Reactive transport of Cd(II) and Cr(VI) in natural porous media: Influencing factors and mechanism

The mechanistic understanding of toxic metals reactive transport in complex porous media is essential for pollution prediction and remediation in soil-groundwater systems. This study aims to experimentally and numerically investigate the transport and transformation of cationic Cd(II) and anionic Cr(VI) in four natural soils under different ionic strengths (IS). A two-site nonequilibrium transport model well-described the transport and retention of Cd(II) and Cr(VI) in soils. Results showed that the order of the migration capacity of Cd(II) in different soils was Jiangxi soil (JX) > Jiangsu soil (JS) > Shanghai soil (SH) > Hebei soil (HB), in contrast to that of Cr(VI). Soil pH, CEC and iron oxide content were the primary factors determining mobility of both Cd(II) and Cr(VI). Higher ionic strength promoted Cd(II) transport but barely affected Cr(VI) mobility, indicating the outer-sphere and inner-sphere complexation mechanisms. Furthermore, Cr(VI) migration process was accompanied with reductive transformation that was jointly controlled by soil minerals, organic matter and redox conditions, with reduction capacity ranked as SH>JS≈JX>HB. This is because the dominant reductant Fe(II) in SH soil efficiently reduced Cr(VI), whereas soil organic matter contributed to limited Cr(VI) reduction in JX and JS soils. Our findings reveal the contrasting mechanisms of metal cations and anions reactive transport in different natural media, providing valuable insights for effective management of the subsurface environment.