Practical considerations and challenges involved in surfactant enhanced bioremediation of oil.

Surfactant enhanced bioremediation (SEB) of oil is an approach adopted to overcome the bioavailability constraints encountered in biotransformation of nonaqueous phase liquid (NAPL) pollutants. Fuel oils contain n-alkanes and other aliphatic hydrocarbons, monoaromatics, and polynuclear aromatic hydrocarbons (PAHs). Although hydrocarbon degrading cultures are abundant in nature, complete biodegradation of oil is rarely achieved even under favorable environmental conditions due to the structural complexity of oil and culture specificities. Moreover, the interaction among cultures in a consortium, substrate interaction effects during the degradation and ability of specific cultures to alter the bioavailability of oil invariably affect the process. Although SEB has the potential to increase the degradation rate of oil and its constituents, there are numerous challenges in the successful application of this technology. Success is dependent on the choice of appropriate surfactant type and dose since the surfactant-hydrocarbon-microorganism interaction may be unique to each scenario. Surfactants not only enhance the uptake of constituents through micellar solubilization and emulsification but can also alter microbial cell surface characteristics. Moreover, hydrocarbons partitioned in micelles may not be readily bioavailable depending on the microorganism-surfactant interactions. Surfactant toxicity and inherent biodegradability of surfactants may pose additional challenges as discussed in this review.

Surfactant aided biodegradation of NAPLs by Burkholderia multivorans: comparison between Triton X-100 and rhamnolipid JBR-515.

Both chemical surfactants and biosurfactants have been effectively used for the degradation of petroleum hydrocarbons. Chemical surfactants are known to enhance biodegradation effectively while activity of biosurfactants is also comparable and they have the additional advantage of being biodegradable. However, the mode of action of chemical surfactants and biosurfactants may vary. This work was conducted to determine the mode of action of Triton X-100 and rhamnolipid JBR-515 by exploring the factors affecting the process of surfactant aided biodegradation of model non aqueous phase liquids (NAPLs) by a naphthalene degrader, Burkholderia multivorans (NG1). Emulsification studies, growth rate and degradation rate studies were conducted and correlated with alteration in cell surface properties including surface hydrophobicity, cell surface charge and cell surface functional groups. Triton X-100 and JBR-515 demonstrated distinct mode of uptake of NAPLs. Triton X-100 enhanced bioavailability by emulsification and supported direct interfacial uptake of model NAPLs by B. multivorans (NG1). Conversely, the biosurfactant rhamnolipid JBR-515 did not demonstrate emulsification of NAPLs and enhanced bioavailability through micellar solubilization. NAPL composition influenced the alteration in the cell surface properties. For both the surfactants, increase in surfactant concentration increased the rate of utilization of aliphatic hydrocarbons from the NAPLs.

Alteration in cell surface properties of Burkholderia spp. during surfactant-aided biodegradation of petroleum hydrocarbons.

Chemical surfactants may impact microbial cell surface properties, i.e., cell surface hydrophobicity (CSH) and cell surface charge, and may thus affect the uptake of components from non-aqueous phase liquids (NAPLs). This work explored the impact of Triton X-100, Igepal CA 630, and Tween 80 (at twice the critical micelle concentration, CMC) on the cell surface characteristics of Burkholderia cultures, Burkholderia cepacia (ES1, aliphatic degrader) and Burkholderia multivorans (NG1, aromatic degrader), when grown on a six-component model NAPL. In the presence of Triton X-100, NAPL biodegradation was enhanced from 21% to 60% in B. cepacia and from 18% to 53% in B. multivorans. CSH based on water contact angle (50-52°) was in the same range for both strains while zeta potential at neutral pH was -38 and -31 mV for B. cepacia and B. multivorans, respectively. In the presence of Triton X-100, their CSH increased to greater than 75° and the zeta potential decreased. This induced a change in the mode of uptake and initiated aliphatic hydrocarbon degradation by B. multivorans and increased the rate of aliphatic hydrocarbon degradation in B. cepacia. Igepal CA 630 and Tween 80 also altered the cell surface properties. For B. cepacia grown in the presence of Triton X-100 at two and five times its CMC, CSH increased significantly in the log growth phase. Growth in the presence of the chemical surfactants also affected the abundance of chemical functional groups on the cell surface. Cell surface changes had maximum impact on NAPL degradation in the presence of emulsifying surfactants, Triton X-100 and Igepal CA630.

Microbial decolorization of reactive black-5 in a two-stage anaerobic-aerobic reactor using acclimatized activated textile sludge.

A two-stage anaerobic-aerobic treatment process based on mixed culture of bacteria isolated from textile dye effluent was used to degrade reactive black 5 dye (RB-5). The anaerobic step was studied in more detail by varying the dye concentration from 100 to 3000 mg l(-1). The results showed that major decolorization was achieved during the anaerobic process. The time required for decolorization by > 90% increased as the concentration of the dye increased. It was also found that maintaining dissolved oxygen (DO) concentration below 0.5 mg l(-1 )and addition of a co-substrate viz., glucose, facilitates anaerobic decolorization reaction remarkably. An attempt was made to identify the metabolites formed in anaerobic process by using high performance liquid chromatography (HPLC) and UV-VIS spectrophotometry. A plate assay was performed for the detection of dominant decolorizing bacteria. Only a few bacterial colonies with high clearing zones (decolorization zones) were found. The results showed that under anaerobic condition RB-5 molecules were reduced and aromatic amines were generated. The aromatic amine metabolite was partly removed in subsequent aerobic bio-treatment. It was possible to achieve more than 90% decolorization and approximately 46% reduction in amine metabolite concentration through two-stage anaerobic-aerobic treatment after a reaction period of 2 days.

A coupled photocatalytic-biological process for degradation of 1-amino-8-naphthol-3, 6-disulfonic acid (H-acid).

There has been growing emphasis on the development of coupled treatment systems (e.g., advanced oxidation-biological) for treating poorly biodegradable wastewater. An attempt has been made in the present study to couple photocatalytic (TiO2/UV) pretreatment with conventional activated sludge process to achieve improvement in the biodegradation of H-acid. The combination of titanium dioxide and UV light has been known to generate strong oxidants that degrade several organic pollutants into carbon dioxide via the formation of some intermediates. The intermediates formed may undergo biodegradation readily. Accordingly, photodegradation experiments were carried out initially at an optimized TiO2 dose and the minimum pretreatment time required for transforming H-acid was identified. For this purpose, UV-vis spectrophotometry and high-performance liquid chromatography (HPLC) were extensively used. Subsequently, it was attempted to biodegrade untreated and pretreated H-acid using activated sludge from the textile industry acclimatized to H-acid. It was found that photocatalytic pretreatment of H-acid for 30 min, during which period approximately 8-10% chemical oxygen demand (COD) removal occurred, can be coupled to second-stage biological treatment for achieving enhanced biodegradation of H-acid.

Preliminary studies on bacterial decolorization of H-acid based azo dye-reactive black 5.

A co-culture acclimatized to H-acid was used to degrade Reactive Black 5 (RB 5), a bis azo dye having central H-acid function. The effect of substrate concentration, pH and medium composition on the decolorization has been investigated. Decolorization was found independent of pH. Luria-Bertani broth favored decolorization over Yeast Extract; however further decolorization experiments have been conducted using Yeast Extract. The Michaelis-Menten Kinetic model is found to describe the dependence of specific decolorization rate on the RB 5 dye concentration.