Bismuth(III) Forms Exceptionally Strong Complexes with Natural Organic Matter

The use of bismuth in the society has steadily increased during the last decades, both as a substitute for lead in hunting ammunition and various metallurgical applications, as well as in a range of consumer products. At the same time, the environmental behavior of bismuth is largely unknown. Here, the binding of bismuth(III) to organic soil material was investigated using extended X-ray absorption spectroscopy (EXAFS) and batch experiments. Moreover, the capacity of suwannee river fulvic acid (SRFA) to enhance the solubility of metallic bismuth was studied in a long-term (2 years) equilibration experiment. Bismuth(III) formed exceptionally strong complexes with the organic soil material, where >99% of the added bismuth(III) was bound by the solid phase, even at pH 1.2. EXAFS data suggest that bismuth(III) was bound to soil organic matter as a dimeric Bi3+ complex where one carboxylate bridges two Bi3+ ions, resulting in a unique structural stability. The strong binding to natural organic matter was verified for SRFA, dissolving 16.5 mmol Bi per gram carbon, which largely exceeds the carboxylic acid group density of this compound. Our study shows that bismuth(III) will most likely be associated with natural organic matter in soils, sediments, and waters.


Supporting information
Bismuth(III) forms exceptionally strong complexes with natural organic matter Dan B. Kleja, Jon Petter Gustafsson, Vadim Kessler, Ingmar Persson Contents Coordination chemistry of bismuth(III) (text).
Redox chemistry of bismuth (text) Table S1. Hydrolysis constants of Bi 3+ in aqueous solution. Table S2. Selected stoichiometric stability constants of bismuth(III) complexes with organic ligands in aqueous solution. Table S3. Physicochemical characteristics of fulvic acid and soil sample. Table S4. Bi-O distances in different coordination numbers. Figure S1. Calculated conditional stoichiometric constants for the Bi 3+ -oxalate and Bi 3+glycine systems as a function of pH.

Coordination chemistry of bismuth(III)
Bismuth(III) exhibits a broad range of coordination numbers from 3 to 10 in complexes and coordination compounds (Table S4). 1 The coordination chemistry of bismuth(III) is strongly influenced by its electron configuration, 5d 10 6s 2 , and show an unusual diversity. The tendency of the heavier main group elements to adopt an oxidation state two steps below being fully oxidized was originally attributed to the effect of the so-called "inert electron-pair". 2 This property was explained by the relativistic stabilization of the 6s orbital, caused by the direct relativistic effect and the presence of the filled 4f subshell. According to the valence bond theory, the inert electron-pair can either occupy a hybrid orbital formed by mixing the 6s and 6p orbitals on the metal ion and as such becoming stereochemically active, or be a pure s 2 electron-pair and thereby stereochemically inactive. The hybrid orbital with a lone electronpair can in terms of coordination number be considered as at least an additional ligand in the coordination sphere normally taking up more space than that of an ordinary ligand. 3,4 However, according to molecular orbital theory the classical concept of 6s/6p orbital hybridization on the isoelectronic lead(II) ion is regarded as incorrect as the energy level of these orbitals are too different and the very different spatial distribution of their wavefunctions. [5][6][7][8] This should certainly also apply for the isoelectronic bismuth(III) ion as it displays a similar kind of coordination chemistry as lead(II) and thallium(I). The coordination chemistry of lead(II) and bismuth(III) is not expected to be identical as the higher charge of latter will favor higher coordination numbers due stronger electrostatic bonds. The strong stereo-chemical activity observed in a large number of bismuth(III) complexes must instead be a result of an anti-bonding bismuth 6s-ligand np (6s/np) interaction which cause structural distortions in order to energetically minimize these unfavorable covalent interactions. [6][7][8][9] Two general structural types of bismuth(III) complexes can be identified, i/ complexes with high symmetry and high coordination numbers, 8-10, in basical square antiprismatic, tricapped trigonal prismatic and bicapped antiprismatic fashion, respectively, ii/ complexes with a severely distorted coordination sphere with large difference between the shortest and longest Bi-O bond distance and generally with a clearly visible gap in the coordination sphere. The six-coordinated complexes can be regarded as distorted eight-coordinated ones with a gap taking up the same space as two ligands where the strongly bound ligands form a 3-legged stool. Between these and the gap are three more ligands much more weakly bound, or a distorted pentagonal pyramidal configuration with a much shorter Bi-O bond distance to the ligand in the apex than to the remaining three ones. Seven-coordinated complexes have either

Redox chemistry of bismuth
Bismuth has four oxidation states with known chemistry, metallic bismuth, bismuth(I) (d 10 s 2 p 2 electron configuration), bismuth(III) (d 10 s 2 ) and bismuth(V) (d 10 ). The most stable oxidized form of bismuth is the oxidation state +III. Metallic bismuth versus bismuth(III) has a positive standard electrode potential, +0.308 V, see Table. Therefore, it does not react with non-oxidizing acids such as hydrochloric acid, but oxygen in air will until a protective layer of Bi 2 O 3 is formed. Bismuth(I) is uncommon oxidation state, but it is stabilized is solvents binding through covalent interactions forming e.g. an unusual dimeric solvate complex in the solvent N,N-dimethylthioformamide (dmtf), [Bi 2 (dmtf)] 2+ . 2 Bismuth(V) is a very strong oxidizing agent and is easily reduced to bismuth(III). Bismuth(V) has no known aqueous chemistry.  Table S2. Selected stoichiometric stepwise stability constants, K n , of bismuth(III) complexes with organic ligands in aqueous solution. K 1 =[BiL (3-
Oxalic acid 2O (Table S2).  Figure S2. Specific UV absorbance of DOM as a function of pH in the batch experiments with the organic soil sample. Specific UV absorbance is equal to the UV absorbance measured at λ=254 nm normalized to mg carbon.   Figure S4. Wavelet transform (WT) results for EXAFS data (left column) and model output (right column) using structural parameters in Table 1 (κ = 12, σ = 2, k range: 2.8-8 Å -1 for pH 1.2, 2.8-10 Å -1 for all others). High-intensity areas at R + ΔR ≈ 3.5 Å -1 are consistent with a Bi … Bi interaction at 4.0 Å -1 . The WT:s were made using the Igor Pro procedure of M. Chukalina (Wavelet2. ipf, a procedure for calculating the Wavelet transform in IGOR Pro, Grenoble, France, 2010).