Insight into the effect of humic acids on transport of Cd²⁺ in biochar-amended saturated porous media

Cadmium (Cd), as a common heavy metal pollutant, has attracted considerable attention due to its ecotoxicity in aquatic and terrestrial ecosystems. However, the impact of humic acid (HA) on the transport of Cd²⁺ in biochar-amended soils remains unclear.

The transport behavior of Cd²⁺ in saturated porous media amended with pomelo peel biochar (PBC) and the effect of HA were examined by column transport experiment, and the breakthrough curves were simulated by the two-site nonequilibrium transport model. The underlying mechanisms were analyzed by batch adsorption experiment and X-ray photoelectron spectroscopy (XPS) characterization.

The addition of PBC significantly inhibited Cd²⁺ transport. After the addition of 0.5% and 1% (w/w) PBC to the sand, the peak breakthough time of Cd²⁺ was delayed by 50% and 125%, respectively. This is mainly due to the microporous structure and high surface area of PBC, which could provide numerous adsorption sites for Cd²⁺. The most notable observation is that in the absence of PBC, HA had a negligible effect on the transport of Cd²⁺. However, in the presence of PBC, 20 mg/L HA inhibited Cd²⁺ transport in PBC-amended saturated porous media, and the peak breakthrough time was delayed by 27.8%. As the concentration of HA increased to 50 mg/L and 100 mg/L, the inhibitory effect weakened.

Low concentrations of HA inhibited Cd²⁺ transport in PBC-amended saturated porous media. As the concentration of HA increased, the inhibitory effect significantly weakened. This study suggests using biochar can be a promising approach for improving the efficacy of soil remediation in heavy metal-contaminated environments. In cases where HA content is high in groundwater and soil, it is necessary to moderately increase the use of biochar to enhance the immobilization of Cd²⁺.