Formation Of Scorodite Research Articles

Mechanism and kinetics of scorodite formation in arsenic-bearing solutions using Fe(OH)3 as a solid iron source

Recently, solid iron sources have been used for scorodite synthesis in arsenic-bearing wastewater from nonferrous metallurgy. Immobilising arsenic-solution as scorodite via iron hydroxide (Fe(OH)3) solid iron source is an important method for controlling arsenic pollution. The evolution behavior of scorodite during its formation in high arsenic solution have been rarely investigated. In this paper, the mechanism and kinetics of scorodite formation using Fe(OH)3 in arsenic-bearing solution were investigated. This work was divided into three parts. Firstly, the influencing parameters were investigated, revealing that the dissolution of Fe(OH)3 and scorodite generation accelerated at lower initial pH and higher reaction temperature. Increasing Fe/As ratio delayed scorodite crystallisation, which was in turn enhanced by elevating arsenic concentration. Secondly, the mechanism of scorodite formation was investigated, revealing that Fe(OH)3 underwent acidic dissolution to form a precursor. Subsequent scorodite formation had a ΔrGmθ ranging from −69.39 kJ·mol−1 to −15.64 kJ·mol−1. Residual As was absorbed and converted into Fe(OH)3@scorodite. Thirdly, the chemical kinetics were investigated, showing that activation energy (Ea) for Fe(OH)3 dissolution was 72.54 and 105.37 kJ·mol−1 at Stages I and II, respectively, whereas it was 105.97 kJ·mol−1 for residual As absorption-conversion at Stage III outweighing the Ea of As-Fe coprecipitation. The restrictive steps were Fe(OH)3 dissolution and residual arsenic absorption-conversion. This proposed method can be applied for environment-friendly treatment of 10－40 g/L of arsenic-bearing industrial effluent for scorodite formation. Overall, this research confirmed the formation of scorodite via Fe(OH)3 and can potentially provide feasible schemes for eliminating arsenic-bearing acidic waste, dust, and anode slime from nonferrous metallurgical processes.

Rapid Oxidative Dissolution of Zerovalent Iron Induced by Sulfite for Efficient Removal of Arsenate and Arsenite: Selective Formation of Scorodite.

In this study, we proposed a moderate oxidation strategy for accelerating the oxidative dissolution of zerovalent iron (ZVI) using sulfite (S(IV)), thereby improving the removal of As(V) and As(III). Results revealed that, in the presence of 2.0 mM S(IV), both As(V) and As(III) were selectively converted into scorodite at pH0 3.0-7.0, while As(III) oxidation and As(V) immobilization were impressed over pH0 8.0-10.0. Batch experiments, radical quenching experiments, and electron spin resonance (ESR) measurements demonstrated that ZVI initially boosted S(IV) activation to generate SO4•-, •OH, and protons, and in turn, ZVI was further oxidized more intensely by these radicals than by oxygen. Concurrently, substantial protons derived from S(IV) oxidation neutralized hydroxyls produced by ZVI oxidation, maintaining an acidic environment conducive to the generation of scorodite rather than iron (hydr)oxides. Characterizations of X-ray diffraction (XRD), Raman, attenuated total reflectance-Fourier transform infrared (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS), field emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (HRTEM) confirmed that scorodite was formed in situ and then exfoliated from the surface of ZVI, and approximately 75% of ZVI could still be recovered, which contributed to efficient As removal in successive runs and real As-polluted wastewater. The application of S(IV) achieved a balance among ZVI reactivity improvement, As(V)/As(III) removal, and raw material consumption, making it a promising approach for addressing arsenic contamination in wastewater treatment.

Local Structure and Crystallization Transformation of Hydrous Ferric Arsenate in Acidic H2O-Fe(III)-As(V)-SO42- Systems: Implications for Acid Mine Drainage and Arsenic Geochemical Cycling.

Hydrous ferric arsenate (HFA) is a common thermodynamically metastable phase in acid mine drainage (AMD). However, little is known regarding the structural forms and transformation mechanism of HFA. We investigated the local atomic structures and the crystallization transformation of HFA at various Fe(III)/As(V) ratios (2, 1, 0.5, 0.33, and 0.25) in acidic solutions (pH 1.2 and 1.8). The results show that the Fe(III)/As(V) in HFA decreases with decreasing initial Fe(III)/As(V) at acidic pHs. The degree of protonation of As(V) in HFA increases with increasing As(V) concentrations. The Fe K-edge extended X-ray absorption fine structure and X-ray absorption near-edge structure results reveal that each FeO6 is linked to more than two AsO4 in HFA precipitated at Fe(III)/As(V) < 1. Furthermore, the formation of scorodite (FeAsO4·2H2O) is greatly accelerated by decreasing the initial Fe(III)/As(V). The release of As(V) from HFA is observed during its crystallization transformation process to scorodite at Fe(III)/As(V) < 1, which is different from that at Fe(III)/As(V) ≥ 1. Scanning electron microscopy results show that Oswald ripening is responsible for the coarsening of scorodite regardless of the initial Fe(III)/As(V) or pH. Moreover, the formation of crystalline ferric dihydrogen arsenate as an intermediate phase at Fe(III)/As(V) < 1 is responsible for the enhanced transformation rate from HFA to scorodite. This work provides new insights into the local atomic structure of HFA and its crystallization transformation that may occur in AMD and has important implications for arsenic geochemical cycling.

Cotreatment strategy for hazardous arsenic-calcium residue and siderite tailings via arsenic fixation as scorodite

Siderite tailings is a potentially cost-free iron (Fe) source for arsenic (As) fixation in hazardous arsenic-calcium residues (ACR) as stable scorodite. In this study, a pure siderite reagent was employed to investigate the mechanism and optimal conditions for As fixation in ACR via scorodite formation, while the waste siderite tailings were used to further demonstrate the cotreatment method. The cotreatment method starts with an introduction of sulfuric acid to the ACR for As extraction and gypsum precipitation, and is followed by the addition of H2O2 to oxidize As(III) in the extraction solutions and finalized by adding siderite with continuous air injection for scorodite formation. The dissolution-oxidation of siderite can slowly produce Fe(III) to control aqueous As(V)-Fe(III) precipitation supersaturation for continuous scorodite crystallization. Chemical analyses show that the extraction efficiency of As from the ACR reaches 94.55%, while the precipitation yield of extracted As via scorodite formation arrives at 99.63% and 99.47%, leading to fixation efficiency of 94.20% and 94.04% in terms of the total As in the ACR by using siderite reagent and tailings, respectively. The final solid products show desirable TCLP stability and long-term stability, meeting the requirement for safe storage (GB 5085.3–2007). XRD, FTIR, and TEM results reveal that such high stability is attributable to the formation of scorodite and the surface adsorption of As on the raw siderite and secondary maghemite. This innovative and economical application of siderite tailings for the treatment of hazardous ACR can be extended to the management of hydrometallurgical wastes.

Electrochemical investigation of scorodite synthesis for arsenic fixation using hematite as an iron source: Elucidation of reaction acceleration by Fe2+ using a local-cell model

Owing to the chemical stability of scorodite, the synthesis of scorodite for arsenic fixation has gained interest as a treatment method for arsenic-containing wastewater generated at nonferrous metal smelting plants and other facilities. Diverse synthesis processes have been investigated such as hydrothermal, oxidation, and solid iron oxide methods. The solid iron oxide method using hematite as an iron source is particularly promising owing to its many advantages. Though the overall reaction equation for this process does not include Fe2+, the reaction is influenced by Fe2+ in the solution, significantly increasing the reaction rate. In the present study, we examine the mechanism of this reaction electrochemically to elucidate the role of Fe2+. By analyzing the open-circuit potentials and the polarization behaviors of each local reaction and conducting short-circuiting experiments based on the predicted local-cell model, we demonstrate the electrochemical nature of the reaction, which involves the reduction of hematite and the oxidation of Fe2+. In addition, the multistep reaction involved in the formation of scorodite is discussed using potential–pH diagrams based on thermodynamic reference data.

As and S speciation in a submarine sulfide mine tailings deposit and its environmental significance: The study case of Portmán Bay (SE Spain)

The dumping of an estimated amount of 57 million tons of hazardous sulfide mine waste from 1957 to 1990 into Portmán's Bay (SE Spain) caused one of the most severe cases of persistent anthropogenic impact in Europe's costal and marine environments. The resulting mine tailings deposit completely infilled Portmán's Bay and extended seawards on the continental shelf, bearing high levels of metals and As. The present work, where Synchrotron XAS, XRF core scanner and other data are combined, reveals the simultaneous presence of arsenopyrite (FeAsS), scorodite (FeAsO₄·2H₂O), orpiment (As2S3) and realgar (AsS) in the submarine extension of the mine tailings deposit. In addition to arsenopyrite weathering and scorodite formation, the, the presence of realgar and orpiment is discussed, considering both potential sourcing from the exploited ores and in situ precipitation from a combination of inorganic and biologically mediated geochemical processes. Whereas the formation of scorodite relates to the oxidation of arsenopyrite, we hypothesize that the presence of orpiment and realgar is associated to scorodite dissolution and subsequent precipitation of these two minerals within the mine tailings deposit under moderately reducing conditions. The occurrence of organic debris and reduced organic sulfur compounds evidences the activity of sulfate-reducing bacteria (SRB) and provides a plausible explanation to the reactions leading to the formation of authigenic realgar and orpiment. The precipitation of these two minerals in the mine tailings, according to our hypothesis, has important consequences for As mobility since this process would reduce the release of As into the surrounding environment. Our work provides for the first time valuable hints on As speciation in a massive submarine sulfide mine tailings deposit, which is highly relevant for similar situations worldwide.

Oxidation of As(III) by pressurized oxygen and the simultaneous precipitation of As(V) as scorodite in acidic sulfate solutions

Due to the lack of affordable and green disposal technologies, high arsenic-containing waste acid from the heavy nonferrous metallurgical smelter causes serious environmental pollution and threatens human health. In this study, a cost-effective and environmentally-friendly solution for simultaneous oxidation and precipitation of arsenic as well as formation of scorodite by adding ferrous sulfate as an iron source is proposed under elevated temperature (170 °C) and pressurized oxygen (1 MPa) conditions. And that the total reaction can be interpreted as 4Fe2+ + 4HAsO2 + 3O2 + 10H2O → 4FeAsO4·2H2O + 8H+. Consequently, the final oxidation and removal efficiency of arsenic reached 98.95% and 97.42%, respectively, resulting in scorodite with an arsenic leaching concentration of 1.4 mg/L in the TCLP leaching test. The results prove that the HO•, Fe(III) and Fe(IV) produced by the oxidation of Fe(II) is of capable oxidizing As(III) to As(V). Most importantly, the synthetic transformation mechanism of scorodite via a four-stage process is revealed, including the oxidation of As(III), formation of a mixture of amorphous hydronium jarosite and iron arsenate as precursors, partial dissolution of the precursor as well as nucleation and growth of scorodite particles.

Arsenic Trioxide Leaching and Scorodite Crystallization in Methanesulfonic Acid

ABSTRACT Arsenic, one of the most toxic elements, is a byproduct generated from metallurgical processes of arsenic-containing ores. Arsenic can be formed into an arsenic trioxide as a byproduct during the decopperization in electro-refining process when processing high arsenic-bearing copper minerals. However, arsenic trioxide has a very high arsenic mobility, so it requires to be re-dissolved and immobilized into scorodite (FeAsO4▪2H2O). In this study, methanesulfonic acid (MSA) was tested for both arsenic trioxide leaching and scorodite precipitation. Arsenic trioxide leaching tests showed the arsenic extraction of 80% at 21°C in MSA with H2O2 as an oxidant within 6 hours. Meanwhile, arsenic precipitation tests were conducted to achieve above 95% arsenic removal within 6 hours by the formation of scorodite at an initial pH of 0.5 and temperature of 80°C. In addition, coarser scorodite particles were formed in higher temperature and lower solution pH. The optimum condition for scorodite crystallization in MSA medium can be suggested to be at 80°C with the initial pH of 0.5.

Evolution of As speciation with depth in a soil profile with a geothermal As origin

High contents of arsenic were detected in soils in Guandu plain, northwest Taiwan. To determine the sources and speciation of As in the soils, the depth profiles of soil properties, elemental composition and As speciation were investigated. The As concentrations in the soil profile ranged from 152 to 1222 mg kg−1, with the highest concentration at the depth of 70–80 cm. The As distribution was found to be positively correlated to Fe, Pb, and Ba. The As(V)-adsorbed ferrihydrite and scorodite were the predominant phases in the top layers (<50 cm), while beudantite was the predominant phase below 50 cm along with As(III)- and As(V)-adsorbed ferrihydrite as the minor components. The results of sequential extraction showed that As-associated with noncrystalline and crystalline Fe/Al hydrous oxides and residual phases were predominant at the depths of 0–60, 60–100 and 100–140 cm, respectively, indicating an increasing As recalcitrance with soil depth. Based on the soil properties, and elemental and mineral compositions at different soil depths, the origin of beudantite in the soils was likely allogenic rather than authigenic or anthropogenic. The formation of scorodite in the surface soils was suggested to be transformed from beudantite. As-associated Fe hydrous oxides may be contributed by the progressive dissolution of beudantite and scorodite, and the continuous influxes of As and Fe. While Fe hydrous oxides were able to immobilize As during the dissolution of As-bearing minerals, the increase of As mobility in soils may imply an increase in the environmental risk of As over time.

Hematite-catalysed scorodite formation as a novel arsenic immobilisation strategy under ambient conditions

Scorodite is an important mineral not only for arsenic (As) removal from industrial wastewaters but also in the mobility and final fate of As in waste rocks, contaminated soils and sediments, and mine tailings. Because of the mineral's high As-loading capacity and stability, numerous studies have been done to understand its formation. Unfortunately, most of these studies were limited to elevated temperatures (>70 °C), so the processes involved in scorodite formation under ambient conditions remain unclear. This study provides evidence of the catalytic effects of hematite on the formation of scorodite at 25 °C in a pyrite-rich natural geologic material. Scorodite peaks were detected in the XRD patterns of the leaching residues with and without hematite, but those in the former were stronger and more pronounced than the latter. These results suggest that the formation of scorodite was catalysed by hematite, a generalisation that is further supported by strong characteristic IR absorption bands of scorodite at 819 cm–1 (As–O bending vibration), 785 and 725 cm–1 (As–O stretching vibrations), and 2990 cm−1 (OH-vibration) as well as the distinct XPS binding energies of Fe(III)–As (709.7 eV), As(V)–O (44.8, 44.31 and 43.7 eV), O2− (530.5 eV) and coordinated water (531.3 eV) in scorodite. This phenomenon could be attributed to three possible mechanisms: (1) more rapid precipitation promoted by the “seeding” effect of hematite particles, (2) additional supply of Fe3+ from hematite dissolution under acidic conditions, and (3) enhanced oxidations of Fe2+ to Fe3+ and As(III) to As(V) on the surface of hematite.

Removal and fixation of arsenic by forming a complex precipitate containing scorodite and ferrihydrite

For the metallurgical industries, the safe disposal of arsenic-bearing waste is animportant issue. In this study, a two-stage arsenic precipitation process was proposed, which consists of formation of scorodite at lower pH followed by precipitation of arsenical ferrihydrite at higher pH. The experimental results show that arsenic can be removed effectively from a solution with arsenic concentration of 5 g/L as precipitate of crystalline scorodite through oxidizing ferrous ions with air stream at pH of 2at first. Then, the pH value was raised to 3 and 4, and almost all the residual arsenic was removed from the solution due to adsorption onto amorphous ferrihydrite at the molar ratio of Fe/As between 2 and 4. The United States Environmental Protection Agency's Toxicity Characteristic Leaching Procedure tests demonstrate that the complex precipitates generated under these conditions (Fe/As ratio of 4, final pH between 3 and 4) were very stable and no leached arsenic can be detected using acetic acid buffer solution (pH 4.93 ± 0.05) in tests of 72 h. Experimental results suggest that a high Fe/As ratio was necessary, so that the excess iron promotes the formation a product of good stability, arsenical ferrihydrite (AsFH: AsO43−·Fe(OH)(H2O)1+x) under a relatively high pH in acidic solution.

Spread and partitioning of arsenic in soils from a mine waste site in Madrid province (Spain)

The formation of scorodite is an important mechanism for the natural attenuation of arsenic in a wide range of environments. It is dumped on site by metallurgical industries to minimize arsenic release. However, the long-term stability of these deposits is unclear. Sequential As extractions and synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy were used to determine both As and Fe speciation in a small catchment area affected by a scorodite-rich waste pile at an abandoned smelting factory. Our results indicate that this deposit behaves as an acute point source of As and metal pollution and confirms the strong association of As(V) with Fe(III) oxide phases, highlighting the important role of ferrihydrite as an As scavenger in natural systems. In this seasonally variable system, other trapping forms such as jarosite-like minerals also play a role in the attenuation of As. Overall, our results demonstrate that scorodite should not be considered an environmental stable repository for As attenuation when dumped outside because natural rainfall and the resulting runoff drive As dispersion in the environment and indicate the need to monitor and reclamate As-rich mine deposits.

Total X-ray scattering, EXAFS, and Mössbauer spectroscopy analyses of amorphous ferric arsenate and amorphous ferric phosphate

Amorphous ferric arsenate (AFA, FeAsO4·xH2O) is an important As precipitate in a range of oxic As-rich environments, especially acidic sulfide-bearing mine wastes. Its structure has been proposed to consist of small polymers of single corner-sharing FeO6 octahedra (rFe–Fe ∼3.6Å) to which arsenate is attached as a monodentate binuclear 2C complex (‘chain model’). Here, we analyzed the structure of AFA and analogously prepared amorphous ferric phosphates (AFP, FePO4·xH2O) by a combination of high-energy total X-ray scattering, Fe K-edge X-ray absorption spectroscopy, and 57Fe Mössbauer spectroscopy. Pair distribution function (PDF) analysis of total X-ray scattering data revealed that the coherently scattering domain size of AFA and AFP is about 8Å. The PDFs of AFA lacked Fe–Fe pair correlations at r ∼3.6Å indicative of single corner-sharing FeO6 octahedra, which strongly supports a local scorodite (FeAsO4·2H2O) structure. Likewise, the PDFs and Fe K-edge extended X-ray absorption fine structure data of AFP were consistent with a local strengite (FePO4·2H2O) structure of isolated FeO6 octahedra being corner-linked to PO4 tetrahedra (rFe–P=3.25(1)Å). Mössbauer spectroscopy analyses of AFA and AFP indicated a strong superparamagnetism. While AFA only showed a weak onset of magnetic hyperfine splitting at 5K, magnetic ordering of AFP was completely absent at this temperature. Mössbauer spectroscopy may thus offer a convenient way to identify and quantify AFA and AFP in mineral mixtures containing poorly crystalline Fe(III)-oxyhydroxides. In summary, our results imply a close structural relationship between AFA and AFP and suggest that these amorphous materials serve as templates for the formation of scorodite and strengite (phosphosiderite) in strongly acidic low-temperature environments.

Arsenic release from arsenopyrite weathering: Insights from sequential extraction and microscopic studies

At a former As mine site, arsenopyrite oxidation has resulted in formation of scorodite and As-bearing iron hydroxide, both in host rock and mine tailings. Electron microprobe analysis documents that arsenopyrite weathers along two pathways: one that involves formation of sulfur, and one that does not. In both pathways, arsenopyrite oxidizes to form scorodite, which dissolves incongruently to form As-bearing iron hydroxides. From a mass balance perspective, arsenopyrite oxidation to scorodite conserves As, but as scorodite dissolves incongruently to iron hydroxides, As is released to solution, resulting in elevated As concentrations in the headwater stream adjacent to the site. The As-bearing iron hydroxide is the dominant solid phase reservoir of As in mine tailings and stream sediment, as suggested by sequential extraction. This As-bearing iron hydroxide is stable under the aerobic and pH 4–6 conditions at the site; however, changes in biogeochemical conditions resulting from sediment burial or future remedial efforts, which could promote As release from this reservoir due to reductive dissolution, should be avoided.

Effects of initial Fe 2+ concentration and pulp density on the bioleaching of Cu from enargite by Acidianus brierleyi

To maximize Cu recovery and minimize As release during the bioleaching of enargite (Cu 3AsS 4) by Acidianus brierleyi at 70 °C, the initial Fe 2+ ion concentration and pulp density were investigated in batch tests. A maximum Cu recovery of 91.0 ± 0.5% was obtained with an initial Fe 2+ ion concentration of 1.8–2.7 g/L and a pulp density of 1.0%. As the immobilization of As depended on the formation of scorodite (FeAsO 4·2H 2O) and cupric arsenate, a more rapid and stable As immobilization was achieved with an initial Fe 2+ ion concentration of 2.7 g/L. Thus the initial Fe 2+ concentration of 2.7 g/L and 1.0% pulp density provided for a combination of As precipitation and > 90% Cu recovery. When the initial Fe 2+ concentration was increased, K-jarosite (KFe 3(SO 4) 2(OH) 6) precipitated, leading to passivation of the enargite surface. When the initial Fe 2+ concentration was lowered, the Cu recovery was incomplete due to insufficient oxidation. Elevating the pulp density to 2.0% showed that the increase in Eh significantly lagged behind the exponential phase in the planktonic cell growth curve. Pulp density also clearly affected the makeup of secondary minerals in solid residues after the bioleaching of enargite. There is a tendency for dissolved Fe 3+ ions to readily precipitate as potassium jarosite at relatively smaller pulp densities and as scorodite at relatively larger pulp densities.

Mechanism of the enhancement of bioleaching of copper from enargite by thermophilic iron-oxidizing archaea with the concomitant precipitation of arsenic

Bioleaching of enargite by the thermophilic iron-oxidizing archaea, Acidianus brierleyi, at 70 °C for 27 days yielded a 90.9% recovery of Cu and 5.9% recovery of As. The preferential dissolution of Cu compared to As was mainly achieved by the formation of crystalline scorodite (FeAsO 4·2H 2O). Although the chemical valence of As in enargite was confirmed to be As(III), most of the dissolved As was in the compound H 3As VO 4. This indicates that the H 3As IIIO 3 released from the enargite was immediately oxidized to H 2As VO 4 − in solution via a catalytic reaction on the surface of the enargite. During the bioleaching process, dissolved Fe 3+ was consumed through the formation of precipitates, such as scorodite and jarosite ((M +)Fe 3(SO 4) 2(OH) 6), as well as the oxidation of enargite. Even after the formation of scorodite had stopped due to the consumption of Fe 3+, As was further passivated in the absence of Fe 3+. The co-sorption of Cu 2+ and H 2As VO 4 − on jarosite led to the formation of copper arsenate by a seed-forming effect. Subsequent secondary mineral precipitation resulted in the formation of multi-precipitation layers consisting of scorodite, potassium jarosite, elemental sulfur and copper arsenate, after most of the Cu had been recovered from the enargite by microbially induced leaching.

Influence of hardpan layers on arsenic mobility in historical gold mine tailings

Abstract Hardpans, or cemented layers, form by precipitation and cementation of secondary minerals in mine tailings and may act as both physical and chemical barriers. Precipitation of secondary minerals during weathering of tailings can sequester metal(loid)s, thereby limiting their release to the environment. At Montague Gold Mines in Nova Scotia, tailings are partially cemented by the Fe arsenate mineral scorodite (FeAsO4·2H2O). Previous studies have shown that the formation of scorodite can effectively limit aqueous As concentrations due to its relatively low solubility (

Multiscale Assessment of Methylarsenic Reactivity in Soil. 2. Distribution and Speciation in Soil

Methylated forms of arsenic (As), monomethylarsenate (MMA) and dimethylarsenate (DMA), have historically been used as herbicides and pesticides. Because of their large application to agriculture fields and the toxicity of MMA and DMA, the distribution, speciation, and sorption of methylated As to soils requires investigation. Monomethylarsenate and DMA were reacted with a soil up to one year under aerobic and anaerobic conditions. Microsynchrotron based X-ray fluorescence (μ-SXRF) mapping studies showed that MMA and DMA were heterogeneously distributed in the soil and were mainly associated with iron oxyhydroxides, e.g., goethite, in the soil. Micro-X-ray absorption near edge structure (XANES) spectra collected from As hotspots showed MMA and DMA were demethylated to arsenate over one year incubation under aerobic conditions. Monomethylarsenate was methylated to DMA, and DMA was maintained as DMA over a 3 month incubation under anaerobic conditions. Arsenic-iron precipitation, such as the formation of scorodite (FeAsO(4)·2H(2)O), was not observed, indicating that MMA and DMA were mainly associated with Fe-oxyhydroxides as sorption complexes.

Synthesis and phase transformations involving scorodite, ferric arsenate and arsenical ferrihydrite: Implications for arsenic mobility

Scorodite, ferric arsenate and arsenical ferrihydrite are important arsenic carriers occurring in a wide range of environments and are also common precipitates used by metallurgical industries to control arsenic in effluents. Solubility and stability of these compounds are controversial because of the complexities in their identification and characterization in heterogeneous media. To provide insights into the formation of scorodite, ferric arsenate and ferrihydrite, series of synthesis experiments were carried out at 70 °C and pH 1, 2, 3 and 4.5 from 0.2 M Fe(SO 4) 1.5 solutions also containing 0.02–0.2 M Na 2HAsO 4. The precipitates were characterized by transmission electron microscopy, X-ray diffraction and X-ray absorption fine structure techniques. Ferric arsenate, characterized by two broad diffuse peaks on the XRD pattern and having the structural formula of FeAsO 4·4–7H 2O, is a precursor to scorodite formation. As defined by As XAFS and Fe XAFS, the local structure of ferric arsenate is profoundly different than that of scorodite. It is postulated that the ferric arsenate structure is made of single chains of corner-sharing Fe(O,OH) 6 octahedra with bridging arsenate tetrahedra alternating along the chains. Scorodite was precipitated from solutions with Fe/As molar ratios of 1 over the pH range of 1–4.5. The pH strongly controls the kinetics of scorodite formation and its transformation from ferric arsenate. The scorodite crystallite size increased from 7 to 33 nm by ripening and aggregation. Precipitates, resulting from continuous synthesis at pH 4.5 from solutions having Fe/As molar ratios ranging from 1 to 4 and resembling the compounds referred to as ferric arsenate, arsenical ferrihydrite and As-rich hydrous ferric oxide in the literature, represent variable mixtures of ferric arsenate and ferrihydrite. When the Fe/As ratio increases, the proportion of ferrihydrite increases at the expense of ferric arsenate. Arsenate adsorption appears to retard ferrihydrite growth in the precipitates with molar Fe/As ratios of 1–4, whereas increased reaction gradually transforms two-line ferrihydrite to six-line ferrihydrite at Fe/As ratios of 5 and greater.

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.