Development and application of imprinted polymers for selective adsorption of metal Ions and flavonols in complex Samples

摘要

Presence of heavy metals in the environment is a worldwide known contaminationudproblem. Depending on their chemistries and level of contamination, these heavyudmetals can have severe effects on the ecosystem, aquatic life and eventuallyudhumans. Researchers have been particularly interested in finding methods for theudremoval of these pollutants from the environment. Several methods have beenudproposed and some have been used with some degree of success. Methods usedudfor trace metal removal include, chemical precipitation, chemical reduction,udsolvent extraction, micellar ultrafiltration, organic and inorganic ion exchange,udadsorption processes, etc. However, the matrix in which these heavy metals areudpresent in is sometimes very complex and some of these heavy metals are presentudin the environment at very low concentrations, say ppb levels. However, they canudhave adverse effects even at such low-level concentrations. The above-mentionedudmethods usually suffer from the effects of the matrix and by-products producedudafter treatment such as sludge in the case of precipitation. Hence, in this studyudmolecularly imprinted polymers (MIPs) were used. MIPs are highly cross-linkedudpolymers prepared with the presence of template molecule. Once the template hasudbeen removed it leaves behind a cavity that can only fit the template, hence MIPsudare very selective for the template molecule. Metals of interest in this study wereuduranium (VI) and chromium (VI). Therefore, two separate imprinted polymersudwere prepared using chromium and uranium as template molecules for selectiveudextraction of these oxy-ions from aqueous samples. Beside removal of heavyudmetals, the study also focussed on developing MIPs for selective recovery of highudvalue compounds from plant materials (onion and Moringa oleifera).udThree separate imprinted polymers using chromium, uranium or quercetinudtemplates were prepared by bulk polymerization method. Functional monomersudused were 4-vinylpyridine; 1-(prop-2-en-1-yl)-4-(pyridin-2-ylmethyl)piperazineud(PPMP) and methacrylic acid; and 4-vinylpyridine for chromium, uranium andudquercetin imprinted polymers, respectively. For all imprinted polymers, ethyleneudglycol dimethacrylate (EDMA) and 1,1‘-azobis(cyclohexanecarbonitrile) (ACCN) were used as the cross-linking monomer and initiator, respectively. Controludpolymers (CP) or non-imprinted polymers (NIP) for each imprinted polymer wereudprepared and treated exactly the same as imprinted polymers but with omission ofudrespective templates. Following removal of respective templates with appropriateudsolutions, various parameters that affect selective adsorption such as solution pH,udinitial concentration, aqueous phase volume, sorbent dosage, contact time,udbreakthrough volumes etc., were optimized to get optimal adsorption of theudimprinted polymers.udOptimal parameters for Cr (VI) adsorption were as follows: solution pH, 3;udcontact time, 120 min; eluent, 20 mL of 0.1 M NaOH; and sorbent amount, 125udmg. Maximum retention capacity of IIP and CP was 37.58 and 25.44 mg g-1,udrespectively. The observed selectivity order was as follows, Cr (VI) > SO4ud2- > F- >udPO4ud3- > NO2ud- > NO3ud- > Cl-. However, in the presence of high concentrations ofudsulphate ions, the selectivity on the CP completely collapsed. For uranium VIudremoval, the optimal pH was 4.0-8.0, sorbent amount was 20 mg, contact timeudwas 20 min and the retention capacity was 120 mg of uranyl ion per g of IIP. Theudselectivity order observed was as follows, UO2ud2+ > Fe3+ Cu2+ > Co2+ > Mn2+ >udZn2+ ~ Ni2+.udThe binding capacity of quercetin MIPs was investigated at 25 and 84°C,udrespectively, in batch mode. The slopes for the effect of extraction time revealedudthat the mass transfer of the analytes was higher at 84°C than at 25°C. Also, theudbinding capacity for the most promising MIP and its corresponding NIP increasedudat 84°C but the MIP had higher binding capacity. The increase in binding capacityudfor the MIP was from ~30 μmol g-1 at 25°C to ~120 μmol g-1 at 84°C. For theudcorresponding NIP, the binding capacity values were ~15 and ~90 μmol g-1, at 25udand 84°C, respectively. A demonstration of MIP selectivity at higher temperatureudusing standard solutions of selected flavonols showed that the MIP still retainedudits selectivity for quercetin. Similar selectivity was observed when preliminaryudapplication studies on aqueous yellow onion extracts were investigated. The studyudclearly demonstrated the suitability of the developed imprinted polymers (for chromium, uranium and quercetin) for selective adsorption of Cr (VI), UO2ud2+ andudquercetin from their respective complex matrices.udBreakthrough volume of molecular imprinted polymer solid-phase extractionud(MISPE) was investigated using a mixture of myricetin, quercetin andudkaempferol. The breakthrough volumes for quercetin, kaempferol and myricetinudwere 22, 27 and 8 mL, respectively. The number of theoretical plates (N) for theudMISPE column corresponding to these volumes were 18, 47 and 4 for quercetin,udkaempferol and myricetin, respectively. Using these results, selectivity of MIPudand its retention capacity was evaluated. The extractions of Moringa leaves andudflowers were carried out using a MISPE cartridge and various solvents wereudinvestigated for the selective elution of quercetin from the MIP sorbents. Forudidentification and quantification of quercetin and other flavonols, a highudperformance liquid chromatography (HPLC) was used. Recoveries of quercetinudfrom different Moringa extracts ranged from 87 – 92% and this demonstrated thatudthe MISPE method can be used for the recovery of quercetin and kaempferol fromudthe Moringa extracts. Amount of quercetin found in Moringa leaves was 1555 mgudkg-1.udAll the imprinted and non-imprinted polymers prepared in the study wereudcharacterized with Fourier Transform Infrared (FTIR) spectroscopy. Scanningudelectron microscopy (SEM) was used for recording surface morphology of all theudpolymers. Surface area and pore size analysis were recorded on MicromeriticudTristar BET. For quercetin MIP, thermogravimetric analysis (TGA) was also usedudin addition to the mentioned techniques.udIn additional studies, the concentrations of metals in the soil and, in the leaves andudflowers of Moringa plant grown in South Africa were examined. Theudinvestigation included heavy metals, major and trace nutrient elements. Theudanalysis of metals was achieved after total digestion of soils or leaves using audmicrowave, and the concentrations of metals were determined using inductivelyudcoupled plasma-optical emission spectroscopy (ICP-OES). These results were compared to those obtained from some selected vegetables like spinach, cabbage,udcauliflower, broccoli, and peas. No toxic heavy metals were detected in the leavesudand flowers of Moringa. On average Moringa contained higher concentration ofudCa (18500 mg kg-1) and Mg (5500 mg kg-1) than other vegetables compared withudin the study. Other major nutrients contained in Moringa were much similar toudother vegetables. Besides metals, the concentrations of flavonols (myricetin,udquercetin, kaempferol) determined from Moringa leaves and flowers were alsoudcompared to selected vegetables. Plant and vegetable materials were extractedudunder reflux using acidified methanol (1% HCl) solution. Following which, theudflavonols were identified and quantified using reverse phased-high performanceudliquid chromatography method equipped with UV detection. Moringa leavesudexhibited highest concentrations of myricetin (1296.6 mg kg-1), quercetin (1362.6udmg kg-1), kaempferol (1933.7 mg kg-1) than vegetables (spinach: myricetin 620.0udmg kg-1, quercetin 17.9 mg kg-1, kaempferol 215.3 mg kg-1).udLastly, the antioxidant activity of Moringa flowers and leaves were compared toudthat of the aforementioned selected vegetables. The antioxidant activity wasudstudies by analyzing the total phenolic content (TPC), total flavonoid contentud(TFC), reducing power, radical scavenging activity, and the 2,2-diphenyl-1-udpicrylhydrazyl free radical (DPPH) method. Moring contained almost twice theudTPC and thrice the TFC than the vegetables. Also, Moringa demonstrated higherudreducing power and lower percentage of free radicals remaining (DPPH method).udHence, Moringa showed to be a good antioxidant source than the selectedudvegetables compared with.