Water Electroneutrality Balanced: ∑ Cations = ∑ Anions ∑ C C x e C = ∑ C A x e A C C = moles / liter of cation c (one mole = 6.02x10 23 ) e C = number of charges per molecule of cation C A subscript is for anions Pure water: [H + ] = [OH - ] Natural water: [H + ] = [OH - ] + [HCO 3 - ] + 2[CO 3 -2 ] blog.ssis.edu.vn/
Why do we care about pH? Biological systems Extremes are disruptive Corrosivity Equilibrium relationships involving H +
Good pH ranges Freshwater... Marine... Drinking... Soda?
Alkalinity Capacity of water sample to neutralize an... Capacity of a water sample to take H + without significant change in... Common ions that give water alkalinity HCO 3 -, CO 3 2- & OH -
Acidity Capacity of water to take OH - without significant pH change Opposite of Alkalinity Capacity of water sample to neutralize a base Common ion: H + For Acid Mine Drainage Acidity =~f(Fe 2+, Fe 3+, Al, Mn, and H + ) csmphotos.files.wordpress.com
pH, Alkalinity & Acidity Waters with same pH can have different amounts of Alkalinity or Acidity Chemicals may be present that can react with or release H + or OH - The lower the alkalinity (or acidity), the easier it is to change pH
Reversible Reactions aA + bB ↔ cC + dD A & B can react to form C and D C & D can react to form A and B After sufficient time has passed, equilibrium is reached Equilibrium can be perturbed by adding more reactant or product
Reversible Reactions: Gas - Solution At equilibrium Gaseous chemical dissolves Dissolved chemical volatilizes At equal rate Henry’s Law Example… Gas Dissolved
Reversible Reactions In Solution At equilibrium Reactants become products Products become reactants At equal rate Example…
Reversible Reactions: Solution - Precipitate At equilibrium Precipitate is formed Precipitate is dissolved At equal rates Example…
Equilibrium Equation aA + bB ↔ cC + dD K is constant at given temperature & relatively low concentration
Example - Pure Water What is water?... Reversible equation?... Equilibrium equation?... At 25 C, K w =... Even if other sources of H + (acids) or OH - (bases) are present Which substance predominates?...
pH of Pure 25 C? Follow these steps Write down species... Write electroneutrality eq... Write equilibrium equation.. Solve equations...
Now, add… OH - ? (add a strong base) Systems finds new equilbrium, with less H + and more OH - pH goes up H + ? (add a strong acid, precipitate iron,…) Systems finds new equilbrium, with more H + and less OH - pH goes down. Either way, [H + ] [OH - ] = K w still holds
Precipitation of Limestone CaCO 3 -2 ↔ Ca +2 + CO 3 -2 [Ca +2 ] [CO 3 -2 ] = K sp K sp = Solubility product Do not include precipitate in equilibrium equation [Ca +2 ] [CO 3 -2 ] < K sp ?... [Ca +2 ] [CO 3 -2 ] > K sp ?...
Carbonate Equilibrium System (CES) Keeps natural waters in good pH range Common Chemicals Carbon Dioxide, CO 2 Carbonic Acid, H 2 CO 3 Bicarbonate, HCO 3 - Carbonate, CO 3 -2 Solids containing Bicarbonate, Carbonate, Hydroxide, and (though rarely) certain Oxides (CaO),…
Main Sources CO 2 from atmosphere Byproduct of biological or chemical reactions combustion Solids containing carbonate e.g., limestone Byproduct of sea life CO 2 g CO 2 aq H 2 CO 3 HCO 3 CO 3 Solid CO 3,… Atmosphere Solution Precipitate
CES Reactions Reversible CO 2 g ↔ CO 2 aq CO 2 aq + H 2 O ↔ H 2 CO 3 H 2 CO 3 ↔ H + + HCO 3 - HCO 3 - ↔ H + + CO 3 -2 Ca +2 + CO 3 -2 ↔ CaCO 3 Equilibrium (at 25 C) CO 2 aq = CO 2 g / 1637 atm [H 2 CO 3 ] / [CO 2 aq] = 1.58 x [H + ] [HCO 3 - ] / [H 2 CO 3 *] = 4.47x10 -7 M [H + ] [CO 3 -2 ] / [HCO 3 - ] = 4.68x M [Ca +2 ] [CO 3 -2 ] = 3.80 x M 2 Where CO 2 g is in atm & CO 2 aq is in mole fraction
Carbonate System & pH Typical Natural Water with 100 mg/L alkalinity as CaCO 3 pH 100 mg/L as CaCO CO 2 CO 3 HCO 3 OH
Carbonate System Scenarios Open / Closed Open - in equilibrium with atmosphere open container, streams & shallow lakes, upper ocean Closed - not in equilibrium Closed tank, quick reactions, deep regions of water bodies, soil gases Homogeneous / Heterogeneous Homogeneous – in equilibrium with CO 3 solids Heterogeneous– not in equilibrium with CO 3 solids Closed & heterogeneous bottom of stratified lake with floor of limestone water treatment unit with soda ash / acid or base / CO 2 & some CaCO 3 precipitates
Example - Natural pH of Rain CO 2 in atmosphere naturally “shifts” pH of rain from 7 to ? Open system No solid source of CO 3 CO 2 g CO 2 aq H 2 CO 3 HCO 3 - CO 3 - H+H+ H+H+ Lowers pH
Atmosphere Nitrogen atm Oxygen atm Argon atm CO atm When this example was created. It’s now atm ( ppm) Misc atm TOTAL atm
Solution: pH of Rain (1) Atmosphere to raindrop - Henry’s Law: CO 2 aq = CO 2 g / 1637 atm Convert to M: [CO 2 aq] = CO 2 aq x M w M w = molar density of water = mol/l
Solution: pH of Rain (2) Reaction with water forms carbonic acid - Equilibrium Equation: [H 2 CO 3 aq] = 1.58x10 -3 M [CO 2 aq] [H 2 CO 3 * aq] = [CO 2 aq] + [H 2 CO 2 aq]
Solution: pH of Rain (3) Dissociation of carbonic acid - Equilibrium Equation: [H + ] [HCO 3 - ] = 4.47 x M [H 2 CO 3 *] Two unknowns? Use electroneutrality: [H + ] = [OH - ] + [HCO 3 - ] + 2[CO 3 -2 ] Rain water is acidic, so [OH - ] & [CO 3 -2 ] will be small giving [H + ] ≈ [HCO 3 - ], leading to…
Solution: pH of Rain (4) pH = - log [H+] = -log (2.13x10 -6 ) =… Check assumptions? Use equilibrium equations: [OH - ] = M 2 / [H + ] = M 2 / 2.13x10 -6 M = 4.68x10 -9 M (small, OK)) [CO 3 -2 ] = 4.68x M [H + ] / [HCO 3 - ] = 4.68x M 2.13x10 -6 M / 2.13x10 -6 M = 4.68x M (small, OK)
pH of Rain CO 2 in atmosphere goes up?... Use this method for different conditions? Need to check assumptions about relative concentration of [OH - ] & [CO 3 -2 ]
Atmospheric CO 2 & Ocean pH Oceans can be CO 2 sink or source Currently: sink, taking ~ 30% of anthropocentric CO 2 Primary mechanisms Carbonate equilibrium system (Henry’s Law,…) Biological Pump Living creatures take up carbon, some are trapped in sea bed Too much CO 2 ? pH drop of oceans could effect sea organisms From 1751 to 1994 surface ocean pH estimated to have dropped from ~ 8.18 to 8.10
Ocean pH Change ~half Anthropogenic CO 2 has been absorbed by ocean so far Currently absorbing ~1/3 22 M tons / day Could drop to 7.6 Drastic effect on shell-forming organisms National Geographic (2014) “Ocean Acidification”, ocean.nationalgeographic.com.
Acid Rain Rain with pH below 5.7 Most prevalent cause: SO x produced from burning coal SO x reacts with water to form sulfuric acid Sulfuric acid dissolves into atmospheric water and dissociates into H - and SO 4 -2
Precipitation pH downwind of Ohio Valley power plants
Sulfur Cap & Trade Program
CES, Alkalinity, Acid Rain’s Effect Carbonate solids Common source of alkalinity in natural waters (as they dissolve) Lakes and streams with alkalinity can accept acid rain without big pH changes Alkalinity buffers acid rain Acid rain can damage water bodies that don’t have alkalinity
Acid Mine Drainage
Red Oak Mine Site
Mine Side View Water Infiltration Anaerobic Conditions Low pH High Acidity High metals FeS 2 + O 2 + H 2 O → Fe 2+ + SO H + Fe 2+ + O 2 + H + → Fe 3+ + H 2 O Fe 3+ + H 2 O → Fe(OH) 3 ↓ + H + Presence of limestone will buffer the mine water, resisting pH change Even lower pH Oxidation of organic matter can lead to high concentrations of CO 2 in mine headspace pH – 4.4 (low) [drops to ~2 after seep] Acidity 434 ppm CaCO 3 (high) Partial Pressure CO 2 in mine headspace - ~6% [200x higher than atmosphere]
Red Oak Seep seep discharge
Plan View Not to scale
Red Oak Remediation Post injection, seep characteristics were pH - 6.3, Alkalinity mg/L as CaCO3, Fe- 120 mg/l; however, this level of treatment lasted only 15 months.