Isotherm-Driven Sorption Dynamics and Breakthrough Behaviour of Chromium in COPR-Impacted Soils: A Field-scale Study from Rania–Khan Chandpur, India

In Kanpur Dehat, India, the Rania–Khan Chandpur villages have multiple Chromite Ore Processing Residue (COPR) dumps that have been open for decades. This procedure has allowed leachate from rain to infiltrate into the soil-groundwater systems and elevated the levels of Chromium (Cr) in groundwater up to 39 mg L⁻¹ in 2023 and 27 mg L⁻¹ in 2024. Despite ongoing contamination, there is limited quantitative understanding of local soil’s retention or release capacity for Cr, making it difficult to predict plume migration or design effective remediation. Therefore, the present study specifically focuses on Rania–Khan Chandpur to generate site-relevant sorption and transport parameters essential for assessing long-term groundwater monitoring. To understand the specific characteristics of Cr in the soil–groundwater system, a batch experiment was performed and fitted with isotherm models. The distribution coefficient (K_d) values for Rania soil ranged from 0.088 to 0.047 L kg⁻¹, indicating substantial Cr adsorption capacity. The K_L values were 0.043 at pH 4, 0.015 at pH 7, and 0.007 at pH 11. Freundlich parameters (K_f and 1/n) further confirmed favorable and heterogeneous surface-controlled adsorption behavior across all pH levels. A rainfall-driven column experiment was conducted to evaluate Cr leaching and transport from a 2 cm COPR layer through 15 cm of soil collected from Rania-Khan Chandpur, simulating natural seasonal recharge conditions at the site. Breakthrough curves (C/C₀ vs Pore Volume (PV)) showed Cr breakthrough at approximately 0.3 PV, reaching a peak near 0.8–0.9 PV, followed by long tailing extending up to around 7 PV, indicating strong initial adsorption and subsequent slow desorption and release over time. This finding suggests that while soil can temporarily restrict Cr movement, it gradually releases retained Cr, acting as a long-term source, which helps explain the persistent groundwater contamination. These findings highlight that although soil temporarily retains Cr, long-term release sustains plume persistence, emphasizing the need for site-specific remediation and improved predictive modelling for soil-groundwater systems.