Direct air capture enabled by low-grade thermal energy via hot water regeneration of a ligand-exchange sorbent

[Geoengineering News]

https://www.sciencedirect.com/science/article/abs/pii/S1385894726020991

Authors: Hao Chen, Xinkai Wu, Haibo Liu, Hang Dong, Arup K. SenGupta 

02 March 2026 

https://doi.org/10.1016/j.cej.2026.174640

Highlights

•Low-grade liquid waste heat was demonstrated as an effective regeneration pathway for DAC using a ligand-exchange sorbent.

•The sorbent captured atmospheric CO2 via a reversible bicarbonate-hydroxide chemistry rather than carbamate formation.

•Efficient CO2 regeneration was achieved at ambient pressure using hot water (~80 °C) without vacuum, carrier gas, or drying steps.

•Stable CO2 capture and regeneration were maintained over 600 aqueous thermal cycles, demonstrating strong hydrothermal durability.

•The combined thermal and air-handling energy demand was preliminarily estimated at ~1.4 MWh/ton CO2.

Abstract

Direct air capture (DAC) offers a geographically flexible pathway toward negative emissions, yet its large-scale deployment remains constrained by the high energy demand and operational complexity of sorbent regeneration. While substantial effort has focused on lowering regeneration temperatures, most existing DAC systems remain poorly matched to the largest and most underutilized fraction of industrial waste heat: low-grade (<100 °C) liquid heat streams, such as hot water. This mismatch arises because conventional sorbents are primarily designed for gas-phase or dry-state regeneration. Such processes often require vacuum operation, inert sweep gases, or repeated dehydration-rehydration cycles. Here, we systematically evaluate a direct liquid-phase DAC regeneration strategy enabled by a hybrid polymeric ligand exchanger (Poly-LigEx-Cu2+), specifically designed to operate under hydrothermal conditions at ambient pressure. The Cu(II)-chelated ligand exchanger captures CO2 predominantly as bicarbonate, exhibiting a high CO2 capacity (~3–5 mol/kg) under ambient conditions (~420 ppm). Over 600 consecutive sorption-desorption cycles using low-grade hot water (~80 °C), the sorbent maintained >90% regeneration efficiency with negligible capacity loss or Cu leaching, demonstrating exceptional durability under aqueous thermal cycling. Mild steam (100 °C) was further examined to validate process versatility. Spectroscopic analysis and thermodynamic evaluation confirmed that the reversible bicarbonate-hydroxide chemistry enables efficient CO2 desorption under mild thermal driving forces without vacuum or inert carrier gases. Based on experimental measurements and engineering analysis, the total energy requirement was estimated to be ~1.4 MWh/ton CO2, including hot-water regeneration and air-handling demands. Overall, these results establish a mechanistically grounded and energy-efficient DAC pathway that is intrinsically compatible with widespread low-grade liquid waste heat, offering a scalable alternative to conventional gas-phase regeneration strategies.

Source: ScienceDirect