Technical NoteQuantification of bioregeneration of activated carbon and activated rice husk loaded with phenolic compounds
Introduction
Bioregeneration is defined as the process by which the adsorbent surface is being renewed through microbial action. The important role of bioregeneration process in extending the adsorption capacity of adsorbents has been reported by many researchers (de Jonge et al., 1996a, Orshansky and Narkis, 1997, Silva et al., 2004, Syamsiah and Hadi, 2004, Lee and Lim, 2005, Aktaş and Çeçen, 2007). Bioregeneration helps to increase the service period of the adsorbents. There are various methods in ascertaining the extent of bioregeneration quantitatively as reported in the literature. These include the direct determination of the substrate content on the adsorbent (Klimenko et al., 2004, Putz et al., 2005, Aktaş and Çeçen, 2006), the indirect measurement of substrate consumption by measuring carbon dioxide production (Speitel et al., 1989, de Jonge et al., 1996a, Putz et al., 2005) and the measurement of oxygen uptake by respirometry (Orshansky and Narkis, 1997, Ivancev-Tumbas et al., 1998, Lee and Lim, 2005).
In an earlier study, Lee and Lim (2005) quantified the bioregeneration efficiency for powdered activated carbon (PAC) in the simultaneous adsorption and biodegradation processes involving phenol and alkyl-substituted phenol, respectively, by calculating the difference between the initial and final substrate loading on PAC. The initial substrate loading on PAC was determined by subtracting the residual substrate concentration from the initial substrate concentration after adsorption equilibrium had been achieved whereas the final loading on PAC was obtained by subtracting the biodegraded substrate and the residual substrate concentrations from the initial substrate concentration. To calculate the initial substrate loading on PAC accurately, the requirement is that the equilibrium between the adsorbent and the substrate must be established before the commencement of biodegradation. This requirement is easily satisfied under any one of the two following conditions: (i) the contact time to reach adsorption equilibrium is relatively short, which was the case for the system involving PAC and the phenolic compounds, and (ii) the lag phase in biodegradation due to the toxicity of substrate is longer than the contact time to achieve adsorption equilibrium. For many other systems involving non-PAC as an adsorbent such as activated rice husk, this requirement may not be easily satisfied as biodegradation might have started before the establishment of adsorption equilibrium. Under this situation, quantification of bioregeneration in simultaneous adsorption and biodegradation is often difficult. A good approach is to allow the adsorption and biodegradation processes to occur sequentially in order to satisfy the above requirement. The question is: Are the results generated from the sequential adsorption and biodegradation approach applicable in the simultaneous adsorption and biodegradation processes?
The objectives of this study are to quantify and compare the bioregeneration efficiencies of PAC and pyrolyzed rice husk (PRH) loaded with phenol and p-nitrophenol (PNP), respectively, by oxygen uptake measurements using the respirometry technique in two approaches: (i) simultaneous adsorption and biodegradation and (ii) sequential adsorption and biodegradation.
Section snippets
Preparation of adsorbents
The PAC and PRH were used as the adsorbents in the study. The PAC, a steam-activated wood-based carbon (MALBON S4), was dried in the oven at 104 °C for 2 h. The raw rice husk was washed with tap water, rinsed with distilled water and dried in an oven at 104 °C. The washed rice husk was ground, sieved to size <250 μm and then pyrolyzed under nitrogen gas flow at 500 °C for 5 h. The PAC and PRH were kept in the desiccator prior to use.
Equilibrium adsorption studies
Adsorption studies were conducted to determine the adsorption
Equilibrium adsorption studies
The equilibrium adsorption data were found to fit fairly well (R2 > 0.9) to the Langmuir model. The limiting adsorption capacities of PAC, PRH and biomass for phenol were found to be 159, 29.8 and 1.0 mg g−1, respectively, at pH 7.9 whereas those of PAC, PRH and biomass for PNP were 261, 29.0 and 2.9 mg g−1, respectively, at the pH range of 6.93–7.62. The relatively low adsorption capacities of biomass for phenol and PNP coupled with the much lower biomass concentration, at 50 mg L−1, compared to those
Conclusions
The following conclusions can be drawn based on the results of this study:
Bioregeneration efficiency quantified using the sequential adsorption and biodegradation approach provides a good estimate of the quantity for the loaded adsorbent in the simultaneous adsorption and biodegradation processes in which the adsorption equilibrium is established before the commencement of biodegradation. This approach also provides a good estimate of the upper limit of the bioregeneration efficiency for the
Acknowledgement
Financial support from the Ministry of Science, Technology and Innovation, Malaysia (MOSTI) under the E-Science Fund, Project No. 02-01-05-SF0069 is gratefully acknowledged.
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