Environmental Engineering Reference
In-Depth Information
family
Desulfobacteriaceae
for the synthesis of sphalerite (ZnS) particles [128]. This complex ZnS biomineralization was found
to significantly reduce the concentration of Zn below acceptable levels for drinking water suggesting a cheaper and eco-friendly
alternative for water purification. In addition to cd, Zn, and Pb sulfides, iron sulfide nanoparticles belong to an important class
of materials known for their magnetic properties. Incidentally, in the biological world, iron sulfide is naturally found in strains of
magnetotactic bacteria [129, 130]. There have been reports using other bacterial systems for the synthesis of iron sulfide nanopar-
ticles that are within the permanent single magnetic domain size. Very recently, Bharde et al. have shown the ability of actinomy-
cete
Actinobacter
sp. for its ability to synthesize magnetic iron sulfide nanoparticles extracellularly under aerobic conditions
[113]. These nanoparticles were synthesized with ferric ions in the presence of exogenous sulfate source and typically encom-
passed Fe
3
S
4
and FeS
2
of 20 nm diameter. The important role of bacterial sulfate reductases for the conversion of sulfate into
sulfide was postulated with several low molecular weight proteins found to be involved in the stabilization of these particles.
20.6
metal carBonate nanoparticleS
earlier strategies of metal carbonates synthesis focused on using a biomimetic approach, wherein an external source of cO
2
along with metal ions was employed in the presence of different biomacromolecules, which react to form carbonate
nanoparticles [9]. Although biomimetic processes have resulted in different mineral polymorph and morphological control, a
biosynthesis approach was proposed to overcome the necessity for an external cO
2
source. Sastry's group, in addition to
engaging fungus and actinomycete for the synthesis of metal, metal oxide, and metal sulfide nanoparticles, also tailored these
organisms for the extracellular synthesis of a range of carbonate materials including cacO
3
, BacO
3
, SrcO
3
, cdcO
3
, and
PbcO
3
. This was achieved as these organisms are known to produce cO
2
as a by-product during the organisms' active aerobic
metabolism process. The versatility of these organisms in synthesizing different kinds of nanoparticles is quite fascinating and
intriguing. Fungi including
F. oxysporum
[131-134],
Verticillium
sp. [135],
Trichothecium
sp. [131], and actinomycete including
Rhodococcus
sp [132] and
Thermomonospora
sp [135] were employed for the synthesis of the aforementioned carbonate
species. Interestingly, these carbonates displayed different crystal phases with different morphologies ranging from flat circular
to cubic to star-shaped depending on the fungus/actinomycete employed for the synthesis. The variability in morphology and
crystal phases could be attributed to the production of species-specific proteins that might be responsible for the synthesis of
these materials. The important fact that the synthesis of these carbonates minerals proceeds via endogenous production of cO
2
makes this biosynthesis approach a truly biogenic method for synthesizing biominerals. The different biological entities
employed for the biosynthesis of different biominerals are summarized in Table 20.4.
20.7
BioleachinG: a trUly “Green” BioloGical approach
The aforementioned synthesis routes for the biosynthesis of technologically important nanomaterials are fascinating, but in
most cases an external metal ion chemical source is essential. In contrast, bioleaching is a tool promoted by algae, mosses,
lichens, plants, animals, actinomycetes, a variety of bacteria, and a few fungi in their natural habitats for the low-cost extraction
of various metals [136-138]. Bioleaching employing microorganisms have been used at a commercial level for the recovery of
metals like gold, copper, and iron. Although bioleaching approaches are employed for metal recovery, this approach was not
originally investigated in the context of nanoparticle formation.
taBle 20.4
list of biological entities employed for biominerals synthesis
Microorganism
Biominerals
References
Fungi
Verticillium
sp.
BacO
3
, cacO
3
[135]
Trichothecium
sp.
cacO
3
[131]
Fusarium oxysporum
cacO
3
, cdcO
3
, PbcO
3
, SrcO
3
[131-134]
Actinomycetes
Rhodococcus
sp.
cacO
3
[132]
Thermomonospora
sp.
BacO
3
and cacO
3
[135]
