Ferrous Iron Treatment of Soils Contaminated with Arsenic-Containing Wood-Preserving Solution

Abstract

This article discusses the results of efforts to reclaim As-contaminated soil from a former timber-treating plant. The study site, commonly referred to as the Rocker Timber Framing site, is located along Silver Bow Creek approximately 7 miles west of the Butte Mining District, MT, USA. The plant operations resulted in contamination of the soils with a highly caustic solution containing 5% As (III). Contaminated soil resulted in the groundwater plumes that contained up to 25 mg L−1 As, with As (V) being the predominant species. The objective of this study was to evaluate the effectiveness of Fe (II) treatment for remediation of As-contaminated soils. Laboratory-treatability studies were conducted on samples of saturated zone (AS1) and va-dose zone (AV1) soils. The AS1 soil was a mixture of coarse alluvium and potentially some mill tailings from adjacent mining operations. The AV1 soil consisted primarily of fill, including soil, construction debris, and timber fragments. Initial concentrations of total As in AS1 and AV1 soils were 683 and 4814 µg kg−1, respectively. Water-soluble As concentrations were 15.4 and 554 µg L−1, respectively, in a 20:1 solution to soil extract. Batch equilibration were performed by placing 10 g of soil into 20 vessels and adding increasing amounts of FeSO4.7H2O. Amendment increments were made as multiples of molar ratios of total As present in each soil. Treatability studies were run with and without a pH buffer of CaCO3 (added at a 2:1 molar ratio to the FeSO4.7H2O treatment). Solution concentrations of As in the AS1 and AV1 soils (without CaCO3) decreased from 554 to 15.4 µ L−1 and 3802 to 0.64 µ L−1, respectively, as the Fe:As molar ratios increased from 0 to 2, whereas for the AS1 soil the solution As concentration increased at the Fe:As molar ratios >2 and reverse trend was observed for the AV1 soils. The decrease in As solution concentration for the AS1 soil is attributable to the dramatic decrease in soil pH with increasing Fe:As molar ratios. In the case of soils treated with CaCO3, the solution concentrations decreased from 564 to 0.65 µg L−1 and 3790 to 0.79 µg L−1 for the AS1 and AV1 soils, respectively,as the Fe:As molar ratios increased from 0 to 50. Generally, in both the soils, the CaCO3-treated soil contained significantly more solution As compared with the non-CaCO3-treated soil at the comparable Fe:As molar ratios. This is attributable to higher solution pH on CaCO3 treatment. Our rapid engineering study indicates that treating both the soils with Fe:As molar ratio of 2 lowered the As water quality limit to <50 µL−1, whereas treating the AS1 and AV1 soils with Fe:As molar ratio of 2 and 3, respectively, lowered the As water quality limit to ≤15 µg L−1. The concentrations of the Cu and Zn were below the instrument detection limits for the AS1 and AV1 soils without CaCO3 treatment. Sequential extraction of Fe-treated soils illustrated that As was relatively stable. Less than 1% of the As was extractable using a modified TCLP approach and <70% of the As was extractable using a harsh acid modified hydroxylamine hydrochloride extraction.