Page 132 - Industrial Wastewater Treatment, Recycling and Reuse

106 Industrial Wastewater Treatment, Recycling, and Reuse

the proton of the acid subsequently attaching the anion to it without any
exchange of ions as shown below:
R 1 R 1

P N : + HCI P NH + Cl –
R 2 R 2
In this respect, it can be said that weak base resin is not a true ion
exchange resin and differs significantly in its form and in action. The differ-
ence mainly arises from the fact that it contains nitrogen with lone pair of
electrons (free ionogenic base group) that can accept a hydrogen ion to
get ionized. The electrostatic attraction forces then allow the binding of
anions from the solution. This process is termed sorption, and the mechanism
of the same in the case of acid removal is well discussed in the literature for
various acids such as strong inorganic acids, weak organic/carboxylic acids,
and polybasic acids (Bhandari et al., 1992a,b, 1993, 1997, 2000). As com-
pared to strong base ion exchange resins, the weak base resins have larger
sorption capacity and are easy to regenerate. This aspect makes them highly
useful in wastewater treatment, especially for the removal of acids from the
wastewater streams.
The theory and practice of ion exchange process is well known and quite
well developed. As mentioned in Section 2.3.1, knowing the kinetics of the
process is essential for final selection of the resin. For example, when capacity
may be high, but rates of exchange are very low, the resin may not be suit-
able for the application, and a comparatively low capacity resin with better
rates of exchange will have to be chosen. Ion exchange kinetics bears no
resemblance to chemical reaction kinetics in the usual sense. For both
adsorption and ion exchange, it is essential to know:
– The mechanism of the process.
– The rate-determining step.
– What rate laws are obeyed.
– How the rate can be predicted.
Similarly to adsorption, ion exchange is also often a diffusion-controlled
process. For simple undissociated species, Fick’s law of diffusion can be con-
veniently used, whereas for ionic species, usually the Nernst-Planck equa-
tion or other equations that allow accounting for ionic interactions such as
Stefan-Maxwell equation are more appropriate. Ion exchange reactions are
instantaneous. If the different rate steps involved in adsorption and ion
exchange are to be compared using order-of-magnitude analysis, the diffu-
sion step is the slowest step in the overall mechanism (Figure 2.9). Thus, in