Catalytic removal of nitrates from waters in a continuous flow process: The remarkable effect of liquid flow rate and gas feed composition

The selective catalytic reduction of nitrates (NO3 -) in water mediums towards N2 formation by the use of H2 and in the presence of O2 (air) in the gas feed has been investigated under a continuous flow process over Pd-Cu supported on various mixed metal oxides, xwt.% MxOy/g-Al2O3 (MxOy=CeO2, MgO, Mn2O3, Cr2O3, Y2O3, MoO2, Fe2O3 and TiO2). It is demonstrated for the first time that a remarkable improvement of both catalysts activity and N2 reaction selectivity can be achieved when increasing the liquid flow rate in the continuous flow process. In particular, it was found that NO3 - reduction rates up to more than two times higher and NH4 selectivity values up to twenty times lower can be obtained when increasing the liquid flow rate from 2 to 6mL/min. Moreover, it was proven for the first time, in a continuous flow process, that the presence of oxygen (or air) has a remarkable positive effect on the reaction's selectivity towards nitrogen. The reaction's selectivity towards NH4 can be decreased by up to three times when 20vol.% air is added in the gas feed of the NO3 -/H2 continuous flow reaction. The Pd-Cu clusters supported on TiO2-, CeO2- and MgO-coated g-Al2O3 spheres showed the best catalytic behaviour compared with the rest of supports examined, both in the presence and in the absence of oxygen in the reducing feed gas stream. In addition, it was found that the initial concentration of nitrates in the liquid feed can significantly affect catalysts activity and reaction's selectivity. A positive apparent reaction order towards nitrates of 0.9 was calculated on Pd-Cu/TiO2/Al2O3. 2010 Elsevier B.V

Mechanistic aspects (SSITKA-DRIFTS) of the catalytic denitrification of water with hydrogen on Pd-Cu supported catalysts

Detailed mechanistic studies (ex-situ SSITKA-DRIFTS) have been performed on 1 wt.% Pd-0.5 wt.% Cu/γ-Al2O3 and 1 wt.% Pd-0.5 wt.% Cu/TiO2-Al2O3 catalysts concerning the NO3−/H2 and NO3−/H2/O2 reactions, in order to elucidate the promoting role of TiO2 and O2 in suppressing the unwanted NH4+ production in water media. It is demonstrated, for the first time ever, that the mechanism of N2 production strongly depends on the nature of the support and the presence of O2 (air) in the gas feed stream. In particular, these parameters were found to significantly affect the formation of different adsorbed active intermediate N-species on the support or/and metal (Pd, Cu) surface, providing documentary information about the signalling pathways leading to the formation of NH4+ and N2. This study provides for the first time ever, an alternative stepwise pathway for the reduction of NO2−(ads) to NO(ads) and further to N2 on the support or metal-support interface (metal cation sites), instead of Pd metal surface. Based on the results of the present work, it is concluded that the reduction of NO2−(ads) species is favoured on partially oxidized Pd (when adequate supply of oxygen is available). In the case of Pd-Cu/TiO2-Al2O3 (NO3−/H2), both adsorption and reduction of NO2−(ads) take place on the support surface leading to the formation of NO(ads) on TiO2 (Ti4+-NO or Ti4+-NO+), which in turns leads to enhanced N2 production. In addition, the latter system exhibits greatly enhanced selectivity towards N2, under oxidizing conditions, possibly due to the interaction of NOx species (e.g., Pd-NO/N, Ti4+-NO/NO+) at the metal-support interface. On the contrary, in the absence of both titanium dioxide in the support and oxygen in the feed, the sequential reduction of NO3−(ads) takes place on different active sites of the catalyst surface, indicating that H2 is easily dissociated on Pd particles and then spills over onto the Cu and the metal oxides (support), and secondly that NO(ads) is possibly diffused from the support to adjusted Pd sites, for further reaction. These mechanistic findings are very important as they reveal, for the first time ever, the active involvement of the support in the reaction mechanism and its positive effect on N2 production

Assessment of toxic heavy metals concentrations in soils and wild and cultivated plant species in Limni abandoned copper mining site, Cyprus

Mine tailings represent a serious source of soil pollution with public health implications. The objectives of this study were(1) to assess the level of toxic and heavy metal mobilization from the tailing spoil-heap of Limni abandoned mine at Cyprus and the extent of soil contamination to the surrounding area by using pollution indicators; (2) to investigate the uptake and accumulation of heavy metals by cultivated crops; and (3) to estimate the potential of native wild plant species grown in the studied area to be used in phytomanagement approaches. The tailing spoil heap exhibited significantly higher S, Zn, Cu and Pb concentrations compared to the ones found in control reference samples (RS). The lateral mobilization of Mg, S, Zn, Cu and Pb resulted to the contamination of the tailing surrounding areas with these metals. Moreover, Mn and Cu concentrations in the tailing and the surrounding areas exceeded the MPLs for agricultural soils. The severe to very severe pollution of the tailing surrounding sides with S, Zn and Cu was also evident by the calculated values of enrichment factor and geoaccumulation index. The values of combined pollution index also uncovered the extremely high pollution of the tailing and the moderate pollution of the surrounding sites with all analyzed elements. The concentration of Cd in fig, peanut and lemon fruits, as well as in the grains and straw of barley exceeded MPLs, highlighting the potential Cd-mediated hazardous effects from the consumption of these produces. The examination of heavy metal content in wild native plant species showed that Inula viscosa L. has the potential to be used for the phytostabilization of Cd and Pb, and Allium ampeloprasum L. for the phytostabilization of Pb. Overall, results suggest that the Limni mine tailing and its surrounding sites are highly polluted; thus agricultural activity in the studied area should be prohibited and phytomanagement should be urgently carried out

Catalytic removal of pharmaceutical compounds in water medium under an H2 stream over various metal-supported catalysts: A promising process

All rights reserved.Abstract: To date, very few prescriptive studies have been reported in the literature concerning the catalytic removal of pharmaceutical substances in wastewater using H2 in the presence of O2 for the in situ formation of H2O2, while the mechanism of the reaction has not been studied in detail yet. Hydrogen peroxide is a potent oxidizing agent used extensively in catalytic wet air oxidation (CWAO) applications and can be used for the elimination of pharmaceuticals from waste water. In the present work, an attempt has been made to elucidate the actual effects of the in situ production of hydrogen peroxide on the CWAO of pharmaceuticals. Therefore, the effects of the nature of the active phase (Pd, Pt, and Rh), as well as the feed gas composition have been examined toward the reaction at hand. The results showed that 1% Pd/Al2O3 and 1% Rh/Al2O3 are the most effective catalysts for the elimination of paracetamol from the reaction medium using hydrogen-rich streams, having a conversion of up to 70% in 2 h. A maximum conversion of paracetamol of 90% was obtained in just 30 min of reaction over 1 wt.% Rh/Al2O3, when using pure hydrogen in the feed. Total organic carbon measurements performed over the latter catalyst showed that practically no organic carbon is removed from the liquid phase, indicating the conversion of paracetamol to a different organic (probably aromatic) compound, through hydrogenation. Toxicity tests that followed showed a dramatic decrease in the toxicity of the products solution, indicating that paracetamol hydrogenation might be a promising method for the elimination of its toxicity

Novel catalytic and mechanistic studies on wastewater denitrification with hydrogen

Presented at: IWA Regional Conference on Waste and Wastewater Management, Science and Technology, 2013, Limassol, Cyprus, 26-28 JuneThe present work reports up-to-date information regarding the reaction mechanism of the catalytic hydrogenation of nitrates in water media. In the present mechanistic study, an attempt is made, for the first time, to elucidate the crucial role of several catalysts and reaction parameters in the mechanism of the NO3-/H2 reaction. Steady-state isotopic transient kinetic analysis (SSITKA) experiments coupled with ex situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were performed on supported Pd-Cu catalysts for the NO3-/H2 and NO3-/H2/O2 reactions. The latter experiments revealed that the formation and surface coverage of various adsorbed active intermediate N-species on the support or Pd/Cu metal surface is significantly favored in the presence of TiO2 in the support mixture and in the presence of oxygen in the reaction's gaseous feed stream. The differences in the reactivity of these adsorbed N-species, found in the present work, adequately explain the large effect of the chemical composition of the support and the gas feed composition on catalyst behaviour (activity and selectivity). The present study leads to solid mechanistic evidence concerning the presence of a hydrogen spillover process from the metal to the support. Moreover, this study shows that Cu clusters are active sites for the reduction of nitrates to nitrites

The effect of several parameters on catalytic denitrification of water by the use of H2 in the presence of O2 over metal supported catalysts

The present paper involves a detailed study of the selective catalytic reduction of nitrates in aqueous mediums by the use of H2 in the presence of O2 over monometallic and bimetallic supported catalysts. In this study, an attempt has been made to improve the denitrification efficiency (XNO3-, SN2) of several catalysts by regulating some experimental parameters that are involved in the process. Therefore, the effects of the type of reactor (semi-batch reactor vs continuous flow reactor), the nature of the active phase (Pd, Cu, and Pd-Cu) and the particle size of γ-Al2O3 spheres (particle diameter= 1.8 mm and 3 mm) on catalytic activity and reaction selectivity, as well as the adsorption capacity of γ-Al2O3 spheres for nitrates, were examined. As the review indicates, most of the research has so far been conducted on batch or semi-batch reactors. This study successfully demonstrates the benefits of using a continuous flow reactor in terms of catalytic activity (XNO3-, %) and reaction selectivity (SN2, %). Another important aspect of this study is the crucial role of bimetallic Pd-Cu clusters for the prevention of NH4+ formation. Moreover, the use of 1.8 mm diameter γ-Al2O3 spheres as a support was proved to significantly enhance the catalytic performance of bimetallic Pd-Cu catalysts towards nitrate reduction compared to 3 mm diameter γ-Al 2O3 spheres. This difference may be attributed to mass (NO3-, OH-) transfer effects (external mass transfer phenomena)