Global warming is intensifying, and coastal regions are concurrently experiencing escalating challenges of marine desertification and pollutant remediation. Seafloor mineral carbonation presents a promising strategy that simultaneously achieves carbon sequestration and marine ecosystem restoration. This study examined the longterm carbon sequestration performance of steel slag (SS)-bentonite composite materials under low-temperature marine conditions, assessed their potential heavy metal risks, and proposed a novel in-situ reactive capping strategy for carbon sequestration. The results revealed that SS-bentonite composites achieved substantial carbon sequestration capacity in low-temperature seawater (15 degrees C, 3 MPa CO2), and CO2 fixation remained above 200 g/ kg throughout long-term mineralization. Low-temperature seawater conditions regulated the crystallization pathways of the minerals monohydrocalcite and magnesite. The incorporation of bentonite enhanced SS's longterm mineralization reaction activity by increasing nucleation sites and accelerating Ca2+ leaching, thereby promoting calcite transformation and magnesite formation. The newly formed calcium-rich shell layer during the mineralization process effectively adsorbed heavy metals. The introduction of bentonite significantly reduced the environmental risks associated with Mn and Cr, promoted the formation of Fe-heavy metal precipitates, thereby mitigating the migration risks of heavy metals. As the bentonite dosage increased from 0% to 30%, the residual Cr in the composite rose significantly from 53.24% to 93.34%. Furthermore, the composite material exhibited pronounced synergistic effects in phenol degradation and heavy metal immobilization, achieving a phenol degradation efficiency of 63.37% and Cr/Cu immobilization efficiencies of 99.9%, with residual fractions below 0.1% (ICP-OES detection limit), thereby demonstrating significant potential for seafloor pollutant capping and remediation.