Development of only-phosphorus-based inhibitors for suppressing the formation of dibenzo-p-dioxins and dibenzofurans during de novo synthesis: A dual approach for preventing secondary pollution and stabilizing heavy metals

Abstract

Although nitrogen–phosphorus- (N–P-) and nitrogen–sulfur-based (N–S-based) inhibitors can effectively suppress the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) during municipal solid waste incineration, secondary pollutants, including NOx, NH3, and SO2, may be generated. Therefore, developing novel green inhibitors is crucial. Herein, a series of only-phosphorus-based (only-P-based) inhibitors were systematically investigated in terms of the inhibitory effect on PCDD/F formation and the corresponding inhibition mechanism within a laboratory-scale system. Compared with N–P- and N–S-based inhibitors, most only-P-based inhibitors demonstrated higher efficiency in inhibiting PCDD/F formation, in the following order: K2HPO4 (87.1%) > MgHPO4·3H2O (77.3%) > Na2HPO4 (72.2%) > polyphosphoric ammonium (60.8%) > (NH4)2SO4 (44.4%). Furthermore, these inhibitors exhibited higher inhibition efficiency for highly chlorinated PCDD/F homologues than N–P- and N–S-based inhibitors. X-ray diffraction and thermogravimetric analysis coupled with mass spectroscopy confirmed that the inhibition mechanism of only-P-based inhibitors exclusively involves passivating the catalytic metal (CuCl2). Moreover, no post-treatment secondary pollutants were detected, underscoring their environmental compatibility. Density functional theory calculations showed that further chlorination of lowly chlorinated PCDD/Fs is suppressed by obstructing CuCl2 catalysis during the Deacon reaction, thereby hindering the generation of Cl2. Additionally, only-P-based inhibitors like MgHPO4·3H2O effectively immobilize heavy metals in real fly ash, enhancing their practical value. Thus, the inhibition mechanism and ecological virtues of only-P-based inhibitors were elucidated, which is essential for realizing their practical application.