Arsenite removal from aqueous solution by a microbial fuel cell–zerovalent iron hybrid process

Abstract

Conventional zerovalent iron (ZVI) technology has low arsenic removal efficiency because of the slow ZVI corrosion rate. In this study, microbial fuel cell (MFC)–zerovalent iron (MFC–ZVI) hybrid process has been constructed and used to remove arsenite (As(III)) from aqueous solutions. Our results indicate that the ZVI corrosion directly utilizes the low-voltage electricity generated by MFC in the hybrid process and both the ZVI corrosion rate and arsenic removal efficiency are therefore substantially increased. The resultant water qualities are compliant with the recommended standards of EPA and WHO. Compared to the ZVI process alone, the H₂O₂ generation rate and output are dramatically improved in MFC–ZVI hybrid process. Strong oxidants derived from H₂O₂ can rapidly oxidize As(III) into arsenate (As(V)), which helps to improve the As(III) removal efficiency. The distribution analysis of As and Fe indicates that the As/Fe molar ratio of the flocs in solution is much higher in the MFC–ZVI hybrid process. This phenomenon results from the different arsenic species and hydrous ferric oxides species in these two processes. In addition, the electrosorption effect in the MFC–ZVI hybrid process also contributed to the arsenic removal by concentrating As(V) in the vicinity of the iron electrode.

Introduction

Arsenic contamination of water has been a worldwide concern due to the toxicity and carcinogenicity of various arsenic species [1], [2], [3]. The predominant species in natural water are inorganic arsenate (As(V)) and arsenite (As(III)). As(III) exists not only in groundwater, but also widely in surface water and sediment [4], [5], [6], [7]. Compared to As(V), As(III) is more mobile and 25–60 times higher in toxicity. The concentrated As(III) in sediment is released continuously into surface water due to the presence of a reducing environment, which renders the task of As(III) removal protracted and arduous [4], [8]. Conventional techniques for arsenic removal, e.g., coagulation, precipitation and adsorption, are generally less effective for As(III) than for As(V) [3], [9], [10]. Therefore, studies on As(III) removal are becoming a focus in arsenic pollution research especially in drinking water treatment.

For efficient arsenic removal, oxidation of As(III) to As(V) has been suggested [9], [11], [12]. Arsenic removal by zerovalent iron (ZVI) technology, a facile and economical process, can simultaneously oxidize and remove As(III) [11], [13], [14], [15]. The ZVI corrosion process in water releases Fe²⁺ ions, which undergo a series of oxydrolysis and finally generate many types of hydrous ferric oxides (HFOs). During this process, arsenic can be removed by means of adsorption and coprecipitation [16]. The newly formed HFOs are generally considered more effective for the pollutant removal as compared to the iron coagulants added directly [10], [17]. Moreover, when the Fe²⁺ released from the ZVI process is further oxidized to Fe³⁺ by O₂, oxidants (H₂O₂, OH) are also generated at the same time. These oxidants can oxidize As(III) to As(V), and the latter is usually more easily removed by adsorption [3], [14]. The ZVI corrosion rate directly determines the generation rates of both HFOs and oxidants, and therefore affects the arsenic removal efficiency in the ZVI process. According to the electrolytic theory, the ZVI will not corrode unless an electric current goes through the electrolyte in the system. The corrosion rate depends on the potential difference between the cathode and the anode as well as the conductivity of the electrolyte solution [18]. Since the potential differences between different sites on the ZVI surface are small, the ZVI corrosion rate is therefore quite slow. As a consequence, the arsenic removal efficiency is seriously restricted.

A microbial fuel cell (MFC) is a device that uses bacteria as the catalysts to oxidize organic and inorganic matter in the environment to produce electricity [19], [20], [21], [22]. Sediment microbial fuel cell (SMFC) and benthic microbial fuel cell (BMFC) are the types that generate modest levels of electrical power in sediment or seafloor environments by a mechanism analogous to the coupled biogeochemical reactions that transfer electrons from organic carbon through redox intermediates to oxygen [23], [24]. Due to the limitation of the theoretical electrogenesis voltage of the MFC (on the order of 1.1 V) and the possible losses, the maximum open circuit voltage of MFC is typically less than 0.8 V [19], [25]. Even in the latest microbial reverse-electrodialysis cell (MRC), the maximum voltage generated is around 1.2–1.3 V [26]. Stacking MFCs in series can improve the voltage output [27], [28]. However, the advantage gained by increasing the number of series-stacked MFCs may be offset by the concomitant electric energy loss [29]. The inferior capability of power generation in the MFCs seriously restricts their applications. Currently, MFCs are mainly used to power miniwatt devices under particular circumstances. One example exploited the voltage difference between the cathode in oxygen rich water and the anode in anaerobic substrate sludge by means of BMFC to power sensors in the ocean [30].

When the MFC is coupled with other instruments or technologies, direct utilization of the generated electricity without electricity collection is possible because of the specific structure of the MFC and its electricity generation characteristics [31]. For example, the MFC can be used to treat the wastewater containing NO₃⁻, CrO₄²⁻, Cu²⁺, etc. [32], [33], [34]. Modified MFC can also be used in seawater desalination, in which the potential difference generated in the MFC was directly used to separate the anions from cations in seawater [35], [36]. These novel ways of MFC utilization not only make full use of the electrons and electricity generated in the MFC, but also expand its range of application.

The marriage of the MFC and ZVI technologies would potentially provide an efficient way of utilizing the low electricity generated by the MFCs for driving the ZVI corrosion process. To the best of our knowledge, no study on the MFC–ZVI hybrid process has been reported.

In this paper, we provide proof of concept of the MFC–ZVI hybrid system for As(III) removal from aqueous solutions. The unique characteristics of this hybrid system as compared to the ZVI process are discussed. The underlying mechanism in the As(III) removal process is investigated in terms of the As(III) removal efficiency, the production of HFOs and H₂O₂ and the distribution profiles of the arsenic and iron in the process.