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Industries all around the world are required to reduce heavy metal contents in water and industrial wastewaters to acceptable levels. The sources of metal pollution in water could be from various sources such as geologic weathering, mining effluents, industrial effluents, domestic effluents, storm water runoff, metal inputs from rural area, atmospheric sources, etc.
With heavy metal contained in the water, it will pose certain health hazards even with trace elements within it. Catastrophic events of metal poisonings from ingestion of polluted waters have been reported all around the world. In an event that occurred in Kyushu, Japan, 1953, a methylmercury have been released into the waters by a chemical firm Chisso Co., a manufacturer of PVC. This have resulted in the mercury poisoning of the aquatic environment in the area and symptoms such as weakening of muscles, loss of visions paralysis and even coma and death has resulted from the ingestion of seafood harvested from the affected area.
The above have shown the importance of removal of heavy metals from industrial effluents and there are more cases of metal poisoning involving other metals such as cadmium, arsenic, lead, chromium, etc, and such event should be avoided by setting a standard for discharge of industrial wastewaters.
Technologies such as precipitation, ion exchange, reverse osmosis and electro dialysis are developed to remove metal ions from wastewaters and drinking waters. Each and every technology has its individual strength and limitations.
2.1.1 Chemical Precipitation
Chemical Precipitation can be performed, for the example of phosphrous with the addition of coagulant material such as alum, lime, iron salts and organic polymers. This will also remove various inorganic ions, together with some heavy metals. This step will remove some heavy metal ions with it, but however will create a large amount of sludge that requires further treatments, which may be tough to treat and dispose.
2.1.2 Ion Exchange
Ion exchange can be used for removal of undesirable anions and cations from wastewaters, and such ions will be displaced from an insoluble exchange material. This process can be used to remove heavy metal ions and the material can be recovered and reused. However, this technology could be further exploited by increasing the surface area that was involved in the ion exchange process. This could be achieved by using the technologies of "Polymer Brushes" which was demonstrated by Liu & Guo using a simple polymer as an ion exchange site.
2.1.3 Membrane Processes
In the membrane filtration techniques, there are a broad range of separation techniques that can be used, such as reverse osmosis, ultra filtration and microfiltration. But for removal of metal ions, we are specifically interested in reverse osmosis techniques. This technique will use a semi permeable membrane and differential pressure to "squeeze" freshwater on one side and concentrating the salts on the rejection side of the cell. This process has a very high operating cost due to the high pressure required.
Electrodialysis will employ the use of a semi permeable ion selective membrane. An electrical potential will be applied between 2 electrodes and resulting in an electrical current passing through the solution and cause a migration of cations towards the negative electrode and a migration of anions towards the positive electrode. Due to the alternative spacing of cation and anion permeable membranes, cells of concentrated and dilute salts will be formed. Wastewater was then pumped through the membranes, which were separated by the spacers and assembled into stacks. The problem associated with this process is that salts with low solubility may clog the membrane.
2.2 Functionalised Polymer Brushes
The idea of using clay particles to remove heavy metal contaminants were used because of its high surface area to volume ratio [7-9]. Also, to improve its selectivity to specific metal of higher toxicity, organo-modified clays were developed [10, 11] and some clay materials were used in conjunction with water soluble polymers [12-16].
Combining the knowledge above, the idea of using functionalised polymer bushes to remove heavy metal ions was plausible. This was demonstrated using PAM as a simple ligand to remove Hg ions from water and it has demonstrated a significant increase in absorption capacity. It was hoped that by using different water soluble polymer and a more complex ligand, the absorption capacity and selectivity towards toxic heavy metal could be increased.
2.2.1 Surface Initiated Atom Transfer Radical Polymerisation (SI-ATRP)
In 1995, Wang and Matyjaszewski first reported on the technologies of Atom Transfer Radical Polymerisation (ATRP). Matyjazsewski et.al. used Cu2 ligand as a deactivator were used without the addition of free radical initiators. Atom Regeneration by Electron Transfer (ARGET)[19-21] ATRP was one of the newer methods of initiator regeneration and has managed to reduce the amount of copper compound in the system to ppm levels[20, 22]. However the disadvantage of ARGET was that the ligand has to be added to the metal in 3-10 times of the molar mass to get a controlled polymerisation. The excess ligand was in place to maintain the catalyst complex and prevent it from destabilising the side reactions.
Figure 1-1 Schematic Mechanism of Atom Transfer Radical Polymerisation
Surface Initiated ATRP (SI-ATRP) was a technique whereby the polymer was growth by surface immobilised initiators [6, 24, 25].
2.2.2 Post Functionalisation
2.2.3 Synthesis of Silica Particles
2.2.4 Characterisation of Polymer Brushes
Synthesis of Functional Polymer Brushes from Silica Particles
2.3 Related Studies
2.3.1 Polyacrylamide grafted attapulgite (PAM-ATP) via surface-initiated atom transfer radical polymerization (SI-ATRP) for removal of Hg(II) ion and dyes by Peng Liu & Jinshan Guo in 2006
Liu and Guo in 2006 have successfully grafted Polyacrylamide (PAM) onto a fibrillar clay using SI-ATRP. Their results have shown an increased adsorption capacity of Hg(II) ions and also cationic methylene blue dyes. Better dispersion of grafted fibrils was observed as compared to the bare attapulgite prior to grafting.
From their studies, the better dispersion could probably be achieved via colloidal stability that was achieved as PAM is water soluble. This has allowed the colloids to be sterically hindered as the PAM expands and "interferes" with another colloid.
PAM grafted fibrils also exhibited higher adsorption capacity due to the chelation of Hg(II) ions in the aqueous solution via monoamido- or diamido-Hg structure.
Figure 2-3. Formation of amido-Hg complexes
From their studies, areas of improvement were identified. (1)PAM can be replaced with a more complex ligand group to enhance the absorption capacity and selectivity. (2) Ligand can be functionalised to another polymer eg.PHEMA. (3) Wider range of heavy metal ions can be investigated for selectivity.
Also from their studies, we have learnt a few methods of characterising the polymer brushes and also measuring its absorption capacity of Hg(II) ions. (1) Fourier Transform Infrared (FTIR) to identify the amide characteristic group after grafting. (2) Thermogravometric Analysis (TGA) to confirm successful grafting. (3) Inductively Coupled Plasma-Optical Emission spectrometer (ICP-OES) to identify absorption capacities of non colourable metallic ions. (4) UV-Visible spectrometer to measure absorption capacity of coloured/colourable metallic ions/dyes. (5) Method of absorption capacity test was via "direct mixing". (6) Morphology can be studied via Transmission Electron Microscope (TEM).
2.3.2 Modification of Poly(hydroethyl acrylate)-Grafted Cross-linked Poly(vinyl chloride) Particles via Surface-Initiated Atom-Transfer Radical Polymerization (SI-ATRP). Competitive Adsorption of Some Heavy Metal Ions on Modified Polymers by Peng Liu et. al. in 2006
Poly(hydroethyl acrylate) (PHEA) was grafted from the surface of crosslinked polyvinyl chloride (PVC) by making use of labile chloride on the surface as sites of initiations. SI-ATRP technology was engaged in this study to create a polymer brush. After grafting PHEA onto PVC (PHEA-PVC), it was then hydrolysed to from carboxyl group. The hydrolysed bead, poly(acrylic acid) grafted PVC (PAA-PVC) and the bare PVC beads were measured for absorption capacities and selectivity against heavy metal ions such as Cu(II), Hg(II), Zn(II) and Cd(II).
Figure 2-4, Synthetic Route of PHEA-PVC and PAA-PVC
From their studies, it was shown that the adsorption capacities for PAA-PVC are significantly better than the other two, which is probably due to the carboxyl group that have a better electrostatic attraction effect on the metal ions tested. For selectivity issues, it was reported that for bare PVC and PHEA-PVC beads, it has a higher Hg(II) absorption, which is probably due to the chelating effects of the metal ions with Cl atom, esters and hydroxyl group. For PHEA-PVC and PAA-PVC beads were found to be saturated by 10 mins of use and it was also found to be regenerative by using acetic acid, which makes it ideal for use via filtration columns. The effect of pH was also tested and it was found that the metal ion absorption capacities of the material increases with decreasing pH, as at higher pH there would probably by more H+ ions to compete for the capacities with the metal ions.
Also learnt from this study, was a few points that can be applied; (1) Absorption capacities can by increased via introduction of a different functional group, such as carboxyl group in this case. (2) Various heavy metal ions were dissolved into a single solution and tested for selectivity. (3) Agitation time of solution mix was done to investigate the saturation time of the absorbent material. (4) ICP-AES was used to examine the level of metallic ion in the solution after absorption.
From their studies, what we can apply was; (1) The choice of functional groups used, (2) Method of measuring metal ion absorption; using multiple metal ions in the same solution. (3) Possibilities of reusing the materials by regenerating of functional groups.
2.3.3 Synthesis and characterization of polymer brushes containing metal nanoparticles by K.Zhang et. al. in 2006
In the study done by K.Zhang et. al. in 2006, they have successfully synthesised PHEMA onto silica particles. Characterisation of the materials were also done to confirm the technique used. The initiators were attached as shown in Figure 2-5. SI-ATRP was performed and HEMA with metal nanoparticles were grown from the surface immobilised initiators.
Figure 2-5, Surface immobilisation of surface initiators
X-ray Photoelectron Spectroscopy (XPS) and ellipsometry were used to confirm the attachment of the chemical modification and the growth of PHEMA from the initiators.
From their studies, the technique used was successful in growing the PHEMA brushes. Also, it was realised that the hydroxyl groups can be used as simple ligands to attach metal ions as well as for further functionalisation of functional groups.
2.3.4 Surface Polymerisation of Colloidal Silica Particles by Jun Ren in 2009
In 2009, a MSc dissertation by Jun Ren has synthesised a polymer brush using using MMA as the polymer. He has used zeta potential, particle size distribution and FTIR to characterise the spherical polymer brushes that was synthesised. Monodisperse silica particles were also synthesised using a modified Stober process.
FTIR was used to investigate the chemical modification at the surface of the particles. At 1100 cm-1 peak, it was used to confirm the hydrolysis of TEOS to silica particles. Other peaks were also used to investigate the chemical modification.
Zeta Potential was used to investigate the changes after the chemical modifications. It was found that after each process, a shift in isoelectric point (IEP) or shift in zeta potential was discovered. This indicated a change in the surface groups.
Particle size and distributions were also used. For silica particles, a monodisperse distribution was observed and the attachment of amino group also yields the same result. However after the attachment of surface immobilised initiators, the distribution changed with the main peak still close to the initial size. After SI-ATRP of MMA, the distribution of the particles shifted towards the higher size range, indicating a growth of PHEMA from the initiators.
From this study, a fast method and relative method for confirming the steps done was established and will pose significant saving of time as compared to using XPS to confirm the growth.
2.3.5 Grafted Polymer for Waste Treatment by X.Zhong in 2009
In a summer experimental project by X.Zhong and supervised by S.Edmondson in 2009, a grafted polymer for waste treatment by removing heavy ions were investigated. Similar techniques to Jun Ren was used but however, PHEMA was used as the polymer and silica gel was used as the particles. The chemical modification techniques were established and were relatively similar to K.Zhang et.al. method for growing PHEMA onto silica particles. A ligand of fairly large size, diethylphosphonoacetic acid was used as the ligand to remove metal ions. The resultant result yields lesser metal ion absorbtion as compared to silica gel. This is probably because that silica gel being a porous particles and by growing brushes within the pores and also attaching such large groups onto the brushes, it was probably tough to attach metal ions or even functionalising the large ligands onto the brush itself.