Rare earth elements sorption to iron oxyhydroxide: Model development and application to groundwater

Abstract

Iron oxyhydroxides are among the most important colloids that control rare earth elements (REE) concentrations and transport in natural hydrosystems. In this study, REE surface complexation to iron oxyhydroxides (Fe(OH) 3 (a)) was described by using the Donnan diffuse layer model and a two-site (i.e. ≡Fe s OH and ≡Fe w OH) model. The specific surface area and pH of zero charge were fixed as 100000 m 2 /mol and 8.0, respectively. The surface site density for weak and strong binding sites were fixed at 0.1 mol/mol Fe (≡Fe w OH) and 0.001 mol/mol Fe (≡Fe s OH) respectively. The two site types were used with pK a1 int = 7.29 and pK a2 int = 8.93. Using linear free energy relationship, the estimated equilibrium surface complexation constants (log K) increased from light REE (LREE) to heavy REE (HREE). Results of REE modeling calculation using the determined log K revealed a good fit of experimental data, showing an order of sorption on iron oxyhydroxides: HREE > MREE > LREE and preferential sorption of HREE at a lower pH. However, sorption edges only showed a slight change with ionic strength (0.1 to 0.7 mol/L) for the whole REE series. The generalized model was subsequently used to evaluate the impact of iron oxyhydroxides on REE speciation in groundwater. Application of the model to "model groundwater" showed that iron oxyhydroxide complexes of REE were significant in near neutral and weakly alkaline pH. This study contributes to putting forward a comprehensive database which would be useful for the application of surface complexation model to describe REE sorption by amorphous ferric hydroxides in nature.

Effect of rare earth elements on rice plant growth

“…In this study, the formation of iron (III) minerals on the rice root was observed both in the presence of Fe(II) chloride or Fe(II) sulfate. (Liu et al, 2017). This suggests that as the radii of the REE decreases the stronger the binding to HFO as shown in Fig.…”

“…5c and 5d for the quantification of REE in concentrations in roots, show a decrease in MREE and HREE concentrations, both in the presence of Fe(II) sulfate and Fe(II) chloride and at 0.5 mg/L and 1 mg/L REE. This result suggests the presence of Fe (III) plaques on the rice root, as log 10 K values propose a stronger binding of the MREE and HREE to hydrous ferric oxide strong sites (Liu et al 2017). In addition, the "flat" patterns observed for the REE series in Fig.…”

“…Ln 3+ ). The Ln 3+ species are prompt to complexation by ligands in solution, such as carbonate, sulphate, soluble organic matter, colloids and solid surfaces such as Fe or Mn oxides (Guthrie et al, 2003;Tang and Johannesson, 2003;Wang et al, 2005;Liu et al, 2017). The partitioning of trace elements, including the REE, among available ligands is controlled by complexation stability constants, reaction kinetics, element concentrations and physicochemical parameters (e.g.…”

“…The speciation calculations were performed using the computer program PHREEQC version 3.1.5 (Parkhurst and Appelo, 2013) and the NAGRA/PSI data base (Hummel et al, 2002) modified to include well-accepted infinite-dilution (25 °C) stability constants for REE inorganic complexes (i.e., in our study, chlorides (Luo and Byrne, 2001) and sulfates (Schijf and Byrne, 2004). Surface complexation of the REE with hydrous ferric oxides (HFO) was modeled as described by Liu et al (2017). Fig.…”

“…SO 4 2and Cl -), and iron (III) oxyhydroxide phases associated with the rice root played a key role in controlling the toxic effects of the REE. In the natural environment, however, the toxic effects of these and other trace elements are dependent on their form in the aquatic soil solution, which is composed of a significant number of competing inorganic and organic ligands able to complex the REE (Tang and Johannesson, 2003;Johannesson et al, 2004;Davranche et al, 2015;Liu et al, 2017). The REE have been shown to interact with iron and manganese oxide surfaces, as well as to complex inorganic ligands.…”