Abstract
Jarosite [(Na+, K+, NH4 +, H3O+)Fe3(SO4)2(OH)6] is an efficient scavenger for trace metals in Fe- and SO4 2--rich acidic water. During the biosynthesis of jarosite promoted by Acidithiobacillus ferrooxidans, the continuous supply of high oxygen levels is a common practice that results in high costs. To evaluate the function of oxygen in jarosite production by A. ferrooxidans, three groups of batch experiments with different oxygen supply levels (i.e., loading volume percentages of FeSO4 solution of 20%, 40%, and 70% v/v in the flasks), as well as three groups of sealed flask experiments with different limiting oxygen supply conditions (i.e., the solutions were not sealed at the initial stage of the ferrous oxidation reaction by paraffin but were rather sealed at the end of the ferrous oxidation reaction at 48 h), were tested. The formed Fe-precipitates were characterized via X-ray powder diffraction and scanning electron microscope-energy dispersive spectral analysis. The results showed that the biosynthesis of jarosite by A. ferrooxidans LX5 could be achieved at a wide range of solution loading volume percentages. The rate and efficiency of the jarosite biosynthesis were poorly correlated with the concentration of dissolved oxygen in the reaction solution. Similar jarosite precipitates, expressed as KFe3 (SO4) 2(OH)6 with Fe/S molar ratios between 1.61 and 1.68, were uniformly formed in unsealed and 48 h sealed flasks. These experimental results suggested that the supply of O2 was only essential in the period of the oxidation of ferrous iron to ferric but was not required in the period of ferric precipitation.
Highlights
Jarosite [(Na+, K+, NH4+, H3O+)Fe3(SO4)2(OH)6] is a ubiquitous Fe(III)-mineral in acid mine drainage (AMD), AMD impacted sediments, and coastal acid sulfate soils [1,2]
A continuous decrease in Fe2+ concentration was observed in all three groups of reaction systems, and the decrease rate increased with decreasing solution loading volume (Fig. 1B), which was attributed to the fact that the presence of higher levels of dissolved oxygen (DO) in the solution was favorable for the growth of A. ferrooxidans LX5 [20,26]
For 20% solution loading volume percentage, the DO concentration was always maintained higher than 6 mg/L in the whole ferrous bio-oxidation period, and 9 g/L of Fe2+ was oxidized completely within 30 h, which was equivalent to a Fe2+ oxidation rate of 300 mg/(LÁh)
Summary
Jarosite [(Na+, K+, NH4+, H3O+)Fe3(SO4)2(OH)6] is a ubiquitous Fe(III)-mineral in acid mine drainage (AMD), AMD impacted sediments, and coastal acid sulfate soils [1,2]. Many investigations focusing on the field of AMD have indicated that a number of metal contaminants (Cu, As, Cd, Ni, etc.) in AMD-polluted sites can be naturally attenuated by newly formed jarosite precipitates [9,10]. This material is an efficient mineralogical control on aqueous concentrations of As in As-contaminated water, such as AMD and As-bearing groundwater [11]. Given the high metal uptake feature of jarosite, the manner by which to synthesize this Fe(III)-mineral efficiently and economically has received a great deal of attention
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