Competitive kinetics versus stopped flow method for determining the degradation rate constants of steroids by ozonation.

Alberto López-López,Elizabeth León-Becerril,Valentín Flores-Payán,Ramiro Vallejo-Rodríguez,Leonel Hernández-Mena

doi:10.1186/s40064-016-2782-4

Abstract

Steroids are classified as endocrine disrupting chemicals; they are persistent with low biodegradability and are hardly degraded by conventional methods. Ozonation process has been effective for steroids degradation and the determination of the kinetics is a fundamental aspect for the design and operation of the reactor. This study assessed two methods: competitive kinetics and stopped flow, for determining the degradation kinetics of two steroids, estradiol (E2) and ethinylestradiol (EE2) in spiked water. Experiments were performed at pH 6, 21 °C, and using tertbutyl alcohol as scavenger of hydroxyl radicals; competitive kinetics method used sodium phenolate as reference compound. For the stopped flow, the experiments were performed in a BioLogic SFM-3000/S equipment. For both methods, the second order rate constants were in the order of 106 and 105 M−1 s−1 for E2 and EE2 respectively. The competitive kinetics can be applied with assurance and reliability but needing an additional analysis method to measure the residual concentrations. Stopped flow method allows the evaluation of the degradation kinetics in milliseconds and avoids the use of additional analytical methodologies; this method allows determining the reaction times on line. The methods are applicable for degradation of other emerging contaminants or other steroids and could be applied in water treatment at industrial level. Finally, it is important to consider the resources available to implement the most appropriate method, either competitive kinetics or the stopped-flow method.

Highlights

Emerging contaminants are chemical compounds comprising products used in anthropogenic activities, including human and veterinary pharmaceuticals, personal care products, surfactants, pesticides, plasticizers, drugs and industrial additives (Snyder et al 2006)
Analytical methods Steroids concentration was determined by solid phase extraction; sodium phenolate concentration in the extracted samples was determined by high performance liquid chromatography (HPLC) using a Varian ProStar 7725 equipment with a Varian ProStar 230 diode array detector (DAD) (Walnut Creek, CA) using maximal absorption wavelengths of 197 nm following the methodology from Vallejo-Rodríguez et al (2011)
Residual steroid was determined by gas chromatography (GC) using an Agilent Technologies chromatograph model 6890 coupled with a mass spectrometer (MS) 5975 and with a quadruple mass filter with an autosampler model 7683, prior the sample was derivatized

Summary

Introduction

Emerging contaminants are chemical compounds comprising products used in anthropogenic activities, including human and veterinary pharmaceuticals, personal care products, surfactants, pesticides, plasticizers, drugs and industrial additives (Snyder et al 2006). López‐López et al SpringerPlus (2016) 5:1105 different concentration levels of ng L−1 and μg L−1 in wastewater with or without treatment (Samaras et al 2010; Santos et al 2005), and in surface and groundwater (Benotti et al 2009, 2010) In surface water such as rivers and lakes, which are considered a source of drinking water, has been detected the presence of EDCs, in particular pharmaceuticals including steroids (Huerta-Fontela et al 2011; Alvarez et al 2008; Benotti et al 2009; Gibson et al 2007), implying a potential public health problem (von der Ohe et al 2011). The second order rate constant (kO3,M) can be determined by competitive kinetics (Deborde et al 2005; Gurol and Nekouinaini 1984; Hoigné and Bader 1983) and stopped flow and quenched flow methods (Chelme-Ayala et al 2010; Huber et al 2005; Ledakowicz et al 2001)

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