Arsenic is a widespread contaminant in soils and groundwater. While various iron-based materials have been studied for immobilizing arsenic in contaminated soils, the feasibility of stabilized iron-based nanoparticles has not been reported. This study investigates the effectiveness of using three types of starch-stabilized iron-based nanoparticles, including zero-valent iron (ZVI), iron sulfide (FeS), and magnetite (Fe 3 O 4 ), for immobilization of arsenic in two representative As-contaminated soils (an orchard soil and a fire range soil). To test the effect of the nanoparticles on the arsenic leachability, As-contaminated soils were amended with the nanoparticles at various Fe/As molar ratios (5 : 1-100 : 1) and contact time (3 and 7 d). After three days' treatments of a field-contaminated sandy soil, the PBET-based bioaccessibility of As decreased from an initial (71.3±3.1)% (mean±SD) to (30.9±3.2)% with ZVI, (37.6±1.2)% with FeS, and (29.8± 3.1)% with Fe 3 O 4 at an Fe/As molar ratio of 100 : 1. The TCLP-based leachability of arsenic in a spiked fire range soil decreased from an initial (0.51±0.11)% to (0.24±0.03)%, (0.27±0.04)% and (0.17±0.04)% by ZVI, FeS, and Fe 3 O 4 nanoparticles, respectively. The Fe 3 O 4 nanoparticles appeared to be more effective (5% or more) than other nanoparticles for immobilizing arsenic. When the two soils were compared, the treatment is more effective on the orchard soil that has a lower iron content and higher initial leachability than on the range soil that already has a high iron content. These results suggest that these innocuous iron-based nanoparticles may serve as effective media for immobilization of As in iron-deficient soils, sediments or solid wastes. particles. Chinese Sci Bull, 2010Bull, , 55: 365−372, doi: 10.1007 Various natural sources (e.g., natural geochemical reactions) and anthropogenic activities (e.g., mining, discharges of industrial wastes, military activities, and application of agricultural pesticides) can lead to arsenic (As) contamination of soils and groundwater. Arsenic is ranked the second most common inorganic pollutant in the U.S. superfund sites [1]. Arsenic-contaminated soils, sediments and waste slurry are major sources of arsenic in food and water. To mitigate the toxic effect on human health, the maximum contaminant level (MCL) for arsenic in the U.S. drinking water was lowered from the previous 50 ppb to 10 ppb, effective in January 2006. Arsenic is a redox active element, with As(V) or As (III) being the two most common stable oxidation states in soils [2]. In general, As(III) is more toxic than As(V), and inorganic arsenic is more toxic than organic arsenic [3], and arsenic in soils is less bioavailable and less bioaccessible than As in water due to soil adsorption effect [4].Arsenate can strongly interact with soils, especially, iron (hydr)oxides. Adsorption of arsenate by iron (hydr)oxides 366 ZHANG MeiYi, et al. Chinese Sci Bull February (2010) Vol.55 No.4-5 have been widely studied [5−13]. These studies have focused on the adsor...