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.