Adsorption of phosphate on functionalised mesoporous silicas for phosphorus recycling
Creators: name: GHOLAMI, Mozhdeh
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340210
subject: Solid state chemistry

description: The accumulation of phosphorus (in the form of phosphates) in water bodies is a concern as this can contribute to excessive growth of aquatic vegetation (eutrophication), which in turn causes a decrease in water quality. Hence processes that can reduce the level of phosphorous in waters containing high levels of phosphorus are of significant interest. One type of process that has good potential for phosphorous removal is adsorption - this is due to its simple and economical operation with less sludge production. A range of metal oxides supported on mesoporous silica have shown potential for adsorbing phosphorous in water, with rare earth oxide based materials showing significant potential. One of the main challenges faced with the aforementioned materials is that they need to be highly efficient, stable and easily regenerated. The main aim of this study was to investigate adsorption of phosphate from aqueous solution using a wide range of rare earth oxides supported on mesoporous silicas. Specific aims included gaining an improved understanding of the mechanism(s) of phosphate adsorption on these materials, studying the efficiency of these materials in the presence of co-existing anions (anions that could compete with phosphate for adsorption sites) and investigating the recyclability of these materials. <br><br>
Initial investigations, which are reported in Chapter 3, involved the preparation and testing of thirteen rare earth oxides (La/KIT-6, Ce/KIT-6, Pr/KIT-6, Nd/KIT-6, Sm/KIT-6, Eu/KIT-6, Gd/KIT6, Tb/KIT-6, Dy/KIT-6, Ho/KIT-6, Er/KIT-6, Tm/KIT-6, and Yb/KIT-6) supported on a relatively recently discovered mesoporous silica, KIT-6. The aforementioned materials were prepared using wet impregnation methods and testing was conducted under one set of conditions (Initial P concentration: 22.80 mg/L; [Ln/KIT-6] = 10 mg/L; pH: 6.40; temperature: 25°C). Characteristics of the materials studied were determined using various techniques which included Small Angle X-Ray Scattering, X-Ray Diffraction, BET surface area analysis, HRTEM and electrostatic Zeta potential measurements. The influence of other anions (chloride, sulphate, nitrate, and bicarbonate) on phosphate removal was also investigated for the best performing materials. The results obtained from the tests conducted on phosphate adsorption using the thirteen rare earth oxides supported on KIT-6 showed a wide range of % phosphate adsorption for the different materials (30 - 100%) under the test conditions used. It was found that the La/KIT-6 and Yb/KIT-6 materials were the best adsorbents in terms of % phosphate removal and rate of phosphate removal. A strong correlation was observed between % phosphate removal and the zeta potentials (surface charges) of the materials with materials having zeta potentials that were closer to neutral obtaining higher phosphate adsorption. The tests conducted on the influence of other anions showed that only bicarbonate had a significant negative effect on phosphate adsorption for all the materials studied.<br><br>
In Chapter 4, detailed studies were conducted on lanthanum and ytterbium oxides supported on different mesoporous silicas (KIT-6, MCM-41 and SBA-15). These studies included investigations on the influence of La and Yb loading on phosphate removal. Characteristics of the materials studied were determined using various techniques such as XRD, BET, HRTEM, EDS, XPS, and zeta potential measurement. The results obtained showed that the most efficient material was 0.1La/KIT-6 under the experimental conditions studied, whilst the worst performing material was 0.05 La/SBA-15. Among the supports that were studied the KIT-6 supported materials showed the highest phosphate removals. Moreover, Zeta potential studies exhibited that the six best performing materials (0.3La/SBA-15, 0.1La/KIT-6, 0.05La/MCM-41, 0.3Yb/SBA-15, 0.1La/KIT-6, and 0.05La/MCM-41) had surface charges closest to neutral, similar to what was found in the initial investigations conducted on the thirteen different rare earth oxides supported on KIT-6. The adsorption data obtained was used to determine what type(s) of adsorption isotherms best described the adsorption occurring for the materials studied. It was found that both Langmuir and Freundlich adsorption isotherm models could be used to fit the adsorption data obtained. The Langmuir model did however fit the data slightly better than the Freundlich model indicating that the adsorption process for the six materials investigated occurred predominantly on the surface of the samples via monolayer adsorption. Analysis of the kinetic data obtained revealed that the adsorption kinetics observed were best fitted by pseudo-second order kinetics.<br><br>
In chapter 5, the lanthanum and ytterbium based materials that were found to be the best materials for phosphate adsorption based on the results obtained in chapters 3 and 4 (0.3La/SBA-15, 0.1La/KIT-6, 0.05La/MCM-41, 0.3Yb/SBA-15, 0.1Yb/KIT-6, and 0.1Yb/MCM41) were studied further by studying the influence of pH, other anions (Cl- , SO4 -2 , NO3 - , and HCO3 -), stability and recyclability /reusability. The phosphate adsorption studies at lower pH (3.0 and 5.0) showed that the adsorption kinetics were improved at lower pH compared to pH 6.4 under the experimental conditions used. The obtained results of the influence of other anions on phosphate showed that only bicarbonate had a significant negative effect on phosphate adsorption for all the materials studied. Based on ICP-MS studies, the materials were stable with respect to leaching of the metal components. Reusability studies revealed the Yb supported silica materials (0.3Yb/SBA-15, 0.1Yb/KIT-6, 0.1Yb/MCM-41) in addition to 0.1La/KIT-6 experienced only a slight loss of adsorption capacity after four adsorption - desorption cycles.<br><br>
In summary it was discovered that there are significant differences in phosphate adsorption between lanthanide oxides supported on KIT-6. The differences in adsorption observed were found to be strongly correlated with the zeta potentials of the respective materials. It was also discovered that the type of mesoporous silica used to support lanthanide oxides for phosphate adsorption does have a significant effect on the rate of phosphate removal obtained. Of the three mesoporous silicas investigated in this study KIT-6 gave the best results.