Plastic is omnipresent in the modern times. From household staffs to sophisticated instruments, the use of plastic has been sky rocketed in last few decades1,4. Approximately 15.0 million tons of plastic is produced annually in India. Though 80% of the plastic produced, is essentially recycled, materials like cheap quality plastic bags, plastic packaging waste and metalized plastic waste are deemed to be unfit for the said purpose. As a consequence, illegal disposal and irresponsible use of plastic bags and other plastic materials has become a serious threat to the environment2,4.
Heavy metals are often used as additives while producing cheap quality plastic bags to increase mechanical strength, durability and introduce colour. Unfortunately, there is no set limit or monitoring of these metal concentrations promoting the possibility of leaching under various stimulants such as rain, sunlight, moisture etc. when plastic waste is dumped in an uncontrolled fashion3. Alongside the visible threat of landfill and clogged sewage, silent leaching of heavy metals to the environment can be fatal for the ecosystem as well as the aquatic life. Moreover high chlorine content of PVC waste is a possible source of toxic dioxin and furan4.
In this work presented herein, we have investigated the heavy metal content of five commercial plastic bags purchased from a municipal super market in Kolkata. These bags which are frequently used by grocery shops, meat and fish vendors, vegetable sellers etc. are characterized and the release of heavy metals under various stimulants such as water, dilute HCl and acetic acid has been monitored.
Materials and methods:
HDPE Plastic bags of five different brands with varying thickness and colours were purchased from a municipal market in Kolkata. HCl, acetic acid and HPLC grade water used for leaching test were procured from a commercial source. Gross calorific values of the plastic bags were measured in a Parr 6200 calorimeter. S content was measured in LECO truespec S analyzer. CHN analysis were carried out using LECO CHN analyzer and heavy metal concentrations were measured using a Perkin Elmer Optima 7000 DV ICP-OES instrument. IR spectra and thermogravimetric analysis were performed in a Nicolet is-10 instrument from Thermo Fisher Scientific and Perkin Elmer STA 6000 instrument respectively.
Sampling and preparation:
Total five samples of HDPE plastic bags of different thickness and colour were chosen for investigation. Furthermore, intended use was also considered during sample collection to identify potential threat associated with plastic bag consumption. Prior to use all the bags were cut into small pieces (< 2 mm) with a new stainless steel scissor and dried at 50°C for 24 hours in an air oven. All the glassware and experimental vessels used were soaked in 10% HNO3 overnight to remove any trace of heavy metals present. The PTFE (Teflon) vessels were oven dried at 55°C for two hours and the glasswares were dried in an air oven. The crucibles used for ash formation were preheated at 900°C to remove any impurities.
Table 1: Classification of different plastic bags
| Sample | Thickness μ |
Size (mm) | Colour | Material | Usage |
| A | 40 | 13×16 | White, opaque | HDPE | Sweet shops |
| B | 20 | 13×16 | White, semi opaque | HDPE | multipurpose |
| C | 20 | 13×16 | White, semi opaque | HDPE | multipurpose |
| D | 20 | 13×16 | White, semi transparent | HDPE | Fish, meat and vegetable market |
| E | 20 | 13×16 | Black, opaque | HDPE | Fish and meat market |
Moisture content measurement:
The size reduced plastic bags were weighed and then heated in an air oven at 55°C for 24 h and weighed again. The weight loss was calculated to measure the moisture content.
Moisture content (%) = [(Initial weight-Final weight) x 100]/ Initial weight
Determination of heavy metals:
0.2 g of size reduced sample was taken in a PTFE vessel and 20 ml of concentrated HNO3 was added to it. The sample was soaked for 30 minutes, heated at 175°C on a hot plate with lid closed for 1.5 hour and cooled to room temperature. Then 2 ml of 30% H2O2 was added and the vessel was heated again for 1 hour with lid. Finally the vessel was uncovered and the solution was almost evaporated to dryness (2-3 ml solution remained). The solution was filtered into 50 ml volumetric flasks and the volume was made up with HPLC grade water. The final solution was analyzed using appropriate blank in ICP-OES to estimate the heavy metals. Only high Ca concentrations were estimated using a titrimetric method as described in our previous work. Results are tabulated in table 2.
Table 2: Heavy metal Concentrations (mg/Kg) in different samples
| Sample | Ca (%) | Zn | Cr | Mn | Cu | Ba | Pb | Cd |
| A | 4.88 | 149.3 | 20.0 | 30.1 | 5.7 | 56.0 | 13.2 | – |
| B | 1.99 | 72.7 | 31.2 | 22.9 | 2.8 | – | 12.7 | – |
| C | 0.61 | 33.7 | 19.8 | 32.2 | 4.6 | 34.1 | 10.8 | – |
| D | 0.025 | 20.5 | 27.4 | 73.5 | 3.9 | – | 11.1 | – |
| E | 2.98 | 73.6 | 22.0 | 70.8 | 9.6 | 164.1 | 49.1 | 1.7 |
Heavy metal leaching test:
All the samples were tested for heavy metal leaching using three stimulants such as HPLC grade water, 0.03 (M) HCl and 0.03 (M) acetic acid respectively. In a 25 ml Erlenmeyer flask, 0.1 g of sample was mixed with 10 ml of a specific stimulant, stirred for 10 minutes in ambient temperature, agitated at 60°C for 2 hours and then kept at room temperature in a dark closet for 24 hours. The final solutions were filtered in PTFE vessels, digested as mentioned earlier and analyzed in ICP-OES using appropriate blanks. Results are tabulated in table 3.
Table 3: Stimulant influenced metal leaching behaviour of plastic bags
| Sample | Cr (mg/L) | Pb (mg/L) | Cd (mg/L) | ||||||
| DW | AA | HCl | DW | AA | HCl | DW | AA | HCl | |
| A | 19.7 | 1.8 | 6.5 | 12.0 | 10.0 | 11.0 | – | – | – |
| B | 13.7 | 12.1 | 28.8 | 10.7 | 10.8 | – | – | – | – |
| C | 14.2 | 3.1 | 10.7 | 6.1 | 8.2 | – | – | – | – |
| D | 10.2 | 5.2 | 1.8 | 10.7 | 8.0 | – | – | – | – |
| E | 13.9 | 4.2 | 19.2 | 13.6 | 14.7 | 7.0 | 1.0 | – | 1.0 |
| US EPA Limits | 5.0 | 5.0 | 1.0 | ||||||
| US EPA
Limits for drinking water (expressed in μg/L) |
100.0 | 15.0 | 5.0 | ||||||
DW= distilled water, HPLC grade, AA = acetic acid
Table 4: Standards for heavy metal content (mg/Kg) in plastic products
| Metals | ASTM 5963 (Toy) | ISO 8124-3 (Toy) | EN71 (Toy) | 94/62/EC
(Packaging) |
Eu.no.10/2011
(Food contact) |
| Pb | 90 | 90 | 90 | 100 | 60a |
| Cr | 60 | 60 | 60 | 100 | 60a |
| Cd | 75 | 75 | 75 | 100 | 60a |
| Ba | 1000 | 1000 | 1000 | – | – |
a migration limits; mg/Kg food.
Results and Discussions:
Elemental composition, calorific value and moisture content:
Elemental composition of plastic bags influences the release of pollutant and recoverable energy from plastic bag waste. As suggested by IR spectra [⁓2915 cm-1 (s), ⁓2847 cm-1(s), ⁓1460 cm-1 (s)], all the samples were of HDPE grade and they exhibited (table 5) high C content (˃ 76%), comparatively lower H content (<15 %) and very low amount of N and S (< 1%). In accordance with high C content and very low moisture content, all the samples showed high gross calorific value of ⁓35-47 MJ/Kg (table) which is comparable to conventional fuels such as gasoline. According to the literature survey these values may vary within a small range due to the presence of different additives and pigments. Keeping in line to the promise shown by plastic waste, pilot plants are operating all over the world to standardize production of cleaner plastic crude oil which can further be distilled to generate commercial fuel.
According to TG analysis, all the samples were stable upto ⁓400°C after which they show a steady weight loss upto ⁓700°C. Interestingly when the residues of the leaching tests were subjected to TG analysis, the thermal decomposition pattern remain almost same confirming the negligible influence of additives on the thermal stability of the test samples.
Table 5: Elemental composition of plastic samples
| Sample | GCV (MJ/Kg) | C (%) | H (%) | N (%) | S (%) | Moisture (%) | Ash (%) |
| A | 40.68 | 83.7 | 12.0 | <1 | <0.1 | <0.1 | 4.76 |
| B | 40.24 | 81.1 | 14.7 | <1 | <0.1 | <0.1 | 5.90 |
| C | 42.25 | 84.5 | 12.4 | <1 | <0.1 | <0.1 | 3.10 |
| D | 46.69 | 86.8 | 13.1 | <1 | <0.1 | <0.1 | 0.41 |
| E | 35.53 | 76.5 | 12.2 | <1 | <0.1 | <0.1 | 9.26 |
| Methane | 53.0 | ||||||
| Gasoline | 46.0 | ||||||
| Fuel Oil | 43.0 | ||||||
| Coal | 30.0 |
Heavy metal composition and leaching:
Manufacturers add varying amount of heavy metals/additives to plastic to increase/introduce their durability, mechanical strength, colour etc. Although there are different standards to limit the use of heavy metals in various plastic made articles (table 4), absence of any such unique standard for plastic bags makes them a potential threat to the environment. Although, the plastic bags used for our study do not contain heavy metal levels greater than those mentioned in the standards that does not make them harmless as the serious threat lies in the leaching behaviour of those plastic bags. Among the three different stimulants, only three samples in acetic acid were found to be leached within the safety limits of USEPA (table 3) showing the possible threat of contaminating soil, drinking water and food materials that come in contact with these plastic bags.
Conclusion:
Proper consciousness towards the use of plastic bags is of high importance. They block sewage systems, act as landfill, make soil infertile and possess a serious threat to the ecosystem. Heavy metals and other additives used in plastic bags may leach to the environment under different stimulants such as sunlight, rain, moisture etc. and in the long run may have catastrophic impact on the ecology. As shown in this study, commercial plastic bags intended for daily use may not contain heavy metals in excess than the standard limits; their leaching behaviour is quite alarming. Proper recycling of plastic bags and alternative uses such as production of fuel or construction material may be the possible way out of this plastic borne disaster.
References:
- Greenfeet, 2004. Paper Vs. Plastic- The shopping Bag Debate; http://www.greenfeet.net/newsletter/debate.shtml
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- Cheng, X., Shi, H., Adams, C. D., Ma, Y., 2010. Assessment of metal contaminations leaching out from recycling plastic bottles upon treatments. Environ. Sci. Pollut. Res. 17 (7), 1323-1330.
- Alam, O., Billah, M., Yajie, D., 2018. Resour. Conserv. Recycl. 132, 121-129.