Page 123 - Industrial Wastewater Treatment, Recycling and Reuse

Advanced Physico-chemical Methods of Treatment for Industrial Wastewaters 97

and regeneration in a cyclic manner. Adsorption is considered today as one
of the most important and crucial operations in secondary and tertiary waste-
water treatment options, especially for the removal of refractory pollutants.

2.3.1 Adsorbent: Screening and Selection
The selection of a suitable adsorbent is the most important step in devising
adsorption separation for effluent treatment, and it is largely dictated by the
nature of pollutants such as acids, organics, and metals. A number of com-
mercial adsorbents, both hydrophilic and hydrophobic types, are available.
These include activated carbon (powder, granules, activated carbon fiber),
zeolites (A, X, Y, ZSM-5, silicalite, ALPO), inorganic oxides (silica,
alumina), and polymers. Adsorbent surface area typically ranges from 100
2
to 1500 m /g. In conventional separation applications, usually adsorbent
with high surface is considered better. However, for effluent treatment
applications, it is the affinity of the surface to the pollutant species that is most
important; thus, the capacity to remove specific pollutants is the criterion for
selection. Adsorbents come in different shapes, such as granules, spheres,
cylindrical pellets, flakes, and/or powders, with size variations from
50 mm to 1.2 cm, porosity 30–85%v, and average pore diameters in the
˚
range 10–200 A. For selection of an adsorbent, adsorption capacity (e.g.,
gram of adsorbate adsorbed per gram of adsorbent at equilibrium), and
adsorbent life (number of adsorption-desorption cycles for which the adsor-
bent can be used without significant loss in its capacity) are important prac-
tical considerations. The choice of a suitable adsorbent is highly case specific,
and careful identification/definition of the problem is essential for devising
the proper solution. Ideally, the adsorbent is required to have the following
properties, which form the basis of selection:
1. High selectivity for pollutants to be removed.
2. High capacity so that quantity of adsorbent required is less.
3. Favorable kinetics for rapid sorption, requiring suitable structural
characteristics.
4. Chemical and thermal stability, low solubility to minimize operational
losses.
5. Hardness and mechanical strength to prevent crushing and erosion to
lower physical losses.
6. Ease of handling.
7. High resistance to fouling for long life, especially important in effluent
treatment.