3.3. Adsorption isotherm
Adsorption isotherm is important for determining the adsorption
behavior of an adsorbent. Therefore, Hg2+ and Pb2+ were
selected as models to determine the adsorption isotherm type
of SH-mSi@Fe3O4. Fig. 5 shows the adsorption isotherm at pH
6.5
±
0.05 for the adsorbent. The isotherm curves demonstrate
the adsorption as a function of the equilibrium concentration of
metal ions in solution. Both Langmuir and Freundlich adsorption
isotherms were used to normalize the adsorption data. The results
are summarized in Table 2. The results showed that Langmuir
model fitted better than the Freundlich model, demonstrating that
the adsorption of Hg2+ and Pb2+ onto SH-mSi@Fe3O4 can be considered
to be a monolayer adsorption process. The adsorption capacity
of Hg2+ and Pb2+ were calculated to be 1.3 and 0.44 mmol/g,
respectively. Taking adsorption of Hg2+ for example, the adsorption
capacity of SH-mSi@Fe3O4 is not only higher than some thiol
functionalized ordered mesoporous silica prepared through the cocondensing
method (0.16 and 0.12 mmol/g of Hg2+ for MP-SBA-15
and MP-MCM-41) [36], but also higher than the reported thiol func-tionalized nanoporous silica (not more than 0.6 mmol/g) [21] and
some other adsorbents [37,38].