Presence of heavy metals in the environment is a worldwide known contamination
problem. Depending on their chemistries and level of contamination, these heavy
metals can have severe effects on the ecosystem, aquatic life and eventually
humans. Researchers have been particularly interested in finding methods for the
removal of these pollutants from the environment. Several methods have been
proposed and some have been used with some degree of success. Methods used
for trace metal removal include, chemical precipitation, chemical reduction,
solvent extraction, micellar ultrafiltration, organic and inorganic ion exchange,
adsorption processes, etc. However, the matrix in which these heavy metals are
present in is sometimes very complex and some of these heavy metals are present
in the environment at very low concentrations, say ppb levels. However, they can
have adverse effects even at such low-level concentrations. The above-mentioned
methods usually suffer from the effects of the matrix and by-products produced
after treatment such as sludge in the case of precipitation. Hence, in this study
molecularly imprinted polymers (MIPs) were used. MIPs are highly cross-linked
polymers prepared with the presence of template molecule. Once the template has
been removed it leaves behind a cavity that can only fit the template, hence MIPs
are very selective for the template molecule. Metals of interest in this study were
uranium (VI) and chromium (VI). Therefore, two separate imprinted polymers
were prepared using chromium and uranium as template molecules for selective
extraction of these oxy-ions from aqueous samples. Beside removal of heavy
metals, the study also focussed on developing MIPs for selective recovery of high
value compounds from plant materials (onion and Moringa oleifera).
Three separate imprinted polymers using chromium, uranium or quercetin
templates were prepared by bulk polymerization method. Functional monomers
used were 4-vinylpyridine; 1-(prop-2-en-1-yl)-4-(pyridin-2-ylmethyl)piperazine
(PPMP) and methacrylic acid; and 4-vinylpyridine for chromium, uranium and
quercetin imprinted polymers, respectively. For all imprinted polymers, ethylene
glycol dimethacrylate (EDMA) and 1,1‘-azobis(cyclohexanecarbonitrile) (ACCN) were used as the cross-linking monomer and initiator, respectively. Control
polymers (CP) or non-imprinted polymers (NIP) for each imprinted polymer were
prepared and treated exactly the same as imprinted polymers but with omission of
respective templates. Following removal of respective templates with appropriate
solutions, various parameters that affect selective adsorption such as solution pH,
initial concentration, aqueous phase volume, sorbent dosage, contact time,
breakthrough volumes etc., were optimized to get optimal adsorption of the
imprinted polymers.
Optimal parameters for Cr (VI) adsorption were as follows: solution pH, 3;
contact time, 120 min; eluent, 20 mL of 0.1 M NaOH; and sorbent amount, 125
mg. Maximum retention capacity of IIP and CP was 37.58 and 25.44 mg g-1,
respectively. The observed selectivity order was as follows, Cr (VI) > SO4
2- > F- >
PO4
3- > NO2
- > NO3
- > Cl-. However, in the presence of high concentrations of
sulphate ions, the selectivity on the CP completely collapsed. For uranium VI
removal, the optimal pH was 4.0-8.0, sorbent amount was 20 mg, contact time
was 20 min and the retention capacity was 120 mg of uranyl ion per g of IIP. The
selectivity order observed was as follows, UO2
2+ > Fe3+ >> Cu2+ > Co2+ > Mn2+ >
Zn2+ ~ Ni2+.
The binding capacity of quercetin MIPs was investigated at 25 and 84°C,
respectively, in batch mode. The slopes for the effect of extraction time revealed
that the mass transfer of the analytes was higher at 84°C than at 25°C. Also, the
binding capacity for the most promising MIP and its corresponding NIP increased
at 84°C but the MIP had higher binding capacity. The increase in binding capacity
for the MIP was from ~30 μmol g-1 at 25°C to ~120 μmol g-1 at 84°C. For the
corresponding NIP, the binding capacity values were ~15 and ~90 μmol g-1, at 25
and 84°C, respectively. A demonstration of MIP selectivity at higher temperature
using standard solutions of selected flavonols showed that the MIP still retained
its selectivity for quercetin. Similar selectivity was observed when preliminary
application studies on aqueous yellow onion extracts were investigated. The study
clearly demonstrated the suitability of the developed imprinted polymers (for chromium, uranium and quercetin) for selective adsorption of Cr (VI), UO2
2+ and
quercetin from their respective complex matrices.
Breakthrough volume of molecular imprinted polymer solid-phase extraction
(MISPE) was investigated using a mixture of myricetin, quercetin and
kaempferol. The breakthrough volumes for quercetin, kaempferol and myricetin
were 22, 27 and 8 mL, respectively. The number of theoretical plates (N) for the
MISPE column corresponding to these volumes were 18, 47 and 4 for quercetin,
kaempferol and myricetin, respectively. Using these results, selectivity of MIP
and its retention capacity was evaluated. The extractions of Moringa leaves and
flowers were carried out using a MISPE cartridge and various solvents were
investigated for the selective elution of quercetin from the MIP sorbents. For
identification and quantification of quercetin and other flavonols, a high
performance liquid chromatography (HPLC) was used. Recoveries of quercetin
from different Moringa extracts ranged from 87 – 92% and this demonstrated that
the MISPE method can be used for the recovery of quercetin and kaempferol from
the Moringa extracts. Amount of quercetin found in Moringa leaves was 1555 mg
kg-1.
All the imprinted and non-imprinted polymers prepared in the study were
characterized with Fourier Transform Infrared (FTIR) spectroscopy. Scanning
electron microscopy (SEM) was used for recording surface morphology of all the
polymers. Surface area and pore size analysis were recorded on Micromeritic
Tristar BET. For quercetin MIP, thermogravimetric analysis (TGA) was also used
in addition to the mentioned techniques.
In additional studies, the concentrations of metals in the soil and, in the leaves and
flowers of Moringa plant grown in South Africa were examined. The
investigation included heavy metals, major and trace nutrient elements. The
analysis of metals was achieved after total digestion of soils or leaves using a
microwave, and the concentrations of metals were determined using inductively
coupled plasma-optical emission spectroscopy (ICP-OES). These results were compared to those obtained from some selected vegetables like spinach, cabbage,
cauliflower, broccoli, and peas. No toxic heavy metals were detected in the leaves
and flowers of Moringa. On average Moringa contained higher concentration of
Ca (18500 mg kg-1) and Mg (5500 mg kg-1) than other vegetables compared with
in the study. Other major nutrients contained in Moringa were much similar to
other vegetables. Besides metals, the concentrations of flavonols (myricetin,
quercetin, kaempferol) determined from Moringa leaves and flowers were also
compared to selected vegetables. Plant and vegetable materials were extracted
under reflux using acidified methanol (1% HCl) solution. Following which, the
flavonols were identified and quantified using reverse phased-high performance
liquid chromatography method equipped with UV detection. Moringa leaves
exhibited highest concentrations of myricetin (1296.6 mg kg-1), quercetin (1362.6
mg kg-1), kaempferol (1933.7 mg kg-1) than vegetables (spinach: myricetin 620.0
mg kg-1, quercetin 17.9 mg kg-1, kaempferol 215.3 mg kg-1).
Lastly, the antioxidant activity of Moringa flowers and leaves were compared to
that of the aforementioned selected vegetables. The antioxidant activity was
studies by analyzing the total phenolic content (TPC), total flavonoid content
(TFC), reducing power, radical scavenging activity, and the 2,2-diphenyl-1-
picrylhydrazyl free radical (DPPH) method. Moring contained almost twice the
TPC and thrice the TFC than the vegetables. Also, Moringa demonstrated higher
reducing power and lower percentage of free radicals remaining (DPPH method).
Hence, Moringa showed to be a good antioxidant source than the selected
vegetables compared with.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/11954 |
| Date | 18 September 2012 |
| Creators | Pakade, Vusumzi Emmanuel |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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