Innovative, ecofriendly biosorbent-biodegrading biofilms for bioremediation of oil- contaminated water

Highlights

• An innovative Biodegradable Biosorbent Biodegrading Biofilm (BBBB) was developed.

• The BBBB is composed of hydrocarbon -degrading biofilm on poly-lactic acid and polycaprolactone nanofiber membranes.

• 100 % of spilled oil is immediately adsorbed and more than 66 % biodegraded in 10 days.

• Bioremediation of contaminated water is obtained at low cost and low impact.

Abstract

Immobilization of microorganisms capable of degrading specific contaminants significantly promotes bioremediation processes. In this study, innovative and ecofriendly biosorbent-biodegrading biofilms have been developed in order to remediate oil-contaminated water. This was achieved by immobilizing hydrocarbon-degrading gammaproteobacteria and actinobacteria on biodegradable oil-adsorbing carriers, based on polylactic acid and polycaprolactone electrospun membranes. High capacities for adhesion and proliferation of bacterial cells were observed by scanning electron microscopy. The bioremediation efficiency of the systems, tested on crude oil and quantified by gas chromatography, showed that immobilization increased hydrocarbon biodegradation by up to 23 % compared with free living bacteria. The resulting biosorbent biodegrading biofilms simultaneously adsorbed 100 % of spilled oil and biodegraded more than 66 % over 10 days, with limited environmental dispersion of cells. Biofilm-mediated bioremediation, using eco-friendly supports, is a low-cost, low-impact, versatile tool for bioremediation of aquatic systems.

Introduction

Petroleum and its derivatives are among the most serious environmental threat for the oceans [1]. New mitigation measures are urgently needed for the remediation of marine contaminated areas and various physical, chemical and biological methods have been proposed [2]. Bioremediation represents a promising, non-invasive and low cost technology that could provide a more effective and sustainable restoration of contaminated water and sediments [3,4]. Bioremediation exploits the ability of microorganisms to degrade and metabolize different environmental pollutants by assimilating organic molecules into cell biomass and converting them to other products such as carbon dioxide and water [4]. Biostimulation and bioaugmentation are the approaches most often applied. Biostimulation uses nutrients to stimulate the growth of autochthonous hydrocarbon (HC)-degrading microorganisms, while bioaugmentation introduces more efficient allochthonous microorganisms to improve biodegradation at a certain site. HC-degrading bacteria isolated from water, sediment and soil with high biodegradation capacities have been characterized [[5], [6], [7], [8], [9]]. In soil, actinobacteria, well known degraders of recalcitrant biomolecules, are generally abundant [10]; they are resistant to drought and good colonizers of organic and inorganic surfaces [11]. In marine environments, the most actively oil-degrading microorganisms are hydrocarbonoclastic bacteria that live almost exclusively on HC [12].

Environmental microbiological resources can be exploited into biotechnological tools to treat pollution caused by oil HC and its derivatives; an example is the immobilization of HC-degrading bacteria, on different supports, that has been used for the bioremediation of environmental pollutants [[13], [14], [15]]. Immobilization of bacteria on carriers preserves their viability and catalytic functions, as well as providing resistance to unfavorable environmental conditions and high concentrations of pollutants, while displaying a longer half-life [15]. Immobilization processes reduce bioremediation costs and eliminate dispersion and dilution of cells in the environment [14].

Oil removal by adsorption is a widely used approach, with polyethylene being the most used polymer sorbent [16], although inorganic materials, such as titania [17], have also been proposed. A superhydrophobic polyethylene-based shish-kebab membrane has been prepared with self-cleaning and oil/water separation properties [16]. Its membrane adsorption capacity ranged from 15 to 32 g/g, depending on viscosity and density of the organic liquids. Porous polyethylene bundles have been proposed with enhanced hydrophobicity and pumping oil-recovery ability via skin-peeling, able to adsorb up to 3 g/g of oil [18]. These products are quite efficient, but one of the main disadvantages is their non-biodegradability.

For microbial approaches, the creation of a strong and reliable biofilm-based remediation technology remains challenging. The carrier material should be biodegradable, insoluble, non-toxic for the immobilized cells and environment, easily accessible, low cost, available in large quantities, stable and suitable for regeneration.

Recently, the properties of biodegradable polymers from either natural or synthetic sources have aroused great interest by finding applications in various technologically advanced fields [[19], [20], [21]]. The success of synthetic biopolymers such as polylactic acid (PLA), and polycaprolactone (PCL) is due to diverse characteristics, including the relative ease of production and low costs. For instance, a super light 3D hierarchical nanocellulose aerogel foam was prepared with low density (1.50 mg/cm³) and high adsorption capacity (145.2–206.8 g/g for different oils) [22].

Combining technological properties of ecofriendly sorbents with their capacity to host a biodegrading biofilm, allows biodegrading biosorbent biofilms to be obtained which can remove oil from water with high efficiency and economically. Among the different approaches proposed to produce porous biopolymeric structures, electrospinning is one of the most investigated, enabling the production of fibers with diameters potentially ranging from nano- to micro-scale [23,24]. Compared to other porous structures, electrospun nanofiber mats show a higher specific surface area and greater porosity [24,25]. Thus, a wide range of electrospun polymers has been extensively studied for application in oil spill remediation [26] or removal of toxic metal ions from wastewater [27], as well as in the biomedical [23,[28], [29], [30], [31]], catalysis [32] and electronic [33] fields. Here, the bioremediation efficiency of membrane-bacterial systems was analyzed on crude oil using four high performance HC-degrading bacterial strains isolated from different environments: two marine hydrocarbonoclastic gammaproteobacteria Alcanivorax borkumensis SK2 [34] and Oleibacter marinus [5], and two soil, long-chain n-alkane degrading actinobacteria, Gordonia sp. SoCg [35] and Nocardia sp. SoB [9]. The crude oil degrading ability of these formulations was measured and compared with that of the bacterial cells in planktonic form.