Electrosynthesis of acetate from inorganic carbon (HCO3−) with simultaneous hydrogen production and Cd(II) removal in multifunctional microbial electrosynthesis systems (MES)
Graphical abstract
Introduction
Carbon dioxide (CO2) is a greenhouse gas and an attractive feedstock for biocatalysis. Many acetogenic bacteria can catalyze a variety of redox reactions and are competent in the formation of acetate from the H2-dependent reduction of CO2 [1]. The microbial metabolisms of CO2 can be realized in microbial electrosynthesis systems (MES), in the presence of an applied electrical current, utilizing hydrogen-producing cathodes interfaced with electrochemically active bacteria (EAB) [2]. Similarly, the indirect conversion of CO2 (via hydrogen) in MES, enables the bioconversion of CO2 to a variety of chemicals, such as acetate [[3], [4], [5]].
In addition to the sequestration and utilization of CO2 to control the greenhouse effect, another environmentally significant problem is the removal and/or recovery of heavy metals from wastewater, including Cd(II), a highly hazardous heavy metal often present in industrial organics-barren wastewaters from chemical manufacturing, mining, extractive metallurgy, nuclear, and other industries [6,7]. For this purpose, bioelectrochemical systems utilizing mixed or pure EAB cultures fed by bicarbonate have been shown to be effective methods for the reduction and recovery of Cd(II) [[7], [8], [9], [10], [11]]. Therefore, it can be envisaged that appropriately designed multifunctional MES using self-regenerative EAB should be able to convert inorganic carbon from CO2 sequestration to building block organics (e.g., acetate) while simultaneously reducing Cd(II) from wastewaters [12]. Such multifunctional systems may be developed to be attractive and sustainable metallurgical methods for the production of biofuels and biomaterials from industrial waste. EAB generally adopt special metabolic pathways which release extracellular polymeric substances (EPS), as a protective mechanism from the harmful effects of Cd(II) [13,14]. Since EPS are composed of variable degrees of saccharides and proteins with negative charge, EPS can electrostatically bind the positively charged Cd(II) ions, in addition to the adsorption of the ions on the functional groups present on the bacterial cell envelopes [13,14].
In this study, the simultaneous production of acetate from bicarbonate (from CO2 sequestration) and hydrogen gas, with concomitant removal of Cd(II) heavy metal in water is demonstrated in multifunctional metallurgical MES incorporating Cd(II) tolerant electrochemically active bacteria. Pure cultures of four well characterized gram-negative EAB including Ochrobactrum sp. X1, Pseudomonas sp. X3, Pseudomonas delhiensis X5, and Ochrobactrum anthropi X7 were selected since they can efficiently remove Cd(II) with a variable degrees of intracellular distribution of different forms of cadmium [7,9,10]. However, the simultaneous production of acetate from inorganic carbon and of hydrogen by these strains has not been shown. Using single strains allows an understanding of the physiological activities of the bacteria at the electrode surface, whereas comparing the bacterial behaviors is beneficial for understanding the impact of each bacterium on the MES products. The EPS release mechanism from the EAB under metal stress is investigated since this is expected to regulate the rate of reduction of Cd(II) from the wastewater. The impact of electrochemically active cytochromes in the EPS that allow extracellular electron transfer are also examined, since these may further enhance the rate of products formation, similarly to nitrate removal and CH4 production [15,16].
The antioxidative activity of glutathione (GSH), catalase (CAT) and superoxide dismutase (SOD) in the bacteria are expected to be altered by either cadmium sequestration or by scavenging O2− induced by Cd(II), as a defense against the harmful effect of Cd(II) [17,18], while the dehydrogenase activity relates to the biocatalytic ability for hydrogen evolution [19]. These intracellular physiological activities are expected to be altered in the presence of Cd(II) and in the presence of a circuital current in the proposed multifunctional metallurgical MES. Therefore, the EPS components and the activities of SOD, CAT, GSH and dehydrogenase in these bacteria were systematically investigated in this study, in response to changes in the initial Cd(II) concentration and in the circuital current. These activities and EPS components were further correlated to the response of the bacteria in the proposed multifunctional metallurgical MES for the microbial electrosynthesis of acetate from organic carbon, with simultaneous hydrogen evolution and Cd(II) removal/recovery.
Section snippets
Reactor assembly
Duplicate two-chamber reactors comprising cylindrical chambers, 2.0 cm long, 3.0 cm in diameter and 14 mL each, as previously described [8,10,11], were used in all experiments. The cathode electrodes were three-dimensional material of graphite felts (Sanye Co., Beijing) [20,21] whereas the anode electrodes were carbon rods. The anodic and cathodic chambers were separated by cation exchange membranes (CEM, Ultrex CMI-7000, Membranes International, Glen Rock, NJ; projected surface area: 7.1 cm2)
System performance
The MES performance was initially evaluated at constant Cd(II) concentration of 20 mg/L. Significantly higher Cd(II) removal rates [1.20 ± 0.03 (X5) ‒ 1.32 ± 0.02 (X1) mg/L/h at 12 h; 0.38 ± 0.01 (X3) ‒ 0.42 ± 0.01 (X1) mg/L/h at 48 h] (Fig. 1A) and hydrogen production [0.0015 ± 0.0001 (X1) ‒ 0.0187 ± 0.0018 (X7) m3/m3/d at 12 h; 0.0019 ± 0.0001 (X1) ‒ 0.0210 ± 0.0005 (X7) m3/m3/d at 24 h] (Fig. 1B) were observed in the presence of EAB in comparison to the controls systems in the absence of
Conclusions
In this study we have demonstrated a sustainable process for the microbial electrosynthesis of acetate from bicarbonate with simultaneous hydrogen evolution and Cd(II) removal in a multifunctional metallurgical MES with tailored EAB. Such multifunctional MES may enable new prospects for the efficient conversion of CO2, from a broad range of sources, to C1 feedstocks, with simultaneous generation of a clean source of energy and heavy metals removal/recovery. The presence of a circuital current
Conflicts of interest
There are no conflicts to declare.
Acknowledgement
The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Nos. 21777017 and 51578104).
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2022, Chemical Engineering JournalCitation Excerpt :The exopolysaccharides in the EPS was measured by phenol–sulfuric acid method, whereas exoproteins were quantified by the Bradford assay (Bio-Rad, Hercules, CA) using bovine serum albumin as a calibration standard. The EPS composition was characterized by EEM fluorescence spectroscopy (F-7000, Hitachi, Japan) with 1.0 cm quartz cell and a thermostat bath [15,21]. The cathodic charges used for acetate production (αacetate), residual H2 (αH2), electrotrophic growth (αgrowth), release of exoproteins (αexoproteins) and exopolysaccharides (αexopolysaccharides) were calculated from Eqs. S1 – 5 [43–45].