Solutions: Environmental Chemistry - a global perspective 4th Edition
Chapter 12

Organic matter in water

PROBLEMS/SOLUTIONS
1.

Use diagrams of a type similar to those in Figs 12.4–12.7 to show the kinds of interactions you might
expect between humic material and phenanthrene, trichloroethylene (TCE, CHCl = CHCl 2), copper(II),
and copper(II) complexed with oxalate.

Solution

CH3
CH2
C
H2

+

HM
O
CH3
humic material

CH3
CH2
Only phenanthrene shown.

...

....
....
....
...
..
..

C
H
2. Explain how molecular size2and oxygen concentration differences could explain the relative solubility
of humic and fulvic acid.
HM
O
Solution
CH

The larger molecular size (comparing
humic acid relative to fulvic acid) tends to increase the hydrophobic
3
portions of the molecule. As such, it is more difficult for the larger macromolecules to disturb the structure
of water, which is necessary for the humic material to dissolve. An increased oxygen content in humic
material is a result of a greater amount of functionality containing oxygen. These functional groups (such
as OH, COOH, C=O, COC, etc.) create polar regions in the molecule and the oxygen can contribute to the
formation of hydrogen bonds with water. This will lead to increased solubility. Furthermore, as molecular
size decreases, the oxygen concentration per mol of material tends to increase. Thus fulvic acid being of a

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Solutions: Environmental Chemistry - a global perspective 4th Edition

Chapter 12

Organic matter in water

smaller size and higher oxygen concentration has a wider range of solubility (at all pHs) compared to humic
acid, which is only soluble at higher pH values.

3.

The 13C NMR spectra of six humic acid samples from marine and estuarine sediments are shown in
Fig. 12.P.1 (From Hatcher, P.G. and Orem, W.H., Structural interrelationships among humic
substances in marine and estuarine sediments, In Organic marine geochemistry (ed. M.L. Sohn),
American Chemical Society, Washington, DC; 1986). The samples are from various locations—the
Potomac River, on the coast of the United States; New York Bight, 16 km off the New York coast, on
the continental shelf; Walvis Bay, from the deep continental shelf on the Namibian coast; Mangrove
Lake, from a marine lake in Bermuda.
We indicated that accurate quantitative analysis of solid samples by NMR is problematic, but it is
appropriate to develop semi-quantitative ideas based on the spectra: Which HA samples appear to be
lowest in aromatic carbons? Where marine algal material is the principal source of HA, the structure is
predominantly aliphatic. Which samples may have the highest aromatic content? Why? The Mangrove
Lake surface sediment HA appears to be enriched in carbohydrate material. Why might the
concentration decline with depth? To what might the small peak at 50 ppm be due?

Fig. 12.P.1

Carbon-13 NMR spectra of six humic acid samples from marine and estuarine sediments.

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Solutions: Environmental Chemistry - a global perspective 4th Edition
Chapter 12

Organic matter in water

Solution
The Mangrove Lake HA material appears to have a much smaller aromatic content than the other samples
as is evident from the small resonances in the 100 to 160 ppm region. In contrast, the Potomac River
samples, especially the fluvial sample shows evidence of much more aromatic content. The river HA would
derive its OM form terrestrial sources (more aromatics) while the marine samples would derive HA from
marine algae. The surface-enriched carbohydrate material (resonance at ~ 75 ppm) may have been derived
from early decomposition of organic matter in the overlying waters. Upon burial, this material would
further degrade and resynthesize into a material that is less like the original cellulosic plant residues. The
peak at 50 ppm could be due to aliphatic esters, ethers or amide groups.

4.

Water from a lake in an area with limestone bedrock has a chemical composition as follows:
Calcium

95 mg L–1

Magnesium

13 mg L–1

Sodium

17 mg L–1
4 mg L–1


Potassium
Hydrogen carbonate (as
Sulfate (as

–1
HCO3 ) 338 mg L

7 mg L–1

SO 24 )

12 mg L–1

Chloride

0.2 mg L–1

Fluoride
Nitrate (as

3 mg L–1

NO3 )

pH

6.8

If the organic matter (HM) content of the water is 12 mg L –1, show that charge balance is adequately

maintained.
Solution
mg L-1

g mol-1

Ca2+
Mg2+
Na+
K+
H+

95
13
17
4
----

40.1
24.3
23.0
39.1
------

HCO3SO42ClFNO3-

338
7
12
0.2

3

61.0
96.0
35.5
19.0
62.0

Concentration
mmol L-1
2.37
0.53
0.74
0.10
1.58 x 10-4
5.54
0.07
0.34
0.01
0.05

factor

[charge]
mmol L-1

2
2
1
1

1

4.74
1.06
0.74
0.10
0.00

1
2
1
1
1

5.54
0.15
0.34
0.01
0.05

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Solutions: Environmental Chemistry - a global perspective 4th Edition
Chapter 12

Organic matter in water

Total positive [charge] is 6.64 mmol L-1 and the total negative [charge] is 6.09 mmol L-1. The excess
positive [charge] is 6.64 – 6.09 = 0.55 mmol L-1.

The humic material has a concentration of 12 mg L-1. Because of the nearly neutral pH, most if not all the
carboxylic acid groups will be deprotonated and carry a negative charge. Phenolic and alcoholic groups,
however, will likely remain protonated under these conditions.
This suggests that the humic material will carry a [charge] of 0.55 mmol L-1. And the concentration of the
function group Xcoo- can be determined by:
12 mg L-1 x Xcoo- = 0.55 mmol L-1
Xcoo- = 46 mmol g-1
46 mmol g-1 is about 10 times the typical concentration for this type of group associated with humic
material. Note that there is a large error associated with the calculation, as the concentrations of species
given do not justify as much accuracy as we have assumed in calculating total positive and negative
charges.

5. The chemical nature of the HM in very deep (up to > 1000 m) cores of sediments from the Black Sea
have been studied in detail in order to understand the processes that occur over very long periods of
time. Average percentage values of carbon, hydrogen, and oxygen concentrations in two depth ranges
are:
C

H

O

Shallow

56.3

5.2

31.9


Deep

60.9

5.2

27.9

What do these results suggest about the long-term processes occurring within the organic sediments?
(These data are summarized from work by A.Y., Huc, B.M. Durand, and J. Monin as reported in
Rashid, M.A., Geochemistry of marine humic compounds, Springer, New York; 1985.)
Solution
The data suggest that the deep environment is a more reducing environment. Oxygen that may have been
part of the original biomass from which the humic material was derived is lost in the reducing environment,
enriching the samples, relatively, in carbon. The long-term processes occurring suggest the beginning of
coal/oil formation.
6.

The equivalent weight of HM is sometimes defined as the molar mass per mol of carboxylic plus
phenolic groups. For a particular sample of fulvic acid, there are 4.2 and 2.2 mmol of carboxylic and
phenolic groups, respectively, per gram of the FA. What is the equivalent weight?

Solution
The total amount of carboxylic and phenolic groups is
4.2 mmol g-1 + 2.2 mmol g-1 = 6.4 mmol g-1

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Solutions: Environmental Chemistry - a global perspective 4th Edition

Chapter 12

Organic matter in water

This can also be expressed as 0.0064 moles of groups per gram of humic material. The equivalent weight of
humic material for 1 mole of groups is then
1 mol ÷ 0.0064 mol g-1 = 156 g
That is, you require 156 g of humic material to obtain 1 mole of total functional groups. Therefore, the
equivalent weight is expressed as 156 g mol-1.

7.

The dissolved organic matter isolated from a natural water sample has the following composition:
C

44.2 % by weight

H

6.1 % by weight

O

46.7 % by weight

N

1.7 % by weight

Molar mass

1900 daltons
Is the DOM likely to be fulvic acid or humic acid? Explain your reasoning.
Solution
The DOM is more likely to be fulvic acid. The C content is at the lower range for C in humic substances
while the O content is at the higher end. Both of these indicate a more oxygen enriched humic substance,
suggesting fulvic acid. As well the ‘small’ molar mass of 1900 daltons also indicates a fulvic acid.

8.

Consider both the degradative and synthesis reaction series for the formation of various types of humic
material. Can both theories be used to explain the relative amounts of carbon and oxygen in the three
classes of HM as described in the text?

Solution
Simply put, yes. The degradative theory provides for the incorporation of oxygen into the molecular
structures as the material is broken down from the larger plant material into humin, humic acid and fulvic
acid. As a result, on a relative basis, the amount of carbon within these structures decreases while oxygen
increases. At the same time the synthetic pathway also provides for the elimination of oxygen as the humic
materials are polymerised. In both cases the increasing C content and decreasing O content with size is
explained.
9.

Look up the structure of the following pesticides and show how they could interact with generic humic
material. atrazine, chlorpyrifos chlorthalonil, malathion fennitrothion, chlordane, alidcarb, and
trifluralin

No solution provided. This is a good exercise for getting students to use various resources to look up these
pesticides and specifically look at their functionality. I usually also have students gather additional
information such as physical constants, especially those relating to transport/mobility and reactivity. These
additional properties also relate to principles covered in Chapter 14 (partitioning) and Chapter 20

(biocides).

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