Scorodite synthesis process using solid iron oxides

The immobilization of arsenic in the form of scorodite (FeAsO4·2H2O), which has excellent chemical stability, is attracting attention as a method for treating wastewater containing high concentrations of arsenic generated at nonferrous metal smelting plants. Scorodite, with its low solubility and high arsenic content per volume, is expected to be an environmentally friendly arsenic fixation method suitable for final disposal. Various scorodite synthesis methods have been studied. The first synthesis method proposed is the hydrothermal method using an autoclave to synthesize highly crystalline scorodite. Although the hydrothermal method is capable of synthesizing scorodite with good crystallinity, from an economic point of view, scorodite synthesis under ambient pressure and at low temperatures is more attractive. As a low-temperature scorodite synthesis method under atmospheric pressure, the oxidation of Fe(II) process by O2 bubbling was proposed. In this method, ferrous sulfate is added to an arsenic-containing solution as a Fe ion source, and in situ oxidation of Fe(II) by O2 or air bubbling to form scorodite with good crystallinity. In addition to temperature, other conditions, pH, Fe/As ratio, and reaction time have been reported to affect scorodite crystallization. Recently, scorodite synthesis using solid iron oxide as the Fe source for scorodite synthesis, instead of aqueous Fe salt solutions, has attracted much attention. In this presentation, we will report on the investigation of reaction parameters, such as the type of iron oxide and reaction temperature, for the scorodite synthesis using solid iron oxide.


Introduction
Arsenic (As) is a highly toxic element that is widely distributed throughout the earth.Non-ferrous metal smelters that process ores containing As emit large amounts of As in the form of slag, sludge, dust and wastewater.Wastewater poses a particular threat to the natural environment and human health due to its high toxicity and mobility.Therefore, efforts have been made to find ways to safely fix As in wastewater in a stable form that can be safely disposed of.The iron co-precipitation method is commonly used to immobilize arsenic, but it produces large amounts of arsenic-containing sludge, which is harmful to the environment.In addition, adsorption of arsenic on iron compounds raises concerns about potential As leaching. 1 2 One promising method that addresses these issues is the immobilization of arsenic as scorodite.Scorodite is a stable compound with low solubility, making it suitable for arsenic immobilization.This method can significantly reduce the amount of sludge produced.Scorodite synthesis is expected to be introduced especially in areas with high concentrations of arsenic-containing wastewater 3 4 .
Several methods for the synthesis of scorodite have been investigated.6][7] However, the synthesis of scorodite at ambient pressure and low temperature is more cost effective.Fujita et al. reported an excellent low-temperature scorodite synthesis method under atmospheric pressure at 95°C using ferrous sulfate as the Fe ion source and in situ oxidation of Fe 2+ by air bubbling to form scorodite with good crystallinity. 8,9 her conditions such as pH, Fe/As ratio and reaction time were found to affect scorodite crystallization.Recently, the use of solid iron oxide instead of aqueous Fe salt solutions as the Fe source for scorodite synthesis has attracted much attention.Hematite [10][11][12] , ferrihydrite 13 , magnetite 14 and limonite 15 have been investigated as solid iron oxides.Scorodite synthesis using these stable iron oxides is expected to be an environmentally friendly arsenic removal process because they are inexpensive and can utilize by-products of other industrial processes.
This study investigates the immobilization behavior of arsenic in solid iron oxides, particularly magnetite (Fe3O4) and wustite (FeO) in comparison with hematite (Fe2O3), and clarifies their characteristics in terms of solution composition, pH, and ORP.The effect of the initial concentration of Fe(II) added on the reaction is also studied in the case of magnetite.

Experimental
Aqueous solutions containing As(V) were prepared using arsenic acid reagent (60 mass% H3AsO4, FUJIFILM Wako Pure Chemical Corporation) and distilled water, and solid iron oxides, hematite (Fe2O3, Alfa Aesar), magnetite (Fe3O4, Alfa Aesar) and wustite (FeO, Alfa Aesar).The initial Fe(II) concentration was adjusted by adding ferrous sulfate heptahydrate (FeSO4•7H2O) to study the effect of Fe(II) concentration.A 1 L multi-port flask was used as the reaction vessel, which is common with the previous report 13 , and solid iron oxide was added to 700 mL of arsenic acid solution [H3AsO4] = 0.67 mol L -1 (i.e., [H3AsO4] = 50 g L -1 ) maintained at 95°C to a pulp concentration of 0.35 mol L -1 .The synthesis was carried out under Ar flow (700 mL min -1 ) to prevent oxidation by atmospheric O2, and stirring was performed with an impeller at 200 rpm.The pH transition of the solution was monitored with a pH meter and an ORP meter (Pt electrode) with the reference electrode of Ag|AgCl in 3.33M KCl aq., and the pH was adjusted by the addition of dilute sulfuric acid (2.0 mol L -1 ) prepared with concentrated sulfuric acid (H2SO4, 98%, FUJIFILM Wako Pure Chemical Corporation).The slurry was sampled at 10, 60, and 420 min after the addition of solid iron oxide, and the powder sample was obtained as the precipitate by vacuum filtration, and the solution sample was obtained as the filtrate.The powder sample was vacuum freeze-dried overnight to remove water.The solution samples were analyzed for As and Fe concentrations by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Spectro Arcos, Spectro Analytical Instruments, Kleve, Germany).The powder samples were characterized using X-ray diffraction (XRD; D2PHASER, Bruker Corp., Billerica, MA, USA) with Cu Kα radiation (1.542 Å), scanning electron microscopy (SEM; SU-6600, Hitachi, Tokyo, Japan) and raman spectroscopy (Renishaw InVia Raman microscope, Renishaw plc., Wottonunder-Edge, UK).

Scorodite synthesis using magnetite
The behavior of scorodite formation using magnetite with/without the presence of Fe(II) was observed.Fig. 1 shows the changes in As concentration, pH and potential of the Pt electrode when magnetite was added to an aqueous solution of As(V) with / without containing Fe(II) at different concentrations.The initial As(V) concentration was 0.67 mol L -1 , and the initial Fe(II) concentrations varied from 0 to 1.0 mol L -1 .The results showed that the As concentration decreased with time and was reduced to less than 0.005 mol L -1 (i.e. less than 0.4 g L -1 ) after 420 min.As concentrations at 10 and 60 min decreased with increasing initial Fe(II) concentration, indicating that Fe(II) enhanced the As removal rate.This was similar to a previous study using solid iron oxide with hematite for arsenic immobilization, shown in Fig. 1(i-Ref) 11 .In such a system, Fe(II) was converted to scorodite by oxidation with hematite, which increased the As removal rate 16 .Similarly, in the present study, Fe(II) in solution could be oxidized and converted to scorodite with magnetite, which increased the As removal rate.The pH of the solution increased rapidly at the beginning of the reaction and then gradually increased or remained constant.This was due to the consumption of protons during the reaction to form scorodite from magnetite, as shown in the following reaction equation.
The pH increase was less pronounced at high initial Fe(II) concentrations due to the pH buffering effect of ferrous sulfate.This suggests that the equilibrium potential of the potential determination reaction is influenced more by pH than by Fe(II) activity.
Fig. 1 (i) As concentration, (ii) pH and (iii) potential of Pt electrodes in the reaction solutions with magnetite added during scorodite synthesis for each initial Fe(II) concentration condition.The reported As 5+ removal behavior by the hematite method is also shown in (i-Ref) 11 .
Initial concentration of Fe(II) Pulp concentration of magnetite 0.35 mol L -1 Pulp concentration of hematite 0.35 mol L -1 Initial concentration of Fe(II) Figure 2 shows SEM images of solid products sampled at 420 min for different initial Fe(II) concentration conditions.Scorodite crystals with facets were formed at high initial Fe(II) concentrations.In contrast, at concentrations below 0.1 mol L -1 , only a powder with a cocoon-like appearance covered with an amorphous membrane was observed.This is a gel-like precursor of scorodite, which crystallized under conditions of high initial Fe(II) concentration due to low pH, however, did not crystallize under conditions of low initial Fe(II) concentration due to high pH.In contrast to the hematite method, Fe(II) addition was not necessary for the scorodite formation reaction to proceed when magnetite was used for arsenic fixation.However, the addition of Fe(II) accelerated the reaction rate, and crystalline scorodite was formed.Fig. 2 SEM images of the precipitates in the reaction solutions with magnetite added during scorodite synthesis for each initial Fe(II) concentration condition obtained at 420 min.
Figure 3 shows the microscopic images and the local Raman spectra of solid products sampled at 420 min for Fe(II) concentration of 0.1 mol L -1 .The faceted crystals observed in the SEM images are identified as scorodite, while the surrounding film-like material shows spectra different from those of scorodite and iron oxide.Although the small size of the mixture of scorodite crystals and gel-like precursor, typically in the range of tens of microns, makes it difficult to perform local elemental analysis and crystal structure analysis, they can be clearly distinguished through local Raman spectroscopic analysis.

2. Scorodite synthesis using wustite
Figure 4 shows the behavior of scorodite formation with wustite in As(V) solution without addition of Fe(II).When only wustite was added, no scorodite was formed because wustite is a divalent iron oxide.In contrast, when both wustite and hematite were added simultaneously, the As concentration decreased with time to 0.01 mol L -1 .The pH and natural immersion potential of Pt were also measured, and the results were similar to those of magnetite addition under Fe(II)-containing conditions.Considering the rapid decrease of As concentration in the presence of Fe(II) and hematite as shown in Figure 1(i-Ref), this suggests that the decrease of As concentration in the presence of wustite and hematite was caused by the wustite dissolution according to the following equation.
This reaction continued because Fe 2+ produced by dissolution and scorodite was synthesized in the presence of hematite and As(V).SEM images of the product after 420 minutes confirmed the formation of faceted scorodite crystals were formed.This result showed that the scorodite formation reaction could proceed by simultaneous addition of solid iron oxide containing divalent iron, such as wustite, without necessarily adding reagent-derived Fe(II) such as ferrous sulfate in solution.
Fig. 4 (i) As concentration, (iii) pH and (iv) potential of Pt electrodes in the reaction solutions with wustite and/or hematite added during scorodite synthesis.(ii) SEM image of the precipitate obtained in the condition of simultaneous addition of wustite and hematite at 420 min.

Conclusion
This study investigated the use of magnetite and wustite for the synthesis of scorodite in As-containing aqueous solutions.When magnetite was used, only an amorphous scorodite precursor was produced due to the increased pH.However, the addition of Fe(II) improved the As removal rate and prevented the pH increase, resulting in the formation of crystalline scorodite.When wustite was added alone, the reaction did not proceed, However, when combined with hematite, crystalline scorodite was obtained.Therefore, it is suggested that a suitable process for synthesizing scorodite using solid iron oxide can be achieved without the need for Fe(II) agents in solution by combining different solid iron oxide raw materials.

Fig. 3
Fig.3The microscopic images and the Raman spectra of the precipitates in the reaction solutions with magnetite added during scorodite synthesis for the initial Fe(II) concentration of 0.1 mol L -1 condition obtained at 420 min for the parts with (a) crystalline-like appearance and with (b) membranelike appearance vs. Ag|AgCl in 3.33M KCl aq.wustite and/or hematite 0.35 mol L -1 Initial concentration of Fe(II) 0 molL-1