Heavy metals behavior of co-combustion ash from sewage sludge and coal slime: Temperature and mixing ratio dependence, interactions and speciation

Co-combustion of municipal sludge (SS) and coal slime (CS) is an effective route to achieve the co-disposal of solid wastes. This work addresses the problem of heavy metal pollution in co-combustion ash, aiming to reveal the effect of interactions in SS-CS co-combustion on the behavior of heavy metals and the control mechanism of minerals using the inductively coupled plasma mass spectrometry (ICP-MS) technique and the Community Bureau of Reference (BCR) method. The residual rate of heavy metals in CS ash monotonically decreased with increasing combustion temperature, while the residual rate of heavy metals in SS tended to increase when it was burned at high temperatures. Increased temperature enhanced the effect of SS-CS interaction on heavy metals. The interaction significantly inhibited the volatilization of As, and the As residue of the blend containing 50% CS (5SS5CS) at 1000 °C was 14.9% higher than the theoretical value. Typical minerals in SS had significant effects on heavy metal migration in CS, and the effects of CaO and Fe2O3 on heavy metals were mainly due to chemical interactions, in which CaO fixed As in the form of Ca3(AsO4). Density functional theory calculations show that the optimal adsorption sites for both As and As2O3 on the CaO(001) surface were O-top sites, and the presence of O defect on the surface led to enhanced interaction of As2O3 with the surface. Co-combustion led to an increase in the residual fractions of heavy metals, but the exchangeable and acid soluble fraction of As in 5SS5CS was still as high as 19.3%, so the ecological impact of As should be considered in further utilization of the co-combustion ash.