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Home Products Technology Photocatalytic Degradation Photocatalytic Degradation - text

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