Influence of CO2 saturation on the compressibility of biochar-modified high-water-content soils

Biochar has been extensively recognized for its potential in carbon sequestration and as a sustainable soil amendment, offering an effective strategy for the valorization of high-water-content dredged slurry. This study develops an iron-modified biochar to simultaneously enhance CO 2 adsorption capacity and improve the geotechnical behavior of dredged soils. A comprehensive experimental program was conducted to evaluate the effects of CO 2 saturation ( S c ), initial water content ( w 0 ), and biochar modification on the compressibility and microstructural characteristics of treated soils. After modification using FeCl 3 ·6H 2 O, HCl, and anhydrous ethanol, the CO 2 adsorption capacity of the iron-modified biochar increased by 484% (5.612 cm 3 /g) compared with that of unmodified biochar (0.961 cm 3 /g), demonstrating significant potential for carbon sequestration. Geotechnical testing revealed that the incorporation of iron-modified biochar increased the Atterberg limits while decreasing the specific gravity ( G s ) and soil pH. Although S c exhibited negligible influence on the Atterberg limits and G s , it further reduced soil pH. The addition of iron-modified biochar notably increased the remolded yield stress ( ), decreased the void ratio at yield stress ( e yr ), and reduced the intrinsic compression index ( ), with these improvements showing a strong positive correlation with S c . However, higher w 0 mitigated the beneficial influence of S c on soil compressibility. Variations in and e yr were primarily governed by physicochemical mechanisms, whereby increased S c modified pore structure and interparticle forces through pH-mediated processes. Physical alterations were mainly responsible for the increased initial void ratio ( e 0 ). Nuclear magnetic resonance (NMR) analysis revealed that changes in σ ′ yr and e yr were closely associated with micropore volume characteristics, suggesting that CO 2 entrapment and enhanced cementation synergistically contributed to improved structural stability. Overall, these findings demonstrate that CO 2 -adsorbed, iron-modified biochar-treated soils not only improve geotechnical performance but also provide a sustainable pathway for carbon sequestration and the management of high-water-content dredged materials.