Environmental photocatalytic processes with POM . The photodecomposition of atrazine and photoreduction of metal ions from aqueous solutions

Polyoxometalate anions (POM), resulting from the condensation of tungstate anions in strongly acidified solutions, can absorb efficiently light in the UV-near Vis region.The produced excited state is a very powerful oxidative reagent which can, mainly via OH radical attack, oxidize and mineralize a great variety of organic pollutants in aqueous solutions, while the photocatalytic circle is closed by reoxidation of the reduced POM, mainly, by O2. Metal ions can also serve as oxidants and close the photocatalytic cycle. In the process metal ions are reduced, precipitated and removed from the solution. Thus photocatalytic treatment for decontamination of waste waters from both organic and inorganic pollutants (metal ions) can be, in principle, achieved. POM+ S hν −→ POMred + Sox , POMred +Mn+ −→ POM+Mred. Atrazine, a widely used herbicide with s-triazine structure, is photodecomposed to cyanuric acid in presence of SiW12O40 4− in aqueous solutions. However, no precipitation of metal ions is obtained when atrazine is the sole organic substrate in aqueous solutions. The low solubility of atrazine produces insufficient concentration of reduced POM to cause precipitation.


INTRODUCTION
Polyoxometalates (POM) is a large category of metal oxygen cluster anions with well defined structures and properties, of diversified interest.Their redox chemistry is characterized by their ability to act as multielectron relays.POM have been termed soluble anodes and indeed they behave so under irradiation at the O −→ M CT band (near visible and UV area), oxidizing a great variety of organic compounds [1].The oxidation leads to the mineralization of organic compounds and for that matter organic pollutants in aqueous solutions [2].The mechanism seems to be mainly via the formation of the highly oxidizing OH radicals from the reaction of the excited POM with H 2 O.In the process electrons accumulate on POM driving the redox potential to more negative values, until a species in solution is able to accept the electrons.Thus, now it can be said that they are converted to soluble cathodes, able to reduce a diversified number of chemicals.The nature of POM and the degree of reduction reflects their reducing capability.Thus H + have been reduced to H 2 , nitrobenzene to aniline, dioxygen to O 2 − etc.
This paper presents an overall view of the ability of POM to: (a) Mineralize (to CO 2 , H 2 O and inorganic anions) a great variety of organic pollutants and (b) to cause, at the same time, reduction (recovery) of metal ions from aqueous solutions.It also presents: (c) the photodecomposition of atrazine in connection with the reduction of various metal ions.

EXPERIMENTAL
Aqueous solution of the organic substrate (2,4 dichlorophenol 5 × 10 −4 M or atrazine 7 × 10 −5 M; 4 ml) containing catalyst (H 3 PW 12 O 40 or K 4 SiW 12 O 40 7 × 10 −4 M) and metal ions (Ag + , Cu 2+ , Ni 2+ or Pd 2+ 1.2 × 10 −3 M) was added in a spectrophotometer cell (1 cm path), deaerated with Ar and covered with a cerum cap.The pH of the solution was 1 for H 3 PW 12 O 40 or 5.5 for K 4 SiW 12 O 40 .Photolysis was performed with an Oriel 1000 W Xe lamp equipped with a cool water circulating filter to absorb the near IR radiation and a 345 or 320 nm cut off filter to avoid direct photolysis of substrates.The photocatalytic process was followed by monitoring the concentration of: a) The organic substrates and intermediates with HPLC reverse phase.The decay of the substrate and the production of intermediates were monitored by HPLC-UV consisted of a Waters Model 600E pump associated with a Waters Model 600 gradient controller, a Rheodyne Model 7725i sample injector equipped with column by Phase Sep (25 cm × 4.6 mm I.D., 5 µm) and Waters Model 486 tunable absorbance detector controlled by the Millenium (Waters) software.Isocratic phase program performed for the determination of atrazine and intermediate photolysis products, was acetonitrile-water (50:50, v/v), at a flow rate of 1.0 ml/min.The wavelength used was 225 nm for the determination of atrazine, CIAT, CAET and CAAT, b) metal ions with Flame Atomic Absorption Spectrometry and c) the reduced catalyst, UV/Vis-spectroscopically (750 nm), by means of a HITACHI U-2000 spectrophotometer.

Overview of photodecomposition of organic substrates and metal recovery upon photolysis of aqueous solutions in the presence of POM. Excita-
tion of POM at the O → M CT band (near visible and UV area) leads, in reality, to electron (e − ) hole (h + ) separation in analogy to metal oxide particulates (semiconductors).The generated photoholes are powerful oxidizing species that either directly or indirectly (through formation of OH radicals) oxidize a great variety of organic compounds and for that matter organic pollutants.Direct oxidation The photodecomposition of organic compounds leads, in most cases, to mineralization, i.e., formation of CO 2 , H 2 O and inorganic anions.So far a great variety of organic pollutants (such as phenols, halophenols, aromatic hydrocarbons, aliphatic compounds etc) have been shown to undergo mineralization upon photocatalytic treatment in the presence of POM [2].
Typical photodecompositions of organic pollutants are shown in Figure 1 [3].Characteristic photodecomposition of organic substrate (lindane) and gradual development of CO 2 and inorganic anions are shown in Figure 2 [3].
A general scheme of photodecomposition of 2,4 dichlorophenol, that seems to work for both POM and metal oxide particulates (TiO 2 ) is presented below (Scheme 1).In the absence of an electron withdrawing species the solution is colored blue from the reduced POM.closes the photocatalytic cycle [1], POM(e − ) + oxidant −→ POM (5) which is shown in Scheme 2. Dioxygen is the most effective oxidant, whereas other oxidants can serve the same purpose [4].
Recently we have demonstrated the ability of photoreduced POM to be oxidized by metal ions

POM(e
The above redox process depends on the reducing ability and the degree of reduction of POM, the oxidizing ability of M n+ and to a lesser extent on the nature of organic substrate.Thus, several metal ions are reduced to lower oxidation states and many, depending on conditions, are recovered (precipitate out) as M 0 [5].recovered when increasing the concentration of the organic substrate (2,4 dichlorophenol or propan-2-ol) by ten times.Thus, by using better oxidizing reagents for reoxidation of the reduced POM, reaction (2) is accelerated and more catalyst in the oxidized form is available to absorb  Figure 7 shows the influence of 2+ , O 2 , Ni 2+ and absence of oxidant in the formation and decay of CIAT, upon photolysis of aqueous solutions of atrazine.

Photodecomposition of atrazine in the presence of
In Figures 8 and 9, the corresponding experiments for CAET and CAAT are exhibited.It should be noted that, under these conditions, no precipitation of metal was observed.The results suggest that: • The main degradation pathway involves dealkylation deamination of the side chains of atrazine, followed by dechlorination.
• The reagent used for reoxidation of the catalyst does not influence the nature of the observed intermediates.
• No precipitation of metals is observed under these conditions.Reaction of Ni 2+ with SiW 12 O 40 5− is thermodynamically forbidden whereas atrazine concentration is too low (15 ppm) to cause Cu removal, as has been observed in other cases.

SiW 12 O 40 4− or H 2 W 12 O 40 6− and M n+ . In
recovered under these conditions.A cloudy solution was formed upon addition of AgNO 3 or PdCl 2 (1.2 mM) in an aqueous solution of SiW 12 O 40 4− (0.7 mM) and atrazine (0.07 mM). be