Page 127 - Industrial Wastewater Treatment, Recycling and Reuse

Advanced Physico-chemical Methods of Treatment for Industrial Wastewaters 101

The sorption isotherms give important information about the process.
Typically, the Langmuir isotherm belongs to a Type I isotherm that indicates
a uniform homogeneous surface with monolayer adsorption without inter-
action between the species getting adsorbed. Type II and Type IV are
typically multilayer adsorptions that are most common in effluent treatment,
while Type III and Type V indicate strong attraction/bonding between the
molecules.

2.3.2.2 Adsorption Kinetics
While equilibrium data is useful in knowing maximum capacity and thereby
aiding preliminary selection of the material, for practical application and for
final selection of the material, it is essential to know the rate of the adsorp-
tion, that is, change of concentration with respect to time. This requires
accounting for various mass transfer steps and evaluating mass transfer coef-
ficients for the same. For adsorption kinetics, understanding of the following
three contributing steps is required.
A. Film transport to surface of particle
B. Pore diffusion
C. Adsorption on the surface.
For desorption, the above steps occur in reverse order. Usually, pore diffu-
sion is a controlling step in most adsorption processes. For commercial
adsorbents that have a large surface area, the maximum contribution is of
micropores (<2 nm), consequently high capacity in micropores. However,
this results in poor kinetics. The kinetics can be improved by mesoporosity
(2–50 nm), where adsorbents typically have moderate capacity with specific
kinetics (pore size specificity), or by macroporosity in materials (>50 nm)
that have less capacity/surface area but better kinetics. A detailed discussion
on surface removal of pollutant species in porous materials can be found in
the section on the ion exchange process.

2.3.3 Recent Advances in Adsorption Processes

The most predominant and extensively researched adsorption process
scheme includes material/surface modification.
A. New materials/material modifications
– New adsorbents from biomass, polymers, nanomaterials
– Immobilization of enzymes
– Chemical Vapor Deposition (CVD)/nano-deposition
– Ion exchanged/impregnated zeolites, carbons, and others