Effect of Coupling Electrochemical and Ozonation Methods on the Degradation of Phenol

Introduction

Currently various industries exhibit high pollution potential because their production processes generate large volumes of refractory type effluents. These effluents are problematic, mainly due to the presence of compounds which are harmful and recalcitrant. Biological processes generally do not remove this type of compounds. In fact, high concentration of these compounds can inhibit the performance or be toxic to biota which is responsible for the removal of pollutants. Advanced Oxidation Processes represent a technological alternative with great potential for the treatment of biodegradable effluents. In this study compared two methods are compared in the degradation of phenol: ozonation and coupled electrochemical-ozonation.

Methods

Solution of Phenol was prepared using Phenol supplied by Merck and deionized water at a concentration of 1000 mgL^-1. Then aliquots of 100 ml were used in 1000 ml of Na₂SO₄  0.1 M solution to obtain a concentration of 100 mgL^-1.

Ozonation. The ozonation experiments were conducted in an up-flow glass bubble column reactor. The gas mixture ozone/air was continuously fed with a flow rate of 0.04 Lmin^-1 through a gas diffuser with a 2 mm pore size at the bottom of the reactor.

Analysis. Concentration of phenol was determined by UV/vis spectrophotometry technique. Samples absorbance was scanned from 200 to 900 nm, and a maximum absorbance at 270 nm was observed. The samples were scanned in a quartz cell. In order to establish the mineralization degree of phenol, Chemical Oxygen Demand of samples was determined by means of the American Public Health Association standard procedures.

Electro-oxidation reactor: The reactor cell contains a pair of BDD electrodes (BDD film supported on a niobium substrate), each electrode was 20.0 cm by 2.5 cm with a surface area of 50 cm². Batch volumes of 0.90 L were treated in the 1.00 L reactor. A direct-current power source supplied the system with 1.0, 2.0 and 3.0 A corresponding to current densities of 20, 40, and 60 mA/cm². All the experiments were carried out at room temperature (20ºC), pH was adjusted at 3.0, 7.0, 9.0 and 12.0 with analytical grade H₂SO₄ and NaOH.

O₃-electrochemical coupled process: For the combined system the pair of BDD electrodes from the electrooxidation reactor were installed in the ozonation reactor. Ozone was introduced at the same rate and the BDD electrodes were given the same current densities as in the individual reactors. Treated samples were taken at the same intervals and analyzed the same way.

Results

This study shows a synergistic effect on the degradation of phenol by combining two advanced oxidation processes: ozone and electrochemical. Figure 1 shows the efficiency of degradation of each process. It is worth noticing that the best efficiency is obtained by a combination of processes, obtaining 99.98% in 1 hour. Figure 2 shows the effect of pH on the treatment in relation to the formation of the main products of mineralization of the molecule. Both technologies, ozonation and electrochemical, remove 97.90% and 99.88%, respectively in a time of 120 minutes. The combined treatment, however, reduces the time down to 60 minutes with a removal of 99.98%. Although ozonation possess characteristics that make it more versatile to be implemented on a larger scale as a single processing system or coupled to biological processes, the use of the electrodes is an effective option because it has a wide potential window, making it more efficient.

Conclusions

Recalcitrant compounds such as phenol require effective combination of advanced oxidation processes to achieve its complete mineralization. The combination of the processes ozone/electrochemical leads to the complete degradation of phenol after 1 hour. The decrease in pH observed in the process suggests the formation of carboxylic acids.