2 research outputs found
Kinetics of PCDD/Fs Formation from Non-Wood Pulp Bleaching with Chlorine
Chlorine bleaching
is still practiced by most of nonwood pulp and
paper mills, resulting in a considerable amount of polychlorinated
dibenzo-<i>p</i>-dioxins and polychlorinated dibenzofurans
(PCDD/Fs) formation and emission. In this study, the effects of primary
chlorination conditions on the formation of PCDD/Fs from nonwood pulp
bleaching with elemental chlorine were investigated. It was found
that low-chlorinated PCDD/Fs were usually formed and then underwent
further chlorination to form highly chlorinated PCDD/Fs with increasing
chlorination time. Higher available chlorine dosages and lower system
pH values greatly accelerated dioxin formation, and pH 3 was the threshold
for the formation of tetra- to octa-CDD/Fs. Higher temperatures promoted
the formation of lower-chlorinated PCDD/Fs, while caused significant
reduction of tetra- to hepta-CDDs and penta- to octa-OCDFs. PCDFs
were formed much faster than PCDDs. A first-order kinetic model showed
a good fit to the data for tetra- to oct-CDFs formation under different
chlorination conditions, indicating that chlorine substitution was
the rate determining step for their formation. Finally, the optimum
chlorination conditions for minimizing and eliminating the formation
of 2,3,7,8-TCDD/TCDF in nonwood pulp bleaching with elemental chlorine
were established
Hazy Weather-Induced Variation in Environmental Behavior of PCDD/Fs and PBDEs in Winter Atmosphere of A North China Megacity
Haze is a serious pollution problem
during the wintertime in North
China. In this study, we investigated how the periodic cycle of winter
haze events affect the environmental behaviors of two typical persistent
organic pollutants, namely, polychlorinated dibenzo-<i>p</i>-dioxins and dibenzofurans (PCDD/Fs) and polybrominated diphenyl
ethers (PBDEs), in the atmosphere of a typical megacity, Beijing.
The average atmospheric concentrations of the total di- to octa-CDD/Fs
(∑PCDD/Fs: 378.0 pg/m<sup>3</sup>) and the total mono- to nona-BDEs
(∑<sub>9h</sub>PBDEs: 166.5 pg/m<sup>3</sup>) during haze episodes
increased by 3.6-fold and 1.9-fold compared with those during the
nonhaze periods, respectively; and their concentrations both linearly
increased with PM<sub>2.5</sub> levels and decreased as a power function
of the atmospheric boundary layer height. The elevated concentrations
could be clearly attributed to the vertically sinking motion of airflow
in the midlower troposphere. When a haze event occurred, the partitioning
rate of PCDD/Fs and PBDEs into particles was reduced; the largest
fraction of the particle-bound ∑PCDD/Fs was shifted from ultrafine
particles to accumulation mode particles; and a steady-state model
(Li-Ma-Yang model) satisfactorily described the gas-particle partitioning
of the PCDD/F and PBDE homologues. The inhalation exposure risk evaluation
indicated that special attention should be paid to the increased cancer
risk induced by the elevated inhalation intake of PCDD/Fs during haze
episodes