Arsenate partitioning from ferrihydrite to hematite: Spectroscopic evidence

Fate of adsorbed arsenate during phase transformation of ferrihydrite in the presence of gypsum and alkaline conditions

“…However, ferrihydrite shows signs of transformation to hematite at slightly higher Fe/As ratios (e.g.,~60). In spite of this transformation, aqueous As concentrations decrease (Das et al, 2014). This observation is supported by other recent studies and attributed to the structural incorporation of As into hematite (Bolanz et al, 2013;Muramatsu et al, 2012).…”

“…Thus, the structure of the ferrihydrite is more easily destabilized after the induction period, and the transformation, once initiated, proceeds even faster than in a gypsum-free system. For example, onlỹ 55% conversion occurs in a gypsum-free system (Das et al, 2014) compared to 100% conversion in the presence of gypsum (current study) after 7 d of aging. The kinetic fit for the % reduction in surface area of ferrihydrite (Fig.…”

“…For example, the aqueous concentration of As measured via ICP-MS at t = 0 was 1.4 mg/L but decreased to 0.5 mg/L at 53 h and then to 0.1 mg/L by the end of the experiment (168 h) (Table 3). Notably, the initial aqueous concentration was slightly lower (~1.4 mg/L) than in the ferrihydrite-arsenate system reported by Das et al (2014) (~1.5 mg/L). This can be explained by the adsorption of Ca 2+ on the phase transforming solids increasing the net positive charge, thus allowing more As (negatively charged ions) to be sequestered (increased/enhanced uptake) at the solid-water interface (Jia and Demopoulos, 2005).…”

“…The rate of ferrihydrite transformation to hematite for the ferrihydritegypsum-arsenate system follows the logistic decay model (Fig. 4a); the ferrihydrite-gypsum (Das et al, 2010) and ferrihydrite-arsenate (Das et al, 2014) systems ( Fig. 4a; Eq. (1)) were described using firstorder decay.…”

“…Kinetic modeling was conducted on the % ferrihydrite remaining as a function of time using Sigmaplot® for the ferrihydrite-arsenate (Das et al, 2014), ferrihydrite-gypsum (Das et al, 2010), and ferrihydritegypsum-arsenate systems (present study) according to:…”