Factors influencing As(V) stabilization in the mine soils amended with iron-rich materials

Chemical stability of As(V) in amended mine-impacted soils was assessed according to functions of incubation period (0, 1, 2, 4, and 6 months), amendment dose (2.5 and 5%), and application timing (0 and 3rd month). Six soils contaminated with 26–209 mg kg⁻¹ of As(V) were collected from two abandoned mine sites and were treated with two alkaline iron-rich materials (mine discharge sludge (MS) and steel-making slag (SS)). Seventeen to 23% of As(V) in soils was labile. After each designated time, As(V) stability was assessed by the labile fractions determined with sequential extraction procedures (F1–F5). Over 6 months, a reduction (26.9–70.4%) of the two labile fractions (F1 and F2) and a quantitative increase (7.4–29.9%) of As(V) in F3 were observed (r² = 0.956). Two recalcitrant fractions (F4 and F5) remained unchanged. Temporal change of As(V) stability in a sample was well described by the two-domain model (k_fast, k_slow, and F_fast). The stabilization (%) correlated well with the fast-stabilizing domain (F_fast), clay content (%), and Fe oxide content (mg kg⁻¹), but correlated poorly with kinetic rate constants (k_fast and k_slow). Until the 3rd month, the 2.5%-MS amended sample resulted in lower As(V) stabilization (25–40%) compared to the 5% sample (50–60%). However, the second 2.5% MS addition on the 2.5% sample upon the lapse of the 3rd month led to a substantial reduction (up to 38%) of labile As(V) fraction in the following 4th and 6th months. As a result, an additional 15–25% of As(V) stability was obtained when splitting the amendment dose into 3-month intervals. In conclusion, the As(V) stabilization by Fe-rich amendment is time-dependent and its efficacy can be improved by optimizing the amendment dose and its timing.