1) Explain the chemistry of acid mine drainage using a net ionic equation. 2) Ex
ID: 709863 • Letter: 1
Question
1) Explain the chemistry of acid mine drainage using a net ionic equation.
2) Explain why the solubility of some metal compounds will be increased in acidic conditions. Use the NIE and Le Chatliers Principle in your explanations.
3) A water sample is analyzed to contain two metals with concentrations above MCL Cd2+ and Al3+. You decide to treat this water with OH and the equilibrium pH is 7.16 . Given the equilibrium pH will you have successfully reduced each metal ion level to below the MLC? Provide detailed calculations to support your answer.
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Explanation / Answer
1)
Acid Mine Drainage is acidic water (pH <5.0), laden with iron, sulfate and other metals, that forms under natural conditions when geologic strata containing pyrite are exposed to the atmosphere or oxidizing environments. AMD can form from coal mining, both in surface and in underground mines. Alkaline mine drainage is water that has a pH of 6.0 or above, contains alkalinity, but may still have dissolved metals that can create acid by oxidation and hydrolysis. The drainage quality (acid or alkaline) emanating from underground mines or backfills of surface mines is dependent on the acid (sulfide) and alkaline (carbonate material) minerals contained in the disturbed geologic material. In general, sulfide-rich and carbonate-poor materials are expected to produce acidic drainage. In contrast, alkaline-rich materials, even with significant sulfide concentrations, often produce net alkaline water.
Upon exposure to oxidizing conditions and in the absence of alkaline materials, some sulfide minerals are oxidized in the presence of water and oxygen to form highly acidic, sulfate-rich drainage. Acidity levels, and metal composition and concentration depend on the type and amount of sulfide mineral and the presence or absence of alkaline materials. In the coal fields when sulfides are present, the oxidation of Fe disulfides and subsequent conversion to acid occur through several reactions. The following four chemical equations are accepted to explain the processes.
Equation 1 FeS2 + 7/2 O2 + H2O = Fe2+ + 2 SO42- + 2 H+
Equation 2 Fe2+ + 1/4 O2 + H+ = Fe3+ + ½ H2O
Equation 3 Fe3+ + 3 H2O = Fe(OH)3 + 3 H+
Equation 4 FeS2 + 14 Fe3+ + 8 H2O = 15 Fe2+ + 2 SO42- + 16 H+
2)
In addition to forming precipitates, some metal ions in solution can react with ligands to form soluble complex ions. The metal ions, ligands, and complex ions are in dynamic equilibrium described by a formation constant. The solubility of normally insoluble metal salts can be increased by the formation of complex ions.
A complex ion is an ion formed from a metal ion with a Lewis base attached to it by a coordinate covalent bond. A complex is a compound containing complex ions. A ligand is a Lewis base that bonds to a metal ion to form a complex ion. For example, Ag(NH3)2+ is a complex ion formed from the Ag+ ion and two NH3 molecules. The NH3 molecules are the ligands.
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