Plastic Waste in the Aquatic Ecosystem

Plastic now accounts for 10% of all waste generated, with global use exceeding 260 million tons per annum [14]. Plastics waste has accumulated in the environment at an uncontrollable rate where it is subjected to wind and river-driven transport ultimately reaching the coast. Owing to its light weight and durable nature, plastic has become a prevalent, widespread element of marine litter [2,15]. The difficulty in eliminating plastics waste is due to the fact that it does not biodegrade in nature but only photodegrades into smaller pieces.

Because of frequent inappropriate waste management practices, or irresponsible human behavior, large masses of plastic items have been released into the environment and have consequently entered the world’s oceans. This process continues and, in some places, is even increasing [4]. Even if plastics are found deep in land, they eventually find their way to the sea or ocean through rivers and streams [16]. The release of plastics into the environment is a result of inappropriate waste management, improper human behavior or incidental pollution [14]. Sources of plastic waste into the aquatic ecosystem include municipal plastic waste (sachet water film bags, etc.), commercial plastic waste (packaging materials, etc.), industrial plastic waste (polyethylene film wrapping, spare parts for cars, PVC pipes and fittings, etc.) and plastic waste from ships [17,18].

Most polymeric materials that enter the environment are subjected to degradation that is caused by a combination of factors, including thermal oxidation, photooxidative degradation, biodegradation and hydrolysis [19]; see Table 2.2. Figure 2.3 shows the pathways and modes of movement for plastic in the marine environment. The common plastics found in the marine environment, however, do not biodegrade and primarily break down through photooxidative degradation. Furthermore, unlike plastics exposed on land, exposed plastics floating on the oceans’ surfaces do not suffer from heat buildup due to absorption of infrared radiation, and the reform barely undergo thermal oxidation [20].

The degradation of negatively buoyant plastics depends on very slow thermal oxidation, or hydrolysis, as a result of most wavelengths being readily absorbed by water. Hence, plastics residing in marine environments degrade at a significantly slower rate than they do on land. Polymer degradation can be categorized as any physical or chemical changes resulting from environmental factors, including light, heat, moisture, chemical conditions and biological activity [19]. It is not clear just how long plastic items remain in their original form. However, some plastic items appear to be broken up into smaller and smaller fragments over time [21]. At sea, this process is thought to occur because of wave action, oxidation and ultraviolet light. On the shore, plastic may break up into smaller

TABLE 2.2 Possible Degradation Routes of Synthetic Polymers

Requirements

Photo-Degradation

Thermal/Thermo- Oxidative Degradation

Biodegradation

Active ingredients

UV-light/high-energy radiation

Heat and/or oxygen

Microbial Agents

Heat requirement

None

Above ambient temperature

None

Degradation rate

Slow initiation, fast propagation

Fast

Moderate

Source: From Cooper DA. Effects of chemical and mechanical weathering processes on the degradation of plastic debris on marine beaches. PhD Thesis, The University of Western Ontario, Ontario, 2012.

Schematic diagram indicating the pathways and modes of movement for plastic in the marine environment, (a) Beaches, (b) coastal waters and sediments and (c) open ocean

FIGURE 2.3 Schematic diagram indicating the pathways and modes of movement for plastic in the marine environment, (a) Beaches, (b) coastal waters and sediments and (c) open ocean.

pieces owing to grinding from rocks and sand [22]. The resulting plastic fragments may be mistaken for prey and ingested by marine organisms. Plastic debris in the oceans may eventually be broken up so much that it becomes microscopic in size like grains of sand, hence the term microplastics. These tiny fragments (about 20 pm in diameter) have been identified in marine sediments and in ocean waters [23].

Various studies have demonstrated occurrence of marine debris worldwide, with plastic making up the major proportion of marine debris. Reports from some of these studies are summarized in Table 2.3. Though the methods were not assessed to ensure that the results were comparable, Table 2.3 clearly indicates the predominance of plastics among the marine litter, and its proportion consistently varies between 60% and 80% of the total marine debris [24].

Green arrows indicate wind-blown debris, gray arrows display vertical movement through the water column including sea floor deposition and burial, black arrows point to ingestion and entanglement by marine organisms and red arrows indicate waterborne debris [25].

 
Source
< Prev   CONTENTS   Source   Next >