Photocatalytic reduction of environmental pollutant Cr(VI) over some semiconductors under UV/visible light illumination
Introduction
Recently, a novel technology, based on photocatalytic process in aqueous suspensions of semiconductors, have received considerable attention in view of solar energy conversion 1, 2. This photocatalytic process was achieved for rapid efficient destruction of environmental pollutants. Upon illumination of the semiconductor-electrolyte interface with light of energy greater than the semiconductor band gap, electron–hole pairs (e-–h+) are formed in the conduction and the valence bands of the semiconductor, respectively. These charge carriers, which migrate to the semiconductor surface, are capable of reducing or oxidising species in solution having suitable redox potentials.
More recently, an interesting technology application of the light driven processes that can occur on irradiated semiconductor photocatalysts, is the recovery of precious metals of strategic and economic importance (platinum, palladium, gold, rhodium, silver, among others), and/or eliminating toxic metals (lead, mercury, chromium, among others) from industrial waste effluents. The metal is reduced on the semiconductor particle surface and is subsequently extracted from the slurry by mechanical and/or chemical means (aqua regia) [3], or can be converted to a less harmful substance 4, 5, 6, 7.
Lead at a certain concentration is a health hazard to both humans and animals, exerting its most toxic effects on the nervous system and kidneys. Lead in the aquatic environment cannot be decontaminated chemically and is not biodegradable. However, it can be removed by photocatalytic methods using photoplatinised titania [8].
Mercury is present mainly in the particulate phase as well as dissolved inorganic and organic mercury species that are accumulatively toxic [9]. The main environmental problem of mercury is the high content of methylated mercury in fresh water making fishes unfit for human consumption [10].
Chromium occurs in two common oxidation states in nature, Cr(III) and Cr(VI). Hexavalent chromium is toxic to most organisms (for concentrations higher than 0.05 ppm), carcinogenic in animals, and causes irritation and corrosion of the skin in humans. It is very soluble in water and forms divalent oxyanions: chromate (CrO42−) and dichromate (Cr2O72−). Because it is only weakly sorbed onto inorganic surface, Cr(VI) is also mobile in nature. On the other hand, Cr(III) is readily precipitated or sorbed on a variety of inorganic and organic substrates at neutral or alkaline pH. Cr(VI) has a toxicity one hundred times higher than that of Cr(III).
Cr(VI) which generates in effluent streams comes mainly from chrome plating and leather tanning industries. Conventional methods for treatment of contaminated Cr(VI) include chemical reduction, ion exchange, adsorption on coal or activated carbon, and bacterial reduction. However, most of these methods require either high energy or large quantities of chemicals, and the photocatalytic process was found superior. The photocatalytic reduction of Cr(VI) in alkaline medium (pH=10) simultaneously facilitates the immobilization of Cr(III) on the photocatalyst surface via Cr(OH)3 formation [11]. A subsequent pH adjustment removes this chromium and generates the catalyst, i.e. this sequence fulfils the reduction, immobilization and concentration of the waste in one step.
The utilization of photocatalysts in reducing Cr(VI) has been reported in literature. The photocatalysts used are mainly CdS 12, 13, ZnS [12], WO3 12, 14, various types of TiO2 14, 15and ZnO [16]. Recently, platinised titanium dioxide (Pt/P25) [17]and MoS2 [18]each in acidic medium, as well as ZnTe [18]in neutral solution, were all found to photocatalyse the reduction of Cr(VI). ZnO is an interesting catalyst and has received much attention due to its low cost of production, its high photoactivity in several photochemical and photoelectro-chemical processes 6, 7, 19, and its UV light response of band gap 3.2 eV. Moreover, in studying the stability of TiO2 in acidic media, photocorrosion of TiO2 is expected, and Cr(VI) can photoetch the catalyst surface [20]. These conclusions seriously undermines the notion that TiO2 would be the effective photocatalyst for the treatment of Cr(VI) in acidic media [21]. The aim of the present work is to study the different factors that affect the photocatalytic reduction of Cr(VI) to correlate the obtained results with that reported in the literature.
Section snippets
Experimental
ZnO and WO3 are BDH grade chemicals. TiO2 (Degussa P25) is predominantly anatase. Hombikat UV100 (Sachtleben Chemie GmbH, Germany) is another commercial titanium dioxide of superior characteristics [22]. The BET surface areas of both WO3 and ZnO were measured experimentally by nitrogen adsorption at −196°C to be ≈9 and 34 m2g−1, respectively. These values are low compared to those reported for the International Samples: Degussa P25 and Hombikat UV100, where the areas are 59 [23]and 250 m2g−1 [22]
Results and discussion
In order to investigate reactions other than the photochemical reduction, a blank test on irradiated reaction solution containing Cr(VI) of 100 ppm concentration under continuous stirring without a photocatalyst was carried out. The results showed that the homogeneous photochemical reaction of Cr(VI) after 6 h irradiation is so small to be negligible, which is in agreement with a previous report [15]. Another blank test on a reaction solution containing Cr(VI) of similar concentration as above
Conclusions
The major conclusions of this study are as follows:
- 1.
Cr(VI) photoreduced was increased with decreasing pH values, suggesting an acid-catalysed behaviour.
- 2.
First-order kinetics were observed from the results at different initial concentrations of Cr(VI).
- 3.
An optimum value of the mass of ZnO of 12 g per dm3 of solution was obtained, where a maximum light absorption by ZnO was ascertained.
- 4.
For the same amount of the different semiconductors and at the same initial Cr(VI) concentration and at pH 6, the
Acknowledgements
We are grateful to Degussa AG, Germany for donating the P25, an anatase TiO2 sample. We are also grateful to Sachtleben Chemie GmbH Germany for supplying the Hombikat UV100, another anatase TiO2 sample.
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