Amendment of arsenic and chromium polluted soil from wood preservation by iron residues from water treatment

► As and Cr polluted soil was successfully amended with iron water treatment residues. ► Adding 5% amendment reduced the leaching of As and Cr in batch experiments. ► 2.5% amendment reduced As in pore water by two orders of magnitude in the field. ► The field experiment revealed pseudo-gley conditions in the soil. An iron-rich water treatment residue (WTR) consisting mainly of ferrihydrite was used for immobilization of arsenic and chromium in a soil contaminated by wood preservatives. A leaching batch experiment was conducted using two soils, a highly contaminated soil (1033 mg kg −1 As and 371 mg kg −1 Cr) and slightly contaminated soil (225 mg kg −1 As and 27 mg kg −1 Cr). Compared to an untreated reference soil, amendment with 5% WTR reduced leaching in the highly contaminated soil by 91% for Cr and 98% for As. No aging effect was observed after 103 d. In a small field experiment, soil was mixed with 2.5% WTR in situ. Pore water was extracted during 3 years from the amended soil and a control site. Pore water arsenic concentrations in the amended soil were more than two orders of magnitude lower than in the control for the upper samplers. An increased release of arsenic was observed during winter in both fields, mostly in the deepest samplers. This is likely due to the formation of a pseudo-gley because of precipitation surplus. Stabilization of arsenic and chromium contaminated soil using WTR is a promising method but the transformation of ferrihydrite in soil proves a concern in case of waterlogged soils. Still the amendment minimized the leaching of arsenic, even in cases of seasonal releases.