**2.4. Chromium retention capacity of soil**

Batch tests were carried out to determine the Cr accumulation capacity of soil; 1 g of dry soil, sieved to a particle size of 0.002 mm, was placed in a glass tube together with 10 ml of a chromium (Cr VI) solution at standard concentrations of 5, 10, 15, 20, and 25 mg/L, in continuous stirring and constant temperature of 25°C. Five tests were performed for each depth of the soil column, with a contact time of half an hour. Subsequently, the tubes were centrifuged at 2500 rpm, and the supernatant was filtered, collected, and acidified for Uv–visible spectroscopy [14].
