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Polycarboxylic acid-based natural deep eutectic solvents for sludge hydrothermal conversion: Unraveling the dual enhancement of dewaterability and 5-methylfurfural production

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Listed:
  • Huang, Cheng
  • Zhai, Yunbo
  • He, Hongkui
  • Liu, Xiaoping
  • Zhou, Yin
  • Hou, Changlan
  • Hu, Jian
  • Liu, Tao

Abstract

This study proposes a novel approach using choline chloride (ChCl)-carboxylic acid natural deep eutectic solvents (NADESs) as bifunctional agents for the simultaneous enhancement of sludge dewaterability and 5-methylfurfural (5-MF) production via hydrothermal treatment. ChCl-polycarboxylic acid NADESs outperformed monocarboxylate analogues, with ChCl-citric acid yielding 7.12 kg 5-MF per ton of volatile solids (180 °C, 30 min) and ChCl-oxalic acid reducing capillary suction time by 87.99 %. Mechanistic analysis revealed that H+ release from strongly acidic NADESs protonates sludge colloids to destabilize extracellular polymeric structures, while multiple -COOH/-OH groups in polycarboxylic acid enhance biopolymer binding compared to monocarboxylates, as demonstrated by DFT calculations. Concurrently, metal ions (Fe, Cr, Zn, etc.) liberated from sludge accelerate carbohydrate hydrolysis/dehydration to 5-hydroxymethylfurfural (HMF), followed by NADES-mediated hydrogenation to 5-MF. The biphasic ethyl acetate–water system further increased 5-MF yield by 26.19 % through in-situ product extraction. By establishing quantitative structure-property correlations between NADES polarity and performance metrics, this work provides molecular design guidelines for multifunctional solvents enabling sludge volume reduction and high-value chemical recovery.

Suggested Citation

  • Huang, Cheng & Zhai, Yunbo & He, Hongkui & Liu, Xiaoping & Zhou, Yin & Hou, Changlan & Hu, Jian & Liu, Tao, 2026. "Polycarboxylic acid-based natural deep eutectic solvents for sludge hydrothermal conversion: Unraveling the dual enhancement of dewaterability and 5-methylfurfural production," Renewable Energy, Elsevier, vol. 256(PG).
    • Handle: RePEc:eee:renene:v:256:y:2026:i:pg:s0960148125021895
    DOI: 10.1016/j.renene.2025.124525
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    References listed on IDEAS

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    1. Wilk, Małgorzata & Śliz, Maciej & Czerwińska, Klaudia & Gajek, Marcin & Kalemba-Rec, Izabela, 2024. "Improvements in dewaterability and fuel properties of hydrochars derived from hydrothermal co-carbonization of sewage sludge and organic waste," Renewable Energy, Elsevier, vol. 227(C).
    2. Liu, Liming & Wang, Hongxia & Zou, Wei & Zhao, Luna & Zhai, Yunbo & He, Hongkui, 2024. "Ionic liquid-catalyzed hydrothermal carbonization of sewage sludge: Effect of residence time and liquid phase circulation on hydrochar characteristic," Renewable Energy, Elsevier, vol. 231(C).
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