Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia StrategyP-05

Behavior of Sulfate Ion During Formation of Bioscorodite
for Arsenite Immobilization
Masahito Tanaka, Tsuyoshi Hirajima, Keiko Sasaki and Naoko Okibe
Department of Earth Resources Engineering, Kyushu University, Fukuoka 819-0395, Japan

Abstract
Removal of arsenic as scorodite (FeAsO4·2H2O) from As(III)-bearing metal refinery
wastewaters has been studied using chemical or biological techniques. Biological scorodite
formation can be effectively applied for dilute As(III) solutions, and we have previously
demonstrated one-step As(III) immobilization using the Fe(II)- and As(III)-oxidizing archaeon
Acidianus brierleyi at pH 1.5, 70°C. Prior to crystallization of pale-green scorodite, yellowish
amorphous ferric-arsenate precursors are typically formed. To clarify the process ofbioscorodite crystallization through the transformation of the precursors into crystalline
scorodite, this study investigated uptake of sulfate ions (SO42−) in the crystal structure.
bioscorodite formation by liquid and solidcharacterization analyses.

1. Introduction
Scorodite is considered as one of theideal As disposal forms due to highthermodynamic stability, high density andlow Fe demand.[1] The past studies started
with hydrothermal methods and developed
to atmospheric and more recently tobiological methods. Compared to otherchemicalmethodologies,
biologicalapproach can become more important for
dilute As solutions, since chemical methods
display difficulties in treating such low As
concentrations. For bioscorodite formation,
our research group has been utilizing thethermo-acidophilic Fe-oxidizing archaeon,
Acidianus brierleyi. We have found that Ac.
brierleyi has not only Fe(II)- but also
As(III)-oxidizing ability to enable one-step
crystalline bioscorodite formation.[2] Prior tocrystallization of pale-green bioscorodite,yellowish amorphous ferric arsenate
precursors are typically formed. To clarify
the process of bioscorodite crystallization,
this study investigated the factors possibly
affecting the formation of precursors andtheirtransformation
intocrystalline
2−scorodite. The effects of SO4 ions werepreviously reported in chemical scorodite
studies.[3][4] Also, the fact that copper
refinery wastewaters often contain highconcentrations of SO42− motivated us toinvestigate behavior of SO42− during

2. Materials and Methods
Pregrown Ac. brierleyi cells wereinoculated (1.0 x 107 cells/ml) in 500 ml
Erlenmeyer flasks containing 200 ml ofheterotrophic basal salts medium (pH 1.5with H2SO4) with 18 mM Fe(II) (asFeSO4·7H2O), 13 mM As(III) (as NaAsO2)
and 0.02% (w/v) yeast extract. Flasks wereincubated at 70°C, shaken at 150 rpm.
Precipitates were regularly taken andwashed once by pure water. Afterfreeze-drying, samples were digested in 35%
HCl to determine the chemical composition.
Liquid samples were regularly taken tomonitor pH, Eh vs SHE, cell density, and theconcentrations of total Fe/As/S by ICP-OES,As(III) by stripping voltammetric method,and Fe(II) by o-phenanthroline method.
Precipitates were freeze-dried overnight for
XRD, SEM, FT-IR (KBr pellet method), andTG-DTA (heated from room temperature to1200°C at 10°C/min with N2 gas 100ml/min).
3. Results and Discussion
3.1.
PrecursorFormation
andTransformation into ScoroditeFe(II) and As(III) were completely
oxidized within 4 days by Ac. brierleyi.

14Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia Strategy

P-05
Consequently, yellowish amorphous ferricarsenate was precipitated as precursors (1ststage As precipitation at day 4–10). The
color then turned pale-green (typical color ofbioscorodite) overnight to form crystalline
scorodite (2nd stage As precipitation at day10~). To observe the behavior of Fe3+,
AsO43− and SO42− ions in solid, amorphousprecipitates were digested in 35% HCl. The
results showed that SO42− ion was one of theconstituents of precursors. The measured[AsO4]im/[Fe]im molar ratio at day 5 wassmaller (0.78) than the theoretical value ofscorodite (1.0), while [AsO4+SO4]im/[Fe]im
was 0.94. At the end of reaction,[SO4]im/[Fe]im decreased from 0.16 to 0.08
as [AsO4]im/[Fe]im increased to 0.94. This
indicates that excess Fe was precipitated due
to incorporation of SO42− ions instead ofAsO43− ions. In order to confirm the uptakeof SO42− ions, chemical scorodite synthesis
was attempted with or without use of H2SO4.
When other acids such as HCl and HNO3
were used instead of H2SO4, no precipitates
were obtained in 1 week. In contrast,
yellowish precipitates readily formed within
1 hour when H2SO4 was used, and weretransformed into scorodite at day 7.
Although inhibitory effects of SO42− ions on
chemical scorodite synthesis have beenreported[3], uptake of SO42− ion was shown
to be the key for precursor formation for
dilute As(III) solutions.
3.2. Behavior of SO42− Ions During
Transformation of Precursors into
Bioscorodite
From changes in FT-IR spectra ofprecipitates, distinct SO42− peaks (1062cm−1) originally present in precursors
disappeared after day 9, corresponding to
the clear color change to pale-green. This
was accompanied by the emergence of OH
peaks deriving from water molecules (3523cm−1), which indicates occurrence ofH-bond cleavage. SO42− ions were once
incorporated in the precursor structure by
weak H-bonding, and then released to form
final scorodite products. In order toelucidate precise chemical formula of theprecipitates, TG-DTA measurement wasconducted. Based on the TG curve
previously reported in the hydrothermal
study[4], the precursors formed in this study
can be considered basic ferric arsenate
sulfate(Fe(AsO4)1-x(SO4)x(OH)x·wH2O).
Consequently, the chemical formula ofprecursors was determined as Fe(AsO4)0.8(SO4)0.2(OH)0.3·2.4H2O. In contrast, thetypical scorodite TG curve was obtainedwith the final bioscorodite product at day 14.
Its chemical formula was calculated asFe(AsO4)0.9(SO4)0.1·1.9H2O. One fifth of theAsO43− locus was occupied by SO42− ions,which were then released from precursors toform final product with a chemical formula
close to theoretical scorodite.
4. Conclusions
Uptake of SO42− ions in scorodite
precursors was shown one of the key factors
for effective bioscorodite crystallizationfrom dilute As(III) solutions (~13 mM).
Amorphous precursors were determined as
basic ferric arsenate sulfate. Transformationof amorphous precursors into crystalline
bioscorodite proceeded via release of SO42−ions.
Acknowledgment
M. T. is grateful for financial assistanceprovided by the Kyushu UniversityAdvanced Graduate Program in Global
Strategy for Green Asia.
References[1] D. Langmuir, J. Mahoney, J. Rowson,
Geochim. Cosmochim. Ac. 2006, 70, 2942.
[2] N. Okibe, M. Koga, S. Morishita, M. Tanaka,
S. Heguri, S. Asano, K. Sasaki, T. Hirajima,
Hydrometallurgy 2014, 143, 34.
[3] G.P. Demopoulos, D.J. Droppert, G. Van Weert,
Hydrometallurgy 1995, 38, 245.
[4] M.A. Gomez, L. Becze, J.N. Cutler, G.P.
Demopoulos, Hydrometallurgy 2011, 107, 74.

Email: m-tanaka13@mine.kyushu-u.ac.jp
This content was published as proceedings ofInternational Symposium on Earth Science andTechnology 2017.

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