Chemical Effects of Air Plasma Species on Aqueous Solutes in Direct and Delayed Exposure Modes: Discharge, Post-discharge and Plasma Activated Water

Abstract

The chemical interaction between non-thermal plasma species and aqueous solutions is considered in the case of discharges in humid air burning over aqueous solutions with emphasis on the oxidizing and acidic effects resulting from formed peroxynitrite ONOO⁻ and derived species, such as transient nitrite and stable HNO₃. The oxidizing properties are mainly attributed to the systems ONOO⁻/ONOOH [E°(ONOOH/NO₂) = 2.05 V/SHE], ^·OH/H₂O [E°(^·OH/H₂O) = 2.38 V/SHE] and to the matching dimer system H₂O₂/H₂O [E°(H₂O₂/H₂O) = 1.68 V/SHE]. ONOOH tentatively splits into reactive species, e.g., nitronium NO⁺ and nitrosonium NO ⁺₂ cations. NO⁺ which also results from both ionization of ^·NO and the presence of HNO₂ in acidic medium, is involved in the amine diazotation/nitrosation degradation processes. NO ⁺₂ requires a sensibly higher energy than NO⁺ to form and is considered with the nitration and the degradation of aromatic molecules. Such chemical properties are especially important for organic waste degradation and bacterial inactivation. The kinetic aspect is also considered as an immediate consequence of exposing an aqueous container to the discharge. The relevant chemical effects in the liquid result from direct and delayed exposure conditions. The so called delayed conditions involve both post-discharge (after switching off the discharge) and plasma activated water. An electrochemical model is proposed with special interest devoted to the chemical mechanism of bacterial inactivation under direct or delayed plasma conditions.

Appendices

Appendix 1

Exchange of n electrons between an oxidizer and a reducer (Ox + ne = Red) is governed by the Gibbs free energy ΔG, i.e.

$$\Delta {\text{G}} = - {\text{n}}F\left[ {E\left( {{\text{Ox}}/{\text{Red}}} \right) - E^\circ \left( {{\text{H}}^{ + } /{\text{H}}_{2} } \right)} \right],$$

(44)

which is related to the potential of the system E(Ox/Red) and to that of the hydrated proton E°(H⁺/H₂). E(Ox/Red) may be expressed in terms of the activities a(i) and approximated by concentrations of the solutes C(i) for dilute solutions:

$$E\left( {{\text{Ox}}/{\text{Red}}} \right) \approx E^\circ \left( {{\text{Ox}}/{\text{Red}}} \right) + \left( {RT/{\text{n}}F} \right){\text{Ln}}\left[ {{\text{C}}\left( {\text{Ox}} \right)/{\text{C}}\left( {\text{Red}} \right)} \right]$$

(45)

$$E\left( {{\text{Ox}}/{\text{Red}}} \right) \approx E^\circ \left( {{\text{Ox}}/{\text{Red}}} \right) + \left( {2.303 \times {\text{RT}}/{\text{nF}}} \right){ \log }_{10} {\text{C}}\left( {\text{Ox}} \right)/{\text{C}}\left( {\text{Red}} \right)$$

(46)

where E°(Ox/Red) is the standard oxidation potential of the system (in volts, versus the standard hydrogen electrode potential, which is conventionally zero for all temperatures) for solute unit activity; F is the Faraday constant (F = 96,484.56 C mol⁻¹), R is the gas constant (8.314 J K⁻¹ mol⁻¹) and T(K) is the temperature. The value of the Nernst–Peters coefficient (2.303 × RT/F) is close to 0.059 V at room temperature.

Most of electrochemical equilibria concerning organic compounds involve protons according to the equation:

$${\text{Ox}} + {\text{ne}} + 2q{\text{H}}^{ + } = {\text{Red}} + {\text{H}}_{2} {\text{O}}$$

(47)

and they are thus accounted by the pH-dependent form of the Nernst law:

$$E\left( {{\text{Ox}}/{\text{Red}}} \right) \approx E^\circ \left( {{\text{Ox}}/{\text{Red}}} \right) + \left( {RT/{\text{n}}F} \right){\text{Ln}}\left[ {\left( {{\text{C}}\left( {\text{Ox}} \right) \times {\text{CH}}^{ + } } \right)^{q} /{\text{C}}\left( {\text{Red}} \right)} \right]$$

(48)

$${\text{or}}\quad E\left( {{\text{Ox}}/{\text{Red}}} \right) \approx E^\circ \left( {{\text{Ox}}/{\text{Red}}} \right) + \left( {2.303 \times RT/{\text{n}}F} \right){ \log }_{10} {\text{C}}\left( {\text{Ox}} \right)/{\text{C}}\left( {\text{Red}} \right) - \left( {2.303 \times RT/{\text{n}}F} \right)q*pH$$

(49)

The solution potential is then pH-dependent since it is governed by the actual concentrations of electron exchangers. Assuming that the considered species present single acid–base properties, (e.g., one proton exchange between the acid and its matching base: Ox = Ox⁻ + H⁺, constant K_a and Red = Red⁻ + H⁺, constant K′_a), the concentrations C(i) in Ox and Red forms are affected by acidity. For introduced concentrations C°(Ox) and C°(Red), the actual concentrations are C(Ox) and C(Red) with

$${\text{C}}^\circ \left( {\text{Ox}} \right) = {\text{C}}\left( {\text{Ox}} \right) + {\text{C}}({\text{Ox}}^{ - } ) = {\text{C}}\left( {\text{Ox}} \right)\left[ {1 + {\text{K}}_{\text{a}} /{\text{C}}\left( {{\text{H}}^{ + } } \right)} \right] = {\text{C}}\left( {\text{Ox}} \right) \times\upalpha\left( {\text{Ox}} \right)$$

(50)

and similarly

$${\text{C}}^\circ \left( {\text{Red}} \right) = {\text{C}}\left( {\text{Red}} \right)\left[ {1 + K^{{\prime }}_{\text{a}} /{\text{C}}\left( {{\text{H}}^{ + } } \right)} \right] = {\text{C}}\left( {\text{Red}} \right) \times\upalpha\left( {\text{Red}} \right)$$

(51)

where the side reaction coefficients relevant to protons α(Ox) and α(Red) are occasionally referred to masking coefficients. The Nernst–Peters equation applied to Ox and Red is then:

$$E \approx E^\circ \left( {{\text{Ox}}/{\text{Red}}} \right) + \left( {2.303 \times RT/{\text{n}}F} \right){ \log }_{10} [{\text{C}}^\circ \left( {\text{Ox}} \right)/{\text{C}}^\circ \left( {\text{Red}} \right)\left] { + \left( {2.303 \times RT/{\text{n}}F} \right){ \log }_{10} } \right[\upalpha\left( {\text{Red}} \right)/\upalpha\left( {\text{Ox}} \right)]$$

(52)

and (after correction for the activity coefficients if necessary) the formal standard potential E′° is clearly pH dependent due to the logarithm term:

$$E^{{{\prime }\circ }} = E^\circ \left( {{\text{Ox}}/{\text{Red}}} \right) + \left( {2.303 \times RT/{\text{n}}F} \right){ \log }_{10} [\upalpha\left( {\text{Red}} \right)/\upalpha\left( {\text{Ox}} \right)]$$

(53)

In the most of cases, the masking coefficients take a simplified expression in a given pH range, so that E′° = f(pH) is linear in discrete pH domains.

Example: The quinone C₆H₄O₂/hydroquinone C₆H₄(OH)₂ system exchanges 2e according to:

$${\text{Q}} + 2{\text{H}}^{ + } + 2{\text{e}} = {\text{QH}}_{2} .$$

(54)

In the acidity range 0 < pH < 7, the oxidizer remains in the molecular form Q, while the reducer QH₂ can release a proton and yield QH⁻ (0 < pK′_a < 7). Then

$$\upalpha\left( {\text{Ox}} \right) = 1\;{\text{and}}\;\upalpha\left( {\text{Red}} \right) = 1 + {\text{K}}^{{\prime }}_{\text{a}} /{\text{C}}\left( {{\text{H}}^{ + } } \right)$$

(55)

and α(i) take simplified expressions, e.g., α(Red) ≈ K_′a/C(H⁺). The resulting E′° expressions are gathered in Table 7.

Table 7 E′° expressions for selected pH parameters

The E′° = f(pH) plot presents two lines of respective slopes −0.06 and −0.03 V per pH unit for Q/QH₂ for pH < pK′_a, and Q/QH⁻ for pK′_a < pH < 7.

Basic information of the E′° versus pH diagrams can be found in [25, 102, 103] and for ammonia, azide, tryptophan and for trimethylamine and aniline in [104].

Appendix 2

A better understanding of aged and diluted PAW efficacy may be illustrated (Fig. 7) with the help of E′° versus pH graphs. Let (P) refer to newly prepared PAW, which contains the oxidising Plasma Active Species in given respective concentrations, and (T) to the target solution. The species P are reduced into R (P + ne + qH⁺ = R) according to the matching Nernst’s equation:

Fig. 7

Plasma activated water (dashed lines) remains a strong oxidiser for organic wastes T (solid line) after ageing (or/and dilution)

$$E\left( {\text{P}} \right) = E^\circ \left( {{\text{P}}/R} \right) + \left( {0.06/{\text{n}}} \right){ \log }\;a\left( {\text{P}} \right)/a\left( {\text{R}} \right){-}\left( {0.06/{\text{n}}} \right)\log \;a\left( {{\text{H}} + } \right)^{\text{q}} .$$

(56)

The associated E′° versus pH plot is thus given by:

$$E^{{{\prime }\circ }} \left( {\text{P}} \right) \approx {\text{E}}^\circ \left( {{\text{P}}/{\text{R}}} \right) - \left( {0.06 \times {\text{q}}/{\text{n}}} \right){\text{pH}}$$

(57)

and is represented by a line with the slope (0.06q/n). Point A on line (P) represents the initial PAW solution. Dilution of PAW often takes place before incorporating a PAW aliquot to (T). Diluting (P) induces a decrease in the activities of the active species a(P) and in the produced species a(R) which usually remain in the same ratio. The resulting plot is thus not modified, so that the representative point remains A. However, in the case that only a(P) is affected (and lowered), the new representative point B is located on a E′° versus pH line parallel to line E(P) i.e. with the same slope 0.06 q/n.

Ageing of the PAW solution implies an increase in acidity (or a decrease in pH). The new position C or C′ of the oxidiser is placed on the E(P) or E′(P) line at a lower pH = pH′. For pH = pH′, the oxidation–reduction system of the target T is represented by point D on the red line. The reaction Gibbs free energy between the P and T species remains largely negative, so that the oxidation of T takes place easily.