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  • ENVR 403Introduction to Environmental Chemistry

    Philip C. SingerDepartment of Environmental Sciences and Engineeringphil_singer@unc.edu919-966-3865April 2009

  • Topics1. Chemistry of Natural Waters2. Purification of Water

  • 1. Chemistry of Natural WatersRedox Equilibria-speciation-dissolved oxygen resources Acid-Base Equilibria-speciation-the carbonate system and alkalinitySolubility Equilibria-metal solubility-aqueous lead (Pb) solubility

  • pH and pE as Master VariablesNO3- + 10H+ + 8e- = NH4+ + 3H2OKred

    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 = EoH + 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 + H2OC6H5OH + 7O2 = 6CO2 + 3H2ONitrogenous oxygen demand (NOD)NH4+ + 2O2 = NO3- + H2O + 2H+Chemical oxygen demand (COD) 3CH2O + 2Cr2O72- +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 watersMajor Cations: Na+, K+, Ca2+, Mg2+ Major Anions: Cl-, SO42- , NO3-, HCO3-, CO32-Charge Balance[Na+] + [K +] + 2[Ca2+] + 2[Mg2+] + [H +] = [Cl- ]+ 2[SO42-] + [NO3-] + [HCO3-] + 2[CO32-] + [OH-]Cb-Ca = Acid neutralizing Capacity = Alkalinity =[HCO3-] + 2[CO32-] + [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[CO32-] + [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[CO32-] + [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 WatersFall 200911:00-12:15 Tu/Th

  • 2. Purification of Water

    Drinking WaterWastewaterContaminated Groundwater

  • Sources of Drinking Water

    Fresh WaterGround WaterSurface Water-Rivers and streams-Lakes and impoundmentsBrackish Water Reclaimed Wastewater?Importance of Watershed Protection

  • Potential Contaminants in Drinking Water

    Pathogenic microorganismsBacteria (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 contaminantse.g. taste and odor-causing organics, pesticides, pharmaceutically active compounds

  • Objectives of Drinking Water TreatmentRemoval of particulate materialRemoval of color (natural organic material, humic substances arising from vegetative decay)Removal of taste and odor-causing substancesRemoval of harmful contaminants - chemical and microbialResidual protection of water during distribution - protect against microbial recontamination, corrosion

  • Chemicals Used in Water TreatmentCoagulantsAluminum, ferric saltsPolymersDisinfectantsChlorine, chlorine dioxide, ozone(UV-Irradiation)Acids/Bases for pH adjustmentLime, causticSulfuric AcidCorrosion Control Phosphates, silicatesOtherFluoride

  • Feed WaterFiltered Water(Permeate)Membrane FiltrationThin barrier or film of material that allows certain substances to pass through while rejecting other substances.

  • Membrane Removal Functions Microfiltration (MF)particles, bacteria, cystsUltrafiltration (UF)viruses and colloidsNanofiltration (NF)viruses; natural organic matter; SOCs; hardnessReverse Osmosis (RO)dissolved minerals (desalination)

  • Membrane Filtration ComparisonFeed WaterRONFUFMFParticles, Giardia, CryptosporidiumSome VirusesDOC, HardnessMineralsWater

  • Objectives of Wastewater Treatment

    Removal of suspended solidsRemoval of oxygen-demanding materialCBOD (e.g. CH2O), NOD (NH3) Removal of nutrients that can promote algal growth, eutrophication Nitrogen, phosphorusRemoval of toxic substancesDisinfection

  • Metal Binding Agents in DetergentsDissolved calcium in hard waters precipitates when the pH is raised during normal laundering operationsDissolved calcium and magnesium in hard waters precipitates the cleansing agent in detergentsHence, detergents contain chelating agents that bind calciumCommon chelating agents polyphosphates, NTA, EDTA, citric acid, succinic acid

  • Groundwater ContaminationNAPLsDNAPLs

  • Groundwater RemediationPump and treatUse of surfactantsIn-situ biodegradation/bioremediationReductive dehalogenation (zerovalent Feo)Isolation


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