1. ENVR 403 Introduction to Environmental Chemistry
Philip C. Singer
Department of Environmental Sciences and Engineering
April 2010

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

3. 1. Chemistry of Natural Waters
Redox Equilibria
-speciation
-dissolved oxygen resources
Acid-Base Equilibria
-the carbonate system and alkalinity
Solubility Equilibria
-metal solubility
-aqueous lead (Pb) solubility

4. pH and pE as Master Variables
H2CO3 = HCO3- + H+ Ka1
pH = pKa1 + Log [HCO3-]/[H2CO3]
NO H+ + 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])
Analogous to Nernst Equation
EH = EoH RT/nF (Log [Ox]/[Red])

5. After M. Benjamin, 2002

6. After M. Benjamin, 2002

7. After M. Benjamin, 2002

8. After M. Benjamin, 2002

18. Electroneutrality Equation (Charge Balance) for Natural waters
Major Cations: Na+, K+, Ca2+, Mg2+
Major Anions: Cl-, SO42- , NO3-, HCO3-, CO32-
Charge Balance
[Na+] + [K +] + 2[Ca2+] + 2[Mg2+] + [H +] = [Cl- ] [SO42-] + [NO3-] + [HCO3-] + 2[CO32-] + [OH-]
Cb-Ca = Acid neutralizing Capacity = Alkalinity =
[HCO3-] + 2[CO32-] + [OH-] – [H +]

19. 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

20. If a body of water has a pH of 7
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 alkalinity
New 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 x [OH-] – [H +]
Find pH = 6.35

22. After M. Benjamin, 2002

24. Solubility of CaCO3(s) Kso = [Ca2+][CO32-]
[Ca2+] = Kso/[CO32-] = Kso/α2[TOTCO3]
Solubility of Ca increases with decreasing pH

28. Lead in Drinking Water
CO3 2-, PO43-
OCl-
Cl-
Pb 2+
Pb(IV)O2, Pb3(CO3)2OH2, PbCO3, Pb5(PO4)3OH
Lead Pipe Pb(0)
CO3 2-, PO43-, Cl-
Pb2+
Particulate
Pb(II) Pb(IV)
Note why we are studying orthophosphate, DIC, chloramines, and pH.
Decrease in oxidizing potential will lead to dissolving of PbO2 with low alk and pH
Increase alkalinity (DIC) and pH to ppt lead carbonates. Add orthophosphate to ppt lead phosphates.
Lead pipe (Pb0) develops scales of corrosion products.
The products formed (oxides, carbonates, and phosphates) depend on the water chemistry.
Changes in distribution system water chemistry can destabilize corrosion products.
Optimization of water chemistry can mitigate lead release to solution. 28

29. Solubility of Potential Lead-containing Phases
Plattnerite (PbO2) -2
Hydrocerussite
(Pb3(CO3)2(OH)2)
with 50 mg/L DIC -3
Litharge (PbO) -4
Hydroxylpyromorphite
(Pb5(PO4)3OH)
with 1 mg/L P
(M)
diss -5 -6
Log[Pb]
15 ug/L Pb -7 -8 -9 4 5 6 7 8 9 10 11 pH

30. Conventional and Waterfree Urinal

31. Operation of Falcon Waterfree cartridge
Urine in
Source: Falcon Waterfree

32. Fed Ex Global Health Building (high use)
Falcon—6 months
Fed Ex Global Health Building (high use)

33. Chemical Composition of Urine (Udert et al. 2003)

34. Underlying Chemical Theory: Hydrolysis of Urea
NH2(CO)NH2+2H2O2NH4++CO3-2
Urease is urea-hydrolyzing enzyme
Urease decomposes urea into ammonium and carbonate, causing the pH to increase
Increase in pH and addition of ammonium and carbonate leads to over-saturation with respect to several different minerals, including struvite (MgNH4PO4) and calcium hydroxyapatite Ca5(PO4)3(OH)2.

36. Comparison of XRD patterns with struvite reference pattern.

37. O P Mg Ca
SEM Results for RDU Terminal A Composition Similar to Struvite, MgNH4PO4 (s)