Arsenic chemistry in municipal sewage sludge dewatering, thermal drying, and steam gasification: Effects of Fenton-CaO conditioning

Minghao Jin; Huan Liu; Hongping Deng; Han Xiao; Geng Lu; Hong Yao

doi:10.1016/j.watres.2022.118140

Arsenic chemistry in municipal sewage sludge dewatering, thermal drying, and steam gasification: Effects of Fenton-CaO conditioning

Water Res. 2022 Apr 15:213:118140. doi: 10.1016/j.watres.2022.118140. Epub 2022 Feb 2.

Authors

Minghao Jin¹, Huan Liu², Hongping Deng³, Han Xiao³, Geng Lu³, Hong Yao¹

Affiliations

¹ State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China.
² State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China; Department of New Energy Science and Engineering, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China. Electronic address: huanliu@hust.edu.cn.
³ State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China; Department of New Energy Science and Engineering, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China.

PMID: 35152134
DOI: 10.1016/j.watres.2022.118140

Abstract

In sludge disposal, Arsenic (As) poses serious secondary pollution due to its high toxicity and low stability. This work systematically studied the effects of Fenton-CaO composite conditioning on As chemistry throughout sludge dewatering, thermal drying, and steam gasification processes. The experimental results showed that, for raw sludge, 40.9% of As was released with filtrate discharging and 26.8-57.3% emitted with flue gas emission. When sludge was conditioned by Fenton-CaO, all of the As in the filtrate was fixed in the sludge cake and the releasing rate of gaseous As was reduced by up to 86.0%. Furthermore, the comprehensive results of the model compounds experiment, sequential extraction, and thermodynamic calculations revealed the effects of Fe/Ca conditioners on As species evolution. In the Fenton pre-oxidation, As(V) was reduced to As(III) due to the decreasing Eh caused by the excessive Fe(II). After adding CaO, As(III)/DMA (dimethyl arsenic) was adsorbed onto the surface of amorphous Fe(OH)₃ that was introduced by Fenton's reagent, 50% and 43% of which were then oxidized or demethylated to form As(V)/MMA (monomethyl arsenic), respectively. In the following drying process at 120-180 °C, the FeOOH and CaO derived by residual Fe/Ca conditioners could promote the oxidation of 30% of the rest As(III) by the catalytic effect or directly reacting with it. In the final steam gasification process, the very little As(III) left in the dry sludge was released with the gas phase and the proportion of As(V) in gasification ash almost reached 100%. In short, Fenton-CaO composite conditioning could achieve the near-zero emission of As and reduce the toxicity of the products throughout the whole sludge treatment process.

Keywords: Arsenic; Dewatering; Fenton-CaO; Sewage sludge; Steam gasification; Thermal drying.