Geochemistry of Bioaccessible Arsenic Bill Cutler, ERM, Honolulu, HI; Nguyen Hue, University of Hawaii, Honolulu, HI The Problem Arsenical pesticides were used in the former Olaa Sugar Plantation from about 1915 to 1950: Soils across a broad area show As at concentrations well above background levels Although bioaccessible As is a small fraction of total As, some locations are above acceptable risk-based standards. Geology and soil development controlled the location of sugar cane cultivation Application of arsenical herbicides led to widespread soil As contamination Geology and Soil Development Controlled Cane Cultivation Geology of the Island of Hawaii consists of a sequence of lava flows and intervening ash deposits Soil development occurs rapidly in the wet climate in eastern Island of Hawaii The former Olaa Sugar Company Plantation was laid out in rich soil on lava flows at least 5000 years old. Younger flows flanking the plantation show poor soil development. Soil Arsenic Distribution Arsenic in surface soils of the former Olaa Plantation have been mapped extensively by the authors. Background soil arsenic is typically less than 10 mg/kg Total arsenic levels in soil exceed 50 mg/kg across approximately 40 square kilometers of former cane plantation land Soil Mineralogy and Arsenic Speciation The volcanic-derived soils are cobbly hydrous loams of soil order Andisol. They are rich in iron oxyhdroxides (ferrihydrite) and alumino-silicates (imogolite, halloysite). Arsenic is present in Olaa Plantation soils in the oxidized (As+5) state Soil samples were collected under nitrogen atmosphere for As speciation Hydride generation/ICPMS and X-ray Adsorption Near Edge Structure (XANES) analysis confirmed inorganic arsenic in As+5 state XANES Spectra for Arsenic Olaa Surface Soils 11850 11860 11870 11880 11890 11900 Energy (eV) Normalized Absorbance w/ offsets As(V) Standard Olaa Soil As(III) Standard Hawaii Andisols strongly sorb arsenic. Desorption is very limited, resulting in low bioaccessible As as measured by PBET Arsenic Sorption and Bioaccessibility Olaa andisols exhibit extremely high oxyanion (i.e. phosphate, arsenate) sorption capacity Soils can sorb more than 1 weight percent arsenic (10,000 mg/kg) Sorption is very rapid and irreversible under field geochemical conditions (pH 6, oxidizing) This strong As sorption propensity resulted in permanent retention of As applied as herbicide Bioaccessible As was measured using a physiologically-based extraction test (PBET), a batch extraction test simulating stomach digestion Test used pH 1.5 HCl @37oC for 60 minutes with 1:100 soil:solution ratio PBET results correlate well with recent monkey bioavailability results for the same Olaa soils Bioaccessible As ranges from 2-25% of total As Arsenate Sorption Isotherm Olaa Ap soil, <2mm, 200ppm As(V) spike, 25 o C y = 437.10x - 6.17 R 2 = 1.00 500 1000 1500 2000 2500 4 6 8 10 ln t (minutes) As Sorbed (ppm) Iron Amendments Reduce Bioaccessible Arsenic Iron compounds are commonly used in water treatment for arsenic removal. This study is evaluating their use as a soil amendment to reduce the bioaccessible arsenic fraction. Olaa soils were amended with a range of dosing using two soluble arsenic salts: Ferrous sulfate Ferric chloride After iron addition, soils were amended with a slurry of 1 weight % slaked lime Ca(OH)2 Batch tests were put through successive “wet-dry” cycles prior to PBET bioaccessible As analysis Arsenic (As) concentrations of 200-800 mg/kg are present in former sugar cane plantation soils on the “Big Island” of Hawaii, due to application of arsenical herbicides from about 1915 to 1950. Volcanic-derived soils are of the Andisol soil order, with abundant iron oxide and alumino-silicate secondary minerals. As strongly binds to these authigenic mineral phases, particularly the iron oxyhydroxides. Based on hydride generation methods and synchrotron X-ray adsorption near-edge structure analysis (XANES), As exists in an inorganic oxidized (As+5) state. Bioaccessible As, as measured by a physiologically-based extraction test (PBET) constitutes only 2 to 20 percent of total As; but can be as high as 100 mg/kg in soils with high total As levels. The bioaccessible As fraction can be significantly reduced with iron amendments. Field Trials Field trials of iron amendments are underway at a school garden in Keaau, Hawaii. Total arsenic in the garden area reaches nearly 1000 mg/kg Bioaccessible arsenic averages 80 mg/kg Work is being performed in conjunction with University of Hawaii and Hawaii Dept. of Health Bioaccessible arsenic is a small fraction of total arsenic in Hawaiian Andisols, but can reach concentrations requiring remedial action. Iron amendments show great promise as a remediation tool to reduce bioaccessible As in surface soils Courtesy of Dr. John Drexler UC Boulder Total Soil Arsenic School Garden Area