Biodegradation of p-nitrophenol using Arthrobacter chlorophenolicus A6 in a novel upflow packed bed reactor
Introduction
The presence of substituted groups in phenol particularly nitro, chloro and bromo increases its toxic effects exerted on the environment as well as on the human health owing to their carcinogenic and recalcitrant properties [1]. The U.S. environmental protection agency (EPA) has listed p-nitrophenol (PNP) as a priority pollutant and recommended its concentrations in natural waters and drinking waters to below 10 ng l−1 [2], [3] whereas, monthly average industrial effluent concentrations of PNP should not exceed 162 μg l−1 [4]. PNP is probably the most important among the mono-nitrophenols in terms of its annual usage which is up to 20 million kg per year [5]. The major sources of wastes that discharge PNP are the industries mainly involved in the management of explosives, drugs, dyes, phosphoorganic insecticides (methyl parathion), pesticides and leather. PNP are also formed in aqueous matrices during pesticides formulation, distribution and field application [6]. In addition, PNP was detected in rain water in Japan, which forms due to photochemical reaction between benzene and nitrogen monoxide in the atmosphere [7]. It may also have the potential to leach through soil and enter groundwater, where they are hardly degradable and hence persist in the environment [8]. All these aspects warrant a high efficiency treatment of wastewaters contaminated with PNP and other substituted phenols prior to their discharge into the environment. Although several techniques such as volatilization, photo-decomposition, physical adsorption, solvent extraction, chemical oxidation and electrochemical methods have been tested for the removal of phenol and phenolic compounds from wastewaters [9], high cost, low efficiency and generation of toxic by-products are some of the limiting factors of these methods. The eco-friendly biodegradation process has gained maximum attention due to its many advantages over the traditional physico-chemical methods. However, the presence of nitro groups enhances the resistance of the aromatic ring against biodegradation by many microorganisms [1], [10], [11], and hence only selective species of bacteria belonging to Flavobacterium, Alcaligenes, Pseudomonas, Rhodococcus and Arthrobacter have shown ability to degrade PNP [10], [11], [12]. Among these microbial species, actinomycetes secrete both extracellular as well as intracellular enzymes and have thus revealed good potential in degrading PNP more effectively. Arthrobacter chlorophenolicus A6 is an aerobic actinomycetes that has been demonstrated to degrade wide different types of toxic substituted phenols in batch shake flask and is also reported to be one of the most efficient strains that completely mineralize 4-chlorophenol (4-CP) even at 300 mg l−1 within 24 h of culture [13]. However, there is also no report available so far on the performance of Arthrobacter chlorophenolicus A6 on biodegradation of PNP in any kind of bioreactor. Moreover, all the studies conducted so far on PNP degradation using microorganism have been largely limited to experiments in simple batch shake flasks except only for a few on stirred tanks, and sequencing batch reactor [14], [15], [16], [17]. Furthermore, the performances of these reactors were largely limited under high PNP loading conditions.
Biofilm reactors have certain advantages over suspended growth bioreactors as it offers higher resistance to shock loads and its ability to survive even at low influent substrate concentrations. Besides, biofilm reactors offer high volumetric biomass concentration in small reactor volume [18]. Packed bed reactor (PBR) is one such biofilm reactor that has gained much popularity in wastewater treatment. Furthermore, packed bed reactor (PBR) operating in upflow mode prevents the suspended biomass wash out as well as reduces frequent clogging in comparison to down flow PBR. However, when a PBR is operated in an upflow mode with the supply of oxygen from the bottom of the reactor, gas channeling occurs leading to the development of liquid-rich and gas-rich regions rather than uniform distribution of oxygen and food throughout the reactor bed [19], [20]. Furthermore, reduction of pressure drop and superficial flow velocity diminish the growth of microorganisms and thereby degradation of organic pollutants due to lack of availability of sufficient oxygen and/or food deep inside the reactor bed material, especially when the bed contains a porous but poorly permeable supporting material for the growth of microorganisms [21], [22]. Therefore, hydrodynamic conditions in a PBR need to be improved to get maximum microbial growth and degradation efficiency. One possible way to achieve this could be through introduction of cross flow system with aeration through multiple numbers of ports, (fragmented approach) along the length of a PBR. The present study investigated the performance of a novel packed bed bioreactor designed with cross flow aeration at multiple ports for PNP removal by Arthrobacter chlorophenolicus A6 at different feeding and operational conditions.
Section snippets
Materials
All the chemicals and reagents used were either of analytical reagent (AR) grade or laboratory reagent (LR) grade. AR grade p-nitrophenol (PNP) was procured from Himedia (India).
Analytical methods
Biomass concentration in samples was determined by measuring optical density at wavelength 600 nm (OD600) using a UV–vis spectrophotometer (Model lambda-45, Perkin Elmer, USA). The absorbance values were expressed as dry cell weight using a calibration curve plotted between the optical density (OD600) versus mixed liquor
Hydrodynamic study in novel PBR
As described earlier, the hydrodynamic experiment was performed in PBR under two different aeration systems viz., aeration only through the bottom aeration port (simple PBR); and aeration through all the 4 ports with the arrangement to get cross flow aeration along with perforated discs wrapped by nylon membrane at the top (novel PBR). Fig. 2 compares the results of hydrodynamic study performed in these two conditions of the reactor. Fig. 2(a) shows the mixing profile in the novel PBR which
Conclusion
The novel PBR showed better hydrodynamic feature than the simple PBR. The polyurethane foams (PUF) with macro pores of sizes as large as 244 μm introduced in the reactor as support material for the growth of Arthrobacter chlorophenolicus A6 had PNP adsorption capacity of 37.84 mg g−1, which could remove PNP for only initial period of reactor operation. Complete removal of PNP was observed using the novel PBR at a maximum PNP loading rate of 2787 mg l−1 d−1, which corresponded to PNP influent
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