The fly ash from municipal sewage sludge (MSS) incineration contains abundant phosphorus and can be used as phosphate fertilizer. However, fly ash also contains arsenic that is extremely poisonous to the environment. The toxicity of arsenic significantly depends on its speciation: As (III) is much more toxic than As (V). Addition of cotton stalk (CS) was proposed to promote the speciation transformation of arsenic to the less harmful As (V) during MSS incineration. In this work, the speciation transformation of arsenic during the co-firing of MSS and CS in a fluidized bed was studied, and the fate of arsenic during flue gas cooling process was also investigated. The results indicate that the chemical oxidation of arsenic vapors (As₂O₃(g)) is greatly stimulated by the co-firing of MSS and CS especially at 40–50% CS. Mineral compounds such as Ca, Fe, and Al compounds in CS can provide chemical adsorption sites for arsenic to facilitate the chemical oxidation of arsenic by forming various arsenates (As(V)) during MSS incineration. Additionally, the oxidation of arsenic is continuously conducted during flue gas cooling at 320 – 610℃. The arsenic toxicity in the fly ash sampled downstream the flue gas was significantly reduced by addition of 40% CS into MSS during incineration.

The effects of cotton stalk on the speciation transformation of arsenic in the fly ash during sludge incineration were studied, and the fate of arsenic in the cooling down process of flue gas was also investigated. Arsenic is immobilized or stabilized in the MSS incineration fly ash by condensation, physical adsorption and chemical oxidation. As (III) is gradually transformed into As (V) in the fly ash from co-firing of MSS and CS, and the oxidation of arsenic is complete when the CS mass share is higher than 40%.The mineral compounds such as Ca, Al and Fe compounds in CS could provide chemical reactive sites of arsenic, and stimulate the oxidization of arsenic by forming various arsenates during MSS incineration. Arsenic in the flue gas is easily captured in the fly ash at around 460℃. The oxidation of arsenic is continuously carried out during flue gas cooling process. Overall, adding CS into MSS is considered to be a prospective approach to reduce the toxicity of arsenic in the fly ash during MSS incineration.