Overview of nanomaterials for cleaning up the environment

Researchers have presented an extensive analysis of the role of nanomaterialsin environmental remediation and monitoring. Nanomaterials can be used to clean up toxins and bacteria from natural waters, wastewaters and the air (see paper in Energy & Environmental Science: “A review on nanomaterials for environmental remediation”).
Nanomaterials’ unique properties allow them to remove pollutants from the environment. The extremely small size of nanomaterial particles, typically in the range between 1 and 100 nanometres, creates a large surface area in relation to their volume, which makes them highly reactive, compared to non-nano forms of the same materials.
Silver, iron, gold, titanium oxides and iron oxides are some of the commonly used nanoscale metals and metal oxides cited by the researchers that can be used in environmental remediation. Silver nanoparticles, for example, have proved to be effective antimicrobial agents and can treat wastewater containing bacteria, viruses and fungi. Nanoscale titanium dioxide can also kill bacteria and disinfect water when activated by light.
Gold nanoparticles may potentially be another useful material for removing contaminants, such as toxic chlorinated organic compounds, pesticides and inorganic mercury, from water. They can also be used to remediate air. In combination with titanium dioxide, goldnanoparticles have been shown to convert the toxic air pollutant, sulphur dioxide, to sulphur. Titanium dioxide nanomaterials are commonly used in some processes to disinfect water, in addition to breaking down halogenated compounds, and removing dyes and metal toxins from drinking water and wastewater.
The researchers point to studies that show that carbon nanomaterials are particularly suited to removing a broad range of pollutants. Carbon nanotube clusters, for example, are used to purify water by adsorbing bacteria that contaminate the water. Heavy metals, such as cadmium, as well as organic pollutants including benzene and 1,2-dichlorobenzene can also be removed from water by carbon nanotube materials.
The researchers suggest that nanoparticles can be attached to host polymer materials, such as porous resins, cellulose and silica, to reduce potential harm to human health and the environment derived from the release of nanoparticles into the environment. The nanoparticles fixed to the host material are thus bulkier and can be more easily removed and captured from wastewater. Nanoparticles, such as nanoscale zinc oxide, fixed in this way, are used, for example, to break down organochlorine pesticides, halogenated herbicides and azo dyes.
In addition to remediating pollution, nanoparticles can be used as sensors to monitor toxins, heavy metals and organic contaminants in land, air and water environments and have been found to be more sensitive and selective than conventional sensors. Sensor strips composed of nylon 6 nano-fibre nets are one example. These are used to detect formaldehyde, a toxic air pollutant widely used in the manufacture of household materials and building products. The yellow sensor strips turn red upon exposure to formaldehyde.
The researchers acknowledge that ongoing work is needed to further improve the shape, sizes, structures, functionality and manufacture of nanomaterials that show promise in cleaning up contaminants that enter water, land and air environments from industries, transport and other human activities. A better understanding of the behaviour of nanomaterials and their potential harm to the environment is also required.

Particle length-dependent titanium dioxide nanomaterials’toxicity and bioactivity

Titanium dioxide (TiO2) nanomaterials have considerable beneficial applications varying from additives in paint, paper, plastics and cosmetics to uses in photocatalysts, solar cells and medical materials and devices. It has been established for many years that pigment-grade TiO2 (200 nm sphere) is relatively inert when internalized into a biological model system (in vivo or in vitro).

For this reason, TiO2 nanomaterials are an attractive alternative in applications where biological exposures will occur. Unfortunately, metal oxides on the nanoscale (one dimension <100 nm) may or may not exhibit the same toxic potential as the original material.

A further complicating issue is the effect of modifying or engineering of the nanomaterial to be structurally and geometrically different from the original material.

Results: TiO2 nanospheres, short (15 um) nanobelts were synthesized, characterized and tested for biological activity using primary murine alveolar macrophages and in vivo in mice. This study demonstrates that alteration of anatase TiO2 nanomaterial into a fibre structure of greater than 15 um creates a highly toxic particle and initiates an inflammatory response by alveolar macrophages.

These fibre-shaped nanomaterials induced inflammasome activation and release of inflammatory cytokines through a cathepsin B-mediated mechanism. Consequently, long TiO2 nanobelts interact with lung macrophages in a manner very similar to asbestos or silica.

Conclusions: These observations suggest that any modification of a nanomaterial, resulting in a wire, fibre, belt or tube, be tested for pathogenic potential.

As this study demonstrates, toxicity and pathogenic potential change dramatically as the shape of the material is altered into one that a phagocytic cell has difficulty processing resulting in lysosomal dysruptiion.