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The following research was aimed to assess the risk associated with dioxin contamination, which is a persistent organic contaminant. The first part of the study is about the possible sources of dioxin generation. Levels of polychlorinated dibenzo-pdioxins and polychlorinated dibenzofurans (PCDD/Fs) were determined in soil and incinerator residues. For this purpose soil from municipal waste (MW) dumping site and residues from hospital waste incinerators (HWI) and brick kilns (BK) were collected. Representative samples were quantified using the high resolution gas chromatography-high resolution mass spectrometry (HRGC/HRMS) 6890 Series gas chromatograph (Agilent, USA). The levels of PCDD/Fs in the samples were found in order of HWI > BK > MW site. Based on concentration of the PCDD/Fs, toxic equivalent quantity (TEQ) was measured for each sample in terms of World Health Organization (WHO) and International (I), toxicity schemes. Higher contamination concentration in the ash samples of HWI resulted in higher values of TEQ; which were found in order of HWI> BK > MW site. Among the HWIs, highest values, 2343.00 ng I-TEQ Kg-1 and 2138.20 ng WHO-TEQ Kg-1 were observed for Khyber Teaching Hospital (KTH) in comparison with Hayatabad Medical Complex (HMC), with values of 1493.53 ng I-TEQ Kg-1 and 1416.47 ng WHO-TEQ Kg-1. Second part of the study deals with the main objective of the research which is related to sorption and desorption. For this part two types of dioxins; dibenzo p dioxin (non chlorinated) and 2 Chloro dibenzo p dioxin (mono chlorinated) were used. Six different soils (SS1, SS2, SS3, SS4, SS5 and SS6), each representing a specific soil series were assessed for their sorption/desorption capacity towards dibenzo p dioxin (DD) and 2 Chloro dibenzo p dioxin (2 Cl-DD). Distribution coefficient (Kd) values were obtained from Batch sorption and desorption studies for 72 hrs (3 days) of equilibrium time. Obtained Kd values for DD and 2 Cl-DD varied from, 58-1583 LKg- 1 and 219-2623 LKg-1 respectively. Relatively high sorption was found for SS1, SS2 and SS3, among the six samples. Little variations were found among the Kd values for DD, in comparison with 2 Cl-DD ones. Freundlich and Langmuir Isotherms were applied to the data, where most of the samples fitted the Freundlich isotherm. The overall sorption capacity of selected soils was found to be low indicating a potential risk of being released to environment. Third part of the study covers the leaching part of research. As it is obvious from the sorption part that the soil series collected from KP, showed relatively low sorption capacities which indicates leaching or transport potential. Their transport or leaching under certain environmental conditions such as preferential flow can increase the risk of groundwater contamination. Breakthrough curves (BTCs) against time were plotted for selected dioxins transport prediction, based on measured distribution coefficient (Kd), dispersion coefficient (D) and retardation factor (R). Dioxin transport was measured at two different linear velocities (20 & 50 cmday-1). Attainment of equilibrium was verified to be dependent upon the Kd, R, D and chlorination on dioxin. Kunda series with low OM (0.6%), clay (0.2%) and R (377) was found to have relatively high DD transport potential under normal velocity. This behavior could be due to high dispersion values for its sandy nature. It was concluded that under rapid water or solute flow, dioxins can be leached or transported irrespective of the soil nature and dioxins chemistry.
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