Pb migration characteristics and mechanism during sludge combustion

The problem of Pb pollution emission from sludge during its combustion has received a widespread attention. The migration and transformation characteristics of Pb during sludge combustion were analyzed by a combination of experimental studies and simulations with the help of inductively coupled plasma mass spectrometry (ICP-MS), specific surface and porosity analyzer (BET), and X-ray diffraction (XRD), etc. The influences of factors such as the addition of CaO, moisture, and acidic gases on the volatilization of Pb and the solid-phase enrichment were investigated, and the activity strength of the adsorption of PbCl₂ molecules on different surfaces were revealed with the combination of the density-functional theory. The experimental results showed that the increase of Pb residue to 61.37% after sludge combustion at 900 °C was mainly influenced by the melting effect. The addition of 4% CaO was unfavorable to the enrichment of Pb in the solid-phase product at combustion temperatures of 700−900 °C, while the residual rate of Pb in the solid-phase product increased by 7.82% at 1 000 °C with the addition of 4% CaO. The Pb residual rate was reduced by 28.43% at 20% sludge moisture content compared to burnt dry sludge. The presence of acidic gases favored Pb immobilization, but the promotion varied inconsistently with concentration. CO₂ had an obvious promotion effect on the fixation of Pb, and the larger the concentration, the more favorable to the enrichment of Pb, when the simulated flue gas was fed with 30% CO₂, the residual rate of Pb increased by 23.37%. While SO₂ had a weak promoting effect on Pb immobilization, and with the increase of SO₂ concentration, the residual rate of Pb was firstly increased and then decreased, and in the combustion condition with 0.15% SO₂, the residual rate of Pb was increased to the best extent, which was only 4.82%. The residual rate of Pb after the combustion of sludge loaded with 4% CaO at 20% H₂O/20O₂/80N₂, 20CO₂/20O₂/60N₂ and 0.15CO₂/20O₂/79.85N₂ increased by −3.61%, 8.77% and 7.65%, respectively. Simulations showed that after the adsorption of PbCl₂ molecules on the clean CaO(001) surface, the Pb atoms of Pb molecules underwent orbital hybridization with Osurf atoms, whereas on the surface of CaO(001) with pre-adsorbed CO₂, SO₂ and H₂O, the Pb atoms of PbCl₂ molecules would undergo orbital hybridization with OH, OC, and OS atoms, which led to an increase of adsorption energies of PbCl₂ molecules after adsorption on the surface of CaO(001) with pre-adsorbed CO₂, SO₂, and H₂O on the surface of CaO(001) with pre-adsorbed CO₂, SO₂ and OS atoms, which resulted in the increase of adsorption energy after adsorption by 90.069, −13.458 and −55.836 kJ/mol, respectively, compared with that on the clean surface. In summary, controlling the combustion temperature and adding an appropriate amount of CaO can effectively improve the residual rate of Pb, the presence of moisture inhibits the immobilization of Pb, and the CO₂ and SO₂ atmospheres are favorable for the enrichment of Pb in the solid-phase products.