Application of central composite design approach for optimisation of zinc removal from aqueous solution using a Flow-by fixed bed bioelectrochemical reactor
Graphical abstract
Introduction
Heavy metal pollution is considered one of the most important environmental issues owing to the toxic and non-biodegradable nature of these metals. Therefore, this type of pollution is a serious threat to the environment and public health [1]. According to the World Health Organization, zinc-containing liquids are considered hazardous because of their acute toxicity, which should be discharged at concentrations lower than 5 mg/L [2]. Furthermore, zinc may accumulate in the environment and inhibit biological treatment processes. Numerous treatment approaches have been utilised to remove zinc from wastewater. However, most of these approaches, such as coagulation flocculation or chemical precipitation, can be used only for treating effluents containing heavy metals at high concentrations (≥1000 ppm) and require large quantities of chemical reagents [3]. These approaches are considered cost operations and generate secondary byproducts during treatment, requiring further disposal [3]. Other methods, such as ion exchange and membrane filtration, can be used efficiently for heavy metal removal from wastewaters; however, they require expensive maintenance and are energy intensive [4]. Therefore, it is essential to be able to treat wastewater containing zinc ions at low-to-medium concentrations using an effective and low-cost method.
The electrolysis method has been used to recover zinc from solutions generated by the fly ash extraction process [5]. This method requires high energy consumption, between 3.1 and 3.25 kWh/kg Zn, and a PbO2 anode to oxidise H2O to O2 at an overpotential of 0.6–0.8 V [5], [6]. Such high energy requirements may make zinc removal from dilute waste solutions expensive. The application of cathodic deposition in the environmental treatment of heavy metal effluents has increased during the last two decades owing to the development of efficient three-dimensional (3D) electrochemical reactors [7]. The main benefit of this type of reactor is the ability of its fixed bed cathode with high specific surface area to deliver a high mass transfer rate. Many different 3D electrode designs have been utilised for removing heavy metals, such as reticulated vitreous carbon, metal particles, metal-plated foams, expanded metals, and screens [8]. Using screens to construct 3D electrodes has proven to be more efficient in comparison with other types of 3D electrodes owing to a number of benefits such as high porosity, lower pressure drop, high turbulence promotion causing high mass transfer, high specific area leading to a high reaction rate, and low cost [8], [9]. Our previous work [9] showed that using screens provides a better performance for cadmium removal. However, the energy consumption is still relatively high using this traditional electrolysis process.
Much attention has recently been paid to microbial electrolysis cells (MECs) as environmentally friendly technologies, which can be used in numerous applications such as bioremediation, chemical production, sensing, and wastewater treatment [10]. The application of MECs to remove or recover heavy metals from dilute wastewaters is a relatively new method that was first investigated by Heijne et al. [11]. MECs are based on electrochemically active microbes at the anode (bioanode), which have the ability to oxidise the organic matter existing in wastewater (treatment action), in addition to providing a reducing potential that supports metal reduction on the cathode (recovery action). However, this system has a drawback owing to the ability of the bioanode to provide a reduction potential of − 0.20 V vs. standard hydrogen electrode (SHE) [12]. This drawback limits the spontaneous deposition on the cathode surface to only metals such as Cu2+, Fe3+, V5+, and Cr6+ because their reduction potentials are higher than − 0.20 V vs. SHE. The reduction potentials of most heavy metal ions (i.e. Cr3+, Cd2+, Ni2+, and Zn2+) are lower than − 0.20 V, leading to the requirement of an external DC power supply to further lower the cathode potential to achieve the recovery or removal of these metals [3], [13].
In MECs, bacteria at the bio-layer on the surface of the anode oxidise organics such as acetates, leading to conversion into CO2 and H2O, producing electrons on the anode. Therefore, the successful operation of MECs is governed by the types and quantities of bacteria present on the surface of the anode. Various starting materials have been used as inocula in MECs, such as soils and activated sludge [14]. Using soil in MECs was found to be a good way to generate electricity. Consequently, results of investigations performed by Bond et al. [15] and Holmes et al. [16] have shown that the main bacteria in soil are mainly Deltaproteobacteria (average 75%), with approximately 60% belong to the Geobacteraceae family. However, diverse soil sources may provide various microbial communities, leading to different abilities to generate electricity. Therefore, the performance of an MEC relies on the type of soil used on the anode [14]. Three different soil types have been used in MECs, including sand, silt, and clay [17], [18], [19]. Fosso-Kankeu et al. [17] showed that clay is the best soil for electricity generation in soil microbial fuel cells, delivering a peak voltage of 644 mV compared to 348 mV from silt and 336 mV from sand. Our previous work [8] demonstrated the good performance of a bioelectrochemical system for removing cobalt using local soil.
Table 1 summarises previous studies on zinc removal or recovery methods that used bioelectrochemical systems (BESs). Most of these studies used synthetic Zn2+-containing wastewater and needed assistance from an external power supply.
These studies confirmed the potential of BESs, especially MECs, for zinc removal. However, no work has been published on zinc removal using a BES with a cathode composed of a stainless steel stack, or on the optimisation of the operation parameters in this system.
Classical optimisation, in which only one factor is changed at a time to measure its effect, requires many experiments that are time-consuming. In practice, this ignores the interactions between individual components. By contrast, response surface methodology (RSM) overcomes the drawbacks of classical optimisation because it considers the combination of effects during optimisation [26]. RSM, as one of the experimental design techniques, is used for statistical modelling as well as optimisation of any process. It is a good approach for examining the impact of parameters and their interactions on the response of interest [27]. Many studies have confirmed that central composite design (CCD) is an excellent experimental design that can be used for the optimisation of any process based on RSM. In addition, it has been used extensively to generate second-order surface-response models [28], [29].
RSM has been extensively applied for optimising the operating variables of several processes. However, its use in the removal of heavy metals from synthetic solutions using MECs is very rare in the literature. In the present work, we report for the first time an optimisation approach for zinc removal using an MEC with a 3D cathode composed of a stack of parallel stainless steel screens and an anode of porous graphite, with local soil used as a source of bacteria.
To our knowledge, no study has been performed on the optimisation of zinc removal using MECs by applying RSM. Therefore, the aim of the present work was to optimise zinc removal using a CCD. The influence of three parameters was investigated, that is, applied voltage, initial concentration of zinc, and pH.
Section snippets
Soil and electrodes characterisations
Soil samples were collected from the surface of soil (0.1 m deep) in a region located at al-Ghwarizm College of Engineering, University of Baghdad, Al-Jadriya, Iraq. Samples were sieved through a 2 mm diameter mesh and stored at 4 °C for two weeks before use. The physiochemical properties of soil, such as maximum water holding capacity (MWHC), electrical conductivity (EC), and pH were determined using routine methods, with more details found in our previous work [8]. The VITEK 2 compact system
Characterisation
The results of physiochemical measurements of the soil were as follows: EC = 290 µScm−1, pH = 8.2, and MWHC = 7.94%. VITEK 2 software results showed that four exoelectrogenic bacteria existed in the soil, namely Klebsiella pneumoniae, Pseudomonas aeruginosa, Aeromonas hydrophila/caviae, and Bacillus cereus/thuringiensis/mycoides, which have the ability to generate electrons on the anode [8]. Fig. 3 shows the powder XRD results of the soil. It is essentially composed of calcium carbonate and
Conclusions
The current research showed that the application of RSM experimental design based on CCD was effective at obtaining the operating parameters that maximised the RE% of zinc from aqueous solutions. Using Minitab-17 software, regression analysis and variable optimisation were performed to predict the significant response in the experimental domain. A simple second-order quadratic model equation was established to predict the response (zinc RE%) on the overall experimental regions and to correlate
CRediT authorship contribution statement
Ziad T. Alismaeel: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing. Ali H. Abbar: Conceptualization, Data curation, Investigation, Methodology, Supervision, Validation, Visualization, Writing – original draft. Osama F. Saeed: Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Writing –
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors thank the technical staff of the Biochemical Engineering Department, Al-Khwarizmi College of Engineering, University of Baghdad for their general support.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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