In Situ Chemical Fixation Of Arsenic Contaminated Soils

This paper reports the results of an experimentalstudytesting a low-cost in situ chemical fixation method designed to reclaimarsenic-contaminated subsurface soils. Subsurface soils from severalindustrial sites in southeastern U.S. were contaminated with arsenicthrough heavy application of herbicide containing arsenic trioxide. Themean concentrations of environmentally available arsenic in soilscollected from the two study sites, FW and BH, are 325 mg/kg and 900mg/kg, respectively. The soils are sandy loams with varying mineralogicaland organic contents. The previous study [Yang L, Donahoe RJ. The form, distribution and mobility of arsenic in soils contaminated by arsenictrioxide, at sites in Southeast USA. Appl Geochem 2007;22:320 341]indicated that a large portion of the arsenic in both soils is associatedwith amorphous aluminum and iron oxyhydroxides and shows very slowrelease against leaching by synthetic precipitation. The soil's amorphousaluminum and iron oxyhydroxides content was found to have the mostsignificant effect on its ability to retain arsenic. Based on thisobservation, contaminated soils were reacted with different treatmentsolutions in an effort to promote the formation of insolublearsenic-bearing phases and thereby decrease the leachability of arsenic. Ferrous sulfate, potassium permanganate and calcium carbonate were usedas the reagents for the chemical fixation solutions evaluated in threesets of batch experiments: (1) FeSO4; (2) FeSO4 and KMnO4; (3) FeSO4,KMnO4 and CaCO3. The optimum treatment solutions for each soil wereidentified based on the mobility of arsenic during sequential leaching oftreated and untreated soils using the fluids described in EPA Method 1311[USEPA. Method 1311: toxicity characteristic leaching procedure. Testmethods for evaluating solid waste, physical/chemical methods. 3rd ed. Washington, DC: U.S. Environmental Protection Agency, Office of SolidWaste. U.S. Government Printing Office; 1992]toxic characteristicsleaching procedure (TCLP) and EPA Method 1312 [USEPA. Method 1312:synthetic precipitation leaching procedure. Test methods for evaluatingsolid waste, physical/chemical methods. 3rd ed. Washington, DC: U.S. Environmental Protection Agency, Office of Solid Waste. U.S. GovernmentPrinting Office; 1994]synthetic precipitation leaching procedure(SPLP). Both FW and BH soils showed significant decreases in arsenicleachability for all three treatment solutions, compared to untreatedsoil. While soils treated with solution (3) showed the best results withsubsequent TCLP sequential leaching, SPLP sequential leaching of treatedsoils indicated that lowest arsenic mobility was obtained using treatmentsolution (1). Treatment solution (1) with only FeSO4 is considered thebest choice for remediation of arsenic-contaminated soil because SPLPsequential leaching better simulates natural weathering. Analysis oftreated soils produced no evidence of newly-formed arsenic-bearing phasesin either soil after treatment. Sequential chemical extractions oftreated soils indicate that surface complexation of arsenic on ferrichydroxide is the major mechanism for the fixation process.