Abstract
The problem of maintaining excellent water quality for residential use and aquatic life continues
to be a challenge. Pollutants from natural sources, such as volcanic eruptions, and human activities,
such as industrial processes, mining, agricultural activities, and a variety of other activities,
frequently contaminate water supplies. Among these pollutants, heavy metals constitute the most
serious environmental challenge owing to their non-biodegradability. Most of these metals have
been linked to some toxic symptoms in humans such as hypoglycemia, interrupted sleep, arthritis,
chronic fatigue, headaches or migraines, dry skin, low blood pressure, muscle cramps, premenstrual
syndrome (PMS), etc. In recent times, adsorption techniques remain the commonly used
method for the removal of these toxic metals from contaminated water. However, various
methodologies have been examined to create a more proficient adsorbent. Still, there is a
considerable need for the development of a green, low-cost, adsorptive material whose small
amount can efficiently remove an exceedingly high amount of toxic metals. This has geared a lot
of enthusiasm in adsorption studies.
In this work, a new, low-cost, biopolymer adsorbent with an increased number of adsorption sites
was developed. This new biopolymer adsorbent with excellent adsorptive properties is another
chitosan derivative, which was synthesized by crosslinking chitosan (CCS) with pyridine-2,6-
dicarboxylic acid (PDC) through ion exchange to introduce more adsorption sites (O^N^O). These
adsorption sites are in the form of a long polymeric chain. The functionalized biopolymer has been
characterized using different instrumental analyses, including elemental (CHN), spectroscopic
(UV-visible, NMR, powder XRD, and FTIR), thermal analyses (TGA and DSC), surface and
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morphological (BET and SEM) analyses. The PDC-CCS was utilized for the recovery of Cu2+ and
other toxic metals (namely: Cd2+, Cr3+, Mn2+, Pb2+, and Ni2+) from contaminated water. The
adsorption limit/capacity of PDC-CCS has been examined for solution pH, temperature, initial
metal ion concentration, and the contact time of the adsorbent. An extreme adsorption limit of
2186, 1258.79, 1118.70 928.52, 829.62, and 580.21 mmol/g has been found for Cu(II), Ni(II),
Cd(II), Pb(II), Mn(II), and Cr(III) respectively. Also, it was discovered that the adsorption
limit/capacity exceedingly relies upon temperature and pH. On testing the experimental data with
the two most popular adsorption models (fundamentally, Freundlich and Langmuir), we found that
Cu(II) ions adsorption, as well as the adsorption of other metal ions, suit both models. Similarly,
the experimental adsorption kinetics is, in reality, second-order. At the RI-PB/def2-SVP level of
theory, the Density Functional Theory (DFT) approach has been used to evaluate the adsorption
energy for the metal ions. During the adsorption study of the Cu(II) ions, DFT calculations suggest
that the main adsorption mechanism is by chelation through charge transfer from the adsorbent to
the metal ions in the solution. A selectivity study performed at varying pH revealed that PDC-CCS
could be utilized for simultaneous removal of the metals at pH 4.2; selective adsorption of Mn(II)
at pH 5.56 as well as simultaneous-selective removal of Ni(II) and Mn(II) at near-neutral pH.
When compared with some relevant previously used adsorbents, PDC-CCS shows an exceptional
adsorption capacity; consequently, a successful biopolymer adsorbent for the treatment of water
contaminated by hazardous metals.
In an attempt to find another useful application of chitosan and the crosslinked chitosan, we explore
the biopolymer as a metal complexation agent for the polymerization of vinyl acetate. As one of
the vinyl polymers with numerous applications, it is desirable to research the catalysis of vinyl
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acetate polymerization from the perspective of cost viability and green chemistry. Chitosan and
transition metal salts were used as precursors to first synthesize the chitosan-metal coordination
biopolymers of Mn(II), Fe(III), Co(II), and Ni(II) which were characterized using different
instrumental techniques such as spectroscopic (UV-visible, FT-IR, XRD, EDS, and ICP-OES),
thermal analysis (TGA and DTA), surface analysis (SEM), and hydrogen-temperature
programmed reduction (H2-TPR) analysis. Spectroscopic studies confirmed the successful
incorporation of the metals into the biopolymer matrix. Thermal analysis and H2-TPR revealed the
reducibility of the Chit-Fe(III) at 120 ℃. While Chit-Fe(III) and Chit-Ni(II) were inactive, Chit-
Co(II) and Chit-Mn(II) were found to be active towards vinyl acetate polymerization in the
presence of aqueous Na2SO3. Furthermore, the polyvinyl acetate (PVAc) produced from Chit-
Co(II) compared perfectly with a commercial PVAc and was in higher yield than PVAc produced
from Chit-Mn(II). The polymerization has been shown to proceed via surface-initiated atom
transfer radical polymerization (SI-ATRP), and the viscosity average molecular weight of PVAc
produced has been measured as 25,078. The density functional theory approach has been used to
ascertain the coordination orientation of the Chit-Co(II) and explain its high efficiency towards
vinyl acetate polymerization. The catalyst reusability test revealed an insignificant loss of activity
for the Chit-Co(II) after seven cycles of polymerization. Kinetics and thermodynamic studies show
that vinyl acetate polymerization is an endothermic process and proceed at ambient temperature.
Upon crosslinking chitosan with pyridine-2,6-dicarbaldehyde before complexation with Co(II) ion
to form the crosslinked chitosan-Co(II) coordination biopolymer and subsequent use as a catalyst,
the induction period of vinyl acetate polymerization was significantly lowered. This effect of the
crosslinker has been attributed to the lowering of the HOMO-LUMO energy gap. The result of
this work also suggests an investigation of chitosan-metal and crosslinked chitosan-metal
coordination biopolymer via a low-ppm ATRP approach for possible biomedical application.