Hexavalent Chromium Immobilization in Soil by Sludge Biochar Tubules Under Fluctuating Groundwater: Performance, Mechanisms and Engineering Potential.

Although biochar has been widely applied for chromium (Cr)-contaminated soil remediation, its direct mixing with soil is limited by poor recoverability and insufficient long-term stability, while the role of groundwater table fluctuation (GTF) is often overlooked. In this study, we developed a biochar tubule-based strategy using Fe, Mg, and Al dual-modified sludge biochar (SBC) for Cr immobilization, and evaluated its performance, mechanisms, and engineering potential under GTF conditions. The demonstrated that modification substantially improved the microstructure and surface chemical properties of biochar. Experiments showed that Fe-SBC exhibited optimal immobilization performance, followed by Mg-SBC, Al-SBC, and SBC. Compared with the controls, Fe-SBC reduced Cr release by 97.66% (slightly contaminated) and 71.40% (heavily contaminated), while the corresponding Cr-retained amounts (predominantly in stable fractions) increased by 155.68% and 257.42%; meanwhile, 7.05% and 9.05% of the total amounts were removed by the biochar tubule, respectively. Mechanistic analysis revealed that Cr(VI) was transported into the biochar tubules via tubule-induced preferential flow and subsequently immobilized through synergistic adsorption, reduction, complexation, and precipitation processes, with redox reactions playing a predominant role (67.37-98.87%). Additionally, the environmental risks, application cost and prospects of biochar were evaluated. This study provides new insights into sludge resource utilization and soil immobilization techniques.