In Situ Fixation of Metal(loid)s in Contaminated Soils: A Comparison of Conventional, Opportunistic, and Engineered Soil Amendments
Citation:
Mele, E., E. Donner, A. Juhasz, G. Brunetti, E. Smith, A. Betts, P. Castaldi, S. Deiana, K. Scheckel, AND E. Lombi. In Situ Fixation of Metal(loid)s in Contaminated Soils: A Comparison of Conventional, Opportunistic, and Engineered Soil Amendments. David L. Sedlak (ed.), ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 49(22):13501-13509, (2015).
Impact/Purpose:
In many countries, regulatory limits for As and Pb in soils are based on their total concentrations. However, because of mineralogical differences and the influence of soil properties, contaminant absorption following incidental soil ingestion is frequently less that the default value of 100%. Importantly, exposure parameters may be refined through the assessment of contaminant relative bioavailability (RBA), i.e., the fraction of the ingested dose that is absorbed into the systemic circulation of the organism in question. Contaminant RBA can be measured using in vivo assays and model organisms, but cost and ethical considerations limit their routine use. As a result, in vitro gastrointestinal methods have been developed as surrogate assays for estimating contaminant RBA. In this context, bioaccessibility refers to the fraction of a contaminant that is soluble in gastrointestinal fluid and therefore potentially available for absorption into systemic circulation. Remediation strategies that promote the formation of stable forms of As and Pb (e.g., in situ immobilization) have been proposed for the remediation and/or management of contaminated soils. One such technology is based on the use of soil amendments that can immobilize contaminants through sorption and/or precipitation reactions. However, remediation of As- and Pb- cocontaminated soils using soil amendments is challenging as Pb is present as a divalent cation while As may be found either as arsenate or arsenite at soil pH values from 4 to 9.5. Nevertheless, a range of materials have been proposed for the in situ immobilization of these contaminants. Some traditional remediation materials, such as phosphate, are particularly useful for metal cations and Pb in particular. However, in the case of soil contaminated by both metals and As, phosphate may induce As mobilization through exchange reactions. Therefore, the objective of this study was to evaluate the ability of a range of amendments added to a metal(loid)-polluted soil to decrease the associated risks to human health and the environment. Pb and As bioaccessibility in a mining soil was compared pre- and postamendment with conventional (phosphate), opportunistic [water treatment residuals (WTRs)], and engineered (nZVI) remediation products. Lead RBA was also investigated in the treated and untreated soil using an in vivo mouse model. In addition, As speciation before and after treatment was assessed using X-ray absorption near-edge spectroscopy (XANES) to elucidate the specific mechanisms underlying the observed reductions in bioaccessibility.
Description:
This study aimed to assess and compare the in vitro and in vivo bioaccessibility/bioavailability of As and Pb in a mining contaminated soil (As 2267 mg kg-1, Pb 1126 mg kg-1), after the addition of conventional (phosphoric acid), opportunistic [water treatment residues (WTRs)], and engineered [nano- and microscale zero valent iron (ZVI)] amendments. Phosphoric acid was the only amendment that could significantly decrease Pb bioaccessibility with respect to untreated soil (41 and 47% in the gastric phase and 2.1 and 8.1% in the intestinal phases, respectively), giving treatment effect ratios (TERs, the bioaccessibility in the amended soil divided by the bioaccessibility in the untreated soil) of 0.25 and 0.87 in the gastric and intestinal phase, respectively. The in vivo bioavailability of Pb decreased in the phosphate treatment relative to the untreated soil (6 and 24%, respectively), and also in the Fe WTR 2% (12%) and nZVI-2 (13%) treatments. The ZVI amendments caused a decrease in As bioaccessibility, with the greatest decrease in the nZVI2-treated soil (TERs of 0.59 and 0.64 in the gastric and intestinal phases, respectively). Arsenic X-ray absorption near-edge spectroscopy analysis indicated that most of the As in the untreated soil was present as As(V) associated with Fe mineral phases, whereas in the treated soil, the proportion of arsenosiderite increased. Arsenite was present only as a minor species (3-5%) in the treated soils, with the exception of an nZVI treatment [~14% of As(III)], suggesting a partial reduction of As(V) to As(III) caused by nZVI oxidation.
