Harnessing Photoelectrochemistry for Wastewater Nitrate Treatment Coupled with Resource Recovery

Abstract

Wastewater is a misplaced resource well suited to recover nutrients, value-added chemicals, energy, and clean water. A photoelectrochemical device is proposed to transform wastewater nitrates to ammonia and nitrous oxide, coupled with water oxidation. Numerical models were developed to quantify the dependence of process efficiencies and nitrogen-removal rates on light absorber band gaps, electrocatalytic kinetic parameters, competing oxygen reduction and hydrogen evolution reactions, and the reacting nitrate species concentrations that affect the mass-transfer limited current densities. With a single light-absorber and state-of-the-art catalysts, optimal solar-to-chemical efficiencies of 7% and 10% and nitrogen-removal rates of 260 and 395 gN m -2 day -1 are predicted for nitrate reduction to ammonia and nitrous oxide respectively. The influence of competing reactions on the performance depends on the nitrate concentration and band gap of the light absorber modeled. Oxygen reduction is more dominant than hydrogen evolution to compete with the nitrate reduction reaction, but is mass-transfer limited. Even with kinetic parameters that enhanced the driving forces for the competing reactions, the performance is only minimally affected by these reactions for optimally selected band gaps and nitrate concentrations larger than 100 mM. Theoretically predicted peak nitrogen removal rates and specific energy intensities are competitive with reported estimates for bioelectrochemical and Sharon-Anammox processes for ammonia recovery and nitrogen removal respectively. This result, together with the added benefit of harnessing sunlight to produce value-added products, indicates promise in the photoelectrochemical approach as a tertiary pathway to recover nutrients and energy from wastewater nitrates.

Combined Effects of Concentration, pH, and Polycrystalline Copper Surfaces on Electrocatalytic Nitrate-to-Ammonia Activity and Selectivity

“…For comparison, Figure also shows the energy intensity for producing ammonia, via the traditional Haber–Bosch process, from N 2 obtained from wastewater treatment using: (1) a traditional nitrification–denitrification process with a process intensity of 45 MJ kg N –1 and (2) the SHARON-Annamox process with an energy intensity of 10–16 MJ kg N –1 . Even when considering parasitic energy requirements such as pumping and aeration, the energy intensities are small, indicating that the microbes are efficient at metabolizing nitrogen contaminants.…”

“…The state-of-the-art nitrate treatments are biological nitrification–denitrification for wastewater treatment and physical separation technologies, such as ion-exchange and reverse osmosis, for drinking water. , These approaches are effective for removing NO 3 – in the form of N 2 (biological nitrification–denitrification) or to displace NO 3 – into a more concentrated secondary waste stream (ion-exchange, reverse osmosis). Electrochemical approaches have been proposed to recover nitrogen nutrients by driving the thermodynamically favorable NO 3 – to NH 3 conversion instead of reducing it to N 2 . ,− Therefore, electrocatalytic NO 3 – reduction to NH 3 has been extensively investigated in recent work. The performances of metallic (Ti, Cu, − Co, , Ru, Ni, Fe, and Bi) and bimetallic (CuCo, CuNi, , and CuPd − ) electrocatalysts have been reported at various pH levels and NO 3 – concentrations.…”

“…Excessive anthropogenic production of nitrogen fertilizers has disrupted the natural nitrogen cycle. This imbalance has resulted in the global contamination of groundwater and surface water with reactive nitrogen contaminants, including nitrates (NO 3 – ), nitrites (NO 2 – ), and ammonia/ammonium (NH 3 /NH 4 + ), causing environmental threats such as dead zones and health risks in humans and aquatic wildlife. − However, these contaminants, especially NO 3 – that is dominantly present in many waste streams, represent an untapped resource for nutrient (nitrogen) recovery. − This study focuses on evaluating the combined effects of NO 3 – concentration, pH, and polycrystalline surfaces on the electrochemical conversion of NO 3 – to NH 3 by copper catalysts.…”

“…. 5 Even when considering parasitic energy requirements such as pumping and aeration, the energy intensities are small, indicating that the microbes are efficient at metabolizing nitrogen contaminants. To these estimates, the Haber−Bosch process, with an energy intensity of 44.4 MJ kg N −1 , was additionally included to convert the recovered N 2 into NH 3 .…”

Powering the Remediation of the Nitrogen Cycle: Progress and Perspectives of Electrochemical Nitrate Reduction

“…To be comparable with the state-of-the-art nitrogen removal/recovery technologies, a numerical model was recently developed to quantitatively predict the optimal efficiencies and nitrogen-removal rates. For a viable process of NO 3 RR to NH 3 , the nitrogen-removal rate was predicted to be 260 g N m –2 day –1 …”