Ind. Eng. Chem. Res. 2010, 49, 5603–5609
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Denitrification of Water with Activated Carbon-Supported Metallic Catalysts Luisa Calvo,* Miguel A. Gilarranz, Jose´ A. Casas, Angel F. Mohedano, and Juan J. Rodriguez Seccio´n Departamental de Ingenierı´a Quı´mica, Facultad de Ciencias, UniVersidad Auto´noma de Madrid, Cantoblanco, 28049 Madrid, Spain
The catalytic reduction of nitrates with hydrogen in aqueous phase was studied in a trickle bed reactor using different activated carbon-supported metallic catalysts. Continuous experiments were performed at ambient conditions (25 °C and 1 atm) at different space-time values in the range of 25.8-103.3 kgcat h/mol. The activity of monometallic catalysts based on Pd, Rh, Cu, and Ni was low in all the cases, yielding nitrate conversions below 15%. The introduction of a second metal produced a synergistic effect, improving the catalytic activity and the selectivity toward N2. That increase in activity was more pronounced when using Pd or Rh combined with Cu, reaching nitrate conversions above 80% in the case of Pd-Cu catalysts. The catalyst with a Pd/Cu mass ratio of 2:1 (Pd050Cu025) also showed the lowest selectivity to nitrite and the highest to N2. Metal leaching was always below detection limits for Pd and Rh, whereas in the case of Cu and Ni, significant metal concentrations were found in the reaction effluent, the leaching of Ni being higher. The initial pH significantly affected both activity and selectivity of the bimetallic catalysts, the maximum activity being achieved within the 6-7 range, which also led to the lowest Cu leaching. The use of a twostep reaction system was checked using two different catalysts, a bimetallic Pd-Cu in the first and a monometallic Pd catalyst in the second. With this two-step system, an important reduction of the nitrite originating in the first stage was observed, and the formation of N2 at the expense of ammonium was favored. 1. Introduction Nitrate concentrations in surface waters and especially in ground waters have increased in many locations in the world and are the main sources of such contamination from the intense use of fertilizers in agriculture, human sewage, and livestock manure.1 Ingestion of nitrate with drinking water causes methemoglobinemia through reduction to nitrite. Some studies suggest that nitrate is a precursor of carcinogenic nitrosamines.2 Although European legislation has established a maximum allowable concentration of 50 mg/L for nitrates in drinking water, the U.S. Environmental Protection Agency has lowered that level to 10 mg/L. Therefore, reducing the nitrate concentration in drinking water is imperative nowadays in many places. Nitrate is a stable and highly soluble ion with low potential for coprecipitation or adsorption. These properties make it difficult to remove using conventional water treatment technologies, such as lime softening and filtration.3 Physical-chemical processes such as ion exchange, reverse osmosis, or electrodialysis allow effective removal of nitrate but give rise to secondary waste streams. Among these methods, ion exchange has the lowest capital and operating costs, but disposal of spent brine from regeneration is fairly difficult and costly in noncoastal locations.4 Reverse osmosis requires a high energy input and has problems associated with the use of membranes, including their high cost, fouling, compaction, and deterioration with time. Since the use of reverse osmosis produces water with low total dissolved solids, it is an interesting option in industries such breweries. Electrodyalisis results are only adequate for soft waters and have limited potential for full-scale application.3 The most promising techniques for nitrate removal without generating secondary wastes are biological denitrification and catalytic denitrification. The first one is an effective method commonly used for the treatment of municipal and industrial wastewaters. Nevertheless, application to drinking water puri-
fication is not likely due to the problems derived from bacterial contamination, the presence of residual organics, and the possible increase in chlorine demand.2 Catalytic denitrification is viewed as a promising emerging technology for the removal of nitrates from water.5,6 One of the advantages is that the process can be performed at ambient temperature and pressure. In most cases, hydrogen has been used as a reducing agent, although formic acid has also been tested.7 The use of formic acid has been shown to require a high amount of this reagent, which makes necessary the removal of the remaining excess. Thus, the use of hydrogen as the reducing agent is better. The widely accepted reaction scheme for catalytic hydrodenitrification (Scheme 1) shows that it is a complex multistage process with nitrate reduction to nitrite in a first stage and then to gaseous N2, ammonium, and hydroxide ions, probably through the formation of NO as an intermediate. Since the undesired product ammonium can be formed simultaneously to N2, the selectivity of the catalyst is a main feature. Catalytic denitrification has been reported over a variety of catalysts based on metals such as Pd,8,9 Pt,10 Pt-Cu,10 Pd-Sn,11 Pd-In,11 and Pd-Cu,7,8,12 the last being the most active. Selection of the support is also of importance, and different materials have been used, including alumina,4,6,12 titania,9,13 niobia,1 zirconia,14 and activated carbon.5 However, there is still a lack of information on the role of the support in both activity and selectivity in the literature. Activated carbon is attractive as a catalyst support due to both its physical and chemical properties’ being advantageous to its high surface area, Scheme 1. Catalytic Reduction of Nitrate with Hydrogen
* To whom correspondence should be addressed. Tel.: +34 914978774. Fax: +34 914973516. E-mail:
[email protected]. 10.1021/ie100838r 2010 American Chemical Society Published on Web 05/24/2010
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Figure 1. Nitrate conversion and selectivities of the monometallic catalysts at 0.5% metal load (τ: 52 kgcat h/mol).
Figure 2. Nitrate conversion and selectivities of the bimetallic catalysts at 0.5% total metal load (τ: 52 kgcat h/mol).
which allows a high dispersion of the metallic phase.15 The recovery of the metal phases from the waste catalysts by burning the carbon material is very easy. Moreover it is a versatile material, since its surface chemistry can be modified depending on the specific needs. The aim of this work is to study the catalytic reduction of nitrate from aqueous solution using hydrogen with different activated carbon-supported metal catalysts, whose activity and selectivity will be checked. Furthermore, since most of the studies on catalytic denitrification have been carried out without pH control, there is scarce information in the literature on the effect of this variable. In this study, we analyze that effect on the activity and selectivity of the catalysts tested. As a final approach, the use of a two-step process is proposed, in which the denitrification is carried out sequentially in a trickle bed reactor. 2. Experimental Section 2.1. Preparation and Characterization of the Catalysts. The catalysts were prepared by incipient wetness impregnation using as the support a commercial activated carbon (AC) supplied by Merck, (1-2 mm size fraction). The elemental analysis of the activated carbon was 86.2% C, 0.9% H, 0.7% N, 0.6% S, and 2.5% ash. Pd, Rh, Cu, Ni, and a combination thereof at a nominal loading of 0.25-0.5 wt % were used as active phases. The precursor salts used were PdCl2 and RhCl3, dissolved in 0.1 and 0.2 M HCl, respectively and CuCl2 · 2H2O and Ni(NO3)2 · 6H2O dissolved in water. Impregnation was followed by drying at room temperature for 2 h and overnight at 100 °C. Finally, the catalysts were calcined at 200 °C and reduced in H2 (50 N mL/min) at 100 °C.
The BET surface area of the activated carbon was obtained from the 77 K N2 adsorption-desorption isotherm (Autosorb1, Quantachrome). The samples were previously outgassed for 8 h at 10-3 Torr and 150 °C. The porous structure of the activated carbon was the following: BET area 917 m2/g, external area 119 m2/g, micropore volume 0.37 cm3/g, and mesopore volume 0.14 cm3/g. The metal content of the catalysts was analyzed by total reflection X-ray fluorescence, using a TXRF EXTRA II spectrometer. The particle size distribution of the metallic phase was determined by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) using a JEOL 2100F microscope with a point resolution of 0.19 nm coupled with an energy-dispersive X-ray spectrometer (EDXS; INCA x-sight, Oxford Instruments) used for chemical elemental analysis. The samples were prepared by suspending the ground catalysts in ethanol and sonicating for 1 h. A drop of the suspension was deposited on a lacey-carbon/ Ni, followed by drying at ambient conditions for 24 h before analysis. The bimetallic catalysts were identified by the symbols of the corresponding metals, followed by the percent metal load (i.e., Pd025Cu025 refers to the catalyst prepared with Pd and Cu with a nominal 0.25 wt % load of each metal). In the case of monometallic catalysts, the metal load was omitted, since 0.5 wt % was always used. 2.2. Reaction Setup and Experimental Procedure. Catalytic nitrate reduction was carried out in a trickle bed reactor (9 mm i.d. borosilicate glass) with cocurrent downflow of both phases, liquid and gas. The runs were performed at ambient conditions (25 °C and 1 atm). An aqueous solution of nitrate (100 mg/L) was continuously fed by means of a chromatographic pump (Gilson 307) at a flow rate between 0.05 and 0.2
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Figure 3. Nitrate conversion and selectivities of the bimetallic catalysts at 1% total metal load (τ: 52 kgcat h/mol). Table 1. Influence of the Total Load and Pd/Cu Ratio on Nitrate Conversion and Selectivity catalyst
XNO3 (%)
SNO2 (%)
SNH4+ (%)
SN2 (%)
Pd025Cu025 Pd050Cu050 Pd050Cu025
77.3 73.2 82.5
22.2 28.8 16.5
50.5 29.3 38.7
27.2 41.9 44.8
mL/min to cover a wide range of space-time (τ: 25.8-103.3 kgcat h/mol). Hydrogen was continuously passed at a flow rate of 50 N mL/min. Samples were periodically taken from the reactor exit. Stabilization of the reactor took in all the experiments less than 20 h. Some experiments were carried out under controlled pH within the 5-8 pH range, using acetic acid/acetate and sodium phosphate hydrate/sodium phosphate dibasic heptahydrate buffer solutions. The concentrations of nitrate and nitrite were determined by anionic suppression IC (Metrohm, mod. 761 Compact IC) with a conductivity detector, using a Supp 5 column (25 cm long, 4 mm diameter) and a mixture of 1 mM NaHCO3 and 3.2 mM NaCO3 aqueous solution as the mobile phase. Ammonium ion and the content of Cu and Ni in the reactor effluent were measured by colorimetric methods (Orbeco-Hellige 975 MP), and some results were confirmed by total reflection X-ray fluorescence (TXRF), using a TXRF EXTRA II spectrometer. This last technique was also used to analyze Pd and Rh in the effluent. The amount of N2 was calculated as the difference between the inlet nitrate concentration and the sum of the nitrate, nitrite, and ammonium concentrations in the effluent. It was assumed that nitrite and ammonium were the only reaction byproducts in the liquid phase. 3. Results and discussion 3.1. Monometallic Catalysts. In a first approach, the catalytic reduction of nitrate was studied using Pd, Rh, Cu, and Ni catalysts supported on activated carbon. The election of these metals was based on the high catalytic activity showed in other reductive processes, such as hydrodechlorination.16-18 As can be seen in Figure 1, the activity of the monometallic catalysts was low in all the cases, since the conversion of nitrate was below 15%. These results are in agreement with those reported by other authors under similar operating conditions.19,20 This behavior can be associated with the low adsorption of nitrate on monometallic metal sites, as confirmed in the case of Pd by Prusse and Vorlop.7 Although the catalytic activity is quite low for all the monometallic catalysts tested, Figure 1 shows different selectivities, Pd being the most selective toward nitrite (the reaction product from the first reduction stage) and N2 (the desired final product). Selectivity toward reaction products was
Figure 4. Influence of space-time on nitrate conversion and selectivity with the Pd25Cu025 catalyst.
always expressed as millimoles product per millimoles of reactant converted. 3.2. Bimetallic Catalysts. In view of the poor results observed for the monometallic catalysts, the option of preparing and testing catalysts with the active phase consisting in combinations of two metals was considered. Thus, bimetallic catalysts were prepared with a total metal load of 0.5 and 1.0 wt %, always using a 1:1 metals mass ratio. The combination of Cu with Pd and Rh produced a significant synergistic effect with a substantial enhancement of the activity compared to the monometallic catalysts, as can be seen from the conversion values of Figures 2 and 3. With the Pd-Cu catalyst, around 80% nitrate conversion was achieved in the conditions of these experiments. The selectivity to N2 was also improved, although the main reaction product was ammonium and the relative amount of nitrate was still quite significant. Increasing the metal load up to 1% allows a certain improvement of nitrate conversion in the case of Rh-Cu catalyst, but not for the Pd-Cu. However, in all cases, the selectivity to N2 improves at the expenses of ammonium, this effect being more pronounced in the case of the Rh-Cu catalyst. Combinations with Ni yielded fairly poor results, comparable to the obtained with monometallic catalysts in terms of nitrate conversion, although with a higher selectivity to N2. These results are in agreement with those reported in the literature, in which Pd-Cu over different supports (alumina, titania, activated carbon, and activated carbon cloth) has been recognized as the preferred catalytic system for denitrification.5,7,12,13,19,21 It must be pointed out that the metal load in the current work is quite a bit lower than that used in most of the studies of the literature. The positive effect of increasing the metal load on
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Figure 5. TEM (a) and STEM (b and c) micrographs of the Pd025Cu 025catalyst and TEM (d) and STEM (e and f) micrographs of the Pd050Cu025 catalyst.
Figure 6. Metal particle size distribution of the Pd025Cu025 (a) and the Pd050Cu025 (b) catalysts.
the selectivity to N2 had been already observed by Yoshinaga et al.19 for 0.5 and 1% total metal load Pd-Cu catalysts. Maintaining the Pd load at 0.5% and reducing the Cu content from 0.5 to 0.25% increased nitrate conversion moderately (from 73 to 83%) and improved slightly the selectivity to N2 (from 42 to 45%), mostly at the expense of nitrite, thus indicating a higher extension of the reductive process. The influence of the Pd/Cu mass ratio was also studied; the results are summarized in Table 1. As can be seen, the catalysts exhibited a good activity with nitrate conversion values ranging from 73.5 to 82.5%. The highest activity was achieved with the catalyst with a 2:1 Pd/Cu ratio (Pd050Cu025). This catalyst also yielded a lower selectivity to nitrite, which can be attributed to a displacement to end chain reaction products. The result shows that increasing the Pd/Cu ratio promotes the reduction
of both nitrate and nitrite; however, the amount of ammonium produced from them is large. With respect to N2 formation, it was observed that a Pd/Cu mass ratio of 2:1 produced a favorable effect, since the selectivity toward N2 was higher, even though the total content of metal was not the highest tested. For a bimetallic catalyst, an increase in metal load increased the nitrate conversion and selectivity toward N2. 3.3. Effect of Space-Time. Figure 4 shows the effect of the space-time on nitrate conversion and selectivity with the Pd025Cu025 catalyst. A pronounced increase in conversion is observed within the space-time range of 25.8-52 kgcat h/mol, and no substantial improvement occurred beyond that range. Space-time also showed a remarkable influence on products distribution. Thus, selectivity toward nitrite, a primary intermediate, decreases as space-time increases, whereas the selectiv-
Ind. Eng. Chem. Res., Vol. 49, No. 12, 2010 Table 2. Metal Leaching of the Catalysts Tested (tos: 28 h) leaching (%) catalyst Ni Cu Rh Pd Rh025Ni025 Rh050Ni050 Rh025Cu025 Rh050Cu050 Pd025Ni025 Pd050Ni050 Pd025Cu025 Pd050Cu050 Pd050Cu025
Rh or Pd
