Citation:

Cutler, W., A. El-Kadi, N. Hue, J. Peard, K. Scheckel, AND C. Ray. Iron Amendments to Reduce Bioaccessible Arsenic. Diana Aga, Wonyong Choi, Andrew Daugulis, Gianluca Li Puma, Gerasimos Lyberatos, Joo Hwa Tay (ed.), JOURNAL OF HAZARDOUS MATERIALS. Elsevier Science Ltd, New York, NY, 279:554-561, (2014).

Impact/Purpose:

At many locations within the Hawaiian Islands, arsenic (As) has been identified in soils at concentrations significantly above naturally-occurring background levels. Arsenic soil contamination is predominantly the result of historic spray application of inorganic As herbicides on sugar cane lands from 1913 to about 1950, and release of As at herbicide storage and mixing areas. Extensive areas of former sugar cane cultivation on the Island of Hawaii show soil total As (As_TOT) concentrations ranging from 40 to 900 mg kg^-1. The highest As_TOT concentrations, some greater than 20,000 mg kg^-1, have been found at former pesticide mixing areas that were operated by various sugar plantations (Cutler, 2011). In vitro bioaccessible As (As_IVBA), a surrogate for human oral relative bioavailability, ranges from less than 1% to more than 50% of As_TOT in Hawaiian soils. The Hawaii Department of Health (HDOH) developed guidance for using the As_IVBA content of soil (not As_TOT) in assessing the potential for human health risk and determining the need for remedial action. Soils with As_IVBA concentrations exceeding an action level of 23 mg kg^-1 typically require some form of mitigation (remediation or controls) in an unrestricted land use setting. Despite the strong natural capacity of Hawaiian Andisols to sequester As, some soils with high As_IVBA require remedial action. Traditional remedial technologies for As-contaminated soil consist of removal and landfill disposal, or capping, which can be costly and unsustainable due to limited clean soil sources. In situ treatment technologies to remediate soil by reducing As_IVBA may prove to be technically robust and cost effective at some As-contaminated sites. The current study builds on prior work showing that Fe amendments may provide significant reductions in As_IVBA. There is little information on the long-term viability of Fe amendments to reduce As_IVBA under field conditions. Our primary goal was to determine whether reduction in As_IVBA could be achieved and maintained over a period of several years in a garden plot setting using Fe-amendment products readily available in Hawaii (ferric chloride and ferrous sulfate). In addition, the reversibility of reduced As_IVBA was explored by application of PO₄, which could occur if Fe-amended soils were used for gardening, landscaping or agricultural purposes, and PO₄-based fertilizers were applied.

Description:

Former sugarcane lands on the Island of Hawaii have elevated soil arsenic (As) from historical use of arsenical pesticides. The bioaccessible fraction of total As (As_TOT), a measure of the potential for human As uptake by incidental ingestion of soil, is used in the assessment of human health risk and the determination of the need for remedial action. Ferric chloride plus lime and ferrous sulfate plus lime were applied to As-contaminated soils in a field plot setting to determine the potential for reducing in vitro bioaccessible As (As_IVBA) by increasing As sequestration by the formation of additional iron (Fe) oxyhydroxides. The two Fe sources performed similarly in reducing As_IVBA over a 2-year observation period, with 30-41% reduction in As_IVBA for 0.25 wt% Fe dosing (dry soil basis) and 59-63% reduction for 0.5 wt% Fe dosing. Addition of phosphate (PO₄) to treated and untreated soils caused a significant increase in As_IVBA. Iron-treated and control soils showed more than twice the As_IVBA after the addition of 1500 mg P kg^-1. The cost of in situ treatment of As-contaminated soil with ferrous sulfate plus lime to lower As_IVBA was estimated to be an order of magnitude less than excavation and landfill disposal on the Island of Hawaii, making the technology a viable alternative when remedial action objectives were based on As_IVBA levels.

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