Role of Fe(II) and phosphate in arsenic uptake by coprecipitation

Journal Article · Tue Jul 08 04:00:00 EDT 2008 · Geochim. Cosmochim. Acta

DOI:https://doi.org/10.1016/j.gca.2007.04.008· OSTI ID:1007578

Sahai, N; Lee, Y J; Xu, H; Ciardelli, M; Gaillard, J -F ^[1]

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1. NWU

Natural attenuation of arsenic by simple adsorption on oxyhydroxides may be limited due to competing oxyanions, but uptake by coprecipitation may locally sequester arsenic. We have systematically investigated the mechanism and mode (adsorption versus coprecipitation) of arsenic uptake in the presence of carbonate and phosphate, from solutions of inorganic composition similar to many groundwaters. Efficient arsenic removal, >95% As(V) and {approx}55% in initial As(III) systems, occurred over 24 h at pHs 5.5-6.5 when Fe(II) and hydroxylapatite (Ca{sub 5}(PO{sub 4}){sub 3}OH, HAP) 'seed' crystals were added to solutions that had been previously reacted with HAP, atmospheric CO{sub 2(g)} and O{sub 2(g)}. Arsenic adsorption was insignificant (<10%) on HAP without Fe(II). Greater uptake in the As(III) system in the presence of Fe(II) was interpreted as due to faster As(III) to As(V) oxidation by molecular oxygen in a putative pathway involving Fe(IV) and As(IV) intermediate species. HAP acts as a pH buffer that allows faster Fe(II) oxidation. Solution analyses coupled with high-resolution transmission electron microscopy (HRTEM), X-ray Energy-Dispersive Spectroscopy (EDS), and X-Ray Absorption Spectroscopy (XAS) indicated the precipitation of sub-spherical particles of an amorphous, chemically-mixed, nanophase, Fe{sup III}[(OH){sub 3}(PO{sub 4})(As{sup V}O{sub 4})] {center_dot} nH{sub 2}O or Fe{sup III}[(OH){sub 3}(PO{sub 4})(As{sup V}O{sub 4})(As{sup III}O{sub 3}){sub minor}] {center_dot} nH{sub 2}O, where As{sup III}O{sub 3} is a minor component. The mode of As uptake was further investigated in binary coprecipitation (Fe(II) + As(III) or P), and ternary coprecipitation and adsorption experiments (Fe(II) + As(III) + P) at variable As/Fe, P/Fe and As/P/Fe ratios. Foil-like, poorly crystalline, nanoparticles of Fe{sup III}(OH){sub 3} and sub-spherical, amorphous, chemically-mixed, metastable nanoparticles of Fe{sup III}[(OH){sub 3}, PO{sub 4}] {center_dot} nH{sub 2}O coexisted at lower P/Fe ratios than predicted by bulk solubilities of strengite (FePO{sub 4} {center_dot} 2H{sub 2}O) and goethite (FeOOH). Uptake of As and P in these systems decreased as binary coprecipitation > ternary coprecipitation > ternary adsorption. Significantly, the chemically-mixed, ferric oxyhydroxide-phosphate-arsenate nanophases found here are very similar to those found in the natural environment at slightly acidic to circum-neutral pHs in sub-oxic to oxic systems, such phases may naturally attenuate As mobility in the environment, but it is important to recognize that our system and the natural environment are kinetically evolving, and the ultimate environmental fate of As will depend on the long-term stability and potential phase transformations of these mixed nanophases. Our results also underscore the importance of using sufficiently complex, yet systematically designed, model systems to accurately represent the natural environment.

• Research Organization: Advanced Photon Source (APS), Argonne National Laboratory (ANL), Argonne, IL (US)
• Sponsoring Organization: USDOE
• OSTI ID: 1007578
• Journal Information: Geochim. Cosmochim. Acta, Journal Name: Geochim. Cosmochim. Acta Journal Issue: (13) ; 07, 2007 Vol. 71; ISSN GCACAK; ISSN 0016-7037
• Country of Publication: United States
• Language: ENGLISH

Related Subjects

58 GEOSCIENCES
ABSORPTION SPECTROSCOPY
ADSORPTION
ARSENIC
BUFFERS
CARBONATES
COPRECIPITATION
GOETHITE
NATURAL ATTENUATION
OXIDATION
OXYGEN
PHASE TRANSFORMATIONS
PHOSPHATES
PRECIPITATION
REMOVAL
SPECTROSCOPY
STABILITY
TRANSMISSION ELECTRON MICROSCOPY