Transitions - Sustainable Design: The Science of Sustainability and Green Engineering - page 136

Table 3.6 Concentrations (mg g − 1 ) of Chlorinated Dioxins and Furans after Heating Mg-Al
Silicate, 4% Charcoal, 7% Cl, 1% CuCl 2 • H 2 Oat300 ◦ C

Reaction Time (hours)

Compound 0.25 0.5 1 2 4

Tetrachlorodioxin 2 4 14 30 100

Pentachlorodioxin 110 120 250 490 820

Hexachlorodioxin 730 780 1,600 2,200 3,800

Heptachlorodioxin 1,700 1,840 3,500 4,100 6,300

Octachlorodioxin 800 1,000 2,000 2,250 6,000

Total chlorinated Dioxins 3,342 3,744 7,364 9,070 17,020

Tetrachlorofuran 240 280 670 1,170 1,960

Pentachlorofuran 1,360 1,670 3,720 5,550 8,300

Hexachlorofuran 2,500 3,350 6,240 8,900 14,000

Heptachlorofuran 3,000 3,600 5,500 6,700 9,800

Octachlorofuran 1,260 1,450 1,840 1,840 4,330

Total chlorinated furans 8,360 10,350 17,970 24,160 38,390

Source : L. Stieglitz, G. Zwick, J. Beck, H. Bautz, and W. Roth, Chemosphere , 19, 283, 1989.

This leads to the formation and chlorination of a chemical intermediate that is
a precursor. The reaction steps after this precursor is formed can be identical to
the precursor mechanism discussed in the preceding paragraph.
De novo formation of dioxins and furans may involve even more funda-
mental substances than those moieties mentioned above. For example, diox-
ins may be generated 16 by heating of carbon particles absorbed with mixtures
of magnesium-aluminum silicate complexes when the catalyst copper chloride
(CuCl 2 ) is present (see Table 3.6 and Fig. 3.12). The de novo formation of chlo-
rinated dioxins and furans from the oxidation of carbonaceous particles seems to
occur at around 300 ◦ C. Other chlorinated benzenes, chlorinated biphenyls, and
chlorinated naphthalene compounds are also generated by this type of mecha-
nism.
Thus, green engineering must account for the potential that dioxin's may be
generated during any of these processes. Good operations and maintenance will
translate into the lower rates of production of hazardous substances, or ideally no
hazardous waste production at all.
Other processes generate dioxin pollution. A source that has been greatly
reduced in the last decade is the paper production process, which formerly used
chlorine bleaching. This process has been changed dramatically, and most paper
mills no longer use chlorine. Dioxin is also produced in the making of PVC
plastics, which may follow chemical and physical mechanisms similar to the
second and third processes discussed above.
Since dioxin and dioxinlike compounds are lipophilic and persistent, they
accumulate in soils, sediments, and organic matter and can persist in solid and
hazardous waste disposal sites. 17 These compounds are semivolatile, so they may