Colloids and Surfaces A: Physicochem. Eng. Aspects 347 (2009) 8–17 Contents lists available at ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa Sorption and speciation of arsenic by zero-valent iron K. Sasaki a,∗ , H. Nakano a , W. Wilopo a , Y. Miura b , T. Hirajima a a Department of Earth Resources Engineering, Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan b Center of Advanced Instrumental Analysis, Kyushu University, Kasugakouen, Fukuoka 816-8580, Japan article info Article history: Received 19 August 2008 Received in revised form 22 October 2008 Accepted 22 October 2008 Available online 31 October 2008 Keywords: Zero-valent iron Arsenic Groundwaters Permeable reactive barriers Speciation abstract Sorption and speciation of arsenic in groundwater remediation using zero-valent iron (ZVI) was inves- tigated by batch and column experiments. In batch tests, arsenite was oxidized to arsenate which was favorably immobilized by co-precipitation with Fe(III)-bearing compounds as corrosion products, leading to the dominant surface species of As(V). In column materials after introducing influent with arsenite for 22 pore volumes(pvs), As(III) was predominant on the surface of ZVI in all segments of column, while intense oxidation of arsenite to arsenate was observed especially near the inlet. It is considered that the surface of ZVI granules was already uniformed and corrosion products are the most accumulated on ZVI granules near the inlet because ZVI has been contacted with high concentrations of arsenite for the longest time in the most bottom parts. This was also supported by the experimental results of XPS that relative intensity of O 1s peak assigned to oxide was the largest in the most bottom segment. Also XPS results revealed that arsenic was immobilized on ZVI granules mainly as arsenite and to a lesser extent arsenate. These observations suggest that the retention mechanism is primarily sorption rather than reduction. The results of geochemical calculations along with the SEM-EDX suggest that arsenite and arsenate were co-precipitated with mainly Fe(III)-bearing minerals and carbonates. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The speciation of arsenic in natural waters is controlled by reduction, oxidation, and methylation reactions that affect its solubility, transport, bioavailability and toxicity [1]. Inorganic spe- ciation is important since the varying protonation and charge of the arsenic species present at different oxidation states has a strong effect on their behavior in natural and anthropological processes. Sorption and co-precipitation are important processes to reduce the concentrations of inorganic contaminants in groundwaters. Per- meable reactive barriers (PRBs) have been developed as passive and in situ groundwater remediation technique for more than 15 years and constructed as modern practice for reduction of a variety of inorganic contaminants, as well as organics [2,3]. In groundwaters the ratio of arsenite and arsenate can vary greatly depending upon the abundance of redox-active solids (especially organic carbon), the activity of microorganisms, and the input of oxygen from the atmosphere. Arsenic(III) typically dom- inates in strongly reducing aquifers in which Fe(III) and sulfate reduction are taking place. For example, the ratio of As(III)/As Total in reducing groundwaters from Inner Mongolia is typically 0.6–0.9 [4]. ∗ Corresponding author. E-mail address: keikos@mine.kyushu-u.ac.jp (K. Sasaki). There have been numerous investigations on the removal of arsenic from groundwater using zero-valent iron (ZVI), typically in the form of iron fillings [5-7]. Zero-valent iron is a powerful reductant with a reduction potential of -0.44 V for the reaction of Fe = Fe 2+ + 2H + , and is reactive with many environmental con- taminants [8]. Since the net cell potential is E ◦ =0.499 V for the reaction of HAsO 4 2- + 7H + + 5e - = As(0) + 4H 2 O, reduction of arsen- ate to arsenic(0) by zero-valent iron is thermodynamically possible. Iron media may also remove arsenate species by adsorption onto iron corrosion products. In some cases, adsorption of arsenate to Fe(II, III) compounds, which are usually observed on the original surface of ZVI granules, may be irreversible due to formation of stable solid phases. The purpose of the present work is to understand the immobi- lization mechanism of arsenite by ZVI and to determine the capacity of the material to retain arsenic in a PRB. 2. Experimental 2.1. Reactive materials Zero-valent iron in the form of iron fillings was provided by Connelly Ltd., Chicago, IL, USA. The elemental composition was as follows: Fe 89.82%, C 2.85%, Mn 0.6%, S 0.107%, P 0.132%, Si 1.85%, Ni 0.05–0.21%, Cr 0.03–0.17%, Mo 0.15%, Ti 0.004%, Cu 0.15–0.20%. The 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.10.033