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ENVIRONMENTAL IMPACT OF NANO MATERIALS

Effects in the environment

The reality of the presence of nanomaterials in our surroundings is not a herald of possible environmental hazards. The propensity towards toxicity as far as nanomaterials are concerned revolves around certain properties such as:

1. Size
2. Geometry
3. Surface properties (charge, surface chemistry)
4. Crystal structure
5. Chemical composition

It has been established that mundane climatic dispositions/parameters could affect the properties of said nanomaterials and consequently alter their bioavailability, hence potential toxicity/hazard. These environmental parameters include but are not limited to salinity and pH value. The current trend in establishing the eco-toxicity of nanomaterials focuses on those with simple structures, some of which have been in circulation for several years now.

The results of these studies have drawn attention to the effect on aquatic life forms although recent developments have brought sediment/soil dwelling organisms to the spotlight. Most nanomaterials currently under studies are reported to exhibit little to no toxic effect on short-term exposure. However the presence of nanomaterials containing silver or zinc oxide is tantamount to high acute toxicity to aquatic life-forms mainly due to their ability to release ions. Thereby, additional effects caused by the particles cannot be excluded. Certain photo-catalytically active forms of titanium dioxide (TiO2) show increased toxicity in laboratory tests under the influence of simulated sunlight. Other studies have brought to the limelight the fact that exposure to some nanomaterials in aquatic life-forms leads to sub-lethal effects such as gill damage and tissue/organ malformation. As if it didn’t suffice, exposure could also culminate in change in behavior such as:

1. Modified energy budget
2. Altered feeding patterns
3. Increased flight behaviours

Due to the inorganic nature of most nanomaterials currently in circulation, it goes without saying that its inability or reticence towards bio-degradation only fuels its persistence in the environment. Hence if we are to take into account the peculiarities and complexities attributed to nanomaterials in view of objectively assessing their ecological toxicity, we must focus solely on the results of short-term exposure. Nanomaterial prototypes that have been the subject of most studies aimed at establishing eco-toxicity after long-term exposure include:

1. Silver (Ag)
2. Titanium Oxide (TiO)
3. Zinc Oxide (ZnO)

The exposure of certain soil and water dwelling invertebrates such as water fleas and nematodes resulted in the paucity of progeny and increased mortality. However such studies have not yet been carried out with vertebrates such as birds and fish. The paucity of data as it relates to the eco-toxicological effects of some nanomaterials is due to the discrepancies/difficulties associated with the methodologies adopted. Some studies present data that proves that no toxic effects thrive while others proffer an opposing point of view. Some findings also propose the alteration of some physiological processes such as reproduction rate in either direction. Studies involving plants have proven the latter’s ability to translocate nanomaterials that have also been reported to influence the germination process of their hosts.The negative effects of titanium oxide (TiO) on soil dwelling organisms have also been reported.

The eco-toxicological effects of most nanomaterials are not just zoned around the direct ones; the indirect effects are also a reality. For instance the common trait amongst nanomaterials being the ability to adsorb to surfaces spells doom for aquatic life forms such as fish. Aquatic photosynthetic plants such as algae could be shielded from ultra-violet light due to the adsorbed nanomaterials. They could also adsorb organic substances which could in turn contribute to the uptake of toxic substances by indigenous organisms.

Although the threatening downsides of nanomaterials are quite glaring, we still face a serious road block in the form of the challenge in adequate assessment of environmental hazards caused by them. A comparison of the many studies on hazard assessment is made more difficult because the development of uniform specifications on application into the test systems and test performance is still under development.

Release into the environment

The advent of nanomaterial use has been followed by several fields of applications for each and every type of nanomaterial. As mentioned earlier on the existence of nanomaterials is not as recent as it sounds. This further buttressed by the fact that nanomaterials in the form of Titanium dioxide (TiO 2 ) and Silicon dioxide (SiO 2 ) have for decades now be produced in tons. Notwithstanding nanomaterials such as carbon tubes and quantum dots are pretty recent. The first step in actually developing a credible/reproducible method for assessing the environmental consequences of nanomaterials, demands that we first establish the sources of said materials and their fate or release pattern over the product life cycle from:

1. Production
2. Use
3. Transport
4. Recycling and
5. Waste disposal

Nanomaterials gain entrance into the environment by either the mechanical wearing-out of coatings or leakage from sunscreens into water bodies during recreational activities. The decontamination of wastelands and water treatment schemes could also contribute to the quantity of nanomaterials that find themselves in the environment. The form in which most nanomaterials come in (final product) make it so that the nanoparticles are embedded in fragments of the product. However it is still a subject under great speculations as to whether these fragment are further degraded in the environment to release the nanomaterials.

Scientific investigations on model water treatment plans have revealed that 90 % of nanomaterials are retained in sewage sludge. It calls for precautions since most agriculturists rely on sewage sludge as a source of natural fertilizer/manure. However as mentioned previously the effects these materials have on the soil and the organisms that dwell therein are not yet crystal clear. Notwithstanding in view of the potential health threats that this poses, the use of such sewage sludge should be prohibited.

It then goes without saying there is the lack of both subjective/qualitative and objective/quantitative data to foster the development of a scheme targeted at eco-toxicological assessment of nanomaterials and the regulation of its use/application. Behaviour and persistence in the environment The mainstream nanomaterials are inorganic in nature and as result are resilient to biodegradation. Certain processes affect/influence the behavior of these nanomaterials once they gain access into the environment. These processes include:

1. Agglomeration
2. Hetero agglomeration
3. Dissolution
4. Adhesion to surfaces
5. Adsorption of substances

These processes are in turn influenced by the properties inherent to nanomaterials such as size, geometry and surface properties and also environmental parameters such as pH and salinity. Agglomerations point towards the attraction between individual nanoparticles due to electrostatic and steric forces. Heteroagglomeration refers to the attraction between the nanomaterials and environmental particles. Over time the density of these agglomerates become high enough that they settle out of the environment.

Released nanomaterials can be transformed in the environment by the following process reduction, oxidation and loss of protective coatings by either mechanical or biological degradation. This transformation most often than not alters: bioavailability, mobility and by extension the toxicity of said nanomaterials. Evidences to support the hypothesis of bioaccumulation and persistence of nanomaterials in living organisms have been obtained. The organisms considered in the studies include invertebrates such as earthworms and water fleas; vertebrates such as fish were also considered. Although bioaccumulation was proven, the findings reveal that the level is minimal. This is due to good but incomplete excretion.

Certain findings have confirmed the uptake and accumulation of nanomaterials, in plants. These findings have also revealed the translocation of nanomaterials within a food chain. This is because of the incomplete elimination of the nanomaterials at the start of the food chain. The pool of information pertaining to the behavior and persistence of nanomaterials has accrued over the years. Be that as it may the usefulness of these data is hampered due to the fact that most of the studies are not founded on reproducible methodologies, so that comparisons are difficult. There is a need for standardised methods that take into account the specific processes for the description of the environmental behavior.