Abstract
In this study, we investigated the extraction of an organo-chlorine herbicide from contaminated soil and its decomposition in water by ultrasound at a frequency of 500 kHz. In particular, (4-chloro-2-methyl phenoxy) acetic acid (MCPA) was employed as an organo-chlorine herbicide sample. An aqueous suspension of model soil, prepared from kaolin contaminated with MCPA, was sonicated under an argon atmosphere in a sonoreactor. During the early period of the sonication experiment, an increase in the MCPA concentration and total organic carbon (TOC) in suspension was observed as MCPA adsorbed on kaolin was extracted out. Subsequently, this was followed by an observed reduction of MCPA, as well as TOC, in suspension as the decomposition of MCPA and its intermediates progressed. During the period of observed decrease of MCPA in suspension, the concentration of chlorine ion increased. In the same period, the sum of MCPA and chlorine ion based on the estimated chlorine present in solution was about constant. By the end of the experiment, the extraction of MCPA based on the estimated chlorine present was about 92 % of the initial MCPA adsorbed on kaolin. The remaining 8 % was presumed to be present as organic chlorine byproducts and residual MCPA adsorbed on soil. Comparison of the removal of MCPA in aqueous solution and in aqueous suspension indicated that the presence of soil particles was deterrent to effective decomposition of MCPA. On the other hand, the importance of ultrasonic irradiation to extract and subsequently decompose MCPA from soil was clearly demonstrated when the results were compared with those obtained in a dissolution experiment of model soil in water subjected to stirring instead of applying ultrasound.
