Speciation transformation of arsenic during municipal sewage sludge incineration with cotton stalk as additive

- Speciation transformation of As during co-firing of MSS and CS was studied.
- As(III) was gradually oxidized to As(V) as relative amount of CS increased.
- Ca, Al, and Fe compounds in CS provided chemical adsorption sites for arsenic.
- Oxidation of arsenic was continuously conducted during flue gas cooling process.

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 °C. The arsenic toxicity in the fly ash sampled downstream the flue gas was significantly reduced by addition of 40% CS into MSS during incineration.