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Nitrate Imbalance-control by TRAnsformative Technologies that are Electrochemically-driven

Periodic Reporting for period 1 - NITRATE (Nitrate Imbalance-control by TRAnsformative Technologies that are Electrochemically-driven)

Reporting period: 2019-09-01 to 2021-08-31

Water quality is one of the great challenges of this century as stated in H2020 objectives. One of the utmost concerns is associated to nitrogen cycle imbalance due to anthropogenic nitrate water pollution. Because of anthropogenic nitrogen fertilizer inputs, nitrate concentrations in surface and ground waters have dramatically increased during the last century. Drinking water with elevated nitrate levels is harmful to human health and is associated with respiratory and reproductive system illness, cancer, thyroid problems and even death. The World Health Organization has set a restrictive maximum concentration level in drinking water of 50 mg/L of nitrate. Unfortunately, nitrate is among the most reported water quality violations worldwide. Nitrate contaminated groundwater impacts both municipal and private groundwater wells, the latter of which receives little attention has no mandatory treatments.
Understanding that one of the main needs is point of use (POU) treatment within homes for people who rely on private groundwater wells or contaminated tap water, is a huge step. POU systems must be user-friendly, reliable, have small physical footprint, and not have waste production. For source waters being treated by large municipalities, units with small physical footprint are preferred for ease of implementation in existing facilities. This project proposes electrochemical processes, which operate at ambient conditions, do not require addition of chemicals, are compact, easy to handle, and cost-effective.
Electrochemical reduction is an encouraging treatment technology to transform and reduce nitrate. Scientific and engineering challenges lie on uncovering alternative electrocatalysts based on high-efficient earth-abundant elements to platinum group element electrodes. Therefore, this research aims to (1) provide a framework for selecting promising earth-low-cost abundant elements to electrocatalytically degrade nitrate in drinking water, (2) the design and construction of different electrochemical reactors for nitrate remediation is expected and (3) to reframe research questions in context of real water matrices to deal with new challenges. The (4) Real-time Scanning Electrochemical Microscopy analysis of nitrate reduction will be interesting to elucidate nitrate removal mechanisms.
The main objective of NITRATE’s project is to advance with science, translating the results into an improvement of the technology readiness level related to the treatment of waters containing nitrate, and making it accessible to everyone.
Platinum group elements (PGEs) are widely used electrocatalysts. However, the low abundance of PGEs in the earth's crust and high environmental impacts to be acquired result in high costs, limiting their use in drinking water treatment. Identifying sustainable alternatives to PGEs is a major barrier in applying electrocatalysis for nitrate reduction. By moving up the periodic table, the first publication (P1) provided a framework for selecting promising earth-abundant elements that can electrocatalytically convert nitrate in water. Next phase of the project was to produce nanoparticles. Earth-abundant electrodes such as nano-Fe3O4 and Cu foam-Pt (Pt loading<0.50 wt%) were synthesized. The encouraging outcomes emphasized the potential of the new electrodes to treat contaminated water sources with nitrate, while allowing a sustainable decentralized ammonia recovery. Enriched water for crops irrigation can therefore be a prospect use for this added value product (Publications P2 and P3).

Prior research attempts at scale-up have faced implementation challenges due to limited knowledge regarding electrochemical reactor design for transformative water treatment applications. Therefore, in the work for publication P4, the researchers experimentally investigated residence time distribution and platinum-sheet electrode mass transfer effects due to (a) the liquid cross-flow velocity through the electrochemical cell, (b) the gas pressure of the air-diffusion electrode (ADE), and (c) the presence of mesh sheet mass transfer promoters between the electrodes. Results revealed a synergistic improvement of mass transfer with the ADE gas flow and the presence of mesh promoters. Engineers could exploit this synergistic effect to design electrochemical cells with significantly lower capital cost.

While most studies have focused on ideal lab made solutions, translation to higher technology readiness levels and commercialization requires reframing research questions in context of real water matrices. In publication P5, we discussed the disconnects that may occur when focusing on synthetic solution treatment rather than on real waters. Therefore, the publication P6 was developed to study the effect of the complexity of different water matrices on the removal of endocrine disruptors using different electrochemical advanced oxidation processes (EAOPs) based on H2O2 electrogeneration. The findings help shed light on the role of coexisting species in water matrix, since this affects the efficiency and competitiveness of EAOPs. Another work emerged with the aim to identify in what extent the components of natural water matrices may affect the catalytic performance during ERN. To gain a better understanding, the ERN was conducted by testing the inorganic ions individually revealing that the species that most influenced the system was magnesium. When softening pre-treatment was applied to the real effluents, the reduction of nitrate become more effective and achieved higher percentages of removal at faster kinetics. This means that the electrochemical reduction technology will be limited for treatment of soft waters at present.
In the ERN field, most of the works are performed potentiostatically to study the mechanism/fundamentals of using a specific cathode material. In contrast, this project uses a galvanostatically approach, which is considered a more realistic attempt to water treatment. The results obtained are paving the way to achieve trustworthy systems which would reduce nitrate at an affordable price, not only as a point-of-use system so that people relying on unregulated wells can have greater safety in their drinking water, but also for source waters being treated by large municipalities, units with small physical footprint would be preferred for ease of implementation in existing facilities.
Even though, in the beginning of the project, the objective was to obtain innocuous nitrogen gas from the ERN, it was verified that ammonia can be obtained as a value-added product and be used for the irrigation of plantations at low-cost expenditures when compared to the Haber Bosh process, which deals with extreme operating conditions and high consume of energy. Besides synthesizing and testing cathodic electrode materials for the ERN, exploring the effect of the water matrices on the ERN is a new challenge not yet addressed in literature that is making it possible to identify where there electrocatalytical systems would be successfully applied.
As results expected for the incoming phase of the NITRATE project, the members envisage to understand in deep the mechanism of the ERN by synthesize and study alternative electrocatalysts and photo-electrocatalytic materials. The electrocatalytic properties of these new materials will be analyzed by Scanning Electrochemical Microscopy (SECM) to promote the next technology readiness level.
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