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INTRODUCTION
Contamination of nitrate in water resources has become a severe environmental problem. Excess nitrate in water can cause water pollution (such as river eutrophication and water quality deterioration). On the other hand, nitrate can also be reduced to nitrite, which poses several health threats to humans, such as liver damage and even cancer (Hosseini et al. 2011). Nitrate in wastewater mainly comes from agricultural activities, domestic and industrial sewage (Hwang et al. 2010). With aggravation of water pollution, efficient technologies for nitrate elimination have attracted much attention.
Among the existing technologies for nitrate removal, physico-chemical denitrification, biological and chemical reductions have been widely used (Kim et al. 2013). However, physico-chemical methods like ion exchange, reverse osmosis and electrodialysis, require frequent regeneration of the medium and further treatment for the secondary waste produced (Soares et al. 2008). Biological denitrification can achieve high nitrate removal. However, this process is complex and requires monitoring of the carbon source (Subramanyan et al. 2010). To date, chemical catalytic reduction of nitrate has been regarded as one of the promising techniques to reduce nitrate in wastewater.
Vorlop & Tacke (1989) proposed the new method of catalytic reduction to reduce nitrate in water. Since then, this potential technology has been gradually accepted by researchers. In previous research, H2 and organic acids (e.g. HCOOH) were employed as a reductant for catalytic nitrate reduction. However, the potential risk of explosion, low solubility of H2 in aqueous media, difficulty in operational conditions and potential health problems limit their wide application for nitrate removal (Choi et al. 2013; Kim et al. 2013).
Studies indicate that nitrate removal with bimetallic catalysts are better than those with monometallic ones. A noble metal (such as Pd, Pt, Ir) and a promoter metal (such as Cu, Ag, In) are usually chosen as the active components to coat on different supports (Soares et al. 2011) and it has been proven that Pd-Cu is much more efficient (Aristizábal et al. 2011). In addition to the active components, the supports also play an important role in nitrate reduction. Therefore, different types of materials (such as CeO2, TiO2, SnO2, ZrO2, membrane, activated carbon) used as the supports have been tested for the reduction of nitrate. However, satisfactory catalytic performance could not be obtained...