Arsenic chemistry in municipal sewage sludge dewatering, thermal drying, and steam gasification: Effects of Fenton-CaO conditioning. (15th April 2022)

• Record Type: Journal Article
• Authors: Jin, Minghao; Liu, Huan; Deng, Hongping; Xiao, Han; Lu, Geng; Yao, Hong
• Is Part Of: Water research. Volume 213(2022)
• Publication Date: 2022-04-15
• Subjects: Sewage sludge -- Fenton-CaO -- Arsenic -- Dewatering -- Thermal drying -- Steam gasification; Water -- Pollution -- Research -- Periodicals; 363.7394
• DOI: 10.1016/j.watres.2022.118140
• Languages: English
• ISSNs: 0043-1354

Abstract

Highlights: Fenton-CaO promoted As accumulation, detoxification and stabilization in sludge. Fenton-CaO induced As oxidation/demethylation by two-step reaction in dewatering process. FeOOH and CaO promote As oxidation by catalyzing/directly reacting with it in drying process. Fe-bound As was less stable than Ca-bound As in char gasification stage of sludge. All the As in gasification ash existed in the form of As(V). 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. Graphical abstract: Image, graphical abstract