(Invited) Molecular Dynamics Simulation Insights into Plasma Treatment of Emerging Pollutants in Water
Authors: Brault Pascal 
Published: February 2024

Eliminating organic pollutant molecules in water is a world challenge, that non-thermal plasmas at atmospheric pressure are aiming to address. Non thermal plasmas are used for treating contaminated water due to their ability to produce reactive oxygen and nitrogen species, the so-called RONS, which diffuse into water, react with pollutant molecules, and degrade them. Many experimental works have been designed, using different plasma sources [1, 2], but in all cases HO● radicals are acknowledged to play the main role in the degradation process.
Classical and ab-initio reactive molecular dynamics (MD) simulations are relevant techniques for describing interactions of HO● with pollutant molecules in water, leading to the knowledge of degradation products, chemical pathways and rates [3]. Basically, MD simulations are solving the Newton equations of motion of a set of species, say atoms, molecules, clusters, etc. It is able to describe system size up to billion of species. It only needs the knowledge of interactions (force fields) between all species as well as initial conditions, preferably consistent with experiments. So it is suitable for addressing chemical reactivity. Classical reactive MD simulations are using available semi-empirical force fields, such as reaxFF, while ab-initio MD are required when no such force field is available. In this case, force fields are calculated quantum mechanically at relevant time steps of MD simulations.
In fact, MD simulations act as a predictive "virtual microscope" provided the force fields are enough precise.
In this work, MD simulations of the interaction of HO● radicals with molecules representative of various emerging pollutant families, as pesticide, antibiotics, PFAS in water, are studied. For mimicking HO● radicals release into water by non-thermal plasma, HO● radicals are periodically injected into a simulation box containing a pollutant molecule surrounded by several tens of water molecules. For obtaining statistically significant results, at least 10 simulations, with different initial conditions are run. The products that are formed will be compared with experiments, especially HPLC, GCMS and MS/MS measurements of plasma treated water.
Another approach, consists in introducing a temperature ramp during the simulation for determining at which temperature products are appearing and in which order. Such an approach is revealing the reaction barriers up to full decomposition of the pollutant molecule. It can thus, by back-analysis, help to define which plasma or other advanced oxidation processing will be relevant [3].

Acknowledgements

Part of this work has been supported by Conseil Régional Centre-Val de Loire with grant #2021-00144786 for project
Perturb’Eau. Monica Magureanu, Corina Bradu, Florin Bilea, Olivier Aubry, Dunpin Hong, Manoj Panyamthatta-Tayaroth are gratefully acknowledged for stimulating discussions and for providing experimental results.

References

[1] P. Vanraes, A. Bogaerts, Plasma physics of liquids—A focused review. Appl. Phys. Rev.
5, 031103 (2018)
[2] M. Magureanua, F. Bilea, C. Bradu, D. Hong. A review on non-thermal plasma treatment of water contaminated with antibiotics, J. Hazardous Materials
417, 125481 (2021)
[3] P. Brault et al, Insight into plasma degradation of paracetamol in water using a reactive molecular dynamics approach, J. Appl. Phys. 129, 183304 (2021)