Degradation products of profenofos as identified by high-field FTICR mass spectrometry: Isotopic fine structure approach

ABSTRACT

This study was performed to identify the degradation products of profenofos “a phenyl organothiophosphate insecticide” in raw water (RW) collected from the entry point of Metropolitan Water Works Authority “Bangkaen, Thailand” and ultrapure water (UPW) with and without TiO₂ under simulated sunlight irradiation. Degradation of profenofos was followed with ultrahigh performance liquid chromatography (UHPLC) and follows pseudo first-order kinetic. Accordingly, high-field FTICR mass spectrometry coupled to an electrospray ionization source was used to reveal the degradation routes of profenofos and the isotopic fine structures (IFS) elucidations to approve the chemical structures of its degradation products. More degradation products were detected in UPW as compared to RW. Consequently, two main degradation pathways namely (i) interactive replacements of bromine and hydrogen by hydroxyl functional groups and (ii) rupture of PO, PS, CBr and CCl bonds were observed. None interactive replacement of chlorine by hydroxyl functional group was detected. Accordingly, mechanistical pathways of the main degradation products were established.

KEYWORDS:

• Profenofos
• degradation products
• FTICR MS
• IFS

Funding

Financial support from the Center of Excellence for Innovation in Chemistry (PERCH-CIC), Office of Higher Education Commission, Ministry of Education. Suan Dusit Rajabhat University scholarship and Kasetsart University Research and Development Institute (KURDI) are gratefully acknowledged.

AppendixMaterials and methods

Ion Chromatography

Ion chromatograph consisted of a Dionex 500 ion chromatograph equipped with an Ion®PacAS11-HC (2×250mm) column. An automated potassium hydroxide eluent generator (EG40) and suppresses conductivity detector were employed to analyse small organic acids and inorganic degradation products. The gradient elution was started from 1.0 mM KOH and linearly increased to 2.0 mM KOH over 7 min and further linearly increased to 50 mM KOH in 10 min. The column was then equilibrated under the initial condition for 10 min prior to the next injection. The flow rate and injection volume were 0.3 mL/min and 25 μL, respectively.

Figure A1. Typical ion chromatogram of the degradation of profenofos after 6 h irradiation.

Table A1. Calculated rate constant of the photodegradation of profenofos at different TiO₂ concentrations. Conditions: Profenofos concentration: 5.35 × 10⁻⁵ mol/L, pH = 5.57.

Table A2. Rate constant of the photodegradation of profenofos at different pH solutions. Conditions: Profenofos concentration: 5.35 × 10⁻⁵ mol/L, TiO₂ concentration: 0.5 g/L.

Table A3. Elaborated FTICR MS data of the photodegradation of profenofos in raw and pure water elaborated as analyzed in positive and negative ionization modes (-: not present a: present only in PW/TiO₂).

Results and discussion

Optimized values of TiO₂ amount and initial pH

Solar radiation is well known as a factor that influences photochemically micropollutants after their use in the environment. Here, dependence of TiO₂ loading on the degradation of profenofos under simulated solar radiation was investigated using different doses of TiO₂ ranging from 0.1 to 1.5 g/L and a concentration of profenofos of 5.35 × 10⁻⁵ mol/L. Table A1 shows the calculated rate constant of the degradation of profenofos and the used concentrations of TiO₂. As shown in Table A1, the rate constant of profenofos increases by increasing titanium dioxide concentrations and tends to converge towards a limit at the titanium dioxide concentration of 1.5 g/L. It is clear that increasing TiO₂ doses influence significantly the rate constant of the degradation of micropollutants. This behavior is likely due to the decrease in light transmission at higher concentrations where light scattering becomes a significant problem since the particulates can filter the solar radiation from the rest of the dispersion.^[1-3] The TiO₂ loading of 0.5 g/L was used for subsequent experiments.

Accordingly, pH is also investigated since it's considered as a principal parameter in the photochemical transformation of micropollutants in aqueous suspensions of TiO₂. pH influence the surface charge properties of TiO₂ by enhancing and/or inhibiting its adsorption-desorption capacity. The point of zero charge (pzc) of TiO₂ is around 6.25.^[4-8] Below this value, TiO₂ surface is positively charged while above this value is negatively charged. The effect of the pH on the photodegradation rate of profenofos was investigated in the pH range from 2.0 to 7.0. Table A2 shows the calculated constant rate of the degradation of profenofos at different pH. The calculated rate constant increases as pH increases up to 4.54 and then slowly increases is observed (Table A2). This is to be expected as profenofos is a neutral molecule and the point of zero charge of the TiO₂ is at a pH around 6.25. This indicates weak dependence of the reaction rate on solution pH around neutral condition. Similar behavior was observed in the case of benzamide.^[9] The rate, expectedly, is therefore high around the pH of 6. Similar observation was mentioned in the photocatalytic degradation of phosalone in aqueous TiO₂.^[10] For subsequent studies, since the initial solution pH is 5.57 it was used without prior pH adjustment.

Ion chromatography analysis

Ion chromatography was used to detect small organic molecules formed ring fragmentation oxidized with TiO₂ and inorganic ions formed from the heteroatoms such as sulfate, bromide and chloride as shown in Figure A1 After 6 h irradiation time a mass balance between profenofos disappeared and phosphate involved was ca 91 %.