254
release kinetics of silver ions. The presence of common ligands reduces the bioavailability of silver ions and mitigates its toxicity [334]. A recent study found that low
concentration (sublethal) of silver ions or nano-Ag enhances E. coli growth, suggesting a hermetic response that could be counterproductive to its antimicrobial applications [335].
CNTs kill bacteria by causing physical perturbation of the cell membrane, oxidative stress, or disruption of a specific microbial process via disturbing/oxidizing a
vital cellular structure/component upon direct contact with bacterial cells. Graphene
and graphite materials exhibit antimicrobial properties through similar mechanisms
[191, 313]. The cytotoxicity of CNTs strongly depends on their physicochemical
properties. Short, dispersed, and metallic CNTs with small diameters are more toxic
[147, 148, 313].
Potential Applications in Water Treatment
Antimicrobial nanomaterials are envisaged to find their applications in three critical
challenges in water/wastewater systems: disinfection, membrane biofouling control, and biofilm control on other relevant surfaces. Nano-Ag has good potential for
application in POU treatment. It can improve water quality for high-end use, or
provide another barrier against waterborne pathogens for vulnerable population.
Commercial devices utilizing nano-Ag are already available, e.g., MARATHON
®
and Aquapure
®
systems. Nano-Ag has also been incorporated into ceramic microfilters as a barrier for pathogens, which can be employed in remote areas in developing
countries [260].
The antimicrobial properties, fibrous shape, and high conductivity of CNTs
enable novel CNT filters for both bacteria and virus removal: The thin layer of
CNTs effectively removes bacteria by size exclusion and viruses by depth filtration;
the retained bacteria are largely inactivated by CNTs within hours [34]. With a small
intermittent voltage (2–3 V), MWNTs can directly oxidize attached bacteria and
viruses and lead to inactivation in seconds [269, 312]. The applied electric potential
also enhances viral transport to the anodic CNTs [269]. Such CNT filters can be
used as high-performance POU devices for water disinfection with minimal to no
power requirement.
The application of nanomaterials in membrane biofouling control is detailed in
section “Membranes and Membrane Processes.” They can also be used in other
water treatment-related surfaces such as storage tanks and distribution pipes to control pathogen contamination, biofilm formation, and microbial influenced corrosion.
Affordable coating techniques that can economize nanomaterial use and maximize
its efficacy while allowing for regeneration are in critical need. An alternative
approach is to employ nanoscale biofouling-resistant surface structures, a strategy
used by marine organisms (dolphins and sharks) and plants (lotus leafs). A common
disadvantage of many nanomaterial-enabled disinfection approaches is the lack of
disinfection residue, which is crucial for controlling microbial growth during water
storage and distribution. Nevertheless, nanotechnology-enabled disinfection can
13 Wastewater
release kinetics of silver ions. The presence of common ligands reduces the bioavailability of silver ions and mitigates its toxicity [334]. A recent study found that low
concentration (sublethal) of silver ions or nano-Ag enhances E. coli growth, suggesting a hermetic response that could be counterproductive to its antimicrobial applications [335].
CNTs kill bacteria by causing physical perturbation of the cell membrane, oxidative stress, or disruption of a specific microbial process via disturbing/oxidizing a
vital cellular structure/component upon direct contact with bacterial cells. Graphene
and graphite materials exhibit antimicrobial properties through similar mechanisms
[191, 313]. The cytotoxicity of CNTs strongly depends on their physicochemical
properties. Short, dispersed, and metallic CNTs with small diameters are more toxic
[147, 148, 313].
Potential Applications in Water Treatment
Antimicrobial nanomaterials are envisaged to find their applications in three critical
challenges in water/wastewater systems: disinfection, membrane biofouling control, and biofilm control on other relevant surfaces. Nano-Ag has good potential for
application in POU treatment. It can improve water quality for high-end use, or
provide another barrier against waterborne pathogens for vulnerable population.
Commercial devices utilizing nano-Ag are already available, e.g., MARATHON
®
and Aquapure
®
systems. Nano-Ag has also been incorporated into ceramic microfilters as a barrier for pathogens, which can be employed in remote areas in developing
countries [260].
The antimicrobial properties, fibrous shape, and high conductivity of CNTs
enable novel CNT filters for both bacteria and virus removal: The thin layer of
CNTs effectively removes bacteria by size exclusion and viruses by depth filtration;
the retained bacteria are largely inactivated by CNTs within hours [34]. With a small
intermittent voltage (2–3 V), MWNTs can directly oxidize attached bacteria and
viruses and lead to inactivation in seconds [269, 312]. The applied electric potential
also enhances viral transport to the anodic CNTs [269]. Such CNT filters can be
used as high-performance POU devices for water disinfection with minimal to no
power requirement.
The application of nanomaterials in membrane biofouling control is detailed in
section “Membranes and Membrane Processes.” They can also be used in other
water treatment-related surfaces such as storage tanks and distribution pipes to control pathogen contamination, biofilm formation, and microbial influenced corrosion.
Affordable coating techniques that can economize nanomaterial use and maximize
its efficacy while allowing for regeneration are in critical need. An alternative
approach is to employ nanoscale biofouling-resistant surface structures, a strategy
used by marine organisms (dolphins and sharks) and plants (lotus leafs). A common
disadvantage of many nanomaterial-enabled disinfection approaches is the lack of
disinfection residue, which is crucial for controlling microbial growth during water
storage and distribution. Nevertheless, nanotechnology-enabled disinfection can
13 Wastewater