Photocatalytic treatment of organic pollutants in a synthetic wastewater using UV light and combinations of TiO2, H2O2 and Fe(III)

In this study, the photocatalytic treatment of an organic wastewater with/without phenolic compounds by means of ultraviolet irradiation, titanium dioxide and hydrogen peroxide was examined in an annular photoreactor. Specifically, the effect of initial total carbon concentration, catalyst loading and H 2 O 2 amount on the removal of total carbon was first examined in the case of a synthetic organic wastewater. The influence of partial carbon substitution by phenol, 2-chlorophenol, 2,4-discholophenol, trichlorophenol, and 4-nitrophenol on total carbon removal and target compounds’ conversion was studied keeping constant the initial organic carbon load. It was shown that the process applied was effective in treating the wastewater for initial total carbon 32 mg L -1 , 0.5 g L -1 TiO 2 , and 66.6 mg L -1 H 2 O 2 . Applying UV/TiO 2 and UV/H 2 O 2 , 58% and 53% total carbon removals were achieved, respectively, but combining TiO 2 and H 2 O 2 did not result in a better performance in the case of the synthetic wastewater without any phenolic compounds. In contrast, when a phenolic compound was added, the addition of H 2 O 2 was beneficial, eliminating the differences observed from one phenolic compound to another. The total carbon removals observed were lower than the corresponding final conversions of the target phenolic compounds. Finally, the electric energy per order values were calculated and found to range in 52–248 kWh/m 3 /order, being dependent from the process applied and the phenolic compound present in the wastewater.

Funding: This work was supported by the Social Policy Grant of Nazarbayev University (Astana, Kazakhstan) awarded to Dr. Stavros Poulopoulos. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Copyright: © 2019 Poulopoulos et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

1 Introduction

Water pollution is a globally pressing ecological problem and has adverse impacts on both natural ecosystems and human daily life. The growth of industry along with the unprecedent increase in the human population have led to a high demand for water resources, whereas at the same time large amounts of various wastewaters are generated and threaten the quality of these resources. Food and energy security, sustainable development, human and ecosystems health rely on water availability and quality [1]. Therefore, it is of outmost importance to make sure that industrial wastewaters are adequately treated prior to their disposal so as to minimize the impact on the human health and environment [2]. Although by means of conventional methods and especially of biological processes, 80–90% of all pollutants are usually removed [3], it has been found that hazardous organic pollutants may escape these processes [4].

Chlorophenols constitute a group of organic compounds that is widely used in the dye manufacture, petroleum refineries, herbicide production, and in pharmaceutical industry [5]. They can be found in the environment through the release of polluted water from these industries [6]. They are harmful to humans and ecosystems and considered as potential carcinogenics [7]. Moreover, they are known to exhibit a bio-resistant nature [8]. Advanced Oxidation Processes (AOPs) can be utilized to eliminate such toxic organic pollutants in wastewaters by converting them into water and carbon dioxide [9].

Specifically, AOPs are an attractive alternative when bioresistant organic pollutants are to be mineralized in wastewaters. In the oxidation process, highly active hydroxyl radicals are involved, which are generated by a variety of mechanisms [10]. Among them, photochemical and photocatalytic methods constitute sustainable treatment technologies with “zero” waste [11]. Hydrogen peroxide (H2O2) is commonly used to provide the active hydroxyl radicals, but its use increases the operating cost of the process, whereas the low oxidation rate when compex wastewaters are processed is the main drawback of titanium dioxide use [12]. The employment of photo-Fenton processes or combining H2O2 with TiO2 could be effective alternatives. Regarding the elimination of phenols and chlorophenols in water, these processes have been proved promising [13]. Particularly, AOPs can effectively decompose toxic and bioresistant compounds in water like phenol, 2-chlorophenol and 2,4-dichlorophenol [13–15]. For example, Andreozzi et al. [16] studied the photochemical oxidation of 2,4-dichlorophenol and 3,4-dichlorophenol using homogeneous photocatalysis with Fe(III) under UV-A irradiation. They reported complete removal of both compounds with increased concentrations of iron and proper adjustment of the solution pH.

There is an extensive list of applications of photochemical processes to eliminate toxic or emerging pollutants in water [17–22], but significantly less research has been conducted to degrade these compounds in complex wastewaters or treat effectively complex wastewaters in terms of carbon removal [23,24]. Photochemical reactions take place through complex and variable pathways based on radical mechanisms that are sensitive to experimental parameters, and thus their efficiency can be adversely influenced by the occurrence of rest compounds in a wastewater [25]. As a result, further research is essential to investigate the application of photochemical processes in the case of complex wastewaters.

In this study, UV/TiO2, UV/H2O2, UV/TiO2/H2O2 and UV/TiO2/H2O2/Fe(III) processes were used to treat a synthetic wastewater containing mainly organic load. The UV/TiO2/H2O2 process was also applied to treat the synthetic wastewater containing the following compounds: Phenol, 2-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, and 4-nitrophenol. Finally, electrical energy consumption due to artificial light irradiation was estimated as it is connected directly to the environmental footprint of photochemical processes.