Ind. Eng. Chem. Res. 2010, 49, 9043–9051
9043
Fenton’s Depuration of Weathered Olive Mill Wastewaters over a Fe-Ce-O Solid Catalyst Rui C. Martins, Teresa Gomes, and Rosa M. Quinta-Ferreira* GERSE, Group on EnVironment, Reaction and Separation Engineering, Department of Chemical Engineering, Faculty of Sciences and Technology, UniVersity of Coimbra, Po´lo II - Rua Sı´lVio Lima, 3030-790 Coimbra, Portugal
The treatment of actual olive mill wastewaters, coming from an evaporation pond subjected to climatic conditions, was studied by the heterogeneous Fenton process with the laboratory catalyst Fe-Ce-O 70/30. Investigating the impact of the solid load, hydrogen peroxide concentration, and pH over the methodology efficiency a full factorial experimental design was followed by results stating that the amount of Fe-Ce-O 70/30 and pH had a high influence on final depuration, while [H2O2] was not statistically relevant. Reduced models based on those data showed some ability to predict chemical oxygen demand (COD) removal, final effluent biodegradability, and iron leached concentration. Also respirometric and luminescence techniques were used to evaluate toxicity. The recommend operational conditions gathered up when each one of the response factors was optimized were diverse. From those, it seemed appropriate to run with high Fe-Ce-O loads (1.5 g/L) allied with pH ) 4 and [H2O2] ) 115 mM since, even if COD degradation (24%) was not the highest one, the final effluent was very biodegradable (BOD5/COD ) 0.54) allowing the application of an activated sludge post-treatment. Furthermore, for these conditions, catalyst stability was safeguarded since Fe elution was low (0.12 mg/L). Finally, the catalytic system involving Fenton’s peroxidation Fe-Ce-O 70/30 was revealed to be promising for real olive mill wastewaters biodegradability improvement and toxicity removal enabling a proper postbioremediation. 1. Introduction Olive mill wastewaters (OMW) are a serious environmental issue with special impact on the Mediterranean area countries, like Portugal, where the most part of the world olive oil is produced. Some strategies have been proposed for these difficult effluents and the most common practice is the direct deposition in evaporation basins where biochemical conversion may occur1 even if soaring risks are associated with this approach of management concerning the often serious issue of contamination of superficial and underground waters.2 Moreover, depending on the weather conditions, the collected OMW may not evaporate making the pond unenviable in the following year. It is, hence, of extreme importance to combine methodologies to depurate these weathered wastewaters. The application of advanced oxidation processes (AOPs) as suitable options for OOMW depuration were reviewed in several published works.3-6 Nevertheless, to the best of our knowledge only one work regards OMW affected by climatic conditions. Karageorgos et al.7 verified a high phenol and color removal by the application of an ozonation process, and even if chemical oxygen demand (COD) was not large, a high toxicity decrease was observed. The classic Fenton process, based on the hydrogen peroxide oxidant power enhanced by the presence of iron ions in solution,8 has been widely applied for the remediation of several industrial water streams.9 Even though high efficiencies are usually attributed to this technology, catalyst recovery is a major drawback since the iron amounts normally requested are much higher than the legal threshold imposed for an effluent discharge into the surroundings (2 mg/L). In this context, efforts have been made by using solid catalysts capable of replacing homogeneous catalysis based on iron oxides10 supported over a number of materials such as zeolites,11-13 clay,14-17 ashes,18 activated carbon,19,20 and recently carbon aerogels.21 Iron is well-known by its role within Fenton chemistry due to its the high capacity to promote hydrogen peroxide decomposition into
highly reactive free radicals, and because the lanthanide ceriabased catalysts are generally considered as promising formulations in the majority of the catalytic treatments,22-24 the conjugation between the two metals was considered. In fact, the latest studies in our research group, concerning the depuration of a simulated effluent comprising a mixture of six phenolic acids usually present in real OMW, pointed out that the laboratory catalyst Fe-Ce-O with a molar proportion between the two metals of 70/30 is appealing in this field. With the eyes put in the final goal of an industrial application, these results were checked out in the present work where the behavior of this catalyst for weathered OMW was analyzed. The main goal of this experimental work was, thus, to analyze the effect of [H2O2], catalyst load, and pH over the efficiency of Fenton’s process using a Fe-Ce-O 70/30 solid catalyst for the depuration of OMW coming from an evaporation pond. In this regard, special attention was devoted to the effluents biodegradability and toxic character besides the organic content removal to infer about the system capacity to be successfully submitted to a posterior efficient activated sludge treatment.25 Moreover, the catalysts stability related with the operational conditions was assessed. A statistical approach based on a design of experiments (DOE) methodology was applied, aiming to manage the effect of the operating variables (and interaction between them) over the process efficiency with a minimal number of experiments.26 2. Experimental Section 2.1. Catalyst Preparation and Characterization. The FeCe-O 70/30 catalyst was prepared by coprecipitation adding sodium hydroxide (3 M) to a solution containing the respective metal nitrate salts (Fe(NO3)3 · 9H2O and Ce(NO3)3 · 6H2O) in the molar proportion preselected ensuring a total amount of 15 g of metal salt/100 mL.27 The resulting precipitate was filtrated and washed with ultrapure water, dried at 105 °C overnight, and calcinated at 300 °C for 3 h.
10.1021/ie101203p 2010 American Chemical Society Published on Web 08/23/2010
9044
Ind. Eng. Chem. Res., Vol. 49, No. 19, 2010
The catalyst Brunauer-Emmet-Teller surface area (SBET ) 188 m2/g) was determined using an accelerated surface area and porosimetry analyzer (ASAP 2000, Miromeritics). The mercury porosimetry (Poresizer 9329, Miromeritics) revealed that the catalyst does not present pores with diameters higher than 70 Å whereas nitrogen adsorption pointed out a microporous/ mesoporous structure with pores with diameters in the range between 20 and 70 Å with an average size of 66 Å. Even if the precursors used to produce the catalyst encompass Ce and Fe in the trivalent oxidation state, according to literature28,29 it is possible that the mixed oxide also presents Ce4+, the remaining electrons transferred to Fe facilitating electronic mobility that is favorable to the oxidation reactions. In fact, Fenton’s reaction efficiency is related to the Fe redox cycle. According to Dantas et al.,19 hydrogen peroxide reacts with the iron on the catalyst surface leading to a high amount of hydroxyl radicals. The oxidized iron on the solid can react with those radicals producing HO2•, regenerating the catalyst. The Fe-Ce-O activity can be related to the role of Ce in this cycle since ceria, due to its oxygen storage capacity, can give or withdraw Fe electrons according to the needs improving the oxidation reactions. 2.2. Oxidation Procedure and Analytical Methods. The experiments were performed in a batch magnetically stirrer reactor. Effluent (400 mL), which was previously filtrated in a vacuum system using 0.45 µm disposable filters (Whatman) to remove the suspended solids, was first introduced, and the pH was set at the desired value using either sulfuric acid (H2SO4) or sodium hydroxide (NaOH). The selected amount of Fe-Ce-O 70/30 and the precalculated volume of hydrogen peroxide (50%, w/w) were added, and the reaction began. Samples were periodically withdrawn, filtrated to remove the catalyst, and alkalinized to quench the remaining H2O2, which interferes with the analytical techniques. The chemical regime was ensured using catalyst particles with diameters in the range 250-500 µm and a stirring speed of 600 rpm. The effluent chemical oxygen demand (COD) and biochemical oxygen demand (BOD5) were determined according to the standard methods30 in a WTW CR 3000 thermoreactor and a WTW MPM 3000 photometer for COD while a WTW Inolab 740 was used to measure the dissolved oxygen for BOD5. Total organic carbon (TOC) was attained in a Shimadzu TOC-5000. The total phenolic content (TPh) was measured using the Folin-Ciocalteau method31,32 using a T60 UV/vis spectrophotometer (PG Instruments) to measure the absorbance.33 Respirometric techniques were used to infer about the effluent biodegradability and toxicity over activated sludge. Biomass was collected from a winery wastewater treatment plant (3000-4000 mg/L of volatile suspended solids). The bacteria oxygen uptake rate (OUR), which corresponds to the slope of the regression line of the oxygen decay along time, was measured when a totally biodegradable compound was fed (acetic acid, OURacetic acid I) and in the presence of the effluent (OURsample), leading to the sample biodegradability calculated according to eq 1. % biodegradability )
(
)
OURsample × 100 OURacetic acid I
(1)
After the sludge was in contact with the effluent, acetic acid was fed a second time (OURacetic acid II) and sample toxicity was assessed by eq 2.
% toxicity )
(
)
OURacetic acid I - OURacetic acid II × 100 OURacetic acid I
(2)
The environmental impact of the raw effluent and the reaction samples was inferred by luminescence techniques based on the light inhibition of marine bacteria Vibrio fischeri after their contact for 15 min at 15 °C with several dilutions of the wastewater (ISO DIS 11348) using the commercial apparatus LUMIStox (Dr. Lange GmbH, Berlin, Germany). The effective concentration that provokes the inhibition of 20% and 50% of the microorganisms population (EC20 and EC50, respectively) was thus attained. pH was determined using a Crison micropH 2000. The solid catalyst carbon content was measured by elemental analysis using a Fisons Instruments EA 1108 CHNS-O. To verify the catalyst metal leaching behavior, the dissolved iron after 120 min of reaction was reached by atomic adsorption in Perkin-Elmer 3300. Cerium is well-known by its low elution capacity24 and, therefore, was not considered. 3. Results and Discussion 3.1. Characterization of the Effluent. The OMW composition is significantly dependent on several parameters such as clime, cultivation techniques, variety of the fruit, and method of oil extraction.2 The effluent used in this experimental work was collected from an evaporation pond located in the center region of Portugal, and its main characteristics are summarized in Table 1. The raw OMW presented brown color and strong olive smell. Even if the organic content is high (1700 mgO2/L) when compared with the legally allowed level for its direct discharge to the natural courses (150 mgO2/L), this value is lower than the typical ones found for this kind of wastewater (which can be up to 220 gO2/L).34 The effluent was collected in January 2010 (almost a year after being disposed in the pond). In the evaporation pond the effluent is subjected to the climatic conditions, namely dilution by rain and some biological stabilization, leading to a reduction in its initial COD content. This explains why while the raw OMW can have a COD charge as much as 220 gO2/L; our weathered effluent has a COD level equal to 1700 mgO2/L. The low BOD5 found reveals a low biodegradable character, which is confirmed by the BOD5/COD (0.28) ratio below the threshold above that a wastewater is considered to be totally bioremediable (0.4).35 This must probably be due to the high content in phenolic compounds (180 mg/L) that are well-known by their biorefractory properties. Respirometric techniques were applied to analyze the effect of OMW over activated sludge, and Figure 1 shows dissolved oxygen vs time in four situations: with endogenous respiration, in the presence of acetic acid (acetic acid I), in contact with OMW (OMW), and with acetic acid fed a second time (acetic acid II). OUR data are then calculated from the slope of each regression line, and a high value is observed for acetic acid (acetic acid I) pointing out its high biodegradability whereas a lower slope is observed when OMW is fed (OMW). These results, when used in eq 1, highlight the low biodegradable character (40%). Furthermore, it was concluded that the contact with the OMW partially damaged the microorganism activity, since the acetic acid II OUR is smaller than acetic acid I OUR. This means that OMW has inhibition properties shown through the high value of the toxicity calculated by eq 2 (90%). These results clearly reveal the low ability of a biological treatment to directly depurate this effluent. Moreover, eco-toxicological
Ind. Eng. Chem. Res., Vol. 49, No. 19, 2010 Table 1. Physicochemical Characteristics of the Olive Mill Wastewater Collected parameter
value
COD (mgO2/L) BOD5 (mgO2/L) TOC (mgC/L) BOD5/COD TSS (mg/L) TPh (mg/L) EC20 (%) EC50 (%) pH
1700 ( 135 473 ( 95 309 ( 9 0.28 250 ( 25 180 ( 20
