Degradation Kinetics and By-Products of Naproxen in Aqueous Solutions by Gamma Irradiation

Abstract

This study focused on the degradation of the emerging contaminant naproxen in aqueous solutions by gamma irradiation. Influence of initial naproxen concentration, solution pH, various additives (CH₃OH, H₂O₂, \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_3^ -$$ \end{document} , and humic acid), and absorbed dose on naproxen degradation were investigated. At a naproxen concentration of 23.6 mg/L and an absorbed dose of 1.0 kGy, the extent of naproxen degradation was 98%. The degradation reaction of naproxen followed first-order like kinetics. Addition of 40 mg/L humic acid or 0.5% H₂O₂ enhanced the naproxen degradation, whereas 1.5% CH₃OH and 1.5% H₂O₂ inhibited naproxen degradation. Added \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_3^ -$$ \end{document} slightly decreased naproxen degradation. The naproxen degradation efficiency was higher under acidic conditions than in neutral or alkaline media. The solution pH value became lower as the absorbed dose increased. Identification of byproducts revealed that decarboxylation is the principal initial process in the degradation of naproxen under the conditions used in these experiments.

Introduction

Naproxen (C₁₄H₁₄O₃) belongs to the nonsteroidal anti-inflammatory class of drugs (Boynton et al., 1988). It is one of the most widely used medicines for mild-to-moderate pain relief in the treatment of osteoarthritis, rheumatoid arthritis, ankylosing spondylitis, primary dysmenorrhea, tendinitis, bursitis, and other conditions (Polat and Korkmaz, 2002).

In recent years, naproxen and naproxen-based products have been detected in surface water, groundwater, wastewater, and even in drinking water at a concentration range from ng/L to several μg/L (Marco-Urrea et al., 2010). Unfortunately, these compounds have adverse effects on biota such as impairing the lipid peroxidation system of bivalves (Gagné et al., 2006). Naproxen contamination is, therefore, becoming a serious problem.

Many studies have been carried out on naproxen removal, mainly via biodegradation and physicochemical treatments. Marco-Urrea et al. (2010) reported that white-rot fungus could degrade naproxen. Felis et al. (2007) used H₂O₂/UV to degrade naproxen in surface water; 90% naproxen degraded, but only 10% mineralization was achieved. Boyd et al. (2005) showed that naproxen could be removed from water by chlorination, but disinfection by-products formed. In the study of Tixier et al. (2003), phototransformation and biodegradation were identified as the main processes for elimination of naproxen in surface water. Rivera-Jimenez et al. (2011) reported that Co-Al-HDTMA-NaBt adsorbent could effectively remove naproxen from water at natural pH.

Gamma irradiation is an advanced oxidation method. It has been employed for decomposition of various pollutants in aqueous solutions (Mohamed et al., 2009; Cao et al. 2011). The first study of gamma-irradiated naproxen was conducted by Polat and Korkmaz (2002). In this study, only the kinetic features of the radicals induced in gamma-irradiated naproxen sodium were investigated, and the effects of operational parameters on naproxen degradation by gamma irradiation were not discussed. Therefore, further studies are needed to determine the feasibility of irradiation as a way of treating naproxen-contaminated solutions.

The major aim of this work was to study the effects of operational parameters, such as solution pH and various additives (H₂O₂, CH₃OH, \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_3^ -$$ \end{document} , and humic acid), on naproxen degradation by gamma irradiation. These tests will assist in determination of experimental conditions for treatment of naproxen by gamma irradiation in natural aquatic environments. In addition, a degradation path for naproxen was proposed.

Materials and Methods

Chemicals and reagents

Naproxen, CH₃CN, CH₃COOH, and CH₃OH (HPLC grade) were purchased from Sigma-Aldrich. High-purity humic acid was also obtained from Sigma-Aldrich. H₂O₂, NaNO₃, HCl, and NaOH (analytical-grade) were obtained from Shanghai Chemicals Factory, China. Ultrapure water was generated with a Milli-Q system (Elix5+Milli-Q A10).

Procedures

Sample preparation

Following on from the studies of Boyd et al. (2005) and Marco-Urrea et al. (2010), a 23.6 mg/L naproxen solution was used to test the effect of absorbed dose on degradation efficiency and to examine the changes of pH and total organic carbon (TOC) after gamma irradiation. Different additives (H₂O₂, CH₃OH, \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_3^ -$$ \end{document} , and humic acid) at various concentrations were added into the naproxen aqueous solutions to examine their effects on naproxen degradation. Dilute HCl and NaOH aqueous solutions were used to vary the pH of the naproxen aqueous solutions and test the effect of pH value on degradation.

Irradiation process

Gamma irradiation of naproxen was carried out by using a ⁶⁰Co source. The initial activity of the source was around 1.85×10¹⁶ Bq. The irradiation time was fixed at 5 h. Naproxen solutions were allowed to equilibrate at atmospheric pressure and room temperature (22°C±2°C) before irradiation. The dose absorbed by the samples was determined with a silver dichromate dosimeter (Zhang et al., 2008).

Analysis

According to the method of Méndez-Arriaga et al. (2008), an HPLC system (Agilent 1200) was employed to determine the naproxen concentration. The naproxen degradation efficiency was calculated from Equation (1). \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} \eta = \frac { C_0 - C_t } { C_0 } \times 100 \% \tag {1} \end{align*} \end{document}

η—The naproxen degradation efficiency (%);

C_t—The residual naproxen concentration after gamma irradiation (mg/L);

C₀—The initial naproxen concentration (mg/L).

According to the method reported by Aresta et al. (2006), LC-MS (ThermoQuestLCQ Duo) was employed to identify naproxen and the products of its radiolytic degradation. TOC was determined by a TOC analyzer (Shimadzu, TOC-5000A). pH values were measured by a pH monitor (Shanghai Kangyi Instrument Co., Ltd., PHS-2C).

Naproxen radiation chemical yield (G-value)

The radiation chemical yield of naproxen (defined by the number of molecules formed or destroyed in solutions absorbing 100 eV of radiation energy) was calculated by using Equation (2) (Woods and Pikaev, 1994; Sánchez-Polo et al., 2009). \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm G } = \frac { \Delta R ( 6.023 \times 10^ { 23 } ) } { { \rm D } ( 6.24 \times 10^ { 16 } ) } \tag {2} \end{align*} \end{document}

ΔR —The amount of reduced naproxen (mol/L);

D —The absorbed dose (Gy).

Results and Discussion

Radiolytic degradation of naproxen

Degradation of water pollutants by gamma irradiation is initiated by the primary products of water radiolysis (e.g.,•OH, H•, \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm e}_{ \rm aq}^ -$$ \end{document} ), as shown in Equation (3). Under certain conditions, reactions between these radicals occur during gamma irradiation: (i) In the presence of O₂, H• and \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm e}_{ \rm aq}^ -$$ \end{document} form \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${{ \rm HO}_2}^{ \bullet}$$ \end{document} and \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${{ \rm O}_2}^{ \bullet - }$$ \end{document} [Eqs. (4) and (5)]; (ii) at higher doses, more•OH and \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm e}_{ \rm aq}^ -$$ \end{document} form, and can react [Eq. (6)]; (iii) under acidic conditions, H⁺ and \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm e}_{ \rm aq}^ -$$ \end{document} form H• [Eq. (7)]; (iv) \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${{ \rm HO}_2}^{ \bullet}$$ \end{document} and its conjugate base \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm O}_2^{ - }$$ \end{document} exist in pH-dependent equilibrium [Eq. (8)]; (v) \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${{ \rm HO}_2}^{ \bullet}$$ \end{document} reacts with either itself or \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${{ \rm O}_2}^{ \bullet - }$$ \end{document} to form H₂O₂ and O₂, [Eqs. (9) and (10)]; and (vi) H• and OH⁻ form \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm e}_{ \rm aq}^ -$$ \end{document} and H₂O [Eq. (11)]. \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} \begin{split}{ \rm H}_2 { \rm O} \rightarrow { \rm e}_{ \rm aq}^ - ( 2.6 ) + { \rm H} \bullet ( 0.6 ) + \bullet { \rm OH} ( 2.7 ) + { \rm H}_2 ( 0.45 ) \\ \quad+ { \rm H}_2{ \rm O}_2 ( 0.71 ) + { \rm H}_3{ \rm O}^ + ( 2.6 )\end{split} \tag{3}\end{align*} \end{document}

(The values in parentheses are the radiation chemical yields of these species per 100 eV of absorbed energy) (Woods and Pikaev, 1994).

\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} & {\rm H} \bullet + { \rm O}_2 = {{ \rm HO}_2}^{ \bullet} \\ &\quad k = 2.1 \times 10^{10}\, { \rm M}^{ - 1}{ \rm s}^{ - 1} \ ( \hbox{\rm Basfar} \ et \ al. , \ 2005 ) \tag{4} \end{align*} \end{document}

\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} &{\rm e}_{\rm aq}^ - + {\rm O}_2 = {{\rm O}_2}^{ \bullet - } \\& \quad k = 1.9 \times 10^{10} \,{\rm M}^{ - 1}{\rm s}^{ - 1} \ ( \hbox{\rm Basfar} \ et \ al. , \ 2005 ) \tag{5} \end{align*} \end{document}

\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} &{\rm e}_{\rm aq}^{ - } + \bullet { \rm OH} = {\rm OH}^ - \\ & \quad k = 3.0 \times 10^{10}\, {\rm M}^{ - 1}{\rm s}^{ - 1}\ ( \hbox{\rm Basfar} \ et \ al. , 2005 ) \tag{6} \end{align*} \end{document}

\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} &{\rm e}_{\rm aq}^ - + {\rm H}^+ = {\rm H} \bullet \\ &\quad k = 2.3 \times 10^{10} \, {\rm M}^{- 1}{\rm s}^{- 1}\ ({\rm S} \acute{\rm a} {\rm nchez} \hbox{-} {\rm Polo} \ \ et \ al., \ 2009) \tag{7} \end{align*} \end{document}

\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} &{{\rm HO}_2}^{ \bullet} = {{\rm O}_2}^{ \bullet - } + {\rm H}^ + \\ &\quad k = 8 \times 10^5 {\rm s}^{ - 1} \ ( \hbox{\rm Basfar}\ et \ al. , \ 2005 ) \tag{8} \end{align*} \end{document}

\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} &{{\rm HO}_2}^{ \bullet} + {{\rm HO}_2}^{ \bullet} = {\rm H}_2{\rm O}_2 + {\rm O}_2 \\ &\quad k = 8.3 \times 10^5 \,{\rm M}^{ - 1}{\rm s}^{ - 1} \ ( {\rm S} \acute{\rm a}{\rm nchez} \hbox{-} {\rm Polo} \ et \ al. , \ 2009 ) \tag{9} \end{align*} \end{document}

\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} &{{\rm HO}_2}^{ \bullet} + {{\rm O}_2}^{ \bullet - } = {\rm H}_2{\rm O}_2 + {\rm O}_2 \ ( {\rm pH} < 7 ) \\ &\quad k = 9.7 \times 10^7\, {\rm M}^{ - 1}{\rm s}^{ - 1} \ ( \hbox{\rm Basfar} \ et \ al. , \ 2005 ) \tag{10} \end{align*} \end{document}

\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} &{\rm H} \bullet + {\rm OH}^ - = {\rm e}_{\rm aq}^ - + {\rm H}_2{\rm O} \\ &\quad k = 2.2 \times 10^7 \,{\rm M}^{ - 1}{\rm s}^{ - 1} \ ( {\rm S} \acute{\rm a}{\rm nchez} \hbox{-} {\rm Polo} \ et \ al. , \ 2009 ) \tag{11} \end{align*} \end{document}

The naproxen degradation efficiency at different absorbed doses is shown in Fig. 1. The naproxen concentration decreased with increasing absorbed dose, and most of naproxen in the aqueous solution degraded when exposed to a dose of 1.0 kGy. Gamma irradiation is an effective method to remove naproxen from aqueous solutions.

FIG. 1.

Degradation of naproxen as a function of absorbed dose.

The G values of naproxen [calculated using Eq. (2)] at different absorbed doses are shown in Table 1, and decreased with increasing absorbed dose. Due to the increase in radical species at higher absorbed doses, the absolute rates for radical–radical reactions [Eqs. (12 )–(15)] also increase, thus reducing the effective concentrations of radicals (e.g., •OH) available to react with naproxen, and, hence, decreasing the G value. The radiation chemical yield trend reported here is similar to the trends reported for radiolytic degradation of malathion and lindane (Mohamed et al., 2009). \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} &{ \bullet}{ \rm OH} + \bullet { \rm OH} = { \rm H}_2{ \rm O}_2 \\ &\quad k = 5.5 \times 10^9\, { \rm M}^{ - 1}{ \rm s}^{ - 1} \ ( \hbox{\rm Basfar} \ et \ al. , \ 2005 ) \tag {12} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} &{ \bullet} { \rm OH} + { \rm H} \bullet = { \rm H}_2{ \rm O} \\ &\quad k = 7.0 \times 10^9\, { \rm M}^{ - 1}{ \rm s}^{ - 1} \ ( \hbox{\rm Basfar} \ et \ al. , \ 2005 ) \tag {13} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} &{ \bullet} { \rm OH} + {\rm e}_{ \rm aq}^ - = { \rm OH}^ - \\ & \quad k = 3.0 \times 10^{10}\, { \rm M}^{ - 1}{ \rm s}^{ - 1} \ ( \hbox{\rm Basfar} \ et \ al. , \ 2005 ) \tag{14} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} &{ \rm H}_2 { \rm O} + { \rm H} \bullet + { \rm e}_{ \rm aq}^ - = { \rm H}_2 + { \rm OH}^ - \\ &\quad k = 2.5 \times 10^{10}\, { \rm M}^{ - 1}{ \rm s}^{ - 1} \ ( \hbox{\rm Basfar} \ et \ al. , \ 2005 ) \tag {15} \end{align*} \end{document}

Table 1.

G Values for Naproxen Removal Under Different Absorbed Doses

	Absorbed dose (kGy)
	0.1	0.3	0.5	0.7	1.0
G (×10⁻⁵ molecules/[100 eV])	5.61	2.40	1.65	1.30	0.97

By plotting the natural logarithm of naproxen residual concentration (C_t) as a function of the absorbed dose, a linear relationship could be derived: C_t=23.6e^−2.778D (R²=0.9929). This indicated that the kinetics of the naproxen degradation were of first order with regard to the absorbed dose.

Influence of operational parameters on the naproxen degradation

Effect of naproxen concentration

Figure 2 shows the results of the radiolytic degradation of naproxen at different initial concentrations. The initial concentration greatly affected the degradation of naproxen by gamma irradiation. The rate constant was largest at the lowest naproxen concentration studied—at a naproxen concentration of 11.8 mg/L, the rate constant was 4.01 kGy⁻¹. However, at initial concentrations of 23.6 and 40.2 mg/L, the rate constants were 2.778 and 2.174 kGy⁻¹, respectively. These results are consistent with published findings for radiolytic degradation of individual polychlorinated biphenyl (PCB) congeners (Mincher et al., 2002).

FIG. 2.

Effect of initial concentration on naproxen degradation.

Effect of solution pH

Figure 3 shows the significant effect of solution pH on naproxen degradation. Acidic conditions enhanced the naproxen degradation, because of the higher relative concentration of H• present [Eq. (7)]. However, in alkaline solutions, OH⁻ readily reacts with the H• to generate \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm e}_{ \rm aq}^ -$$ \end{document} [Eq. (11)], thereby increasing the concentration of \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm e}_{ \rm aq}^ -$$ \end{document} , enhancing the probability of recombination between •OH and \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm e}_{ \rm aq}^ -$$ \end{document} [Eq. (14)], and reducing the effective •OH concentration, thus leading to a decrease in the degradation efficiency of naproxen (Sánchez-Polo et al., 2009).

FIG. 3.

Effect of pH on naproxen degradation.

Effects of H₂O₂ and \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_3^ -$$ \end{document} on naproxen degradation

It is well known that H₂O₂ is an •OH promoter that can accelerate the degradation of pollutants in aqueous solutions (Basfar et al., 2005). Figure 4 shows the effect of H₂O₂ on naproxen degradation. In fact, 0.5% H₂O₂ accelerated the naproxen degradation as a result of increased •OH in the medium. However, 1.5% H₂O₂ inhibited naproxen degradation, thus indicating that a massive dosage of H₂O₂ does not lead to more available •OH in the aqueous solutions. Instead, it may compete with naproxen for •OH and, thus, reduce the naproxen degradation efficiency. These results are similar to those previously reported results by Sánchez-Polo et al. (2009).

FIG. 4.

Effects of H₂O₂ and \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_3^ -$$ \end{document} on naproxen degradation.

According to Fig. 4, 0.2484 and 0.4968 mM added \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_3^ -$$ \end{document} slightly inhibited the naproxen degradation, which is consistent with published findings for the radiolytic degradation of PCBs (Singh and Kremers, 2002). This is because \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_3^ -$$ \end{document} is an \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm e}_{ \rm aq}^ -$$ \end{document} scavenger that can compete with naproxen for \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm e}_{ \rm aq}^ -$$ \end{document} and, therefore, reduce the naproxen degradation efficiency.

Effects of CH₃OH and humic acid on naproxen degradation

As shown in Fig. 5, when 1.5% CH₃OH was added, the degradation efficiency of naproxen significantly decreased. The effect of CH₃OH is because of the well-known fact that it reacts more rapidly than naproxen with •OH and \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm e}_{ \rm aq}^ -$$ \end{document} in aqueous solutions [Eqs. (16) and (17)]. These reactions will reduce the yield of •OH and \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm e}_{ \rm aq}^ -$$ \end{document} , thus resulting in a decrease of naproxen degradation efficiency. \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} &{ \bullet} { \rm OH} + { \rm CH}_3{ \rm OH} = { \rm H}_2{ \rm O} + \bullet { \rm CH}_2{ \rm OH} + { \rm CH}_3{ \rm O} \bullet\\ &\quad k = 4.7 \times 10^8\, { \rm M}^{ - 1}{ \rm S}^{ - 1}\ ( \hbox{Wu and Bao} , \ 2002 ) \tag {16} \end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} &{ \rm e}_{ \rm aq}^ - + { \rm CH}_3{ \rm OH} = { \rm H} \bullet + { \rm CH}_3{ \rm O}^ -\\ &\quad k = 1.0 \times 10^4\, { \rm M}^{ - 1} { \rm S}^{ - 1} \ ( \hbox{Wu and Bao} , \ 2002 ) \tag {17} \end{align*} \end{document}

FIG. 5.

Effects of CH₃OH and humic acid on naproxen degradation.

When 40 mg/L humic acid was added, naproxen degradation efficiency was enhanced. The results are similar to those previously reported by Mohamed et al. (2009). Irradiated humic acid can generate excited triplet states (³HA*) and various reactive oxygen species, including •OH, ¹O₂, and H₂O₂ (Sandvik et al., 2000), thus increasing the concentrations of radicals for reaction with naproxen.

Variation of solution pH

The effect of gamma irradiation on solution pH value is shown in Fig. 6. The pH value decreased with increasing absorbed dose, going from a value of 6.56 without irradiation to 6.09, 5.81, 5.53, 5.36, and 5.07, at absorbed doses of 0.1, 0.3, 0.5, 0.7, and 1.0 kGy, respectively. This decrease in pH is possibly due to the production of a large amount of H₃O⁺ during the irradiation process [Eq. (3)].

FIG. 6.

Change of solution pH value with irradiation.

Variation in TOC during irradiation

Figure 7 describes the TOC change of naproxen aqueous solutions after irradiation. The TOC value decreased with increasing absorbed dose, which indicated that gamma irradiation could lead to both degradation and partial mineralization of naproxen in aqueous solutions.

FIG. 7.

Change of solution total organic carbon (TOC).

Identification of degradation products

The full-scan ESI mass spectrum of naproxen (negative ions) is reported in Fig. 8. The [M]⁻ (m/z 228.8) ion was clearly observed in the spectrum, along with the corresponding isotope peak. Fragmentation of deprotonated naproxen in the ion source leads to the formation of two main product ions, arising from the loss of CO₂ (m/z 184.9) and the consecutive losses of CO₂ and CH₃ (m/z 169.9).

FIG. 8.

Full-scan electrospray ionization (ESI) mass spectrum of naproxen (negative ions).

According to Fig. 9, the major molecular ion ([M]⁻) that correlated with the degradation of naproxen by gamma irradiation was at m/z 184.9, 44 u lower than the naproxen ion, thus indicating that gamma irradiation caused decarboxylation. The results are consistent with published findings for the photocatalytic degradation of naproxen (Méndez-Arriaga et al., 2008). The degradation pathway of naproxen by gamma irradiation is shown in Fig. 10.

FIG. 9.

Full-scan ESI mass spectrum of the m/z ion 184.9.

FIG. 10.

The degradation pathway of naproxen by gamma irradiation.

Conclusions

Gamma irradiation effectively degraded naproxen in aqueous solutions. At an absorbed dose of 1.0 kGy, naproxen degradation efficiency was 98%. The naproxen degradation process followed first-order like kinetics. The degradation yield was higher under acidic conditions than in neutral or alkaline media. Addition of 40 mg/L humic acid or 0.5% H₂O₂ enhanced the naproxen degradation. However, naproxen degradation was significantly inhibited by addition of 1.5% H₂O₂ or 1.5% CH₃OH, and slightly inhibited by 0.2484 or 0.4968 mM \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $${ \rm NO}_3^ -$$ \end{document} . The solution pH and TOC decreased after gamma irradiation. The degradation pathway of naproxen was decarboxylation.

Footnotes

Acknowledgments

The authors are grateful for support from the Major Project on Control and Rectification of Water Body Pollution, P.R. China (Foundation item no. 2008ZX07101-004) and the National Natural Science Foundation of China 11075039.

Author Disclosure Statement

All authors report no conflicts of interest.

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Degradation Kinetics and By-Products of Naproxen in Aqueous Solutions by Gamma Irradiation

Abstract

Abstract

Introduction

Materials and Methods

Chemicals and reagents

Procedures

Sample preparation

Irradiation process

Analysis

Naproxen radiation chemical yield (G-value)

Results and Discussion

Radiolytic degradation of naproxen

Influence of operational parameters on the naproxen degradation

Effect of naproxen concentration

Effect of solution pH

Effects of CH3OH and humic acid on naproxen degradation

Variation of solution pH

Variation in TOC during irradiation

Identification of degradation products

Conclusions

Footnotes

Acknowledgments

Author Disclosure Statement

References

Effects of CH₃OH and humic acid on naproxen degradation