Oxidation of ubiquitous aqueous pharmaceuticals with pulsed corona discharge

Highlights

• Diclofenac, ibuprofen, metformin and tramadol are effectively oxidized with PCD.

• Alkaline media promotes PCD oxidation of pharmaceuticals.

• Drugs removal efficiency correlates with electronic molecular structure and hydrophobicity.

• Pulsed corona discharge surpasses other AOPs in energy efficiency.

Abstract

Aqueous pharmaceuticals present an emerging environmental problem for their potent character. Advanced oxidation processes (AOPs) present efficacious abatement strategy, being, however, unaffordable in mass application. Energy efficient gas-phase pulsed corona discharge was experimentally studied for oxidation of aqueous ibuprofen, diclofenac, metformin and tramadol showing the energy yields of 53, 21, 15 and 89 g kW-1 h-1, respectively, at 10 mg L-1 initial concentrations, surpassing other AOPs. Impacts of alkaline pH and starting concentrations on oxidation energy efficiency are beneficial to various extents to all considered pharmaceuticals. The removal efficiency was found to correlate with molecular structure and hydrophobicity of medications.

Introduction

Among aqueous organic pollutants, pharmaceutical residues in surface and ground waters present a major environmental concern with growing World's urban population [1]. These have been detected in water ways as micropollutants ranging from ng L⁻¹ to μg L⁻¹ concentrations threatening the environment with their bioaccumulation in the aquatic life [[2], [3], [4], [5], [6]]. Nonsteroidal anti-inflammatory drugs diclofenac (DCF) and ibuprofen (IBU), analgesic tramadol (TMD) and antidiabetic medication metformin (MTF) are among the most consumed pharmaceuticals frequently detected in the aquatic environment [[1], [2], [3],7].

Diclofenac (sodium 2-[2-(2,6-dichloroaniline)phenyl]acetate), one of the most widespread drugs with a global annual consumption of about 1.5 thousand metric tons, is often found as a persistent aquatic pollutant [8]. Average concentrations in low μg L⁻¹ range were detected in influents and effluents of municipal sewage treatment plants and surface waters in Europe, Asia and the USA [9]. Adverse effects of exposure to DCF at concentrations as low as 1 μg L⁻¹ have been observed in aquatic biota, including rainbow trout [10]. Diclofenac residues have been reported to cause vulture population declined due to kidney damage following feeding on the corpses of domestic animals contaminated with DCF [11,12]. The main source of DCF pollution is its disposal with municipal sewage inadequately treated by conventional wastewater treatment methods [[13], [14]].

Moderately toxic IBU [18], 2-[4-(2-methylpropyl)phenyl]propanoic acid, exceeding DCF in consumption for about twelve times [3], has been reported in surface waters in concentrations from 5 ng L⁻¹ to 28 μg L⁻¹ [[15], [16], [17]]. Significant concentrations of IBU were found downstream of wastewater treatment plants (WWTPs) for its poor removal [18,19].

With regard to use of painkiller TMD, 2-(dimethyl amino)-methyl)-1-(3ʹ-methoxyphenyl) cyclohexanol hydrochloride, it increased tenfold in global consumption from 13.8 in 1990 to 149 tons in 2000 [20], maintaining the level thereafter [21], inducing, however, genotoxic and cytotoxic effects [22]. Tramadol is detected in household and hospital effluents as well as in surface waters [23,24] in concentrations exceeding 200 ng L⁻¹ [25]. European-wide monitoring of organic micro-pollutants in WWTPs effluents exposed TMD as one of the most frequent contaminants in concentrations ranging from low nanograms to micrograms per litre being only partially removed at WWTPs [26,27].

Global production of antidiabetic MTF, 3-(diaminomethylidene)-1,1-dimethylguanidine hydrochloride, increased from 20 to 30 thousand metric tons from 2011 to 2015, respectively, at annual growth rate about 10% [28]. Metformin is introduced into the environment with wastewaters in both metabolized (guanylurea) and non-metabolized forms [29,30] acting as an endocrine disruptor at environmentally relevant concentrations [31], lacking, however, a solid environmental risk assessment [32]. Concentrations of MTF in the range of several μg L⁻¹ were detected in surface waters in the USA [33], Europe [34] and China [35]; the average MTF concentration in 139 stream sampling sites over the USA was 110 ng L⁻¹ [36]. The influent sewage at German and American WWTPs contain around 129 and 88 μg L⁻¹, respectively [37].

Advances in water and wastewater treatment have led to the development of promising technologies, involving highly reactive hydroxyl radicals (HO^•), termed as advanced oxidation processes (AOPs). Ozonation, Fenton and electro-Fenton treatment, electrochemical and photocatalytic oxidation have been studied for aqueous DCF [[38], [39], [40], [41]], IBU [38,[42], [43], [44], [45]], TMD [[46], [47], [48], [49], [50]] and MTF [[51], [52], [53], [54], [55], [56], [57]] removal.

Gas-phase pulsed corona discharge (PCD) producing active hydroxyl radicals at the gas-liquid interface is a modern energy-efficient wastewater treatment method, which proved its energy efficiency in oxidation of various pollutants [58]. Hydroxyl radicals formed at the surface of drops and films react directly with the drugs in a fast reaction [59]. Besides, reactive species include ozone, hydrogen peroxide and other reactive species formed in the discharge.

Energy efficiency of PCD oxidation of organic pollutants depends on controllable parameters including reactor's geometry, gas-liquid mass transfer, current-voltage characteristics and external factors determined by the treated media properties – content of pollutants, temperature, pH, electric conductivity etc. Molecular structure of organic pollutants characterized by electronic energies is evidently the most important factor affecting the oxidation rate. Hydrophobicity was considered as influencing the position of pollutants relative to interface. The impact of these factors is reported for PCD-treated aqueous media for the first time.

The current study assesses the applicability of PCD to removal of aqueous DCF, TMD, MTF and IBU as compared its energy efficiency with other AOPs at operation parameters variations. The drugs removal efficiency was found to correlate with electronic molecular structure and hydrophobicity.