EXAFS Studies about the Sorption of Cadmium Ions on Montmorillonite

Abstract

We investigated local structures around cadmium (Cd) sorbed on montmorillonite at various pH values by EXAFS (extend X-ray absorption fine structure). We found two sorption types, an outer-sphere complex and a surface precipitate. The outer-sphere complex was mainly formed at a lower pH which had an identical structure to aqueous Cd 2þ , while the surface precipitate was found at high pH which had a one-layerCadmium (Cd) is a toxic heavy metal that results in significant pollution of many soil and groundwater systems. Furtherworse, Cd is known to bioaccumulate. Since the mobility and bioavailability of Cd in polluted systems are strongly related to its sorption structures, it is important to understand the sorption structures on clay minerals which are main soil components. Montmorillonite is a dioctahedral smectite mineral that is prevalent in soil environments and has a high surface area and cationexchange capacity (CEC). Over past several decades, macroscopic investigation on the sorption mechanisms of metals on montmorillonite presented many thermodynamic models such as ''surface complexation models'' to describe metal sorption reactions at the solid-water interface.1 The models explained the pH-dependent sorption properties on the montmorillonite. 2-4Outer-sphere complex is mainly formed on permanent charge sites on the interlayer of montmorillonite at low pH values while inner-sphere complex formation occurs at high pH values, where the metals are fixed to deprotonated surface hydroxides on the edge of montmorillonite. However, we have to determine the molecular structures of the sorbed species directly in order to reveal the real sorption mechanism. Recent studies using EXAFS (extend X-ray absorption fine structure) have shown that heavy metals are sorbed on clay and oxide surfaces accompanied by the formation of multinuclear or polynuclear surface complexes (i.e., surface precipitate) much more frequently than previously expected based on the thermodynamical models. 5-9 These microscopic studies have promoted our understanding of the real sorption mechanism. EXAFS analyses of Cd species on the montmorillonite, however, have never been reported in spite of its importance for soil pollution mainly because the K-edge EXAFS requires X-rays with higher energy than 23 keV. We carried out K-edge Cd EXAFS measurements for Cd sorbed on montmorillonite in a vertical Wiggler beam line in SPring-8 and Photon Factory beam line, which provide a high energy X-ray. EXAFS provides only averaged local structure information for all coexisting metal species having the same absorption edge. We varied the pH values in order to change the sorption species continuously and applied a regression analysis to quantitatively evaluate the composition of each sorption structure.Cd was sorbed on Na-montmorillonite (Kunipia-F, Kunimine Industries Co.) using a batch technique at several different pH values. Cd(NO 3 ) 2 (0.002 mol L À1 ) and NaNO 3 (0.004 mol L À1 ) solutions were applied and the reaction time was 24 h (295 AE 1 K). The p...

Cd(II) Sorption on Montmorillonite-Humic acid-Bacteria Composites

“…Table 2 shows that the entropy of Cd adsorption on Mont is negative (−14.7 J/mol/K), indicative of outer-sphere adsorption. Similar outer-sphere Cd-complex on pure montmorillonite is also confirmed by EXAFS data at pH ≈ 5 13 . However, the entropy for Cd adsorption on Mont-HA is slightly positive (9.8 J/mol/K), indicative of inner-sphere adsorption.…”

“…3a . The first and second oscillations of the three spectra show no much difference, which is due to the same first coordination shell atoms, i.e., O atoms, seen eleswhere 13 32 33 . A few differences are pronounced at k -values of 6–10 Å −1 .…”

“…Clay minerals can sorb notable quantities of a variety of heavy metals in geologic systems due to their large specific surface areas, reactive surface properties and high cationic exchange capacities 11 12 . Previous work shows that Cd ions mainly form outer-sphere complexes on the permanently charged sites of montmorillonite 13 , and inner-sphere surface complexes on the variable charged edge sites (≡SOH) at pH > 7 14 . Bacterial cells provide a significant reactive surface for heavy metal adsorption due to the abundance of anionic functional groups of carboxyl, amine, hydroxyl, phosphoryl, and sulfhydryl 15 16 17 .…”

Interaction of Nitrate, Barium, Strontium and Cadmium Ions with Fused Quartz/Water Interfaces Studied by Second Harmonic Generation

“…45 Extended X-ray absorption fine structure (EXAFS) studies also provide valuable information regarding the local structure of an interfacial species. EXAFS studies have been carried out on cadmium sorbed to montmorillonite, 47 cryptomelane (KMn 8 O 16 ), 48 goethite, 49 and hydrous ferric oxide. 50 These studies indicate Cd sorption can occur through outer-sphere complexation, inner-sphere complexation, or surface precipitation, and the exact mechanism of binding was reported to depend on the chemical identity of the substrate, the bulk solution pH, and the electrolyte and metal ion concentration.…”

Mechanisms of heavy metal sorption on alkaline clays from Tundulu in Malawi as determined by EXAFS

“…An example on the use of this technique is provided by the interaction of cadmium(II) solution with thiol-functionalised 2:1 clay minerals where cadmium was found to give monomer complexes or CdO molecules on the surfaces or interlayers of the untreated clay minerals with Cd-O distance of 2.24Á while Cd-S clusters were formed on the thiol-functionalised clay minerals [3]. However earlier EXAFS studies about cadmium sorption on montmorillonite revealed outer sphere complex formation at lower pH and surface precipitate at higher pH as Cd 5 (OH) 18 (NO 3 )(H 2 O)-like structures [12]. In the study of mercury(II) interaction with goethite surface [13] the distance between the adsorbed mercury and the two closest oxygen atoms in the mineral was found to be 2.04Á and the distance between Hg and Fe was 3.28Á giving ∠ Fe-O-Hg = 108 • .…”