envr 403 introduction to environmental chemistry philip c. singer department of environmental...
TRANSCRIPT
ENVR 403Introduction to Environmental
Chemistry
Philip C. SingerDepartment of Environmental
Sciences and [email protected]
919-966-3865April 2009
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]
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
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