Influence of phosphate on the oxidation kinetics of nanomolar Fe(II) in aqueous solution at circumneutral pH

Fe hydroxyphosphate precipitation and Fe(II) oxidation kinetics upon aeration of Fe(II) and phosphate-containing synthetic and natural solutions

“…In experiments with (P/Fe) ini values between 0.12 and 0.3, the t 3/4 values increase with increasing PO 4 concentration but with (P/Fe) ini = 0.9, the t 3/4 is shorter than in the experiments with lower (P/Fe) ini values (Table 1). In earlier work on Fe(II) oxidation in presence of PO 4 it was reported that dissolved PO 4 accelerates the oxidation of aqueous Fe(II) in homogenous systems (Tamura et al, 1976;Mitra and Matthews, 1985;Mao et al, 2011) but slows down oxidation of adsorbed Fe(II) in heterogeneous systems (Wolthoorn et al, 2004). The acceleration of homogenous Fe(II) oxidation in the presence of PO 4 may be an explanation for the less pronounced or not noticeable initial stage with retarded Fe(II) oxidation rates during our experiments with (P/Fe) ini ratios greater than zero.…”

“…2). Tamura et al (1976) and Mao et al (2011) studied Fe(II) oxidation in the presence of excess PO 4 and concluded that the kinetics are first order with respect to dissolved PO 4 concentration. Tamura et al (1976) proposed following rate law:…”

“…Conversely, Mitra and Matthews (1985) concluded that HPO 4 2À was the sole phosphate species controlling the Fe(II) oxidation rate. Mao et al (2011) found that the aqueous phosphate complex FePO 4 À was the most important Fe(II) species contributing to the oxidation kinetics of nanomolar Fe(II) at circumneutral pH. Cumplido et al (2000) showed that the rate of Fe(II) oxidation in presence of Fe oxyhydroxides phases accelerates with increasing PO 4 concentrations.…”

“…Stumm and Lee, 1961;Sung and Morgan, 1980;Davison and Seed, 1983;Millero, 1985), but there is a limited number of studies that addressed the kinetics of Fe(II) oxidation in presence of PO 4 (Tamura et al, 1976;Mitra and Matthews, 1985;Cumplido et al, 2000;Wolthoorn et al, 2004;Mao et al, 2011). These studies indicate that the presence of PO 4 affects the kinetics of Fe(II) oxidation.…”

“…These studies indicate that the presence of PO 4 affects the kinetics of Fe(II) oxidation. Phosphate accelerates the oxidation of dissolved Fe(II) (Tamura et al, 1976;Mitra and Matthews, 1985;Mao et al, 2011). Tamura et al (1976) found that the rate of oxidation of micromolar Fe(II) could be expressed as a function of the concentration of H 2 PO 4 À .…”