Unexpected Dioxin Formation During Digestion of Soil with Oxidizing Acids

Abstract

Dioxins, such as polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs), are among the most toxic unintentionally produced persistent organic pollutants, and their emission is of great concern. Herein, we discovered abundant dioxin formation in soil and various organic carbon-containing matrices after digestion with aqua regia. Σ17PCDD/Fs concentrations were in the range of 66.6–142,834 pg/g dw (5.6–17,021 pg WHO2005-TEQ/g dw) in 19 soil samples after digestion with aqua regia for 6 h. Σ17PCDD/Fs concentration was significantly and positively correlated with soil organic carbon content (R 2 = 0.89; p < 0.01). Compared with cellulose and lignin, humic acid served as an important organic matter component that was converted to PCDD/Fs during soil digestion. Strong oxidation and production of reactive chlorine by aqua regia may be the key factors in the formation of PCDD/Fs. The yearly emission of PCDD/Fs due to digestion with strong acids by the inspection and testing industry was estimated to be 83.8 g TEQ in China in 2021 based on the highest level, which was ∼0.9% of the total dioxin inventory in China. Great attention should be paid to unexpected dioxin formation during digestion processes considering the potential risk of release from laboratories and enterprises.

Br-to-Cl Transformation Guided the Formation of Polyhalogenated Dibenzo-p-dioxins/Dibenzofurans

“…Polychlorinated dibenzo- p -dioxins/dibenzofurans (PCDD/Fs), polybrominated dibenzo-p-dioxins/dibenzofurans (PBDD/Fs), and mixed polybrominated/chlorinated dibenzo- p -dioxins/benzofurans (PBCDD/Fs) share a similar chemical structure and physicochemical properties, collectively known as polyhalogenated dibenzo-p-dioxins/dibenzofurans (PXDD/Fs). − These compounds are highly toxic secondary organic pollutants that pose a severe threat to human and ecological health, which are usually emitted to the environment as byproducts of the combustion process with flue gases or dust. − Extensive research has been dedicated to investigating the PXDD/Fs formation mechanisms, and two classical mechanisms are considered to elucidate the formation of PXDD/Fs in combustion systems: (i) De novo synthesis pathway, where PXDD/Fs are formed from residual carbon in the presence of a chlorine (Cl) or bromine (Br) source. − (ii) Precursor synthesis pathway, where PXDD/Fs are formed by condensation reactions of chlorinated or brominated precursor compounds (Cl-precursors/Br-precursors), such as chlorobenzene (CBzs) and bromobenzene (BBzs). , It is evident that both the Cl and Br source are necessary factors for PXDD/Fs formation, and the Cl-precursors and Br-precursors have a non-negligible effect on the formation of PXDD/Fs. − However, the specific contributions of Cl-precursors and Br-precursors to the formation of PXDD/Fs have not been fully elucidated.…”