Organochlorine pesticides removal from groundwater by citrus coal and UV/O-3: a hybrid pilot-scale study

Abstract

In this laboratory-scale study, organochlorine pesticides (OCPs) reduction using citrus coals and O-3 assisted by solar UV as an integrated treatment of groundwater under visible UV to reach sustainable development were traced. The experimental parameters were set up throughout standard methods and analysis protocols. The best performance of citrus coal was observed at a reaction time of 90 min, pH: 6.5, adsorption bed height = 40 cm, initial OCPs concentration, respectively, as 100 mu g/L for alachlor (ALC), alpha (+/-)-BHC, atrazine (ATZ), and alpha-chlordane, 150 mu g/L for methoxychlor, 50 mu g/L for heptachlor, and 90 mu g/L for dieldrin. The best performance of citrus coal was observed at a reaction time of 90 min, pH: 6.5, adsorption bed height 40 cm, initial OCPs concentration of 100 mu g/L for ALC,alpha (+/-)-BHC, ATZ, and alpha-chlordane, 150 mu g/L for methoxychlor, 50 mu g/L for heptachlor, and 90 mu g/L for dieldrin. Under the above-mentioned operating conditions, the removal rates were estimated at more than 75 for all these compounds. The adsorption outputs were fitted to the Freundlich model with the average R-2 > 0.97, while for the Langmuir model this value was >0.86. Thereby, it is concluded that OCPs adsorption occurs on a heterogeneous surface by multilayer sorption. With regard to K-f(1/n) parameter, for the Freundlich adsorption model, the order of OCPs adsorption was appeared to be methoxychlor > dieldrin > heptachlor > alpha-chlordane > ALC >alpha (+/-)-BHC > ATZ. The optimum variables for UV/O-3 were obtained as 0.4 mg/L ozone, pH = 9, and 35 min reaction time, so that, more than 98 of these substances were degraded from the solution under this condition. The reaction kinetics were fitted with R-2 value of more than 0.9 for most OCPs. The kinetic reaction constants (kappa) in pH values of 7 and 9.5 were higher than other ranges for all OCPs. The progress of pH at the alkaline area improved the pseudo-first-order constants. The kinetic model indicated that the reaction rates will be restricted by the initial pH value and the concentration of OH center dot. This evidence was well-established by monitoring dissolved O-2 at the various pH ranges during the experiment runs. Eight intermediates were detected by gas chromatography-tandem mass spectrometry detection with chemical formula as C14H23ClNO3, C13H16ClNO3, C14H18ClNO4, -NHCH(CH3)(2), -NHCOCH3, -NH2, C13H9ClO, and C7H5ClO from the samples taken in the non-optimized UV/O-3 effluent. However, all of them disappeared in the final effluent at the optimized condition. Altogether, this integrated strategy helped to remove OCPs more than 86 on average, with residues less than 1 mu g/L and even 0.05 mu g/L in some cases. Due to the generation of a large amount of citrus wastes (46,000 ton/y) in the north of Iran and the abundance of solar energy especially solar UV in the middle east, this hybrid approach to treat groundwater contaminated with agricultural OCPs as a type of EDCs can be an effective alternative for conventional and current removal strategies to provide safety and treat groundwater by this agricultural compound.