Abstract
M.Sc.
Water is often contaminated with organic and inorganic compounds by natural means and through human activities. Once contaminated, water of this nature has little or no use. However, water that is free from toxic chemicals is essential to both human health and the environment. Current water treatment techniques such as separation by membranes (reverse osmosis), adsorption (activated carbon) and ion exchange are not always very efficient at removing contaminants which may be present in parts per billion (ppb) levels. Techniques need to be developed that are reasonably inexpensive and easy to use, and yet effective at removing both organic and inorganic pollutants to acceptable levels. Adsorption is a technique that has the potential to meet these criteria.
In our laboratories, insoluble beta cyclodextrin (β-CD) polymers have been used to remove pollutants from water at concentrations as low as ppb levels. However, they exhibited some disadvantages, such as poor structural integrity and difficulty in recovery. This project sought to deal with these limitations by incorporating functionalized multi-walled nanotubes (f-MWNTs) into the polymer, and then embedding polymer particles in an alginate matrix as small beads for ease of use.
The polymer composites, 1% f-MWNTs with β-CD polyurethane, were synthesised and embedded in alginates to form alginate composite beads. Composite beads were tested against a model organic and heavy metal pollutants, namely p-nitrophenol and Pb2+, respectively. The composites were characterized using Fourier Transform Infra Red spectroscopy (FT-IR), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDXS), among other techniques.
The absorption capacity of f-MWNTs, β-CD, and alginates combined proved to be effective and stable adsorbents. They showed an adsorption efficiency of at least 95% for p-nitrophenol at a concentration of 10 mg/L and 98% for Pb2+ at a concentration of 50 mg/L.
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The novel adsorbents show a good thermal stability and maintain their structural integrity after repeated (thirty times) use in recycling experiments. The recycled beads maintained a high average adsorption efficiency of 96%, indicating the potential cost benefit of these materials. As a comparison, the plain calcium alginate (CaAG) beads and plain β-CD polyurethane beads showed an average adsorption of 55% and 74%, respectively, but their structural integrity was significantly compromised during similar recycling. Brunauer Emmett Teller (BET) surface area analysis showed that incorporation of f-MWNTs (1% loading) increased the surface area of the composite beads, and adsorption isotherms showed a good fit with both Langmuir and Freundlich models.
This project has demonstrated the use of f-MWNTs as copolymer which improves the structural stability of the polymers, and that the combination of these polymers and alginates provide a potentially useful material for water treatment applications.