Effects of Calcium on Arsenate Adsorption and Arsenate/Iron Bioreduction of Ferrihydrite in Stimulated Groundwater

Abstract

The reduction and transformation of arsenic-bearing ferrihydrite by arsenate-iron reducing bacteria is one of the main sources of arsenic enrichment in groundwater. During this process the coexistence cations may have a considerable effect. However, the ionic radius of calcium is larger than that of iron and shows a low affinity for ferrihydrite, and the effect of coexisting calcium on the migration and release of arsenic in arsenic-bearing ferrihydrite remains unclear. This study mainly explored the influence of adsorbed Ca²⁺ on strain JH012-1-mediated migration and release of arsenate in a simulated groundwater environment, in which 3 mM ferrihydrite and pH 7.5. Ca²⁺ were pre-absorbed on As(V)-containing ferrihydrite with a As:Fe ratio of 0.2. Solid samples were analyzed by X-ray diffraction (XRD), scanning electron microscopic (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The results show that calcium and arsenate can synergistically adsorb on ferrihydrite due to the electrostatic interactions, and the adsorbed Ca²⁺ mainly exists on the surface through the outer-sphere complex. Adsorbed Ca²⁺ entering the stimulated groundwater was easily disturbed and led to an extra release of 3.5 mg/L arsenic in the early stage. Moreover, adsorbed Ca²⁺ inhibited biogenic ferrous ions from accumulating on ferrihydrite. As a result, only 12.30% Fe(II) existed in the solid phase, whereas 29.35% existed without Ca²⁺ adsorption. Thus, the generation of parasymplesite was inhibited, which is not conducive to the immobilization of arsenic in groundwater.

Keywords: As bioreduction; calcium; electrostatic adsorption; ferrihydrite; groundwater.

Figure 1

Initial XRD patterns (a) and FTIR spectra (b) of Fh, As-Fh, and As-Ca-Fh.

Figure 2

SEM morphology image of As-Fh (a) and As-Ca-Fh (b) before bioreaction. Selected EDS mapping areas (c) and elemental distribution of Fe (d), As (e), and Ca (f) of As-Ca-Fh.

Figure 3

Arsenate adsorption on ferrihydrite as a function of pH. The pentagram and circle shapes indicate arsenate adsorption in the presence (☆ for simulated data, ★ for measured data) and absence (○ for simulated data, ● for measured data) of 5 mM Ca²⁺, respectively (a). The surface speciation distribution of As and Ca on ferrihydrite as a function of pH with (c,e) or without (b,d) the presence of Ca²⁺.

Figure 4

Morphology of strain JH012-1 (a). Iron (b) and arsenate (c) reduction ability of JH012-1; growth of JH012-1 under different concentrations of arsenate (d).

Figure 5

Measured total Fe(II) concentrations (a), solution Fe(II) concentration (b), proportion of Fe(II) in the solid phase (c), and XRD patterns of post-reaction ferrihydrite * stands for parasymplesite (d). Triangles ▲, circles ●, and squares ■ represent Fh, As-Fh, and As-Ca-Fh, respectively, and the corresponding hollow shape represents sterile treatment.

Figure 6

SEM morphology image of As-Fh (a) and As-Ca-Fh (b) after bioreaction; selected EDS mapping areas (c) and elemental distribution of Fe (d), As (e), and Ca (f) of As-Ca-Fh.

Figure 7

Measured total As (a), As(III) (solid line), and As(V) (dotted line) concentrations (b); XPS analyses of As species in the surface region of As-Fh (c) and As-Ca-Fh (d) where the blue and green color lines stands for photoelectron spectral lines of arsenic and the baseline. Squares ■ and circles ● in picture (a) and (b) represent As-Fh and As-Ca-Fh, respectively; the corresponding hollow shape in picture (a) (□,○) represents sterile treatment. The hollow shape in (c,d) means the same thing as the blue color line in it (the photoelectron spectral of arsenic). The difference is that hollow shape circles stand for the data point and the blue color line is the fitted line for hollow shape circles.