Environmental Engineering Reference
In-Depth Information
6.5.2 Silanized Monolayers on Two-Step Silica-Coated g -Fe
2
O
3
6.5.2.1 Cu
2+
and Zn
2+
Removal
The silanized magnetic particles with reactive amine groups are found to confer
specific adsorption for heavy metal ions, including divalent mercury, lead,
copper, zinc, manganese, and monovalent silver in contaminated aqueous and
non-aqueous solutions [130].
It was shown (Section 6.4.2) that APTES films directly silanized onto g-Fe
2
O
3
were unstable in alkaline solutions, although the films silanized from toluene
were relatively stable in acidic solutions. An idea to coat g-Fe
2
O
3
with a thin
silica layer before silanation was proposed in order to obtain a stable silanized
film. In Section 6.4.3, it was shown that dense thin silica films were successfully
coated onto g-Fe
2
O
3
using the sol-gel process followed by DLS coating. The
purpose of silica coating is to make the surface more amenable for silanation. It
has been well-documented that silanized films on silica are relatively stable
compared to those directly silanized on other metal oxides [131]. The silanation
of silica-coated magnetic particles using APTES in toluene is performed. Toluene is
used as a solvent simply due to the fact that the films formed on the magnetic
particles are relatively stable compared to those formed from water as shown in
Section 6.4.2. The silanized films on silica-coated magnetic particles were charac-
terized by XPS, DRIFTS, zeta-potential measurements, leaching test, and thermal
gravimetric analysis (TGA) as well-documented in literature [48]. It is found that
the silanized films on silica-coated magnetic particles were more stable in acid
solution than on bare magnetic particles, but both unstable in alkaline solutions.
Copper loading on the silanized silica-coated magnetic particles of reactive
amine groups is shown in Fig. 6.15. For comparison, copper loading on silica-
coated magnetic particles is also shown in this figure. It is evident that at a given
pH 5.3, copper can be removed effectively from low concentration solutions (e.g.,
100% removal from a 12 ppm solution). With increasing initial copper ion
concentrations, the copper removal efficiency decreases as copper loading
approaches the capacity limit of about 0.18 mmol of Cu per gram of particles.
In contrast, the copper loading on the silica-coated magnetic particles is signifi-
cantly lower than that on the silanized particles, suggesting the important role of
reactive amine groups in this application. It should be noted that the metal ion
loading is pH-dependent. The protonation of amino groups and detachment of
silane-coupling agents are both influenced by pH, thus they will affect metal ion
adsorption. In addition, various metal ions such as copper and zinc showed
different loading performance, leading to selective adsorption and removal/
recovery of various metals. This is demonstrated in our previous publication [48].
6.5.2.2 Stripping of Metal Ions and Recycling of Magnetic Sorbents
Stripping of metal ions from loaded magnetic particles is a major step for the
subsequent recovery of metals by electrowinning, while the recycling of magnetic
particlesisanecessarysteptooffsetthehigh price of magnetic sorbents by lowering
