Electrochemical and Photo-Assisted Nitrogen Recovery

Abstract

Eutrophication occurs when excess nutrients, primarily nitrogen and phosphorus, accumulate in water bodies, promoting excessive algal growth and disrupting ecosystems. While naturally occurring, human activities such as agricultural runoff, sewage discharge, and fertilizer use accelerate this process, leading to biodiversity loss and water quality degradation. Conventional nitrogen recovery from municipal wastewater can be energy-intensive and costly. In this work, we use electrochemical and photochemical approaches to enable nitrogen recovery from synthetic wastewater with the overall goal of further minimizing energetic consumption. In the first portion of this work, we studied proton-coupled electron transfer active (PCET-active) behavior in an electrochemical system. Proton-coupled electron transfer (PCET) involves the transfer of both electrons and protons. This phenomenon enables the formation of pH gradients in electrochemical systems. This pH gradient has high utility in applications such as CO2 capture and nitrogen recovery. However, most PCET-active molecules in solution phase do not undergo protonation beyond certain pH conditions, limiting their utility in applications that require high pH swings. To address this, we opted to tune the proton affinity of a PCET agent - 1-aminoanthraquinone-2-sulfonic acid (AAQS) - by functionalizing it to carbon electrodes. Molecular AAQS exhibits protonation only in pH buffers between 0–10. However, when conjugated onto a glassy carbon electrode via radical-mediated diazonium reduction, we observed partial protonation above pH 10. We then integrated a PCET-conjugated electrode into an electrochemical flow cell for ammonia removal from synthetic wastewater. We used a different class of molecule called phenazines conjugated onto flexible carbon cloth electrodes. The PCET-conjugated electrode was able to create the necessary pH swing to aid ammonia removal at an average energetic cost and nitrogen flux of 3.91 pm 1.73 kWh kg-1 N and 16.4 pm 0.71 g N m-2 d-1 respectively with a PCET electrode capacity of 9 C at an applied current density of 16 A m-2. Continuous separation of ammonia via a membrane contactor showed an average coulombic efficiency of 61.4 pm 3.5%. While our work compares reasonably with work done using a similar initial nitrogen concentration for PCET molecule dissolved in solution, additional studies would focus on improving active material loading on the conjugated electrode to boost processing capacity of our system. The final section of this work investigates photo-sensitive molecules called photobases, specifically malachite green carbinol base (MGCB), for ammonium conversion to ammonia. Upon illumination, MGCB transitions to an excited state, cleaving hydroxide ions into the solution and inducing a pH swing from acidic to alkaline conditions (pH 6–10). The observed pH swings enabled ammonium conversion to ammonia in both single-phase and two-phase systems. In the single-phase system, MGCB directly reacts with ammonium ions to form ammonia, achieving about 40% conversion efficiency. In the two-phase system, where hydroxide ions diffuse through an anion-exchange membrane, the conversion efficiency is about 6%, primarily due to degradation of the photobase. Ongoing work will focus on improving efficiency by exploring photobase derivatives with better solubility, resistance to photodegradation, and longer lifetimes under illumination.