REMEDIATION OF ARSENIC CONTAMINATED SOILS BY IN SITU CHEMICAL FIXATION METHOD

Subsurface soils from several industrial facilities are contaminated with arsenic because of the application of arsenic containing herbicide. Low cost in-situ chemical fixation treatment is designed to react contaminated soils directly with treatment solutions to cause the formation of insoluble arsenic-bearing phases and thereby decrease the environmental leachability of arsenic. Combinations of ferrous sulfate, potassium permanganate and calcium carbonate were used as major reagents for the chemical fixation solutions. Sequential leaching with an extraction fluid described in the EPA synthetic precipitation leaching procedure (SPLP) was used to simulate the long-term leaching behavior of treated soils under natural conditions. The results indicate that the fixation solution with only ferrous sulfate had the best effect among all the reagent combinations, reducing SPLP-leachable arsenic by as much as 90%. Although XRD and SEM-EDX microanalysis were performed to identify new phases formed during the treatment, no detectable arsenic-bearing phases were found in any of the treated soils.

A modified 4 step sequential extraction procedure was used to further study the chemical fractionation of soil As before and after chemical treatment. Sequential extraction data show that soil treatment greatly reduced the most readily labile portion of arsenic which is extracted in the 1st step of the sequential extraction, with its value lowered to less than one tenth that of untreated soil. The potentially mobile fraction of soil arsenic extracted in the 2nd step of the sequential extraction is also considerably smaller after treatment. It is shown that after treatment, most of the As in the soil is transferred to amorphous Fe oxyhydroxides which are the major phases extracted in the 3rd step. X-ray absorption near edge spectra show As is present as As(V) in the treated soil. Extended X-ray absorption fine structure (EXAFS) spectral analysis indicates that a large portion of the total soil arsenic is coprecipitated and incorporated into newly formed amorphous Fe oxides after treatment.