About

William Abraham Tarpeh is an Assistant Professor of Chemical Engineering at Stanford University. He is also a courtesy faculty member of Civil and Environmental Engineering and a Center Fellow at the Precourt Institute for Energy. His research focuses on areas related to chemical engineering, energy, and environmental sustainability, contributing to the development of innovative solutions in these fields. Dr. Tarpeh's work is recognized for its interdisciplinary approach, integrating chemical engineering principles with environmental and energy sciences to address pressing global challenges.

Research topics

• Environmental engineering
• Environmental science
• Waste management
• Chemical engineering
• Chemistry
• Engineering
• Computer Science
• Pulp and paper industry
• Organic chemistry
• Inorganic chemistry
• Environmental economics
• Composite material
• Business
• Nanotechnology
• Environmental resource management
• Ecology
• Materials science
• Process management
• Nuclear chemistry
• Knowledge management

Selected publications

• Translating Fundamental Insights into Ag-Based Bimetallic Electrocatalysts to Anion-Exchange Membrane Fuel Cells

  ACS Energy Letters · 2026-03-03 · 1 citations

  article

  To address the challenges of high costs and scalability for fuel cells, it is essential to develop affordable and earth-abundant materials that are Pt-group-metal (PGM)-free. Recent advancements in PGM-free electrocatalysis for the oxygen reduction reaction (ORR) in alkaline media show that high catalyst loadings are needed to achieve high reaction rates; however, there are associated mass transport limitations. To address this issue, we developed low-loading ionomerless Ag-bimetallic thin films by alloying with 3d block elements or Sn. We bridge knowledge from fundamental rotating disk electrode (RDE) studies to gas-diffusion cathodes in high-temperature anion-exchange membrane fuel cells (HT-AEMFCs), resulting in high-activity ORR catalysis and peak power densities up to 1.2 W cm–2geo for the Ag-Sn and Ag-Co alloys. Experimental postcharacterization and theoretical calculations reveal small Co or Sn oxide nanoislands on Ag as likely active sites enhancing ORR activity, ultimately offering these low-loading PGM-free ORR materials as a viable path toward sustainable energy conversion.

  PublisherDOI

• Fate of organic contaminants in electrochemical nitrogen recovery from urine

  ChemRxiv · 2026-01-06

  articleSenior author

  This study compared the fate of pharmaceuticals and disinfection byproducts across three electrochemical nitrogen recovery processes treating urine: electrochemical stripping (ECS), electrodialysis (ED), and bipolar electrodialysis (BPED). ECS achieved greater TAN recovery efficiencies than ED and BPED and was the only process that recovered a urine-derived ammonium sulfate product rather than a mixed concentrate (containing TAN, sodium, and potassium). Enrichment and removal ratios based on target pharmaceutical quantification and suspect screening were used to evaluate the fate of organic compounds relative to nitrogen recovery. These metrics suggested that ECS prevented organic contamination of the product more effectively than the other two processes and that both ECS and BPED achieved greater organic removal than ED. No disinfection byproducts were detected in the product for any process, but formation in other chambers may require mitigation. Our findings inform pre- and post-treatment for electrochemical nitrogen recovery, as well as modifications to reactor configuration and operating conditions. With future work using suspect screening and identification of additional compounds to build greater mechanistic understanding of compound fate and characterize the toxicity of contaminants detected in treated urine and recovered products, our work will advance electrochemical technologies that recover high-purity, safely applied products and enable a circular nitrogen economy.

  PublisherDOI

• High Detection Frequency of Enteric Pathogens: Insight from Wastewater-Based Epidemiology (WBE) Surveillance Approach in Dakar, Senegal

  International Journal of Environmental Research and Public Health · 2026-03-04

  articleOpen access

  Despite the importance of wastewater environmental monitoring in disease prevention and response strategies, its use remains poorly documented in Senegal. In addition, there is more onsite sanitation than sewer networks in Dakar, and open drains channel for rainwater are also used as clandestine wastewater discharge into the sea. This study aimed to assess the presence of specific pathogens in wastewater, faecal sludge, and bathing water (the sea). Samples were taken at treatment plants, an open drain, and in the receiving environment (the sea) from June to December 2023. Total nucleic acid was subjected to multiplex qualitative qPCR using SeeGene Allplex™ kits targeting 34 gastrointestinal pathogens. Descriptive statistics, multiple correspondence analysis (MCA) and logistic regression were performed. Considering all matrices, across 51 analysed samples, the results revealed strong bacterial (96.08%, n = 49), parasitic (84.31%, n = 43), and viral (68.63%, n = 35) presence. These results showed high levels of Aeromonas spp. (96.08%), Blastocystis hominis (80.39%), Enterocytozoon (58.82%), and Norovirus GII (74.51%) among bacteria, protozoa, helminths, and viruses, respectively. Moreover, faecal sludge and pumping station samples show more identified pathogen than wastewater treatment plant and seawater samples. The MCA revealed that the dry season is spatially associated with a greater number of pathogens than the rainy season, but the latter showed a greater species diversity. Logistic regression showed that certain physicochemical parameters, including BOD5, turbidity, pH, and suspended solids, influence pathogen detection. However, qualitative detection and sampling period may constitute limitations. These results reveal that wastewater and bathing water can serve as sources of information on the circulation of pathogens of interest with epidemic potential. Therefore, this valuable epidemiological tool could serve as an adjunct to clinical surveillance in order to prevent future epidemics.

  PublisherOA PDFDOI

• Dynamic Quantification of Nitrogen Species in Acidic Titanium-Catalyzed Nitrate Reduction toward Ammonia

  ACS electrochemistry. · 2026-01-12

  article

  The electrochemical reduction of NO3– to NH3 offers a sustainable pathway for nitrogen recycling and can have significant implications for future chemical production. Key challenges in this domain include identifying the transient nature of reactive intermediates and the quantitative measurement of reaction products. These areas are critical for mechanistic insights that can inform future technological development. Here, we report an analysis of NO3– reduction on a metallic Ti surface, enabled by experimental methodologies with exceptional sensitivity and time resolution for the identification and quantification of NO3– reduction species. Combining gas chromatography, ion chromatography, and electrochemical mass spectrometry, we identified and quantified key reduction products (NH4+, H2, NO3–, NO2–, N2O), including difficult-to-detect species NO. Ti electrodes achieved high NH4+ Faradaic efficiency, averaging 82.3% at −1.0 V vs RHE, while for the first time, quantifying detectable NO and N2O. The appearance of NO supports its role as a reaction intermediate, while N2O formation points to an alternative N pathway that does not produce NH3. We also observed increased H2 production, likely linked to Ti hydride formation, that coincides with decreases in NO and N2O production. These findings advance our understanding of NO3– reduction and offer new avenues for rationally designing catalysts that enhance NH4+ selectivity and efficiency.

  PublisherDOI

• <i>Operando</i> Scanning Electrochemical Microscopy of Electrocatalytic Nitrate Reduction Intermediates

  Environmental Science & Technology Letters · 2026-02-20

  articleSenior authorCorresponding

  Efficient electrochemical conversion of wastewater nitrate to sustainable ammonia requires comprehensive mechanistic understanding and stringent control over competing pathways among multiple aqueous nitrogen intermediates─a challenge that underscores the need for in situ and operando surface analyses of electrocatalysts. In this study, we describe scanning electrochemical microscopy (SECM)-based approaches to characterize the nitrate reduction reaction (NO3RR) on Ti and Cu. These methods facilitate the assessment of catalytic activity, product selectivity, and surface-adsorbed intermediates. By combining tailored chronoamperometric protocols, we achieved operando visualization of NO3RR onset potentials (−0.35 V vs RHE for Ti and −0.16 V vs RHE for Cu) and verified potential-dependent nitrogen product distributions through stepwise quantification of ammonia, hydroxylamine, and nitrite. Furthermore, in situ surface interrogation SECM directly titrates surface-adsorbed H* and O*, revealing their coverage shifts under varying potentials and reaction times during the NO3RR and providing evidence for a hydrogen-mediated pathway on Ti. The methodology establishes a robust SECM-based platform for systematically benchmarking electrocatalyst performance and is readily applicable to the NO3RR on other catalysts and even other reactions that produce a complex array of intermediates.

  PublisherDOI

• Probing the Mechanism of Selective Phosphate Adsorption from Wastewater Using Aqueous and Synchrotron X-ray Characterization

  Journal of the American Chemical Society · 2025-08-11 · 1 citations

  articleSenior authorCorresponding

  Ion exchange shows promise for recovering phosphate from wastewater as value-added products but requires high phosphate selectivity to compete with conventional treatment. Hybrid anion exchange (HAIX) resins, which contain nonselective basic functional groups and selective iron oxide nanoparticles (FeOnp), can remove phosphate from wastewater. However, knowledge gaps remain regarding the mechanisms of phosphate selectivity and influence of competing ions, hindering efforts to model adsorption dynamics and design adsorption processes for varying wastewaters. To address these gaps, we integrated aqueous-phase adsorption analysis with solid-phase, synchrotron-based X-ray characterization; this integration facilitated elucidation of the distribution and speciation of iron, phosphate, and competing anions on HAIX resins. We compared a quaternary ammonium-functionalized HAIX resin (strong base anion exchange, SBA) to a tertiary amine version (weak base anion exchange, WBA) to determine the role of functional groups. X-ray radiography revealed differences in FeOnp speciation (goethite vs ferrihydrite) and distribution (peripheral vs homogeneous) between the resins, resulting in varied phosphate affinity and intraparticle diffusion resistance. Using micro-X-ray fluorescence (μ-XRF) and micro-X-ray absorption near-edge structure (μ-XANES) spectroscopy, we identified differences in where and how phosphate binds across resin types and wastewaters. Across wastewater compositions, FeOnp sites in WBA contribute more to phosphate adsorption than in SBA, possibly due to variations in Fe distribution and speciation. Phosphate adsorption densities calculated from quantitative μ-XRF maps matched those from aqueous analysis, demonstrating the effectiveness of this integrated approach. Overall, results demonstrate the use of synchrotron-based X-ray characterization for investigating adsorption mechanisms and advance HAIX as a phosphate recovery technology from wastewaters.

  PublisherDOI

• <i>(Invited)</i> Electrochemical Wastewater Refining: Reactive Separations to Covert Nitrate into High-Purity Ammonia Products

  ECS Meeting Abstracts · 2025-11-24

  article1st authorCorresponding

  The nitrogen cycle has been drastically perturbed by humans. In particular, aqueous nitrate and ammonia discharges have outpaced removal at wastewater treatment plants. These nitrogen species are more than pollutants, they are critical products in the manufacturing of fertilizers, pharmaceuticals, disinfectants, and other chemical commodities. Thus, water reuse can play a critical role in rebalancing the nitrogen cycle and reducing the environmental impacts of chemical manufacturing. This presentation focuses on valorizing agricultural runoff by converting nitrate to ammonia and recovering high-purity ammonia because of ammonia’s status as the precursor for almost all nitrogen-containing commodity chemicals. We pursue electrochemical methods because they facilitate replacement of fossil fuels with renewable energy inputs and enable distributed implementation that matches the distributed nature of our target wastewaters. Ultimately, electrochemically refining wastewater NO3– and NH4+ to NH3 can (1) remediate legacy Nr pollution in the environment, (2) recover valuable Nr resources, and (3) reduce the need for virgin Nr production and related emissions from Haber-Bosch facilities. This study describes electrodialysis and nitrate reduction (EDNR), a reactive electrochemical separation architecture that combines catalysis and separations to remediate nitrate and ammonium-polluted wastewaters while recovering ammonia. By engineering operating parameters (e.g., background electrolyte, applied potential, electrolyte flow rate), we achieved near-complete recovery and conversion of Nr in both simulated and real wastewaters. EDNR process demonstrated long-term robustness and recovered &gt;100 mM ammonium fertilizer solution from 8.2 mM Nr-containing agricultural runoff. EDNR is the first reported process to our knowledge that remediates dilute real wastewater and recovers ammonia from multiple Nr pollutants, with an energy consumption (245 MJ/kg NH3-N in simulated wastewater, 920 MJ/kg NH3-N in agricultural runoff) on par with the state-of-the-art. In addition to the titanium catalysts used in proof-of-concept EDNR, we also compare the performance of cobalt catalysts that exhibit higher selectivity towards ammonia. Overall, our reactive separations achieved superior performance in real agricultural runoff as direct treatment of simplified solutions. Observed rates exceeded those of conventional wastewater nitrogen removal technologies, and enable energy-efficient distributed ammonia production as a fuel, fertilizer, or precursor for other commodity chemicals.

  PublisherDOI

• Titanium-, nitrogen-doped carbon flowers catalyze the electrochemical nitrate reduction reaction to ammonia

  ChemRxiv · 2025-04-16

  preprintOpen accessSenior author

  An emerging design heuristic for electrochemical nitrate reduction (NO3RR) catalysts is synthesizing electron-deficient sites to facilitate binding of electron-rich NO3-. However, this rule has rarely been applied to metal-, nitrogen-doped carbon (MNC) catalysts. Titanium (Ti), with low electronegativity and high NO3RR reactivity, is a compelling MNC candidate. To date, atomically-dispersed TiNx motifs have eluded synthesis due to the strong oxophilicity of Ti. Here, we leverage nitrogen-rich carbon flowers (CF) to overcome synthetic challenges and produce Ti-, N-doped carbon flower (TiCF) catalysts. Advanced materials characterization demonstrates that TiCF catalysts are a mixed phase material with ¾ of Ti atoms in TiO2-like nanoparticles and ¼ of Ti atoms in novel atomically-dispersed TiNx sites. TiCF achieves 61 ± 8% NH3-selectivity of at −0.70 V vs. RHE and 14 ± 5 mA/cm2 to NH3 formation (|jNH3|) at −0.85 V vs. RHE in (0.1 M NaOH + 0.1 M NaNO3 + 0.45 M Na2SO4) electrolyte. Control studies show both CF morphology and Ti sites are essential for high NO₃RR activity. Density functional theory calculations attribute the NO3RR reactivity to TiNx, which facilitates multiple bond formation with surface intermediates to promote favorable NH₃ synthesis pathways. Thus, TiCF exhibits 60x higher |jNH3| values than bulk Ti and NH₃ yield rates (&gt; 0.06 mmol NH₃/hr/cm²) competitive with state-of-the-art MNC catalysts (e.g., FeNC, CuNC). TiCF introduces a new class of Ti electrocatalysts, advancing the MNC design space and sustainable NH3 production.

  PublisherOA PDFDOI

• Unraveling Foulant Deposition Mechanisms on Cation Exchange Membranes during Electrochemical Stripping for Wastewater–Ammonia Recovery

  Environmental Science & Technology · 2025-12-08 · 2 citations

  articleSenior authorCorresponding

  Recovering nutrients as valuable products from wastewater can alleviate environmental issues, including algal bloom formation and greenhouse gas emissions from chemical manufacturing, while creating a circular nutrient economy. However, integrating nutrient recovery into treatment trains requires long-term stability of recovery-focused technologies. Electrochemical stripping (ECS) shows potential for ammonia recovery across diverse wastewaters but is hindered by cation exchange membrane (CEM) fouling due to accumulation of wastewater constituents. This study examined the distribution and speciation of dominant foulants on CEMs during ECS in different wastewaters using advanced X-ray imaging and spectroscopy. Micro-X-ray fluorescence showed substantial deposition of Ca on CEMs, while Ca-K-edge spectroscopy indicated the presence of calcium carbonate polymorphs, calcium phosphates, and calcium organics. Building on these insights, we evaluated the efficacy of complexing agents (citric acid and ethylenediaminetetraacetic acid [EDTA]) and antiscalants (acrylate- and phosphonate-based) in preventing fouling. Aqueous measurements showed that EDTA and an acrylate-based antiscalant inhibited Ca precipitation without compromising ammonia recovery. Ex situ μ-XRF and μ-XANES quantified the spatial distribution and chemical forms of residual foulants, demonstrating effective mitigation by selected additives. These results provide mechanistic insight into inorganic fouling on CEMs and demonstrate that targeted additives can mitigate scaling, enhancing ECS stability for sustainable nutrient recovery.

  PublisherDOI

• Unit process modeling to benchmark and translate electrocatalytic reactive separations of wastewater nitrate

  ChemRxiv · 2025-08-12

  preprintSenior author

  Reactive separations systems that leverage the nitrate reduction reaction (NO3RR) promise to extract value from nitrate-laden wastewater treatment by refining nitrate pollutants into ammonia products. Despite recent fundamental advances in NO3RR electrocatalysis and some proof-of-concept demonstrations of promising reactive separations unit processes, a rational approach to scale-up and deployment of these critically needed technologies remains elusive. This imperative is challenged by the impure and variable nature of nitrate-laden wastewaters. To address these challenges, and to narrow the broad engineering design space for electrocatalytic nitrate refining (that necessarily includes integrated processes for nitrogen extraction, conversion, and recovery), we pursue process modeling that bridges subunit kinetic behavior with overall process performance. In this work, we establish characteristic rate equations for the subunit processes that comprise a reactive separation called electrocatalyst-in-a-box (ECaB), which facilitates nitrate reduction in an engineered environment separate from wastewater. Integrating these equations into a continuous ECaB process model predicts performance and lays the groundwork for further optimization toward practical application in wastewater treatment. The model also enables technoeconomic analysis that facilitates comparison of component- and subunit process-level cost contributions to the overall system and provides a benchmark for reactive separations against conventional wastewater nitrogen treatment. In sum, this work provides a framework to translate fundamental unit process studies toward integrated reactive separation system design, and to uncover unit process insights within the context of an integrated system to prioritize research and development needs in membrane, catalysis, and reactor design research.

  PublisherDOI

Frequent coauthors

• Thomas F. Jaramillo

  28 shared

• Jinyu Guo

  Interface (United States)

  25 shared

• Kara L. Nelson

  24 shared

• Elizabeth R. Corson

  21 shared

• Valerie A. Niemann

  SLAC National Accelerator Laboratory

  18 shared

• Peter Benedek

  17 shared

• Ileana Wald

  University of California, Berkeley

  17 shared

• Dean M. Miller

  Stanford University

  16 shared