The pH value of the solution was an important parameter influencing
the adsorption process of metal ions on the sorbent because
of the protonation or deprotonation reaction to the sorbent and the
hydrolysis reaction to the metal ions in the alkaline condition.
Therefore, solution pH is the first parameter to be optimized on
the adsorption of lead by the SH-Fe3O4/Cu3(BTC)2 with a lead concentration
of 10 lg L1 (Fig. 4a). It was found that the signal intensity
increased with increasing solution pH and then remained
nearly constant from 3 to 7. This could be due to the reason that
lead species might be changed in the adsorption process with the
pH value of the solution varied. From the pH dependence, different
species of lead, such as Pb2+, PbOH+
, and Pb(OH)2 existed with pH
value varied. The corresponding complexation reactions among
different lead species with mercapto groups can be expressed as
follows:
—SH þ Pb2þ ! —S—Pbþ þ Hþ