Thermal Degradation of Octachloronaphthalene over As-Prepared Fe₃O₄ Micro/Nanomaterial and Its Hypothesized Mechanism

Abstract

Decomposition of octachloronaphthalene (CN-75) featuring fully substituted chlorines was investigated over asprepared Fe 3 O 4 micro/nanomaterial at 300°C. It conforms to pseudo-first-order kinetics with k obs = 0.10 min −1 as comparable to that of hexachlorobenzene and decachlorobiphenyl. Analysis of the products indicates that the degradation of CN-75 proceeds via two competitive hydrodechlorination and oxidation pathways. The onset of hydrodechlorination producing lower chlorinated naphthalenes (CNs) is more favored on α-position than β-position. Higher amounts of CN-73, CN-66/67, CN-52/60, and CN-8/11 isomers were found, while small content difference was detected within the tetrachloronaphthalene and trichloronaphthalene homologues, which might be attributed to lower energy principle and steric effects. The important hydrodechlorination steps, leading to CN-73 ≫ CN-74 in two heptachloronaphthalene isomers contrary to that in technical PCN-mixtures, were specified by calculating the charge of natural bond orbitals in CN-75 and the energy of two heptachloronaphthalene radicals. On the basis of the molecular electrostatic potential of CN-75, the nucleophilic O 2− , and eletrophilic O 2 − and O − , present on the Fe 3 O 4 surface, might attack the carbon atom and π electron cloud of naphthalene ring, producing naphthol species with Mars−van Krevelen mechanism, and formic and acetic acids.

Thermal dechlorination of PCB-209 over Ca species-doped Fe2O3

“…(2) where k obs is the pseudo-first-order initial rate constant (min −1 ), as calculated from the experimental data. The pseudo-first-order rate constant for the degradation of PCB-209 over 1%Ca-Fe 2 O 3 was 0.103 min −1 (R 2 = 0.94), which was comparable with that of PCN-75 degradation over the prepared Fe 3 O 4 , with pseudo-first-order rate constant of 0.10 min −1 (R 2 = 0.89) (Su et al, 2014). The existence of strong π -conjugated effects both in PCB-209 and PCN-75 resulted in their similar reaction rate constants.…”

“…The driving force of PCB hydrodechlorination reaction during different steps was discussed on the basis of the isomer distribution pattern in combination with thermodynamic data and steric effect. The results generated in the present study were compared with those obtained from PCN-75 degradation reported in our previous work under similar reaction conditions (Su et al, 2014).…”

“…Furthermore, the TE, H 0 f , and G 573.15 f values of −4140.0277, 80.31, and 348.57 kJ mol −1 , respectively, were lower than those of other detected OctaCB isomers, except that of PCB-201 and PCB-202 which was closed to PCB-197 . As compared to the degradation of PCN-75 over Fe 3 O 4 , the hydrodechlorination at the second step also happened preferentially at the ortho-position but at the other side to mainly form PCN-66/67 (Su et al, 2014). As shown in Fig.…”

“…As compared to the degradation of PCN-75 over Fe 3 O 4 , the main hydrodechlorination pathway of PCN-75 after the elimination of two chlorines at ortho-position happened at meta-positions. At the onward step, no predominant PCN isomers distinguished among the detected TetraCN and TriCN isomers, due to their similar thermodynamic parameters (Su et al, 2014).…”

“…Therefore, higher formation rate and lower degradation rate of produced PCB-207 led to its largest amount among the three NonaCB isomers. However, as compared to the degradation of PCN-75 over Fe 3 O 4 , the hydrodechlorination proceeded primarily at the ortho-position to mainly produce PCN-73 (Su et al, 2014).…”