Efficient degradation of selected polluting dyes using the tetrahydroxoargentate ion, Ag(OH)₄⁻, in alkaline media

Highlights

• We applied the reported tetrahydroxoargentate ion Ag(OH)₄⁻ for oxidation of surrogate organic recalcitrant dyes, by indirect redox oxidation.

• Through UV-Vis and chromatography techniques, we showed the possibility to degrade recalcitrant compounds such as fluorescein and rhodamine.

• Possible mechanisms of this indirect oxidation dye process using Ag(OH)₄⁻ are suggested.

Abstract

The use of soluble and highly oxidizing Ag(III) in the form of the tetrahydroxoargentate ion Ag(OH)₄⁻ is reported for the oxidation of surrogate organic recalcitrant dyes (i.e., rhodamine 6G (Rh6G) and fluorescein (Fl)). The possible use of Ag(OH)₄⁻ for the treatment of these and other refractory compounds is assessed. Such dyes were selected due to their common occurrence, stability, refractory nature, the relatively high toxicity of Rh6G, and their structural similarity to Fl. Several reaction intermediates/products were identified. The results showed that the highly oxidizing tetrahydroxoargentate anion was capable of degrading these recalcitrant dyes. Furthermore, the final degradation products do not represent a higher environmental risk than the original surrogates themselves. In addition, the partial mineralization of the dyes was proven.

Introduction

Textile industries often release large volumes of problematic wastewaters associated with intense coloration and adverse effects on human health (Farida et al., 2009). More than 10⁵ dyes are available today; the industrial use of these dyes generates over 7 × 10⁵ tons of dyes in wastewaters worldwide (Wu and Jane, 2003), particularly from the textile, paper, food, cosmetic, and pharmaceutical industries (Natarajan et al., 2011). Their elimination has prompted the use of new technologies. The most common physicochemical remediation methods include membrane filtration, coagulation, flocculation, oxidation and photoprocesses (such as UV, Fenton, H₂O₂ and O₃ oxidation), and adsorption (Farida et al., 2009, Natarajan et al., 2011, Yu et al., 2009, Peralta et al., 2008, He et al., 2009a, He et al., 2009b, Li et al., 2007). Ozonation is particularly suited for this undertaking, but it is expensive. Membrane filtration, coagulation, flocculation and adsorption can remove color, but they are typically associated with bulky sludge production and intense requirements for added chemicals.

The photocatalytic approach using UV-irradiated semiconductor suspensions offers high chemical stability, high catalytic activity, and relatively low toxicity and cost (Natarajan et al., 2011, Yu et al., 2009, He et al., 2009a, He et al., 2009b, Li et al., 2007). Unfortunately, most photocatalysts are active only in the UV region (Li et al., 2007). NaBiO₃, Bi₂WO₆, and Bi₂O₆ extend the usable sunlight wavelength range, but their photoactivity is relatively low, with concomitant slow degradation rates (Yu et al., 2009, He et al., 2009a, He et al., 2009b, Li et al., 2007).

Electrochemical methods offer promising alternatives. In this context, Ag(III) can be explored as a possible redox mediator in its soluble form, which is the tetrahydroxoargentate anion Ag(OH)₄⁻ in alkaline media (Cohen and Atkinson, 1968). Its notably high standard potential (1.8 V vs. NHE) makes it attractive for the treatment of many organic compounds. We have previously reported its preparation by electrolysis from metallic silver in alkaline solutions and its characterization (Zamora-Garcia et al., 2013). The usage of the tetrahydroxoargentate ion has not been reported for the transformation of recalcitrant organic species in water.

In the present work, we report the indirect electrodegradation with Ag(OH)₄⁻ of two types of dyes as pollutant models: rhodamine (Rh6G) and fluorescein (Fl). Rh6G has a rigid structure, extraordinary photostability, and a bio-refractory nature (Yu et al., 2009, Li et al., 2009, Zheng et al., 2012, He et al., 2009a, He et al., 2009b, Fu et al., 2005), but low solubility in alkaline media. Conversely, Fl exhibits a similar structure and properties but with high solubility in such media.

Rhodamines and their derivatives contain fluorophores and chromophores that have attracted considerable attention due to specific photophysical properties, such as high molar extinction coefficients (e.g., that for Rhodamine 6G is 116,000 M⁻¹ cm⁻¹) (Farida et al., 2009), which allow their use in dye-lasers, molecular imaging, and selective ion chemosensing (He et al., 2009a, He et al., 2009b, Fang-Jun et al., 2010). A highly sensitive, rhodamine-based colorimetric off-on fluorescent chemosensor has been developed for Hg²⁺ in aqueous solution and for live cell imaging (Wang et al., 2011, Hochberger et al., 1998). Rh6G is a very soluble cationic dye (Zheng et al., 2012) that is typically used as a dye for paper and for natural (e.g., silk, cotton, wool, leather, bast fibers) and synthetic fibers, as well as a water pollution tracer, an absorption indicator, and in personal care and cosmetic products. Criminologists use it for latent printing and identification purposes (Masters, 1990).

Since most synthetic dyes pose environmental risks to some degree, we selected Rh6G as a model pollutant considering that its rigid closed ring structure contributes to its stability and bio-refractivity (Zheng et al., 2012, He et al., 2009a, He et al., 2009b, Farida et al., 2009, Yu et al., 2009, Fu et al., 2005), and its relative toxicity. For example, Rh6G is a potent inhibitor of oxidative phosphorylation (Gear, 1974); rat mortality after Rh6G injection is well documented (French, 1989) as well as its toxic effect on rat retinal ganglion cells (Thaler et al., 2008).

Even though its toxicity is generally regarded as low (TOXNET, 2017), the yellow dye fluorescein was also selected for the present study due to its structural similarity with Rh6G and to its much higher solubility in alkaline media. Fluorescein is used in many applications: as a presumptive reagent for dilute blood detection; in cosmetics, cleansers, and other household products; as a tracer for water leaks and water pollution sources; as an adsorption indicator; as a chemical intermediate for other dyes; and as a fluorescent pH indicator (Lide, 2007). Its sodium salt has several uses in ophthalmology (Dolak et al., 2008).

In the present work, UV–Vis spectrophotometry, total organic carbon (TOC), and high-pressure liquid chromatography (HPLC) were used to identify the species involved at different reaction stages and to evaluate the degree of mineralization. Gas chromatography with flame ionization detection (GC-FID) was used as a qualitative way to monitor the rupture and degradation of the dyes. Lastly, gas chromatography with mass spectrometry (GC-MS) detection was used to identify the final reaction products.