ENVR 403Introduction to Environmental

Chemistry

Philip C. SingerDepartment of Environmental

Sciences and Engineeringphil_singer@unc.edu

919-966-3865April 2010

Topics

1. Chemistry of Natural Waters2. 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

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])

Analogous to Nernst EquationEH = Eo

H + 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

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

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

• 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.

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)

Solubility of Potential Lead-containing Phases

Litharge (PbO)

Hydroxylpyromorphite

(Pb5(PO4)3OH)

with 1 mg/L P

Hydrocerussite

(Pb3(CO3)2(OH)2)

with 50 mg/L DIC

-9

-8

-7

-6

-5

-4

-3

-2

4 5 6 7 8 9 10 11

pH

Log

[Pb]

diss

(M

)

15 ug/L Pb

-16

-14

-12

-10

-8

-6

-4

-2

4 5 6 7 8 9 10 11

pH

Lo

g[P

b] d

iss

(M)

15 ug/L Pb

Plattnerite (PbO2)

Conventional and Waterfree Urinal

Operation of Falcon Waterfree cartridge

Urine in

Source: Falcon Waterfree

Falcon—6 monthsFed Ex Global Health Building (high use)

Formula Avg Concn (mol/L)

Ammonia NH3 0.034

Phosphate PO4-3 0.024

Calcium Ca+20.0046

Magnesium Mg+20.0039

Sodium Na+ 0.12

Potassium K+ 0.056

Carbonate CO3-2

0

Sulphate SO4-2

0.016

Chloride Cl- 0.11

Oxalate (COO)2-2 0.00023

Citrate C3H5(COO)3-3

0.0026

Urea (NH2)2CO 0.27

pH 6.2

Chemical Composition of Urine (Udert et al. 2003)

Underlying Chemical Theory: Hydrolysis of Urea

• NH2(CO)NH2+2H2O2NH4++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.

Urea Hydrolysis

6

6.5

7

7.5

8

8.5

9

9.5

0 0.04 0.08 0.12 0.16 0.2 0.24 0.28

Urea hydrolyzed, mol/L

pH

Standard (IS = 0)

Corrected (IS = 0.239)

5 10 15 20 25 30 35 40 45 50 55 60

2-Theta

Co

un

t In

ten

sity

Sample 1

Sample 2

Struvite Reference

Comparison of XRD patterns with struvite reference pattern.

Sample StruviteMg 16.18 17.7O 47.8 46.6P 25.48 22.6

Ca 10.43 0Total 100 87

Weight %Element

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

O

Mg

P

Ca