Transformation dynamics and predictive modeling of Cr(VI) in agricultural soils under different pollution levels

Quantifying hexavalent Chromium [Cr(VI)] transformation in soils under different pollution levels is critical for accurate environmental risk assessment. This study investigated Cr(VI) transformation dynamics over a 180-day aging period in eight contrasting soils at four pollution levels. Elevating pollution level from 20 to 300 mg kg −1 increased the available Cr(VI) in the eight soils from 1 to 40 % to 29–81 % (with an average increase of 34.1 %) following 10 days of aging; After 180 days, available Cr(VI) rose from 0 to 3 % to 0–36 % (13.3 %) across the same pollution gradient, indicating weaker influence for long-term aging period. Kinetic analysis indicated that, Cr(VI) reduction was better fitted by second-order and Elovich models than by first-order or parabolic diffusion models at a given pollution level, highlighting a chemisorption-dominated process. Increasing pollution levels enhanced the fit of the parabolic diffusion model, suggesting that diffusion plays a progressively significant role in Cr(VI) reduction at higher pollution level. Correlation analyses identified pH, organic matter, pollution level, and aging time as key factors controlling Cr(VI) reduction. Accordingly, a multiple linear regression model incorporating pollution level was developed to predict Cr(VI) reduction percentage with R 2 = 0.84 (RMSE = 9.77 %). The model was validated against independent datasets, yielding an R 2 of 0.74, notably outperforming models based on single pollution level conditions (R 2 = 0.64). These results advance mechanistic understanding of long-term Cr(VI) behavior in soils across diverse pollution levels and provide a predictive framework to support risk assessment and soil quality standard development. • Elevating pollution level from 20 to 300 mg kg −1 increased available Cr(VI) percentage by up to 49 %. • Cr(VI) reduction dynamic was strongly influenced by pollution level within 10 days. • Pollution level coupled with pH, OM, and aging time governed Cr(VI) reduction process. • Pollution level-based model was developed and validated for Cr(VI) transformation. • Each 100 mg·kg −1 increase caused an average 9 % drop in Cr(VI) reduction percentage.