Enhanced Arsenic (III and V) Removal in Anoxic Environments by Hierarchically Structured Citrate/FeCO3 Nanocomposites

Abstract

Novel citrate/FeCO3 nanocomposites (CF-NCs) were synthesized for effective arsenic (III and V) sorption with constant addition of Fe2+ into HCO3− solution in the presence of citrate. This paper is the first report on the formation of CF-NCs, and in this study we investigate the mechanisms of arsenic uptake by the sorbent under anoxic conditions through various solid- and liquid-phase spectroscopic methods, including X-ray absorption spectroscopy. In CF-NCs, citrate was found to be incorporated into the structure of siderite (up to 17.94%) through (Fe2+citrate)− complexes. The crystal morphology of rhombohedral siderite was changed into hierarchically nanostructured spherical aggregates composed of several sheet-like crystals, which improved the surface reactivity in the presence of sufficient citrate. Compared to pure siderite (15.2%), enhanced removal of As(III) in the range of 19.3% to 88.2% was observed, depending on the amount of incorporated citrate. The maximum sorption capacities of CF-NCs for As(III) and As(V) were 188.97 and 290.22 mg/g, respectively, which are much higher than those of previously reported siderite-based adsorbents. It was found that arsenic (III and V) sorption on CF-NCs occurred via bidentate corner-sharing surface complexation, predominantly without changes in the arsenic oxidation states. These results suggest that arsenic (III and V) can be attenuated by siderite in anoxic environments, and this attenuation can be even more effective when siderite is modified by incorporation of organic compounds such as citrate.

Effects of Preferential Incorporation of Carboxylic Acids on the Crystal Growth and Physicochemical Properties of Aragonite

“…In addition, the amount of citric acid incorporated increased steadily from 0.23 to 0.65 wt % as the carboxylic acid concentration in the synthetic solution increased from 0.1 to 1.0 mM, but malic acid reached a maximum at 0.19 wt %. These results can support the XRD and TEM/SAED results, which showed a disorder in the lattice structure of carbonate mineral upon the incorporation of citric and malic acids [34]. Therefore, it is suggested that only the two carboxylic acids can be favorably incorporated into aragonite, and this tendency is stronger in citric acid than malic acid.…”

“…This means that some of the deprotonated carboxylic acids can exist in the form of Ca-organic ligand complexes (i.e., chelate). According to the previous studies, the metal chelate complex species such as (Ca-citrate) − can be strongly involved in its incorporation into carbonate minerals such as calcite or siderite [33,34,52]. Therefore, we deduced that the Ca-organic ligand complexes have an important role in the incorporation of carboxylic acids into aragonite.…”

“…First, the weak shoulders positioned near 1455 cm −1 (indicated by *) are within the banding range corresponding to the typical stretching modes of the carboxyl group, indicating the existence of citrate in the citric-arag sample [33]. Second, the broad feature around 3300 cm −1 might be attributable to the stretching modes of structural water [53], which could have been inserted together during the process of citrate incorporation into the aragonite structure [33,34]. These FT-IR results can support the fact that citrate was incorporated into the aragonite structure.…”

“…The possible co-precipitation of carboxylic acids with aragonite was investigated by high-performance liquid chromatography (HPLC), Fourier transform-infrared (FT-IR), and thermogravimetric and differential scanning calorimetry (TG-DSC) analyses of the precipitates. The contents of the incorporated carboxylic acids in the co-precipitated aragonite samples were measured by HPLC (1200 series, Agilent Technologies, Waldbronn, Germany) at the Korea Institute of Science and Technology (KIST) [34]. Briefly, each organic co-precipitated solid sample was treated with 1 M of HCl to dissolve the solid sample and ionize the organic acid contained in the solid into liquid phase.…”

“…Therefore, we designed a seeded co-precipitation system for growth of aragonite in the presence of different types of carboxylic acids considering the number of carboxyl groups, structure, and arrangement of functional groups to investigate the effects of carboxylic acids on the crystal properties of aragonite. Furthermore, we performed speciation modeling to explain the possible mechanism(s) of confirmed preferential incorporation of certain carboxylic acids into aragonite, because the form in which the organic acid exists (i.e., metal-organic complexes or organic species) is relevant for the incorporation into carbonate mineral during crystallization [33,34]. This is the first systematic investigation using a seeded co-precipitation method on (1) the incorporation of carboxylic acids into synthetic aragonite and (2) the subsequent changes in the crystal properties of the co-precipitated aragonite.…”