USING FERROUS SULFATE SOLUTION TO TREAT ARSENIC-CONTAMINATED SOILS – A COLUMN STUDY

Arsenic contamination of soil and groundwater is a pervasive global environmental problem. Effective, long-term treatment of arsenic in subsurface materials is difficult because it readily changes valence states and reacts to form species with varying toxicity and mobility. Although iron salts have been successfully used as a coagulation agent in treating wastewater containing arsenic, their use in treating arsenic-contaminated soil is rarely mentioned in other published studies.

An in situ chemical fixation method for treating arsenic-contaminated soils has been developed at The University of Alabama, in cooperation with Southern Co. Services, Inc. and EPRI. In this method, an inexpensive treatment solution, made from ferrous sulfate heptahedrate, is used to (1) accelerate the oxidation process of As(III) species to As(V) species, which generally have lower solubilities and are more strongly bonded to soil substrates; (2) coprecipitate arsenic by forming ferric oxyhydroxide; (3) adsorb arsenic species from solution.

Column experiments were performed using soil samples collected from two study sites contaminated with arsenic due to the previous application of herbicide containing As₂O₃. The two soil samples were packed in duplicate into plexiglass columns. One of the duplicate columns was treated with the ferrous sulfate solution and the other column was leached with DI water for comparison. After treatment, the treated and untreated soil columns were exposed to three months of SPLP (synthetic precipitation) leaching. The effluent solution chemistry was analyzed by ICP-OES and ion chromatography. The leached soil columns were sectioned and the solid phases analyzed by microwave-assisted acid digestion, XRD, and SEM.

It was found that calcium-rich minerals present in the soil can significantly retard the movement of iron within the column, ensuring the stability of the fixed portion of arsenic. During the treatment process, leachable arsenic was reduced by roughly 45% compared with the same soil leached with DI water. During the subsequent 3 months of SPLP leaching, no detectable arsenic (LOD~10ppb) was leached from the treated soil columns. In contrast, the arsenic concentration within the untreated soil column leachate peaked at 5000 ppb and then stabilized at 1000 ppb.