Andrzej ŁODYGA ? Fertilizer Research Institute, Puławy, Poland; Dorota MINDA-DATA? Grupa Azoty, Poland; Magdalena KOZIOŁ, Piotr TYŃSKI ? Fertilizer Research Institute, Puławy, Poland

Please cite as: CHEMIK 2013, 67, 7, 648?653

Abstract:
Oxidation reaction with use of Fenton?s reagent is one of the most efficient and common methods of removing organic compounds from industrial wastewater. Fenton?s reagent is a mixture of Fe2+ and H2O2 ions and a source of hydroxyl radicals (OH?) with a high oxidizing potential. The paper presents examinations of Fenton process efficiency depending on solution pH, temperature and oxidation duration as well as on H2O2/Fe2+ ratio of the added Fenton?s reagent.

 

Formaldehyde is an organic compound commonly used in various industrial branches such as the textile, tanning, rubber, paper, photographic, founding and construction industry, as well as in biology and medicine. It is applied in the production of resins, adhesives, binders, dyes, paints and lacquers; it is also a disinfectant and a preservative, and constitutes an important substrate in organic synthesis, mainly in the synthesis of acetylene derivatives.

Formaldehyde is one of the most chemically active compounds found in industrial wastewater. It is classified as a substance with carcinogenic action and causes skin damage, inflammatory lesions and allergic reactions. Formaldehyde vapours strongly irritate skin, eyes and the respiratory tract. Since formaldehyde exerts a toxic effect on living organisms, it is necessary to remove it from wastewater directed to natural waters [1].

The technological process of producing polyacetals, in which one of raw materials is formalin, yields wastewater containing various organic compounds, i.a. formaldehyde. In order to be directed to an industrial wastewater treatment plant, that wastewater cannot contain more than 0.0006% of formaldehyde. In an industrial wastewater treatment plant, CH2O is removed to the level below 0.0002% because this is the highest permissible value of contamination with formaldehyde for treated wastewater as regards all industrial branches and wastewater types. In order to achieve a particular level of formaldehyde concentration in wastewater which was directed to an industrial wastewater treatment plant and in which CH2O concentration was much higher than the permissible level of 0.0006%, research works were conducted with a view to removing organic substances via the deep oxidation process with Fenton?s reagent.

Oxidation with use of Fenton?s reagent is quite a common method of removing undesirable organic compounds from industrial wastewater or other utilities. Many studies and patent applications concerning this topic have been developed [2÷5]. Fenton?s reagent is a mixture of Fe2+ and H2O2 ions and a source of hydroxyl radicals (OH?). The latter have the highest oxidizing potential of all known oxidants, so the reaction with this reagent is one of the most efficient methods of removing organic contaminations from industrial wastewater. Its basic advantages are: high efficiency of the oxidation reaction, cheap and easily available substrates and a simple way of conducting the process [8, 9]. The Fenton reaction mechanism includes many stages; these are mainly radical reactions. In the first stage, H2O2 decomposition initiated and catalyzed by Fe2+ ions in the acidic environment takes place according to the following reaction [6]:

Further oxidation of temporary products leads to formation of carbon dioxide and water [6 ÷ 8]. If Fenton?s reagent is used in the reaction with formaldehyde, CH2O will be oxidized to formic acid and then to carbon dioxide and water. A disadvantage of removing organic contaminations via the Fenton oxidation process is the formation of Fe(OH)3 residue which contains large quantities of adsorbed organic compounds. Therefore, works are performed which aim at lowering the Fe2+ content in Fenton?s reagent and conducting the process with minimizing the onerous formation of iron (III) hydroxide [10]. This paper presents the results of examinations concerning the Fenton oxidation reaction conducted on model and real solutions of industrial wastewater. The examinations inspected the influence of solution pH and process temperature on the degree of formaldehyde decomposition. It also analyzed the influence of the added amount of hydrogen peroxide and the H2O2/Fe2+ ratio on the degree of removing formaldehyde and formic acid from the wastewater solution. Depending on the initial wastewater composition, i.e. on the concentration of formaldehyde and formic acid, optimal conditions were established so as to ensure total decomposition of formaldehyde by applying an appropriate proportion of Fenton?s reagent components and correct process duration.

Experimental part

The formaldehyde oxidation reaction with use of Fenton?s reagent was conducted in a thermostated container with a volume of 1000 ml. An appropriate amount of process iron sulphate (FeSO4×7H2O) was added to a 500 ml wastewater sample containing formaldehyde. The solution was mixed with a magnetic agitator with a velocity of 300 rpm. After FeSO4×7H2O dissolved, an appropriate amount of 30% hydrogen peroxide solution (H2O2) was added. The contents were mixed again for 1, 2 and 3 h. Before and after each hour of conducting the Fenton process, the content of formaldehyde and formic acid was determined in the wastewater. Formaldehyde content was determined via titration with sodium (IV) sulphate. Formic acid concentration was determined via potentiometric titration with use of Titrator Titrino 702 SM automatic titration set manufactured by Metrom. The first part of the examinations aimed at determining an optimal pH for the oxidation reaction. That experiment was conducted with use of model solutions. Therefore, 1000 ml of model solution were prepared whose formaldehyde concentration reached 0.054% and whose pH equalled 5.47. The solution was divided into two equal parts. One of them was acidified to a pH of 3.01 with use of sulphuric acid with a concentration of 1.0 mol/dm3. The other part of the solution was left unchanged. Oxidation was conducted using Fenton?s reagent whose H2O2/Fe2+ mass ratio was 10:1 and whose preparation utilized 2 ml of H2O2 (30%) and 0.3307g of FeSO4×7H2O. The performed experiment allowed us to determine the most beneficial pH for decomposing formaldehyde in wastewater. The further part of the examinations was conducted with use of real wastewater coming from an industrial system and directed to a biological wastewater treatment plant. The oxidized wastewater contained 0.05% of formaldehyde and 0.47% of formic acid and did not require a correction of pH because the latter equalled 3.4. The conducted examinations determined the efficiency of the Fenton process in temperatures of 25 and 40°C. The experiment also aimed at optimizing Fenton?s reagent composition in respect of iron (II) ion content. The dose of introduced Fe(II) ions was optimized so as to produce the smallest possible amount of the onerous iron (III) hydroxide as a result of the oxidation reaction without decreasing reaction efficiency. The applied amount of hydrogen peroxide (30%) was constant: it equalled 10 ml per 500 ml of the examined wastewater solution.

Discussion of results

Table 1 show the results obtained in the first stage of the examinations concerning the optimal pH for the formaldehyde oxidation reaction in model wastewater solutions. Formaldehyde oxidation efficiency was determined via the decomposition degree (R) expressed in % and calculated using the following equation:

The obtained results show that the oxidation process is more efficient in the environment whose pH is ca. 3. That value was assumed as optimal for Fenton oxidation reaction of formaldehyde.

The conducted examinations concerning temperature influence on the course of the oxidation process showed that for the constant mass ratio of Fenton?s reagent components (H2O2/Fe2+), formaldehyde decomposition degree is higher in the temp. of 40°C than 25°C (Fig. 1). The experiment was conducted in the temp. of 40°C because that value is the temperature of the examined real wastewater directed to a treatment plant. The dependence of the amount of H2O2 and Fe2+ ions on formaldehyde and formic acid decomposition degree in real wastewater in the temp. of 40°C is shown in Table 2.

The data in Table 2 show that formaldehyde and formic acid decomposition degree in real wastewater (pH=3.4) increases together with reaction duration. A nearly complete decomposition of formaldehyde in the examined wastewater was obtained after 3 hours of conducting the reaction with Fenton?s reagent in which H2O2/Fe2+ mass ratio was 150:1. However, we observed that in such conditions of conducting the reaction, the colourless wastewater solution became yellow and cloudy. This was caused by a by-product of the oxidation process ? iron (III) hydroxide. Therefore, 300:1 was deemed the most beneficial H2O2/Fe2+ mass ratio; after 3 hours of conducting the reaction, formaldehyde decomposition degree reached 99.5% and the onerous Fe(OH)3 residue was not formed. Formaldehyde concentration in the examined wastewater was lowered from 0.05% to 0.00025%.

Summary and conclusions

The performed examinations allowed for selecting optimal conditions ensuring a nearly complete decomposition of formaldehyde with use of Fenton?s reagent and without formation of iron (III) hydroxide residue in industrial wastewater with pH of ca. 3.4, containing 0.05% of formaldehyde and 0.47% of formic acid. That effect was obtained after conducting the wastewater oxidation reaction for 3 hours in a temp. of 40°C, using 10 ml of H2O2 (30%) per 500 ml of wastewater and maintaining the H2O2/Fe2+ mass ratio. Formaldehyde concentration in wastewater was lowered to a value below 0.0006%. Such wastewater can be directed to an industrial wastewater treatment plant. In the conducted optimization of the oxidation process with Fenton?s reagent, formaldehyde decomposition degree in the examined wastewater reached 99.5%, while formic acid decomposition degree ? ca. 42%. The applied method of oxidizing the wastewater with Fenton?s reagent does not require using complicated apparatuses or expensive, hardly available reagents. It has high efficiency and yields non-toxic products (because organic compounds are decomposed to carbon dioxide and water during the oxidation process) or simpler biodegradable particles.

Literature
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Andrzej ŁODYGA ? Ph.D., graduated from the Faculty of Mathematics, Physics and Chemistry of the Maria Curie-Skłodowska University (UMCS) in Lublin in 1972. He obtained his Ph.D. degree in 1993 at the Faculty of Chemistry of the UMCS. He works at the Fertilizer Research Institute (Pulawy)as a head of the Organic Technologies Department. Specialty ? chemistry, chemical analysis, organic technology.

Dorota MINDA-DATA ? M.Sc., graduated from the Faculty of Chemical Engineering and Technology of the Cracow University of Technology in 2004. She currently works in Grupa Azoty.

Magdalena KOZIOŁ ? M.Sc. , graduated from the Faculty of Materials Science and Ceramics of the AGH University of Science and Technology in 2008. She currently works as an engineering and technical specialist at the Organic Technologies Department of the Fertilizer Research Institute. Specialty – laboratory analysis and quality control.

Piotr TYŃSKI ? M.Sc., graduated from the Faculty of Chemistry of the Rzeszow University of Technology in 2002. He is a research and technical specialist at the Organic Technologies Department of the Fertilizer Research Institute (Puławy). Specialty ? organic technology.