Elsevier

Water Research

Volume 133, 15 April 2018, Pages 47-59
Water Research

Plasma-activation of tap water using DBD for agronomy applications: Identification and quantification of long lifetime chemical species and production/consumption mechanisms

https://doi.org/10.1016/j.watres.2017.12.035Get rights and content

Highlights

  • Cold plasma activation of tap water induces modification of ion concentrations.

  • Production/consumption of ions is influenced by operating in hermetic enclosure.

  • Electrical conductivity of activated tap water can be deduced from ion concentrations.

  • Irrigation of lentils seeds with plasma-activated tap water increases plant growth.

Abstract

Cold atmospheric plasmas are weakly ionized gases that can be generated in ambient air. They produce energetic species (e.g. electrons, metastables) as well as reactive oxygen species, reactive nitrogen species, UV radiations and local electric field. Their interaction with a liquid such as tap water can hence change its chemical composition. The resulting “plasma-activated liquid” can meet many applications, including medicine and agriculture.

Consequently, a complete experimental set of analytical techniques dedicated to the characterization of long lifetime chemical species has been implemented to characterize tap water treated using cold atmospheric plasma process and intended to agronomy applications. For that purpose, colorimetry and acid titrations are performed, considering acid-base equilibria, pH and temperature variations induced during plasma activation. 16 species are quantified and monitored: hydroxide and hydronium ions, ammonia and ammonium ions, orthophosphates, carbonate ions, nitrite and nitrate ions and hydrogen peroxide. The related consumption/production mechanisms are discussed. In parallel, a chemical model of electrical conductivity based on Kohlrausch's law has been developed to simulate the electrical conductivity of the plasma-activated tap water (PATW). Comparing its predictions with experimental measurements leads to a narrow fitting, hence supporting the self-sufficiency of the experimental set, I.e. the fact that all long lifetime radicals of interest present in PATW are characterized.

Finally, to evaluate the potential of cold atmospheric plasmas for agriculture applications, tap water has been daily plasma-treated to irrigate lentils seeds. Then, seedlings lengths have been measured and compared with untreated tap water, showing an increase as high as 34.0% and 128.4% after 3 days and 6 days of activation respectively. The interaction mechanisms between plasma and tap water are discussed as well as their positive synergy on agronomic results.

Introduction

Civilization is founded on agriculture, remaining as important today as its beginning 10,000 years ago. Even if mechanization, technological innovations and chemicals have ensured higher productivity in regard of the two last centuries, modern agriculture must face new challenges today. The United Nations Food and Agriculture Organization (FAO) has indicated that global food shortages will become three times more likely owing to climate change and the rapid development of urbanization, industrialization and world population. In parallel, micropollutants (food additives, industrial chemicals, pesticides, pharmaceuticals and personal care products) constitute a class of hazardous products which rise major concerns. Despite their very low concentrations (detectable in the ng/L-μg/L range), many studies have shown their harmful effects on the environment (Milla et al., 2011, Rizzo et al., 2013) and in particular on agriculture (Jampeetong and Brix, 2009, Roosta et al., 2009).

According to the FAO, one of the most viable processes to limit food shortages is to increase crop yields, so far mainly limited by seed surface and by water and soil contamination with bacteria, microorganisms, fungi and various chemical compounds. There are also environmental concerns like insect pests, adverse weather conditions or human as storage and fertilization.

Among the solutions considered to face such challenges, cold atmospheric plasmas (CAP) appear more and more as an innovative and ecofriendly approach. CAP are weakly ionized gases that can be generated in ambient air using simple devices called dielectric barrier discharges (DBD) reactors and that can easily be coupled with green electricity. Applying a high voltage between the two electrodes – with at least one of them covered by a dielectric barrier – enables the production of an intense electric field which in turn ionizes the gas (e.g. ambient air) confined in the interelectrode gap as described in Fig. 1 a. The dielectric barrier is of major importance as it prevents any arc formation and permits only the generation of cold microdischarges. As a consequence, energetic species (e.g. electrons, metastables) are generated as well as reactive oxygen species including atomic oxygen, hydroxyl radicals, ozone, but also radical ions, namely O2+., O2−., O3−. (Marotta et al., 2012).

DBD reactors can be utilized for the treatment (also called activation) of liquids: the gaseous radicals can directly interplay at the liquid interface or even diffuse in depth, leading to complex chemical mechanisms that are still being deciphered. In addition, the CAP can generate transient electric field, flow kinetics and heating effects which can directly impact on the chemical mechanisms. Such plasma-liquid interactions have recently drawn considerable attention in the field of water treatment: numerous studies have indeed shown the effectiveness of plasma DBD activation to decontaminate water, e.g. removing pesticides (Vanraes et al., 2017), phenols (Marotta et al., 2012) or pharmaceutical compounds (Magureanu et al., 2015). Other methods than the direct interaction between plasma and water are also used. Examples include the use of water drop plasma for Ibuprofen removal (Wang et al., 2017) or plasmas directly generated in water to produce “plasma bubbles” for E. coli inactivation (Ma et al., 2017b) and degradation process of chitosan (Ma et al., 2017a).

At the same time, the number of publications dealing with plasma-activated liquids (PAL) has dramatically increased. If PALs are widely used in the biomedical field with applications in the treatment of cancer (Tanaka et al., 2016, Boehm et al., 2016, Merbahi et al., 2017), inactivation of bacteria (Zhang et al., 2016, Shen et al., 2016) or fungi (Panngom et al., 2014), they are still poorly investigated in agriculture applications. Still, the potential of CAP seems tremendous in regard of emerging works performed on various agronomic models (lentils, radishes, tomatoes and sweet peppers) and showing important effects on seeds germination promotion and seedlings stems elongations (Zhang et al., 2017, Sivachandiran and Khacef, 2017, Lindsay et al., 2014). In that framework, CAP activations could find several additional benefits to boost crop yields: first as a cleaning process of water allowing the reduction of micro pollutants and microorganisms and second as a process permitting the synthesis of radicals-based green fertilizers. The objective of this work is to precisely evaluate the chemical interactions between cold atmospheric plasmas and tap water to identify the long lifetime species that could have a beneficial impact in the treatment of seeds.

In this study, we present a simple set of techniques enabling the quantification of 11 chemical species by colorimetric method (ammonia, ammonium, orthophosphates, nitrites, nitrates and hydrogen peroxide), 3 species by acid titration (carbonate ions) and 2 species by temperature and pH measurements. The predominance of the acid-base pairs is evaluated considering selective protocols and expression of Ka equilibria constants. All the concentrations measurements include uncertainties calculations taking into account variations in pH and temperature resulting from the liquid activation. Chemical reactions leading to variations in concentrations of these chemical species are discussed as well as the influence of plasma activation time and configuration of the DBD reactor (with/without hermetic enclosure).

A chemical model of plasma-activated Tap water “PATW” electrical conductivity (σPATW) has been developed according to the Kohlrausch's law: each individual conductivity of the ions present in the PATW is calculated, their sum leading to the theoretical σPATW. The values predicted by our model are compared with experimental measurement of σPATW. The appropriate fitting between simulation and experimental curves clearly demonstrate the self-sufficiency of our experimental approach, meaning that the experimental set includes the necessary techniques for the characterization of all relevant long lifetime chemical species in PATWs.

Finally, to demonstrate the potential of plasma-activated water in life sciences applications and in particular in agriculture, PATW has been utilized as an irrigation liquid for coral lentils (lens culinaris). This agronomical model has already been used by our team to investigate biological phenomena such as seeds dormancy, germination and early-stages of seedlings development (Zhang et al., 2017). They present a very low dormancy and a high germination rate. Results dealing with germination and stems elongation are presented.

Section snippets

Plasma source & diagnostics of the plasma phase

Water has been activated using a dielectric barrier discharge (DBD) in a configuration dedicated to agronomical applications. An AC voltage delivered by a function generator (ELC Annecy France, GF467AF) is augmented by a power amplifier (Crest Audio, 5500 W, CC5500) and supplies this DBD at 500 Hz. A ballast resistor (250 kΩ, 600 W) protects the power supply from any excessive currents, as shown in Fig. 1 b. The excitation electrode is composed of a metal plate from which a matrix of 4*4 metal

Temperature

Upon its plasma activation, tap water is gradually heated, inducing an increase in its temperature as reported in Fig. 2. Starting at ambient room temperature (27 °C), the tap water under plasma exposure shows an increase of 3.60 ± 0.72 °C after 5 min and 9.03 ± 0.78 °C after 30 min, which corresponds to an absolute temperature of 36.15 ± 1.26 °C. The rise in temperature is not linear: after 25 min of activation (90% response time), water heating slows down and its temperature reaches a plateau

Electrical conductivity

Variations of σPATW result from charged species production and consumption mechanisms occurring in the liquid phase. These mechanisms result from a complex interaction between the plasma and liquid phases. This interaction includes (i) the diffusion of gaseous species from the plasma to the liquid bulk, (ii) the stimulation of the liquid interface giving rise to new species in the subinterface layer and then to their in-depth diffusion and (iii) PATW heating effects induced by the plasma source.

Conclusion

In this article, we have characterized and quantified 16 long lifetime chemical species in PATW. We have shown that the chemical composition of PATW has no impact on pH but a major one on electrical conductivity. Reactions between gas phase species and tap water lead to the formation of aqueous species like hydrogen peroxide, nitrite, nitrate or ammonia. Conversely this interaction activates the consumption of bicarbonates ions.

Due to acid/base equilibria and the increase of water temperature

Acknowledgements

This work has been achieved within the LABEX Plas@Par project, and received financial state aid managed by the Agence Nationale de la Recherche, as part of the Programme Investissements d'Avenir (PIA) under the reference ANR-11-IDEX-0004-02. Also, this work was supported by grant number 265356 from CNRS (Mission pour l'interdisciplinarité). The authors thank P. Auvray for assembling the AC generator.

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