Environmental Engineering Reference
In-Depth Information
(a)
(b)
Aquaporins: cellular “water”
channels transport as many as
4 × 10
9
H
2
O·S
-1
figurE 23.3
Pure water is forced through the aquaporin Z (AqpZ) network under pressure [16]. (a) A depiction of interaction of water
molecules with AqpZ network and (b) the 3D view of the aqpZ.
23.4
fErritins
The biological systems in recent years have attracted much attention for inspiring research and development in water purifica-
tion. Ferritin is a cage-like iron-storing protein found in animals, plants, and microbial systems and is involved in the sequestra-
tion and storage of protein [18]. Ferritin is composed of self-assembled 24 polypeptide subunits; apo-ferritin, an iron-free
assemblage, has an inner cavity of approximately 8 nm diameter. When iron molecules are diffused into this cavity, they miner-
alize the apo-ferritin into nanoparticles of ferrihydrite, which is actually a ferric oxyhydroxide. Ferritin then becomes a potential
candidate for the remediation, and particularly the photoreduction, of contaminants such as toxic metals like chlorocarbons [19].
As we know, when iron oxide undergoes photoreduction from Fe (III) to Fe (II), it makes the catalyst inactive, and since ferritin
makes this conversion naturally, the encaged iron oxide is prevented from undergoing photoreduction, thus maintaining its sta-
bility. In addition to the conventional use of ferritin for the remediation and removal of contamination, a recent development is
to use it to synthesize nanoparticles. It is evident that both metallic and nonmetallic hydroxide particles can be synthesized using
ferritin. Recently, both iron and cobalt nanoparticles were synthesized using ferritin, which facilitates the assemblage of parti-
cles in a solution. After assembling, ferritin was dried on a solid support and cleaned with ozone to remove the protein cage. This
leaves well-dispersed nano oxide particles with a relatively high degree of control for particle sizes between 2 and 8 nm. Further
exposure of the particles to hydrogen and high temperature can be used to convert the metal oxides to metallic particles [20].
Water contamination with oxyanions (arsenate and phosphate) and metal ions over wide ranges of concentrations is a problem
for water and related systems that clean water. Eutrophication of surface water, caused by a high level of phosphate, is an envi-
ronmental issue. Further, arsenate (HAso
4
/H
2
Aso
4
) is a health- and life-threatening contaminant of drinking water in many
areas of the world, notably in Asia and also in parts of the United States and Europe. There are numerous techniques available
for the removal of phosphate and arsenate from water. In general, all the available techniques suffer from a low-affinity problem
and, therefore, are inefficient in the low-concentration range (below 5 ppb). In addition, current water purification installations
suffer from biofouling due to the accumulation of phosphate in the Ro systems at concentrations favorable for microbial growth.
Recently, this problem was addressed by using ferritin (hyperthermophilic protein nanocage), which can form and hold an iron-
based nanoparticle inside the protein. The nanoparticle thus formed is capable of adsorbing oxyanions with higher affinity even
below 1 ppb. The experimental program used a wide range of aqueous phosphate concentrations to monitor the phosphate
removal by ferritin. Concentrations ranged from 5 to 200 ppb. The specific ferritin concentrations were also varied. The approach
for the adsorption experiment was a standard batch liquid-phase equilibration. Radioactively labeled phosphate solution was
equilibrated with ferric iron-loaded ferritin solution. The system was left overnight for equilibration. The aqueous phosphate
solution and ferritin were separated using a column filter with a cutoff filter of 3 kDa. The
32
P concentration in the permeate was
measured using a liquid scintillation counter, thus producing the final aqueous phase equilibrium concentration [21, 22].
23.5
singlE EnzymE nanoParticlEs
Enzymes are the most versatile candidates for biosensing and bioremediation in areas of chemical conversions. only certain
limitations like short life span and lack of stability underscore their ability to provide cost-effective options. However, nowa-
days, advancements in technology like enzyme immobilization and genetic modification make them more suitable candidates
