Indirect carbonation of steel slag has gained increasing attention for simultaneous CO₂ sequestration and resource valorization, since the waste exhibits significant potential due to its high calcium content and large-scale availability. However, process efficiency is strongly constrained by leaching selectivity and system complexity. This review critically evaluates recent advances in indirect carbonation technologies for steel slag, with particular focus on the process bottleneck of calcium extraction and integration of the technology with life cycle assessment (LCA) and technoeconomic analysis (TEA). The effects of key leaching agents and operational parameters, including temperature, particle size, and liquid-to-solid ratio, are systematically analyzed to elucidate their roles in controlling extraction efficiency. Emerging process intensification strategies, such as multi-step leaching, ultrasound and microwave assistance, and chemical additives, are introduced as effective approaches to kinetic limitations. The carbonation stage is examined in terms of CO 2 source, reaction conditions, and crystallization pathways, enabling controlled synthesis of different CaCO 3 polymorphs (calcite, vaterite, and aragonite). Environmental and technoeconomic assessments indicate that indirect carbonation can achieve net negative CO 2 emissions, with an estimated net sequestration of approximately 238 kg CO 2 per ton of CaCO 3 , while generating high-value products with substantial market potential. Despite these advantages, challenges remain in leaching selectivity and kinetics, energy and chemical efficiency and process integration for scale-up. Future development should prioritize low-energy leaching systems, closed-loop solvent recycling, and digital optimization strategies. Overall, this work provides a reference for industrialization of indirect carbonation using steel slag.
Photothermal conversion refers to the transformation of absorbed optical energy into heat through non-radiative relaxation processes. In this study, the effect of Chlorella microalgal biomass on the photothermal response of magnetite nanoparticles (Fe₃O₄) was investigated under 808 nm laser irradiation. Fe₃O₄ suspensions at concentrations of 125, 250, and 500 µg/mL were first irradiated at a power density of 1 W/cm² for 5 min. The corresponding final temperature increases were 8.6, 12.8, and 18.5 °C, respectively, demonstrating a concentration-dependent photothermal response. To evaluate the effect of Chlorella , its concentration was varied at 50, 100, and 200 µg/mL while the Fe₃O₄ concentration was fixed at 250 µg/mL. After 300 s of irradiation, the combined suspensions exhibited temperature increases of 13.1, 14.4, and 15.6 °C, respectively. Compared with pristine Fe₃O₄ at the same concentration, the mean final temperature increase was higher by approximately 2.3%, 12.5%, and 21.9%, respectively, although the 0.3 °C difference at 50 µg/mL is small relative to the measurement uncertainty. In contrast, Chlorella alone at 200 µg/mL produced a temperature increase of only approximately 1.2 °C under identical conditions. SEM imaging and EDS elemental mapping revealed extensive accumulation of Fe₃O₄ nanoparticles on the algal biomass. Concentration-dependent fluorescence quenching, a zeta-potential shift from −11.3 mV for Fe₃O₄ to −6.28 mV for the combined system, and FTIR spectral changes, including a shift in the Fe–O band from approximately 576–564 cm⁻¹, provided complementary evidence of interfacial association between the two components. However, the mixture showed no superadditive absorbance at 808 nm; therefore, the origin of the additional heating remains unresolved, and changes in nanoparticle distribution, aggregation, sedimentation, the local optical environment, and interfacial heat transfer are possible explanations that were not quantified in this study. Overall, under the tested conditions, incorporating Chlorella increased the thermal response of Fe₃O₄ suspensions without increasing the nanoparticle concentration.
