Scorodite Particles Research Articles

Coprecipitation of Large Scorodite Particles from Aqueous Fe(II) and As(V) Solution by Oxygen Injection

A novel method for synthesizing large scorodite (FeAsO4 .2H2O) particles was recently developed for the fixation of arsenic. This method involves the coprecipitation of scorodite particles from an Fe(II) and As(V) aqueous solution at approximately 95 � C by oxygen injection. In order to understand the process of coprecipitation of scorodite particles by this method, X-ray diffractometry (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) were used for characterizing reaction products extracted from the suspension during the reaction. The SEM observation showed that the formation of large scorodite particles was almost completed after a reaction time of 3 h, and then, fine particles precipitated on the large particles by further reactions. The XRD results indicated that scorodite particles with specific lattice parameters were formed in the reaction. The XPS results indicated that the arsenic composition on the surface of the scorodite particles decreased until 3 h from the start of precipitation reaction and increased thereafter. These results correspond to the results on the morphology of the scorodite particles obtained by SEM. Furthermore, X-ray absorption spectra (XAS) in the range of X-ray absorption near edge structure (XANES) were measured for gel-like reaction products formed in the initial stages of the reaction. The spectra revealed that the gel-like reaction products were composed of Fe(II) and Fe(III). The coprecipitation of scorodite particles synthesized by the novel method is discussed on the basis of these results together with previous results on the analyses of iron and arsenic concentrations in solution. [doi:10.2320/matertrans.M-M2009804]

Immobilization of Arsenic from Novel Synthesized Scorodite—Analysis on Solubility and Stability

The solubility of scorodite synthesized in a novel atmospheric process was investigated. Stable scorodite (FeAsO4� 2H2O) particles were produced by introducing oxidizing gas into a reaction mixture containing ferrous sulfate and high-concentration arsenic (V) to convert ferrous ion to ferric ion. The obtained scorodite crystals released a very low, almost negligible concentration of arsenic in the pH range of 3 to 6, suggesting its long-term stability. The thermodynamic parameters for the dissolution reaction of scorodite, Ksp andGf, were similar to those reported previously. The results show scorodite released a considerably high concentration of arsenic under specific leaching conditions. In particular, the combination of CaO and NaCl present in the alkaline leaching solution had a significant effect on the mobilization of arsenic. Although this study did not cover all possible natural environments in which scorodite would be stored, it seems practically impossible to develop conditions that would completely prevent the dissolution of arsenic from scorodite. It is crucial to develop scorodite storage methods that will minimize the risk of environmental arsenic contamination based on accurate evaluation of the conditions for the leaching of arsenic. (doi:10.2320/matertrans.M-MRA2008844)

Effects of zinc, copper and sodium ions on ferric arsenate precipitation in a novel atmospheric scorodite process

A novel process has been developed for the fixation of arsenic using the formation of scorodite. Stable scorodite particles were produced by introducing an oxidizing gas into a reaction mixture containing ferrous sulfate and arsenic(V) at high concentration to convert the ferrous ion to ferric ion. This approach to scorodite synthesis enabled the reaction to proceed in the presence of sulfuric acid under atmospheric pressure. The influence of zinc, copper, and sodium ions in the solution was investigated; as well as the effect of pure O 2 gas and air as the oxidizing agent and the effect of arsenic concentration and pH of the reaction mixture. Scorodite grains were precipitated even though the reaction system contained co-existing ions at high concentration. It was found that zinc did not affect the synthesis and that the presence of copper yielded a good scorodite product. However, the presence of sodium inhibited the formation of scorodite. An initial pH under 2 was significant for producing well-crystallized scorodite and avoiding conditions favorable for jarosite formation.

A study of the crystallization kinetics of scorodite via the transformation of poorly crystalline ferric arsenate in weakly acidic solution

Abstract In this work, the transformation of poorly crystalline ferric arsenate nanoparticle aggregates to polycrystalline scorodite particles (FeAsO4·2H2O) in an acidic nitrate medium (pH 2–4) and elevated temperature (40–80 °C) is described. Monitoring of the transformation process via different parameters such as arsenic and iron concentration in solution, iron to arsenic molar ratio in solid phase, or crystalline fraction in product, exhibited “S”-shape or sigmoidal kinetic curves, characteristic of an autocatalytic-type reaction. The lengths of the induction period and self-acceleration stage were found to depend strongly on pH and reaction temperature with the fastest transformation achieved at pH 2 and 80 °C. The transformation kinetics have been described by the Avrami–Erofeev nucleation-growth model. The selected nucleation-growth model along with FEG-SEM and FEG-TEM observations supplemented with SAED and microanalysis suggested that the mechanism of transformation involved crystallization within the amorphous precursor phase aggregates. The poorly crystalline ferric arsenate was found to contain an excess of iron, which was attributed to ferrihydrite co-precipitation. The minor ferrihydrite phase was found to convert to crystalline scorodite via a postulated mechanism that involves AsO4 ion adsorption, surface build-up of ferric arsenate and crystallization. TGA and XRD analysis revealed the final crystallization stage to be rather slow not leading to full development of crystalline structure even after 24 h ageing at pH 2 and 80 °C.