Impact/Purpose:

of enhanced photo-reduction of metal ions.

Proposed is a set of studies to prove the concept that addition of surfactants can modify the photo activity of TiO2 towards various pollutants, Preliminary work shows that both anionic surfactants (Sodium dodecyl sulfate, SDS) and non-ionic surfactants (Brij-35) adsorb on Degussa P25 TiO2 and form aggregates capable of solubilizing the dye pinacyanol chloride. of enhanced photo-reduction of metal ions.

Proposed is a set of studies to prove the concept that addition of surfactants can modify the photo activity of TiO2 towards various pollutants, Preliminary work shows that both anionic surfactants (Sodium dodecyl sulfate, SDS) and non-ionic surfactants (Brij-35) adsorb on Degussa P25 TiO2 and form aggregates capable of solubilizing the dye pinacyanol chloride.

Advanced Oxidation Processes (AOPs) are of considerable interest for treating industrial wastewaters containing organic carbon and halocarbon pollutants, in the pulp and paper industries, municipal wastewaters, wastewater containing pesticides and mixed organic/heavy metal wastes. The use of TiO2 and other semiconductors to generate hydroxyl radicals in situ is one particular AOP that is generating interest because of the potential use of sunlight, the low pollutant concentrations obtainable, and because such methods have the potential to treat wastewaters that are not readily treated by other AOPs. Because the hydroxyl radical is generated at the TiO2 surface, these heterogeneous photocatalytic methods are sensitive to the adsorption characteristics of the target pollutants, particularly when a mixture of different components is present. Addition of surfactants can alter the adsorption characteristics. Even at low solution concentrations surfactants adsorbed on the surface can form aggregates which have many of the characteristics of micelles, including the ability to solubilize hydrophobic pollutants. Addition of surfactants can result in enhanced photocatalysis by increasing the concentration of targeted pollutants near the TiO2 surface. For solutions containing a mixture of pollutants and other benign solutes, addition of appropriate surfactants could be used to produce selective decomposition. Because the TiO2 surface is positively charged, positively charged pollutants including most metal ions, are repelled from the surface. Formation of aggregates of negatively charged surfactants on the surface provides charge compensation which causes binding of positively charged pollutants. This provides a mechanism for enhancing photo-oxidation of positively charged organic pollutants, and

Description:

Initial research projects using the nonionic surfactant Brij-35 established that this surfactant could successfully adsolublize aromatic organic pollutants such as anthracene, naphthalene, benzoic acid, chlorophenol, and benzene onto the surface of TiO₂ particles.  Adsolublization was shown to enhance the rate of degradation of anthracene; however, the analytical procedure required a series of steps that proved to be very time consuming, including a separation of solution from TiO₂ particles.  The procedure gave reproducibility of data points that did not consistently meet the data quality criteria of the project.  Efforts then were made to improve the particle separation step, particularly by encapsulating the TiO₂ particles being used inside a film of mesoporous silica.

The mesoporous silica preparation method chosen was based on the use of block copolymers of the poly(oxyethylene)-poly(propylene oxide)-poly(oxyethylene) type.  The mesoporous silica synthesized by this method is known as SBA-15 and consists of hexagonal arrays of cylinderical pores of approximately 6 nm diameter.  Addition of ethanol as a cosolvent allows thin films of this material to be cast or coated onto substrates of various forms.  The thickness of the films was determined by variable angle spectroscopic ellipsometry, which also gave an estimate of 46 percent void space in the films, consistent with a formation of a porous silica film.  The incorporation of the TiO₂ particles was monitored by UV-visible optical microscopy, scanning electron microscopy and transmission electron microscopy (TEM).  The mesoporous structure of the silica films was established initially by small angle X-ray diffraction, and recently confirmed by TEM.  TEM also confirmed that pores in the mesoporous silica do not bend around the TiO₂ particles; therefore, the pores in the silica do provide a pathway for the pollutants to diffuse from the surface of the film to the TiO₂ particles and for degradation products to escape into the aqueous phase.

All films were prepared with a film thickness of 380 nm.  Experiments varying the amount of TiO₂ incorporated into the mesoporous silica films showed that films incorporating 12.5 percent by weight  TiO₂ particles achieved the maximum photodegradation efficiency for 2,4-dichlorophenol (2,4-DCP), which was used throughout this phase of the project as the test pollutant.  The TiO₂/mesoporous silica films were coated onto 4 mm outer diameter tubes, which were placed in 10 mL vials containing 5 mL of 150 ppm 2,4-DCP solutions in water.  Irradiations were carried out at 365 nm.  High performance liquid chromatography analyses of the irradiated solutions as a function of time showed that only trace amounts of 2,4-DCP were detectable after 7.5 hours irradiation.  Calculations showed that on a per photon absorbed basis the mesoporous silica/TiO₂ films had approximately two-thirds the efficiency of an equivalent TiO₂ dispersion.

Experiments with alternative types of mesoporous silica showed that the lamellar phase was not stable after removal of the templating polymer.  The cubic phase was stable but showed much lower photoactivity, approximately the same as for TiO₂ incorporated into a nonporous silica film.  Some effects on the distribution of the TiO₂ particles within the films also were examined.  The TiO₂ particles can aggregate within the silica films.  Samples were prepared in which the solutions were sonicated to break any possible aggregates of TiO2.  These films were shown to be only approximately 4 percent more efficient than the films that had not been sonicated.  Experiments were also undertaken to study the long-term photoactivity of the mesoporous silica/TiO₂ films.  Repeated irradiations of 7 hour duration showed that after eight such irradiations the loss of 2,4-DCP decreased from 95 percent to a steady state value of approximately 85 percent.

Subsequent to the completion of the research project, we have been evaluating the photoactivity of the mesoporous silica/TiO₂ films in photovoltaic cells.  We are continuing this aspect of the project by synthesizing and evaluating out mesoporous materials as hosts into which the TiO₂ particles are incorporated.

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In preliminary experiments, low concentrations of Brij-35 were found to result in an increase in decomposition of pinacyanol chloride. It is planned to study a set of hydrophobic aromatic pollutants consisting of benzene, chlorobenzene, phenol, anthracene, 2-chlorophenol, pentachlorophenol, benzoic acid, pyridine, Indigo Blue, and Erichrome Red in the presence of different surfactants to determine which pollutants are solubilized by the surfactant aggregates and for which solubilization results in enhanced photodecomposition. The extent of photodecomposition will be determined by extraction with CH2C12 from the aqueous dispersion followed by analysis by UV-vis, fluorescence and LC as appropriate.

Cost :$.00

Research Component :Hazardous Waste/Remediation

Approach :

In preliminary experiments, low concentrations of Brij-35 were found to result in an increase in decomposition of pinacyanol chloride. It is planned to study a set of hydrophobic aromatic pollutants consisting of benzene, chlorobenzene, phenol, anthracene, 2-chlorophenol, pentachlorophenol, benzoic acid, pyridine, Indigo Blue, and Erichrome Red in the presence of different surfactants to determine which pollutants are solubilized by the surfactant aggregates and for which solubilization results in enhanced photodecomposition. The extent of photodecomposition will be determined by extraction with CH2C12 from the aqueous dispersion followed by analysis by UV-vis, fluorescence and LC as appropriate.

Cost :$.00

Research Component :Targeted Research

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