We evaluated the use of Fenton reactions induced by solar radiation in the treatment of effluent from a factory of paints for buildings, after prior removal of the suspended solids. The increase of H2O2 concentration from 100 to 2500 mg L−1 for a [Fe2+] = 105 mg L−1 contributed to the reduction of DOC, COD, and toxicity. Our best results were achieved using 1600 mg L−1 H2O2, with 90% of DOC and COD removal and a complete removal of the toxicity with respect to
Wastewaters containing toxic compounds may originate from household and personal care activities as well as from industrial processes. Most of the compounds present in these wastewaters are not treatable by conventional technologies due to the high stability, toxicity, and/or low biodegradability [
Having regard to the above information, many studies aimed at the elimination of toxic and recalcitrant compounds are based on the application of advanced oxidation processes (AOPs) [
Although the photo-Fenton process has been applied in the treatment of wastewaters of different compositions (containing pesticides, pharmaceuticals, petroleum, dyes, and colorants, among other compounds), no attempts have been published concerning the use of the photo-Fenton process combined with solar radiation as an alternative to the treatment of effluent from factories of paints for buildings, searching its decontamination and detoxification. Most of the articles published concerning the treatment of wastewaters containing dyes and pigments involve the study of synthetic effluents containing one or more target compounds in ultrapure water and/or using artificial irradiation, a situation far from the reality [
Therefore, the aim of the present work is to evaluate at lab scale the influence of H2O2 concentration on the soluble fraction of the organic load (dissolved organic carbon (DOC) and chemical oxygen demand (COD)), as well as on the toxicity removal during the application of the solar photo-Fenton process in the treatment of a real effluent, seeking to meet discharge standards and/or reuse in the manufacturing process or in other activities inherent to the industrial plant. Under the best concentration defined in this study for H2O2, the influence of Fe2+ on the kinetics of DOC removal was evaluated, aiming to know the time of treatment and estimate its potential of application. The evaluation of the concentration of Fe2+ and H2O2 is determinant to establish the process efficiency. This information is necessary for an economic and operational evaluation before scaling up to a large scale. In addition, the knowledge about the behaviour of the effluent toxicity during application of the photocatalytic process allows us to make the necessary adjustments so that the process is feasible in large-scale applications.
All solutions, except the effluent, were prepared using ultrapure water.
FeSO4·7H2O (Vetec) was used to prepare aqueous 0.25 mol L−1 Fe2+ stock solution; H2O2 (30% w/w), H2SO4, NaOH, and Na2SO3, all from Vetec, were used as received. A solution of ammonium metavanadate (Vetec) was prepared at a concentration of 0.060 mol L−1 in 0.36 mol L−1 H2SO4 and used for H2O2 quantification.
The effluent was kindly provided by a factory of paints for buildings, whose main activity is the production of acrylic paints. The composition of the effluent was not made available by the company. In general, this kind of effluent must contain acrylic resin, organic and inorganic pigments (titanium dioxide, phthalocyanines, etc.), charges (carbonates, silicates, etc.), additives (dispersants, humectants, surfactants, etc.), and organic and inorganic salts, among others [
All experiments were carried out between 10 am and 14 pm, in a range of temperatures varying between 27 and 33°C, during autumn using a batch reactor located in our lab, in Uberlândia, Brazil (18°55′08′′S; 48°16′37′′W). The solutions containing reagents and effluent, after adjustment of pH and filtering, were exposed to sunlight under clear sky conditions, in open vessels of dark glass (same deep, but with different surface area), and maintained under constant magnetic stirring. Three types of glass containers were used: the first one with 0.21 × 0.21 m (volume of 3.3 L and surface area of 0.044 m2), the second with 13 cm of diameter (volume of 0.98 L and surface area of 0.013 m2), and the third with diameter of 6 cm (volume of 0.21 L and surface area of 0.0028 m2), all with 7.2 cm of depth. Each glass container was filled with the effluent, resulting in a depth of 7 cm.
Two sets of experiments involving the photo-Fenton treatment were performed. One is controlled dosing of H2O2 (ranging from 100 up to 2500 mg L−1), monitoring Fe2+ and Fe3+ concentrations, the removal of DOC and COD as well as the toxicity. The second set is evaluating the effect of different iron concentrations (15, 45, 75, and 105 mg L−1) on the kinetics of DOC removal and consumption of H2O2.
The first one was done using containers of 0.21 × 0.21 m and 105 mg L−1 Fe2+. For these experiments, H2O2 was added to the containers in small portions, in the order of a few mg L−1. The consumption of H2O2 was monitored during solar exposure without control of the dose of the incident UVA radiation. After consuming all H2O2, an aliquot was taken for analyses. A new portion of H2O2 was added, and another sample was taken after consumption of all the H2O2. This cycle was repeated until a cumulative concentration of 2500 mg L−1 of H2O2 has been added. The procedure described as “addition, total consumption, sample collection, and new addition” is important as it prevents the occurrence of reactions in the dark during storage after collection of the sample and possible interference in toxicity tests and the analysis of COD. In these experiments, aliquots (70 mL) of the photolysed solution were collected immediately after the confirmation of the total consumption of the added H2O2. Considering this procedure, it is recommended that the experimental data must be expressed in terms of H2O2 consumption and not on the basis of the accumulated radiation dose or even the reaction time.
In the second set of experiments (kinetic experiments), containers of 13 cm diameter were used. Two sets of experiments were performed: the first one involves the correlation between reaction time and accumulated dose of UVA, and the other relates the reaction directly to the dose of UVA radiation. In the first set of experiments, aliquots of 30 mL were collected at intervals of 20 min up to 60 min of reaction and at very 30 min above 60 min of reaction up to 150 min. For the second, aliquots were collected after exposure to the same dose of UVA radiation. This procedure was done to facilitate the comparison of results, since for the same dose of UVA, the same amount of photons is obtained, providing the same level of degradation for a similar exposition [
We used an initial concentration of H2O2 equal to 1600 mg L−1, determined in previous experiments, which is able to induce an almost complete reduction in DOC and COD. Due to the fast consumption of H2O2, new additions were done at each 78.3 kJ m−2 accumulated, in the experiments when 45 mg L−1 Fe2+ were used, and at each 47.1 kJ m−2 of UVA, when 75 or 105 mg L−1 Fe2+ were used.
Before photo-Fenton experiments, the initial pH of the effluent was adjusted between 2.5 and 2.8, the optimum pH range for Fenton reactions [
After sampling and before analysis, for the experiments involving the controlled dosage of H2O2, the pH of the aliquots was adjusted to the range between 6 and 8 before filtration through membranes with mean pore size of 0.45
Control experiments (direct photolysis, H2O2/solar, H2O2 dark, Fe2+/solar, and Fenton) were also performed in containers with 6 cm diameter using 1600 mg L−1 H2O2 and 105 mg L−1 Fe2+.
The efficiency of the photo-Fenton process was evaluated by monitoring the following experimental parameters: electronic absorption, consumption of H2O2, amount of Fe2+ and Fe3+, COD, DOC, and toxicity, evaluated using
The DOC decay was followed using a TOC analyser (Shimadzu TOC-VCPH/CPN) equipped with an ASI-V autosampler. COD determinations were carried out according to the 5220D Standard Methods [
Paints for buildings can be considered as a stable mixture, in basic medium, of solids in a volatile component (water or organic solvent, in this case, water). A third group of components is the additives. Although they represent a small fraction of the composition, they are responsible for important features for the paints, as wetting and dispersing action for pigments and charges and corrosion protection, among others.
A brief characterization of the effluent used in this study is presented in Table
Composition of the studied effluent.
Parameter | Value |
---|---|
Dissolved organic carbon (mg C L−1)b | 307 |
Dissolved organic carbon (mg C L−1)c | 237 |
Dissolved inorganic carbon (mg C L−1)b | 83.5 |
Dissolved inorganic carbon (mg C L−1)c | 6.2 |
Chemical oxygen demand (mg O2 L−1)b | 1232 |
Chemical oxygen demand (mg O2 L−1)c | 672 |
Toxicity ( |
100 |
pHa | 7.6 |
Solids in suspension (mg L−1)a | 2040 |
Total dissolved iron (mg L−1)b | 0.05 |
Total chloride (mg L−1)b | 8.2 |
Total chloride (mg L−1)c | 4.1 |
Total phosphorus (mg L−1)c | 3.2 |
Total N (mg L−1)b | 34.9 |
Total N (mg L−1)c | 28.1 |
N- |
28.5 |
N- |
26.9 |
Colora | Lilac |
Colorb | Lilac |
Colorc | Colorless |
Odora | Fetid |
Odorb | Fetid |
Odorc | Similar but less intense |
bRaw effluent, after filtration using 0.45
cAfter pH adjustment between 2.5 and 2.8, precipitation, and filtration using 0.45
Considering that the optimum pH range for Fenton reactions occurs between 2.5 and 2.8 [
From this point, the authors decided to apply the photo-Fenton process as an alternative to degrade only the soluble fraction of the organic load to discharge standards and/or its reuseain, and not the solid phase. So, before all photodegradation experiments and after pH adjustment, the effluent was submitted to decantation and filtration using 0.45
The UV-Vis absorption spectrum of the effluent after pretreatment reveals a high absorbance only in the UV region (190–400 nm), probably due to the presence of aromatic compounds (Figure
Influence of the H2O2 concentration on the soluble fraction of the (a) organic load DOC and COD, (b) UV absorbance removal of the effluent, obtained during solar photo-Fenton reaction, using different H2O2 concentrations. Experimental conditions: initial DOC = 237 mg CL−1; initial COD = 672 mg O2 L−1; Fe2+ = 105 mg L−1; and pH = 2.5–2.8.
The H2O2 concentration is an important operational parameter for Fenton reactions, since an excess of this reagent, or its absence, tends to reduce drastically the efficiency of the process. The excess of H2O2 favors self-decomposition reactions (
For the experiments with controlled H2O2 dosage, after addition of the first dose of 100 mg L−1 of this reagent, the color of the effluent changed from light orange to dark gray, suggesting the formation of dark colored intermediates. New additions of H2O2 up to 2500 mg L−1 were done. Although there has not been a complete removal of color of the solution, there was a significant reduction in its intensity. This suggests the persistence of the colored intermediates. Despite that the color of the effluent can limit the action of the incident radiation, a fast COD decay from 672 to 250 mg O2 L−1 was achieved using 400 mg L−1 H2O2, and 105 mg L−1 Fe2+, under solar irradiation. Using 1600 mg L−1 H2O2 a COD of 70 mg O2 L−1 was reached (Figure
Only a slight improvement in COD removal was observed when increasing the H2O2 concentration from 1600 up to 2500 mg L−1, respectively, 70 to 57 mg O2 L−1, probably due to the formation of acids of low molecular weight, refractory to oxidation [
Parallel to DOC, COD, and absorbance analysis, the concentrations of Fe2+, Fe3+ and the total dissolved iron (Fe2+ + Fe3+) were also evaluated for different concentrations of H2O2 (Figure
Dependence on Fe2+, Fe3+, and total dissolved iron concentrations and different concentrations of hydrogen peroxide, during photo-Fenton treatment of the soluble fraction of the effluent. Experimental conditions: DOC = 237 mg C L−1; COD = 672 mg O2 L−1; Fe2+ = 105 mg L−1; and pH = 2.5–2.8.
As before mentioned, the presence of colored intermediates was observed for concentrations of H2O2 up to 2500 mg L−1. This tends to affect the photochemical regeneration of ferrous ions since the absorption of photons by Fe3+ complexes is more difficult in strongly colored solutions, even under stirring (Figure
As presented in Figure
A probable consumption of H2O2 (Figure
The role of H2O2 concentration in the toxicity during the photo-Fenton process was also evaluated, once the soluble fraction of the effluent presented 100% toxicity to
Evaluation of the toxicity to
As presented and discussed above, the results demonstrate the feasibility and potential of photo-Fenton process both in decontamination (Figure
The initial Fe2+ concentration is important in two aspects, one is related to the penetration of light through the solution, and the other is related to the efficiency of the photo-Fenton process in generating hydroxyl radicals since Fe2+ catalyzes the decomposition of H2O2, generating hydroxyl radicals (
The effect of the iron concentration on Fenton and photo-Fenton processes was evaluated in previous studies. It was observed that the rate of degradation increases with a given concentration of iron ions, regarded as the concentration that results in the best experimental conditions, which, on the other hand, is dependent on the composition of the effluent and type of reactor [
In this study, four sets of experiments using different iron concentrations (15, 45, 75, and 105 mg L−1) were done using an initial H2O2 concentration of 1600 mg L−1. As the initial dose of H2O2 was rapidly consumed, new additions were done at each 78.3 kJ m−2 of accumulated dose of UVA radiation in the assays done with 45 mg L−1 Fe2+ and at each 47.1 kJ m−2 of accumulated dose of UVA radiation in the assays done with 75 and 105 mg L−1 Fe2+. Using the lower iron concentration (15 mg L−1), a low rate for DOC removal was verified when compared to the other iron concentrations (45, 75, and 105 mg L−1) (Figure
Influence of Fe2+ concentration on (a) kinetics of DOC removal of the effluent (solid symbols) and consumption of H2O2 (open symbols); (b) dissolved iron species (solid symbols: Fe2+ concentration, open symbols: Fe3+ concentration) during the solar photo-Fenton treatment of the soluble fraction of the effluent under study. Experimental conditions: DOC = 247 mg C L−1; H2O2 = 1600 mg L−1 replaced in 78.3 kJ m−2 to 45 mg L−1 Fe2+ and 47.1 kJ m−2 to 75 and 105 mg L−1 Fe2+; and pH = 2.5–2.8. Irradiation time:
It can be observed in Figure
The curve of DOC removal can be divided into two parts (up to 78.3 kJ m−2 and above 78.3 kJ m−2), Figure
An increase in the rate of DOC removal by a factor of 11 was achieved when the Fe2+ concentration increased from 15 to 45 mg L−1, considering an accumulated dose of 78.3 kJ m−2 of UVA radiation (Figure
After an accumulated dose of 130 kJ m−2 of UVA (
Control experiments using the soluble fraction of the effluent were also carried out under the following conditions: (a) absence of Fe2+ and H2O2 (direct photolysis at pH 2.5–2.8); (b) photolysis without adding Fe2+, in presence of 1600 mg L−1 H2O2 at pH 2.5; (c) photolysis involving a hydrated complex of iron, using 45 mg L−1 Fe2+ at pH 2.5–2.8, without H2O2; and (d) Fenton process in the dark using 105 mg L−1 Fe2+ and 1600 mg L−1 H2O2. These experiments were important to assure that the results found during the photocatalytic assays were consistent and not due to direct photolysis and/or thermal Fenton reactions (Fenton reactions in the dark). No degradation was observed in the conditions from (a) to (c), while 40% of DOC removal was reached in the thermal Fenton reaction in the presence of 105 mg L−1 Fe2+, after 150 min of reaction (data not shown), equivalent to the period required to accumulate a dose of UVA radiation of 238 kJ m−2 in the experiments under solar irradiation. The lower DOC removal obtained in the thermal Fenton reaction, (d), when compared to the photo-Fenton process induced by solar radiation (90% of DOC removal), suggests a synergetic effect caused by both Fenton processes.
For thermal Fenton reactions, the concentrations of Fe2+ and Fe3+ and total dissolved iron (Fe2+ + Fe3+) were also monitored. During the reaction interval (150 min), the concentration of Fe2+ decreased from 102 to 20 mg L−1 after a consumption of 1260 mg L−1 H2O2. At the same time, no increase in the concentration of Fe2+ has been observed (data not shown). This agrees with the results obtained using controlled dosing of H2O2 (Figure
Although, respectively, 62 and 75% of DOC and COD removal were reached using 800 mg L−1 H2O2 and [Fe2+] = 105 mg L−1, no reduction in the toxicity was observed. This suggests that the earliest intermediates of oxidized pretreated effluent obtained using a low dose of H2O2 (up to 800 mg L−1) still present acute toxicity to
The curve of DOC removal is consistent with apparent pseudo first-order kinetics (
These results demonstrate the viability of application of Fenton reactions to the decontamination and detoxification of effluent from factories of paints for buildings, using solar radiation.
The authors thank CNPq (Project no. 2011-EXA022) for the scholarship to O. Gomes Jr. and FAPEMIG (CEX-APQ-00915-11, CEX-APQ-01798-11, CEX-APQ-02425-10, and CEX-APQ-01217-12), CAPES/PVNS, CNPq (304.576/2011-5) for the financial support of this work. The authors also thank Mr. Paulo Souza Müller Jr. for technical support. A. E. H. Machado is particularly indebted to CNPq and CAPES for his research grants. This work is a collaborative research project between members of the Rede Mineira de Química (RQ-MG), partially supported by FAPEMIG (Project no. REDE-113/10).