Comparison of hydroxyl radical yields between photo- and electro-catalyzed water treatments

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Highlights

  • The OHradical dot concentration is about 4.6 × 10−3 M by the photocatalysis.

  • The OHradical dot concentration is about 6.6 × 10−3 M by the electrocatalysis.

  • Yield rate of hydroxyl radicals is in linear correlation with energy consumption.

  • The OHradical dot yield rate was 3.9 × 10−5 M/W cm2 and 4.5 × 10−5 M/W cm2, respectively.

Abstract

In this study, we investigate the production of hydroxyl radicals in photo- and electro-catalytic systems. A model compound, 4-hydroxybenzoic acid (4-HBA), served as a trapping (scavenger) reagent for the determination of hydroxyl radical (OHradical dot) concentrations in both systems. Experiments were conducted by controlling different operation parameters (i.e., reaction time, electrolyte, applied voltage, power, and solution pH) to understand their respective impact on OHradical dot production. The results show that a number of similar characteristics exist between the photo- and electro-catalytic systems, including the electrolytes Na2SO4 and NaNO3, which are both suitable for photo- and electro-catalysis and increase the applied energy, proportionally enhancing the OHradical dot yields. These alkaline conditions are beneficial for generating OHradical dot in either the photo- and electro-catalytic system. Notably, electrolyte Na2CO3 is a strong scavenger of OHradical dot, which induces the failure of the catalytic system. In summary, the electro-catalytic system demonstrates better OHradical dot production rates than those of the photo-catalyst system under the same applied energy and with respect to the catalyst surface area.

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 (OHradical dot) based on the following two approaches: (i) the holes in the catalytic material react with water to generate OHradical dot with a high oxidative capacity and (ii) a reaction between hydroxyl ions and the holes in the system generates OHradical dot. Due to the high reduction potential (2.8 V) [4] of OHradical dot, a strong and non-selective oxidative capacity is generated. This capacity makes it extremely useful for the rapid degradation of organic compounds. Thus, the OHradical dot yield rate serves a key role in degrading organic molecules in wastewater. Accordingly, understanding the OHradical dot yield rate is critical for the application of photo- and electro-catalytic systems.

Several direct methods have been used to measure the OHradical dot concentration. Ma et al. [5] successfully measured OHradical dot 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 OHradical dot content. These authors optimized the dosage of salicylic acid, pH, dissolved oxygen, and illumination to produce an OHradical dot concentration of 5.1 × 10−17 M. Direct measurements of OHradical dot content are very difficult due to the extremely short half-life of the OHradical dot (approximately 10−10 s) and expensive instrumentation [7]. An indirect method to measure OHradical dot 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 OHradical dot radical to generate a stable derivative. The formation of the derivative is then analyzed by HPLC, and the OHradical dot concentration is subsequently calculated. Thus, this method can effectively reflect OHradical dot content. Frequently used OHradical dot 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 OHradical dot production is one of the critical criteria. Therefore, we compared the rate of OHradical dot production in photo- and electro-catalytic systems and investigated the impact of varying the operating parameters on OHradical dot generation in this study. For the determination of OHradical dot concentration, we used 4-hydroxybenzoic acid (4-HBA) as the OHradical dot 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 OHradical dot 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 OHradical dot concentration

Because the method of determining the OHradical dot 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 OHradical dot production rates upon the application of photo- and electro-catalytic systems. The radical scavenger approach was adopted to determine OHradical dot 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 OHradical dot in a photo-catalytic and an electro-catalytic system has been established. Experimental results show the stable

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