Modeling the mass transfer coefficients (MTCs) of PCBs, PAHs, OCPs, and PCDD/Fs using a water surface sampler (WSS)

Abstract

In this study, the mass transfer coefficients (MTCs) characterizing the air-water exchange of PCDD/Fs were investigated. Evaporation and dissolved oxygen (DO) experiments were carried out using a water surface sampler (WSS), and the MTCs of semi-volatile organic compounds (SVOCs) were then modeled based on the data obtained from these experiments. As a result of the calculations made using the evaporation flux observed in the WSS, the average air-side individual MTC [k<inf>g</inf> <inf>(H2O)</inf>] was determined as 1.33 ± 0.58 cm/s k<inf>g (H2O)</inf> values were correlated against u<inf>10,</inf> and a model for the WSS was derived. The water-side individual MTC [(k<inf>w</inf> <inf>(O2)</inf>] for oxygen was calculated using two methods, flux and absorption. According to the results of the calculations, k<inf>w</inf> <inf>(O2)</inf> values were determined as 9.35 × 10⁻⁴ ±7.65 × 10⁻¹ cm/s and 1.30 × 10⁻³ ±6.60 × 10⁻¹ cm/s by flux and absorption methods, respectively. The obtained k<inf>w (O2)</inf> values were also correlated against u<inf>10</inf>, and k<inf>w(O2)_flux</inf> and k<inf>w(O2)_abs</inf> models were constructed for the WSS. Consequently, by applying the findings to the two-film theory, the overall MTC [K<inf>g</inf> <inf>(SVOCs)</inf>] in the gas phase was calculated as 0.03 ± 0.03 cm/s for PCBs, 0.19 ± 0.18 cm/s for PAHs, 0.11 ± 0.08 cm/s for OCPs, and 0.07 ± 0.01 cm/s for PCDD/Fs. MTC values showed seasonal variation, with higher values observed during cold periods. There was also an oscillation in the MTCs among the species of SVOCs. When the chlorine number and molecular weight increased, the MTCs also increased. The results found by modeling are in line with the measurement results obtained by using the WSS but are somewhat lower.