DENSITY SEPARATION OF BFR PLASTICS THROUGH SINK/ FLOTATION

An excellent analysis of the use of the sink float method of separation using polymer density is described in an analysis of chemical additives as barriers to recycling of plastics in the informal sector in India. The study confirms that the sink/float method of separation for pre-shredded plastics has a high degree of effectiveness, and is the only method that has minimal occupational health concerns. Other methods, such as the Beilstein test which involves applying hot copper wire to plastic fragments to see if the flame glows green (halogen positive), can generate potentially harmful emissions and are not considered environmentally sound for this reason.

Haarman and Gasser investigated methods already employed by the informal sector in India to separate different polymer groups. They found freshwater can be used to separate polyolefins (PE, PP) from other plastics, and that water/ethanol mixtures can separate plastics with densities lower than freshwater (e.g. PE from PP). For brominate flame retarded plastics, salt solutions (e.g. with NaCl) can be used to separate polymers that have much higher densities (e.g. ABS/HIPS/PP20 from heavier plastics). They also describe how sink/float baths of different densities can be arranged in series to obtain several homogenous fractions.

They report that when a bath solution with a density of 1.08-1.10 g / cm3 is used, the floating fraction of plastics will be free of plastics where BFRs have been intentionally added during manufacture. It may not segregate plastics that have been contaminated with trace levels of BFRs as a result of the use of contaminated recycled plastic in their production. Those plastics that sink are virtually all contaminated with BFRs as an intentional production additive. Some other heavier plastics can also end up in the polymers that sink including PVC, PC, and PET due to overlap in the density of the polymers, but carefully establishing different solution densities in subsequent baths can then separate these plastics into homogenous polymer groups.

The investigation of Indian informal sector techniques by Haarman and Gasser are largely supported by laboratory studies on flotation density separation of brominated fractions of European WEEE plastics. Sink/float density separation techniques are not 100% effective but do have high rates of recovery and separation that may be applicable in countries with large informal sectors, limited resources for relatively expensive optical separator systems, and large stockpiles of plastic waste that must be sorted. The range of optical and frequency sorting technologies and vendors are not limited to those discussed above, which are presented as examples of different approaches that are available.

POPs IN MARINE PLASTIC LITTER

One of the more unusual categories of POPs-contaminated plastic waste is marine plastic litter. Numerous studies have demonstrated the occurrence of adsorption of persistent organic pollutants onto marine plastic litter. The mechanism is via the inherent hydrophobic nature of POPs which wish to move from a marine or aquatic matrix toward one where oils, fats, and hydrocarbons are present. The petrochemical basis of most plastics means that they are particularly vulnerable to lipophilic POPs adsorption. The adsorption of POPs onto the surfaces of plastic at up to one million times the concentration

Figure 30. Density and solution application for different polymers.

in marine water has been identified as a significant new source of POPs contamination of the food chain.

‘Pellet Watch’ is a unique scientific undertaking by Dr. Hideshige Takada at the Tokyo University of Agriculture and Technology.

Dr. Takada and his team organize volunteers around the world to collect small plastic pellets from beaches according to a uniform protocol. Dr. Takada explains, “Plastic resin pellets are small granules generally with shape of a cylinder or a disk with a diameter of a few mm. These plastic particles are industrial raw material transported to manufacturing sites where “user plastics” are made by re-melting and moulding into the final products.” Because of dumping, spills, and the ubiquitous nature of plastic manufacture these pellets are now distributed in oceans across the globe.

These plastic pellets absorb persistent organic pollutants and other toxic chemicals such as PCBs, DDE, and nonylphenol. Sea creatures consume them mistaking them for food, and they move through the ocean washing up in estuaries and on beaches. When the volunteers return the pellets, they are analyzed at the Pellet Watch labs in Japan and the results are mapped on a global database showing relative levels of contaminants such as PCBs in different geographic areas (Figure 31). The results are

Figure 31. Pellet Watch mapping of PCB levels in globally dispersed plastic pellets. Numbers indicate ng/g-pellet.

then documented in peer-reviewed papers . The Pellet Watch58 program is still underway, analyzing POPscontamination of plastic from around the globe.

In addition to plastics that may enter the marine environment ‘pre-loaded’ with POPs additives such as brominated flame retardants, there is now a growing need to manage POPs-contaminated plastics that have adsorbed ambient POPs pollution in the marine environment onto their surfaces. Some well-meaning organizations have collected this type of marine litter only to burn it on beaches, promote crude pyrolysis conversion to fuels, or attempted other environmentally unsound management methods – often unaware of the contamination issues and the spread of POPs and UPOPs that these approaches involve. The technologies for environmentally sound management and destruction of POPs-contaminated waste described below are equally applicable to marine litter plastics impacts by POPs and plastics that have deliberate POPs additives.