Applications of Equilibrium Thermodynamics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 129

In water, without added salts, K aw = C i a /C i w .C i a is not affected by ionic strength in
water, but C i is modified using the McDevit-Long theory (Section 3.2.4). Let C ww

i

represent the new wastewater concentration. Then new K aw = C i a /C ww . By McDevit-

i

i 41 /x i 41 /C i w , we can write C i w =
C w i exp( φ V H C s ) . Therefore K aw = K aw exp( φ V H C s ) . From Table 3.3, for phenan-
threne, φ V H = ( 0.00213 )( 182 ) = 0.38. From Appendix 1, K aw = 3.16 kPa dm 3 /mol.
Hence K aw = 3.16 exp ( 0.38 × 1 ) = 4.62.

ww
i

i exp ( φ V H C s ) . Since γ

Long theory, γ = γ

Colloids and particulates are ubiquitous in both natural waters and wastewaters.
They are characterized by their size, which can range from a few nanometers to
thousands of nanometers. Examples of colloidal material encountered in natural and
wastewaters are given inTable 4.4. Organic colloids are characterized by sizes 10 3 nm
or less. They are dispersed phases composed of high molecular weight ( > 1000)
macromolecules of plant origin. They are composed of C, H, and O with traces of
N and S. They possess ionizable groups (OH or COOH) and are macro-ions. These
are classified as dissolved organic compounds (DOCs) and range in concentrations
from a few mg/L in oceans to as large as 200 mg/L in peaty catchments or swamps,
and are known to give a distinct brownish tinge to water. DOCs are also observed
at concentrations ranging from 10 to 200 mg/L in atmospheric water (fogwater and
rainwater). DOCs are known to complex with inorganic metal ions and bind organic
pollutants. The concentration of metals and organic compounds bound to a single
macromolecule can be large (Wijayaratne and Means, 1984). The effective solubil-
ities of both inorganic and organic species can, therefore, be several times larger in
the presence of DOCs.
Hydrophobic pollutants will preferentially associate with DOCs since they pro-
vide an organic medium shielding the pollutants from interactions with water. It
has been observed that the association of organic compounds is correlated to their
hydrophobicity ( K ow ) . The more hydrophobic the compound, the greater its sorp-
tion to DOCs. Chiou et al. (1986, 1987) determined the effect of natural humic

TABLE 4.4
Sizes of Organic and Inorganic Particulates
in Natural and Wastewaters

Type of Particle Size (nm)

Organic macromolecules (humics) 1-10

Virus 10-100

Oxides (iron and aluminum) 10-1000

Clays 10-1000

10 3 -10 4 Bacteria

10 3 -10 6

Soil particles

10 4 -10 6

Calcium carbonate, silica

Note: 1nm = 10 − 3

μ m.