The mechanism of sonochemical degradation of a cationic surfactant in aqueous solution

Abstract

The sonochemical degradation of the cationic surfactant, laurylpyridinium chloride (LPC), in water was studied at concentrations of 0.1–0.6 mM, all below its critical micelle concentration (15 mM). It has been found that the initial step in the degradation of LPC occurs primarily by a pyrolysis pathway. Chemical analysis of sonicated solutions by gas chromatography, electrospray mass spectrometry, and high performance liquid chromatography reveals that a broad range of decomposition products, hydrocarbon gases and water-soluble species, are produced. Propionamide and acetamide were identified as two of the degradation intermediates and probably formed as the result of the opening of the pyridinium ring following OH radical addition. Most of the LPC is eventually converted into carboxylic acids. The complete mineralization of these carboxylic acids by sonolysis is however a comparatively slow process due to the hydrophilic nature of these low molecular weight products.

Introduction

Cationic surfactants [1] are widely used in industries and household chemicals, some examples include textile softeners, pharmaceuticals, emulsions, disinfectants and human hair cosmetics. Upon release into the environment, cationic surfactants pollute the environment because their positive charge results in a strong affinity for the surface of particulates in sewage sludge, soil and sediments that are predominantly negatively charged. Hermann et al. [2] investigated the reaction of humic substances (end products of decayed organic matter) from different sources with a cationic surfactant laurylpyridinium chloride (LPC) and found strong interactions between humic substances and LPC. Once bound to the surface of humic substances, LPC could be desorbed only in small amounts even with strong extractants. These reactions affect the overall biodegradation process because bound surfactants are mineralized slowly compared to the free molecules.

Although cationic surfactants are not as widely used as anionic and non-ionic surfactants they are known to be 10 times to 100 times more toxic than anionic and non-ionic surfactants even at very low concentrations. With regard to human oral toxicity, cationic surfactants are found to be of somewhat higher in toxicity than that of anionic and non-ionic surfactants. In case of human eye irritation, cationic surfactants are the most irritating of all the surfactants [3].

Janicke and Hilge [4] have reported that quaternary ammonium salts exhibit little or no degradation under anaerobic conditions. Ginkel [5] has reported that cationic surfactants can be degraded aerobically. Though readily biodegradable in aerobic environments, even at low concentrations cationic surfactants are found to potentially disturb the ecological balance by harming aquatic organisms and soil biota [6]. Battersby and Wilson [7] observed that concentrations of 200 mg L⁻¹ hexadecyltrimethyl ammonium bromide inhibit the production of methane, suggesting that such concentrations are inhibitory to resident microbes.

At present there is not a great deal of data on the degradation of cationic surfactants and their fate in the environment. The reason for this is the lack of sensitive and accurate methods for the determination of cationic surfactants in environmental water and sediments. Cationic surfactants easily form stable ion-pair complexes with the ubiquitous anionic groups on natural constituents found in environmental water, rendering conventional analytical methods inapplicable [8].

The purpose of this study was to investigate the degradation of a cationic surfactant, LPC, by means of an effective advanced oxidation method, i.e., sonolysis. To our knowledge, the only previous study examining the degradation of LPC in aqueous solution was by heterogeneous photocatalysis, and the principal interest was on the formation of ammonium and nitrate ions [9].

Sonochemistry, as an advanced oxidation process, has received considerable interest over the past few decades [10]. Localised “hot-spots”, produced from inertially collapsing bubbles in water exposed to ultrasound reach temperatures of about 4500 K [11], [12]. These extreme conditions generated within the cavitation bubbles results in the dissociation of water vapour molecules into H and OH radicals (see Reaction 1). The concentration of OH radicals at the bubble/solution interface has been estimated to be as high as 1 × 10⁻² M upon bubble collapse [13].

$${\text{H}}_2\text{O})))))\mathbf{H}+\mathbf{OH}$$

$${\text{O}}_2)))))2\mathbf{O}$$

$${\text{N}}_2)))))2\mathbf{N}$$

$$\mathbf{H}+{\text{O}}_2\to {\mathbf{HO}}_2$$

$$\mathbf{O}+{\text{H}}_2\text{O}\to 2\mathbf{HO}$$

$$\mathbf{HO}+\mathbf{OH}\to {\text{H}}_2{\text{O}}_2$$

From the reaction of O, H and OH radicals with each other and with H₂O and O₂ during the rapid cooling phase, HO₂ radicals and H₂O₂ molecules are formed (see (Reaction1), (Reaction2), (Reaction3), (Reaction4), (Reaction5), (Reaction 6)). In this molecular environment organic compounds are decomposed and inorganic compounds are oxidized or reduced [14]. The decomposition of N₂ in the presence of oxygen usually leads to the formation of nitrous and nitric acid [15].

In addition to this oxidative degradation pathway, pyrolysis of solutes may also occur within cavitating bubbles. Volatile solutes adsorbed at the bubble/solution interface can evaporate into the core of the bubble during bubble growth. Under the high-temperature conditions of a sonochemically active bubble, entrained solutes decompose and solute decomposition products may accumulate over many bubble oscillation cycles [11], [16].