Aqueous Atrazine Removal by Activated Carbon / Metal Oxides Composites

In this work the removal of atrazine herbicide from aqueous medium was investigated using different catalysts based on activated carbon impregnated iron or chromium oxides. The materials were characterized by powder XRD, scanning electron microscopy (SEM) and infrared spectroscopy (FTIR). The characterization showed the formation of small particle size goethite (α-FeOOH) phase on activated carbon surface. For the catalyst with chromium it can be observed by XPS analyses the Cr2O3 phase. The atrazine oxidation intermediates were monitored by ESI-MS ion trap and the oxidation products distribution is shown to be dependent on the catalitic system employed. Introduction Atrazine (2-chloro-4-ethylamine-6-isopropylamino-s-triazine) is a triazine selective herbicide widely used throughout the world in the control of weeds in various crop cultures (Royal Society of Chemistry; The British Crop Protection Council) and also in nonagricultural areas (Commission of the European Communities). Atrazine was introduced in the 1950s, and since then it has become the most used herbicide in agricultural and forestry applications, with 70,000–90,000 tonnes applied annually worldwide (Ta N., et al.). It is classified as a possible human carcinogen by the U.S.EPA (Belluck, D. A., et al.) and main source of human exposures is from the consumption of contaminated groundwater. Atrazine’s resistance to microbial degradation, slow hydrolysis, low vapor pressure, and moderate aqueous solubility enhance its potential for contaminating groundwater (U.S. EPAOffice of Pesticide Programs). Considering these facts, there is a need of developing efficient treatments for contaminating groundwater. The use of advanced oxidation processes (AOPs) is among of the most cited in literature for atrazine oxidation (Nélieu, S. et al.; Pratap, K. et al.; Acero, J. L. et al.; Hiskia, A. et al.). Advanced oxidation processes (AOPs) are based on the activation of oxidizing agents like H2O2, O3 or O2 for generation of very reactive non-selective transient oxidizing species such as the hydroxyl radicals (OH ), which can degrade the organic compounds in water. Nevertheless, for all the works mentioned above, the authors utilize homogeneous systems. This homogeneous system requires stoichiometric amounts of Fe and large quantities of acid, usually H2SO4, to adjust the optimum pH to 3. After the process the effluent must be neutralized with a base to be safely discharged. Upon neutralization significant amounts of sludge are formed, which is serious limitation of the process due to its disposal problems. The spent acid, base and the formed sludge, are evident drawbacks of the Fenton process. The development of active heterogeneous systems to promote a Fenton-like chemistry which can operate at near neutral pH has a considerable interest since it could offer some advantages, such as no need of acid or base, no sludge generation and the possibility of recycling the catalyst (Cuzzola A. et al.; Ferraz W. et al.). Herein heterogeneous systems, the support choice is undoubtedly very important on developing of a good catalyst and activated carbon has been mostly used for this aim since the 70 decade (Reinoso F.R. et al.). Due to its high surface area and porous structure it can efficiently adsorb gases and compounds dispersed or dissolved in liquids (Oliveira L.C.A. et al.; Culp GL et al.; Ruthven DM. et al.). The adsorption of several organic contaminants in water, such as pesticides, phenols and chlorophenol, has recently been also reported (Oliveira, L.C.A.; Baup S.et al.; Garner I.A. et al.; Martýn-Gullon I. et al.). Methods Preparation of the composites The composites of activated carbon (AC)/iron oxide were prepared from a suspension of activated carbon in a 100 mL solution of FeCl3 (5.8 g, 21.55 mmol) and FeSO4 (12 g, 43.1 mmol) at 343 K. NaOH solution (20 ml, 5 mol L) was added drop wise to precipitate iron oxides. The amount of activated carbon was adjusted in order to obtain AC/iron oxide weight ratios of 5/1 and 1/1. The obtained materials were dried in an oven at 333 K for 24 h. The composite of AC/chromium oxide was prepared from a aqueous suspension of 7.7 g Cr(NO3)3.9H2O and 10 mg activated carbon at 343 K for one hour and dried in an oven at 333 K for 24 h. Characterization of the composites The prepared composites were characterized by powder XRD (Co Kα, λ = 1.78897 Å), infrared spectroscopy (FTIR), using an Excalibur FTS 3000 series (Digilab), scanning electron microscopy (SEM) (Leo-Evo 40XVP) with Au sputtering coated samples fixed on a carbon tape and XPS analysis. XPS data were obtained using a XSAM 800 cpi ESCA (KRATOS Analytical) equipped with a Mg anode (Mg K radiation, 1256.6 eV) and spherical analyzer at 15 KV and 15 mA. The performance of the catalysts was investigated by monitoring the oxidation kinetics of the methylene blue dye (a model molecule). The influence of organic acids (dipicolinic acid and formic acid), in a Fentonlike system was also studied. A typical test was made using 10 mg of the catalyst, 9.9 mL methylene blue (10 mg Kg), 0.1 mL H2O2 (50% v/v) and dipicolinic acid (5 mg, 1/2 proportion, dipicolinic acid/catalyst, w/w ratio) or formic acid solution (44 μL, 1/1 proportion, mol H2O2/mol formic acid) with a reaction time of 4 h. These conditions were studied previously by the authors in another work [12]. The dye decomposition was monitored by measuring absorbance at 665 nm with a UVPC 1600 UV/Vis spectrophotometer (Shimadzu), all reactions were carried out at 278 K. It was also studied the metal ion leaching from the composites in the presence of the organic acids. The tests were made using 9.9 mL distilled water, 10 mg of the composite, 0.1 mL hydrogen peroxide (50% v/v) and 5 mg of dipicolinic acid or 44 μL of formic acid solution. The mixture was kept in contact for two hours. Then the precipitate was centrifuged off and the resulting material was used to prepare 19.8 mL of methylene blue solution (10 mg kg). The homogeneous catalyst activity was tested by adding 0.2 mL of H2O2 (50% v/v) to the methylene blue solution, prepared in the later step, and kept reacting for one hour. The concentration of methylene blue was measured by UV/Vis spectrophotometry. The catalyst has been recovered by filtration and a second leaching test was carried out, repeating procedure described above. Atrazine oxidation tests The atrazine oxidation tests were carried out in aqueous medium using the prepared composites as a catalyst and hydrogen peroxide as oxidizing agent. In a typical experiment were used 9.9 mL atrazine solution (20 mg kg), 10 mg of the catalyst and 0.1 mL of hydrogen peroxide (50% v/v). The atrazine stock solution used herein was prepared from Gesaprin 500 commercial herbicide. The mixture was stirred with a magnetic stirrer and the atrazine intermediates products were monitored by mass spectroscopy ESI/MS Trap (Agilent1100 ion trap VL). The reaction time was 2 h and 5 h for the composite AC/iron oxide 1/1 and 5/1 and 2.5 h and 5 h for the composite AC/chromium oxide. Typical ESI conditions were as follows: dry gas temperature of 600 K; dry gas (N2) flow rate of 5 L min; nebulizer pressure of 10 psi. capillary voltage 3.5 kV; skimmer voltage of 35 V, capillary exit of 125 V, the target mass set to 300, and ICC set to 30000 with a maximum accumulation time of 300 ms. Results and Discussion Characterization of the composites The XRD analysis of the AC/iron oxide 1/1 and AC/iron oxide 5/1 and AC/chromium oxide composites are shown in Figure 1. 20 30 40 50 60 70 80 Gt