Post by Roger Clifton on Apr 7, 2019 12:55:06 GMT 9.5
I read an article recently that said that a survey showed that the plastic sitting on the ocean floor is only about a quarter of the plastic that went into the ocean. The article indicated that it may be good news, implying that the stuff is being mopped up somehow on the way down. It could be being dissolved, oxidised or digested.
Whereas dissolution would be largely a function of the surface area, oxidation would vary according to the chemistry of the polymer, and digestion would vary with its toxicity. So I guess a analysis of what plastic reached the ocean floor, or more exactly, what failed to reach the ocean floor, would tell us something about where it went.
There is another possibility – that it is being eaten, broken up and expelled undigested. That would mean that the stuff is remaining in suspension in the oceans, cycling endlessly through scavengers' diets. But at least its surface area would have been increased in the process, and its other modes of decay accelerated.
On land, termites have been known to munch their way through PVC insulation. In this case, they are not after the vinyl chloride monomer, but after the plasticiser, typically phthalate. Plasticiser use has changed over the years, as presses and extruders have become more powerful, but they are still used in different forms in different plastics – and in different products.
A smooth surface would present little opportunity for a microbe or crustacean to eat into, breaking up the object. That presupposes that the bug can digest it, as one might expect of polythene and other CH or CHO polymers. Or just the plasticiser in the case of certain termites. There is a lurking hazard that the biosphere will evolve a widespread means of breaking down the vinyl chloride monomer. In the first instance that would mean widespread collapse of electrical installation and greywater piping, in the environments favouring the bug concerned. Further, the possibility that they will excrete the relatively indigestible C-Cl bond into the atmosphere presents a direct threat (as Freon did) to the ozone layer.
Post by Roger Clifton on Apr 8, 2019 10:40:11 GMT 9.5
Got it. Your link connects me to the abstracts, dense with information...
The most common plastic, polythene (or more correctly, "polyethylene") ages with time by a crystallisation process that must open up cracks into the interior, because the outgassing of short chains (methane, ethene, ethane) increases with the ageing. Polypropylene chains also leach out, suggesting that colloidal dissolution may be significant decay pathway for all immersed plastics. Skin weathering is due primarily to exposure to sunlight, countered partially by UV blockers lingering in the artefact. So yes, plastics do decay. Eventually.
Colloidal dissolution may well be the process that keeps soils clean of charcoal in forested areas that are frequently burned over. Similarly, it casts a time limit on how long "soil sequestration" schemes can keep biochar buried.
Crystallisation to denser forms is aided by pressure, so would be expected to be faster at depth.
Post by Roger Clifton on Apr 17, 2019 10:05:23 GMT 9.5
It says... "bio-based plastics could also drive down emissions. ... the material itself carbon-neutral, although manufacturing still generates a small amount of greenhouse gasses (sic).... Ultimately, Suh and Zheng found that replacing fossil-based energy with renewable sources had the greatest impact on plastic's greenhouse gas emissions overall. Transitioning to 100% renewable energy — a purely theoretical scenario, Suh concedes — would reduce emissions by 51%"
"Ultimately 51%"? I would have thought that the word "ultimately" should lead us to an ultimate scenario, where 100% noncarbon energy is used to manufacture goods from 100% bio-plastic. But he did not say "noncarbon", he said "renewable", which inevitably ends up being backed by carbon-based gas. It would seem that the author is surrounded by an audience that will not tolerate anything less than 50% carbon-based fuel, dooming us to 50% emissions. However, are they all really so intolerant of researchers saying otherwise?
Post by David B. Benson on Apr 18, 2019 15:41:55 GMT 9.5
What's happened to all the plastic rubbish in the Indian Ocean? 2019 Apr 17 Phys.org
There doesn't seem to be a so-called garbage patch in the Bay of Bengal. The Southern Indian Ocean is blown over to the South Atlantic Ocean, where there is a small gyre. The authors from University of Western Australia are left with a mystery.
The first step in this process regenerates dimethylterephthalate monomer by reaction of PET with methanol. The regenerated DMT precipitates out when the temperature is lowered. I did a bit of digging and found that PET is made by reacting DMT with ethylene glycol, producing MeOH as a byproduct. It appears that this could allow complete recycling of PET, not just conversion into fuels.
...this could allow complete recycling of PET, not just conversion into fuels.
Before this or that plastic can be recycled, it must be sorted from the municipal waste stream. Currently the sorting is done almost entirely by human hand, with a necessary collapse of efficiency, limiting recycling to only the most valuable plastic/glass/etc. However this might be one of the most useful applications of AI (artificial intelligence).
If the waste stream is crushed, then a series of sensors might supply the characterising information to the AI, so that it may choose which side stream each fragment should go. Thus the different plastics and other materials might form a reliable source of raw materials for recycling.
Before this or that plastic can be recycled, it must be sorted from the municipal waste stream.
In regions with bottle laws, much of this is already done for PET. The Tomra and other machines accept bottles, crush them and put them into bins. What you get is a pre-sorted supply of slightly dirty PET, plus whatever the labels and bottle caps are made of (HDPE and/or PP for the caps, from what I dug up).
It seems likely to me that the methanol reaction also yields ethylene glycol. It looks like the initial reaction would be MeOH + PET bottles in, MeOH + DMT + glycol solution plus other solids out. HDPE and PP appear to be immune to methanol and will separate by filtration. The HDPE/PP can be recycled separately, the DMT precipitates when the temperature is lowered, and the MeOH and glycol separate from residual soda by distillation.
Hmmm... residual soda is sugary. If you can strip the MeOH well enough to avoid inhibiting yeast you might have a product suitable for fermentation. Voila, a fuel stream (albeit a very small one).
What's happened to all the plastic rubbish in the Indian Ocean?
There is no doubt that flotsam is routinely blown south-west clean across the span of the Indian Ocean. Wreckage from the missing Malaysia Airlines flight MH 370 that crashed off Australia has been found in several places across the southern African coast and islands on the way. (1)
The same strong tradewinds drive the Agulhas current (2) close along that coast and past the Cape, where its momentum (not IMHO wind) carries it a short distance into the Atlantic. It is undercut by the colder and denser eastbound Antarctic Circumpolar Current (3). The latter is the world's strongest current, its eastward momentum drawing water up from the entire ocean column and the Ekman force (a Coriolis effect referred to in the paper (4)) driving a near-surface flow northwards into each of the three great ocean basins. At a conference I heard that if the Current were magically stopped, it would take 400 years to rebuild its momentum. The Current is maintained by the relentless winds of the Roaring Forties and Fifties, raising a sea that I imagine would fragment and sink any plastics entering it.
The paper is behind a pay wall, but its abstract is accessible (4). I'm particularly interested to hear of research from The University of Western Australia as it has given me two degrees. Similarly the nearby Curtin University, which also has a strong oceanographic section. Both focus on the Indian Ocean, including the search for the MH 370.
Strong acids are widely used in industrial chemistry. Concentrated H2SO4 travels well by railroad tank car. HNO3, HCl and HF find uses all over the place.
I find the diagram in the article a bit unclear but it looks like the acid is a proton donor which reverses the polymerization reaction by turning the amine group into ammonium. It's likely that the acid can be regenerated and recycled once the ammonium salts are separated.
If the application of the plastic does not require acid resistance, it looks like this could be the thermosetting resin counterpart to the methanolysis recycling of PET.
Post by Roger Clifton on May 17, 2019 12:28:58 GMT 9.5
The Great Ocean Conveyor Belt is a flow of surface water that en route, passes from the North Pacific Ocean, through Indonesia, into the northern Indian Ocean. Close to the equator, the trade winds then blows some of it eastward. Consequently, one would expect that Cocos and Keeling Islands (south-west of Sumatra) would collect plenty of North Pacific flotsam.
Much of the flow through Indonesia passes through the Lombok Strait. Forty-odd years ago I visited the beach at Lombok Strait, more or less to pray at the Wallace Line. The beach had collected hulks of palm tree trunks, many coconuts in various stages of sprouting and dying, and the broken timber that was thrown off cargo ships in those days. About five years ago I visited the same beach for the same reason. However, this time it was littered with human jetsam, mainly plastic bottles with their lid still on, labelled in Japanese, Korean, Chinese, Bahasa, Thai, Tagalog and English. But no Hindi.
I would bet that the plastic found on Cocos Island originates from the North Pacific rather than the Indian Ocean rim.