Natural organic matter inhibits Ni stabilization during Fe(II)-catalyzed ferrihydrite transformation

Trace metals, such as nickel (Ni), are often found associated with ferrihydrite (Fh) in soil and sediment and have been shown to redistribute during Fe(II)-catalyzed transformation of Fh. Fe(II)-catalyzed transformation of Fh, however, is often inhibited when natural organic matter (NOM) is associated with Fh. To explore whether NOM affects the behavior of Ni during Fe(II)-catalyzed transformation of Fh, we tracked Ni distribution, Fe atom exchange, and mineral transformation of Fh and Fh coprecipitated with Suwannee River natural organic matter (SRNOM-Fh). As expected, in the absence of Fe(II), Fh and SRNOM-Fh did not transform to secondary Fe minerals after two weeks. We further observed little difference in Ni adsorption on SRNOM-Fh compared to Fh. In the presence of Fe(II), however, we found that Ni associated with SRNOM-Fh was more susceptible to acid extraction than Fh. Specifically, we found almost double the amount of Ni remaining in the Fh after mild extraction compared to SRNOM-Fh. XRD showed that Fh transformed to goethite and magnetite whereas SRNOM-Fh did not transform despite ⁵⁷Fe isotope tracer experiments confirmed that SRNOM-Fh underwent extensive atom exchange with aqueous Fe(II). Our findings suggest that Fe atom exchange may not be sufficient for obvious Ni stabilization and that transformation to secondary minerals may be necessary for Ni stabilization to occur.

Stability of Fe-As composites formed with As(V) and aged ferrihydrite

During the aging process, ferrihydrite was transformed into mineral mixtures composed of different proportions of ferrihydrite, goethite, lepidocrocite and hematite. Such a transformation may affect the fixed ability of arsenic. In this study, the stability of Fe-As composites formed with As(V) and the minerals aged for 0, 1, 4, 10 and 30 days of ferrihydrite were systematically examined, and the effects of molar of ratios Fe/As were also clarified using kinetic methods combined with multiple spectroscopic techniques. The results indicated that As(V) was rapidly adsorbed on minerals during the initial polymerization process, which delayed both the ferrihydrite conversion and the hematite formation. When the Fe/As molar ratio was 1.875 and 5.66, the As(V) adsorbed by ferrihydrite began to release after 6 hr and 12 hr, respectively. The corresponding release amounts of As(V) were 0.55 g/L and 0.07 g/L, and the adsorption rates were 92.43% and 97.50% at 60 days, respectively. However, the As(V) adsorbed by the transformation products aged for 30 days of ferrihydrite began to release after adsorbed 30 days. The corresponding release amounts of As(V) were 0.25 g/L and 0.03 g/L, and the adsorption rates were 84.23% and 92.18% after adsorbed 60 days, for the Fe/As=1.875 and 5.66, respectively. Overall, the combination of As(V) with ferrihydrite and aged products transformed from a thermodynamically metastable phase to a dynamically stable state within a certain duration. Moreover, the aging process of ferrihydrite reduced the sorption ability of arsenate by iron (hydr)oxide but enhanced the stability of the Fe-As composites.

Coupled Kinetics of Ferrihydrite Transformation and As(V) Sequestration under the Effect of Humic Acids: A Mechanistic and Quantitative Study

In natural environments, kinetics of As(V) sequestration/release is usually coupled with dynamic Fe mineral transformation, which is further influenced by the presence of natural organic matter (NOM). Previous work mainly focused on the interactions between As(V) and Fe minerals. However, there is a lack of both mechanistic and quantitative understanding on the coupled kinetic processes in the As(V)-Fe mineral-NOM system. In this study, we investigated the effect of humic acids (HA) on the coupled kinetics of ferrihydrite transformation into hematite/goethite and sequestration of As(V) on Fe minerals. Time-resolved As(V) and HA interactions with Fe minerals during the kinetic processes were studied using aberration-corrected scanning transmission electron microscopy, chemical extractions, stirred-flow kinetic experiments, and X-ray absorption spectroscopy. Based on the experimental results, we developed a mechanistic kinetics model for As(V) fate during Fe mineral transformation. Our results demonstrated that the rates of As(V) speciation changes within Fe minerals were coupled with ferrihydrite transformation rates, and the overall reactions were slowed down by the presence of HA that sorbed on Fe minerals. Our kinetics model is able to account for variations of Fe mineral compositions, solution chemistry, and As(V) speciation, which has significant environmental implications for predicting As(V) behavior in the environment.

Simultaneous and continuous stabilization of As and Pb in contaminated solution and soil by a ferrihydrite-gypsum sorbent

For the increasing need of stabilization both cationic and anionic metal(loid)s simultaneously, we newly developed a metal sorbent (FIXALL), consisting mainly of ferrihydrite and gypsum. The objectives of this study were to determine the molecular mechanisms of Pb and As stabilization in an aqueous system and to examine a simultaneous and long-term (up to 754days) effect on Pb and As stabilization in an anthropogenically contaminated soil using the FIXALL sorbent. When the solution contained a low concentration of Pb (5mgL^-1), the mechanisms of Pb removal by FIXALL were based chiefly on the formation of inner-sphere surface complex with ferrihydrite. In the highly concentrated Pb solution (1200mgL^-1), contrarily, the removal of Pb by FIXALL was the direct consequence of the dissolution of gypsum and subsequent precipitation of PbSO₄, which strengthens the drawback of low capability of ferrihydrite for Pb removal. Regardless of initial concentrations, the primary mechanism of FIXALL for As stabilization is attributed to the formation of inner-sphere surface complex with ferrihydrite. A contaminated soil study demonstrated that FIXALL could decrease the concentration of water soluble As and Pb simultaneously and continuously for 754days without notable changes in their chemical species and soil pH.

Structural incorporation of As5+ into rhomboclase ((H5O2)Fe3+(SO4)2 · 2H2O) and (H3O)Fe(SO4)2

Iron sulfates represent an essential sink for the toxic element arsenic in arid and semi-arid mining areas with high evaporation rates. Information about the structural incorporation of As(5+) in iron sulfates, however, remains scarce. Here we present evidence for the heterogeneous substitution of S(6+) by As(5+) in the crystal structure of rhomboclase ((H5O2)Fe(3+)(SO4)2 · 2H2O) and its dehydration product (H3O)Fe(SO4)2. Rhomboclase (Rhc) was synthesized in the presence of As(5+) with molar As/Fe ratios of 0, 0.25, 0.5, 0.75 and 1.0, resulting in As loads of 0.0, 0.93, 1.44, 1.69 and 1.87 wt.%, respectively. The unit cell parameters of Rhc increase from 9.729(6), 18.303(2), and 5.432(1) Å for a, b, and c, to 9.745(9), 18.332(5), and 5.436(8) Å when Rhc is crystallized at a molar As/Fe ratio of 1. Simultaneously, the crystallite size decreased from 304 to 176 nm. In situ dehydration of Rhc to (H3O)Fe(SO4)2, investigated by powder X-ray diffraction, shows that Rhc starts to dehydrate at 76 °C, which is completed at 86 °C. The presence of As(5+) does not impact the start or end temperatures of Rhc dehydration but does accelerate the dehydration. X-ray absorption fine structure spectroscopy (EXAFS) reveals that S(6+), in the Rhc and (H3O)Fe(SO4)2 structure, is replaced by As(5+), while the polymerization of AsO4-tetrahedra and FeO6-octahedra during the formation of (H3O)Fe(SO4)2 results in a strong distortion of the AsO4-tetrahedron.

Arsenate-ferrihydrite systems from minutes to months: a macroscopic and ir spectroscopic study of an elusive equilibrium

Sorption by ferrihydrite is an important control on As(V) concentrations in many oxic aquatic systems. There are significant discrepancies in reported sorption constants (log(KAs)), which presents a problem for quantifying and understanding this important system. A review of reported ferrihydrite-As(V) sorption studies indicated a positive correlation between reaction time used in the experiments and the log(KAs) values derived from the data. In this paper, we study the kinetics of As(V) sorption over ≈3000 h in nine systems with varying pH and As(V)/Fe. Ferrihydrite was stable in all systems containing As(V), and the [As(V)] in solution decreased linearly as a function of log(t) (termed Elovich kinetics) over the full 3000 h in most systems. A stable [As(V)] was only observed in systems with low As(V)/Fe and low pH. Apparent As(V) sorption constants were derived from the data at specific time intervals using the diffuse layer model and equations describing log(KAs) values as a function of time provide a way to describe this elusive equilibrium. IR spectra support the hypothesis that slow interparticle diffusion is responsible for the slow approach to equilibrium. This work resolves discrepancies in previous studies of As(V)-ferrihydrite and provides equations to allow for system appropriate log(KAs) values to be used.