Wednesday, August 25, 2010

Cleaning up the Pacific Ocean

It is out of the box idea time again. As noted many times here on Bastion of Reason, the ocean has a number of problems ranging from increasing acidity, increasing temperature, decreasing phytoplankton and decreasing oxygen concentration just to name a few. However, another problem, although not on the scale of the aforementioned problems, is the growing amount of plastic that is being inappropriately deposited in the ocean. In fact most people have probably heard about the growing ‘plastic graveyard’ in the Northern portion of the Pacific Ocean, otherwise commonly known as the Great Pacific Garbage Patch. For the duration of this post the Great Pacific Garbage Patch will be referred to as the Patch. Scientists that have studied this man-made monstrosity do not hold out much hope that it can become manageable in the near future as things currently stand.

The chief problem is that the advantages of plastics as a transport storage medium work as disadvantages in its disposal. The most effective way that plastics are broken down is through photodegradation. Photodegradation involves molecules breaking and contorting their molecular bonds as a result of photon absorption. The most common form of photodegradation reaction is oxidation. Unfortunately in effort to ensure a sufficient shelf-life most plastics are manufactured to include a blend of antioxidants which increases the total time before the plastic succumbs to photodegradation. In addition these antioxidants can interfere with plastic recycling. Add to that information the concern that it is highly probable the sheer size of the Patch also reduces the effectiveness of photodegradation by reducing the total time a given region of the plastic is exposed to enough light to initiate sufficient photodegradation; such a concern is especially true for smaller pieces of plastic. There is preliminary rudimentary evidence which demonstrates that certain species of Sphingomonas and Pseudomonas can degrade plastics like polystyrene and polyethylene, but it seems unlikely that even if these bacteria can do the job that the Pacific Ocean is an appropriate environment for their deployment leaving photodegradation as the best recourse.

Although there is significant uncertainty surrounding the environmental effects of the Patch, a comprehensive study, the British Antarctic Survey, was recently published in the Philosophical Transactions of The Royal Society B outlining some of the potential problems that could be caused by the Patch. For instance: 1. various impurities and valued additives could breakdown and contaminate local Pacific Ocean environments; 2. plastic debris typically breakdown to microscopic sizes that can then be ingested by marine animals shortening their lifespans and damaging progeny; 3. floating plastic waste can ride the ocean currents and become man-made vectors for invasive species;

Even if a new generation of plastics is developed that is either very easily photodegradable or easily recyclable (there are a number of prototypes for a higher class of recyclable plastics) and these new plastics actually capture a significant market share, if nature is left alone to deal with the problem of plastics by itself it will be hundreds of years until the Patch is eliminated, if at all. With the concern of the environmental damage illustrated above waiting hundreds of years for the demise of the Patch is unacceptable. Therefore, humans must intervene to hasten the elimination of the Patch. Unfortunately straight net/trawler collection of plastic appears to be incredibly inefficient, financially costly and time-consuming; also as long as ships continue to utilize fossil fuels their operation may prove more environmentally damaging than leaving the Patch to its own devices. With these restrictions, it seems necessary to device a technological strategy to reduce the size of the Patch. This post will present an idea that perhaps could evolve into an effective strategy to deal with the Patch.

First, because photodegradation is the principle means of plastic removal a quick review of the process is in order. Most people remember from their education that light can be divided into three general categories relevant to most human interactions due to energy levels: infrared (IR), visible and ultra-violet (UV). With respect to plastic-based photodegradation only UV light is relevant because the bonds between the atoms in many plastics have dissociation energies similar to the quanta of UV light. UV light is typically divided into three classifications: UVA (320-400 nm wavelengths), UVB (280-320 nm wavelengths) and UVC (100 to 280 nm wavelengths) [remember that energy and wavelength tend to be inversely proportional]. However, due to the protective effects of the ozone layer no UVC and only a very significantly reduced amount of UVB reach the surface of the Earth. The general threshold of plastic degradation varies based on the type of plastic (typically 300 to 370); so exposure to UV light on the ocean surface is not guaranteed to facilitate degradation resulting in the slower degradation rates seen in areas like the Patch. Side note: fluoropolymers are almost impossible to degrade naturally due to the strength of the C-F bond and the lack of UVC light.

The first step of photodegradation is initiation. Initiation begins with the absorption of UV light at a sufficient energy to break a given chemical bond in the main polymer chain of the plastic in question (example: generic CH3-CH3 bonds typically break at >= 340 nm). Free radicals are generated from bond breakage along with a hydrogen atom holding on to an unpaired electron. The second step is propagation, a fast moving free radical catalytic process. The free radicals produced through initiation can react with oxygen forming a peroxy radical which can then react with a hydrogen atom attached to the polymer chain forming a hydroperoxide and another free radical. The hydroperoxide can then split into two more free radicals which can further breakdown the polymer backbone of the plastic. The final step is termination. Termination typically occurs when there are so many free radicals relative to polymer components that the free radicals neutralize themselves by binding to each other making inert products. Free radical binding is common in termination over stabilizer neutralization (removal of free radicals via pre-treated antioxidants) because most of the antioxidants are eliminated in the initial stage of propagation.

The main problem with photodegradation seems to be a lack of enough high-energy particles to begin initiation. Basically the supply of plastic entering the Patch heavily outpaces the amount of plastic that is fully broken down by photodegradation. Clearly removing the ozone layer to allow more UVB and UVC light to reach Earth is an irresponsible and stupid idea, so any strategy to increase the number of high-energy particles must occur inside the ozone layer. Due to the potential damaging nature of high-energy particles, it would make the most sense to highly localize them around the Patch. Unfortunately direct application of UV light could create a number of problems.

First, while adding UV light would significantly increase the probability of photodegradation initiation, it could also increase cellular damage to various aquatic life in the region. However, the depth of this potential damage is unclear. Second, the intense focus of UV light on a concentrated region could increase ocean temperatures in that region which will weaken bio-diversity as well as reduce the ability of the region to retain gases including oxygen and carbon dioxide. Third, there is unclear information to whether or not such a strategy would even work on the more problematic plastic pieces (those that are small enough to be ingested by aquatic life).

With the problem of manual collection and what could be termed ‘enhanced’ photodegradation having problems, the best option may be an autonomous collection system. The most important element when considering the design of such a system would be to ensure the safety of aquatic lifeforms. Fortunately the most troublesome pieces of plastic are micro to nano-sized so a vast majority of aquatic life should be spared if the proper filter systems are used. The idea of the collection unit is to situate itself under various portions of the Patch allowing water to flow down through the initial filter. Once inside the unit, a pair of vacuums on either side of the upper portion of the unit will pin the plastic pieces against two respective walls. After a certain period of time after the water in the upper portion of the unit is drained out naturally through a smaller set of filters and the collection walls will give way forcing the collected plastic down into two side containers.

Once the plastic is in the side containers one of the two strategies can be executed. First, no additional action can be taken and the plastic can accumulate until the entire collection unit is extracted from the ocean and the plastic can be removed manually. Second, a set of UV emitting LEDs can be situated along the walls near the bottom of each of the side units and can eliminate the plastic through localized photodegradation. If the second option is used then power considerations for the LEDs must be made.

One hopeful means of power generation is through the natural removal of the water, generating a miniature hydroelectric plant. If this natural water flow is not sufficient or plausible then powering the device will be a little trickier. Using a lithium battery could potentially add too much weight to the device pushing it deep enough into the ocean where it could not longer be recovered. Using solar cells may be difficult depending on the total unit depth as the cells would have to be positioned above the ocean surface. Using more conventional batteries in series may generate more waste than the collector removes creating a net loss.

Another important element to consider when creating this type of device is how it will influence the behavior of phytoplankton. For the above proposed device most aquatic life is undisturbed by the device due to the small filter size, but there is a possibility that phytoplankton could be caught up in the device as well. Now the margin of error in the phytoplankton population is not zero because taking no action against the Patch will also have a seriously adverse effect on phytoplankton population, so if a small population is lost in the process of clean-up there is still a net benefit. Unfortunately there is no real definitive way to know what type of influence such a device would have on phytoplankton populations. The best solution seems to devise an additional element within the device that can bind the plastic, but not the phytoplankton.

Overall if the Great Plastic Garbage Patch and its other lesser plastic garbage patch ‘associates’ are going to be removed, the most viable strategy at this point seems to require a generally autonomous collection/degradation unit that can eliminate the plastic while limiting any damage to existing aquatic life. The figure below outlines a crude vision of what the above device may look like –

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