envr 403 introduction to environmental chemistry philip c. singer department of environmental...

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ENVR 403 Introduction to Environmental Chemistry Philip C. Singer Department of Environmental Sciences and Engineering [email protected] 919-966-3865 April 2009

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ENVR 403Introduction to Environmental

Chemistry

Philip C. SingerDepartment of Environmental

Sciences and [email protected]

919-966-3865April 2009

Topics

1. Chemistry of Natural Waters2. Purification of Water

1. Chemistry of Natural Waters

•Redox Equilibria-speciation-dissolved oxygen resources

•Acid-Base Equilibria-speciation-the carbonate system and alkalinity

•Solubility Equilibria-metal solubility-aqueous lead (Pb) solubility

pH and pE as Master Variables

NO3- + 10H+ + 8e- = NH4

+ + 3H2O Kred

pE = -1/8 (pKred) + 1/8 (Log [NO3-] [H+]10/[NH4

+])

pEo = -1/n (Log Kred)

pE = pEo + 1/n Log [Ox]/[Red])

Analogy to Nernst EquationEH = Eo

H + 2.3 RT/nF (Log [Ox]/[Red])

H2CO3 = HCO3- + H+Ka1

pH = pKa1 + Log [HCO3-]/[H2CO3]

After M. Benjamin, 2002

After M. Benjamin, 2002

After M. Benjamin, 2002

After M. Benjamin, 2002

Dissolved Oxygen Depletion(Oxygen Demand)

Carbonaceous oxygen demand (CBOD)CH2O + O2 = CO2 + H2O

C6H5OH + 7O2 = 6CO2 + 3H2O

Nitrogenous oxygen demand (NOD)NH4

+ + 2O2 = NO3- + H2O + 2H+

Chemical oxygen demand (COD) 3CH2O + 2Cr2O7

2- +16H+ = 3CO2 + 4Cr3+ + 11H2O

d[C]/dt = -kL[CBOD] - kN [NOD] + kH (Cs-C)where C = dissolved oxygen concentrationCBOD = carbonaceous oxygen demandNOD = nitrogenous oxygen demandCs = dissolved oxygen saturation concentration

Dissolved oxygen sag curve

Nutrient Enrichment of Surface Waters and Eutrophication

•PhotosynthesisCO2 + NO3

- + PO43- + H2O Algae biomass + O2

(C106H263O110N16P)

•RespirationAlgal biomass + O2 CO2 + NO3

- + PO43- + H2O

Electroneutrality Equation (Charge Balance) for Natural waters

Major Cations: Na+, K+, Ca2+, Mg2+

Major Anions: Cl-, SO42- , NO3

-, HCO3-, CO3

2-

Charge Balance[Na+] + [K +] + 2[Ca2+] + 2[Mg2+] + [H +] = [Cl- ]+ 2[SO4

2-] + [NO3-] + [HCO3

-] + 2[CO32-] +

[OH-]Cb-Ca = Acid neutralizing Capacity = Alkalinity =

[HCO3-] + 2[CO3

2-] + [OH-] – [H +]

What is the pH of pure rainwater?

Pure rainwater contains only dissolved CO2 in equilibrium with the atmosphere. H2CO3 = KH PCO2 ~ 10-5M

What is the pH of a 10-5M solution of H2CO3?

Cb-Ca = Alkalinity = 0

[H+] = [HCO3-] + 2[CO3

2-] + [OH-] = K1[H+]C/D + 2 K1K2C/D

where D = [H+] 2 + K1 [H+] + K1K2

Answer: pH 5.65

If a body of water has a pH of 7.5 and an alkalinity of 2x10-3 equivalents/L, what is the equilibrium pH of the water if 10-3 eq./L of acid is added to the water?

Use initial pH and given alkalinity to find total carbonate concentration C C = (Alkalinity + [H+] - [OH-]) /((K1[H+]/D) + 2K1K2/D) = 2.12x10-3 M

Added acid consumes alkalinityNew alkalinity = 2x10-3 – 10-3 = 10-3 eq/L

Find new pH from alkalinity expression

Alkalinity = [HCO3-] + 2[CO3

2-] + [OH-] – [H +]10-3 = ((K1[H+]/D) + 2K1K2/D) 2.12 x10-3 + [OH-] – [H +]

Find pH = 6.35

After M. Benjamin, 2002

ENVR 419Chemical Equilibria of Natural Waters

Fall 200911:00-12:15 Tu/Th

2. Purification of Water

•Drinking Water•Wastewater•Contaminated Groundwater

Sources of Drinking Water

•Fresh Water•Ground Water•Surface Water

-Rivers and streams-Lakes and impoundments

•Brackish Water •Reclaimed Wastewater?•Importance of Watershed Protection

Potential Contaminants in Drinking Water

•Pathogenic microorganisms•Bacteria (salmonella, cholera), viruses (Hepatitis A virus, poliovirus), protozoan cysts (Giardia, Cryptosporidium)

•Suspended particles (turbidity)•Dissolved inorganic contaminants

•e.g. Fe, Mn, As, Cr, Cu, Pb, NO3-, ClO4

-

•Dissolved organic contaminants•e.g. taste and odor-causing organics, pesticides, pharmaceutically active compounds

Objectives of Drinking Water Treatment

•Removal of particulate material•Removal of color (natural organic material, humic

substances arising from vegetative decay)•Removal of taste and odor-causing substances•Removal of harmful contaminants - chemical and microbial•Residual protection of water during distribution - protect against microbial recontamination, corrosion

Chemicals Used in Water Treatment•Coagulants

•Aluminum, ferric salts•Polymers

•Disinfectants•Chlorine, chlorine dioxide, ozone•(UV-Irradiation)

•Acids/Bases for pH adjustment•Lime, caustic•Sulfuric Acid

•Corrosion Control •Phosphates, silicates

•Other•Fluoride

MEMBRANE

Feed Water Filtered Water(Permeate)

Membrane Filtration Thin barrier or film of material that allows certain Thin barrier or film of material that allows certain

substances to pass through while rejecting other substances to pass through while rejecting other substances.substances.

Membrane Removal Functions • Microfiltration (MF)

– particles, bacteria, cysts

• Ultrafiltration (UF)– viruses and colloids

• Nanofiltration (NF)– viruses; natural organic matter; SOCs; hardness

• Reverse Osmosis (RO)– dissolved minerals (desalination)

Membrane Filtration ComparisonFeed WaterFeed Water

RO

NF

UF

MF

Particles, Giardia, Cryptosporidium

Some Viruses

DOC, Hardness

Minerals

Water

Objectives of Wastewater Treatment

•Removal of suspended solids•Removal of oxygen-demanding material

•CBOD (e.g. CH2O), NOD (NH3)

•Removal of nutrients that can promote algal growth, eutrophication

•Nitrogen, phosphorus

•Removal of toxic substances•Disinfection

Metal Binding Agents in Detergents• Dissolved calcium in hard waters precipitates

when the pH is raised during normal laundering operations

• Dissolved calcium and magnesium in hard waters precipitates the cleansing agent in detergents

• Hence, detergents contain chelating agents that bind calcium

• Common chelating agents – polyphosphates, NTA, EDTA, citric acid, succinic acid

Groundwater Contamination

•NAPLs•DNAPLs

Groundwater Remediation

•Pump and treat•Use of surfactants

•In-situ biodegradation/bioremediation•Reductive dehalogenation (zerovalent Feo)•Isolation