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Cases of inhibition of microbial degradation due to surfactant-induced change in surface h -drophobicity have also been reported. Chen et al48 observed that low concentration (0.09 CMA) of Triton X-100 inhibited the growth on solid anthracene of a Mycobacterium sp. strain and a Pseudomonas sp. strain. The causes of inhibition were believed to be the sorption of the surfactant onto both microbial cell surfaces and anthracene particles.

Desorption of Contaminants

Organic compounds can often strongly bind to particles on porous materials, such as soils therefore, becoming trapped into micropores. This, usually, does not allow rapid remediation and can lead to extended remediation periods. Several studies have shown that the mass transfer from ab/adsorbed phase to liquid is the controlling mechanism of biodégradation rate.49 In these cases, biosurfactants can enhance the bioavailability of contaminants even at concentrations below the CMC.28 Phenomena associated with this mechanism include a reduction of surface and interfacial tensions, capillary force and wettability and an increase of contact angle. At concentrations below CMC, surfactants reduce the surface and interfacial tensions between air/water, oil/water and soil/water systems. In a soil/oil system, surfactants increase the contact angle and reduce the capillary force holding together oil and soil particles due to the reduction of the interfacial force. Surfactants have been used to stimulate the dissolution of non-aqueous phase liquids initially present in soils,50 the dissolution of solid contaminants51 and the desorption and transport of soil-sorbed contaminants.52,53

Noordman et al54 investigated the effect ofthe rhamnolipid biosurfactant on hexadecane degradation in the case ofsubstrate entrapped in small soil pore sizes (6 nm). Even in low mixing conditions, rhamnolipids stimulated the release of entrapped substrates and enhanced uptake by cells.

Soil Washing

Hydrocarbon Contaminated Soils

The prospects of using biosurfactants in hydrocarbon-contaminated soil washing depend on the capacity of these compounds to enhance the desorption and dissolution of the polluting organic compounds and increase the rate oftransport of contaminants in soils. The mechanisms involved in the hydrocarbon removal from soils are related to the mechanisms involved in increasing bioavail-ability for bioremediation purposes. The properties of stabilizing oil/water emulsions and increasing hydrocarbon solubility may enhance both the biodegradation rate and the hydrocarbon removal rate from soils.55 These mobilization and solubilization effects occur at both concentration below and above the CMC. The application of microbial SACs to remove contaminants from soils is a technology characterized by some minor degree of uncertainty than the SAC-enhanced bioremediation, since only the chemicophysical properties of the biosurfactants and not their effects on cell surface properties and microbial metabolisms drive the removal efficiency.

The use of chemical surfactants has been reported to be efficient in removing hydrocarbons from soils. Lee at al.56 reported that non ionic surfactants removed more than 80% oftotal hydrocarbons from soils. Billingsley et al41 demonstrated interesting differences in the effects of non-ionic and anionic surfactants on the removal and bioavailability of PCBs. Non-ionic surfactants washed more PCBs from soils while the substrate into anionic surfactants micelle cores were more available for biodegradation by a PCB-degrading Pseudomonas sp. Microbial SACs often exhibited better capacity of removing hydrocarbons than their synthetic counterparts. The more commonly studied biosurfactants, such as rhamnolipids and surfactin, have been successfully evaluated in washing of soils contaminated by crude oils, PAHs and chlorinated hydrocarbons.28 In several cases, the removal efficiency was very high (up to 80%) and depended on both the contact time and biosurfactant concentration.50,57 Rhamnolipids have been reported to release three times as much oil as water alone from the beaches in Alaska after the Exxon Valdez tanker spill.58 Van Dyke et al59 have reported that rhamnolipids, at a concentration of 5 g/1, could remove approximately 10% more hydrocarbons from a sandy loam soil than sodium dodecyl sulfate. Biosurfactants appeared to be more effective in increasing the apparent solubility of PAHs by up to five times as compared to chemical surfactants.60,61 Biosurfactants have also found applications in aquifer remediation due to their ability to reduced interfacial tension between dense the non-aqueous phase liquids and groundwaters.62,63

Metal Contaminated Soils

The interactions between surfactants and metals are not fully understood. It is known that surfactants can remove metals from surfaces by different mechanisms. Non ionic metals can form complexes with biosurfactants, enhancing their removal from porous media.64 Anionic surfactants interact with cationic metals leading to their desorption from surfaces.27 Nevertheless, also cationic surfactants can play a role by competitive binding to negative charged binding sites. The first studies on biosurfactant-metal complex were carried out by Tan et al65 They demonstrated the rapid formation of monorhamnolipid-metal complex. Rhamnolipids have been evaluated for their affinity to metal cations.66 K+ < M 2+ < Mn2+ < Ni2+ < Co2+ < Ca2+ < H 2+ < Fe3+ < Zn2+ < Cd2+ < Pb2+ < Cu2+ < Al3+ are the cations in order (from lowest to highest) of affinity with rhamnolipids. Mulligan and coworkers extensively studied the potential of rhamnolipids, sophorolipids and surfactin in washing of metal-contaminated soils and sediments.26 Mulligan and Young67 studied the effect of biosurfactants by Pseudomonas sp., Bacillus sp. and Candida sp. on zinc and copper removal from soils and demonstrated that anionic surfactants are able to selectively remove metals oxide, carbonate and organic fraction from soils. Rhamnolipids successfully removed heavy metals from an oil cocontaminated soil68 and heavy metal contaminated sediments.26 Batch soil washing experiments were carried out to evaluate the feasibility of using surfactin for the removal ofheavy metals from contaminated soils and sediments. By a series of five soil washings, removals of 70% and 22% of copper and zinc, respectively were reported.26 Surfactin was able to remove the metals by both sorption at the soil particle interphase and metal complexation.

Future applications of bioemulsifiers in remediation of heavy metals and radionuclides can be now envisaged. Several microbial polysaccharides have been shown to bind heavy metals. Emulsan by A. lwoffii RAG-1 forms stable oil-in-water emulsions. In this system, metal ions bind primarily at the oil/water interphase enabling their recovery and concentration from relatively dilute solutions. Cations bound to the emulsion can be completely removed to the water phase when pH was lowered.69

Conclusion and Prospects

The heterogeneity of SAC structural types and properties results in a broad spectrum of potential applications in environmental remediation as well as in the oil industry, agriculture, medicine, cosmetic and food industries.29 Our increasing ability to analyze the microbial diversity in natural environments is expected to expand our knowledge on microbial SACs with respect to their exploitation for commercial applications and their roles in the physiology of the producing microorganisms. During the past few years, high throughput methods have been generated for the systematic screening of SAC-producing microorganisms.70,71 Unfortunately, only a small percentage ofmicroorganisms can be cultivated from environmental samples using traditional cultivation techniques.72 In order to overcome the problems associated with cultivation of microorganisms, new cultivation methods have been developed in order to increase the number of culturable bacterial species and investigate the previously inaccessible resources that these microorganisms potentially have.73

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