Combined effect of adsorption and biodegradation of biological activated carbon on H2S biotrickling filtration
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 (H2Sx). 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.
Section snippets
Biofiltration system
Fig. 1 shows the schematic diagram of a four-column biofilter system that was designed and constructed for this study. The four glass columns, which are identified as A, B, C, and D, could be run simultaneously and controlled separately. The biofilter bed material is enclosed in each glass tube which has an inner diameter of 4 cm and packing height of 5 cm. Column A (biotrickling filter with 20% BAC + 80% glass beads), B (trickling filter with 20% VAC + 80% glass beads), C (20% VAC + 80% glass beads
Performance comparison of the biofiltration systems
The performance of the biofiltration systems is shown in Fig. 3. Fig. 3a illustrates the H2S removal profiles of column A, B and C during the experimental duration of 120 h. Besides the inlet H2S curve (set constant at 45 ppmv), the other three curves represent the outlet concentration versus time profiles from columns A, B, and C. Results from column D was not plotted here as its breakthrough happened within 3 min after passing through the H2S gas stream. This indicated that the adsorption of H2S
Conclusions
The combined effects of carbon adsorption and biodegradation of BAC in treating H2S are investigated through four parallel columns in bench scale experiments. The physical/chemical adsorption of H2S on carbon surface tended to produce complex sulphur-bearing species (other than sulphate) as confirmed by other researchers (Bandosz, 1999, Yan et al., 2004). However, biological degradation of H2S produces mostly sulphate. The BAC used in this study, compared to VAC, was found to produce mainly
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