Remediation and Selective Recovery of Metals from Acidic Mine

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Remediation and selective recovery of metals from acidic mine waters using novel modular bioreactors Sabrina Hedrich, and D. Barrie Johnson Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es5030367 • Publication Date (Web): 24 Sep 2014 Downloaded from http://pubs.acs.org on October 3, 2014

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Remediation and selective recovery of metals from acidic

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mine waters using novel modular bioreactors

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Sabrina Hedrich1,* and D. Barrie Johnson

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School of Biological Sciences, College of Natural Sciences, Bangor University, Deiniol Road,

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Bangor LL57 2UW, U.K.

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2, 30655 Hanover, Germany

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current address: Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg

*Corresponding author: e-mail: [email protected], Tel +49 0511-6423187

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ABSTRACT

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Mine waters are widely regarded as environmental pollutants, but are also potential sources

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of valuable metals. Water draining the Maurliden mine (Sweden) is highly acidic (pH 2.3) and

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rich in zinc (~460 mg L-1) and iron (~400 mg L-1), and contains smaller concentrations (0.3 -

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49 mg L-1) of other transition metals and arsenic. We have developed novel techniques that

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promote the concurrent amelioration of acidic waste waters and selective recovery of metals,

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and have used these systems to treat synthetic Maurliden mine water in the laboratory. The

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two major metals present were removed via controlled biomineralization: zinc as ZnS in a

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sulfidogenic bioreactor, and iron as schwertmannite by microbial iron oxidation and

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precipitation of ferric iron. A small proportion (~11%) of the schwertmannite produced was

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used to remove arsenic as the initial step in the process, and other chalcophilic metals

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(copper, cadmium and cobalt) were removed (as sulfides) in the stage 1 metal sulfide

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precipitation

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biomineralization units can be effective at processing complex mine waters and generating

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metal products that may be recycled. The economic and environmental benefits of using an

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integrated biological approach for treating metal-rich mine waters is discussed.

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Keywords: Acid mine drainage; bio-mineralization; bioremediation; iron oxidation; sulfate

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reduction; metal recovery, metal recycling

reactor.

Results

from

this

work

have

demonstrated

that

modular

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INTRODUCTION

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Waters draining abandoned metal mines and mine wastes are often acidic (pH 99.9% of the

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arsenic present could be successfully removed within 2 hours residence time, by adding 80

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mg (dry weight) of the mineral per liter of synthetic mine water (Table S1, Figure S1). This

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amount of schwertmannite was equivalent to ~11% of that produced from synthetic water in

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module I.

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Selective precipitation of transition metal sulfides. Hydrogen sulfide was produced in the

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sulfidogenic bioreactor (stage 2 metal sulfide precipitation) in excess of that required to

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precipitate zinc (as described below). This was transferred to the stage 1 metal sulfide

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precipitation vessel in a N2-gas stream, at a rate of 4.5 µmoles h-1 (calculated from the mean

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flow rate and the pH differential between that within the bioreactor and the liquor that flowed

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into it, as described below). Over 99.9% of cadmium and copper in the synthetic mine water,

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together with ~50% of nickel and ~3% of zinc, were precipitated (as sulfides) in the stage 1

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metal sulfide precipitation vessel, but no removal of manganese or cobalt was detected

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(Table S1, Figure 3).


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The final stage in the mine water treatment protocol involved precipitating zinc (as ZnS)

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within the acidophilic sulfidogenic bioreactor vessel (stage 2 metal sulfide precipitation;

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Figure 1). By maintaining the pH within this reactor at 4.0, co-precipitation of aluminum (e.g.

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as basaluminite) and manganese, both of which were still present in the part-processed

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water at similar concentrations to those of Maurliden mine water, was avoided (data not

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shown). Some members of the acidophilic sulfidogenic consortium had been found in

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previous work to be sensitive to aluminum11, and therefore this metal was added

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incrementally to the test liquor. As shown in Figure 4a, increasing the aluminum

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concentration to 81 mg/L caused the performance of this module to decline initially (indicated

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by slower inflow rates and less complete biomineralization of ZnS) but the consortium quickly

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adapted to tolerate aluminum concentrations of up to 132 mg L-1 (the concentration in

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Maurliden mine water).

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Sixteen days after the biosulfidogenic module had been operating in continuous flow mode,

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analysis of liquor inside the reactor showed that, although 93% (6.5 mM) of the glycerol in

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the influent liquor had been oxidized, 5 mM acetic acid was present (Figure 4b). This was

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due to the dominant aSRB in the sulfidogenic consortium at that time being

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Desulfosporosinus M1 (Figure S2a) which is known to be an “incomplete oxidizer” of

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glycerol, generating equimolar concentrations of acetic acid as a waste product17. Although

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the concentration of acetic acid declined (to 3.4 mM) by day 20, the presence of such large

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concentrations of acetic acid implied that the efficiency of H2S production was far less than

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what was desirable. To alleviate this problem, the acidophilic heterotroph Acidocella

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aromaticaT,

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previously been shown to grow in syntrophic culture with Desulfosporosinus M1, converting

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acetic acid into hydrogen and carbon dioxide, and that the hydrogen so-generated was used

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as a secondary electron donor by the sulfidogen18. The introduction of Ac. aromatica PFBC

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resulted in concentrations of acetic acid within the bioreactor to be lowered and maintained

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at 98% of

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that provided) with no loss in the efficiency of the module, in terms of both zinc precipitation

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and production of net alkalinity.

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T-RFLP analysis of the acidophilic sulfidogenic consortium within module II on day 16 (Figure

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S2a) indicated that the dominant bacteria were both aSRB: Desulfosporosinus sp. M1 (70%

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relative abundance) and strain CEB3 (24% relative abundance), a firmicute previously

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detected in the acidophilic sulfate-reducing consortia used in these experiments11. The other

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bacterium detected in module II with minor abundance (6%) was the facultatively aerobic

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chemolithotroph Acidithiobacillus ferrooxidans (~5% relative abundance). A T-RFLP profile

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obtained at day 63 (Figure S2b), after Ac. aromatic PFBC had been added to the consortium,

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showed that, although all three of the previously detected bacteria were still present, their

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relative proportions had changed (38% Desulfosporosinus M1, 30% Ac. aromatica PFBC,

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24.5% strain CEB3 and 1.5% At. ferrooxidans). One minor T-RF, of 130 nt (6%) length was

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not identified.

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DISCUSSION

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Mine water generated as a waste product at the Maurliden mine contains relatively high

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concentrations of two metals (iron and zinc), and smaller concentrations of several other

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cationic metals, as well as anionic arsenic. The objectives in these trials were to recover zinc

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and iron as potentially saleable products and to remove arsenic and most of the other metals

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present in the mine water as a remediation strategy. The integrated bioreactor modules were

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highly effective in meeting these objectives. Zinc was removed from the synthetic mine water

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as ZnS, from which the metal could be recovered, as is the case at the Budel zinc refinery in

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The Netherlands19.The much lower price of iron on the commodities market would suggest

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that making a bio-mineral (schwertmannite) that has potential value both as a pigment and

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as an adsorbent of (anionic) pollutants20,

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is a more commercially-viable alternative for 11

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recovering and recycling this metal. This was illustrated to some extent in the present study,

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where ~11% of the schwertmannite produced was used to remove arsenic upstream of the

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bioreactor modules, thereby concentrating this toxic metalloid rather than allowing it to co-

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precipitate as a diffuse toxin in a sludge or spent compost. Therefore schwertmannite

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harvested from module 1 can easily be applied in immobilized beds to adsorb arsenic from

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mine waters in a similar setup as described by Janneck et al.21.

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The water generated from the bio-processing system described was moderately acidic (pH

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4.0) and contained only aluminum and manganese as residual (non basic) metals. Removing

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these two metals was not an objective of the current research program, but further controlled

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pH amelioration of the mine water (e.g. by addition of sodium hydroxide) would allow both

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metals to be selectively precipitated, e.g. as basaluminite (Al4(SO4)(OH)10*4-5H2O) and

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rhodochrosite (MnCO3). Manganese can also be removed using microbially-catalyzed

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oxidation of Mn(II) to Mn(IV) and mineralization of MnO2 22.

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The rationale of the module configuration used in the present trials was dictated by the mine

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water chemistry. About 50% of the soluble iron in Maurliden mine water is present as ferric

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iron. If the sulfidogenic bioreactor was used as the first treatment module, much of the

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hydrogen sulfide generated would be used to reduce this to ferrous iron, generating

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elemental sulfur as a co-product (2Fe3+ + H2S → 2Fe2+ + S0 + 2H+). This would both be

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wasteful of the electron donor (glycerol) used to generate H2S, and also induce unnecessary

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iron cycling. This necessitates the oxidation of the ferrous iron in the mine water which, at the

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pH of the mine water, needs to be microbially-mediated.

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The iron oxidation/precipitation module was highly effectively in generating relatively pure

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schwertmannite, with most (>99.9%) of the iron present in the synthetic mine water

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precipitated as schwertmannite. Residual ferrous iron could, if required, be removed using a

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“polishing” packed bed bioreactor, as described by Hedrich and Johnson9. Soluble iron would

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not, in any case, be precipitated in the sulfidogenic bioreactor as the pH is too low for this11.



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Even so in this study the iron-oxidation module was operated under sterile conditions, this

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would not be possible in large scale applications. Studies in our laboratory using the same

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setup but operated under non-sterile conditions, however, shown that ferrous iron was

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oxidized with the same efficiency and the microbial streamer community was still dominated

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by “Fv. myxofaciens” but accompanied by another iron-oxidizer Acidithiobacillus ferrooxidans

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and the heterotroph Acidiphilium sp.23.

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The sodium hydroxide used to precipitate the schwertmannite in module I has not only the

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advantage of lower reagents costs, compared to lime (calcium oxide) frequently used in

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neutralization of acidic mine waters, but also avoids formation of gypsum in the high sulfate

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mine waters and is more compatible for the microorganisms in the sulfidogenic system

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(module II).

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The low pH sulfidogenic bioreactor is a novel and integral component of the

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bioremediation/metal recovery system described, as it serves both to capture transition

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metals (as sulfides) and to increase mine water pH. It has an empirical operational design,

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whereby acidic mine water is pumped into the bioreactor at a rate required to maintain pH

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homeostasis, achieved using a pH electrode and meter coupled to a pump11. Most of the

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hydrogen sulfide generated was used to precipitate ZnS in the bioreactor vessel itself, in a

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theoretically pH-neutral reaction (the alkalinity of sulfate reduction being counterbalanced by

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protons generated by the reaction between Zn2+ and H2S, equation 1b). Production of small

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amounts of H2S in excess of that required to precipitate Zn2+ (“free H2S”) allowed the pH of

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the reactor to be poised above that of the mine water (a pre-requisite of the modus operandi

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of the module). This free H2S was used to remove copper and cadmium (also as sulfides)

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upstream of the main sulfidogenic reactor in another metal sulfide precipitation stage,

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thereby avoiding co-precipitation within and contamination of, the main ZnS product. The

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very small solubility products of cadmium and copper sulfides (log Ksp values of -28.9 and -

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35.9, respectively10) mean that these precipitate at lower pH than many other metals,

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including cobalt, nickel and zinc. However, the much larger concentration of zinc than other 13 ACS Paragon Plus Environment

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chalcophilic metals resulted in some formation of small concentrations of ZnS in the pH 3.2

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stage 1 metal sulfide precipitation vessel, even though its log Ksp value (-24.5) is greater than

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those of cadmium and copper. The amount of free H2S generated by the sulfidogenic reactor

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was controlled by the flow rate and the pH differential between the influent liquor and that

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within the reactor. For the trial period, the pH differential (3.2 vs. 4.0) was equivalent to 430

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µM H+, and the mean flow rate was 47.5 ml h-1. Since net proton consumption was due to

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H2S production (7SO42- + 4C3H8O3 + 14H+ → 7H2S + 12CO2 + 16H2O) this was equivalent to

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4.53 µmoles of free H2S being generated h-1. Over two days, 76 µmoles of divalent transition

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metals (2 µmoles Cd, 25 µmoles Cu and 49 µmoles Zn) were precipitated in the stage 1

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metal sulfide precipitation bottle, whereas ~217 µmoles of H2S were calculated to have been

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generated in the same time period.

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The flow rates of mine water through the aerobic and anaerobic modules, which were of

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similar working volumes, were very different (equivalent to a HRT of 1.96 H in the iron

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oxidation/precipitation reactor and 41.7 H in the sulfidogenic bioreactor) since rates of iron

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oxidation were much greater than those of sulfate reduction. This would necessitate the

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sulfidogenic bioreactor being much larger than the schwertmannite-generating module in a

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full-scale system. Based on the chemical data listed in Table 1, treating 1 m3 of Maurliden

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mine water h-1 would require a 2 m3 iron oxidation bioreactor (and a similar sized

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schwertmannite mineralization reactor) coupled to a 42 m3 sulfidogenic reactor. The reagent

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costs (m-3 mine water) would be ~ $0.60 for sodium hydroxide and ~ $0.50 for glycerol. The

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value of the zinc in the ZnS product is ~ $0.80 (on the basis that 0.46 kg can be recovered

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from each m3 of mine water). Schwertmannite (~0.69 kg produced m -3 mine water) could be

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sold, e.g. as a pigment, though the commercial value of this product is unpredictable. The

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production of materials that have commercial value can therefore offset, at least in part, the

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cost of the treatment process. Remediation of mine waters, such as that at the Maurliden

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mine, using conventional chemical treatment, and disposal of the mixed metal sludges

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produced has a substantial financial cost and inherent environmental risk (long-term

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remobilization of metals and metalloids). Using an integrated biological process like that 14 ACS Paragon Plus Environment


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described can, by generating separate mineral products, eliminate most if not all of the costs

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and risks involved in disposing of mine water waste products.

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Although described here for application to a particular mine water, the modular systems

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described can be modified and configured to optimize remediation and metal recovery in

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mine waters with very different chemistries, and represents an alternative, more

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environmentally-benign and sustainable approach to mine water treatment for the 21st

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century.

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ACKNOWLEDGEMENTS

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This work was carried out in the frame of ProMine (European project contract NMP-2008-

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LARGE-2:# 228559). SH and DBJ acknowledge the financial support given to this project by

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the European Commission under the Seventh Framework Program for Research and

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Development, and to help give by colleagues at Boliden AB. We would like to thank Nils-

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Johan Bolin (Boliden) for his advice and helpful comments.

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SUPPORTING INFORMATION

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The Supporting information contains two additional figures presenting the results of arsenic

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adsorption onto schwertmannite and T-RFLP analysis of the sulfidogenic bioreactor

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consortium.

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REFERENCES

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(1) Nordstrom, D.K. Advances in the hydrogeochemistry and microbiology of acid mine

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waters. Int. Geol. Rev. 2002, 42, 499 515.

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(2) Langdahl, B. R.; Ingvorsen, K. Temperature characteristics of bacterial iron solubilisation 15 ACS Paragon Plus Environment

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C assimilation in naturally exposed sulfide ore material at Citronen Fjord, Greenland

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(83°N). FEMS Microbiol. Ecol. 1997, 23, 275 283.

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(3) Johnson, D. B. Chemical and microbiological characteristics of mineral spoils and

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drainage waters at abandoned coal and metal mines. Water Air Soil Poll: Focus 2003, 3, 47

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(4) Schippers, A.; Breuker, A.; Blazejak, A.; Bosecker, K.; Kock, D.; Wright, T.L. The

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biogeochemistry and microbiology of sulfidic mine waste and bioleaching dumps and heaps,

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and novel Fe(II)-oxidizing bacteria. Hydrometallurgy 2010, 104, 342 350.

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(6) Tabak, H.H.; Govind, R. Advances in biotreatment of acid mine drainage and biorecovery

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biotreatment of acid mine drainage and biorecovery of metals: 1. metal precipitation for

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recovery and recycle. Biodegradation 2003, 14, 423 436.

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the selective recovery and recycling of metals from acid mine drainage and mine process

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bio-oxidation of iron and precipitation of schwertmannite from mine-impacted waters.

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Bioresource Technol. 2012, 106, 44 49.

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(10) Stumm, W., Morgan, J. J., 1996. Aquatic chemistry: chemical equilibria and rates in



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waters by novel consortia of acidophilic sulfidogenic bacteria. Microbial. Biotechnol. 2012, 5,

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34 44.

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(12) Hedrich, S.; Schlömann, M.; Johnson, D.B. The iron-oxidizing Proteobacteria.

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Microbiology 2011, 157, 1551 1564.

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(13) Lovley, D.R.; Phillips, E.J.P. Rapid assay for microbially reducible ferric iron in aquatic

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sediments. Appl. Environ. Microbiol.1987, 53, 1536 1540.

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(14) Lane, D. J. 16S/23S rRNA sequencing. In Nucleic acid techniques in bacterial

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characterization of acidophilic microorganisms isolated from a pilot plant stirred tank

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bioleaching operation. Appl. Environ. Microbiol. 2003, 69, 1936 1943.

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(17) Jones, R. M.; Hedrich, S.; Johnson, D. B. Acidocella aromatica sp. nov.: an acidophilic

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heterotrophic alphaproteobacterium with unusual phenotypic traits. Extremophiles 2013, 17,

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841-850.

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(18) Kimura, S.; Hallberg, K. B., Johnson, D. B. Sulfidogenesis in low pH (3.8 – 4.2) media by

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Table 1. Concentration of the major components of Maurliden mine water (data obtained

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from Boliden AB, Sweden). Analyte

Concentration

Analyte

[mg/L]

Concentration [mg/L]

Zn

464

Cd

1.0

Fe (II)

200

Co

0.4

Fe (III)

203

Ni

0.3

Al

132

Ca

271

Mn

49

Mg

123

Cu

7.7

Na

13.8

As

1.3

K

4.0

446


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Figure 1. Schematic representation of the integrated system used to remediate synthetic

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Maurliden mine water and to recover iron and zinc as mineral products.

450



451 452 453 454 455 456

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Figure 2. Changes in (a) pH (•) and concentrations of ferrous (□) and ferric iron (■), and (b) of aluminum (◊), copper (∆), manganese (●) and zinc (▲) during passage of synthetic Maurliden mine water through the iron oxidation/precipitation module I. Effluent (i) is water draining the iron oxidation bioreactor and effluent (ii) is water draining the schwertmannite precipitation vessel. The hydraulic residence time in both vessels was 1.96 H.

457 458 459 460


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461 462 463 Mn

464 Cu

465 Cd

466

Co

467

Ni

468 469 470 471 472 473 474 475

Figure 3. Changes in concentrations of transition metals in synthetic Maurliden mine water in the stage 1 metal sulfide precipitation vessel receiving H2S generated in module II. Zinc is not shown, but declined from 464 to 450 mg L-1 during the 2 day experiment.



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476 477 478

(a)

479 480 481 482 483 484 485 486 487 (b)

488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503

Figure 4. Changes in (a) flow rates (□) and concentrations of soluble zinc (●), and (b) concentrations of glycerol (●) and acetic acid (□), in the sulfidogenic bioreactor (module II). The downward-pointing arrows indicate addition of aluminum: (i) 1 mM, (ii) 2 mM, (III) 3 mM), (iv) 4 mM, (v) 5 mM. The upward-pointing arrow indicates the addition of an active culture of Acidocella aromaticaT to the sulfidogenic bioreactor.

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Remediation and Selective Recovery of Metals from Acidic Mine

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