School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemBiosorption of heavy metal cations from water solution using microwave-modified barks of Eucalyptus globulus Labill and Pinus radiata D.Don( 2011)The utilization of bark from Eucalyptus globulus Labill and Pinus radiata D.Don as adsorbents for Cu(II) and Zn(II) in solution was investigated in this study. Analysis of the chemical composition of the two bark species enabled a determination of their potential end use application for heavy metal removal. The higher lignin (49.4%) and extractive (20.5%) content of pine bark as well as the higher hemicellulose (35.7%) content of eucalypt facilitated the use of these barks as suitable adsorbents for heavy metal cations. The release of water soluble extractives (phenolic compounds), however, may impede the use of the barks as metal chelating agents because the leachates may under some circumstances be detrimental to aquatic ecosystems. In order to fix the extractives within the bark and prevent them from leaching, the eucalypt and pine barks were ground, air dried and then heated for 60 seconds using three different microwave power levels to obtain three determined bark temperatures (100, 150 and 200°C). UV spectroscopy showed that the absorbance value of untreated bark at 280 nm was not significantly different from that of the corresponding bark treated with microwave. This was observed for both bark species. This indicates that microwave heating for 1 minute using the three microwave energies examined did not result in any fixation of phenolic compounds within the bark. FTIR spectroscopy, however, indicated the occurrence of chemical bond transformations in various functional groups in bark hemicelluloses and lignin as a result of microwaving. Eucalypt and pine barks were evaluated in a Cu(II) and Zn(II) adsorption study with the following initial treatments: (1) Microwaving to various pre-determined temperatures (100, 150 and 200°C), (2) extracting with water (to remove leachable compounds) and (3) combined processing, incorporating both microwaving and extraction with water. Pressure steamed pine bark, ground pine wood and cotton were examined for comparison. All the samples were analyzed using UV spectroscopy to measure the absorbance value at 280 nm in order to elucidate the leachability of phenolic compounds from the absorbents. Batches of each sample were then soaked in heavy metals solutions (10, 20, 40 mg/L of a single metal element - Cu(II) and Zn(II)). After an hour of shaking, the adsorbent was filtered from the suspension and the solute was analyzed for residual copper using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) to determine the efficiency of each material in binding the metal cations from the solution. The results showed that pressure steamed pine bark and water extracted eucalypt bark exhibited the highest adsorption capacity for Cu(II) and Zn(II) at lower concentrations (10, 20 and 40 mg/L). As observed for eucalypt and pine bark, microwave treated bark generally showed lower uptake capacity compared to the corresponding untreated bark due to their lower reactivity. Microwave treated bark also removed lower amounts of Cu(II) and Zn(II) than those exposed to the combined treatment. Cotton adsorbed the lowest amounts of Cu(II) and Zn(II). This performance was also observed for the bark when in contact with a mixed Cu(II)-Zn(II) solution. At higher Cu(II) concentration (3000 mg/L), there was a change in the performance of the absorbents examined. Untreated eucalypt bark exhibited the highest Cu(II) adsorption (11.1 mg/g), followed by pressure steamed pine bark (10.4 mg/g) and water extracted eucalypt bark (9.2 mg/g). Microwave treated eucalypt bark chelated more Cu(II) compared to the bark subjected to the combined treatment of microwave and water extraction, but this was not observed for microwave treated pine bark. Adsorption equilibrium parameters were generated using the graphical interpretation of the equations of the Freundlich and Langmuir models. On the basis of these models, Cu(II) and Zn(II) adsorption by both bark species were monolayer. The Freundlich isotherm model provided a better fit to the experimental data in describing both Cu(II) and Zn(II) adsorption by both eucalypt and pine barks. The type of metal species did not significantly affect the metal adsorption in the single cation solutions, but in a binary-metal solution, Cu(II) was adsorbed more than Zn(II). Initial concentration of the metal cations in solution influenced adsorption ability of the absorbent examined, in either single or multi metal solutions. Increasing metal concentration in solution increased the uptake capacity and decreased the uptake efficiency of adsorbents. In relation to bark species, eucalypt bark had a higher adsorption capacity than pine bark. Adsorbent particle size did not appear to affect the adsorption process, while adsorbent dosage did. An hour of reaction time was sufficient to reach adsorption equilibrium between the adsorbents and the metal solution. Pressure steam treatment on bark may have resulted in the insolubilisation of leachable compounds of the material, potentially rendering the bark to be more reactive with metal cations. Another possible reason for the improved capture of cations is the absence of leachable extractives. A proportion of leachable extractives had been removed during pressure steaming. The removal of soluble extractives may increase the number of adsorption sites available to heavy metals. Alternatively, steaming may result in the swelling of the bark material, leaving more accessible adsorption sites, thereby improving the adsorption ability of the bark. Water extraction treatment of bark is found to be beneficial for metal adsorption. In addition to the generation of an effective adsorbent for metal cations, water soluble extracts can be isolated for a range of purposes, including tannin adsorbents for heavy metals, proteins and oils, natural additives and a chemical source. This study has shown that eucalypt and pine bark have potential for binding heavy metals. The plentiful availability of eucalypt and pine bark in Australia allows their viable utilization for enhancing water quality in a variety of applications.