Effects of nZVI on the migration and availability of Cr(VI) in soils under simulated acid rain leaching conditions

Hexavalent chromium, Cr(VI), is a ubiquitous toxic metal that can be reduced to Cr(III) by nano-zero-valent iron (nZVI). Finding out effects of continuous rainfall leaching on the Cr(VI) release and availability remains a problem, needing to be addressed. Whether the Cr(VI) reduction by nZVI and continuous rainfall leaching lead to localized heterogeneity in soil is unclear. Therefore, two in situ high-resolution (HR) techniques of the diffusive gradients in thin-films (DGT) and planar optode were combined with ex situ sampling experiments here. Results demonstrate that nZVI decreased Cr(VI) leaching by 5.60–8.50 % compared to control soils. DGT-measured concentrations of Cr(VI), CDGT-Cr(VI), ranged from 7.31 to 19.4 μg L-1 in the control soils, increasing with depth while CDGT-Cr(VI) in nZVI-treated soils (2.41–6.18 μg L-1) decreased or remained stable with depth. However, simulated acid-rain leaching increases CDGT-Cr(VI) by 1.61-fold in nZVI-treated soils, negatively affecting the remediation. DGT measurements in bulk soils using disc devices are better at capturing the change of Cr(VI) availability at different conditions, whereas 2D-HR DGT mappings did not characterize significant mobilization of Cr(VI) at the micro-scale. These findings emphasize the importance of monitoring Cr(VI) release and availability in remediated soil under acid-rain leaching conditions for effective environment management. [Display omitted] •Investigate soil dynamic under acid rain leaching using an in situ deployment system.•Significant decreases in Cr(VI) availability observed in nZVI-treated soils.•Long-term acid rain leaching increased the Cr(VI) resupply potential in soils.•2D distribution of Cr(VI) availability in leaching soil columns was revealed.•No significant hotspots of labile Cr(VI) in nZVI-treated soils after leaching.