3.7. Stability and reclamation
Stability in different conditions is important for the feasible
application of an adsorbent. Furthermore, leaching of adsorbent
components to water environment may induce secondary contamination.
The iron of bare Fe3O4 core is easier to leach into
the aqueous solution than those containing a shield coating [44].
For SH-mSi@Fe3O4, a thin layer of coated silica helped Fe3O4 to
resist harsh conditions. No iron ion leached after the adsorbent
was dispersed in natural water samples for 48 h. The percentage
of leached iron in 0.1 M HCl and NaOH aqueous solutions were
0.422 and 0.425%, respectively. There were just 12.47 and 8.88%
of iron leached in extreme strong acid (1 M HCl) and alkaline (1 M
NaOH) conditions after 48 h (Table S1 in supplementary information).
Therefore, it can be expected that this material is stable in
natural water matrices and wastewater even under extreme conditions.
Fig. 8 shows that the saturation magnetizations of SHmSi@
Fe3O4 and Fe3O4 were 38.4 and 75.5 emu/g, respectively. The
absence of remanence confirm the superparamagnetism of SHmSi@
Fe3O4, and the magnetization property enables the adsorbent
to be easily separated from aqueous solution under a magnetic field
in less than 30 s. As SH-mSi@Fe3O4 is resistant to relatively strong
acid solutions, SH-mSi@Fe3O4 loaded with Hg2+ which shows
strong affinity to thiol group was treated in 1 M HCl under sonication
for 30 min. Results showed that 91% of the adsorbed Hg could be released. Then, the regenerated adsorbent was treated with
deionized water to neutralize for adsorption of Hg2+ in succeeding
cycles. We have repeated the above procedure for six cycles.
As shown in Fig. 9, the removal efficiency is slightly reduced in the
subsequent cycles. However, it is still above 90% in the sixth cycle.