Predicting Outcomes of Mine Water Treatment
The Math of Chemistry
Stoichiometry is the math of chemistry. You might remember that word from high school chemistry and still cringe, but rest assured, it really isn’t much different than figuring out a recipe of how to bake a cake. Stoichiometry is important to mine water treatment because we can use the equations to predict what reactions will happen and calculate what reagents are needed.
Let’s Bake a Cake
In every stoichiometry equation, the mass (elements, electrons, etc.) must balance between the two sides.
Stoichiometry is very similar to baking a cake. A recipe for a specific cake includes all the ingredients and their relative proportions. Similarly, a balanced chemical equation is the ‘recipe’ for a chemical reaction, and it contains a list of all the reactants (the ingredients), products (the cake), and their relative proportions. If we want to bake two cakes, or only half of a cake, a recipe allows us to scale our products up or down, depending on our desired outcome. The process of using a balanced chemical equation (recipe) to calculate the amounts of reactants (ingredients) and products (cake) is called stoichiometry.
The ratios of reactants in a stoichiometric equation are in a chemical unit called moles (similar to use of a ‘cup’ or ‘tablespoon’ as a unit in baking) which can be converted to weight (e.g., grams, milligrams, etc.). Stoichiometry allows us to predict the chemical inputs required to produce an output, by counting the atoms in different compounds. This can be used to identify treatment mechanisms and determine the treatability of the water and the required and interfering constituents
Examples of Stoichiometry Reactions in Water Treatment
Alkalinity and Oxygen Demand of Ammonia (NH3)
Step 1) Ammonia oxidizing bacteria or archaea oxidize ammonia to nitrate
Step 2) Alkalinity is consumed
Step 3) Some recently identified organisms can perform both steps 1 and 2 simultaneously
The overall reaction is summarized in the bottom equation. The full equation indicates that treatment of 1 mg of ammonia as N requires 4.6 mg of dissolved oxygen and 7.1 mg of alkalinity.
Cyanide (CN) Treatment
There are multiple pathways for CN- treatment including oxidation, reduction, hydrolysis, assimilation, or substitution. We can determine how much oxygen and water is consumed by these reactions by using the known stoichiometry for the breakdown of a cyanide complex. When CN- is oxidized by microbes to ammonia, the overall equation of this treatment mechanism indicates that oxidation of 1 mg of CN- requires 0.6 mg of dissolved oxygen for treatment and produces 0.5 mg of ammonia (as N).
The overall treatment of SCN- through biological oxidation as outlined in the equation indicates that oxidation of 1 mg of SCN- requires 1.1 mg of dissolved oxygen for treatment and produces 0.2 mg of ammonia (as N) and 1.6 mg of sulphate. In oxidative bioreactors, SCN- will often be oxidized to ammonia before the ammonia is oxidized to nitrate.
Treatment Potential of Metals and Carbon Needed
Metals can be treated by precipitation as metal-sulphide minerals. Microbes can reduce sulphate to sulphide using an organic carbon source as an electron donor (e.g., wood chips, methanol, ethanol, molasses, plant material). The sulphide then binds with a metal, removing the metal from the water column and forming a less bioavailable metal-sulphide.
