Environ. Sci. Technol. 2009, 43, 7198–7204
Effects of Fulvic and Humic Acids on Arsenate Adsorption to Goethite: Experiments and Modeling LIPING WENG,* WILLEM H. VAN RIEMSDIJK, AND TJISSE HIEMSTRA Department of Soil Quality, Wageningen University, P.O. Box 47, 6700 AA Wageningen, The Netherlands
Received January 05, 2009. Revised manuscript received March 10, 2009. Accepted March 13, 2009.
Data for interactions between arsenate (AsO43-) and fulvic acids (FA) or humic acids (HA) at the surface of goethite are presented (pH 3-7, ionic strength 2 mM and 10 mM). Adsorption of FA and HA leads to desorption of arsenate and a correspondingly strong increase of arsenic concentration in solution. Adsorption of both FA and HA is mutually decreased by the competition with arsenate. The competition between FA and arsenate is much stronger than that between HA and arsenate. Using an advanced model, the LCD model (Ligand and Charge Distribution), arsenate adsorption to goethite in the presence of both adsorbed FA and HA can be predicted reasonably well. The stronger effects of FA on arsenate adsorption are caused, according to the model, by its spatial location which is closer to the oxide surface, and as a consequence, the electrostatic interactions between adsorbed FA particles and arsenate ions are much stronger than those for HA particles. The results show that site and electrostatic competition are the major mechanisms explaining the effects of natural organic matter on the arsenic speciation, whereas other possible mechanisms, such as a chemical reduction of arsenate to arsenite and formation of ternary organic arsenic complexes, are of minor significance.
Introduction Arsenic (As) can directly and indirectly cause health problems (e.g., cancer and skin changes) to human beings when Ascontaminated groundwater is used as drinking water or for irrigation. Areas with high As in groundwater are found in Argentina, Chile, Mexico, China, Hungary, Vietnam, Bangladesh, and India (1). In Bangladesh and India, millions of people suffer from arsenic-related illness. In natural waters, inorganic arsenic is found in two oxidation states, i.e., As (V) for arsenate (AsO43-) and As (III) for arsenite (As(OH)3). Both arsenate and arsenite adsorb to iron (hydr)oxides and the adsorption of arsenic to metal oxides effectively limits the As concentration in groundwater, especially under oxic or slightly reduced conditions (1). Under strongly reducing conditions, the formation of arsenic containing sulfide minerals controls the arsenic concentration (2, 3). In spite of recent intensive research, the processes that mobilize As in the sediments are still unclear (2). Reduction of iron oxides, reduction of arsenate to arsenite, or oxidation of sulfide have been suggested as the underlying causes for As mobilization (1). * Corresponding author phone: 31-317-482332; fax: 31-317-419000; e-mail:
[email protected]. 7198
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An important factor that may influence As concentration in groundwater is the presence of natural organic matter (NOM). NOM is ubiquitous in aquatic and terrestrial systems. NOM particles, such as fulvic acid (FA) and humic acid (HA) particles bind very strongly to oxide minerals. Studies have demonstrated that the presence of FA and HA can decrease As adsorption to a range of minerals and soils (4-8). Data of a typical aquifer in southern Bangladesh suggest that arsenic mobilization is associated with recent inflow of carbon (2). In addition to the competition effects for adsorption, NOM may influence As distribution via some other mechanisms. Degradation and oxidation of NOM may be coupled with reduction of arsenate to arsenite (9), which can lead to a decrease in As adsorption due to a lower adsorption affinity of arsenite compared to arsenate in many situations. In addition, some studies also postulated that As can form organic complexes with NOM possibly via metal ion bridging (9-12). The formation of organic arsenic complexes at the mineral surface or in the solution may decrease or increase As solubility in groundwater (7). Beuer and Blodau (13) have identified the competition between arsenic and organic anions for sorption sites as the primary mechanism for the arsenic release from solid phases, whereas redox reactions were probably of minor importance. The monocomponent adsorption of arsenate and arsenite to various oxide particles has been studied frequently. Series of studies have been conducted on the surface structure of adsorbed As (V) and As (III) using EXAFS (Extended X-ray Absorption Fine Structure). The monocomponent adsorption behavior of arsenate and arsenite has been interpreted using the CD (Charge Distribution) model (14, 15). For As (V), Stachowicz et al. (15) have assessed the reactivity of a bidentate (B) (-(FeO)2AsO2-2), a protonated monodentate (MH) (-FeOAsO2OH-1.5), and a protonated bidentate (BH) (-(FeO)2AsOOH-1) arsenate surface complex. The CD was derived from using MO/DFT (Molecular Orbital/Density Functional Theory) optimized geometries of surface complexes. The adsorption of arsenate to goethite in both the monocomponent system containing AsO43- and multicomponent systems containing AsO43-, phosphate (PO43-), and bicarbonate (CO32-) can be well-described with the CDMUSIC model (15-17). Their calculations show that the nonprotonated bidentate surface complex (B) is dominantly present at all pH values (pH 4-10) and loading conditions (0.5-1.7 µmol/m2). The protonated monodentate (MH) complex is found in the subneutral pH range below pH ∼7. The protonated bidentate complex (BH) is found only at very low pH (pH
