Production of a bacterial biosurfactant in an electrochemical environment as a prelude for in situ biosurfactant enhanced bio-electrokinetic remediation

https://doi.org/10.1016/j.psep.2021.01.041Get rights and content

Highlights

  • Microbial survival and growth under electrochemical conditions.

  • Production of a biosurfactant under electrochemical conditions.

  • Effect of electrolytic conditions on biosurfactant production.

  • Secretion of a defensive metabolite by bacteria under electrical stress.

  • Characterization of rhamnolipid congeners.

Abstract

The possibility of producing biosurfactants in an electrochemical environment was studied at three different currents of 0.5 A, 1 A and 1.5 A. The Pseudomonas aeruginosa strain was able to produce a biosurfactant in all the three experiments with a yield of 36.25 ± 3.75 mg/mL, 22.5 ± 5 mg/mL, 6.25 ± 1.25 for 0.5 A, 1 A and 1.5 A respectively. A rhamnolipid biosurfactant with 7 mono-rhamnolipid and 4 di-rhamnolipid homologues was detected in all experiments. However, the survival and yield of the biosurfactant was affected by the intensity of the electric field applied. The highest electric field of 1.5 A led to the total inactivation and elimination of the bacteria within the first 12 h while the bacteria survived to the end of the experiment in 96 h when 0.5 A and 1 A was applied.

Introduction

Electrochemical systems have gained increasing interest in recent years for various environmental applications such as precipitation of dissolved ions for solid/liquid separation (electrocoagulation), generation of oxidising species for contaminant degradation (electrooxidation), production of microbubbles for the separation of solids by flotation (electroflotation), separation and concentration of dissolved ions and molecules (electrodialysis), extraction and separation of metals from aqueous contaminated streams (electrolytic metal separation), in situ generation of active chlorine species for disinfection (hypochlorite electrolysis) and combined electrochemical and microbiological processes for mineralisation of organic load (bio-electrochemical oxidation) (Muddemann et al., 2019; Almomani et al., 2020; Radjenovic and Sedlak, 2015). Electrochemical systems have various advantages over other approaches used in environmental applications, such as their small foot print, they do not produce waste, auxiliary chemicals are generally not required and can easily be combined with other technologies to have more efficient processes (Radjenovic and Sedlak, 2015; Almomani and Baranova, 2013). The combination of electricity from renewable resources with the in-situ production of chemicals in electrochemical systems also enables sustainable solutions for the future (Muddemann et al., 2019). Electrokinetic/electrochemical remediation is a promising technology for remediation of environmental media contaminated with organics, inorganics, radioactive and mixed contaminants (Yeung and Gu, 2011). The electrokinetic method of remediation employs the use of direct current applied across an electrode pair (anode and cathode) placed on either side of a porous medium to cause electroosmosis of the liquid phase, electrophoresis of charged particles and electromigration of ions to oppositely charged electrodes (Tang et al., 2018). In the process of electrokinetic remediation, decomposition of electrolytes occurs at the electrodes (Mena Ramirez et al., 2015). Oxidation reactions occur at the anode creating highly acidic conditions due to the production of hydrogen ions while reduction reactions occur at the cathode creating highly alkaline conditions due to the production of hydroxyl ions (Mena et al., 2016). Similar to other technologies of remediation, effective decontamination of contaminated environmental media using electrokinetic remediation is achieved when the contaminants are in mobile phase (Yeung and Gu, 2011). But contaminants mostly exist as precipitates, species sorbed on solid matter and species sorbed on colloidal particles suspended in pore fluid or dissolved in pore fluid (Kingston, 2002). Therefore, solubilization enhancement strategies are always developed to aid in the decontamination process because extraction of contaminants is only possible if they exist as dissolved or colloidal suspended species in the pore fluid (Yeung and Gu, 2011).

In recent years, biosurfactants have greatly been used in combination with electrokinetic remediation to enhance the process of electrokinetic remediation (Pourfadakari et al., 2019). Promising results have been obtained in the remediation of soil, sludge and dredged marine sediments contaminated with heavy metals, polycyclic aromatic hydrocarbons and their mixtures (Tang et al., 2018; Ammami et al., 2015). Through micellisation, surface tension reduction, solubilization and increased adsorption, biosurfactants increase the rate of contaminant removal by altering the surface properties of the matrix leading to an enhanced electrokinetic remediation process (Tang et al., 2017). Biosurfactants are not only effective in enhancing the electrokinetic remediation process but have been rendered a better substitution for synthetic surfactants because of greater environmental compatibility, biodegradability, high foaming capacity, low toxicity, higher selectivity, able to function at extreme pH, temperature and salinity (Pourfadakari et al., 2019).

Electrokinetic remediation has further been combined with bioremediation and biosurfactants (Li and Yu, 2015). The results have shown that the synergy significantly enhances the degradation of organic compounds and greatly improves the remediation process (Gidudu and Chirwa, 2020b; Kim et al., 2011; Ossai et al., 2020). In fact, great prospects of in situ field scale electrokinetic remediation have been pronounced in the recent times (Yeung and Gu, 2011; Kim et al., 2011). If remediation of contaminated sites can be performed in situ then production of biosurfactants can also be done in situ (Mulligan, 2005). Successful production of biosurfactants in situ for the sole purpose of improving bioremediation have been reported in the past (Zhao et al., 2018; Youssef et al., 2013). There is however lack of studies that have investigated the possibility of in situ production of biosurfactants within a complex electrokinetic/electrochemical environment. In previous studies were biosurfactants have been used to enhance the electrokinetic remediation process, biosurfactants were separately produced under optimum conditions in absence of electrochemical conditions then the biosurfactants were introduced to electrokinetic systems to aid in enhancing the process of remediation (Tang et al., 2018; Ammami et al., 2015; Pourfadakari et al., 2019; Gidudu and Chirwa, 2020a). But to advance the system to the level of having in situ biosurfactant enhanced bio-electrochemical remediation, the possibility of producing biosurfactants in toxic electrochemical environments must be evaluated. This study investigated the possibility of in situ production of biosurfactants as a prospect for in situ synergetic application of electrokinetic remediation and biosurfactants in field scale applications.

Section snippets

Microbial culture, media, and growth conditions

A Pseudomonas aeruginosa strain with great biosurfactant production and hydrocarbon degrading ability was used in this study. The strain was obtained from API (Atmospheric tank) tank sludge of a refinery in South Africa by selective enrichment as reported in previous research (Gidudu and Chirwa, 2020a). The mineral salt medium (MSM) sterilized by autoclaving at 121 °C for 15 min was used. The medium was prepared as was reported by Trummler et al. (2003) by dissolving in 1 L of distilled water:

Viable cell count

In Fig. 2 viable cells started with the initial inoculation of 11.07 ± 1.00 CFU/mL in all the experiments. Microbial growth was then observed to increase in 12 h in all experiments with the 0 A experiment having the highest growth followed by 0.5 A while 1 A had the least. As electrokinetic reactions increased leading to the production of OH- ions at the cathode and H+ ions at the anode, viable cells increased in the 1 A experiment to 26.7142 ± 1.0782 CFU/mL in 48 h but started decreasing

Conclusion

The electrochemical environment affects the survival of bacteria and the production of biosurfactants but a Pseudomonas aeruginosa strain can still produce biosurfactant within an electrochemical environment. The survival of the bacteria and yield of the biosurfactant entirely depends on the intensity of the electric field applied. The highest yield of the biosurfactant was obtained when a current of 0.5 A was applied followed by 1 A and the least was obtained when a current of 1.5 A was

CRediT authorship contribution statement

Brian Gidudu: Conceptualization, Investigation, Validation, Writing - original draft, Software, Formal analysis, Writing - review & editing. Evans M. Nkhalambayausi Chirwa: Supervision, Methodology, Writing - review & editing, Resources, Project administration.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgment

This research was financially supported by the National Research Foundation (NRF) of South Africa, Rand Water, and the University of Pretoria. It is indeed in the authors at most interest to appreciate the financial support offered in that regard.

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