Experimental and mechanistic study on the enrichment of heavy metals by modified calcium oxide during MSW pyrolysis

ABSTRACT

Experimental and density functional theory (DFT) calculations show that NaCl modified can promote the adsorption of heavy metals Cr, Cd, and Pb by CaO in the municipal solid waste (MSW) pyrolysis. CrCl₃ (g), CdCl₂ (g) and PbCl₂ (g) are the main form of Cr, Cd and Pb in MSW pyrolysis gas. The DFT calculations show that the adsorption energy of NaCl modified CaO (NaCl/CaO) increased from 285.37 kJ/mol, 130.58 kJ/mol and 145.72 kJ/mol to 373.80 kJ/mol, 216.03 kJ/mol and 214.67 kJ/mol for Cr, Cd and Pb compared with CaO. Similarly, the experimental average adsorption rates increased from 43.32%, 40.13% and 25.54% to 56.42%, 46.44% and 46.22%. The bondings between CaO and CrCl₃, CdCl₂, PbCl₂ molecules are the outermost 2p orbitals of O in CaO hybridizes with the 3d and 3p orbitals of Cr, the 5s and 4d orbitals of Cd, and the 6p and 6s orbitals of Pb. After the adsorption of NaCl, the total charge change on the CaO surface changed from 0.12e to 0.01e, and the number of electrons obtained by CrCl₃, CdCl₂ and PbCl₂ molecules from the adsorbent increased from 0.51, 0.26 and 0.22 e to 0.94, 0.53 and 0.34 e. NaCl promoting the electron-losing ability of calcium oxide surface. Meanwhile the Cl atom of NaCl form new chemical bonds with heavy metal atoms, increasing adsorption sites of heavy metal molecules with calcium oxide. This study contributes to the adsorption of heavy metals during MSW pyrolysis by CaO and NaCl/CaO.

KEYWORDS:

• Municipal solid waste
• pyrolysis
• heavy metals
• CaO
• density functional theory

Acknowledgements

The authors gratefully acknowledge the support from the National Key Research and Development Program of China (grant number 2018YFC1901202).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/15567036.2023.2242310.

Additional information

Funding

The work was supported by the National Key Research and Development Program of China [2018YFC1901202].

Notes on contributors

Shan Zhang

Shan Zhang is a master student of Southeast University. Her research direction is air pollution control.

Zhaoping Zhong

Zhaoping Zhong is a professor at the Department of Environmental Science and Engineering of Southeast University. His research direction is air pollution control.

Yuxuan Yang

Yuxuan Yang is a Ph.D candidate of Southeast University. His research direction is air pollution control.

Jiefei Li

Jiefei Li is a master student of Southeast University. His research direction is air pollution control.

Haoran Du

Haoran Du is a Ph.D candidate of Southeast University. His research direction is air pollution control.

Qian Li

Qian Li is a doctoral candidate of Southeast University. His research direction is air pollution control.