Long-term evaluation of novel pilot-scale membrane systems for drinking water production

Researcher:
  • Yuichi Tomioka 
　

COE Post-Doctoral Fellow
Water Quality Control Eng. Lab.


Introduction

On weathering of arsenic bearing sulfide minerals, arsenic is dissolved and diffused throughout the environment. Arsenopyrite is a very common mineral containing arsenic. There are many studies about the arsenopyrite oxidation mechanism. However, the mechanism to control the valence of arsenic ions extracted from arsenopyrite is not well known. In this study, the factors affecting arsenopyrite dissolution and valence of arsenic ions were investigated.


Mineral Sample

Arsenopyrite (Toroku mine, Miyazaki, Japan) was ground with a ball mill and sieved to obtain the 38-75 μm size fraction. The sieved sample was pretreated as mentioned elsewere 1). The results of the XRD analysis of the pretreated sample are shown in Fig. 1. Most peaks can be attributed to arsenopyrite, and quartz was detected as impurity. The chemical composition of the sample is shown in Table 1.


Experimental Leaching Experiments

To investigate the effect of Fe(II) and Fe(III), 50 cm3 erlenmeyer flasks were filled with 10 cm3 of 0.1M sulfuric acid containing Fe(II) and Fe(III) (0-100 mM) and 0.1 g of the arsenopyrite. For the experiments under aerobic conditions, the flasks were capped with porous plugs. For the anaerobic experiments, air in the flasks was purged with N2 gas (1 L/min for 3 min) and the flasks were sealed with butyl-rubber plugs. The flasks were shaken in a water bath shaker at 25 ℃ under light shielding conditions. After 24 h, solution was collected by 0.2 μm membrane filter and analyzed to determine the total soluble As (T-As) and As(III) concentration by ICP-AES or the thionalide extraction method 2). The As(V) concentration was calculated by subtracting the As(III) concentration from the T-As concentration.


As(III) Oxidizing Experiment

The 50 cm3 of erlenmeyer flasks were filled with 10 cm3 of 0.1 M sulfuric acid containing known concentrations of As(III), Fe(II) and Fe(III), and 0.1 g of the arsenopyrite.
Flasks were capped with porous plugs and shaken for 24 h under aerobic conditions.
Two series of experiments were carried out: in “series A”, 1 mM As(III) was added, and in “series B”, As(III) was not added. In series A, a part of the added As(III) is oxidized after 24 h. The amount of the oxidized As(III), XAs(III), is estimated by the following equation.

XAs(III) = [As(III)]d + [As(III)]0h － [As(III)]24h
[As(III)]d: As(III) concentration at 24h in the experiments in series B.
[As(III)]0h: Initial As(III) concentration at 24h in the experiments in series A
[As(III)]24h:As(III) concentration at 24h in the experiments in series A


3. RESULTS AND DISCUSSIONS

Fig. 2 shows the Fe(II) concentration dependence of arsenopyrite dissolution. Under aerobic conditions, the T-As concentration increased with increasing Fe(II) concentrations. The As(III) concentration also increased with increasing Fe(II) concentration though the As(V) concentration decreased. Under anaerobic conditions, the T-As concentrations were very low and independent of Fe(II) concentrations.

Fig. 3 shows the Fe(III) concentration dependence of arsenopyrite dissolution. Under aerobic conditions, with increasing Fe(III) concentration the As(III) concentration increased, but the As(V) concentration decreased. Under anaerobic condition T-As concentration increased with increasing Fe(III) concentration. These results indicate that oxidant Fe(III) and oxygen enhance arsenopyrite dissolution, and that significant amounts of As(V) were generated only in the presence of oxygen.

Fig.4 shows the effect of Fe(II) and Fe(III) concen-tration on As(III) oxidation. The amount of oxidized As(III), XAs(III), decreased with increasing Fe concentra-tions. In another oxidation experiment without arsenopyrite, As(III) oxidation did not take place even in the presence and absence of Fe species. These results indicate that arseno-pyrite enhances As(III) oxidation and high concentrations of Fe(II) or Fe(III) suppress the As(III) oxidation. Based on this, the results of the leaching experiments under aerobic conditions (Fig. 2, Fig. 3) may be explained as follows; when Fe(II) or Fe(III) was not added, As(III) extracted from arsenopyrite was oxidized to As(V), causing high concentration ratios of As(V) to As(III). With high concentrations of Fe(II) or Fe(III), oxidation of As(III) is suppressed and this causes high As(III) concentrations and decreases in As(V) concentration with increasing Fe(II) or Fe(III) concentrations.


Conclusions

Concentrations of Fe(II) or Fe(III) are an important factor in determining the valence of arsenic ion extracted from arsenopyrite under acidic conditions. The As(III) is oxidized to As(V) in the presence of arsenopyrite, but high concentrations of Fe(II) or Fe(III) suppress the oxidation, and reduce the As(V) concentration.


References

1) K. Sasaki, M. Tsunekawa, S. Tanaka, M. Fukushima, H. Konno: Shigen-to-Sozai, 115(1999), 233-239
2) S. Nakaya: Bunseki Kagaku, 12(1963), 241-24.