Environmental concerns often require finding a balance between environmental and performance concerns, as illustrated in the following case study:
Use Prudent Specifications with Titanium Dioxide
Michael Chusid, RA FCSI CCS – Letter to Editor,
Vol.17 No.7 www.buildinggreen.com/op-ed/use-prudent-specifications-titanium-dioxide
I read with interest your article on photoactive titanium dioxide (TiO2) in concrete [see EBN Vol. 16, No. 5]. I have been following the product launch of the material for several years, have published articles on the technology, and have spoken about the material at an American Concrete Institute's nanotechnology conference. My research has satisfied me that the material performs as the manufacturer claims with regards to the removal of organic grime (but not inorganic stains) from the surface of concrete and the depollution of nearby air.
White concrete at Jubilee Church, Rome
I note one inaccuracy in your article. Due to the small size of the particles and their low density on the surface of the concrete, TiO2 does not affect the visual properties of the concrete. The whiteness of the Jubilee Church results not from photoactive TiO2 but from white portland cement, white marble aggregate, and metakaolin—a bright white pozzolanic concrete additive.
CONCERNS
Despite my enthusiasm for photoactive concrete, I have some environmental concerns with the current generation of photoactive TiO.
First, does photoactive TiO2 pose a threat to microorganisms when it enters into surface water or groundwater? The class of TiO2 used as a pigment—rutile—has a record of safe use. The class of TiO2 used in photoactive products, however—anatase—is comparatively rare in nature, and the behavior of its nano-sized particles in ecosystems has not been fully studied.
The catalytic characteristics of the material do not diminish with time.. Consider what could happen, therefore, if a photoactive structure is demolished and concrete from the structure is used as riprap on the shore of a shallow sea. As the concrete erodes, what damage could be done when photoactive particles settle onto coral polyps or other vulnerable species?
I know this risk seems blown out of proportion in light of the product's proven benefits and given the small use of photoactive compounds today. But consider that the compounds are already finding inroads into many products worldwide—including disposable consumer products.
Must we wait for another Silent Spring before establishing guidelines?
RECOMMENDATIONS
Prudence suggests that concrete producers and the project team should minimize the release of photoactive material into the environment. At the very least, a designer should discuss the risks with the building owner. Specifications should require the concrete supplier and the contractor to dispose of waste TiO2 in a manner that will not contaminate the environment. And a permanent plaque should be mounted on the building notifying future generations that the concrete may require special handling upon demolition.
We also need to study the byproducts of depollution. As photoactive TiO2 removes pollutants, it forms new chemical compounds. While these chemical byproducts are considerably less environmentally hazardous than the pollutants they replace, there is no such thing as a free lunch in a closed ecosystem, and the potential runoff from a photoactive surface should be studied for environmental impact.
Thirdly, the photocatalytic reaction can draw calcium out of the calcium silicate hydrate (CSH) that is the cementitious component of concrete, accelerating erosion. This degradation is estimated to be “just a fraction of a millimeter per decade,” a rate that can amount to a fraction of a centimeter per century and could be especially visible at corners or other surface relief. Adding metakaolin to the concrete mixture enriches the CSH content of the concrete and may retard erosion. Moreover, the erosion of the concrete can also release photoactive compounds into the environment.
I remain optimistic that photoactive titanium dioxide is a valuable tool for making the environment cleaner and healthier. This goal, however, will be achieved only if we make informed tradeoffs between environmental benefits and risks.
******************UPDATE **********************
The first comment below draws our attention to a recent study by the EU* that finds nano "TiO2 nanomaterials with the characteristics as indicated below, at a concentration up to 25% as a UV-filter in sunscreens, can be considered to not pose any risk of adverse affects in humans after application on healthy, intact or sunburnt skin." While tightly worded and qualified, it sounds like a positive assessment.
The report, however, DOES NOT ADDRESS MY CONCERNS. My blog post concerns environmental impacts of construction use of anatase TiO2, the highly photocatalytic type; the EU report considers human health, regards TiO2 that "are mainly the rutile form" and taht "do not have a substantially high photocatalytic activity." It still leaves unaddressed the question I posed about corals and other vulnerable species.
There are many differences between human health and environmental health. One is that a building will stand for many years while consumer products like sunscreens can be removed distribution channels quickly. The report points out that, "If any new evidence emerges in the future... then the SCCS may consider revising this assessment." It is not so easy to revise a building or highway.
If you are the one that posted the report, please contact me to discuss this topic further.
*Scientific Committee on Consumer Safety (SCCS)
Opinion on Titanium Dioxide (nano form), COLIPA No. S75, 22 July 2013.