June 7, 2021. By Kolemann Lutz
Researchers from the University of California Riverside developed a NH2-MoOx-Pd/Catalyst that is significantly more active than any other chemical catalyst and rapidly reduces 99.99% of the perchlorate into chloride ions and oxygen in a liquid water suspension with hydrogen gas at 1atm and room temperature.
Perchlorate (ClO4-) is produced in the stratosphere and occurs naturally in some arid soils on Earth, and is especially abundant in Martian soil (up to 1% by wt). The source of the Cl is most likely from volcanic exhalations and/or Chlorine released through the aqueous weathering of basaltic material. Oxygen atoms later bind to form perchlorates via oxidation (gain of oxygen) at grain surfaces mediated by mineral catalysts, electrostatic dust, and UV light to form perchlorates. Several ground and space-based instruments have confirmed the presence of different perchlorate salts and magnesium perchlorate in Martian regolith, which suggests that perchlorate brine flows might be the cause of channelling and weathering observed on the planet's surface.
When humans are exposed to enough chlorine, there is a disruption of iodide uptake in the thyroid gland due to its similarity in ionic radius to iodide, which regulates how the human body develops and uses energy. On Earth, the ubiquity of Cl in soil and industrial goods leads to perchlorate contaminants in water sources that can cause certain thyroid disorders and other toxic effects in plants, microbes, and animals.
Perchlorate is primarily reduced through biologically mediated reduction pathways, into harmless chloride ions (Cl-) and oxygen gas (O2). Although, chlorates could also be reduced by naturally occurring Fe(II) minerals on the Martian surface. As perchlorate salts are highly water soluble and fully ionize in H2O, the alternative method is to reduce perchlorates chemically, although this usually involves harsh conditions or multi-component enzymatic processes and produces waste brine that can be challenging to manage. Additionally, many other metal catalysts either require harsh conditions or are not compatible with water.
A research experiment funded by University of California Riverside (UCR), NSF, and Department of Energy (DOE) emulated the microbial perchlorate reduction process by designing, building, and testing a bio-inspired Molybdenum Catalyst for aqueous perchlorate reduction. Anaerobic microbes use molybdenum in their enzymes to reduce ClO4 and harvest energy in low oxygen environments.
UCR researchers discovered that by simply mixing a common fertilizer known as sodium molybdate (Na₂MoO₄), an abundant organic ligand called bipyridine ((C5H4N)2 to bind the molybdenum (Mo), and a common hydrogen-activating catalyst called palladium on carbon (Pd), they produced a powerful catalyst that quickly and efficiently broke down the perchlorate in H2O using hydrogen gas at room temperature with no combustion involved.
The porous carbon Pd/C catalyst mimics the protein pocket of the enzyme that accommodates the oxygen atom transfer (OAT) metal site. Pd nanoparticles simplify enzymatic electron transfer by attracting electrons from H2. During ClO4 reduction, the ratios of leached Mo and NH2 into water were 1.5% and .2% of the total amount in the catalyst, respectively. The new catalyst reduces perchlorate in a wide concentration range, from less than 1 milligram per liter to 10 grams per liter was reused 10 times without any noticeable loss of activity
Researchers noted that the rapid perchlorate reduction can be attributed to the OAT energy barrier lowered by organic ligand, redox cycling of Mo is sustained by electron transfer from hydrogen gas, and the Mo-bound ClO4 requires the activation via protonation. As the NH2 ligand changed the structure and activity of MoOx ions, the MoOx-Pd/C catalyst could not reduce ClO4 without the ligand.
At ambient temperature and pressure, "the NH2-MoOx-Pd/Catalyst is much more active than any other chemical catalyst reported to date and reduces more than 99.99% of the perchlorate into chloride regardless of the initial perchlorate concentration, mentioned by” doctoral student Changxu Ren and Jinyong Liu, an assistant professor of chemical and environmental engineering at UC Riverside's Marlan and Rosemary Bourns College.
The carbon-supported catalyst demonstrated promise for perchlorate reduction in brine solutions via ion exchange or from reverse osmosis for water treatment. The excess dirty water can be pumped to nearby wastewater treatment systems or further purified on site nearby the hydrogen fuel cell and water production plant. Thus, perchlorate reduction infrastructure and instrumentation should be tested before, during, or shortly after water treatment or fuel cell plant is operational, prior to the remediation and synthesis of plant ready soil with cyanobacteria, mycelia, and other organisms.
Further research will be demanded to design systems and materials involved in separating Cl atoms from magnesium or other chlorine-binding minerals and to investigate large scale perchlorate reduction methods.
The low power physicochemical or abiotic perchlorate reduction at 1 atm and room temperature holds the potential to circumvent biological-induced challenges around microbial perchlorate reduction in the Martian environment and other perchlorate-abundant worlds.
Changxu Ren et al, A Bioinspired Molybdenum Catalyst for Aqueous Perchlorate Reduction, Journal of the American Chemical Society (2021). https://doi.org/10.1021/jacs.1c00595
