Facilitated transport of cadmium by biochar colloids aged with ultraviolet-irradiation in saturated paddy soils.

Little is known about the transport of heavy metals such as cadmium (Cd(II)) with aged biochar colloids in natural soils. Herein, we investigated the cotransport behaviors of Cd(II) with ultraviolet-irradiation aged biochar colloids pyrolyzed at 400 °C (ABC400) and 700 °C (ABC700) in saturated paddy soils. Pristine biochar colloids were included for comparison. Our results showed that Cd(II) transport was significantly facilitated by pristine and aged biochar colloids in saturated paddy soils, compared to the negligible breakthrough of Cd(II) without biochar colloids. This is likely because biochar colloids acted as vehicles carrying adsorbed Cd(II) during cotransport. Compared with pristine biochar colloids, the aged biochar colloids (especially ABC400) exhibited a greater enhancement effect, with 1.4-3.7 times Cd(II) transport in soils, likely due to stronger sorption affinity and higher mobility of aged biochar colloids towards Cd(II). Synergistic transport of aged biochar colloids with Cd(II) was relatively lower in the red soil than that in the huangni soil, probably related to the higher content of iron oxides, larger specific surface area, and lower content of soil organic matter in the red soil. A two-site kinetic retention model was employed to successfully simulate the cotransport of aged biochar colloids with Cd(II) in paddy soils. Our findings illustrate that light irradiation could accelerate the mobility of biochar colloids, as well as their synergistic carrier of Cd(II). This could trigger the potential cotransport risks when biochar is applied for field remediation of Cd-contaminated soils over a long period of time.