Envr 403 introduction to environmental chemistry
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ENVR 403 Introduction to Environmental Chemistry. Philip C. Singer Department of Environmental Sciences and Engineering phil_singer@unc.edu 919-966-3865 April 2010. Topics. 1. Chemistry of Natural Waters 2. Purification of Water. 1. Chemistry of Natural Waters. Redox Equilibria

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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@unc.edu

919-966-3865

April 2010


Topics

1. Chemistry of Natural Waters

2. 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

H2CO3 = HCO3- + H+Ka1

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

NO3- + 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])

Analogous to Nernst Equation

EH = EoH + 2.3 RT/nF (Log [Ox]/[Red])


After M. Benjamin, 2002


After M. Benjamin, 2002


After M. Benjamin, 2002


After M. Benjamin, 2002


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- ]+ 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 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 x10-3 + [OH-] – [H +]

Find pH = 6.35


After M. Benjamin, 2002


Solubility of CaCO3(s)

  • Kso = [Ca2+][CO32-]

  • [Ca2+] = Kso/[CO32-] = Kso/α2[TOTCO3]

  • Solubility of Ca increases with decreasing pH


Lead in Drinking Water

Pb(II) Pb(IV)

OCl-

Cl-

Particulate

CO32-, PO43-

CO32-, PO43-, Cl-

Pb2+

Pb 2+

Pb(IV)O2, Pb3(CO3)2OH2, PbCO3, Pb5(PO4)3OH

Lead Pipe Pb(0)

  • 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.


Solubility of Potential Lead-containing Phases

Hydrocerussite

(Pb3(CO3)2(OH)2)

with 50 mg/L DIC

Hydroxylpyromorphite

(Pb5(PO4)3OH)

with 1 mg/L P

Plattnerite (PbO2)

-2

-3

Litharge (PbO)

-4

(M)

-5

diss

-6

Log[Pb]

15 ug/L Pb

-7

-8

-9

4

5

6

7

8

9

10

11

pH


Conventional and Waterfree Urinal


Operation of Falcon Waterfree cartridge

Urine in

Source: Falcon Waterfree


Falcon—6 months

Fed Ex Global Health Building (highuse)


Chemical Composition of Urine (Udert et al. 2003)


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.


Comparison of XRD patterns with struvite reference pattern.


O

P

Mg

Ca

SEM Results for RDU Terminal A Composition Similar to Struvite, MgNH4PO4 (s)


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