Environmental suitability, carbon footprint and cost savings of recycled plastic for railway applications

ABSTRACT

With the aim of achieving a zero-waste society, this paper presents the environmental suitability of reusing recycled plastic instead of using virgin aggregate materials for railway sub-ballast. Recycled plastic collected from a local recycling company in the state of Victoria, Australia were tested to evaluate any potential contaminant leaching from such material to the surrounding environment. Leaching tests were conducted for 238 potential contaminants following Australian Standard Leaching Procedure (ASLP) using pH-neutral water. From the series of leaching test results, it is clear that concentrations of all the assessed contaminants, except Lead in the tested samples, are either below their individual detection limits or below the Environmental Protection Authority (EPA) Victoria defined limits to label these materials as hazardous. Apart from one pollutant, the material is safe to be reused for different engineering applications. In addition, a carbon footprint analysis was conducted considering a sustainable approach of using such recycled plastic with recycled concrete and recycled glass. It is found that through using such recycled materials total carbon footprint savings for 1 km railway track ballast material would be up to 52,211 kg CO₂e. Finally, a cost analysis is presented to demonstrate economic benefit of using recycled plastic.

KEYWORDS:

• Plastics
• recycled materials
• railways
• pavements
• ground improvement
• carbon footprint

1. Introduction

In the contemporary world, it is undeniable that through different recycling practices we can reduce our exceedingly high ecological footprint. As such different countries, authorities, organisations and individuals adopting innovative ways and one of the salient ways is to minimise the use of virgin material through increasing the use of recycled materials. Alike many other countries in the world, Australian federal and state government authorities are increasingly adopting the use of different recycled materials instead of virgin materials for different civil engineering constructions. To achieve the aimed ‘zero waste’ strategy, it is imperative that we have to explore all the avenues of recycling our generated waste in every sector. A general view of the ‘sustainability’ concept is that we have to accept recycled materials as a resource, rather than a waste destined for landfills. As this view is gaining its momentum around the world, in the recent past many researchers have been investigating reuse potentials of many different recycled materials. Among the published studies on recycled materials some recent ones are; Tamanna, Tuladhar, and Sivakugan (2020) on reuse of recycled glass, Donrak et al. (2020) on reuse of soil-melamine debris blend, Imteaz, Arulrajah, and Maghool (2020) on reuse of waste from plasterboard manufacturing, Mohammadinia et al. (2019) on reuse of blended recycled glass and plastic, Kua et al. (2019) on environmental suitability of using blended slag, fly ash and spent coffee ground, Rahman et al. (2015) with recycled demolition materials; Arulrajah et al. (2015) with recycled foamed glass.

Plastic is one of the heavily dumped used materials, causing several environmental concerns. In 1950, two million tons of plastic were produced globally and in 2015, this amount was increased by 161 times (EA 2020). Until 1992, China was welcoming 45% of the world’s plastic scraps. However, since 1992 it started to put caps and eventually in 2018, it has stopped importing waste plastics. Some other nations followed the same restriction, triggering the Australian government to decide to end the mixed plastic waste exports by mid-2021 (EA 2020). High endurance of the thermosets like plastic materials against high temperatures makes these plastics difficult to break down during the recycling process. Moreover, the inconsistency in composition, colour, transparency and size of plastic aggregates imposes aesthetic limitations in reusing them. The amount of plastics recycled in Australia in 2008–2009 financial year was 287,600 tons versus 1,789,400 tons of waste generated, which is about 16% (Mohammadinia et al. 2019). This rate is quite low compared to recycling rates of other waste materials, that is, 90% for metals, 62% for paper and cardboard and 69% for glass (Brulliard et al. 2012). As such, it is necessary to explore more ways of using recycled plastic.

There have been several studies on reusing recycled plastic with other construction materials for different structures. Hama and Hilal (2019) investigated fresh properties of concrete containing plastic aggregate and reported that workability of such concrete increases with the use of recycled plastic. Also, they have reported that the density of plastic-filled aggregate concrete is lower compared to the concrete without plastic. To further ascertain the potential risks of using shredded tyre, a series of chemical and leachate tests were conducted on shredded tyre samples, which are described in the following section. Polyvinyl chloride (PVC) products are significant portion of plastic wastes dumped in our environment. Mohammed (2019) investigated uses of different forms (aggregate, powder, and chopped insulating wire) of PVC in concrete and reported that presence of PVC in the concrete reduces the workability as to maintain the workability use of superplasticizer is needed. Also, it is found that replacing normal aggregate with PVC aggregate considerably reduces the concrete density and compressive strength. Khatib, Herki, and Elkordi (2019) investigated the use of expanded polystyrene, EPS, (which is mainly used as a packaging/insulating material) in lightweight concrete replacing natural aggregate. They have reported that EPS can be used as lightweight aggregate; however, due to its very light weight can cause segregation during mixing with concrete. As such addition of bonding agent, heat treatment, and/or coating is required as treatment to prevent segregation. Due to its light weight, which makes it unsuitable for heavy load-bearing concrete, Mohammadinia et al. (2019) investigated mixture of recycled plastic waste (RPW) and recycled glass (RG) as aggregate in the concrete for footpath construction. They have found that compressive and splitting tensile strength values of such concrete containing RPW and RG aggregate are reduced due to low adhesion between the recycled aggregates and the cement gel matrix. However, replacement of up to 20% (by volume) for RPW and up to 30% (by volume) for RG is still viable for footpath construction.

Most of the relevant studies with recycled plastic focused on the strength properties of the finished concrete (Siddique, Khatib, and Kaur 2008; Yang et al. 2015), it has not been popular for adaptation in transportation structure due to lower stiffness of recycled plastic granulates. However, the energy absorption capacity of recycled plastic and improving the impact resistance of concrete utilising these aggregates (Saxena et al. 2018) has led to the utilisation of plastic waste in construction of transportation structures. The recovered plastic in the form of PET, HDPE and LDPE granulate and fibres have been mixed query aggregates aggregate (Awoyera and Adesina 2020; Valipour, Shourijeh, and Mohammadinia 2021). Similar to rubber granulates used by Indraratna et al. (2018) for railway capping layer construction, recycled plastic can perform as natural damping agent mixed with glass fines and recycled concrete to increases the fatigue life of mixed blend of aggregates (Mohammadinia et al. 2020).

Also, there is a scarcity of in-depth studies on the contaminants’ leaching properties of such sample. Moreover, in many cases, collecting, processing and reusing such waste material becomes more expensive compared to the virgin material. Akbar and Liew (2020) conducted cost analysis of using carbon fibre reinforced plastic waste in the cement-based construction. They have reported that plastic reinforced concrete will have almost same cost only if 10% of cement is replaced by silica fume, which is necessary to maintain the strength of concrete. Other options of increasing strength through adding carbon fibre will turn out to be 2.5 ~ 8.0 times more expensive compared to plain concrete. Another issue is the total carbon footprint of the materials as in many cases collection, processing and conversion of such waste materials yield to higher carbon footprints compared to the traditional practice of landfilling those waste materials and such recycling option having higher carbon footprint is not desirable. Akbar and Liew (2020) reported that reduction (137 kg CO₂e per m³ of recycled product) of global warming potential is achievable only with the recycled product having up to 1% of recycled carbon fibres, which is necessary to avoid strength reduction. Faraca, Martinez-Sanchez, and Astrup (2019) reported that recycling plastic is viable as per reduction (717 kg CO₂e per ton of hard plastic waste) in global warming potential only if the waste materials are processed through an advanced material recovery facility, where Polypropylene (PP), Polyethylene (PE), Polyethylene terephthalate (PET) and Polystyrene (PS) are targeted to be recycled. It is obvious that recycled plastic is to be reused as a substitute of a portion of the natural aggregate/material. As such, the final strength can be regulated through changing the proportion of recycled plastic in the mix. Among the other associated aspects, all the above-mentioned studies investigated one/two aspects from environmental, sustainability and cost. None of those considered all the aspects. With the above-mentioned gaps in contemporary research, this paper presents thorough study on contaminants’ leaching from recycled plastic material, along with brief insight on carbon footprint savings and economic benefit of recycled plastic for a practical reuse option in railway track capping layer.

2. Materials and methods

For this research, recycled plastic samples (Figure 1) having maximum particle size of 9.5 mm were collected from GT recycling (https://gtrecycling.com.au/) facility in Victoria, Australia. The sizes of the samples were varying from 0.08 mm – 9.5 mm, having a mean particle size (D₅₀) of 5.15 mm. Particle size distribution of the collected samples is shown in Figure 2. Proportions of different sized particles in the samples are; gravel-sized 92.2%, sand-sized 7.6% and fines content 0.2%. This study first investigated leaching properties of recycled plastic samples, to ascertain that the materials can be used without any concern regarding contaminating surrounding environment based on the criteria provided by EPA Victoria (2009). Leaching tests were conducted by an independent state-of-the art NATA certified commercial laboratory, Australian Laboratory Services Ltd. (www.alsglobal.com) in Melbourne following Australian Standard Leaching Procedure, AS 4439 (1997). All the leachate tests were separately performed with deionised water (pH = 7.0) solution. Each test was performed with three random samples and average values from all the tests are presented. The leachates were tested for most of the potential contaminants including metals and metallic compounds, PAH’s (Polynuclear Aromatic Hydrocarbons), Monocyclic Aromatic Hydrocarbons, Halogenated Aliphatic Compounds and Halogenated Aromatic Compounds. Test methods used were recommended by the Chemical Abstracts Services, CAS (https://www.cas.org/), a division of the American Chemical Society. Names of the tested pollutants, their groups and individual test method as identified by CAS numbers are tabulated in Table 1.

Table 1. Names of the tested contaminants and test methods used

Pollutant/Analyte	CAS Number
Turbidimetric Sulphate
Sulphate as SO4	14,808–79-8
Chloride	16,887–00-6
Leachable Metals
Arsenic	7440–38-2
Cadmium	7440–43-9
Chromium	7440–47-3
Copper	7440–50-8
Lead	7439–92-1
Nickel	7440–02-0
Selenium	7782–49-2
Silver	7440–22-4
Tin	7440–31-5
Zinc	7440–66-6
Mercury	7439–97-6
Fluoride	16,984–48-8
Organochlorine Pesticides (OC)
alpha-BHC	319–84-6
Hexachlorobenzene (HCB)	118–74-1
beta-BHC	319–85-7
gamma-BHC	58–89-9
delta-BHC	319–86-8
Heptachlor	76–44-8
Aldrin	309–00-2
Heptachlor epoxide	1024–57-3
trans-Chlordane	5103–74-2
alpha-Endosulfan	959–98-8
cis-Chlordane	5103–71-9
Dieldrin	60–57-1
4.4`-DDE	72–55-9
Endrin	72–20-8
beta-Endosulfan	33,213–65-9
4.4`-DDD	72–54-8
Endrin aldehyde	7421–93-4
Endosulfan sulphate	1031–07-8
4.4`-DDT	50–29-3
Endrin ketone	53,494–70-5
Methoxychlor	72–43-5
Organophosphorus Pesticides (OP)
Dichlorvos	62–73-7
Demeton-S-methyl	919–86-8
Monocrotophos	6923–22-4
Dimethoate	60–51-5
Diazinon	333–41-5
Chlorpyrifos-methyl	5598–13-0
Parathion-methyl	298–00-0
Fenthion	55–38-9
Malathion	121–75-5
Pollutant/Analyte	CAS Number
Chlorpyrifos	2921–88-2
Parathion	56–38-2
Pirimphos-ethyl	23,505–41-1
Chlorfenvinphos	470–90-6
Bromophos-ethyl	4824–78-6
Fenamiphos	22,224–92-6
Prothiofos	34,643–46-4
Ethion	563–12-2
Carbophenothion	786–19-6
Azinphos Methyl	86–50-0
Phenolic Compounds (Halogenated)
2-Chlorophenol	95–57-8
2.4-Dichlorophenol	120–83-2
2.6-Dichlorophenol	87–65-0
4-Chloro-3-methylphenol	59–50-7
2.4.5-Trichlorophenol	95–95-4
2.4.6-Trichlorophenol	88–06-2
2.3.5.6-Tetrachlorophenol	935–95-5
2.3.4.5 & 2.3.4.6-Tetrachlorophenol	4901–51-3/58-90-2
Pentachlorophenol	87–86-5
Phenolic Compounds (Non-halogenated)
Phenol	108–95-2
2-Methylphenol	95–48-7
3- & 4-Methylphenol	1319–77-3
2-Nitrophenol	88–75-5
2.4-Dimethylphenol	105–67-9
2.4-Dinitrophenol	51–28-5
4-Nitrophenol	100–02-7
2-Methyl-4.6-dinitrophenol	8071–51-0
Dinoseb	88–85-7
2-Cyclohexyl-4.6-Dinitrophenol	131–89-5
Polynuclear Aromatic Hydrocarbons
Naphthalene	91–20-3
2-Methylnaphthalene	91–57-6
2-Chloronaphthalene	91–58-7
Acenaphthylene	208–96-8
Acenaphthene	83–32-9
Fluorene	86–73-7
Phenanthrene	85–01-8
Anthracene	120–12-7
Fluoranthene	206–44-0
Pyrene	129–00-0
N-2-Fluorenyl Acetamide	53–96-3
Benz(a)anthracene	56–55-3
Benzo(b + j) & Benzo(k)fluoranthene	205–99-2 207–08-9
7.12-Dimethylbenz(a)anthracene	57–97-6
Chrysene	218–01-9
Benzo(a)pyrene	50–32-8
3-Methylcholanthrene	56–49-5
Indeno(1.2.3.cd)pyrene	193–39-5
Dibenz(a.h)anthracene	53–70-3
Benzo(g.h.i)perylene	191–24-2
Phthalate Esters
Dimethyl phthalate	131–11-3
Diethyl phthalate	84–66-2
Di-n-butyl phthalate	84–74-2
Butyl benzyl phthalate	85–68-7
bis(2-ethylhexyl) phthalate	117–81-7
Di-n-octylphthalate	117–84-0
Total Cyanide	57–12-5
Monocyclic Aromatic Hydrocarbons
Styrene	100–42-5
Isopropylbenzene	98–82-8
n-Propylbenzene	103–65-1
1.3.5-Trimethylbenzene	108–67-8
sec-Butylbenzene	135–98-8
1.2.4-Trimethylbenzene	95–63-6
tert-Butylbenzene	98–06-6
p-Isopropyltoluene	99–87-6
n-Butylbenzene	104–51-8
Oxygenised Compounds
Vinyl Acetate	108–05-4
2-Butanone (MEK)	78–93-3
4-Methyl-2-pentanone (MIBK)	108–10-1
2-Hexanone (MBK)	591–78-6
Sulphonated Compounds
Carbon disulphide	75–15-0
Fumigants
2.2-Dichloropropane	594–20-7
1.2-Dichloropropane	78–87-5
cis-1.3-Dichloropropylene	10,061–01-5
trans-1.3-Dichloropropylene	10,061–02-6
1.2-Dibromoethane (EDB)	106–93-4
Halogenated Aliphatic Compounds
Dichlorodifluoromethane	75–71-8
Chloromethane	74–87-3
Vinyl chloride	75–01-4
Bromomethane	74–83-9
Chloroethane	75–00-3
Trichlorofluoromethane	75–69-4
1.1-Dichloroethene	75–35-4
Iodomethane	74–88-4
trans-1.2-Dichloroethene	156–60-5
1.1-Dichloroethane	75–34-3
cis-1.2-Dichloroethene	156–59-2
1.1.1-Trichloroethane	71–55-6
1.1-Dichloropropylene	563–58-6
Carbon Tetrachloride	56–23-5
1.2-Dichloroethane	107–06-2
Trichloroethene	79–01-6
Dibromomethane	74–95-3
1.1.2-Trichloroethane	79–00-5
1.3-Dichloropropane	142–28-9
Tetrachloroethene	127–18-4
1.1.1.2-Tetrachloroethane	630–20-6
trans-1.4-Dichloro-2-butene	110–57-6
cis-1.4-Dichloro-2-butene	1476–11-5
1.1.2.2-Tetrachloroethane	79–34-5
1.2.3-Trichloropropane	96–18-4
Pentachloroethane	76–01-7
1.2-Dibromo-3-chloropropane	96–12-8
Hexachlorobutadiene	87–68-3
Halogenated Aromatic Compounds
Chlorobenzene	108–90-7
Bromobenzene	108–86-1
2-Chlorotoluene	95–49-8
4-Chlorotoluene	106–43-4
1.3-Dichlorobenzene	541–73-1
1.4-Dichlorobenzene	106–46-7
1.2-Dichlorobenzene	95–50-1
1.2.4-Trichlorobenzene	120–82-1
1.2.3-Trichlorobenzene	87–61-6
Trihalomethanes
Chloroform	67–66-3
Bromodichloromethane	75–27-4
Dibromochloromethane	124–48-1
Bromoform	75–25-2
Total Recoverable Hydrocarbons
C6 – C10 Fraction	C6_C10
C6 – C10 Fraction minus BTEX (F1)	C6_C10-BTEX
BTEXN
Benzene	71–43-2
Toluene	108–88-3
Ethylbenzene	100–41-4
meta- & para-Xylene	108–38-3 106–42-3
ortho-Xylene	95–47-6
Total Xylenes	1330–20-7
Naphthalene	91–20-3

Figure 1. Photo of recycled plastic sample

Figure 2. Typical dual gauge railway track section

In the second phase of the study, a partial life cycle assessment of the studied materials was conducted through calculating carbon footprint savings due to the use of such recycled plastics. For this investigation, assessment of recycled materials’ end of cycle environmental impacts was considered based on available information in the literatures such as Racusin and McArleton (2012), SASA (2016) and US EPA (2003). As it is not suitable to solely use the recycled plastic, blends with other recycled materials (recycled concrete and recycled glass) are considered for both the carbon footprint and economic analyses. Arulrajah et al. (2020) have demonstrated that such mix of recycled materials is suitable for the railway track sub-ballast material. For this study a blend of 40% recycled concrete, 40% recycled glass and 20% recycled plastic was considered. Selected recycled plastic and proposed blends’ environmental impacts (in regard to carbon dioxide release) were compared with those of virgin materials, which are traditionally used for railway track capping material. In regard to environmental impact, carbon footprints of the selected virgin (rock and gravel) and blend of materials were considered. Also, carbon footprint due to disposal of recycled materials to the traditional landfill was considered in order to calculate carbon footprint savings while using recycled materials instead of dumping them in the landfill. Individual carbon footprint for each of the considered materials for different stages (material sourcing and end-of-life disposal) were added to calculate cumulative carbon footprints. In case of carbon footprint from using materials, which are all recycled, the carbon footprint from end-of-life disposal was not added. The difference between carbon footprint of using virgin material with carbon footprint of using recycled material(s) is the carbon footprint savings.

For the economic analysis, a comparison was done considering construction of a 1 km length typical rail track section (Figure 3) having a dual-gauge rail tracks with a top width of 5 m (at the top of ballast layer), side slope of 2(H) to 1(V), and a ballast thickness of 750 mm. A typical thickness of 250 mm for the sub-ballast layer was considered. The unit costs were considered based on the Australian market price in the year 2020.

Figure 3. Particle size distribution of collected recycled plastic sample

3. Results and discussions

3.1 Leaching properties

A series of environmental leaching tests on all the potential contaminants of different categories were conducted. Table 2 shows the detailed leaching test results performed on the samples for different heavy metals (including Mercury) along with the limits specified by the EPA Victoria (2009) for specific waste to be defined as hazardous. From the table, it is clear that most of the metals’ concentrations in the leachate were below corresponding detection limits. All the metals’ concentrations, except Lead, were found to be far below the individual hazardous limits. The concentration of Lead was found to be 2.4 mg/L; EPA Victoria (2009) defines a waste material as hazardous if the concentration of Lead in the leachate is more than 0.50 mg/L. More testing from a similar sample needs to be performed before defining the material (i.e. recycled plastic) as hazardous. In addition to common metals, the concentrations of Chloride, Fluoride, Nitrite as N, Nitrite plus Nitrate as N (NOx), Formaldehyde and Total Polychlorinated biphenyls in the leachate were found to be either below detection limit or below the individual hazardous limits specified by the EPA Victoria (2009) as shown in Table 3.

Table 2. Concentrations of metals in the leachates

Contaminant	Unit	Measuring limit	Concentration	EPA Victoria Limit Victoria Limit
Antimony	mg/L	0.1	<0.1	1
Arsenic	mg/L	0.1	<0.1	0.35
Barium	mg/L	0.1	<0.1	35
Beryllium	mg/L	0.05	<0.05	0.5
Boron	mg/L	0.1	<0.1	15
Cadmium	mg/L	0.05	<0.05	0.1
Copper	mg/L	0.1	<0.1	100
Lead	mg/L	0.1	2.4	0.5
Nickel	mg/L	0.1	<0.1	1
Selenium	mg/L	0.05	<0.05	0.5
Silver	mg/L	0.1	<0.1	5
Tin	mg/L	0.1	<0.1	-
Zinc	mg/L	0.1	0.3	150
Molybdenum	mg/L	0.1	<0.1	2.5
Mercury	mg/L	0.001	<0.0010	0.05
Hexavalent Chromium	mg/L	0.1	<0.10	2.5

Table 3. Concentrations of Chloride, Fluoride, Nitrite, Formaldehyde and Total Polychlorinated

Contaminant	Unit	Measuring limit	Concentration	EPA Victoria Limit Victoria Limit
Chloride	mg/L	1	10	12,500
Fluoride	mg/L	0.1	<0.1	75
Iodide	m/L	0.01	<0.500	-
Nitrite as N	mg/L	0.01	<0.01	150
Nitrite + Nitrate as N	mg/L	0.01	0.12	2650
Formaldehyde	mg/L	0.1	0.2	25
Total Polychlorinated biphenyls	µg/L	1	<1	-

Table 4 shows the concentrations of different Halogenated Phenolic Compounds in leachate. From the table, it can be seen that all the components were below the detection limits, as well as far below the available hazardous limit set by the EPA Victoria (2009). Table 5 shows the concentrations of Non-halogenated Phenolic Compounds in the leachate. Like Halogenated Phenolic Compounds, all the non-halogenated Phenolic Compounds were below the detection limits and far below the available hazardous limit set by the EPA Victoria (2009). Same in the case of Phthalate Esters (Table 6), Organochlorine Pesticides (Table 7), Monocyclic Aromatic Hydrocarbons (Table 8), Halogenated Aromatic Compounds (Table 9), Trihalomethanes (Table 10) and Total Cyanide, concentrations are below the detection limits and far below the available hazardous limits. In regard to Total Petroleum and Recoverable Hydrocarbons, some of those were above detection limits (Table 11); however, EPA Victoria (2009) has not defined any safe/hazardous limit for those contaminants. Also, some BTEXN (Table 12) were above the detection limits, however were either far below the hazardous limit or not defined to be hazardous by EPA Victoria (2009). In addition to these, following categorised contaminants were found to be below the individual detection limits:

• Polynuclear Aromatic Hydrocarbons: Naphthalene, 2-Methylnaphthalene, 2-Chloronaphthalene, Acenaphthylene, Acenaphthene, Fluorene, Phenanthrene, Anthracene, Fluoranthene, Pyrene, N-2-Fluorenyl Acetamide, Benz(a)anthracene, Chrysene, Benzo(b + j) & Benzo(k)fluoranthene, 7.12-Dimethylbenz(a)anthracene, Benzo(a)pyrene, 3-Methylcholanthrene, Indeno(1.2.3.cd)pyrene, Dibenz(a.h)anthracene, Benzo(g.h.i)perylene, Sum of PAHs, Benzo(a)pyrene TEQ (zero)

• Nitrosamines: N-Nitrosomethylethylamine, N-Nitrosodiethylamine, N-Nitrosopyrrolidine, N-Nitrosomorpholine, N-Nitrosodi-n-propylamine, N-Nitrosopiperidine, N-Nitrosodibutylamine, N-Nitrosodiphenyl & Diphenylamine, Methapyrilene

• Nitroaromatics and Ketones: 2-Picoline, Acetophenone, Nitrobenzene, Isophorone, 2.6-Dinitrotoluene, 2.4-Dinitrotoluene, 1-Naphthylamine, 4-Nitroquinoline-N-oxide, 5-Nitro-o-toluidine, Azobenzene, 1.3.5-Trinitrobenzene, Phenacetin, 4-Aminobiphenyl, Pentachloronitrobenzene, Pronamide, Dimethylaminoazobenzene, Chlorobenzilate

• Haloethers: Bis(2-chloroethyl) ether, Bis(2-chloroethoxy) methane, 4-Chlorophenyl phenyl ether, 4-Bromophenyl phenyl ether

• Chlorinated Hydrocarbons: 1.4-Dichlorobenzene, 1.3-Dichlorobenzene, 1.2-Dichlorobenzene, Hexachloroethane, 1.2.4-Trichlorobenzene, Hexachloropropylene, Hexachlorobutadiene, Hexachlorocyclopentadiene, Pentachlorobenzene, Hexachlorobenzene (HCB)

• Anilines and Benzidines: Aniline, 4-Chloroaniline, 2-Nitroaniline, 3-Nitroaniline, Dibenzofuran, 4-Nitroaniline, Carbazole, 3.3`-Dichlorobenzidine

• Organophosphorus Pesticides: Dichlorvos, Dimethoate, Diazinon, Chlorpyrifos-methyl, Malathion, Fenthion, Chlorpyrifos, Pirimphos-ethyl, Chlorfenvinphos, Prothiofos, Ethion

• Phenoxyacetic Acid Herbicides: 4-Chlorophenoxy acetic acid, 2.4-DB, Dicamba, Mecoprop, MCPA, 2.4-DP, 2.4-D, Triclopyr, Silvex (2.4.5-TP/Fenoprop), 2.4.5-T, MCPB, Picloram, Clopyralid, Fluroxypyr, 2.6-D, 2.4.6-T

• Oxygenised Compounds, Sulphonated Compounds, Fumigants: Vinyl Acetate, 2-Butanone (MEK), 4-Methyl-2-pentanone (MIBK), 2-Hexanone (MBK), Carbon disulphide, 2.2-Dichloropropane, 1.2-Dichloropropane, cis-1.3-Dichloropropylene, trans-1.3-Dichloropropylene, 1.2-Dibromoethane (EDB)

• Halogenated Aliphatic Compounds: Dichlorodifluoromethane, Chloromethane, Vinyl chloride, Bromomethane, Chloroethane, Trichlorofluoromethane, 1.1-Dichloroethene, Iodomethane, trans-1.2-Dichloroethene, 1.1-Dichloroethane, cis-1.2-Dichloroethene, 1.1.1-Trichloroethane, 1.1-Dichloropropylene, Carbon Tetrachloride, 1.2-Dichloroethane, Trichloroethene, Dibromomethane, 1.1.2-Trichloroethane, 1.3-Dichloropropane, Tetrachloroethene, 1.1.1.2-Tetrachloroethane, trans-1.4-Dichloro-2-butene

• Halogenated Aliphatic Compounds: cis-1.4-Dichloro-2-butene, 1.1.2.2-Tetrachloroethane, 1.2.3-Trichloropropane, Pentachloroethane, 1.2-Dibromo-3-chloropropane, Hexachlorobutadiene

Table 4. Concentrations of Halogenated Phenolic Compounds

Contaminant	Unit	Measuring limit	Concentration	EPA Victoria Limit
2-Chlorophenol	µg/L	2	<2	15,000
2.4-Dichlorophenol	µg/L	2	<2	10,000
2.6-Dichlorophenol	µg/L	2	<2	-
4-Chloro-3-methylphenol	µg/L	4	<4	-
2.4.5-Trichlorophenol	µg/L	2	<2	200,000
2.4.6-Trichlorophenol	µg/L	2	<2	1000
2.3.5.6-Tetrachlorophenol	µg/L	2	<2	-
2.3.4.5 & 2.3.4.6-Tetrachlorophenol	µg/L	2	<2	-
Pentachlorophenol	µg/L	2	<2	-
Sum of Phenols (halogenated)	µg/L	2	<2	-

Table 5. Concentrations of Non-halogenated Phenolic Compounds

Contaminant	Unit	Measuring limit	Concentration	EPA Victoria Limit
Phenol	µg/L	4	<4	-
2-Methylphenol	µg/L	4	<4	-
3- & 4-Methylphenol	µg/L	4	<4	-
2-Nitrophenol	µg/L	4	<4	-
2.4-Dimethylphenol	µg/L	4	<4	-
2.4-Dinitrophenol	µg/L	100	<100	-
4-Nitrophenol	µg/L	50	<50	-
2-Methyl-4.6-dinitrophenol	µg/L	50	<50	-
Dinoseb	µg/L	50	<50	-
2-Cyclohexyl-4.6-Dinitrophenol	µg/L	50	<50	-
Sum of Phenols (non-halogenated)	µg/L	4	<4	7000

Table 6. Concentrations of Phthalate Esters

Contaminant	Unit	Measuring limit	Concentration	EPA Victoria Limit
Dimethyl phthalate	µg/L	2	<2	-
Diethyl phthalate	µg/L	2	<2	-
Di-n-butyl phthalate	µg/L	2	<2	-
Butyl benzyl phthalate	µg/L	2	<2	-
bis(2-ethylhexyl) phthalate	µg/L	10	<10	500
Di-n-octylphthalate	µg/L	2	<2	-

Table 7. Concentrations of Organochlorine Pesticides

Contaminant	Unit	Measuring limit	Concentration	EPA Victoria Limit
alpha-BHC	µg/L	0.5	<0.5	-
Heptachlor	µg/L	0.5	<0.5	15
Aldrin	µg/L	0.5	<0.5	-
cis-Chlordane	µg/L	0.5	<0.5	-
trans-Chlordane	µg/L	0.5	<0.5	-
4.4`-DDE	µg/L	0.5	<0.5	-
Dieldrin	µg/L	0.5	<0.5	-
4.4`-DDD	µg/L	0.5	<0.5	-
4.4`-DDT	µg/L	0.5	<0.5	-
DDT+DDD+DDE	µg/L	0.5	<1.5	1000
Sum of Aldrin + Dieldrin	µg/L	0.5	<0.5	15
Chlordane	µg/L	0.5	<0.5	50

Table 8. Concentrations of Monocyclic Aromatic Hydrocarbon (MAH)

Contaminant	Unit	Measuring limit	Concentration	EPA Victoria Limit
Styrene	µg/L	5	<5	1500
Isopropylbenzene	µg/L	5	<5	-
n-Propylbenzene	µg/L	5	<5	-
1.3.5-Trimethylbenzene	µg/L	5	<5	-
sec-Butylbenzene	µg/L	5	<5	-
1.2.4-Trimethylbenzene	µg/L	5	<5	-
tert-Butylbenzene	µg/L	5	<5	-
p-Isopropyltoluene	µg/L	5	<5	-
n-Butylbenzene	µg/L	5	<5	-

Table 9. Concentrations of Halogenated Aromatic Compounds

Contaminant	Unit	Measuring limit	Concentration	EPA Victoria Limit
Chlorobenzene	µg/L	5	<5	15,000
Bromobenzene	µg/L	5	<5	-
2-Chlorotoluene	µg/L	5	<5	-
4-Chlorotoluene	µg/L	5	<5	-
1.3-Dichlorobenzene	µg/L	5	<5	-
1.4-Dichlorobenzene	µg/L	5	<5	2000
1.2-Dichlorobenzene	µg/L	5	<5	75,000
1.2.4-Trichlorobenzene	µg/L	5	<5	-
1.2.3-Trichlorobenzene	µg/L	5	<5	-

Table 10. Concentrations of Trihalomethanes

Contaminant	Unit	Measuring limit	Concentration	EPA Victoria Limit
Chloroform	µg/L	5	<5	3000
Bromodichloromethane	µg/L	5	<5	-
Dibromochloromethane	µg/L	5	<5	-
Bromoform	µg/L	5	<5	-

Table 11. Concentrations of Total Petroleum and Recoverable Hydrocarbons

Contaminant	Unit	Measuring limit	Concentration	EPA Victoria Limit
C10 – C14 Fraction	µg/L	50	<50	-
C15 – C28 Fraction	µg/L	100	200	-
C29 – C36 Fraction	µg/L	50	<50	-
C10 – C36 Fraction (sum)	µg/L	50	200	-
>C10 – C16 Fraction	µg/L	100	<100	-
>C16 – C34 Fraction	µg/L	100	160	-
>C34 – C40 Fraction	µg/L	100	<100	-
>C10 – C40 Fraction (sum)	µg/L	100	160	-
>C10 – C16 Fraction minus Naphthalene	µg/L	100	<100	-

Table 12. Concentrations of BTEXN

Contaminant	Unit	Measuring limit	Concentration	EPA Victoria Limit
Benzene	µg/L	1	<1	50
Toluene	µg/L	2	2	40,000
Ethylbenzene	µg/L	2	<2	15,000
meta- & para-Xylene	µg/L	2	<2	-
ortho-Xylene	µg/L	2	<2	-
Total Xylenes	µg/L	2	<2	30,000
Sum of BTEX	µg/L	1	2	-
Naphthalene	µg/L	5	<5	-

3.2 Carbon footprint savings

Selected recycled materials’ and their adopted blends’ environmental impacts were compared with those of virgin materials, which are traditionally used for rail track sub-ballast. With regard to environmental impact, carbon footprints of selected virgin and recycled materials were considered. Also, carbon footprints due to disposal of recycled materials to the traditional landfill were considered in order to calculate carbon footprint savings while using recycled materials instead of dumping them in the landfill. Table 13 shows carbon footprints of different materials and processes (i.e. landfilling, transportation). From the table, if recycled material is used instead of virgin material (rock/gravel), then a carbon footprint savings of 1.20 (=5.20–4.0) kg CO₂e/ton can be achieved. Moreover, a savings of 10.5 kg CO₂e/ton can be achieved through diverting recycled materials from landfilling. As such a total footprint savings of using recycled materials is 11.70 (=10.5 + 1.20) kg CO₂e/ton.

Table 13. Carbon footprints of different materials and processes

Material/Process	Carbon Footprint (kg CO2e/ton)
Virgin rock/gravel aggregate	5.20^a
Recycled aggregate	4.0^b
Landfilling of recycled concrete, glass	10.5 ^c

^aRacusin and McArleton (2012)

^bSASA (2016)

^cUS EPA (2003)

Considering a railway sub-ballast section of 0.25 m thickness having a top width of 8 m and bottom width of 9 m as shown in Figure 3, for a 1-km length railway pavement, total amount of virgin materials needed is 4,462.5 ton (considering the unit weight of virgin material as 2.1 t/m³). So, total carbon footprint savings per kilometre railway track would be 52,211 kg CO₂e.

3.3 Economic analysis

Costing analysis was conducted for the typical dual-gauge rail track section (as shown in Figure 3) having same dimensions. The volume of materials needed for the capping layer construction for 1 km of track is V = 2,125 m³. Assuming that for this typical km length of sub-ballast the typical virgin capping material will be replaced with a mixture of 40% crushed concrete + 40% recycled glass + 20% recycled plastic, the approximate financial benefit was calculated based on current material prices in Australia. Unit prices and final monetary savings calculations are shown in Table 14. The dry densities of these materials were obtained from geotechnical testing, which were 2.75 ton/m³ for virgin material, 2.68 ton/m³ for crushed concrete, 2.46 ton/m³ for recycled glass and 0.97 ton/m³ for recycled plastic. It is found that through using proposed recycled material instead of virgin material will provide a net benefit of 25,542 USD (Australian dollar) for a typical rail track of 1 km length.

Table 14. Unit prices and summary of costing ana

Material	Cost $/ton	Volumetric percentage %	Density at Dr = 90% t/m^3	Volume/km m^3	Cost/material A$	Total cost A$
Virgin aggregate	20	100	2.29	2,125	97,325	97,325
Crushed concrete	16	40	1.85	850	25,840	71,782
Recycled glass	13	40	0.6	850	20,442
Recycled plastic	100	20	1.9	425	25,500
					Savings =	25,542

4. Conclusions

To accelerate processes towards achieving global sustainability through increasing reuse of different waste materials, this study investigated the environmental suitability of using recycled plastic. As a potential reuse option, the use in railway track as capping material instead of using traditional material (virgin aggregates) was investigated.

To ascertain environmental suitability, a series of chemical leaching tests following Australian Standard Leaching Procedure (ASLP) were conducted with the collected recycled plastic samples. Tests were conducted for 238 selected contaminants of different categories. Contaminants’ concentrations in the leachate solutions were compared with the EPA Victoria defined upper limits (if available) for waste material to be defined as hazardous. From ASLP testing of all the tested contaminants, it is found that except one all the tested contaminants’ concentrations were either below the individual detection limits or the upper limits provided by local regulatory authority (EPA Victoria) to categorise the waste aggregates as hazardous.

In the leachate from recycled plastic, the average concentration of Lead was found to be 2.4 mg/L, while EPA Victoria (2009) defines a waste material as hazardous if the concentration of Lead in the leachate is more than 0.50 mg/L. It is to be noted that this concentration was found in the collected sample, which is likely to vary for recycled plastic samples from other areas/countries. Before scrapping the material for reuse, more testing is required to be done for recycled plastic from another batch and/or other recyclers.

As recycled plastic solely is not expected to fulfil other engineering requirements as railway track sub-ballast material, it was proposed that the recycled plastic should be blended with other recycled materials, such as recycled crushed concrete and recycled glass. With regard to carbon footprint savings, it is found that through using mentioned recycled materials with recycled plastic, net carbon footprint savings per kilometre of dual-gauge railway track sub-ballast would be 52,211 kg CO₂e. Also, through using such recycled materials a monetary savings of A$25,542 would be achieved. It is to be noted here that this price saving was calculated based on current prices of the subject materials in Australia. It is likely to vary with time and for different countries.

Acknowledgments

This research project was funded by Sustainability Victoria (Contract ID: C-11569) in partnership with Metro Trains Melbourne, Rail Projects Victoria and the Level Crossing Removal Projects. This research was conducted by the Australian Research Council Industrial Transformation Training Centre for Advanced Technologies in Rail Track Infrastructure (IC170100006), which is funded by the Australian government.

Disclosure Statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the Sustainability Victoria [C-11569].