Co-combustion of Zn/Cd-hyperaccumulator and textile dyeing sludge: Heavy metal immobilizations, gas-to-ash behaviors, and their temperature and atmosphere dependencies

•Oxy-fuel co-combustion enriched Zn and Cd in ash with a low environmental risk.•Al-rich textile dyeing sludge immobilized Zn and Cd via enhanced chemisorption.•Cl directly promoted Zn and Cd volatilizations.•Oxy-fuel co-combustion enhanced S retention mainly as CaSO4 in ash.•Possible reaction and immobilization pathways of the main species were proposed.