Photocatalytic reduction and recovery of mercury by polyoxometalates.

Photocatalytic reduction of mercury in aqueous solutions using PW12O40(3-) or SiW12O40(4-) as photocatalysts has been studied as a function of irradiation time, concentration of Hg(II), polyoxometalate, and organic substrate in the presence or absence of dioxygen. The photocatalytic cycle starts with irradiation of polyoxometalate, goes through the oxidation of, for instance, propan-2-ol (used as sacrificial reagent), and closes with the reoxidation of reduced polyoxometalate by Hg2+ ions. Mercury(II) is reduced to mercury(I) and finally to Hg(0) giving a dark-gray deposit, following a staged one-by-one electron process and a first-order kinetics in [Hg2+]. The process is slightly more efficient in the absence of dioxygen, while the increase of either catalyst or propan-2-ol concentration results in the augmentation of the rate of reduction till a certain point where it reaches a plateau. The results show that this method is suitable for a great range of mercury concentration from 20 to 800 ppm achieving almost complete recovery of mercury up to nondetected traces (<50 ppb). In addition, this homogeneous process demonstrates advantages such as the lack of necessity for separation of the zero state metal from the catalyst and ensures that the precipitation of metal will not poison the catalyst or hinder its photocatalytic activity.

Photocatalytic reduction and recovery of copper by polyoxometalates.

A series of polyoxometalates PW12O40(3-), SiW12O40(4-), and P2Mo18O62(6-) have been used as photocatalysts for recovery of copper and production of fine metal particles. The process involves absorption of light by polyoxometalates, oxidation of an organic substrate, for instance, propan-2-ol as sacrificial reducing reagent, and reoxidation of the reduced polyoxometalates by Cu2+ ions, closing the photocatalytic cycle. Copper(II) ions are reduced to copper(I) and finally to zero-state particles in a 2-electron process, as also suggested by the half-order dependence. Increase of catalyst or propan-2-ol concentration, or both, accelerates the photodeposition of copper until a saturation value is reached. The method is operational at a wide range of copper concentrations varying from 3 to 1300 ppm, leading to very low final concentrations (<0.2 ppm). The presence of dioxygen suppresses the initiation of copper recovery, though the process is equally effective after dioxygen is consumed. The process is independent of pH within the range 0.3-5.0. Addition of ClO4-, NO3-, or CH3COO- has no effect on the removal of copper ions. Chloride ions retard the enhancement of copper precipitation through stabilization of copper(I). This homogeneous, polyoxometalate-based process exhibits some benefits in comparison with the semiconductor-based (heterogeneous) recovery of metals: The final zero-state metal particles are obtained in pure form. No separation from the catalyst is needed, and moreover, the process is catalytic as the photodeposited metal particulates do not hinder the photocatalytic action of polyoxometalate anions.

Sonolytic, photolytic, and photocatalytic decomposition of atrazine in the presence of polyoxometalates.

Aqueous solutions of atrazine [2-chloro-4-(isopropylamino)-6-(ethylamino)-s-triazine] (CIET) decompose upon illumination with a low-pressure Hg-arc lamp (254 nm). However, no decomposition takes place with lambda > 300 nm. On the other hand, addition of polyoxometalates (POM), PW12O40(3-) or SiW12O40(4-), into a solution of atrazine photodecomposes the substrate within a few minutes (cutoff fiter 320 nm). Ultrasound (US) treatment also decomposes aqueous solutions of atrazine within a few minutes. Both methods, sonolysis and photolysis with POM, give common intermediates, namely, 2-hydroxy-4-(isopropylamino)-6-amino-s-triazine (OIET), 2-chloro-4-(isopropylamino)-6-amino-s-triazine (CIAT), 2-chloro-4-amino-6-(ethylamino)-s-triazine (CAET), 2-hydroxy-4,6-diamino-s-triazine (OAAT), and 2-hydroxy-4-hydroxy-6-amino-s-triazine (OOAT) among others. The final products for both methods, US and photolysis with POM, were cyanuric acid (OOOT), NO3-, Cl-, CO2, and H2O. OOOT showed no signs of decomposition by sonication and/or photolysis with POM. It also resisted degradation upon photolysis with plain UV light (254 nm). However, it has been reported to decompose upon photolysis with lambda > 200 nm. Combination of US and photolysis with POM produces only a cumulative effect.