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

Abstract

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.

Keywords: EXAFS spectroscopy; X-ray diffraction; bismuth(III); dimeric bismuth(III) complex; electron microscopy; fulvic acid; metallic bismuth; mor layer.

Figure 1

Dissolved Bi(III) and DOC in soil suspensions as a function of pH at different equilibrium times after initial additions of 0.1 and 1.0 mmol bismuth(III) L^–1 or a combination of 0.1 mmol bismuth(III) L^–1 and 1.5 mmol iron(III) L^–1 (a–d). The experiment with and without the addition of Fe(III), as shown in (e,f), was carried out using an equilibration time of 30 d.

Figure 2

Dissolved bismuth(III) in soil suspensions as a function of DOC at different equilibrium times after initial additions of 100 or 1000 μmol bismuth(III) L^–1.

Figure 3

Binding of bismuth(III), iron(III), chromium(III), and aluminum(III) by the Risbergshöjden Oe soil in dilute NaClO₄ (Bi) or NaNO₃ solutions. The suspension density was ∼10 g L^–1 on a dry weight basis. Bismuth(III) data are from the present study, iron(III) data from Gustafsson et al., chromium(III) data from Gustafsson et al., and aluminum(III) data from Gustafsson and van Schaik. Bismuth and chromium(III) data were obtained using 30 d equilibration times, whereas an equilibration time of 5 d was used in iron(III) and aluminum(III) experiments.

Figure 4

Left: stacked k³-weighted K-edge EXAFS spectra for bismuth for the organic soil obtained for different pH values. Right: Fourier transforms (FT magnitudes) of the k³-weighted EXAFS spectra. Lines are raw data, and dashed red lines are model fits.

Figure 5

Proposed structure of the dimeric bismuth(III) carboxylate complex. “R” denotes continuation of the carbon chain.

Figure 6

SEM pictures of bismuth beads exposed to (a) deionized water or (b) 100 mg L^–1 FA solution adjusted to pH 5.6 during 24 months.