Resource recovery from paddy field using plant microbial fuel cell

Highlights

• Plant-MFCs were integrated with paddy field to recover “Nutrients, Energy, & Water”.

• Terracotta cylinders were used as chassis and proton exchange membrane of PMFCs.

• PMFC performance, catholyte recovery, and paddy growth are found to be correlated.

• Nutrients like ammonium and clay minerals were recovered throughout the experiment.

Abstract

The plant microbial fuel cell (PMFC) is a recently developed energy-generating technology that supports sustainable agriculture. Design configuration is the key bottleneck in optimization and upscaling of paddy based PMFCs. In this study, two designs (Type-I (horizontal) and Type-II (vertical)) of terracotta based ceramic PMFCs (C-PMFC) are incorporated into the paddy field to recover nutrients, energy, and water (“NEW”) resources. The peak voltage generated in Type-I and Type-II C-PMFC was 292.1 mV and 321.7 mV respectively. The polarisation study in the ripening phase of paddy showed a maximum power density of 9.1 mW/m² (type-I) and 16.8 mW/m² (type-II). The volume of catholyte recovered is observed to be dependent on the C-PMFC performance and growth phases of the paddy. In the entire 10 weeks of the experimental period, a total of 451 mL and 943 mL of catholyte was collected at 100Ω load in type-I and type-II, respectively. The collected catholyte is alkaline in nature and maximum catholyte recovery is achieved at the active tillering phase and declined with the advancement of the development stages of the plant. Osmotic and electro-osmotic migration of various nutrients like ammonium from the paddy field to cathode chamber of C-PMFC is observed throughout the experiment.

Introduction

Plant microbial fuel cell (PMFC) is an eco-friendly integration of microbial fuel cell (MFC) and plant root zone for the production of concurrent green energy and biomass. The proof of principle for PMFC as the source of promising bioelectricity was first demonstrated by Strik et al., [1]. Water-saturated environments such as wetlands and flooded agricultural fields such as paddy fields are ideal places for the incorporation of PMFCs, as water plays a key role in maintaining anoxic conditions at the anode that is a prerequisite condition for power generation in PMFC [2,3]. Since the inception of PMFC as the source of power generation, plant selection is based on rhizodeposition, photosynthetic pathway, and its adoption to the water-saturated condition [4,5].

Due to the inefficient water management practices in a low-land rice production system, the majority of the water used is unproductive and outflows via evaporation, percolation, and seepage [6], which leads to depletion of water resources and causing severe water crisis. Therefore there is a need to adopt new water-saving irrigation practices [7]. The other aspect that has been reported is that fertilizer N-use efficiency of irrigated lowland rice is very low, with an average N recovery efficiency of 30–40 % [8,9]. Hence nitrogen loss occurs in flooded paddy fields by denitrification, ammonia volatilization, leaching, and agricultural runoff that accounts for increased production cost and leads to environmental pollution. Only a few studies have focused on onsite reduction and recovery of excess applied fertilizer from subsurface drainage water by installing the denitrifying walls or trenches and bioreactors in agricultural fields [[10], [11], [12]]. In this context, the paddy field equipped with electrochemical systems such as MFCs and PMFCs can offer solution for some of the existing problems in low land paddy cultivation.

The performance of paddy based PMFC depends on various factors including meteorological conditions, amount of root exudates, the microbial community in the rhizosphere [13], soil characteristic [4], soil amendments [14], electrode material [15], size of the electrodes [16], the relative position of anode and cathode [17], and different design configurations [18], etc. Among all, design configuration is a key consideration in the optimization of paddy based PMFCs. The initial outdoor experiments on paddy based PMFCs with sediment MFC (sMFC) configuration resulted in less power density (6 mW/m²) since cathode was submerged and covered with soil because of active agitation of the flooded field [19]. Whenever cathodes were provided with polystyrene-foam bars to maintain buoyancy, it resulted in five times more power density [20]. The use of dual-chamber PMFC with earthenware membrane under the greenhouse showed 87 % more power density when compared to the single-chamber sediment type configuration [18].

Tubular design with anode directly placed in the root rhizosphere is the preferred configuration for PMFCs integrated with crops and wetlands [21]. The tubular or cylindrical design made of less expensive, and durable porous material such as ceramics as membrane cum structural component with an internal cathode chamber and external anode directly placed in the root rhizosphere of paddy avoid excavation of the agricultural field and will provide easy installation, decrease the distance between electrodes, and also provide the additional advantage of self-hydration for the cathode electrode and ion exchange membrane [22]. Moreover, the internal cathode chamber will provide the benefit of water production (passive and active transfer) in the form of catholyte along with possible nutrient/heavy metal recovery by osmotic and electro-osmotic drag.

The materials and membrane units used in the construction of MFC and PMFC should be inexpensive to achieve the goal of low-cost configuration [[23], [24], [25]]. In this context, even though there is a need for optimization in terms of porosity, thickness, and chemical composition, yet clay and earthen based ceramics are extensively used as membranes and structural framework of microbial fuel cells because of its low cost (5 $US/m2), high durability [26], suitability [27], and relatively moderate power performance (6.85 W/m²), and considered to be ideal materials for the advancement of MFC research [28]. Daud et al. [29], compared the ionic gradient concentration and power performances of MFCs with ceramic membranes (CM) with that of MFCs equipped with cation exchange membranes (CEM) and Nafion 117 where, under the batch mode, MFCs with CM outperformed the MFCs with CEM, due to improved diffusion of protons in the presence of other high concentration cations. Clay and earthenware based ceramic materials as membrane and anode chambers in the PMFC research have been explored in the laboratory [30], in a greenhouse [18], and ambient condition [31], but till date, to our knowledge, no studies have been reported on the use of ceramics in the PMFC under real field conditions. Researchers from the University of the West of England (UWE) developed ceramic-based MFCs (named as Pee power) to recover energy and nutrients from real-time urine treatment and further field trials of Pee power were conducted at UWE campus in February–May 2015 [32,33]. In the present study, an attempt has been made to use the simple and feasible design of terracotta based ceramics as structural component-cum-membrane in paddy based PMFC applied to the real paddy field for green electricity, and catholyte production.

This study aims to: (1) evaluate the performance of horizontal (type-I) and vertical (type-II) designs of terracotta based ceramic-PMFCs (C-PMFC) with internal cathode installed in a paddy field; (2) present for the first time in situ production and recovery of water in the form of catholyte from C-PMFC installed in the real field; (3) evaluate the influence of C-PMFC performance and growth stages of rice plant on the volume of actively transported catholyte; and (4) characterize the produced catholyte in terms of pH, conductivity and ionic concentration.