Save the whales. Collect the whole setPlan to be spontaneous tomorrowAmbition is a poor excuse for not having enough sense to be lazyLife is too short not to be in a hurry

U6220: Environmental Chem. & Tox.Thursday, June 16 2004 Announcement:http://www.columbia.edu/itc/sipa/envp/louchouarn/courses/ Solutions Acid-Bases:Carbonate systemsAlkalinity The legacy of acid-rain in a changing climate

Polarity of moleculesBasic Chemistry Review

The C-H bond is one of the most common in organic compounds. The electronegativity difference between these two atoms is 0.4 (weakly polar).The electronegativity difference in the O-H bond, however, is 1.4 (polar bond)Dipole bonding results from polar substances in a polar solution (solubility)

Apolar molecules will tend to have low solubility in polar solutions

The Water (Mickey Mouse) Molecule

Water: H2O! How simple can that be?Dipole (slightly charged at each end!) Uneven charge Hydrogen bonds!Higher energy requirement for change of state (i.e. to separate Mickey and Minnie!)

SolubilityBasic Chemistry Review

Solubility of compounds in water (or any other liquid/solvent) influences their dispersal and fate in the environment (water exists in liquid form on land, under land, and in the atmosphere).Solution of an ionic compound in a polar solvent (ie. NaCl in water).

SolubilityC) Basic Chemistry Review

Solubility of nonpolar compounds in nonpolar solvents:like dissolves like

NaCl will NOT dissolve in hexaneBut PCBs, oil, etc, will!Solubility of nonpolar solutes in water decreases with size of solute

Partition behaviorBasic Chemistry ReviewPartition coefficient: Ratio of a concentrations of a chemical in two different phases organic pollutants - PCBs, PAHs and lipidshow solutes behave with respect to two solvents (polar vs. non polar)The partition coefficient is constant for a given solute and two specific solvents (under constant environmental conditions: T & P)

Partition coefficient is dependent on: Polarity of solute Its molecular weight Relationship to the polarity of solvents

Partition behaviorBasic Chemistry ReviewOctanol (CH3-(CH2)7-OH)/water partition (imitates lipid/water partition)[C]o = [C]w KowKow = [C]o/[C]wAs Kow increases, the lipophilicity of a chemical increasesKow for various homologous series is related to molecular surface area size

Solutions

Kind of Mixture

Particle Size

Example

Characteristics

Solution

500 nm

Blood, paint, sediments, aerosols

opaque or murky, separation upon standing, filtrable

Molarity (M)Moles of solute/Liters of solutionsMass per Volume (g/L)mass of solute/Liters of solutionsMass per Mass (ppm)mass of solute/mass of solutions (g/g)Mass per Mass per Volume (ppmv)percent volume solute/volume of solutionSolutions

Aquatic equilibria are important in environmental processesAt equilibrium:Solution Equilibria and AcidsaA + bB cC + dDKc = [C]c[D]d/[A]a[B]bWhere Kc is the equilibrium constant and the right hand of the equation, the equilibrium quotient, is the ratio of products to reactants

Le Chatelier Principle (useful indications of shifts in equilibrium):When a system in equilibrium is subjected to change, the system will alter in such a way as to lessen the effect of that change Adding product C to the system will make the rxn shift to the left (consumption of C and D) and the position of the equilibrium changes (Kc remains unchanged)

Dissociation of water and the pH scale2H2O H3O+ + OH-orH2O H+ + OH-whereKw = ([H+] [OH-]/[H2O])eqWhere [H2O] is equal to unity (pure substances)Kw = ([H+] [OH- ])eq

Dissociation of water and the pH scaleKw = ([H+] [OH-]/[H2O])eqExperimental determination shows thatKw = 1.8x10-16 mol/liter (at 25C)It is conventional to omit the concentration of water from this K expression. Why?[H2O]eq = 55.5 MolarWhy?Kw = 10-14 Why?

Dissociation of water and the pH scaleKw = ([H+] [OH- ])eqExperimental determination shows thatKw = 10-14 (at 25C)log Kw = log 10-14 log Kw = -14 log 10log Kw = 14pKw = -log KwSo pKw = 14[H+] [OH-] = 1x10-14 and [H+] = [OH-] = 1x10-7So pH of natural waters = -log [H+] = -log 10-7 = 7

Dissociation of water and the pH scaleKw = [H+] [OH- ]And for any acid (HB) in solutionHB H+ + B-(Henderson-Hasselbach equation)

Dissociation of water and the pH scaleFor waterlog Kw = log [H+] + log [OH-]- log Kw = - log [H+] - log [OH-]pKa = pH + pOHand pH + pOH = 14

For other acids:Ka = [H+] [A- ]/[HA]HCl H+ + Cl- where pKa = -3CH3COOH + H2O CH3COO- + H+ Ka = ([CH3COO-]+[H+])/[CH3COOH] = 10-4.76pKa = ?pKa = 4.76

Dissociation of water and the pH scaleFor other acids:Ka = [H+] [A- ]/[HA]HCl H+ + Cl- where pKa = - log Ka or Ka = 10-pKa

Speciation of metals:Species distribution based on environmental conditions What is the most abundant species of iron in natural waters?Single variable diagrams

pH and mineral surface chargeThe point of zero charge (PZC) is point at which a surface charge changes sign

pHpzc will influence sorption capacity of minerals (and organic substances) in natural environmentspH and minerals

Single Variable Diagrams: pHWhat is the most abundant species of arsenic in natural waters?How does pH influence As distribution?

Carbonate system - Acids in the environmentCO2(aq) + H2O = H2CO3 (total CO2(aq) + H2CO3)KCO2 = [H2CO3 ]/[PCO2] = 10-1.47First dissociation step for carbonic acid:K1 = [H+] [HCO3- ]/[H2CO3] = 10-6.35Second dissociation step for carbonic acid:K2 = [H+] [CO32- ]/[HCO3-] = 10-10.33

Carbonate system - Acids in the environmentDominant carbonate species are related to K1 and K2H2CO3 dominates below pH = pK1 = 6.35HCO3- dominates between pH = pK1 = 6.35 & pH = pK2 = 10.33CO32- dominates above pH = pK2 = 10.33

AlkalinityAlkalinity is a measure of the ability of a water body to neutralize acids and is very important in predicting the extent of acidification in natural waters (i.e. lakes and rivers)

Alkalinity = [OH-] + [HCO3-] + 2[CO32-] - [H+]ANC = [OH-] + [HCO3-] + 2[CO32-] + [B(OH)4-] + [H3SiO4-] + 2[HPO42-] + [HS-] + [NOM-] - [H+] - 3[Fe3+] -

Contribution of all these species tends to be minimal in natural fresh waters (concentrations are too small. With the exception of NOM!). The alkalinity tends then to be equal to ANC only proton accepting species, present in substantial concentration, are carbonates and/or hydroxyl ion.

AlkalinityAlkalinity is a capacity factor measure of the ability of a water sample to sustain reaction with added acid or basepH is an intensity factor measure of the concentration of protons (acids) immediately available for reaction

Buffer capacity: is the capacity of a solution (or water-rock system) to resist pH change when mixed with a more acid or alkaline water (rock)

AlkalinityTitration: a procedure for determining the amount of acid (or base) in a solution by determining the volume of base (or acid) of known concentration that will completely react with it

Acidification of a lake in a natural setting is analogous to a macro-scale titration and lakes are sometimes termed buffered, transitional, and acidic depending on their position on the titration curve:

AlkalinityAn alternative way to report alkalinity is to express it in terms of neutralization reaction between carbonate and protons and given in values of mg/L of CaCO3 (or mg/L of Ca2+)1 mg CaCO3 = 1000 g which is 1000 g/100 g/mol = 10 molSince each carbonate (CO32-) is capable of neutralizing two OH- ions 10 mol of CO32- is equivalent to 20 mol proton-accepting capacity (20 equivalents or eq). A sensitivity classification of water bodies may thus be expressed in terms of alkalinity using units of mol/L of proton accepting capacity (eq)

The legacy of acid-rain

Coal and Acid rainSO2 + 2OH H2SO4 2H+ + SO42- SO2 + H2O2 H2SO4 2H+ + SO42-N2 + O2 2NO (>2000C)NO + O3 NO2 + O2 NO2 + OH HNO3 H+ + NO3-

Acid rainThe main sources of acid deposition are emissions from oil and coal-burning power plants and automobiles

Acid rain?In the US, seven states in the Ohio valley account for ~40% of all SO2 emissions. (emissions travel downwind to N.E.)

Midwest states are the most significant emitters of NOx and NH4

Acid rain?In the US, SO2 emissions have been declining since about 1980 (still above background).

NOx emissions have not changed

Acid rain?Acidity of rain is neutralized by CaCO3

Acid rain?Change in stream sulfate concentrations, but delayed!Yellow: wet depositionBlue: surface waters

Acid rain?Role of hydrodynamic forcing

Large-scale spatial variability in streamflow is explained by precipitation/evaporation balance to a large extent (~90%) and additional processes to a smaller one (soil water storage, seasonality)Spatial Variability of Streamflow (U.S. North East)

What are the seasonal predictions for N. America?Seasonal predictions of precipitation for North America. Clockwise from upper left: Winter, Spring, Fall, Summer. Coupled Model Intercomparison Project (CMIP) using 1% CO2 increase per year and the perturbations for the last 20 years of an 80 years run (Ting, pers. Comm.)

Drought Temporal and spatial variability

For the Catskill region, the change in evapotranspiration and snowpack amount will offset any increase in precipitation that may occur. Temporal Variability of Streamflow

Fluctuations in streamflow patterns (particularly drought incidence) has strong impact of re-acidification of lakes Increased Al inputs (toxic to aquatic biota) Decreased pH Decreased acid neutralizing capacity (ANC)What does it mean for acidified lake recovery?

Redox Potential (Acid Mine Drainage)Sulfate reduction:SO42- + 2CH2O + 2H+ H2S + 2H2O + 2CO2 With the presence of Fe2+Fe2+ + H2S FeS + 2H+ And FeS + S FeS2FeS2 + H2O + 7/2O2 Fe2+ + 2SO42- + 2H+AndFeS2 + 14Fe3+ + 8H2O 15Fe2+ + 8H2SO4Later4Fe2+ + O2 + 10H2O 4Fe(OH)3 + 8H+Sulfide oxidation

Natural Organic Matter (Humic Matter) Fulvic Acids (Fa): small fractions soluble in aqueous solution Humic Acids (Ha): larger fractions soluble in alkaline sol. Humin (Hu ): larger insoluble fractions

Fluctuations in streamflow patterns (particularly drought incidence) has strong impact of re-acidification of lakes Decrease in DOC concentrations (Al3+ + flocculation) Deeper penetration of UVB (factor of 3 in some lakes)Delayed lake recovery

Extra slides

Covalent bondsBasic Chemistry Review

Bonds in such compounds are formed by the sharing of e- rather than by the complete transfer of e- from one atom to another.The e- in covalent compounds exist in molecular orbitals formed by overlapping of two atomic orbitals.Sometimes, sharing two e- is not enough

ElectronegativityC) Basic Chemistry Review

When two different atoms are joined by a covalent bond, the bonding e- are not necessarily shared equally Atoms have different abilities to attract e-

Carbonate system - Acids in the environmentDominant carbonate species are related to K1 and K2 - But Why?Remember:Ka = [H+] [A- ]/[HA]log Ka = log [H+] + log ([A- ]/[HA])-log Ka = -log [H+] - log ([A- ]/[HA])pKa = pH - log ([A- ]/[HA]) pK1 = pH - log ([HCO3- ]/[H2CO3])

Carbonate system - Acids in the environmentpK1 = pH - log ([HCO3- ]/[H2CO3])At pH = pK1pK1 - pH = - log ([HCO3- ]/[H2CO3])0 = - log ([HCO3- ]/[H2CO3])0 = - log[HCO3- ] + log[H2CO3]log[HCO3- ] = log[H2CO3][HCO3- ] = [H2CO3]

What is the acidity of natural rain?

What is the acidity of natural rain?CO2(aq) + H2O = H2CO3 (total CO2 + H2CO3)KCO2 = [H2CO3 ]/PCO2 = 10-1.47 (1)First dissociation step for carbonic acid:K1 = [H+] [HCO3- ]/[H2CO3] = 10-6.35 (2)So, by combining equation (1) and (2)[H+] [HCO3- ] = K1 KCO2PCO2(where PCO2 = 360 ppmv)

What is the acidity of natural rain?[H+] [HCO3- ] = KCO2K1 PCO2(where PCO2 = 360 ppmv)Solving this yields[H+]2 = 10-1.47 x 10-6.35 x 10-3.44[H+]2 = 1x10-11.26 pH = ?[H+] = (1x10-11.26)1/2[H+] = 10-5.63 pH = 5.63