CHAPTER 8: GENERAL DISCUSSION AND PERSPECTIVES
244
sulfuric acid and subsequent lowering the pH. Likewise, this mechanism has
also a partial role
when microbially-derived metabolites assist in the process. These organic compounds may
lead to local acidification of leachate and metal removal from the solid phase. However, the
complexation mechanisms plays even more prominent role when organic molecule is present.
Proton detachment from organic molecule creates binding sites of high affinity to different
metals. Complexation phenomenon leads then to transfer of metal from insoluble form
incorporated in slag into soluble organic complexes present in aqueous leachate. However, the
mechanisms of primary importance depend on microbial organisms employed to assist as
leaching catalyst. A prime example of organic complex affecting stability of poorly soluble
silicates is siderophore compound. Buss et al. (2007) demonstrated enhanced dissolution of
silicates when exposed to siderophores. Likewise, Torres et al. (2014) demonstrated that Fe-
binding siderophores can accelerate dissolution of olivine compared to abiotic rates. Breaking
silica network of silicate minerals may be attributed to selective microbial iron acquisition
when nutrient availability from other sources is otherwise limited (Rogers & Bennett, 2004).
An important evolvement has been made towards understanding the interactions of microbial
organisms with minerals. It is known that the extend of degradation varies from one mineral
to another and is also specific to deteriorative potential of microorganism acting. Moreover,
minerals can serve as energy source for microorganisms or play a vital role in nutrient
acquisition. Demonstrated so far potential of bioleaching unfurls opportunities of slags
treatment and is therefore worth of evaluating.
The objective of the part of dissertation focusing on dynamic and efficiency of metal leaching
with the assistance of bacteria (Chapter 7) is to ensure future management of slags in
environmentally friendly manner. Moreover, such an approach is of strong importance for
satisfying metal demand of modern day society.
Since bioleaching process depends upon number of factors, preliminary study of bio-leaching
process embraced consideration of two crucial parameters including particle size and pulp
density on metal leaching. Two different bacterial strains were implemented as the leaching
agents in order to make a comparison between acidic and circumneutral bioleaching.
Acidithiobacillus thiooxidans and
Pseudomonas fluorescens were found to have different
strength of contribution to metal extraction, whereby
A. thiooxidans demonstrated much
higher efficacy as compared to
P. fluorescens. The results clearly indicated that
implementation of bacteria to metal leaching plays a catalyzing role and is promising for
potential recovery of valuable elements remaining in slags (Figure 8.9). An interesting
outcome of this study is that reduced particle size of the slag which is expected to increase
their contact with leaching solution is not always appropriate. Granulated (amorphous) slag
was found to exhibit higher leachability when larger particle size was used. That was
attributed to neutralization capacity of the smaller fraction of slag which likely created
unfavorable conditions for bacteria and hindered its performance. The reduction of fraction
CHAPTER 8: GENERAL DISCUSSION AND PERSPECTIVES
245
size had nevertheless a good implication for crystalline (historical) slag. The explanation for
this phenomenon is that the main mineral phases hosting Cu are Cu-sulfides. Although
present in low volumetric proportions compared to silicates, they carry major part of Cu. An
encapsulation of these sulfides between the crystals of fayalite and within glassy matrix
makes necessary to reduce particle size, hence increase the sulfides exposure leaching
solution.
The results obtained in this part of the thesis point out the possibility of application of the
bioleaching process for slags. That approach would allow to remove metals from the slags and
consequently enable their use for a variety of applications afterwards.
A. thiooxidans was
found to efficiently catalyze metal extraction. The gainful outcome of the extraction
highlights perspectives arising from the process.
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