Adsorption (As III,V ) and oxidation (As III ) of arsenic by pedogenic Fe–Mn nodules Zheng Chen a,b,c , Kyoung-Woong Kim a, ⁎ , Yong-Guan Zhu b, ⁎ , Ron McLaren d , Fan Liu e , Ji-Zheng He b a Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Korea b Research center for Eco-environmental Sciences, Chinese Academy of Sciences, China c Graduate School of Agriculture, Hokkaido University, Sapporo 060-8589, Japan d Soil and Physical Sciences Group, Agriculture and Life Sciences Division, PO Box 84, Lincoln University, Canterbury, New Zealand e College of Environmental and Resource Sciences, Huazhong Agricultural University, Wuhan, China Received 29 September 2005; received in revised form 17 April 2006; accepted 19 April 2006 Available online 22 May 2006 Abstract Determining the nature of As adsorption and oxidation by naturally occurring iron and manganese oxides in soil is critical for understanding the fate of contaminant As in soils and formulating effective remediation strategies. In batch experiments, oxidation of As(III) and adsorption of As(III) and As(V) by pedogenic Fe–Mn nodule material were examined at various solution pH and As concentrations, and after pre-treatment with humic acid. An experiment using a factorial design showed that most of the variability in As adsorption could be explained by varying solution pH and As concentrations. A semi-mechanistic model was used to fit the adsorption capacity for As(III) and As(V) over a pH range from 6 to 8. Arsenic speciation analysis and reductive dissolution of metal oxides proved that almost all the As(III) added to untreated Fe–Mn nodule material was oxidized by manganese oxides. The adsorption and oxidation of As(III) by Fe–Mn nodule material was greatly reduced by a pre-adsorbed coating of humic acid. These results are useful in predicting the chemical reactions and fate of arsenic in soil. © 2006 Elsevier B.V. All rights reserved. Keywords: Sorption isotherms; Langmuir; Freundlich; Iron oxide; Manganese oxide 1. Introduction Arsenic is a naturally occurring toxic element, and is also introduced into environments through a number of industrial processes and agricultural practices, including mining, poultry farming and the production and use of arsenical pesticides. A decision by the US Environmen- tal Protection Agency in 1999 to reduce the maximum contamination level (MCL) for As in drinking water from 50 μgL - 1 to 10 μgL - 1 reflects a re-evaluation of health risks associated with ingestion of this metalloid. Background soil concentrations of arsenic are typically below 15 mg kg - 1 , but can exceed 2000 mg kg - 1 in some contaminated areas (Smith et al., 1998). Because of this threat, the environmental fate and behavior of As has thus received increasing attention in recent years. Geoderma 136 (2006) 566 – 572 www.elsevier.com/locate/geoderma ⁎ Corresponding authors. Kim is to be contacted at Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Korea. Zhu, Research Center for Eco-environmental Sciences, Chinese Academy of Sciences, China. E-mail addresses: kwkim@gist.ac.kr (K.-W. Kim), ygzhu@mail.rcees.ac.cn (Y.-G. Zhu). 0016-7061/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2006.04.012