Industrial wastewater treatment using a bubble photo-Fenton reactor with continuous gas supply.

The present study assesses the treatability of a real industrial wastewater (WW) with a high organic load (chemical oxygen demand (COD) above 5800 mgO2 L-1) by photo-Fenton's oxidation with the goal of improving the organic matter degradation reached previously, in another work, where the Fenton process was applied in a bubbling reactor. Thus, the process was carried out in a bubble photo reactor (BPR) wherein continuous air supply ensures an efficient mixing of the liquid phase. The effect of the main operatory parameters that influence the WW treatment (i.e., H2O2 and Fe2+ concentrations, initial pH, and UV-Vis radiation intensity) were evaluated, being found that in the best conditions tested (pH0 = 4.6, [Fe2+] = 0.1 g L-1, [H2O2] = 18 g L-1, Qair = 1.0 L min-1-measured at room temperature and atmospheric pressure-and irradiance of 500 W m-2), removals of 95% and 97% for total organic carbon (TOC) and COD, respectively, were achieved. Still, a high reduction of the concentration of the main constituents of this WW was reached, being total for aniline and 86% for sulfanilic acid. The continuous air supply reactor configuration was compared with magnetic stirring; similar mineralization was achieved. However, the air bubbling promotes a good heat transfer within the reactor, minimizing temperature gradients, which is quite advantageous due to the strong exothermicity of the oxidation process during the treatment of such highly loaded real effluents.

Simultaneous treatment of toluene-containing gas waste and industrial wastewater by the Fenton process.

This study reports a new perspective for the simultaneous oxidation of a volatile organic compound (VOC) - a toluene gas stream - and a real industrial liquid effluent by the Fenton's process; for that, a lab-scale bubbling reactor, operating in semi-continuous mode, was used. A parametric study was carried out to evaluate the effect of the aqueous matrix (water vs. real effluent), catalyst species nature (Fe2+ vs. Fe3+), concentration of organic matter in the liquid, and inlet toluene concentration in the gas phase. Their effects in the simultaneous gas-liquid treatment were assessed in terms of the toluene removal (from the gas stream) and wastewater mineralization (removal of dissolved organic carbon - DOC). The presence of organic matter in the liquid phase decreased toluene absorption. However, the simultaneous oxidation in the liquid phase extended the period of absorption until its saturation (and inherently the amount of toluene transferred) while still oxidizing 25% of the organic matter present in the industrial effluent. The application of the Fenton-like (H2O2 + Fe3+) process yielded a slightly reduced toluene transfer as compared to the Fenton one (H2O2 + Fe2+) - ca. 10%, although the overall mineralization has been similar. As expected, increasing the inlet toluene concentration reduces the process duration until liquid saturation, at the same time that a higher accumulation of by-products in the liquid due to oxidation was observed. Finally, a sequential treatment approach was performed, wherein liquid oxidation follows the previous simultaneous gas-liquid treatment, representing a strategy for long term operation, providing an opportunity for further VOC abatement in subsequent cycles. The main compounds resulting from oxidation remaining in the liquid phase after each stage were identified, allowing to close the carbon balance by ca. 80%.

Insights into real industrial wastewater treatment by Fenton's oxidation in gas bubbling reactors.

In the present study, bubbling reactors (BRs) were chosen to design a new procedure for real industrial wastewater (WW) treatment by Fenton's oxidation. The process was carried out in BRs under batch mode for the treatment of a WW with a high organic load (chemical oxygen demand (COD) above 7000 mgO2/L), being the efficient mixing of the liquid phase ensured by the gas bubbling. The parameters that influenced the WW treatment (i.e., H2O2 and Fe2+ concentration, and initial pH) were optimized in a smaller BR (0.5 L volumetric capacity); the maximum oxidation efficiency (dissolved organic carbon (DOC) removal = 52% and COD removal = 83% after 60 min) was reached under the following conditions: Qair = 1.0 L/min (measured at room temperature and atmospheric pressure), [H2O2] = 22.5 g/L, [Fe2+] = 0.75 g/L, and pH = 4.6 - original WW pH. It was not detected any significant effect in the process efficiency of the air flow rate and gas phase composition (i.e., N2, and air), but when the process was performed with continuous O2 bubbling an increase in the DOC removal (from 43% to 53%) was observed after 5 min of oxidation. Even so, the high costs discourage the use of pure oxygen streams in real WWTPs. To understand the dynamics of the process, the continuous air bubbling was compared to another mixing mode (mechanical stirring), and similar mineralization was achieved, proving the feasibility of Fenton's process in a BR. In addition, the gas bubbling proved to be more efficient in terms of heat dissipation during the treatment, decreasing temperature profiles along the oxidation of heavily charged real effluents. An effective scale-up with a bubble column reactor with a higher volumetric capacity by a factor of almost one order of magnitude was also proved, providing similar mineralization. The final effluent was non-toxic and more biodegradable.

Application of the Fenton's process in a bubble column reactor for hydroquinone degradation.

The aim of this study was to assess the degradation and mineralization of hydroquinone (HQ) by the Fenton's process in a bubble column reactor (BCR). The effect of the main operating variables, namely, air flow rate, effluent volume, hydrogen peroxide (H2O2) concentration, catalyst (Fe2+) dose, initial pH, and temperature, were assessed. For all air flow rates tested, no concentration gradients along the column were noticed, evidencing that a good mixing was reached in the BCR. For the best conditions tested ([H2O2] = 500 mg/L, [Fe2+] = 45 mg/L, T = 24 °C, Q air = 2.5 mL/min, pH = 3.0, and V = 5 L), complete HQ degradation was reached, with ~ 39% of total organic carbon (TOC) removal, and an efficiency of the oxidant use-η H2O2-of 0.39 (ratio between TOC removed per H2O2 consumed normalized by the theoretical stoichiometric value); moreover, a non-toxic effluent was generated. Under these conditions, the intermediates and final oxidation compounds identified and quantified were a few carboxylic acids, namely, maleic, pyruvic, and oxalic. As a strategy to improve the TOC removal, a gradual dosage of the optimal H2O2 concentration was implemented, being obtained ~ 55% of mineralization (with complete HQ degradation). Finally, the matrix effect was evaluated, for which a real wastewater was spiked with 100 mg/L of HQ; no reduction in terms of HQ degradation and mineralization was observed compared to the solution in distilled water.

p-Nitrophenol degradation by Fenton's oxidation in a bubble column reactor.

This paper reports on a study of the oxidation of p-nitrophenol (PNP) in a bubble column reactor (BCR). The use of the air stream aimed to provide perfect mixing in the liquid phase, which was successfully achieved and checked experimentally; there were no concentration gradients along the column, even at the lowest air flow rate used (Q = 1 mL/min at room temperature and atmospheric pressure). The effect of the operating variables was assessed, and a total reduction of PNP was reached, as well as mineralization of 49.2%, oxidant consumption of 90.3%, and with an efficiency of use - ηH2O2 - of 0.09 mg C/mg H2O2, under the best operating conditions found - Q = 1 mL/min, [H2O2] = 1.6 g/L, [Fe2+] = 80 mg/L, pH = 3.0 and T = 22-24 °C - (after 120 min of reaction). Following this, various strategies were developed for improving the mineralization rate; it was found that the addition of H2O2 every 5 min and readjusting the pH after 30 min of reaction allow the attainment of a much higher TOC removal (75.1%) and efficiency of oxidant use (ηH2O2 = 0.17 mg C/mg H2O2) with less oxidant. A reaction mechanism was proposed, based on intermediates identified that include p-nitrocatechol - PNC, p-benzoquinone - PB, hydroquinone - HQ - and carboxylic acids (oxalic, maleic and fumaric). Since the performance achieved in the BCR was good, and very similar to that obtained in a conventional batch reactor, it was possible to verify the efficacy of carrying out the Fenton process in this reactor configuration, which in our future work will focus on the treatability of industrial effluents.