Enhancing Water Purity: A Leap with Palladium Nanoparticles in Biofilm Denitrification


Hydrogen gas (H2)-driven denitrification is a microbiological process in which H2 is the electron donor for the reduction of nitrate (NO3) to nitrogen gas (N2). Denitrification is used to remove nitrate and nitrite (NO2) from industrial and municipal wastewaters to prevent eutrophication and hypoxia, and it also is applied in drinking-water purification to eliminate human-health risks, including methemoglobinemia or “blue baby syndrome.” While denitrification is mature technology, research and technological development are essential for making the process more efficient and reliable.  Towards this goal, a new study published in Environmental Science & Technology — led by Professors Min Long and Bruce Rittmann and conducted by Drs. Jie Cheng, Chen Zhou, Zehra-Esra Ilhan, and Diana Calvo from the Arizona State University and Tongji University, investigated how the efficiency and selectivity of NO₃⁻ reduction in a biofilm could be enhanced with palladium nanoparticles (Pd⁰NPs). The core objective was to explore how the integration of Pd⁰NPs into the biofilm of a H2-based membrane biofilm reactor (H₂-MBfR) could increase denitrification rates while ensuring selectivity towards N₂ gas over ammonium (NH₄⁺).

The researchers set up two parallel H₂-based MBfR systems: one was a control with only a microbial biofilm (H₂-MBfR), and the other was enhanced with biogenically synthesized Pd⁰ nanoparticles within the biofilm (Pd-H₂-MBfR). The biofilms inoculated with anoxic biomass from a wastewater treatment plant, and the reactors were operated continuously with a consistent hydraulic retention time (HRT).  The authors in situ synthesized and deposited Pd⁰NPs within the biofilm matrix by feeding the reactor with palladium (II) chloride (PdCl₂), which was reduced to Pd⁰NPs within the biofilm. Both reactors were continuously fed with a mineral salt medium containing NO3, and parallel operation was maintained while varying the H₂-supply pressure and the influent NO₃⁻ concentration. They regularly measured the concentrations of NO3, NO₂⁻, and NH4+ in the effluent to assess the denitrification performance of each reactor.

The authors documented that the Pd-H₂-MBfR had a substantially higher rate of NO₃⁻ reduction compared to the control H₂-MBfR, particularly for higher influent NO₃⁻ concentrations.  This advantage was attributed to the catalytic activity of the Pd⁰NPs, which provided a non-biological pathway for electron transfer to reduce NO2⁻. This enhancement was attributed to the catalytic activity of Pd⁰NPs, thereby accelerating the biological reduction of NO3 to NO2.  Additionally, the study highlighted the ability to achieve 100% N₂ selectivity by adjusting the H₂/NO₃⁻ flux ratio to less than 1.3 e⁻ equiv of H₂/e⁻ equiv N.  This finding is particularly significant, as it demonstrates the potential to manipulate the denitrification process to favor the formation of environmentally benign N₂ gas over NH₄⁺, which is a water pollutant.

The authors used advanced solid-state analytical tools to characterize the biofilm and its Pd0NPs.   Scanning electron microscopy and energy-dispersive X-ray spectroscopy confirmed the synthesis and uniform distribution of Pd⁰NPs within the biofilm matrix, which is critical for their effectiveness in catalyzing NO2⁻ reduction.  Furthermore, they performed DNA extraction and sequencing to analyze the microbial community structure and functional potential.  The presence of Pd⁰NPs in the biofilm influenced the microbial community structure by enriching certain denitrifying bacteria — such as Dechloromonas, Azospira, Pseudomonas, and Stenotrophomonas — increasing the abundance of genes affiliated with NO₃⁻-N reductases.  These finding suggests that Pd⁰NPs created a more favorable environment for bacteria able to reduce NO3.

In conclusion, the new study by Professors Min Long and Bruce Rittmann and their colleagues, who documented advantageous incorporation of Pd⁰ nanoparticles (Pd⁰NPs) into the biofilm matrix of an MBfR system, introduces a novel strategy to leverage the catalytic properties of Pd⁰NPs together with the metabolic capabilities of the microbial community within the biofilm.  The accelerated rate of NO₃⁻ reduction, along with selectivity towards N₂, in the Pd-H₂-MBfR opens new avenues for the development of more efficient and environmentally friendly technologies for water and wastewater treatment.  Future studies focusing on the optimization of Pd⁰NP synthesis, biofilm integration, and reactor design will be essential to scale up this technology for practical applications in water and wastewater treatment facilities that help achieve sustainability goals.

Enhancing Water Purity: A Leap with Palladium Nanoparticles in Biofilm Denitrification - Advances in Engineering

About the author

Dr. Min Long is a Professor in the College of Environmental Science and Engineering at Tongji University in China. She earned her bachelor’s and Ph.D. degrees in Environmental Engineering from Tongji University in China in 2014 and 2019, respectively. From 2019 to 2023, she held positions as a postdoctoral research scholar and assistant research scientist in the group of Dr. Bruce E. Rittmann at the Biodesign Swette Center for Environmental Biotechnology, Arizona State University in the United States. Her current research focuses on utilizing coupled catalysts with biological methods in wastewater treatment and resource recovery, with a specific emphasis on the remediation of Per- and Polyfluorinated Substances (PFAS).

About the author

Dr. Jie Cheng received her Ph.D. at Zhejiang University, under the supervision of Dr. Yan He in 2023. She was a Visiting Scholar in the Biodesign Swette Center for Environmental Biotechnology under the supervision of Dr. Bruce E. Rittmann at Arizona State University from 2021 to 2023. She was a recipient of the Chinese National Scholarship and was awarded an Outstanding Young Scholar several times. She is a Research Fellow in the Institute for Functional Intelligent Materials at the National University of Singapore with Dr. Guillermo Bazan from 2023. Her research interests focus on kinetic research and mechanism analysis of environmental pollutant dissipation and CO2 reduction reaction in the hybrid inorganic-biological composite system.

About the author

Dr. Bruce E. Rittmann is Regents Professor of Environmental Engineering and Director of the Biodesign Swette Center for Environmental Biotechnology at Arizona State University.  His research focuses on the science and engineering needed to “manage microbial communities to provide services to society.”  Services include generating renewable energy, cleaning water and soil, and improving human health.  Dr. Rittmann is a member of the National Academy of Engineering; a Fellow of AAAS, WEF, IWA, AEESP, and NAI; and a Distinguished Member of ASCE.  Dr. Rittmann was awarded the first Clarke Prize for Outstanding Achievements in Water Science and Technology from the NWRI, the Walter Huber Research Prize and the Simon Freese Award from ASCE, the G.M. Fair Award from AAEES, the Perry L. McCarty/AEESP Founders Award, and the Camp Applied Research Award from WEF.  He is the co-winner of the 2018 Stockholm Water Prize.  Dr. Rittmann has published over 830 journal articles, books, and book chapters, and he has 21 patents.  With Dr. Perry McCarty, Dr. Rittmann co-authored the textbook Environmental Biotechnology: Principles and Applications (McGraw-Hill Book Co.), which is now out in its second edition.


Cheng J, Long M, Zhou C, Ilhan ZE, Calvo DC, Rittmann BE. Long-Term Continuous Test of H2-Induced Denitrification Catalyzed by Palladium Nanoparticles in a Biofilm Matrix. Environ Sci Technol. 2023 ;57(32):11948-11957. doi: 10.1021/acs.est.3c01268.

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