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

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.