REMOVAL OF ATRAZINE FROM AQUEOUS SOLUTION USING ADVANCED OXIDATION TECHNIQUES

Abstract

The degradation of atrazine, a widely used herbicide, a known endocrine disrupting agent, carcinogenic and persistent, in aqueous solution was investigated by various advanced oxidation and reduction technologies. The radiolytic degradation of atrazine has been observed to be an efficient technology for removal of atrazine from water. A detailed kinetic study was performed with respect to G-value, % degradation, observed dose constant (k), and quantum efficiency under various experimental conditions. Hydrogen peroxide, iso-propanol, tert-butanol, nitrate, nitrite, sulfate, carbonate, bicarbonate and bromide ions as well as chloroform have shown inhibitory impact on % degradation of atrazine. The relative contributions of aqueous electron, hydroxyl radical and hydrogen radical towards atrazine degradation were determined as ratio of k for these reactive species and were found as 5: 3 : 1 for ke‒aq: k•OH: kH•. The quantum efficiency ratios of e‒aq, •OH and H• for the degradation of atrazine was calculated as 2: 1: 1. The degradation efficiency was higher under extreme pH conditions (high or low) and lower under neutral conditions. The G-values were observed to decrease with increase in accumulated absorbed dose under all tested experimental conditions. Taking the relative contributions of oxidative and reductive radical species to atrazine degradation into account, reductive pathway proved to be a better approach for the radiolytic treatment of waste-water contaminated with atrazine. In addition to radiolytic degradation, the degradation of atrazine was investigated by 253.7 nm UV irradiation alone or in combination with hydrogen peroxide, peroxymonosulfate (PMS, HSO5−) or persulfate (PS, S2O82−) with an enhanced removal efficiency observed using photochemical processes. UV/PS was found to be the more efficient process than UV/PMS and UV/H2O2 with pseudo-first-order rate constant (kobs) of 6.70 × 10‒3 cm2 mJ‒1 at 4.64 μM atrazine and 92.80 μM PS initial concentrations. The pH showed no significant effect on direct photolysis of atrazine, with kobs of 7.59 × 10‒4, 7.73 × 10‒4 and 6.72 × 10‒4 cm2 mJ‒1 at pH 3.0, 5.7 and 11.0, respectively. UV/H2O2 and UV/PMS performances were independent of pH. However, UV/PS was affected by pH when it was changed from 5.3 to 7.4. The second order rate constant for reaction of atrazine with sulfate radicals was determined as 2.6 × 109 M‒1 s‒1 whereas that for hydroxyl radicals was obtained as 2.3 × 109 M‒1 s‒1. The quantum yield and molar absorption co-efficient at 253.7 nm were determined to be 0.045 mol einstein‒1 and 3504 M‒1 cm‒1, respectively. Product analysis for these reactions was carried out using GC-MS and HPLC/MS/MS and several new degradation by-products of atrazine were identified. A possible degradation mechanism is proposed based on the identified degradation by- products. Due to the presence of various organic and inorganic species in natural water samples, lower removal efficiency of atrazine by UV/H2O2, UV/PMS and UV/PS in real water was observed. The additions of various additives and radical scavengers to aqueous atrazine solution showed a decrease in the removal efficiency of atrazine by UV/PS. The degradation of atrazine was also examined by photo-Fenton and photo-Fenton- like advanced oxidation technologies (AOTs): UV/H2O2/Fe2+, UV/S2O82−/Fe2+ and UV/HSO5−/Fe2+. The study was carried out at two pH value conditions, i.e., pH 3.0 and pH 5.8. At pH 3.0, UV/HSO5−/Fe2+ was found to be the most efficient technology whereas UV/S2O82−/Fe2+ was observed to be the most effective at pH 5.8. The degradation of atrazine followed pseudo-first-order reaction with the highest observed rate constant of 2.00 × 10−2 cm2 mJ‒1 in UV/HSO5−/Fe2+ system at the initial concentrations of 4.64 μM atrazine, 46.4 μM HSO5− and 35.81 μM Fe2+. The UV fluence required for the complete removal of 4.64 μM atrazine at initially 92.80 μM of oxidant and 8.95 μM of Fe2+ concentrations at pH 3.0 was found to be 480, 720 and 960 mJ cm−2 in UV/HSO5−/Fe2+, UV/S2O82−/Fe2+ and UV/H2O2/Fe2+ systems, respectively. Humic and fulvic acids were found to negatively impact the degradation of atrazine. The removal of TOC was not significant unless a high UV fluence was applied. At an initial concentration of 18.56 μM atrazine, 1856.00 μM oxidant and 17.91 μM Fe2+, a 62.94 %, 47.10 % and 44.09 % decrease in TOC was achieved at a UV fluence of 6000 mJ cm−2 in UV/PS/Fe2+, UV/PMS/Fe2+ and UV/H2O2/Fe2+ systems, respectively. Nevertheless, it is suggested in this study that photo-Fenton and photo-Fenton-like technologies are capable of removing atrazine from water efficiently. Besides, Cobalt ions were found to more efficiently activate the PMS and greatly enhanced the degradation of atrazine by UV/PMS/Co2+ but silver ions were observed to decrease the removal rate of atrazine by UV/PS/Ag+ system. The electrical energy per order (EE/O) was calculated for various applied AOTs as an important Figure-of-Merit to explore the potential applications feasibility of the studied systems. Among the studied UV-assisted AOTs, UV/PS was suggested as an effective and economically favorable process for water remediation concerning toxic organic compounds such as pesticides. The degradation of atrazine exhibited second-order kinetics by Fenton reagent. The low activation energy of atrazine, i.e., 23.72 kJ mol ̅1, through Fenton oxidation suggested the easier removal of atrazine from water through this process. This study also reports the preparation of P-doped (P-TiO2), F-doped (F-TiO2) and PF-codoped anatase TiO2 (PF-TiO2) nanoparticles via a conventional sol-gel method. UV-vis diffuse reflectance results showed that doping with phosphorous and fluorine simultaneously could efficiently increase the absorption of TiO2 in visible light region. The BET surface area of 211.96, 174.98, 88.76 and 79.67 m2 g‒1 were obtained for PF- TiO2, P-TiO2, F-TiO2 and reference TiO2 nanoparticles, respectively. The smallest crystal size, highest surface area and absorption in UV-visible region are responsible for the highest photocatalytic activity of PF-TiO2 nanoparticles for atrazine degradation under UV-visible light irradiation. The degradation of atrazine for PF-TiO2, P-TiO2, F-TiO2 and reference TiO2 nanoparticles after 6 h UV-visible light irradiation was 80.9, 71.0, 48.3 and 31.3%, respectively. Therefore, these modified photocatalysts could be considered as potential photocatalysts sources in wastewater treatment. Keywords: Atrazine; Gamma Ray Irradiation; UV radiation; Advanced Oxidation Technologies (AOTs); Photocatalysis; Doping; Degradation Mechanism; Water treatment.