Gema Pliego

Trends in the Intensification of the Fenton Process for Wastewater Treatment: An Overview

The implementation of increasingly stringent regulations for wastewater discharge has enforced research efforts toward either the implementation of novel treatments or the improvement of those presently available. The literature on the use of Fenton oxidation in wastewater treatment has established this method as one of the most effective and suitable process for the abatement of recalcitrant water pollutants. However, despite the many advantages of the conventional Fenton process, there are issues relative to pH modulation, the cost associated to H2O2 and catalyst consumption as well as to sludge disposal that limit a more extended full-scale application. In recent years, several solutions have been developed for the sake of improving Fenton (or Fenton-like) oxidation as a cost-effective technology. This paper presents a thorough review on the different ways of intensifying the Fenton by using radiation, electrochemistry, and/or heterogeneous catalysts, as well as by optimizing the main operating conditions in the conventional homogeneous system. The application of these enhanced technologies to synthetic and real industrial wastewaters is described and discussed.

Case study of the application of Fenton process to highly polluted wastewater from power plant.

This work investigates the application of Fenton process to the treatment of a highly polluted industrial wastewater resulting from the pipeline cleaning in a power plant. This effluent is characterized by a high chemical oxygen demand (COD>40 g/L), low biodegradability and quite a high iron concentration (around 3g/L) this coming from pipeline corrosion. The effect of the initial reaction temperature (between 50 and 90 °C) and the way of feeding H2O2 on the mineralization percentage and the efficiency of H2O2 consumption has been analyzed. With the stoichiometric amount of H2O2 relative to initial COD, fed in continuous mode, more than 90% COD reduction was achieved at 90 °C. That was accompanied by a dramatic improvement of the biodegradability. Thus, a combined treatment based on semicontinuous high-temperature Fenton oxidation (SHTF) and conventional aerobic biological treatment would allow fulfilling the COD and ecotoxicity regional limits for industrial wastewaters into de municipal sewer system. For the sake of comparison, catalytic wet air oxidation was also tested with poor results (less than 30% COD removal at 140 °C and 8 atm oxygen pressure).

Application of intensified Fenton oxidation to the treatment of sawmill wastewater

The application of the Fenton process for the treatment of sawmill wastewater has been investigated. The sawmill wastewater was characterized by a moderate COD load (≈3gL(-1)), high ecotoxicity (≈ 40 toxicity units) and almost negligible BOD/COD ratio (5×10(-3)) due to the presence of different fungicides such as propiconazole and 3-iodo-2-propynyl butyl carbamate, being the wastewater classified as non-biodegradable. The effect of the key Fenton variables (temperature (50-120°C), catalyst concentration (25-100 mg L(-1) Fe(3+)), H2O2 dose (1 and 2 times the stoichiometric dose) and the mode of H2O2 addition) on COD reduction and mineralization was investigated in order to fulfill the allowable local limits for industrial wastewater discharge and achieve an efficient consumption of H2O2 in short reaction times (1h). Increasing the temperature clearly improved the oxidation rate and mineralization degree, achieving 60% COD reduction and 50% mineralization at 120°C after 1h with the stoichiometric H2O2 dose and 25 mg L(-1) Fe(3+). The distribution of H2O2 in multiple additions throughout the reaction time was clearly beneficial avoiding competitive scavenging reactions and thus, achieving higher efficiencies of H2O2 consumption (XCOD ≈ 80%). The main by-products were non-toxic short-chain organic acids (acetic, oxalic and formic). Thus, the application of the Fenton process allowed reaching the local limits for industrial wastewater discharge into local sewer system at a relatively low cost.

Mineralization of naphtenic acids with thermally-activated persulfate: The important role of oxygen

This study reports on the mineralization of model naphtenic acids (NAs) in aqueous solution by catalyst-free thermally-activated persulfate (PS) oxidation. These species are found to be pollutants in oil sands process-affected waters. The NAs tested include saturated-ring (cyclohexanecarboxylic and cyclohexanebutyric acids) and aromatic (2-naphthoic and 1,2,3,4-tetrahydro-2-naphthoic acids) structures, at 50mgL(-1)starting concentration. The effect of PS dose within a wide range (10-100% of the theoretical stoichiometric) and working temperature (40-97°C) was investigated. At 80°C and intitial pH=8 complete mineralization of the four NAs was achieved with 40-60% of the stoichiometric PS dose. This is explained because of the important contribution of oxygen, which was experimentally verified and was found to be more effective toward the NAs with a single cyclohexane ring than for the bicyclic aromatic-ring-bearing ones. The effect of chloride and bicarbonate was also checked. The former showed negative effect on the degradation rate of NAs whereas it was negligible or even positive for bicarbonate. The rate of mineralization was well described by simple pseudo-first order kinetics with values of the rate constants normalized to the PS dose within the range of 0.062-0.099h(-1). Apparent activation energy values between 93.7-105.3kJmol(-1) were obtained.

An overview on the application of advanced oxidation processes for the removal of naphthenic acids from water

ABSTRACT Although the exploitation of new energy sources like oil sand or shale may efficiently relieve the urgency of energy shortage, it can also have significant and adverse environmental impacts since huge volumes of oil-containing wastewaters are produced yearly worldwide due to such activities. Naphthenic acids (NAs), which are reported to be bio-recalcitrant due to their structural complexity and toxicity, are the main harmful components of oil shale fracking processes and oil sand process-affected waters. Identification techniques are being continuously improved to deal with the growing analytical need for traditional NAs and emerging ones like oxy-, aromatic, and diamondoid NAs. Meanwhile, treatment approaches focused on technical solutions have been investigated in the past decades. Among these, advanced oxidation processes are the most studied. Different oxidizing agents, like ozone, hydrogen peroxide, and persulfate, among others, have been used, giving rise to a diversity of techniques. The current work presents an updated overview of these techniques with regard to their application for the removal of NAs from water.

Nanoscale Fe/Ag particles activated persulfate: optimization using response surface methodology

This work studied the bimetallic nanoparticles Fe-Ag (nZVI-Ag) activated persulfate (PS) in aqueous solution using response surface methodology. The Box-Behnken design (BBD) was employed to optimize three parameters (nZVI-Ag dose, reaction temperature, and PS concentration) using 4-chlorophenol (4-CP) as the target pollutant. The synthesis of nZVI-Ag particles was carried out through a reduction of FeCl2 with NaBH4 followed by reductive deposition of Ag. The catalyst was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) surface area. The BBD was considered a satisfactory model to optimize the process. Confirmatory tests were carried out using predicted and experimental values under the optimal conditions (50 mg L-1 nZVI-Ag, 21 mM PS at 57 °C) and the complete removal of 4-CP achieved experimentally was successfully predicted by the model, whereas the mineralization degree predicted (90%) was slightly overestimated against the measured data (83%).

Landfill leachate treatment by sequential combination of activated persulfate and Fenton oxidation

This work assesses the feasibility of sequential persulfate and Fenton oxidation for the decolorization and mineralization of landfill leachate (5600 mg L-1 TOC; pH0: 8.6) in a continuous batch-recirculation system. Firstly, it was analyzed the role of the operational conditions upon the persulfate activation evaluating the effects of electrolysis, ilmenite (FeTiO3) as a source of Fe(II) and UV-LED (at 365 nm). The studied variables include current density (j) (50-200 mA cm-2), persulfate dose (46.8-234 mM) and mineral concentration (500-1500 mg L-1). The increase in j enhanced the hypochlorite generation and PS conversion to SO4- and, consequently, decolorization efficiency increasing the penetration of light through the solution and the photoreduction of Fe(III) to Fe(II) in the FeTiO3 surface. The combined electrolysis/FeTiO3/UV-LED showed synergetic effect compared to the individual processes, achieving mineralization around 53% under the optimum operating conditions (1 g L-1 of FeTiO3, using 234 mM of PS at 200 mA cm-2 under UV-LED radiation). The subsequent Fenton oxidation once the pH decreased up to around 3, led to overall mineralization above 90% after 480 min, confirming the suitability of this combined treatment to deal with recalcitrant and highly colored effluents.