Reductive dissolution of As(V)-bearing Fe(III)-precipitates formed by Fe(II) oxidation in aqueous solutions

Table 1 Precipitate synthesis, composition, and structure

Label	Synthesis solution^a				Precipitate^b			Structure^c
Label	Cation	P/Fe (−)	Si/Fe (−)	As/Fe (−)	P/Fe (−)	Si/Fe (−)	As/Fe (−)	Structure^c
Ca-00-00	Ca	−	−	0.014	−	−	0.013	100% pcLp
Ca-01-00	Ca	0.09	−	0.013	0.09	−	0.013	21% HFO; 79% pcLp
Ca-02-00	Ca	0.20	−	0.014	0.20	−	0.014	15% CaFeP, 26% HFO, 59% pcLp
Ca-05-00	Ca	0.53	−	0.016	0.53	−	0.016	59% CaFeP, 16% HFO, 25% pcLp
Ca-15-00	Ca	1.49	−	0.014	1.04	−	0.007	100% CaFeP
Na-15-00	Na	1.46	−	0.014	0.60	−	0.005	100% FeP
Ca-02-05	Ca	0.21	0.48	0.014	0.21	0.06	0.014	(25% CaFeP, 75% Fh-lowSi)
Ca-02-10	Ca	0.20	0.96	0.014	0.21	0.11	0.014	25% CaFeP, 75% Fh-Si
Ca-00-10	Ca	−	0.96	0.014	−	0.13	0.014	100% Fh-Si
2L-Fh^d	Ca	−	−	−	−	−	−	2L-Fh

^aInitial Fe(II) concentration 0.5 mM; pH 7.0 adjusted with CO₂ in 4 mM CaCO₃ (Ca) or 8 mM NaHCO₃ (Na)
^bMolar P/Fe and Si/Fe of precipitates calculated from difference between initial total and final dissolved P, Si and Fe concentrations measured by ICP-MS; values are average of triplicate samples (duplicate for Ca-00-00)
^cStructure based on Fe K-edge EXAFS analysis of dried samples from Ref. [20] (pcLp poorly crystalline lepidocrocite, HFO hydrous ferric oxide, CaFeP amorphous Ca–Fe(III)-phosphate, FeP amorphous Fe(III)-phosphate), Fh-Si silicate-containing ferrihydrite. Precipitate Ca-02-05 has not been examined by EXAFS spectroscopy and the indicated structural composition is tentative (see text)
^d2-line ferrihydrite synthesized by neutralization of 0.2 M Fe(NO₃)₃ × 9H₂O with 1 M KOH [31]. Freshly prepared 2L-Fh was suspended in Ca electrolyte and wet and dry precipitates were prepared as for other Fe(III)-precipitates

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