Concepts from Chemical Reaction Kinetics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 228

Theseorganiccompoundsgenerallydonotenterintochemicalreactionswiththecolloid
that acts as an inert sink for organic pollutants. The concentration of a compound truly
dissolved in water will be considerably small if organic compounds adsorb to colloids
(particulates). If the total amount of an organic in water is W i and W ci is the mass
adsorbed from a solution containing ρ s mass of a sorbent per unit volume of solution,
then from the equations developed in Section 4.3.2, we have
W ci
W i =

1
1 + ρ s K sw .
There are two competing rates to consider—the organic adsorption rate to the sorbent
and its hydrolysis rate in water. In most cases, the rate at which the organic is adsorbed
to the sorbent is relatively fast compared with the rate of hydrolysis. Hence hydrolysis is
therate-limitingstep.Therateconstantforhydrolysiswillbereducedby ( 1 + ρ s K sw ) − 1
since the concentration of a truly dissolved organic that can hydrolyze will be reduced
to the same extent. Hence the modified rate constant is k ∗ = k( 1 + ρ s K sw ) − 1 , where k
is the hydrolysis rate constant in the absence of solids.
Consider the hydrolysis of DDT.At 298 K, k = 2.95 × 10 − 8 s − 1 at a pH of 8 (Wolfe
et al., 1977). If K sw for DDT on a sediment is 10 4 L/kg, then for a ρ s of 10 − 5 g/cm 3 ,
k ∗ = 2.3 × 10 − 8 s − 1 , whereas for a ρ s of 10 − 3 g/cm 3 , k ∗ = 2.3 × 10 − 9 s − 1 . The
hydrolysis of DDT in slightly alkaline solutions is dramatically reduced in the presence
of solid sorbents. These calculations are valid only if the sorbents do not participate as
heterogeneous catalysts as discussed in the next section.

5.7.3 H ETEROGENEOUS C ATALYSIS
In the atmosphere, hydrosphere, and lithosphere there are numerous reactions that
occur at the surfaces of solids or liquids. These gas-solid, gas-liquid, and solid-
liquid interface reactions are influenced by the nature and property of the surface.
For example, the hydrolysis rates of organic esters and ethers are accelerated in the
presence of sediment particles. Particle-mediated catalysis plays a large role in many
atmospheric photochemical reactions. Many waste treatment processes also rely on
reactionsatsurfaces.Someexamplesarecatalystsforairpollutioncontrol.Removalof
volatile organics from automobile exhaust involves the use of sophisticated catalysts.
Reactions such as the scrubbing of stack gases using solvents can be accelerated if
the gaseous species reacts at the gas-liquid interface. Thus, the removal of ammonia
by gas scrubbing is enhanced if the solution is slightly acidic.

5.7.4 G ENERAL M ECHANISMS OF S URFACE C ATALYSIS
Reactionsonsurfacesdifferconsiderablyfromthoseinbulkphases.Asurfacereaction
involves a series of successive steps. These are shown schematically in Figure 5.12.
The first step, namely, bulk phase diffusion, is generally fast. Hence it is unlikely to be
a rate-determining step, except for diffusion in solutions. It is difficult to differentiate
between steps 2, 3, and 4. For example, a molecule can simultaneously react with the
surface as it adsorbs. Hence the entire process of adsorption, reaction, and desorption
is usually considered a single rate-limiting step.