}$$

$$\begin{array}{rcl} 4Fe^{3+} + & 12H\_2O \rightarrow & 4Fe(OH)\_3^- & + & 12H^+ \\ \text{ferric iron} & \text{Water} & \text{ferrichydrooxide} & \text{acidity} \end{array} \tag{3}$$

$$\begin{array}{ccccccccc}\text{FeS}\_2 & + & 14\text{Fe}^{3+} & + & 8\text{H}\_2\text{O} & \rightarrow & 15\text{Fe}^{2+} & + & 2\text{SO}\_4^{2-} & + & 16\text{H}^+\\\text{pyruvate} & \text{ferric iron} & & \text{water} & \text{ferrous iron} & & \text{sulfate} & & \text{acidity}\end{array} \tag{4}$$

Many procedures have been developed to assess the acid forming characteristics of mine waste materials. The most widely used methods are Acid-Base Accounting (ABA) test and the Net Acid Generation (NAG) test. These procedures are described below.

#### **3.1 Acid-base accounting**

94 Environmental Monitoring

Fig. 2. Tailing pond (left photo) for disposal of slurry-like waste produced from gold dressing and sample collection (right photo), a routine monitoring program which

Due to tailings comprise both solid wastes mixing with water during the operation period of mine and they will become drying after the mining end, redox reaction would be taking place and may in turn change stability of some elements which can be leached out to the environment by accidence. Moreover, their property may also cause AMD. Therefore, routine monitoring plans for both water and tailing must be designed and continuously followed up. Monitoring data should be used for protection at the end and may be very

Acid Mine Drainage (AMD) is the problem of acid drainage, traditionally referred in Australia and North America as Acid Rock Drainage (ARD). It seems to be a significant environmental impact of mining activities especially in opencast mines. It may damage long after the operation has ended because process and reaction have taken time. Runoff passing through the sourcing area can then give rise to severe threat. Moreover, AMD potentially dissolves and leaches out some toxic metals from the heap, mining waste dump and even natural soil and rock prior to contamination of surface water and groundwater. AMD is usually generated by the oxidation of sulfides in mining wastes; consequently, water supply from the area would be sulfide-rich drainage with acidic leaching property that may lead to mobilization of metals. Sulfides bound up in the waste rocks and tailings usually have various forms. Mineral sulfides are crystalline substances that contain sulfur combined with metal or semi-metal without oxygen. The most general form is "pyrite" (FeS2), moreover, other forms also include Fe1-xSx, Fe3S4. FeS, CuFeS4, ZnS, PbS, HgS, CoAsS etc. After these sulfide minerals are exposed to the air and water, the sulfide ions are oxidized into soluble sulfates as well as toxic metal ions and hydrogen ions may in turn be released into the environment. Initial factors for acid generation are: 1) sulfide minerals in the solid wastes (e.g., rocks and tailings); 2) water or a humid atmosphere; 3) an oxidant (usually oxygen in the form of O2). Therefore, processes of acid generation and metal release would be taken place together during the formation of AMD which are closely related to oxidation of pyrite and precipitation of Fe hydroxides. There are four common chemical reactions represent AMD formed from pyrite. The first equation shows that an important oxidant of pyrite is

has to be carried out regularly

**3. Acid mine drainage** 

useful for development of the mineral processing.

Characterization of rock types and geologic setting in the mine should be initially concerned prior to determination of capacity acid drainage generation of these rocks (Environment Australia, 1997). Acid-Base Accounting (ABA) is the most commonly-used static procedure that has been used for estimation/qualification of the acid generation potential of mine wastes (Furguson & Erickson, 1988). This procedure was developed at West Virginia University in late 1960s. ABA tests are designed to measure the balance between potentially acid-generating potential, particularly oxidation of sulfide materials and acid neutralizing potential in sample such as dissolution of alkaline, carbonates, displacement of exchangeable bases and weathering of silicate. The values arising from ABA are referred to the Maximum Potential Acidic (MPA) and the Acid Neutralizing Capacity (ANC), respectively. After MPA and NAC have been determined for a sample, both values are compared with set criteria. Two methods of combination commonly used are: 1) The difference in value between MPA and ANC or Net Acid Producing Potential (NAPP) where NAPP = MPA-ANC; 2) The ratio of ANC to MPA (ANC/MPA). NAPP is a theoretical calculation commonly used to indicate where a waste material has potential to produce acidic drainage. NAPP values represent balance between capacity of acid generation and capacity of acid neutralization. Unit of NAPP is also expressed as kg H2SO4/t in MPA and ANC. In addition, ANC/MPA ratio is also considered for assessment of acid generation from mine waste material. The main purpose of ANC/MPA ratio is to indicate relatively

Geochemical Application for Environmental Monitoring and Metal Mining Management 97

represents the inherent acid neutralizing capacity of the sample. Calculation will be carried

Net Acid Generation (NAG) test was developed as an assessment tool for acid producing potential of sample for longer than 20 years ago. The NAG test is usually used in association with NAPP. It is direct method to measure ability of sample to produce acid via sulfide oxidation. Hydrogen peroxide (H2O2) is used to activate and complete oxidation process of the sulfide minerals contained in the sample. H2O2 added during the NAG test leads to simultaneous reactions of acid generation and acid neutralization. Then pH measurement of solution has to be carried out after the completion of reaction. The acidity of solution under the NAG is a direct measurement of net acid generation of sample. Shu et al. (2001) studied the effect of lead/zinc mine acidity on heavy metal mobility using both NAG test and ABA method. They concluded, based on their results that NAG test, direct measurements of ANC from acid produced from oxidized sulfide, yields more accurate than that of ABA method. This is because prediction of acid forming potential from the total pyritic sulfur content as done for ABA method may overestimate amount of acid generation due to uncompleted

However, classifications of waste rock have generally used NAPP estimation based on ABA method in combination of NAG pH testing. Schematic classification is present in Fig. 4. Three types of west rocks from mining activity can be grouped as No Net Acid Forming (NAF), Potentially Net Acid Forming (PAF), and Uncertainly Net Acid Forming (UC).

Fig. 4. NAG pH plot against NAPP for classification potential of net acid formation of waste

**No Net Acid Forming (NAF):** either there is minimal or no sulfides present or the neutralization potential exceeds the acid potential. This type of waste rock gives a negative

out and expressed in terms of kg H2SO4/t.

**3.2 Net acid generation** 

acidification of pyritic sulfur.

rock

Definitions of these groups are given below.

NAPP and NAG pH greater than or equal to 4.5.

safety margin of material. Safe values for prevention of acid generation are reported with different ANC/MPA values ranging from 1 to 3. The higher ANC/MPA value indicates high probability of the material that may remain circum-neutral in pH and should not be problematic by acid rock drainage. Both NAPP value and ANC/MPA ratio are usually used together for placement planning of rock waste and other overburdens (Skousen et al., 1987). Sulfur and ANC data are often used in combination with ANC/MPA ratio as presented in Fig. 3.

Fig. 3. Plots of all parameters considered in Acid-Base Accounting (ABA)

*Maximum Potential Acidic:* MPA is the maximum amount of acid that can be produced from the oxidation of sulfur-containing minerals in the rock material. It can be measured and calculated from the sulfur content. Total sulfur content of a sample is commonly determined by the LECO high temperature combustion method or other appropriate methods. For instant, it is assumed that all sulfurs occur as iron-sulfide (or pyrite; FeS2) and this iron-sulfide reacts under oxidizing condition to generate acid according to the following reaction:

$$\text{FeS}\_2 + 15/4 \,\text{O}\_2 + 7/2 \,\text{H}\_2\text{O} \quad \Delta \quad \text{Fe(OH)}\_3 \quad + 2 \,\text{H}\_2\text{SO}\_4$$

According to the stoichiometry, the maximum amount of acid that could be produced by a sample containing 1%S as pyrite would be 30.6 kilograms of H2SO4 per ton of material. The MPA is calculated from the total sulfur content as:

$$\text{MPA (kg H}\text{\u3}\text{\u3}\text{\u4/t)} \quad = \text{ (Total \u3}\text{\u3}\text{\u4}\text{\u4)}$$

*Acid Neutralizing Capacity:* ANC is calculated from the amount of acid neutralizer in the sample and it is expressed in metric tons/1000 metric tons of material. Acid generated from pyrite oxidation will be partly reacted by acid neutralizing minerals contained within the sample. This inherent acid buffering is resulted in term of the ANC. Most of the minerals which contribute the acid neutralizing capacity usually are carbonates such as calcite and dolomite. The modified Sobek method is the most common method used to determine ANC. This method is determined experimentally by reaction of a known amount of standardized acid (hydrochloric acid, HCL) with a known amount of sample and then the mixed solution sample is back-titrated by sodium hydroxide (NaOH). The amount of acid consumed represents the inherent acid neutralizing capacity of the sample. Calculation will be carried out and expressed in terms of kg H2SO4/t.

#### **3.2 Net acid generation**

96 Environmental Monitoring

safety margin of material. Safe values for prevention of acid generation are reported with different ANC/MPA values ranging from 1 to 3. The higher ANC/MPA value indicates high probability of the material that may remain circum-neutral in pH and should not be problematic by acid rock drainage. Both NAPP value and ANC/MPA ratio are usually used together for placement planning of rock waste and other overburdens (Skousen et al., 1987). Sulfur and ANC data are often used in combination with ANC/MPA ratio as presented

Fig. 3. Plots of all parameters considered in Acid-Base Accounting (ABA)

under oxidizing condition to generate acid according to the following reaction:

MPA is calculated from the total sulfur content as:

*Maximum Potential Acidic:* MPA is the maximum amount of acid that can be produced from the oxidation of sulfur-containing minerals in the rock material. It can be measured and calculated from the sulfur content. Total sulfur content of a sample is commonly determined by the LECO high temperature combustion method or other appropriate methods. For instant, it is assumed that all sulfurs occur as iron-sulfide (or pyrite; FeS2) and this iron-sulfide reacts

FeS2 + 15/4 O2 + 7/2 H2O Fe(OH)3 + 2 H2SO4 According to the stoichiometry, the maximum amount of acid that could be produced by a sample containing 1%S as pyrite would be 30.6 kilograms of H2SO4 per ton of material. The

MPA (kg H2SO4/t) = (Total %S) X 30.6 *Acid Neutralizing Capacity:* ANC is calculated from the amount of acid neutralizer in the sample and it is expressed in metric tons/1000 metric tons of material. Acid generated from pyrite oxidation will be partly reacted by acid neutralizing minerals contained within the sample. This inherent acid buffering is resulted in term of the ANC. Most of the minerals which contribute the acid neutralizing capacity usually are carbonates such as calcite and dolomite. The modified Sobek method is the most common method used to determine ANC. This method is determined experimentally by reaction of a known amount of standardized acid (hydrochloric acid, HCL) with a known amount of sample and then the mixed solution sample is back-titrated by sodium hydroxide (NaOH). The amount of acid consumed

in Fig. 3.

Net Acid Generation (NAG) test was developed as an assessment tool for acid producing potential of sample for longer than 20 years ago. The NAG test is usually used in association with NAPP. It is direct method to measure ability of sample to produce acid via sulfide oxidation. Hydrogen peroxide (H2O2) is used to activate and complete oxidation process of the sulfide minerals contained in the sample. H2O2 added during the NAG test leads to simultaneous reactions of acid generation and acid neutralization. Then pH measurement of solution has to be carried out after the completion of reaction. The acidity of solution under the NAG is a direct measurement of net acid generation of sample. Shu et al. (2001) studied the effect of lead/zinc mine acidity on heavy metal mobility using both NAG test and ABA method. They concluded, based on their results that NAG test, direct measurements of ANC from acid produced from oxidized sulfide, yields more accurate than that of ABA method. This is because prediction of acid forming potential from the total pyritic sulfur content as done for ABA method may overestimate amount of acid generation due to uncompleted acidification of pyritic sulfur.

However, classifications of waste rock have generally used NAPP estimation based on ABA method in combination of NAG pH testing. Schematic classification is present in Fig. 4. Three types of west rocks from mining activity can be grouped as No Net Acid Forming (NAF), Potentially Net Acid Forming (PAF), and Uncertainly Net Acid Forming (UC). Definitions of these groups are given below.

Fig. 4. NAG pH plot against NAPP for classification potential of net acid formation of waste rock

**No Net Acid Forming (NAF):** either there is minimal or no sulfides present or the neutralization potential exceeds the acid potential. This type of waste rock gives a negative NAPP and NAG pH greater than or equal to 4.5.

Geochemical Application for Environmental Monitoring and Metal Mining Management 99

geological classification as well as mining operation. Placement and disposal may be designed based on this classification in cooperation with other testing methods. Rock powdering using appropriate crusher and miller must be done prior to further analyses. Subsequently, the powdered rock samples may be fused to glass beads or pressed as pellet for X-ray Fluorescence (XRF) analyses of 9 major oxides (i.e., SiO2, TiO2, FeOt, MnO, MgO, CaO, Na2O, K2O and P2O5) and perhaps some trace elements (e.g., Ba, Zn, Sr, Rb, Zr, Co, Cr, Ni, Y and V). Rock standards should be used for calibration at the same analytical condition. Moreover, loss on ignition (LOI) should also be measured by weighting rock powders before and after ignition at 900º C for 3 hrs in an electric furnace. Trace and rare earth elements may be additionally analyzed using advanced instruments such as Inductively Coupled Plasma (ICP) Spectrometer, Atomic Absorption Spectrometer (AAS) and other spectrometric techniques. Rock samples have to be digested totally without remaining of rock powders. About 0.1000 g (±0.0001 g) of powdered samples are weighted and then dissolved in a concentrate HF-HNO3-HClO4 acid mixture in sealed Teflon beakers. The digested samples were diluted immediately and added mixed standard solution to all samples. Proportion of these concentrate acids is usually adapted in laboratory as well as time of digestion. Hotplate has been engaged traditionally but it may take long time. Alternatively, microwave has been applied to shorten the digestion time. This method is total digestion which most elements including toxic elements and non toxic ones are dissolved for analyses. However, these contents do not clearly reflect environmental impact. Microwave-assisted acid solubilization has been proved to be the most suitable method for the digestion of complex matrices such as sediments and soil. This method shortens the digestion time, reduces the risk of external contamination and uses smaller quantities of acid (Wang et al., 2004). However, there are different procedures required for appropriate sample types. Some standard digestion techniques are usually used for soil, sediment and

sludge; for example, EPA 3052, EPA 3050B and EPA 3051 are described below.

should be obtained from EPA (1996).

*EPA 3052:* This method is an acid digestion of siliceous matrices, and organic matrices and other complex matrices (e.g., ashes, biological tissues, oils, oil contaminated soils, sediments, sludges and soils) which they may be totally decomposed for analysis. Powdered sample of up to 0.5 g is added into 9 ml of concentrated nitric acid and usually 3 ml hydrofluoric acid for 15 minutes using microwave. Several additional alternative acids and reagents have been applied for the digestion. These reagents include hydrochloric acid and hydrogen peroxide. A maximum sample of 1.0 g can be prepared by this method. Mixed acids and sample are placed in an inert polymeric microwave vessel then sealed prior to heating in the microwave system. Temperature may be set for specific reactions and incorporates reaching 180 ± 5 ºC in approximately shorter than 5.5 minutes and remaining at 180 ± 5 ºC for 9.5 minutes to complete specific reactions. Solution may be filtered before appropriate volume is made by dilution. Finally, the solution is now ready for analyses (e.g., AAS or ICP). More details

**EPA 3050:** Two separate procedures have been proposed for digestion of sediment, sludge and soil etc. The first procedure is preparation for analysis of Flame Atomic Absorption Spectrometry (FLAA) or Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) whereas the other is for Graphite Furnace AA (GFAA) or Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Appropriate elements and their detection limits must be concerned and designed for selection of both methods (EPA, 2009). Alternative determination techniques may also be modified as far as scientific validity is proven. This method can also be applied to other elements and matrices but performance need to be

**Potentially Net Acid Forming (PAF):** the acid potential exceeds the neutralization potential. These rocks are described as potentially acid forming. They may generate AMD if they are exposed to sufficient oxygen to allow sulfide oxidations. Geochemical tests usually yield positive NAPP and NAG pH below 4.5.

**Uuncertain Net Acid Forming (UC):** uncertain classification is obtained when there is an apparent conflict between the NAPP result and NAG pH; for example, NAPP is negative but NAG pH lower than 4.5 or NAPP is positive but NAG pH higher than 4.5. However, further testing work would be performed for such rock types to determine proportion between NAF and PAF if they occur.

Recently, this classification has been using widely for geochemical study of waste rock and assessment of acid forming potential. Tran et al. (2003), for an example, also used NAG together with NAPP tests to figure out key criteria for construction design of waste rock dumps to avoid AMD. They collected samples from 2 sites in which have different temperatures. NAG and NAPP tests were applied to classify PAF, NAF and UC materials prior to placement control of waste rocks within the dumps. They succeeded to have reduced AMD load that may be generated from both dumps.
