Comparison of hydroxyl radical yields between photo- and electro-catalyzed water treatments
Introduction
Along with the industrial and commercial developments in Taiwan and with the improvement in quality of life, the ingredients of daily discharged wastewater have become far more complicated than in the past. Industrial and domestic wastewater contains high concentrations of recalcitrant organic pollutants, providing new challenges for designing wastewater treatment protocols. To overcome these problems, numerous promising wastewater treatment technologies have been developed [1]. Among these technologies, AOPs such as the Fenton process, photo-catalysis, UV, ozone, and electro-catalysis have become the most widely utilized wastewater treatment methods due to their non-selective pollutant degradation and rapid reaction rates.
Recently, the integration of catalytic materials (i.e., TiO2, CdS, and ZnS) with photo- and electro-catalytic technologies has shown great promise in the degradation of organic pollutants. These catalytic materials can be employed under normal temperatures and pressures, and only require simple equipment and operating procedures. Some studies have presented satisfactory results of utilizing catalytic materials for various wastewater types, including dyeing wastewater, surfactants, and leachates [2], [3]. Theoretically, the principles of organic pollutant degradation by photo- and electro-catalysis are similar to each other. In both cases, the energy introduced into the system (photonic or electronic) is higher than the energy band gap of the catalytic materials. This phenomenon enables the electrons in the valence band of the catalytic materials to jump to the conduction band, thus forming an electron–hole pair. The photo- or electro-catalytic system subsequently generates hydroxyl radicals (OH) based on the following two approaches: (i) the holes in the catalytic material react with water to generate OH with a high oxidative capacity and (ii) a reaction between hydroxyl ions and the holes in the system generates OH. Due to the high reduction potential (2.8 V) [4] of OH, a strong and non-selective oxidative capacity is generated. This capacity makes it extremely useful for the rapid degradation of organic compounds. Thus, the OH yield rate serves a key role in degrading organic molecules in wastewater. Accordingly, understanding the OH yield rate is critical for the application of photo- and electro-catalytic systems.
Several direct methods have been used to measure the OH concentration. Ma et al. [5] successfully measured OH concentrations by combining electrochemistry with electron spin resonance (ESR) spectroscopy. Chang et al. [6] used photo-catalytic reaction tests based on UV/TiO2 to determine OH content. These authors optimized the dosage of salicylic acid, pH, dissolved oxygen, and illumination to produce an OH concentration of 5.1 × 10−17 M. Direct measurements of OH content are very difficult due to the extremely short half-life of the OH (approximately 10−10 s) and expensive instrumentation [7]. An indirect method to measure OH content that has shown much potential is the “Radical scavenging method”. This strategy is very useful for continuous analysis of batch experiments based on its advantages of simple instrumentation, good sensitivity and stability, and low cost. This system consists of a scavenger that reacts with an OH radical to generate a stable derivative. The formation of the derivative is then analyzed by HPLC, and the OH concentration is subsequently calculated. Thus, this method can effectively reflect OH content. Frequently used OH scavengers include salicylic acid, hydroxybenzoic acid, dimethyl sulfoxide (DMSO) and phenol [8], [9], [10], [11].
In consideration of AOP selection for wastewater treatment, the yield efficiency of OH production is one of the critical criteria. Therefore, we compared the rate of OH production in photo- and electro-catalytic systems and investigated the impact of varying the operating parameters on OH generation in this study. For the determination of OH concentration, we used 4-hydroxybenzoic acid (4-HBA) as the OH scavenger due to its high capture rate and the fact that only 3,4-dihydroxybenzoic acid (3,4-DHBA) is generated after the reaction. We then analyzed the appearance of the 3,4-DHBA using HPLC as a function of operation time. The experimental results are expected to provide applicable information for practical wastewater treatment methods.
Section snippets
Materials and methods
The OH generation experiments can be divided into two categories: electro-catalyst and photo-catalyst.
A schematic diagram of the electro-catalytic system is shown in Fig. 1. The central round reactor (PYREX no. 3140) had a dimension of 15.0 cm (diameter) × 7.5 cm (high), and a total capacity of 1.2 L. The catalytic materials of the anode included commercially available TiO2, dimensionally stable anodes (DSA), and graphite. We used a commercially available graphite cathode that had dimensions of 5.0
Method of determining OH concentration
Because the method of determining the OH concentration is critical for this study, the method's feasibility should be verified. Fig. 5 shows HPLC chromatograms of the radical scavenger (4-HBA) and the derivative 3,4-DHBA, using NaNO3 as an electrolyte. No overlap nor interference between the 4-HBA and 3,4-DHBA peaks was generated at 5.0 min and 3.7 min. That is, the 4-HBA and 3,4-DHBA peaks can be effectively distinguished to determine their respective concentrations. The calibration curve of
Conclusions
The purpose of this study is to understand the variations in OH production rates upon the application of photo- and electro-catalytic systems. The radical scavenger approach was adopted to determine OH concentrations in terms of electrolyte, applied energy, pH, and catalytic materials. Several of the following conclusions can be drawn:
- 1.
A standard procedure for the measurement of OH in a photo-catalytic and an electro-catalytic system has been established. Experimental results show the stable
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