ENVR 403 Introduction to Environmental Chemistry

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ENVR 403Introduction to Environmental Chemistry

• Philip C. Singer
• Department of Environmental Sciences and
  Engineering
• phil_singer_at_unc.edu
• 919-966-3865
• April 2009

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Topics

• 1. Chemistry of Natural Waters
• 2. Purification of Water

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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

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pH and pE as Master Variables
H2CO3 HCO3- H Ka1 pH pKa1 Log
HCO3-/H2CO3
NO3- 10H 8e- NH4 3H2O Kred pE -1/8
(pKred) 1/8 (Log NO3- H10/NH4) pEo
-1/n (Log Kred) pE pEo 1/n Log
Ox/Red) Analogy to Nernst Equation EH EoH
2.3 RT/nF (Log Ox/Red)
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After M. Benjamin, 2002
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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 2Cr2O72- 16H 3CO2 4Cr3
  11H2O

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Dissolved oxygen sag curve
dC/dt -kLCBOD - kN NOD kH
(Cs-C) where C dissolved oxygen
concentration CBOD carbonaceous oxygen
demand NOD nitrogenous oxygen demand Cs
dissolved oxygen saturation concentration
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Nutrient Enrichment of Surface Waters and
Eutrophication

• Photosynthesis
• CO2 NO3- PO43- H2O ? Algae biomass O2
• (C106H263O110N16P)
• Respiration
• Algal biomass O2 ? CO2 NO3- PO43- H2O

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Electroneutrality Equation (Charge Balance) for
Natural waters

• Major Cations Na, K, Ca2, Mg2
• Major Anions Cl-, SO42- , NO3-, HCO3-, CO32-
• Charge Balance
• Na K 2Ca2 2Mg2 H Cl-
  
  2SO42- NO3- HCO3- 2CO32- OH-
• Cb-Ca Acid neutralizing Capacity Alkalinity
• HCO3- 2CO32- OH- H

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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-
2CO32- OH- K1HC/D 2 K1K2C/D where
D H 2 K1 H K1K2 Answer pH 5.65
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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-) /((K1H/D)
2K1K2/D) 2.12x10-3 M Added acid consumes
alkalinity New alkalinity 2x10-3 10-3 10-3
eq/L Find new pH from alkalinity
expression Alkalinity HCO3- 2CO32-
OH- H 10-3 ((K1H/D) 2K1K2/D) 2.12
x10-3 OH- H Find pH 6.35
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ENVR 419 Chemical Equilibria of Natural
Waters Fall 2009 1100-1215 Tu/Th
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2. Purification of Water

• Drinking Water
• Wastewater
• Contaminated Groundwater

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Sources of Drinking Water

• Fresh Water
• Ground Water
• Surface Water
• -Rivers and streams
• -Lakes and impoundments
• Brackish Water
• Reclaimed Wastewater?
• Importance of Watershed Protection

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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

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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

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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

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Membrane Filtration

• Thin barrier or film of material that allows
  certain substances to pass through while
  rejecting other substances.

Feed Water
Filtered Water (Permeate)
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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)

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Membrane Filtration Comparison
Feed Water
Particles, Giardia, Cryptosporidium
Some Viruses
MF
DOC, Hardness
UF
Minerals
NF
RO
Water
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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

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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

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Groundwater Contamination

• NAPLs
• DNAPLs

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Groundwater Remediation

• Pump and treat
• Use of surfactants
• In-situ biodegradation/bioremediation
• Reductive dehalogenation (zerovalent Feo)
• Isolation