Photocatalytic Water And Wastewater Treatment And Purification

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Due to the nonselective attack of hydroxyl radicals, TiO2 photocatalysis can decompose virtually all organic contaminants and oxidize various inorganic anions. The quality of polished water depends on the properties of water and wastewater and the treatment parameters, as discussed in Section 3.5. The extent of the treatment should be decided according to the purpose of treatment and regulations.

Table 2 summarizes the photocatalytic oxidation and degradation of some compounds [7-9,18]. A wide range of anions such as nitrate, sulfide

Table 2 Photocatalytic oxidation, reduction, and degradation of various compounds



Inorganic anions


Heavy metals

Dichromate, Cr(VI), Pb2+, Mn2+, Ti+, Co2+, Hg2+, Hg(II), HgCl2, CHaHgCl, Ag(I), Cu(II), Cd(II), Au(III), Pt(VI), Pd(II), Ni(II)


Isobutene, pentane, heptane, cyclohexane, paraffins, chloromethane, bromomethane, chloroethane


Methanol, ethanol, propanol, glucose, acids (formic, ethanoic, propanoic, oxalic, butyric, malic)


Cyclohexene, chloroethylene, chloroethene


Bezene, toluene, xylene, naphthalene, chlorobenzene, chloronitrobenzene, biphenyls, polychlorinated biphenyls, acids (benzoic, aminobenzoic, phthalic, salicylic, hydroxybenzoic, chlorobenzoic)

Phenolic compounds

Phenol, chlorophenol, fluorophenol, hydroquinone, catechol, methylcatechol, cresol, nitrophenols


sodium dodecyl sulfate, polyethylene glycol, sodium dodecyl benzene sulfonate, trimethyl phosphate

Herbicides and pesticides

Atrazine, prometron, propetryne, bentazon, monuron, DDT, parathion, lindane


Methylene blue, rhodamine B, methyl orange, fluoroscein, Congo red

Activated sludge

Mixture of various organic compounds


Escherichia coli, Bacillus pumilus, phage

Biological toxins

microcystin-LR (RR, YR, LA)

Source: Adapted and modified from Refs. [7-9,18].

Source: Adapted and modified from Refs. [7-9,18].

and sulfite, and cyanide can be easily oxidized to harmless or less toxic compounds [19-22]. The elements, (N, S, and P) are typically transformed to their maximum oxidation state. For example, nitrite is oxidized to nitrate and sulfide and sulfite are converted to sulfate. Metals found in water resources (especially industrial wastewater, groundwater, and surface water in mining areas) impose a serious concern. Their toxicity depends on their valence states. It is practical for TiO2 photocatalysis to convert such toxic metals to their less toxic or nontoxic forms, or remove them from the water streams. Metal ions (M), if present, are reduced by the electrons generated in the conduction band of TiO2 (Equation 19) [23]:

The treatment feasibility counts on the standard reduction potential of the metals. Ag(I), Cr(VI), Hg(II), and Pt(II) were easily treated by TiO2 photocatalytic redox reaction, while Cd(II), Cu(II), and Ni(II) were not removed effectively [24]. Another metal removal approach is the photodeposition of metal ions on Pt-loaded TiO2 [25]. For example, Pb2+ concentration in Pt-TiO2 suspension was reported to decrease over time while the color of TiO2 became brown, implying deposition of Pd to the surface of Pt-TiO2. Similar results were observed with other metal ions such as Mn2+, Ti+, Hg2+, and Co2+.

Most of the common organic compounds found in water and wastewater, including phenolic compounds, chlorinated chemicals, surfactants, dyes, and pesticides, can be decomposed regardless of their molecular structure due to the nonselective attack of hydroxyl radicals. Many of these organic chemicals are classified as priority pollutants by the U.S. Environmental Protection Agency (EPA) or are in the drinking water contaminant candidate list [26-31]. Their structure typically affects reaction kinetics and intermediates formation. Many research studies have focused on the photocatalytic degradability and reaction mechanisms of organic compounds, and identification of reaction by-products and their toxicity. In many cases, the reaction pathways are too complicated to elucidate the detailed steps.

Degradation of target compounds is fast. Typically, the double bonds in organic chemicals are susceptible to hydroxyl radical attack. The dearomatization of aromatic compounds (e.g., toluene, benzene, and phenol) is quick even in the presence of substituents such as Cl, NO2, and OCH3 on the aromatic ring [13,32,33]. An aliphatic chain bound to the aromatic ring is easy to fragment but its mineralization to CO2 is significantly slow since intermediates formed during the reaction such as formate and acetate ions are relatively stable [34,35]. So far, complete mineralization of almost all organic compounds was observed. Due to the high stability of the triazine aromatic ring, s-triazine herbicides were reported to be transformed to nontoxic cyanuric acid as a final product [36]. For persistent chlorinated compounds, such as chlorophenol and chlorobenzoic acid, the chloride ions are easily removed and the dechlorinated compounds are then available for biological treatment system [13,37]. Nitrogen-containing compounds are transformed to NH+ and NO" [38]. Sulfur-containing chemicals are mineralized to sulfate ions [39]. Organophosphorus pesticides produce phosphate ions [35,40].

Recent advances in analytical instruments such as high-performance liquid chromatography, gas chromatography, and mass spectrometry make it possible to detect and identify reaction intermediates during the photocatalytic degradation event of organic compounds. One of the most extensive studies so far on intermediate identification and degradation pathways in TiO2 photocatalysis was conducted by Jenks and his coworkers [41,42]. Using 4-chlorophenol (4-CP) as a model compound, they experimentally proved that after the photoexcitation process and the generation of reacting species, a series of cascade reactions (demonstrated in Section 3.3) could lead to complete mineralization of 4-CP. Initially, 4-CP undergoes bifurcation forming either hydroquinone by substitution [41] or 4-chlorocatechol by hydroxylation [42] at ratios that depend on reaction conditions. Hydroquinone undergoes further reactions to from hydro-xybenzoquinone, 1,2,4-benzenetriol, 1,2,4,5-benzenetetraol (traces), and oxidative cleavage of the benzene ring at either the C1-C2 or C3-C4 bonds to form acyclic derivatives. Oxidative cleavage of the ring of 1,2,4-benzenetriol occurs via electron transfer to form a radical cation of this compound followed by trapping of the radical cation by superoxide. This mechanism results in the formation of dioxetanes, which subsequently collapse to form open-ring six-carbon compounds of either acid-aldehyde (break of C3-C4 bond) or diacid (break of C1-C2 bond). Major acyclic intermediates are (E)- or (Z)-4-oxohex-2-enedioic acids, oxobutanedioic acid, propanedioic acid, and ethanedioic acid. Other smaller intermediates, prior to the formation of acetic acid, formic acid, and formaldehyde, include 1,2-ethanediol and hydroxyacetic acid. While degradation of 4-chlorocatechol undergoes a degradation pathway that includes formation of 5-chloro-1,2,4-benzenetriol by hydroxylation, 1,2,4-benzenetriol by substitution, and small quantities of 4-chloropyrogallol by hydroxylation. Ring cleavage forms compounds as previously discussed and chlorine-bearing compounds such as (E)-3-chloro-4-oxohex-2-enedioic acid, 3-chloro-4,5-dioxopent-2-enoic acid, and chlorofumaric acid. Many other smaller intermediates in the degradations were identified by Jenks and his coworkers [41,42].

In addition, TiO2 photocatalysis has strong disinfection function toward microorganisms. The widespread use of antibiotics and the emergence of more resistant strains of microorganisms in water induce an immediate need to develop alternative disinfection systems. The TiO2 photocatalytic process is practically useful for killing pathogenic microorganisms. The hydroxyl radicals are highly toxic and reactive to microorganisms like other organic substances. Photocatalytic inactivation of bacteria such as Escherichia coli and Bacillus pumilus, as well as several phages has been investigated [43-45]. TiO2 photocatalysis initially promotes peroxidation of the polyunsaturated phospholipid component of the lipid membrane and thus induces a major disorder in the cell and damages essential functions, leading to death of microorganisms [46]. In addition, TiO2 photocatalysis has demonstrated high decomposition and detoxification efficiency toward biological toxins, especially cyanobacterial toxins in drinking water resources [47-49].

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