Synthesis Of Thermo-Magneto-Responsive Poly(N-Isopropylacrylamide)-Based Composite Hydrogels For Adsorption-Desorption Of Chromium (III) Ions
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Date
2019-07-01
Authors
Chen, Jian Jie
Journal Title
Journal ISSN
Volume Title
Publisher
Universiti Sains Malaysia
Abstract
Stimuli-responsive composite hydrogels have been in the vanguard of researches for
their application in metal ion adsorption and its release via conformational change.
The preparation of composite hydrogels with both thermo- and magneto-responsiveness requires careful layer-by-layer coatings of functional shells onto the
core of iron oxide magnetic nanoparticles (MNPs). However, multiple stages of shell
encapsulation of MNPs remains a major setback on the production of composite
hydrogels with adequate colloidal stability and well-functioned dual-responsiveness.
In this study, homo-polymeric poly(N-isopropylacrylamide)-encapsulated magnetite
nanoparticles (PNIPAM-MNPs) cross-linked composite hydrogels were facilely
synthesized via layer-by-layer coatings with and without employing silanization
precursor, 3-(trimethoxysilyl)propyl methacrylate (MPS). It was found that PNIPAM
could be gelated directly onto the silica-coated, poly(vinylpyrrolidone) (PVP)
functionalized MNPs(silica-PVP-MNPs) via free radical polymerization without MPS
to improve its colloidal stability and both thermo-magneto-responsive. Besides, co-polymeric poly(N-isopropylacrylamide-co-acrylic acid)-encapsulated MNPs ((PNIPAM-co-AA)-silica-PVP-MNPs) composite hydrogels were prepared for elucidating the difference in adsorption mechanisms between chelating groups of
carboyxlates (-COO-) contained by AA moiety and amides (-CONH) of NIPAM moiety. In the temperature manipulated adsorption-desorption tests, desorption of Cr3+gradually predominated as temperature increased from 298 K to 323 K for PNIPAM-silica-PVP-MNPs. Re-adsorption of Cr3+ by the composite hydrogel took place as being quenched to 298 K for lower initial Cr3+ concentration (20 – 80 mg L-1) which
showed that desorption can be realised for surface adsorption. Before heating, the
equilibrium adsorption data of Cr3+ fitted well into Flory-Huggins and Frumkin models,
that elucidated the chelation of Cr3+ ions occurred via replacement of water molecules
on the binding sites. Moreover, PNIPAM-silica-PVP-MNPs had higher maximum
adsorption capacity, qm (434.78 mg g-1
) compared to (PNIPAM-co-AA)-silica-PVP MNPs (qm = 243.90 mg g-1
) as extrapolated by Langmuir isotherm model in which the
data of both composite hydrogels also showed good fit to the model. The adsorption
kinetic analysis indicated that Cr3+ adsorption on PNIPAM-silica-PVP-MNPs was
governed by intra-particle diffusion and reversible surface physisorption as its data
followed pseudo-first, pseudo-second- and intra-particle diffusion models. On the
other hand, surface chemisorption predominated over (PNIPAM-co-AA)-silica-PVP-MNPs
as it followed only pseudo-second model.