Combined effect of adsorption and biodegradation of biological activated carbon on H₂S biotrickling filtration

Abstract

In order to evaluate the combined effect of adsorption and biodegradation of H₂S on activated carbon surface in biotrickling filtration, four laboratory-scale biofiltration columns were operated simultaneously for 120 h to investigate the mechanisms involved in treating synthetic H₂S streams using biological activated carbon (BAC). The first three columns (A, B, C) contained a mixture of activated carbon and glass beads, with the carbons (BAC or virgin activated carbon (VAC)) and conditions (with or without liquid medium recirculation) differentiated. The last column (D) used 100% glass beads with liquid medium recirculation. Air streams containing 45 ppmv H₂S were passed through the columns at 4 s of gas retention time (GRT) and liquid flow rate was set at 0.71 ml min⁻¹. Column D got its breakthrough in 3 min of operation, indicating a negligible contribution of glass beads to the adsorption of H₂S. The removal efficiency (RE) of Columns B and C using VAC dropped quickly to 30% within the first 8 h, and afterwards continued to drop further but slowly. Column A using BAC stayed at 25% of RE throughout the operation time. A thorough investigation of the H₂S oxidation products, i.e., various S species in both aqueous (recirculation media) and solid phases (BAC and VAC), was conducted using ICP-OES, IC, XRF, and CHNS elemental analyzer. BAC demonstrated a better performance than columns with adsorption only. Water film was found to enhance H₂S removal. The percentage of sulphate in the total sulphur of the BAC system improved to twice of that of VAC system, indicating sulphate is the main product of H₂S biofiltration. The observed pH drop in BAC system double confirmed that the presence of biodegradation in the biofilm over carbon surface did profound effect on the oxidation of H₂S, compare to the systems with adsorption only.

Introduction

Biofiltration has recently been recognized as one of the most popular and efficient technologies for odour treatment. Two types of biofiltration reactor show the most promise as alternatives to physical and chemical treatments of odour: biofilters and biotrickling filters (Oyarzun et al., 2003, Gabriel and Deshusses, 2004, Chung et al., 2005). Typical biofiltration process consists of two steps: first, the pollutant is transferred from the air stream into liquid film and adsorbed on a solid medium; then the pollutant is biodegraded by microbes living in the liquid phase or on the packing material. Subsequently, the operating conditions of biofilter, supporting material, and inoculated microbes are important parameters to consider.

In terms of the supporting materials, soils are first used as medium of bacteria. However, their tendencies to short-circuit and clog have limited their effectiveness (Carlson and Leisner, 1966). Although compost has good water retention properties, a large density of microorganisms, and a suitable organic content, it suffers from aging effects that require replacement over time (Langenhove et al., 1992). Fibrous peat has been demonstrated to be preferable to soil or compost, but it is naturally hydrophobic, and moisture content of the peat beads is difficult to control (Leson and Winer, 1991). Activated carbon, widely applied in wastewater treatments, has excellent structural properties and good resistance to crushing (Ehrhardt and Rehm, 1985, Walker and Weatherley, 1999, Sirotkin et al., 2001). It has substantial water-holding capacity and provides a good surface for microbial attachment. Furthermore, biological activated carbon (BAC) has been showing the ability to remove gaseous contaminants in biofiltration (Abumaizar et al., 1998, Kim et al., 2002, Moe and Qi, 2004, Duan et al., 2005a). In our previous work, pellet carbon-based BAC has been proved to remove H₂S efficiently at as short as 4 s of gas retention time, indicated by the high elimination capacity (113 g H₂S m⁻³ h⁻¹) and removal efficiency (over 98%) in a biotrickling filter (Duan et al., 2005a). The mechanism of H₂S removal using BAC was also investigated preliminarily with focuses on the changing tendencies of BAC surface properties along the bed (Duan et al., 2005b).

So far, a number of studies have investigated the mechanism of H₂S oxidation by oxygen over different adsorbents and at different temperatures (Steijns et al., 1976, Ghosh and Tollefson, 1986, Yan et al., 2002). In general, H₂S can be oxidized to form sulphur and water (Steijns et al., 1976, Ghosh and Tollefson, 1986, Yan et al., 2002). Steijns et al. (1976) indicated that elemental sulphur (one of H₂S intermediate oxidation products) could be further oxidized to SO₂ at temperatures above 200 °C, and H₂S might be oxidized auto-catalytically when it deposited in the micropores of the adsorbents. This process involves a reaction on the surface layer between the chemisorbed oxygen and the dissociatively adsorbed hydrogen sulphide. Katoh et al. (1995) studied the oxidation mechanism of a mixture of H₂S, methanethiol and dimethylsulphide gases when adsorbed on wet activated carbon fiber (ACF). They found that H₂S was oxidized to form elemental sulphur in micropores, and then the element sulphur reacted with H₂S to form polysulphide (H₂S_x). Moreover, the polysulphide and oxygen can produce a polysulphide radical on the surface of the ACF and react with H₂S to form polysulphide and SO₂. Mikhalovsky and Zaitsev (1997) used the X-ray photoelectron spectroscopy to analyze the H₂S adsorption on activated carbon in an inert atmosphere which resulted in the formation of surface oxygen-containing complexes and elemental sulphur. Furthermore, the surface functional groups contributed significantly to the formation of SO₂ in H₂S oxidation. Bandosz and her coworkers (Bagreev and Bandosz, 2001, Bandosz, 2002) investigated the effect of surface chemistry and pH on the H₂S adsorption on virgin and spent activated carbon, pointing out that the rate-limiting step is the reaction of the adsorbed hydrogen sulphide ion with oxygen. Overall, the identifications of H₂S oxidation products using different approaches have contributed significantly to a better understanding to the mechanism involved in H₂S adsorption on carbon.

However, the mechanism of H₂S oxidation in a process that combines biodegradation and physical/chemical adsorption on activated carbon surface, is rarely studied. Particularly the differences of those oxidation products of H₂S via adsorption or biodegradation are unclear yet, as the full identification of different S species in H₂S biofiltration is really difficult. In this work, a series of laboratory-scale biofiltrations of H₂S using four columns packed with different materials were conducted to differentiate the effect of biodegradation and adsorption; surface chemistry of the used carbons (BAC or virgin activated carbon – VAC) in all these columns were studied, together with a thorough investigation on the oxidation products of H₂S in liquid and solid phases. A better understanding to the combined effects of biodegradation and adsorption of using BAC in treating H₂S was thus achieved.