Towards a consistent geochemical model for prediction of uranium(VI) removal from groundwater by ferrihydrite

Abstract

2 Uranium(VI), which is often elevated in granitoidic groundwaters, is known to adsorb 3 strongly to iron (hydr)oxides under certain conditions. This process can be used in 4 water treatment to remove uranium(VI). To develop a consistent geochemical model 5 for uranium(VI) adsorption to ferrihydrite, batch experiments were performed and 6 previous data sets reviewed to optimize a set of surface complexation constants using 7 the three-plane CD-MUSIC model. To consider the effect of dissolved organic matter 8 (DOM) on uranium(VI) speciation, new parameters for the Stockholm Humic Model 9 (SHM) were optimized using previously published data. 10The model, which was constrained from available X-ray absorption fine structure 11 (EXAFS) spectroscopy evidence, fitted the data well when the surface sites were 12 divided into low-and high-affinity binding sites. Application of the model concept to 13 other published data sets revealed differences in the reactivity of different 14 ferrihydrites towards uranium(VI). Use of the optimized SHM parameters for 15 uranium(VI)-DOM complexation showed that this process is important for 16 uranium(VI) speciation at low pH. However in neutral to alkaline waters with 17 substantial carbonate present, Ca-U-CO 3 complexes predominate. 18The calibrated geochemical model was used to simulate uranium(VI) adsorption to 19 ferrihydrite for a hypothetical groundwater in the presence of several competitive 20 ions. The results showed that uranium(VI) adsorption was strong between pH 5 and 8. 21Also near the calcite saturation limit, where uranium(VI) adsorption was weakest 22 according to the model, the adsorption percentage was predicted to be > 80 %. Hence 23 uranium(VI) adsorption to ferrihydrite-containing sorbents may be used as a method 24 to bring down uranium(VI) concentrations to acceptable levels in groundwater. 25 3 1 Keywords: uranium, surface complexation, speciation, dissolved organic matter, 2 ferrihydrite, groundwater 3 4 Introduction 5In recent years it has been revealed that many groundwaters in granitoidic 6 environments contain excessive amounts of dissolved uranium. The recommended 7 guideline value for uranium in groundwater is 15 µg/L (WHO, 2004). In Sweden it 8 has been estimated that about 17 % of all private bedrock wells contain higher 9 concentrations of dissolved uranium (Ek et al., 2007). Also many larger waterworks 10 contain excessive uranium concentrations (Falk et al., 2004) and research into 11 technologies for uranium removal is ongoing. One possible technique could be 12 uranium removal by iron-oxide-coated sand, for which the chemical mechanism is 13 surface complexation of uranium(VI) onto the Fe (hydr)oxides ferrihydrite and 14 goethite (Logue et al., 2004). Of these, ferrihydrite may often be more important 15 because of its much larger surface area and content of reactive surface groups. 16 17To evaluate the possible use of iron-oxide-based sorption media for uranium 18 removal, we need to understand the various mechanisms that affect it. Uranium occ...

Uranium theoretical speciation for drinking water from private drilled wells in Sweden – Implications for choice of removal method

“…Uranium chemistry is complicated, Langmuir (1978) identified 42 different aqueous species in thermodynamic calculations. Complexes with dissolved organic matter (DOM) and Ca-uranyl carbonate complexes were not considered in the calculations, but have recently been shown to be crucial for U speciation in water (Bernhard et al, 1996(Bernhard et al, , 2001Fox et al, 2006;Gustafsson et al, 2009;Prat et al, 2009). DOM-UO 2 complexes have been considered in some previous studies, but mainly in connection with U transport in the subsurface environment (Artinger et al, 2002;Ranville et al, 2007;Yang et al, 2012).…”

Modelling lead(II) sorption to ferrihydrite and soil organic matter

“…showed that the contribution of inorganic phases in this mor layer is likely to be small 21 In this work we used the 3-plane CD-MUSIC model [13] using the surface charging 10 parameters of Gustafsson et al [33] In the model approach, singly coordinated ≡FeOH 11 groups are assumed to determine proton charging. The site density is set to 6.3 sites nm -2 12 using a molecular weight of ferrihydrite of 89 g/mol and a specific surface area of 750 13 m 2 /g.…”

“…The implementation of the CD-MUSIC model for uranium(VI) [33] considered the 22 possible existence of surface site heterogeneity by dividing the sites into high-and low-23 affinity sites with 99 % and 1 % of the number of sites, respectively. This approach is 1 similar to the one Dzombak and Morel [14] used for the Diffuse Layer Model.…”

Speciation and hydrological transport of metals in non-acidic river systems of the Lake Baikal basin: Field data and model predictions

“…Comparisons between speciation predictions and observations have been made for different metals in various types of natural systems (Butler et al 2009;Gustafsson et al 2009;Wällstedt et al 2010;Nystrand et al 2012). However, most of these studies have focused on the behaviour of limited number of metals, in acidic waters.…”

“…Frequently used speciation models include WHAM (Tipping 1994), PHREEQC (Parkhurst and Appelo 1999) and Visual MINTEQ (Gustafsson et al 2009). Although these models have been shown to provide excellent representation of metal speciation in laboratory environments (e.g.…”