Development and application of imprinted polymers for selective adsorption of metal Ions and flavonols in complex Samples
Year: 2012
Type: dissertation
Abstract: Presence of heavy metals in the environment is a worldwide known contamination \nproblem. Depending on their chemistries and level of contamination, these heavy \nmetals can have severe effects on the ecosystem, aquatic life and eventually \nhumans. Researchers have been particularly interested in finding methods for the \nremoval of these pollutants from the environment. Several methods have been \nproposed and some have been used with some degree of success. Methods used \nfor trace metal removal include, chemical precipitation, chemical reduction, \nsolvent extraction, micellar ultrafiltration, organic and inorganic ion exchange, \nadsorption processes, etc. However, the matrix in which these heavy metals are \npresent in is sometimes very complex and some of these heavy metals are present \nin the environment at very low concentrations, say ppb levels. However, they can \nhave adverse effects even at such low-level concentrations. The above-mentioned \nmethods usually suffer from the effects of the matrix and by-products produced \nafter treatment such as sludge in the case of precipitation. Hence, in this study \nmolecularly imprinted polymers (MIPs) were used. MIPs are highly cross-linked \npolymers prepared with the presence of template molecule. Once the template has \nbeen removed it leaves behind a cavity that can only fit the template, hence MIPs \nare very selective for the template molecule. Metals of interest in this study were \nuranium (VI) and chromium (VI). Therefore, two separate imprinted polymers \nwere prepared using chromium and uranium as template molecules for selective \nextraction of these oxy-ions from aqueous samples. Beside removal of heavy \nmetals, the study also focussed on developing MIPs for selective recovery of high \nvalue compounds from plant materials (onion and Moringa oleifera). \nThree separate imprinted polymers using chromium, uranium or quercetin \ntemplates were prepared by bulk polymerization method. Functional monomers \nused were 4-vinylpyridine; 1-(prop-2-en-1-yl)-4-(pyridin-2-ylmethyl)piperazine \n(PPMP) and methacrylic acid; and 4-vinylpyridine for chromium, uranium and \nquercetin imprinted polymers, respectively. For all imprinted polymers, ethylene \nglycol dimethacrylate (EDMA) and 1,1‘-azobis(cyclohexanecarbonitrile) (ACCN) were used as the cross-linking monomer and initiator, respectively. Control \npolymers (CP) or non-imprinted polymers (NIP) for each imprinted polymer were \nprepared and treated exactly the same as imprinted polymers but with omission of \nrespective templates. Following removal of respective templates with appropriate \nsolutions, various parameters that affect selective adsorption such as solution pH, \ninitial concentration, aqueous phase volume, sorbent dosage, contact time, \nbreakthrough volumes etc., were optimized to get optimal adsorption of the \nimprinted polymers. \nOptimal parameters for Cr (VI) adsorption were as follows: solution pH, 3; \ncontact time, 120 min; eluent, 20 mL of 0.1 M NaOH; and sorbent amount, 125 \nmg. Maximum retention capacity of IIP and CP was 37.58 and 25.44 mg g-1, \nrespectively. The observed selectivity order was as follows, Cr (VI) > SO4 \n2- > F- > \nPO4 \n3- > NO2 \n- > NO3 \n- > Cl-. However, in the presence of high concentrations of \nsulphate ions, the selectivity on the CP completely collapsed. For uranium VI \nremoval, the optimal pH was 4.0-8.0, sorbent amount was 20 mg, contact time \nwas 20 min and the retention capacity was 120 mg of uranyl ion per g of IIP. The \nselectivity order observed was as follows, UO2 \n2+ > Fe3+ >> Cu2+ > Co2+ > Mn2+ > \nZn2+ ~ Ni2+. \nThe binding capacity of quercetin MIPs was investigated at 25 and 84°C, \nrespectively, in batch mode. The slopes for the effect of extraction time revealed \nthat the mass transfer of the analytes was higher at 84°C than at 25°C. Also, the \nbinding capacity for the most promising MIP and its corresponding NIP increased \nat 84°C but the MIP had higher binding capacity. The increase in binding capacity \nfor the MIP was from ~30 μmol g-1 at 25°C to ~120 μmol g-1 at 84°C. For the \ncorresponding NIP, the binding capacity values were ~15 and ~90 μmol g-1, at 25 \nand 84°C, respectively. A demonstration of MIP selectivity at higher temperature \nusing standard solutions of selected flavonols showed that the MIP still retained \nits selectivity for quercetin. Similar selectivity was observed when preliminary \napplication studies on aqueous yellow onion extracts were investigated. The study \nclearly demonstrated the suitability of the developed imprinted polymers (for chromium, uranium and quercetin) for selective adsorption of Cr (VI), UO2 \n2+ and \nquercetin from their respective complex matrices. \nBreakthrough volume of molecular imprinted polymer solid-phase extraction \n(MISPE) was investigated using a mixture of myricetin, quercetin and \nkaempferol. The breakthrough volumes for quercetin, kaempferol and myricetin \nwere 22, 27 and 8 mL, respectively. The number of theoretical plates (N) for the \nMISPE column corresponding to these volumes were 18, 47 and 4 for quercetin, \nkaempferol and myricetin, respectively. Using these results, selectivity of MIP \nand its retention capacity was evaluated. The extractions of Moringa leaves and \nflowers were carried out using a MISPE cartridge and various solvents were \ninvestigated for the selective elution of quercetin from the MIP sorbents. For \nidentification and quantification of quercetin and other flavonols, a high \nperformance liquid chromatography (HPLC) was used. Recoveries of quercetin \nfrom different Moringa extracts ranged from 87 – 92% and this demonstrated that \nthe MISPE method can be used for the recovery of quercetin and kaempferol from \nthe Moringa extracts. Amount of quercetin found in Moringa leaves was 1555 mg \nkg-1. \nAll the imprinted and non-imprinted polymers prepared in the study were \ncharacterized with Fourier Transform Infrared (FTIR) spectroscopy. Scanning \nelectron microscopy (SEM) was used for recording surface morphology of all the \npolymers. Surface area and pore size analysis were recorded on Micromeritic \nTristar BET. For quercetin MIP, thermogravimetric analysis (TGA) was also used \nin addition to the mentioned techniques. \nIn additional studies, the concentrations of metals in the soil and, in the leaves and \nflowers of Moringa plant grown in South Africa were examined. The \ninvestigation included heavy metals, major and trace nutrient elements. The \nanalysis of metals was achieved after total digestion of soils or leaves using a \nmicrowave, and the concentrations of metals were determined using inductively \ncoupled plasma-optical emission spectroscopy (ICP-OES). These results were compared to those obtained from some selected vegetables like spinach, cabbage, \ncauliflower, broccoli, and peas. No toxic heavy metals were detected in the leaves \nand flowers of Moringa. On average Moringa contained higher concentration of \nCa (18500 mg kg-1) and Mg (5500 mg kg-1) than other vegetables compared with \nin the study. Other major nutrients contained in Moringa were much similar to \nother vegetables. Besides metals, the concentrations of flavonols (myricetin, \nquercetin, kaempferol) determined from Moringa leaves and flowers were also \ncompared to selected vegetables. Plant and vegetable materials were extracted \nunder reflux using acidified methanol (1% HCl) solution. Following which, the \nflavonols were identified and quantified using reverse phased-high performance \nliquid chromatography method equipped with UV detection. Moringa leaves \nexhibited highest concentrations of myricetin (1296.6 mg kg-1), quercetin (1362.6 \nmg kg-1), kaempferol (1933.7 mg kg-1) than vegetables (spinach: myricetin 620.0 \nmg kg-1, quercetin 17.9 mg kg-1, kaempferol 215.3 mg kg-1). \nLastly, the antioxidant activity of Moringa flowers and leaves were compared to \nthat of the aforementioned selected vegetables. The antioxidant activity was \nstudies by analyzing the total phenolic content (TPC), total flavonoid content \n(TFC), reducing power, radical scavenging activity, and the 2,2-diphenyl-1- \npicrylhydrazyl free radical (DPPH) method. Moring contained almost twice the \nTPC and thrice the TFC than the vegetables. Also, Moringa demonstrated higher \nreducing power and lower percentage of free radicals remaining (DPPH method). \nHence, Moringa showed to be a good antioxidant source than the selected \nvegetables compared with.
Author: Vusumzi Emmanuel Pakade
Language: English
Subfield: Analytical Chemistry
Field: Chemistry
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2274613690