H-ESD : Environmental and Sustainable Development Mine water chemistry 11.12.2014 H-ESD : Environmental and Sustainable Development Michael Staudt, GTK

Table of contents Properties of water Dissociation of water, pH value Solubility of solids in water Solubility of gases in water Oxidation and reduction potential

Properties of water Water (H2O) is extraordinary in that it coexists on Earth in 3 distinct forms or states, 1: dry solid (ice), 2: wet liquid (water) and, 3: dry, invisible gas (vapor). Dipol Van der Waals bonds strong cohesion max density at 4 degrees C

water compounds: acids Acids: are substances which, when dissolved in water , give rise to positive H+ and negative anions HCl = H+ + Cl- H2SO4 = 2 H+ SO4 -

water compounds: bases Bases: are substances which give positively charged cations and negative charged hydroxide anions (OH-) NaOH = Na+ + OH- CaOH2 = Ca++ 2OH -

Salts When acids and bases react with each other salts are formed together with water e.g. HCl +NaOH = NaCl+ H2O

Solubility Of a substance is the maximum amount of that substance which can be taken up by a determined amount of water (slovent) at given Temp. and pressure conditions, forming a homogeneous mixture (saturated solution).

Solubility – cont. generally increases with rising temp. controlled by the ph value few ions ( Na+, K+, NO3- or Cl-) are equally soluble at nearly all pH values Most metal cations are soluble in acid water with increasing pH they form hydroxides

Amphoterism If a strong increase of the pH value occurs, the solid hydroxides or basic salts of many metals pass to complex compounds which may be dissolvable amphoteric elements e.g. : Zinc oxide (ZnO) reacts with both acids and with bases: In acid: ZnO + 2H+ → Zn2+ + H2O In base: ZnO + H2O + 2 OH- → [Zn(OH)4]2-

pH The pH of a liquid is the negative logarithm of the concentration of hydrogen ions (cations) in the solution. pH = - log [H+] Because the pH scale is logarithmic, every single unit change in pH actually represents a ten fold change in the number of hydrogen ions in a solution. The pH of natural water depends on several factors, which include the bicarbonate buffering system, types of rock, types of soil, and nature of discharged pollutants. The concentration of carbonates and carbon dioxide is the main influence on the pH of clean water. High concentrations of bicarbonate produce alkaline waters (high pH), while low concentrations usually produce acidic waters (low pH).

pH – cont. Acidic and alkaline compounds can be weathered into the stream from the different types of rock present. When limestone (CaCO3) is present, carbonates can be released, affecting the alkalinity of the water. The types of soil in the drainage area also affect the pH. Drainage water from forests and marshes is often slightly acidic, due to the presence of humic acids produced by decaying vegetation. Nitrogen oxides (NO, NO2) and sulfur dioxide (SO2) from automobile and power plant emissions are converted into nitric acid (HNO3) and sulfuric acid (H2SO4) in the atmosphere. These acids can affect the pH of streams by combining with moisture in the air and falling to the earth as acid rain or snow.

pH - cont. Surface waters can sometimes act as weak buffer solutions depending on the concentrations of carbonates and bicarbonates. The pH values of natural surface waters usually range from 5.5 to 8.5. Extremely high (9.6) or low (4.5) values are unsuitable for most aquatic organisms. Young fish and immature stages of aquatic insects are extremely sensitive to pH levels below 5. Changes in pH can also affect aquatic life indirectly by altering other aspects of water chemistry. Low pH levels accelerate the release of heavy metals from sediments on the stream bottom. The heavy metals can reduce the chance of survival of most aquatic organisms

Dissolution During the process of dissolution the pH value normally changes, causing a shift of the chemical equilibrium e.g. Ca 2+ + CO3 2- + 2 H + 2 OH- = Ca (OH)2 + H2CO3 = Ca 2+ + 2 OH- + H+ + HCO3- : rise of pH

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Conductivity The ability of an aqueous solution to carry an electric current is called conductivity. The current is conducted in the solution by the movement of ions. Conductivity increases with increasing amounts and mobility of ions. In natural water, the dissociation of inorganic compounds is the main source of ions in the solution. Therefore, measuring conductivity reveals the concentration of dissolved salts in water. Conductivity is also affected by heavy metal ions released into water by acid mine drainage

Hardness Water hardness is the total concentration of cations, specifically calcium (Ca2+), magnesium (Mg2+), iron (Fe2+) and manganese (Mn2+) in water. A stream's hardness reflects the geology of the catchment area and sometimes provides a measure of the influence of human activity in a watershed. For instance, acid mine drainage often releases iron (Fe2+) into a stream, resulting in extraordinarily high hardness readings. For this reason, hardness is a useful water quality indicator. For the most part, however, hardness is a measure of the calcium and magnesium that enters the stream through the weathering of rock.

Dissolved Oxygen There are two main sources of dissolved oxygen in stream water: the atmosphere and photosynthesis. Waves and tumbling water mix air into the water where oxygen readily dissolves until saturation occurs. Oxygen is also introduced by aquatic plants and algae as a byproduct of photosynthesis The amount of oxygen that dissolves is limited by physical condition, such as water temperature and atmospheric pressure. Assuming a constant atmospheric pressure, low water temperatures produce a higher potential dissolved oxygen level than do high water temperatures. Some pollutants, such as acid mine drainage, produce direct chemical demands on oxygen in the water. Dissolved oxygen is consumed in the oxidation-reduction reactions of introduced chemical compounds, such as nitrate (NO31-) and ammonia (NH41+), sulfate (SO42-) and sulfite (SO32-), and ferrous (Fe2+) and ferric (Fe3+) ions.

Alkalinity is a measure of the buffering capacity of a solution. Chemical buffering is the resistance of a solution to changes in pH. Because pH is a measure of the number of hydrogen ions in a solution, alkalinity represents the ability of a solution to absorb or take up hydrogen ions. Hydrogen ions are taken up by molecules, such as carbonates and bicarbonates, because the negatively charged ions bond with the positively charged hydrogen ions thereby removing the hydrogen from the solution. The bicarbonate buffering system works in stream ecosystems much in the same way as it does in your blood stream.

Dissociation constant gw is generally regulated by the presence of CO2 and its reaction products in water: H+ * HCO3- = k1, H+ = H2CO3- * k1 -------------- --------------- H2CO3 HCO3-

Acid Mine Drainage: Chemistry Acid mine drainage impacts stream and river ecosystems through acidity, ferric ion (Fe3+) precipitation, oxygen depletion, and release of heavy metals associated with coal and metal mining, such as aluminum (Al3+), zinc (Zn2+), and manganese (Mn2+). When mineral deposits that contain sulfides are mined, they have the potential to produce acid mine drainage. This includes the mining of coal, copper, gold, silver, zinc, lead, and uranium. The mineral pyrite, more commonly known as "fool's gold," is iron disulfide (FeS2). Pyrite is one of the most important sulfides found in the waste rock of mines. When exposed to water and oxygen, it can react to form sulfuric acid (H2SO4). The following oxidation and reduction reactions express the breakdown of pyrite that leads to acid mine drainage.

AMD - Chemical reactions 1. 2FeS2 + 7O2 + 2H2O -> 2FeSO4 + 2H2SO4 2. 2Fe2+ + 1/2 O2 + 2H+ -> 2Fe3+ + H2O 3. Fe3+ + 3H2O -> Fe(OH)3 + 3H+ 4. FeS2 (s) + 15/4 O2 + 7/2 H2O <--> 4H+ + 2SO4- +Fe(OH)3 (s)

AMD - cont. Production of acid mine drainage can occur long after mines have been abandoned if piles of waste rock are in contact with air and water. The red color often seen in streams receiving acid mine drainage is actually a stain on the rocks called "Yellow-Boy," or ferrous hydroxide (Fe(OH)3) formed during Reaction 3 above. Equation 4 shows that by introducing hydrogen ions, acid mine drainage affects the acidity of a stream. Acidity is commonly measured by pH values, which are easy to collect and compare. pH, however, is not always a good indicator of acid mine drainage because it only indicates the concentration of hydrogen ions. When evaluating the extent of acid mine drainage, it is important to know the amount of hydrogen ions remaining in solution after the natural buffering of the stream is completed. For example, pH measurements may not detect heavy acid mine drainage in a stream because of high alkalinity due to dissolved carbonates. Assessing the excess of hydrogen ions over basic ions,"total acidity," is a better measurement of acid mine drainage.

References Balke, K-D: Applied Hydrochemistry, Lecture notes, University of Tübingen, 2000 Younger, P.L . , Banwart, S. A. & Hedin R. S. : Mine Water: Hydrogeology, Pollution, Remediation, Kluwer Academic Publishers, 2002