Kinetics of competitive adsorption/desorption of arsenate and phosphate at the ferrihydrite–water interface Ghanashyam Neupane a,b, ⁎, Rona J. Donahoe c , Yuji Arai d a University of Idaho-Idaho Falls, 1776 Science Center Drive, Suite 306, Idaho Falls, ID 83402, United States b Center for Advanced Energy Studies, 995 University Boulevard, Idaho Falls, ID 83401, United States c Department of Geological Sciences, University of Alabama, Box 870338, Tuscaloosa, AL 35487, United States d Department of Natural Resources and Environmental Sciences, 1102 South Goodwin Avenue, University of Illinois at Urbana-Champaign, Urbana, IL 61801, United States abstract article info Article history: Received 21 July 2013 Received in revised form 24 December 2013 Accepted 29 December 2013 Available online 9 January 2014 Editor: J. Fein Keywords: Arsenate Phosphate Ferrihydrite Adsorption Desorption As K-edge EXAFS Metal hydroxides (e.g. ferrihydrite) present in geomedia play significant roles in regulating the environmental mobilities of arsenate (As(V)) and inorganic phosphate (P i ) because of their high adsorption affinities for these oxyanions. In this study, results are presented of experiments aimed at determining individual and competitive adsorption/desorption kinetics of As(V) and P i on ferrihydrite at pH 4 and 8. Selected samples were also subject- ed to As K-edge EXAFS study for understanding the changes with time in As(V) complexation on ferrihydrite in the presence/absence of P i . Both oxyanions showed similar behavior in single ion adsorption experiments. How- ever, when both oxyanions were loaded together, more As(V) was adsorbed than P i . Furthermore, more pre- equilibrated P i was desorbed by sequentially added As(V) than vice versa. Interactions of As(V) and P i with fer- rihydrite slowed down after the initial rapid adsorption/desorption. The experimentally determined adsorption/ desorption kinetic data for As(V) and P i showed good compliance with pseudo-second order, Elovich, and power- function equations. Both oxyanions competed for adsorption on ferrihydrite, and each of them showed a limited capacity to desorb the other. EXAFS analysis of selected samples indicated the presence of mononuclear (2E) and binuclear (2C) bidentate As(V) surface complexes. The Fe coordination numbers (CN) increased with increasing time and decreased with addition of P i into the system. A higher proportion of Fe CN associated with 2E As(V) surface complexes decreased after the addition of P i , compared to Fe CN associated with 2C As(V) surface com- plexes. The competitive desorption study indicates that the excessive input of P i due to the overuse of fertilizers could mobilize As(V) from contaminated geomedia. Furthermore, insights into P i -induced desorption of As(V) could also provide an opportunity for developing chemical treatment methods to intercept the mobilized As(V) by co-precipitation in apatite-like phases. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Despite being in the same periodic group, As is a toxic element while P is an essential element to the biosphere. Elevated concentrations of As in the environment can result from natural and anthropogenic activities (Smedley and Kinniburgh, 2002). For example, high levels of As in soil at several industrial sites in the southeastern United States have been traced to the heavy application of As-based herbicides in the past (Yang and Donahoe, 2007). In the environment, inorganic As can exist in several valance states (0, +3, +5), depending on the prevailing pH and redox conditions (Cullen and Reimer, 1989). In natural waters, As mostly occurs as inorganic arsenite (As(III)) and arsenate (As(V)), al- though some organic As compounds are also reported in water severely impacted by industrial pollution (Smedley and Kinniburgh, 2002). Most of the P contamination of surface waters is linked to the input of excess P from agricultural land runoff (Withers et al., 1999). The P contamination of surface waters is reported to cause severe environmental problems such as eutrophication (Correll, 1998). Phosphorous only occurs in pen- tavalent form in the environment, as ortho-phosphates (P i ), pyro- phosphates, longer-chain polyphosphates, and several organic phos- phates (Correll, 1998). The mobility of these oxyanions in the environment, particularly in geomedia, depends on the redox potential and pH conditions, the min- eralogy and organic matter content of the substrate material, and the competing oxyanions present (Smedley and Kinniburgh, 2002). Both As(V) and P i show a high degree of adsorption on Fe-, Mn-, and Al- hydroxides in geomedia (Violante and Pigna, 2002). Adsorption of As(V) and P i on ferric hydroxides involves ligand exchange for OH 2 and OH ‐ in the coordination sphere of surface-exposed structural Fe atoms. Individual ion adsorption experiments showed a rapid initial uptake of both As(V) and P i on ferric hydroxide followed by slow sorption (Fuller et al., 1993; Strauss et al., 1997). During competitive adsorption Chemical Geology 368 (2014) 31–38 ⁎ Corresponding author at: University of Idaho-Idaho Falls, 1776 Science Center Drive, Suite 306, Idaho Falls, ID 83402, United States. Tel.: +1 208 526 7562; fax: +1 208 282 7929. E-mail address: gneupane@uidaho.edu (G. Neupane). 0009-2541/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chemgeo.2013.12.020 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo