Environment & Energy
Related: About this forumA Minor Problem With Visible Light Up-conversion to UV for Pollutant Destruction: It's a Myth.
Since the mid-20th century, a large amounts of seriously recalcitrant organic pollutants - most often organohalides - have been accumulating, sometimes at dangerous levels, particularly in the tissue of animals high in the food chain, including but not limited to human beings. One can read lots of papers about these molecules in breast milk (and the milk of other species) for example.
The long term persistence of these molecules is related to the strength of the chemical bonds that define them, and these in turn are defined, in quantum mechanical terms by a set of molecular orbitals, most often characterized mathematically as three dimensional wave functions. In quantum mechanics, the transition from one state (represented by its wave function) to another state (represented by a different wave function) requires a certain minimal energy, to wit: It is not possible for a large number of energy packets (quanta of light) to break a bond whose energy transition exceeds the energy associated with its wavelength/frequency.
The energy of light is of course, described by the famous Planck equation, which most people who take science courses encounter in high school, or in the worst case, early in their college careers:
An unfortunate feature of our times, wherever environmental issues are discussed, is sun worship - an enthusiasm for all things "solar" - that may hark back to some ancient religious fervor, I don't know, as I'm not a participant.
Scientists are not magical beings separate from humanity; they are pretty much like everyone else, and as such they can get caught up in popular fads, and run with ideas that prove to be nonsensical. Some years back, stuck in some airport somewhere and bored out of my mind with nothing to read, I came across a delightful little book all about these kinds of adventures in scientists kidding themselves, this one: Yes, We Have No Neutrons: An Eye-Opening Tour through the Twists and Turns of Bad Science. It's all about "Cold Fusion," "IQ testing" "N-rays" etc, etc, etc...
A fun read, take it as you will...
By the way, bad science does not always involve bad scientists. Highly intelligent and highly competent people can often mislead themselves terribly. Martin Fleischmann of "Cold Fusion" fame was throughout his career highly regarded as an electrochemist, the author of more than 250 scientific papers, which is why when he erroneously reported "cold fusion" he was taken - initially at least - seriously.
To return to the point of recalcitrant pollutants, it should be pretty clear that since they often appear in surface waters - the Hudson River has long been polluted with polychlorobiphenyls (PCBs) - solar energy doesn't do a very good job at decomposing them. If it did, they would tend to disappear, at least on sunny days in summer.
They, um, don't.
This is because visible light, although it is sufficiently energetic to break some chemical bonds - notably the hydrogen-oxygen bond in water, which is, of course, how photosynthesis works - is insufficiently energetic to break the (aromatic) carbon chlorine bond in a PCB.
At a minimum, higher frequency radiation is required to break these bonds, at a minimum short wavelength ultraviolet radiation, although x-rays and gamma rays are even better. Now even though humanity is working hard at destroying the ozone layer which absorbs and degrades UV radiation, it's a good thing that we can't have "solar remediation" of PCB's, since energy at the wavelength that destroys them would also destroy many of the molecules in our flesh; a fact that is connected with the ever rising rates of melanoma, a largely still incurable cancer that is involved in primitive or more sophisticated sun worship.
However the culture of enthusiasm of all things solar so much as it appears in science has lead many people to speculate that it is possible to "upconvert" visible wavelengths of light into UV light which is known to work, particularly in the presence of titanium dioxide catalysts to degrade recalcitrant pollutants.
I am hardly an expert in the theory of this stuff, by the way. In fact the first time I heard of it is in the paper from the primary scientific literature written by scientists at Yale and Clemson, that I will now discuss, which claims, that the whole affair is, in fact, a myth: The Myth of Visible Light Photocatalysis Using Lanthanide Upconversion Materials. (Cates et al, Environ. Sci. Technol., 2018, 52 (5), pp 29732980)
From the introduction, which covers the very real basics about titanium dioxide catalysts and the mechanism of their actions, and a brief summary of the controversy:
Here's a picture of the proposed mechanism that people reporting "upconversion" claim:
The caption:
In the paper the authors describe the synthesis of some of these putative upconversion catalysts and their characterization and the proposed mechanism under which they work.
The authors discuss the mechanism of upconversion, which by the way is according to them, as I understand it, a known phenomenon, but one which has very limited intensity:
The caption:
Anyway, they make the catalysts and then test them against a series of well known persistent organic pollutants, dyes used in the preparation of clothing.
They compare these unimpressive results with actual direct irradiation of "MB" methylene blue with UV radiation not resulting from "upconversion:"
In their discussion the authors write:
They offer some reasons for why they think that many of the papers on this subject are not reproducible or valid:
Finally, UC-PC authors rarely include UC emission spectra of the materials in their publications, which is certainly a minimum requirement for demonstrating UC capability prior to applying this phenomena to environmental technology. Studies unrelated to photocatalysis have shown visible-to-UV conversion by the same aforementioned Er3+-doped systems, though they used single crystals under pulse laser excitation with sophisticated detection systems.35?37,51 Furthermore, the UC spectrum of YSO: Pr3+ provided by Wu et al., as well as one in a more recent YSO: Pr3+ UC-PC paper, were measured with fluorescence spectrometers equipped with a xenon lamps.22,52 Based on our experience in characterizing YSO: Pr3+, cyan or blue laser excitation in excess of 50 mW and phase-sensitive detection are required to resolve the visible-to-UV anti-Stokes emission spectrum.13,26 Their spectra instead appear to be normal Stokes emission that resulted from unintentional UV excitation of the sample by second order diffraction from the source monochromator in the absence of appropriate long-pass filters. This same instrumental blunder has been implicated in false reports of UC by carbon quantum dots,53 and illustrates the importance of avoiding accidental UV excitation when attempting to measure upconverted visible light.
Too bad. One kind of wishes it worked. We could just dump titanium oxide doped with lanthanides in the Hudson River and be done with it, but we won't done with it.
Right now the PCB contaminated riverbed of the Hudson River is being dredged, but the real problem is that this is just moving a pollutant from one place to another: There is no safe place to dispose of the dredged soils really.
Speaking of another "solar will save us" degradation, the degradation of the planetary atmosphere, the vast "solar revolution" has been notoriously ineffective on that score as well. The Mauna Loa observatory seemed to be off line for a few days - I thought Trump and Pruitt had defunded it in my less than inappropriate paranoia, but it came back on line. The reported concentration on April 21, 2018 was 411.14 ppm which is, to my knowledge, the highest value ever measured there.
Be that as it may, we seem to be falling asleep worshiping the sun.
Metaphors aside, don't do it yourself. Melanoma is a tough disease: I lost two good friends to it.
Have a pleasant Sunday.