Photocatalytic Degradation of Pollutants
Download Fotokatalytická degradace polutantů.pdf, Photocatalytic Degradation of Pollutants.pdf
Photocatalytic degradation is based on ability of TiO2 particles to perform on their surfaces for oxidation and reduction. The basic precondition is that TiO2 particles have adequate crystallographic form and they must be excited by suitable UV radiation. TiO2 particles have diameters well below one micron.
Excited TiO2 particles suspended in water have hydroxyl radicals on their surfaces. These radicals have exceptionally high oxidation potential:
|
| Oxidation Potential [V] |
| Hydroxyl Radical | 2.8 |
| Ozone | 2.1 |
| Permanganate | 1.7 |
| Chlorine | 1.4 |
The oxidation potential is the key reason why different oxidations of organic pollutants take place in water. The end products are usually either CO2 or a mixture of CO2, acids and partially degraded pollutant(s). The partially degraded pollutant(s) is (are) usually biodegradable. Relative oxidation velocity is significantly different for different pollutants. This is clearly demonstrated by the following table:
|
| Relative Velocity |
| Chlorinated alkenes | 1 |
| Phenols | 1 |
| Compounds containing N | 1 |
| Aromatic Compounds | 0.1 |
| Ketones | 0.1 |
| Alcohols | 0.01 |
| Alkanes | 0.01 |
It is clear that different chemicals are differently suitable for TiO2 degradation.
Electrons surplus integrated with hydroxyl radicals on surface of TiO2 particles creates optimal environment for different reduction processes. Dissolved oxygen is the prime target for reduction. The following metals cannot be reduced: Cd2+, Cr3+ a Fe2+.
A continuous degradation flowsheet is simple:
Excitation of TiO2 by UV and degradation are integrated into one unit operation called photocatalytic Degradation.
Particles of TiO2 are separated from water by microfiltration and recycled. TiO2 particles are photo catalyst. In other words they are not consumed and can be reused many times.
The key unit operation is the degradation as it requires the dominant amount of energy. The whole process is nearly always efficient if the pollutant concentrations are in tens of ppm. In this case the specific energy demand to treat on cubic meter of polluted water is approximately 1 kWh/m3.
The following example of chlorinated hydrocarbons indicate that if the pollutant are suitable for oxidation then the process is efficient
Concentrations (mg/L)
| Input | Output |
| DCM | 11 | 0.01 |
| TCM | 6 | 0.2 |
| PCE | 0.15 | 0.0008 |
| TCE | 0.04 | 0.0002 |
| cis-12-DCE | 0.2 | 0.0025 |
Spectrum of pollutant suitable for TiO2 oxidising is relatively broad. For example:
| ids | humic, chlorobenzoic, formic, gluconic, lactic, malic, propionic, tartaric, oxalic, butanoic, octanoic, cumaric, salicylic, polycarboxylic |
| Alcohols | Benzyl, tert-butyl, ethanol, ethylene glycol, glycerol,isopropanol, methanol, propenediol |
| Aldehydes | Acetaldehyde, benzaldehyde, formaldehyde, glyoxal, isobutyraldehyde, trichloroacetaldehyde |
| Aromatics | Benzene, chlorobenzene, chlorophenol, creosote, dichlorophenol, hydroquinone, p-nitrophenol, phenol, toluene, trichlorophenol, xylene, trinitrotoluene |
| Amines | Aniline, cyclic amines, diethylamine, dimethylformamide, EDTA, propanediamine, n-propylamine |
| Dyes | Anthraquinone, diazo, monoazo |
| Ethers | Tetrahydrofuran |
| Ketones | Dihydroxyacetone, methyl ethyl ketone |
| Aromatic Hydrocarbons | benzene , toluene, naphtalene) |
| Halogenated compounds | CH2Cl2, CHBr3,TCE, 1,2-dibromo-3-chlorpropane, monochlorbenzene, dichlorobenzene, chlorobiphenyl |
| Hydroxylated compounds | methanol, rpropanol, phenol, propylphenol, cresols, bisphenol A, 4,4´-ethylidenebisphenol, 4,4´-methylenebisphenol |
| Ethers | methoxyphenols, meta and para substituted methoxybenzenes,- NH2, NO2,- F,- Cl |
| Sulphur- containing compounds | 2-methylthiophene, 3-nitrobenzenesulphonic acid, 2,5-anilinedisulfonic acid, o-phenolsulfonic acid, sulfosalicylic acid, 2-mercaptobenzothiazole |
| Nitrogen-containing compounds | CH3CN, C2H5NH2, (C2H3)2NH2, aniline, nitrobenzene, phenyltetrazole |
| Halogen and nitrogen – containing compounds | cetylpyridinium chloride, C21H38NCl |
| S-N containing componds | phenylmercaptotetrazole, mercaptotetrazole |
| Aldehydes, ketones | formaldehyde, acetophenone, salicylaldehyde, methylsalicyl ketone |
| Amide | benzamide |
| Esters | K hydrogen phtalate, dimethyl phtalate, diethyl phtalate, di-n-butyl phtalate |
| complexed cyanides, chlorinated solvents, pesticides, halogenated micropollutants, THM precursors, tri- and tetrachloroethylene, humic substances, chloroform, bromodichloromethane, atrazine, simazine, methyl tertiary-butyl ether (MTBE), N-nitroso-dimethylamine (NDMA), endocrine disruptors | |
