Catalytic Wet Air Oxidation of Aqueous Solutions of Ammonia in a Continuous-flow Trickle-bed Reactor over Metal Supported on Carbon Materials

In this study, aqueous solutions of 600 mg L of ammonia were oxidized in a trickle-bed reactor using Cu-activated carbon fiber (ACF) catalysts, which were prepared by incipient wet impregnation with aqueous solutions of copper nitrate that was deposited on ACF substrates. Experimental results showed that the total conversion efficiency of ammonia was 93% during wet oxidation over the catalyst at 463 K at an oxygen partial pressure of 3.0 MPa. Moreover, the effect of the initial concentration of ammonia and the reaction temperature on the removal of ammonia from the effluent streams was also studied at a liquid space velocity of less than 5.4 h. The catalytic behavior is related to the Cu(II) oxide supported by ACF. *Corresponding author Email: [email protected] INTRODUCTION Ammonia is used widely and in large quantities for various processes, including manufacture of ammonium nitrate, ammonia, urea, ammonium phosphate, petroleum refineries and coke, among others. Wastewaters containing ammonia are commonly either toxic or have concentrations or temperatures that prevent direct biological treatment. The conventional biological, physical and chemical treatment processes, including biological nitrification, activated carbon adsorption, ozonation and ion exchange processing, produce only phase transformations and may generate contaminated sludge and/or adsorbent, which require further disposal. Therefore, the removal of ammonia from air and waste streams is an important problem. Wet oxidation technology was originally developed to oxidize organic substances in solution into intermediate products with low molecular weights at temperatures between 398 and 623 K under pressures ranging from 0.5 to 20 MPa. However, ammonia is normally an end product of the wet oxidation process and thus is difficult to be oxidized. Mishra et al. [1] explored wet oxidation processes and it is a promising means of pretreating wastewater containing ammonia at concentrations of up to 600 mg L [2]. However, the efficient elimination of ammonia by noncatalytic wet oxidation stipulates excessive pressures of up to 4.0 MPa and a high temperature (513 K), seriously affecting the economic workability of this technology. Catalytic wet oxidation (CWO) is known to increase the applicability of wet oxidation technology using dedicated catalysts, which potentially promote oxidation in a shorter reaction period under milder operating conditions. Furthermore, ammonia pollution can be eliminated by the selective catalytic oxidation of ammonia-containing water to molecular nitrogen and water [3-6]. For instance, Levec and Pintar [7] showed that the wet oxidation of aqueous solutions of organics from wastewaters at low temperatures and pressures was easier with heterogeneous catalysts than without them. Moreover, Inazu et al. [8], who developed several heterogeneous catalysts for wet oxidation, found the 2 wt% Pt/ activated carbon catalyst to be more active than a Pt/MgO catalyst in the wet oxidation of ammonia. The 2 wt% Pt/AC catalysts were active in the reaction at a temperature above 453 K and a pressure of 0.5 MPa. Similarly, Ding et al. [9] studied 252 Sustain. Environ. Res., 20(4), 251-255 (2010) the oxidation of ammonia in supercritical water in the temperature range 683-743 K at 27.6 MPa. They observed that ammonia conversion reached 96% on an MnO2/CeO2 catalyst. The activated carbon fiber (ACF) carbonaceous material as a support for the catalyst is attracting increasing interest because it has small fiber diameters, large contact surface areas, highly dispersed impregnated metal and a narrow range of sizes [10,11], all of which can be exploited in various industrial processes. However, little research has been conducted on the application of Cu-ACF catalyst in CWA. Consequently, this study explores the activity of the CuACF catalyst of the oxidation of ammonia solutions as a function of various parameters. The characteristic of Cu-ACF catalyst was also discussed. MATERIALS AND METHODS Cu-ACF catalysts used in this work were prepared by the incipient wet impregnation of copper(II) nitrate (GR grade, Merck, Darmstadt, Germany) with copper contents 5 wt%. A copper was coated on ACF (around 1100 m g of the specific surface area) substrate (AW-1132, KoTHmex, Taiwan Carbon Technology Co.). The ACF was washed five times with deionized water and dried at 393 K, before being cooled to room temperature. The catalysts were then calcined at 573 K in an airstream for 4 h. The resulting powder was made into tablets using acetic acid as a binder. The tablets were later reheated at 473 K to burn the binder out of the Cu-ACF composite tablets. The tablets were then crushed and sieved into various particle sizes ranging from 0.25 to 0.15 mm for later use. Figure 1 presents the textural characteristics of the Cu-ACF catalyst. The reflectance was measured in relation to a barite (BaSO4) standard. Diffuse reflectance Fourier transformed infrared spectra (FTIR) of species adsorbed on the catalyst were measured at room temperature using a Bruker Vector 22 FTIRattenuated total reflection (FTIR-ATR) with 4 cm resolution (Bruker, USA). Diffuse reflectance FTIRATR spectra of species adsorbed on the catalyst were measured at room temperature using a Bruker Vector 22 FTIR spectrometer, equipped with a diffuse reflectance attachment with a resolution of 4 cm (Bruker, Germany). The changes in the sizes of the catalytic particles were measured using a laser light-scattering particle size analyzer (Coulter LS100, USA). Scanning electron microscopy (SEM, JEOL, JSM-6400, Kevex, DeltaII) elucidated the morphology of the catalysts. All feed solutions were made using Millipore (Bedford, Massachusetts) water (18 MΩ cm), and the pH value of the ammonia aqueous solution was adjusted to 11.5 ± 0.2 using 1 M sodium hydroxide. Figure 2 schematically depicts the CWO system used in this study. The wet oxidation of the ammonia solu100μm Fig. 1. Optical photograph of Cu-ACF catalyst.