Environmental Engineering Reference
In-Depth Information
20.2
Bioremediation
During the detailed study of nature's strategies involved in fabricating materials, one of the pivotal discoveries was the impor-
tant role of microbial activity in transforming elements in the periodic table, which is a result of assimilatory, dissimilatory,
or detoxification processes [21]. This process, also referred to as bioremediation, forms a cornerstone of many biogeochem-
ical cycles and typically involves a change in the oxidation state of at least one-third of the elements in the periodic table by
biological entities, and includes highly toxic elements such as Hg, cr, Se, Te, and As [22]. Microorganisms such as bacteria,
fungi, algae, and plants are known for their inherent ability to withstand high concentrations of toxic metal ions through
specific resistance mechanisms and therefore these are commonly employed toward the remediation of different metal ions
[23-25]. The detoxification by these resistance mechanisms typically occurs either by reduction/oxidation of metal ions or by
the formation of insoluble complexes that results in the intracellular accumulation of metal ion complexes/nanoparticles
[23-25]. Some of the earliest reports that outline the accumulation of inorganic materials date back to the 1960s when ele-
mental gold was reported in Precambrian algal blooms [26], algal cells [27] and bacteria [28]; cdS in bacteria [29] and yeast
[30], and magnetite in magnetotactic bacteria [31]. Although a microbiologist's interest purely lies in understanding the
biochemical processes involved in the resistance machinery, materials scientists are conversely fascinated by the ability of
these biological entities to make technologically important inorganic materials. This has opened up a new avenue of research
wherein biological organisms known for withstanding high concentrations of metal ions are deliberately explored as “nano-
factories” to achieve potential large-scale “green” synthesis of nanomaterials. This is similar to the commercially successful
fermentation technology, which is regularly employed for the large-scale synthesis of enzymes, drugs, etc., and has potential
to be employed for large-scale nanoparticle synthesis. Although a great deal of work has been performed in this emerging area
of bionanotechnology with organisms including bacteria, fungi, and plants being regularly employed for the synthesis of a
wide range of nanomaterials ranging from metals and metal oxides to metal sulfides, the biological synthesis (or biosynthesis)
of inorganic nanomaterials is still in its infancy phase, as the actual biochemical processes involved in the formation of inor-
ganic materials largely remain poorly understood [13]. The following sections will outline some of the biological entities
employed for nanomaterial biosynthesis.
20.3
metal nanoparticleS
The synthesis of metal nanoparticles especially that of noble metals is of enormous importance due to their interesting optical,
electronic, catalytic, and sensing properties [5, 32-39]. Although solution-based chemical routes have dominated the field of
nanomaterial synthesis, the recent past has seen an increasing trend in employing biological entities toward the synthesis of
metal nanoparticles. These biological entities includes bacteria, fungi, algae, plants, and plant extracts, each of which have their
own potential advantages and associated drawbacks in the context of the synthesis of nanoparticles [9-13]. The different
biological entities employed for the reduction of metal ions to their nanoparticulate counterparts are summarized in Table 20.1.
Some of the earliest studies in the field date back to the late 1970s, wherein gold was reported to interact with Precambrian
algal blooms and bacteria [26]. Following this report, a wide range of bacterial species were reported for their ability to form
nanoparticles. Beveridge and Murray showed the ability of
Bacillus subtilis
to accumulate gold nanoparticles of 5-25 nm in the
cell wall when incubated with precursor gold salt [28]. The organophosphate compounds secreted by the bacterium were believed
to play an important role in the reduction of gold ions [40]. Additionally, Beveridge and co-workers also showed the unique
ability of bacterial (
Pseudomonas aeruginosa
) biofilms toward intracellular precipitation of gold nanoparticles with particle
diameter of 20 nm [41]. Several groups have further shown the ability of
Shewanella
spp. (
S. algae
,
S. oneidensis
), a marine
bacteria belonging to
Proteobacteria
for the synthesis of gold and platinum nanoparticles [42-44]. Bacteria that belong to this
genus have been reported for their unique ability to reduce/oxidize metal ions efficiently as these organisms are found under
harsh environmental conditions [45].
S. algae
, an iron-reducing bacterium, showed the ability to reduce gold ions under anaer-
obic conditions that typically yields gold nanoparticles of 10-20 nm in diameter. This metal salt reduction ability of these metal-
reducing organisms through dissimilatory pathways was attributed to the presence of c-type cytochromes. A few other alternative
strategies were also hypothesized wherein the involvement of a species-specific hydrogenase when hydrogen was used as an
electron donor was speculated [42]. Additionally, another fascinating hypothesis proposed the expression of a gold reductase
enzyme in bacteria in response to gold ion stress that might be involved in the direct reduction of gold ions extracellularly
[42, 46]. However, these hypothesized biochemical mechanisms for the reduction of gold ions remains invalidated till date.
In a series of reports, lengke and co-workers investigated the ability of filamentous cyanobacterial strain
Plectonema boryanum
UTeX 485 for its ability to reduce metal ions (gold, silver, platinum, and palladium) [47-50]. Although this cyanobacterial
strain had an interesting ability to reduce all these metal ions to their metallic counterparts, the nanoparticles synthesized were
