microporous adsorbents are used. Microporous adsorbents have more surface area
than that of mesoporous and macroporous adsorbents.
2.1.2 Iodine Number
The ability of the biosorbent to take up smaller molecules is denoted as iodine
number. It is defined as the mg of iodine adsorbed per g of adsorbent and is
expressed in mg/g of adsorbent. Generally the iodine number varies in the range
of 500–1,200 mg/g for activated carbon adsorbents [1].
2.1.3 Point of Zero Charge
Another important characteristic of biosorbents is the point of zero charge (pH PZC ).
It is defined as the pH at which the adsorbent surface has no charge, i.e. net electrical
neutrality. When pH is greater than pH PZC , adsorbent surface has a net negative
charge and favours the uptake of cations, and when pH is lesser than pH PZC , the
surface will have a net positive charge and have higher affinity towards anionic
particles.
The point of zero charge can be determined by the series of adsorption experiments with 0.01 mol/L of NaCl conducted at different pH ranges from 2 to 12. The
zeta potential was measured and the graph of zeta potential versus initial pH is used
to determine the pH PZC (Fig. 1).
Fig. 1 Calculation of point of zero charge (pHPZC)
Material and Process Selection for Biosorption
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