Upcycling of Cigarette Butt Fibers for Dual-Action Remediation of Carbonate Soft Clay: Heavy Metal Precipitation and Strength Enhancement

Soft carbonate soils with low undrained shear strength (Su<25kPa) pose a threat to both infrastructure and ecosystem health. Despite previous studies on fiber reinforcement or chemical stabilization separately, the simultaneous use of a material that physically reinforces while chemically interacting through its inherent heavy metals (HMs) has remained unexplored. This study addresses this gap by presenting a dual-purpose strategy using cigarette-butt fibers (CBF) for physical reinforcement and their inherent HMs as ion exchange facilitators. The combined effect was investigated at critical moisture content through a multi-scale analysis including mechanical, chemical, and microstructural tests. The optimum 1% CBF addition enhanced mechanical performance, increasing secant modulus, strain energy, and UCS by approximately 18, 13, and 8 times, respectively, due to fiber-soil interlocking. Chemically, concurrent decreases in pH and EC reflected competitive Ca 2+ release, enhancing precipitation of up to 54% of leachable HMs. However, limited long-term stability of the new chemical bonds—inferred from the silica-sesquioxide ratio and SEM analysis—necessitated the introduction of magnesium oxide powder (MgO-P) as a chemical regulating agent. The addition of 1% MgO-P reduced HMs leachability by a further 62.25% compared to CBF alone, while promoting the formation of stable Mg/Cu-carbonate minerals. Moreover, MgO-P modified the chemical environment, enabling CBF to interact more independently with the soil matrix and enhancing its physicochemical reinforcement efficiency. Collectively, these findings fill the identified gap by advancing the mechanistic understanding of HM absorption in engineered soils and provide a sustainable, dual-remediation protocol for simultaneous mechanical and environmental stabilization of problematic carbonate soils.