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

Periodic Reporting for period 2 - NITRATE (Nitrate Imbalance-control by TRAnsformative Technologies that are Electrochemically-driven)

Reporting period: 2021-09-01 to 2022-08-31

Water quality is one of the great challenges of this century. Nitrate concentrations in surface and ground waters have dramatically increased during the last century due to the anthropogenic nitrogen fertilizer inputs. Drinking water with elevated nitrate levels is harmful to human health in terms of respiratory and reproductive system illness, cancer, thyroid problems, and 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, impacting 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 alternatives to reduce the process cost. Therefore, this research aimed to (1) identify promising earth-abundant elements to electrocatalytically degrade nitrate in drinking water, (2) the design and construction of different electrochemical reactors for nitrate remediation is expected, (3) to reframe research questions in context of real water matrices to deal with new challenges, and (4) to scale-up the process and understand in deep nitrate removal mechanisms.
The NITRATE project allowed to advance science, translating, and improving the technology readiness level related to the treatment of waters containing nitrate, and making it accessible to everyone.
The low abundance of platinum group elements (PGEs) in the earth's crust and high environmental impacts to be acquired result in high costs, limiting their use in drinking water treatment as electrocatalysts. 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 (P2), Cu foam-Pt (Pt loading <0.50 wt%) (P3), Cu-Co(OH)2 and Cu foam (-Co,-Cu,-Ni,-Sn) 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.
Prior research attempts at scale-up have faced implementation challenges due to limited knowledge regarding electrochemical reactor design for transformative water treatment applications. In the work of publication P4, the researchers experimentally investigated residence time distribution and electrode mass transfer effects due to (a) liquid cross-flow velocity through the electrochemical cell, (b) 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. Microfluidic devices were also developed to lower the energy requirements for electrolysis through innovation in reactor geometry that control mass transfer.
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. Then, 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). Work P7 explored water matrix effects under cathodic polarization required for ERN. The ERN was conducted by testing inorganic ions individually revealing water hardness (i.e. Ca2+, Mg2+) as the main challenge. Softening pre-treatment reinstated ERN performance. The ERN technology might be limited for treatment of soft waters at present.
The impact of the nature of the cations of the supporting electrolyte on the by-products selectivity during the electrochemical reduction of nitrate and nitrite was explored to acquire deeper understanding on nitrate mechanistic reactions. A significant shift in products selectivity towards NH3 by tuning the electrode-electrolyte interface through the electrolyte in solution was achieved. Within the same manuscript, the ERN performance has been successfully translated from low TRL reactors into higher TRL using a membrane-less flow cell type reactor, at natural pH, and in the presence of dissolved O2 in solution.
The major technology constraint of the ERN is associated to the capital cost of electrocatalysts. Using Scanning electrochemical microscopy (SECM), an emerging characterization technology for combinatorial electrocatalytic studies at the nanometer scale, we can map and evaluate electrocatalytic response of different materials and structures. With this in mind, a review article based on SECM applications to environmental remediation is being developed.
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 using units with small physical footprint. 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 the electrocatalytic systems would be successfully applied. During the incoming phase of the NITRATE project, understanding in deep the mechanism of the ERN and the concepts of the Scanning Electrochemical Microscopy (SECM) will promote the ERN to the next technology readiness level.
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