Journal of Colloid and Interface Science 267 (2003) 302–313 www.elsevier.com/locate/jcis Fluoride sorption and associated aluminum release in variable charge soils Laura F. Harrington, Ellen M. Cooper,and Dharni Vasudevan ∗ Nicholas School of the Environment and Earth Sciences, Duke University, Box 90328, Durham, NC 27708-0328, USA Received 29 January 2003; accepted 29 May 2003 Abstract Fluoride sorption and related aluminum (Al) release are evaluated in two iron-oxide-rich soils as a function of soil depth, composition, and physical–chemical properties and potential mechanisms of fluoride–surface interaction are suggested. Measured Al concentrations at equilibrium fluoride sorption, reflective of the net balance between Al dissolution and sequestration of the released Al by the solid phase, suggest net fluoride-assisted dissolution of Al-bearing amorphous and crystalline soil minerals. Strikingly, soils of similar depth and hori- zonation from the same soil order but of distinct soil series exhibited markedly different susceptibility to Al loss in the presence of fluoride, possibly a combined result of differences in the mechanism of fluoride sorption, soil mineralogy, reactivity of the surficial Al and Fe, and soil solution chemistry. Fluoride sorption is strongly correlated with soil Al and Fe present as high-surface-area amorphous and crystalline oxide phases. Fluoride complexation to surficial Al and Fe ions via ligand exchange with surficial OH groups and water molecules appears to be the dominant sorption mechanism. At high dissolved fluoride concentrations (>7 mM), other mechanisms of fluoride retention including adsorption of AlF solution complexes, entrapment in the interparticle pore fluid, and precipitation into solution and/or onto the soil surface are also likely.  2003 Elsevier Inc. All rights reserved. Keywords: Iron oxide; Metal oxide; Adsorption; Dissolution; Precipitation; Complexation; Ultisols 1. Introduction Anion–soil interactions play an important role in deter- mining soil weathering, nutrient availability, and contami- nant fate and transport. To this end, the retention of inorganic anions, including arsenate, chloride, chromate, fluoride, ni- trate, phosphate, selenate, and sulfate, on soils has been extensively studied [1,2]. The potential for soil contamina- tion by fluoride originating from aluminum (Al) smelting operations has provided the impetus for several studies of fluoride sorption to soils [3–8]. Typically, fluoride sorption is well correlated with soil amorphous Al and iron (Fe) oxide content and crystalline Al oxide content [6,9], with poorly ordered amorphous Al oxides serving as the most impor- tant sink for fluoride [10]. Furthermore, fluoride retention by exchangeable Al [11] and lack of sorption by quartz sur- faces [4] has reinforced the concept of preferential interac- tion with surficial Al and Fe. Studies of fluoride sorption onto pure phase minerals (Fe and Al oxides and aluminosilicates) have evaluated * Corresponding author. E-mail address: dharni@duke.edu (D. Vasudevan). (i) mechanistic aspects of the nature and extent of fluo- ride sorption [12–18], (ii) the use of fluoride as a probe for quantifying the number of reactive sites on a mineral surfaces [19–23], and (iii) fluoride-assisted surface disso- lution [24–27]. These studies have emphasized that sorp- tion occurs via multiple mechanisms including electrosta- tic attraction, anion exchange, ligand exchange with surface hydroxyl groups and water molecules, and precipitation. Additionally, fluoride sorption is observed to be strongly pH-dependent: sorption onto soils and pure phase minerals increases with decrease in pH, with maximum sorption at pH values close to the pK a of HF (3.2) [1,2,7,10,28]. Al loss from soils (Al release into soil solution), a phe- nomenon particularly significant in acidic soils, is often closely associated with fluoride sorption to soils [5,8,29]. Fluoride sorption is a precursor to Al dissolution: the elec- tronegative fluoride ion replaces –OH/H 2 O groups bound to surficial Al atoms (ligand exchange), loosens other Al–OH bonds, and facilitates the dissolution of Al from the sur- face [27,29]. Fluoride-facilitated Al dissolution from amor- phous phases is typically more prominent than dissolution from crystalline phases [14]. A noteworthy study by Nordin et al. [27] identified several sets of reactions involved in the 0021-9797/$ – see front matter  2003 Elsevier Inc. All rights reserved. doi:10.1016/S0021-9797(03)00609-X