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Chemical and physical changes of microplastics during sterilization by chlorination
Water Research 163 (2019) 114871
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Chemical and physical changes of microplastics during sterilization by chlorination Varun P. Kelkar a, Charles B. Rolsky a, Anupum Pant c, Matthew D. Green b, Sefaattin Tongay c, Rolf U. Halden a, * a
Center for Environmental Health Engineering, The Biodesign Institute, Arizona State University, 1001 S. McAllister Avenue, Tempe, AZ, 85287-8101, USA Chemical Engineering, School for Engineering of Matter, Transport and Energy, Arizona State University, 551 E. Tyler Mall, Tempe, AZ, 85287-6106, USA Materials Science and Engineering, School for Engineering of Matter, Transport and Energy, Arizona State University, 551 E. Tyler Mall, Tempe, AZ, 852876106, USA b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 February 2019 Received in revised form 9 July 2019 Accepted 14 July 2019 Available online 15 July 2019
Wastewater treatment plants are known to release microplastics that have been detected in aquatic and terrestrial organisms constituting part of the human diet. Chlorination of wastewater-borne microplastics was hypothesized to induce chemical and physical changes detectable by Raman spectroscopy and differential scanning calorimetry (DSC). In the laboratory, virgin plastics (~0.05 2 2 mm) were exposed to differing sterilization conditions representative of dosages used in the disinfection of drinking water, wastewater, and heavily contaminated surfaces. Polypropylene (PP) was most resistant to chlorination, followed by high density polyethylene (HDPE) and polystyrene (PS). Polystyrene showed degradation, indicated by changes in Raman peak widths, at concentration-time regimes (CT values) as low as 75 mg min/L, whereas HDPE and PP remained unaltered even at chlorine doses characteristic of wastewater disinfection (150 mg min/L). However, HDPE and PS were not completely resistant to oxidative attack by chlorination. Under extremely harsh conditions, shifts in Raman peaks and the formation of new bonds were observed. These results show that plastics commonly used in consumer products can be chemically altered, some even under conditions prevailing during wastewater treatment. Changes in polymer properties, observed for HDPE and PP under extreme exposure conditions only, are predicted to alter the risk microplastics pose to aquatic and terrestrial biota, since an increase in carbonchlorine (CeCl) bonds is known to increase toxicity, rendering the polymers more hydrophobic and thus more prone to adsorb, accumulate, and transport harmful persistent pollutants to biota in both aquatic and terrestrial environments. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Wastewater treatment plants Microplastics Chlorination Raman spectroscopy Degradation
1. Introduction The presence and accumulation of microplastics (MPs) in both aquatic and terrestrial environments is a growing concern worldwide, with freshwater, coastal, and marine polymer debris posing risks to aquatic animals and birds which constitute part of the human diet (Wang et al., 2016). Moreover, MPs also have been found in other food products for human consumption and even in bottled mineral water (Obmann et al., 2018). Primary MPs include microbeads (typically <1 mm in diameter) and manufactured pellets (~1e5 mm in diameter) that serve as an industrial feedstock for
* Corresponding author. E-mail address: [email protected] (R.U. Halden). https://doi.org/10.1016/j.watres.2019.114871 0043-1354/© 2019 Elsevier Ltd. All rights reserved.
commercial plastic products. Microbeads are still being used in certain countries as an ingredient in personal care products such as shampoo, face scrubs, and toothpaste. However, environmentally more prevalent are secondary microplastics originating from discarded consumer products, plastic packaging and synthetic fabrics become fragmented as a result of physical stress imparted by harsh environmental conditions, including wave and wind energy, sunlight, erosion, and temperature stress (Andrady, 2011; Cole et al., 2011; Ng and Obbard, 2006; Van Cauwenberghe et al., 2013). These altered microplastics are known to carry harmful contaminants such as polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs), dichlorodiphenyltrichloroethane (DDT) and dexachlorocyclohexane (HCH) via sorption mechanisms (Wang et al., 2018). In addition to the above environmental material alterations, of plastic polymers potentially also could be chemically
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Nomenclature MPs HDPE PP PS WWTPs PCBs PAHs DDT HCH DSC CT MDL Mp DHm UV FTIR
Microplastics high density polyethylene polypropylene polystyrene wastewater treatment plants polychlorinated biphenyls polyaromatic hydrocarbons Dichlorodiphenyltrichloroethane Hexachlorocyclohexane differential scanning calorimetry concentration-time method detection limit melting point enthalpy of melting ultraviolet Fourier transform infrared spectroscopy
altered during the application of bleach onto plastic surfaces for decontamination and cleanup of septic spills on boats and in households, leading to very harsh sterilization regimes exceeding those extant at water and wastewater treatment plants (Doi et al., 2017; Martin and Richards, 2010; Perez et al., 2017; Rothlisberger et al., 2010). In 2013, it was estimated that 299 million metric tons of plastic were generated of which around 12.5 million tons annually end up in freshwater resources and the oceans (Wang et al., 2016); (Hahladakis et al., 2018). With increasing world population, these numbers are expected to grow. Research has shown that microplastics, defined as polymer structures measuring 5 mm or less in diameter, are emitted into surface waters contained in the effluent of wastewater treatment plants (WWTPs) (Carr et al., 2016; Dris et al., 2015; Lares et al., 2018). Most WWTPs in the U.S. and Canada feature no designated pollution control mechanism for the removal of microplastics (Driedger et al., 2015), although European studies show that removal efficiencies of up to 97% can be achieved for microplastics (Dris et al., 2015). Studies examining the loading and removal efficiencies of microplastics, as conducted for example at WWTPs in Los Angeles, USA and Paris, France are still rare (Carr et al., 2016; Dris et al., 2015) and little is known about physical and chemical changes in polymers during water, wastewater and surface sterilization. Chlorine is the most widely used disinfection agent in WWTPs around the world and also finds application as a cleaning agent for sterilizing fish-processing work areas and surfaces having come in contact with urine, stool and vomit. Because of its strong oxidizing nature, chlorine has the potential to affect physical and chemical properties of microplastics, potentially breaking existing bonds and introducing new ones. Introduction of chlorine-carbon bonds would be of particular concern, as substitution of hydrogen with chlorine atoms in organic compounds tends to increase their environmental persistence and ecotoxicity (El-Shahawi et al., 2010). Degradation of MPs with chlorine can be potentially dangerous since the aged MPs have a higher sorptive affinity to harmful chemicals than do virgin MPs (Wang et al., 2018). Multiple studies have investigated the impact of oxidants such as Cl2 gas, Cl2O, NaOCl on polymers (Khatua and Hsieh, 1997; Kwon and Leckie, 2006; Mitroka et al., 2013; Pages et al., 1996; Whelton and Dietrich, 2009; Zebger et al., 2003); but the fate of wastewater-borne microplastics during chlorination has not yet been investigated in detail. The impact of chlorinated water on
HDPE piping used in residential water supply was studied in the past, revealing the formation of unwanted chemicals, including 4chloro-2-methylbutan-2-ol and 2,3-dichloro-2-methylbutane at high chlorine doses (50 mg/L, 250 mg/L and 500 mg/L) (Mitroka et al., 2013). Whereas the chlorinated byproducts released from pipes into water were concluded to have potential adverse health implications for humans and aquatic ecosystems, potential physical and chemical changes in the pipe material itself were not investigated. The present study employed a combination of non-destructive Raman spectroscopy (Araujo et al., 2018) and differential scanning calorimetry (DSC) to quantitatively examine oxidative damage to polystyrene (PS), polypropylene (PP), and high-density polyethylene (HDPE) microplastics in conditions of varying concentrations of chlorine and exposure times. 2. Materials and methods 2.1. Chemicals Reagent grade sodium hypochlorite solution (12.5 ± 2.5% available chlorine), hydrogen chloride solution (12M), sulfuric acid solution (18.4 M) and sodium thiosulfate powder (99.5%) were purchased from Sigma-Aldrich Corp., St. Louis, MO, USA. Plastic polymer fragments were obtained by modifying authentic consumer products (milk jugs, coffee cup lids etc.) manufactured from virgin, food-grade plastics using a pair of scissors. Used plastics were obtained from samples received from a beach from a south east Asian country and consisted of high density polyethylene and polypropylene beads. The authenticity of the polymer materials was confirmed by Raman spectroscopy via spectral library matching. 2.2. Chlorination of microplastics Experiments simulating chlorination conditions during wastewater treatment, drinking water treatment, and two extreme conditions by gradually increasing the chlorine concentration, contact time and temperature were carried out with select types of polymers based on their abundance in the environment. Extreme conditions were used to observe the physical and chemical changes plastic particles can undergo in worst case scenarios which are not realistic or representative of the conditions prevailing in engineered water environments. Impact of chlorination on HDPE, PP, and PS was investigated. Plastic particles in the microplastics range of less than 5 mm (Andrady, 2011; Arthur et al., 2009; Wessel et al., 2016); measuring approximately ~0.05 mm 2 mm x 2 mm in size were washed with ultra-pure 18 MU cm water, air dried and weighed. Aliquots of microplastic particles of a combined mass of about 10e12 mg were placed in 20-ml scintillation glass vials measuring 28 61 mm (Sigma- Aldrich). Each vial was amended with a 20-mm magnetic stir bar to enable mixing. Reagent grade water and sodium hypochlorite solution (12.5% Cl2 by weight) were added to the vials to achieve the desired concentrations, while the solution pH was kept in the range of 6e7 (California State Water Resources Control Board, 1981; USEPA, 1999). Vials were closed with a lid and incubated at the conditions specified. Upon incubation, microplastics were rinsed with DI water, dried and stored in clean 20-ml glass scintillation vials prior to analysis. 2.3. Chlorination chemistry and dosage Both virgin plastics and environmentally weathered plastics were exposed to a Cl2 concentration of 5 mg/L for 30 min at a CT value of 150 mg min/L (WWTP conditions) (USEPA, 1999) followed
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by the common dechlorination procedure (Procedure, 2004) employing sodium thiosulfate as well as Cl2 concentration of 2.5 mg/L for 30 min at a CT value of 75 mg min/L (drinking water conditions) (World Health Organization, 2004) both at room temperatures. Two additional sets of experiments at extreme conditions which are much higher in concentration and contact time than those extant in WWTPs, including: one set of plastics exposed to Cl2 concentrations of 25,000 mg/L for 1 day equivalent to a CT value of 25 g d/L incubated at ambient temperature, and a second set to test even harsher conditions using Cl2 concentrations of 5.5 104 mg/L for 1e9 days, equivalent to CT values of 55e500 g d/ L at 65 C; additional information is provided in the supporting information (SI). Environmentally weathered plastics sourced from an Asian beach, available only for HDPE and PP, were analyzed according to the same protocol detailed above. The pH range of 6e7 was maintained for all of the experiments to ensure the maximum availability of hypochlorous acid. H2O þ NaOCl / HOCl þ NaOH
(1)
HOCl þ HCl )/ Cl2 þ H2O
(2)
2 NaOCl þ H2SO4 / Na2SO4 þ 2 HOCl
(3)
2.4. Concentration-time (CT) values Effective CT values were calculated using CT ¼ C T
(4)
Where C is the concentration of chlorine mg/L and T is the time in minutes (mg min/L).
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before and after chlorine exposure. Given the widespread use of both plastic polymers and chlorine disinfection, one would expect conventional polymers to be fairly resistant to oxidative attack during sterilization. Indeed, no changes visible to the naked eye were apparent in plastics exposed to chlorine at CT values of 75 mg min/L and 150 mg min/L. However, use of Raman spectroscopy revealed that plastics are by no means completely resistant to chlorine attack. Whereas the susceptibility of plastics to chemical degradation varied by polymer type, all three materials investigated showed some characteristic alterations, detectable as changes in Raman intensity, complete disappearance of Raman peaks (correlating to the breaking of a chemical bonds) and emergence of new Raman peaks indicative of the formation of new chemical bonds particularly under very aggressive chlorine dosage regimes not extant during water treatment. 3.1. HDPE degradation The chemical structure of HDPE was not altered at dosages reflective of drinking water treatment and wastewater treatment. However, notable peak shifts and new peaks were observed at escalated dosages of 25 g d/L (Fig. 1). Notable Raman peaks for HDPE included 1064 cm1 (CeCeC asymmetrical chain), 1130 cm1 (CeCeC symmetrical chain), 1295 cm1 (CH2 twist), and 1416 cm1 (CH2 bend) (Coluthup et al., 1975; Larkin, 2011; Pigeon et al., 1991), which shifted at a higher chlorine dose, hinting at a force of compression on the peaks due to intense chlorination (Eichhorn et al., 2001). No significant shifts were observed at 75 and 150 mg min/L which signified that chlorine doses used during drinking water and wastewater treatment are not strong enough to cause chemical changes in HDPE microplastics. The method detection limit (MDL) for each new bond formed was calculated as three times the average signal to noise ratio of the 0 mg min/L HDPE spectra for the three replicates. Introduction of a new chlorinecarbon bond (CleCH2eCeH) is evidenced by the peak at 678 cm1 in Fig. 1A.
2.5. Dechlorination dosage 3.2. Polypropylene degradation Dechlorination, or quenching of free chlorine, was carried out using powdered sodium thiosulfate (Na2S2O3 at a dose of 12 mg/L) immediately after the chlorination reaction was concluded. Exposed microplastics then were removed, rinsed with DI water, air dried, and analyzed using Raman spectroscopy. 2.6. Raman spectroscopy Micro-Raman imaging was carried out using a Renishaw InVia confocal microscope and Raman spectrometer (Renishaw InVia, London, UK) at magnifications of 5x, 20x, 50x and 100x and a numerical aperture of 0.75. Samples were analyzed using a 15-mW laser of a 488-nm wavelength at 5e10% laser intensity with exposure times of 10e15 s. Silicon (111) wafer material served as the calibration standard in all experiments. Focusing was achieved incrementally using the 5x, 20x and finally using 50x lenses. Obtained spectra were exported to Origin Pro data analysis software for data processing. The baseline was corrected using the baseline correction feature in Origin Pro, and peak heights were normalized to 1.0 to study relative changes in peak height ratios. Absolute intensity plots were obtained as an average of multiple readings of Raman intensity plotted against the time (n ¼ 3). 3. Results Chemical changes in the microplastics were investigated with Raman spectroscopy by comparing obtained spectra from polymers
Polypropylene (PP) is used in many commercial, scientific, and industrial applications. The widespread use of PP motivated its selection for studying its potential for oxidative degradation during chlorination. In contrast to HDPE, PP was found to be essentially resistant to chlorination. No new peaks or disappearance of bonds were observed in the Raman spectra of PP even at very high chlorine dosages and extended exposure times (see Fig. 2). The minor changes in peak intensity observed in these extreme conditions can be considered as being negligible. 3.3. Polystyrene degradation Polystyrene is widely used in manufacturing of foams, coffee cup lids, restaurant to-go containers, and floatation materials at docks because of its low cost of production and vast utility. The twin peaks at 400 cm1 and 445 cm1 were observed to widen at moderate CT values (75 and 150 mg min/L), eventually giving rise to a new peak at 348 cm1 in extreme chlorination conditions. Widening of a peak is synonymous with the partial degradation of a bond, in this case, as a result of oxidative stress. In extreme conditions, a peak shift was detected at the higher wave numbers, indicating a compression action (Eichhorn et al., 2001) on the bond corresponding to 445 cm1. Shifting of the aliphatic CeH backbone from 2901 cm1 to 2940 cm1 also was observed (Fig. 3B) at the highest chlorine dosage and a widening of peaks at the more modest chlorination dosages, indicating polymer degradation. The
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Fig. 1. Raman shifts observed for high density polyethylene (HDPE) as a result of chlorination. (A) In the spectral range of 600-800 cm-1, formation of a new peak at 678 cm-1 and peak shifts were observed only at high chlorine doses of 25 g d/L. (B) Furthermore, a 20% decrease in CH2 stretch intensity was observed upon high chlorine dose, while no significant effects were observed at CT values of 75 and 150 mg min/L.
Fig. 2. Raman shifts observed for polypropylene (PP) because of chlorination. (A) The spectral range of 250 to 1500 cm-1. highlighting the bonds at 801 cm-1 (C-C-C stretch) and 1454 cm-1 (CH2 bend), is shown. (B) The spectral range of 2700 to 3000 cm-1, highlighting bonds at 2839, 2851 and 2873 cm-1, is shown.
Fig. 3. Raman shifts observed for polystyrene (PS) as a result of chlorination. (A) Detected changes included the formation of peak at 348 cm-1 and a peak shift at 440 cm-1 within a spectral range of 300-700 cm-1. (B) A widening observed at the backbone peak at 2901 cm-1 (aliphatic C-H group was notable even at low chlorine doses of 75 and 150 mg min/L.
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detected shift gradually progressed to 2940 cm1 over time. This shift of the backbone bond towards a higher wavenumber signifies the force of compression on the backbone bond towards a state of higher energy (Eichhorn et al., 2001). Additionally, a study done on chlorine degradation of PS and poly (styrene-b-butadiene-b-styrene), FTIR analysis revealed the presence of chlorine at both the aliphatic and aromatic positions of PS (Zebger et al., 2003). A direct oxidative attack on the backbone of a polymer was possible because of the absence of both plasticizers and annealing of the plastics before chlorination. Thus, the presence of plastic additives and thermal treatments can play a major role in altering the plastic structure even with a strong oxidant like chlorine. 3.4. Differential scanning calorimetry (DSC) Differential scanning calorimetry is a useful technique for assessing the physical characteristics of polymers. The DSC analysis of plastics exposed to extreme chlorination conditions yielded a significant change, indicated by altered melting point characteristics of stressed plastics (see Supporting Information). No such changes were expected to occur under the softer disinfection techniques that lacked such strong structural changes, as determined previously by Raman spectroscopy. 3.5. Dechlorination Dechlorination is practiced primarily in the treatment of wastewater to remove residual disinfectant prior to reclaiming of treated effluent via discharge to surface waters. Exposure of the three polymers to the one dechlorination agent primarily used in wastewater treatment, powdered sodium thiosulfate (Na2S2O3) at a dose of 12 mg/L, did not result in any noticeable changes as determined by Raman spectrometry.
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and UV) used at treatment plants may yield different effects on polymer structure and properties. Additionally, this study focused exclusively on polymers in isolation, leaving uncertain any potential interactions of chlorine with plasticizers that may be formulated into plastic polymers (Hahladakis et al., 2018; Mitroka et al., 2013). 4. Conclusions This research demonstrates that commercial plastic polymers, which are in widespread use globally, have the potential to be structurally and physically altered when contacted with chlorine as a sterilization agent. Favorable findings were that all plastics exhibited an appreciable degree or even a very strong degree of resistance to oxidative attack by chlorine. Specifically, for HDPE and PP it was found that disinfection conditions prevailing at drinking water and wastewater treatment plants are not aggressive enough to lead to detectable and important changes in these polymeric materials. However, PS was an exception to this conclusion, as this polymer exhibited detectable changes in structure even at CT values as low as 75 and 150 mg min/L. From an ecotoxicological perspective, the discovery of the formation of new chlorine-carbon bonds during chlorination of HDPE under extreme dosages was the most noteworthy finding. Whereas the harsh conditions tested here will be rarely met in typical industrial settings, collected data suggest that chlorination of polymers does have the potential to introduce new structural features of elevated ecotoxicological and human health concern, specifically, the de novo formation of chlorine-carbon bonds, as observed here in HDPE microplastics. Overall, the data collected in this work may inform a better assessment of types and magnitudes of risks posed by wastewaterborne and sewage sludge-borne microplastics destined, respectively, for discharge into receiving surface waters and for application on land.
3.6. Chlorination of environmentally weathered plastics Funding Microplastics in water and wastewater may have diverse origins and potentially may have been subjected to weathering in one or multiple environmental settings. When environmentally weathered plastics made from HDPE and PP were exposed to chlorination and dechlorination agents, the resultant spectra yielded no observable changes different from what was observed with virgin materials (see SI). However, the spectra of environmental MPs in general displayed more baseline variation, potentially indicating rougher surface characteristics that caused the Raman laser focal point to deviate, producing more noisy signals.
This project was supported in part by award LTR 05/01/12 of the Virginia G. Piper Charitable Trust. The content is solely the responsibility of the authors and does not necessarily represent the official views of the sponsor. Conflicts of interest The authors declare no known or perceived conflict of interest. Acknowledgements
3.7. Study limitations This study yielded new data and insights into the behavior of commercial polymers during chlorination, but it also had some limitations. All experiments were carried out with reagent grade water. In real-world chlorination applications, the presence of other pollutants, microorganisms and biofilms may alter the extent of structural changes observable due to competitive reactions and chlorine quenching. Thus, the effective dosage of chlorine available for interaction with drinking water- and particularly wastewaterborne microplastics may be smaller than the dosages used here. Therefore, the results reported are conservative and represent a worst-case scenario. As mentioned previously, the extreme doses of 2.5 104 mg/L at 24-h contact time and 5.5 104 mg/L at 1e9 days contact time are not representative of the conditions prevalent in wastewater or drinking water treatment plants. This study focused on the effects of NaClO on three plastic types based on their abundance. Other disinfectants (Cl2 gas, ozone, chlorine dioxide
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Chemical and physical changes of microplastics during sterilization by chlorination Varun P. Kelkar a, Charles B. Rolsky a, Anupum Pant c, Matthew D. Green b, Sefaattin Tongay c, Rolf U. Halden a, * a
Center for Environmental Health Engineering, The Biodesign Institute, Arizona State University, 1001 S. McAllister Avenue, Tempe, AZ, 85287-8101, USA Chemical Engineering, School for Engineering of Matter, Transport and Energy, Arizona State University, 551 E. Tyler Mall, Tempe, AZ, 85287-6106, USA Materials Science and Engineering, School for Engineering of Matter, Transport and Energy, Arizona State University, 551 E. Tyler Mall, Tempe, AZ, 852876106, USA b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 February 2019 Received in revised form 9 July 2019 Accepted 14 July 2019 Available online 15 July 2019
Wastewater treatment plants are known to release microplastics that have been detected in aquatic and terrestrial organisms constituting part of the human diet. Chlorination of wastewater-borne microplastics was hypothesized to induce chemical and physical changes detectable by Raman spectroscopy and differential scanning calorimetry (DSC). In the laboratory, virgin plastics (~0.05 2 2 mm) were exposed to differing sterilization conditions representative of dosages used in the disinfection of drinking water, wastewater, and heavily contaminated surfaces. Polypropylene (PP) was most resistant to chlorination, followed by high density polyethylene (HDPE) and polystyrene (PS). Polystyrene showed degradation, indicated by changes in Raman peak widths, at concentration-time regimes (CT values) as low as 75 mg min/L, whereas HDPE and PP remained unaltered even at chlorine doses characteristic of wastewater disinfection (150 mg min/L). However, HDPE and PS were not completely resistant to oxidative attack by chlorination. Under extremely harsh conditions, shifts in Raman peaks and the formation of new bonds were observed. These results show that plastics commonly used in consumer products can be chemically altered, some even under conditions prevailing during wastewater treatment. Changes in polymer properties, observed for HDPE and PP under extreme exposure conditions only, are predicted to alter the risk microplastics pose to aquatic and terrestrial biota, since an increase in carbonchlorine (CeCl) bonds is known to increase toxicity, rendering the polymers more hydrophobic and thus more prone to adsorb, accumulate, and transport harmful persistent pollutants to biota in both aquatic and terrestrial environments. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Wastewater treatment plants Microplastics Chlorination Raman spectroscopy Degradation
1. Introduction The presence and accumulation of microplastics (MPs) in both aquatic and terrestrial environments is a growing concern worldwide, with freshwater, coastal, and marine polymer debris posing risks to aquatic animals and birds which constitute part of the human diet (Wang et al., 2016). Moreover, MPs also have been found in other food products for human consumption and even in bottled mineral water (Obmann et al., 2018). Primary MPs include microbeads (typically <1 mm in diameter) and manufactured pellets (~1e5 mm in diameter) that serve as an industrial feedstock for
* Corresponding author. E-mail address: [email protected] (R.U. Halden). https://doi.org/10.1016/j.watres.2019.114871 0043-1354/© 2019 Elsevier Ltd. All rights reserved.
commercial plastic products. Microbeads are still being used in certain countries as an ingredient in personal care products such as shampoo, face scrubs, and toothpaste. However, environmentally more prevalent are secondary microplastics originating from discarded consumer products, plastic packaging and synthetic fabrics become fragmented as a result of physical stress imparted by harsh environmental conditions, including wave and wind energy, sunlight, erosion, and temperature stress (Andrady, 2011; Cole et al., 2011; Ng and Obbard, 2006; Van Cauwenberghe et al., 2013). These altered microplastics are known to carry harmful contaminants such as polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs), dichlorodiphenyltrichloroethane (DDT) and dexachlorocyclohexane (HCH) via sorption mechanisms (Wang et al., 2018). In addition to the above environmental material alterations, of plastic polymers potentially also could be chemically
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Nomenclature MPs HDPE PP PS WWTPs PCBs PAHs DDT HCH DSC CT MDL Mp DHm UV FTIR
Microplastics high density polyethylene polypropylene polystyrene wastewater treatment plants polychlorinated biphenyls polyaromatic hydrocarbons Dichlorodiphenyltrichloroethane Hexachlorocyclohexane differential scanning calorimetry concentration-time method detection limit melting point enthalpy of melting ultraviolet Fourier transform infrared spectroscopy
altered during the application of bleach onto plastic surfaces for decontamination and cleanup of septic spills on boats and in households, leading to very harsh sterilization regimes exceeding those extant at water and wastewater treatment plants (Doi et al., 2017; Martin and Richards, 2010; Perez et al., 2017; Rothlisberger et al., 2010). In 2013, it was estimated that 299 million metric tons of plastic were generated of which around 12.5 million tons annually end up in freshwater resources and the oceans (Wang et al., 2016); (Hahladakis et al., 2018). With increasing world population, these numbers are expected to grow. Research has shown that microplastics, defined as polymer structures measuring 5 mm or less in diameter, are emitted into surface waters contained in the effluent of wastewater treatment plants (WWTPs) (Carr et al., 2016; Dris et al., 2015; Lares et al., 2018). Most WWTPs in the U.S. and Canada feature no designated pollution control mechanism for the removal of microplastics (Driedger et al., 2015), although European studies show that removal efficiencies of up to 97% can be achieved for microplastics (Dris et al., 2015). Studies examining the loading and removal efficiencies of microplastics, as conducted for example at WWTPs in Los Angeles, USA and Paris, France are still rare (Carr et al., 2016; Dris et al., 2015) and little is known about physical and chemical changes in polymers during water, wastewater and surface sterilization. Chlorine is the most widely used disinfection agent in WWTPs around the world and also finds application as a cleaning agent for sterilizing fish-processing work areas and surfaces having come in contact with urine, stool and vomit. Because of its strong oxidizing nature, chlorine has the potential to affect physical and chemical properties of microplastics, potentially breaking existing bonds and introducing new ones. Introduction of chlorine-carbon bonds would be of particular concern, as substitution of hydrogen with chlorine atoms in organic compounds tends to increase their environmental persistence and ecotoxicity (El-Shahawi et al., 2010). Degradation of MPs with chlorine can be potentially dangerous since the aged MPs have a higher sorptive affinity to harmful chemicals than do virgin MPs (Wang et al., 2018). Multiple studies have investigated the impact of oxidants such as Cl2 gas, Cl2O, NaOCl on polymers (Khatua and Hsieh, 1997; Kwon and Leckie, 2006; Mitroka et al., 2013; Pages et al., 1996; Whelton and Dietrich, 2009; Zebger et al., 2003); but the fate of wastewater-borne microplastics during chlorination has not yet been investigated in detail. The impact of chlorinated water on
HDPE piping used in residential water supply was studied in the past, revealing the formation of unwanted chemicals, including 4chloro-2-methylbutan-2-ol and 2,3-dichloro-2-methylbutane at high chlorine doses (50 mg/L, 250 mg/L and 500 mg/L) (Mitroka et al., 2013). Whereas the chlorinated byproducts released from pipes into water were concluded to have potential adverse health implications for humans and aquatic ecosystems, potential physical and chemical changes in the pipe material itself were not investigated. The present study employed a combination of non-destructive Raman spectroscopy (Araujo et al., 2018) and differential scanning calorimetry (DSC) to quantitatively examine oxidative damage to polystyrene (PS), polypropylene (PP), and high-density polyethylene (HDPE) microplastics in conditions of varying concentrations of chlorine and exposure times. 2. Materials and methods 2.1. Chemicals Reagent grade sodium hypochlorite solution (12.5 ± 2.5% available chlorine), hydrogen chloride solution (12M), sulfuric acid solution (18.4 M) and sodium thiosulfate powder (99.5%) were purchased from Sigma-Aldrich Corp., St. Louis, MO, USA. Plastic polymer fragments were obtained by modifying authentic consumer products (milk jugs, coffee cup lids etc.) manufactured from virgin, food-grade plastics using a pair of scissors. Used plastics were obtained from samples received from a beach from a south east Asian country and consisted of high density polyethylene and polypropylene beads. The authenticity of the polymer materials was confirmed by Raman spectroscopy via spectral library matching. 2.2. Chlorination of microplastics Experiments simulating chlorination conditions during wastewater treatment, drinking water treatment, and two extreme conditions by gradually increasing the chlorine concentration, contact time and temperature were carried out with select types of polymers based on their abundance in the environment. Extreme conditions were used to observe the physical and chemical changes plastic particles can undergo in worst case scenarios which are not realistic or representative of the conditions prevailing in engineered water environments. Impact of chlorination on HDPE, PP, and PS was investigated. Plastic particles in the microplastics range of less than 5 mm (Andrady, 2011; Arthur et al., 2009; Wessel et al., 2016); measuring approximately ~0.05 mm 2 mm x 2 mm in size were washed with ultra-pure 18 MU cm water, air dried and weighed. Aliquots of microplastic particles of a combined mass of about 10e12 mg were placed in 20-ml scintillation glass vials measuring 28 61 mm (Sigma- Aldrich). Each vial was amended with a 20-mm magnetic stir bar to enable mixing. Reagent grade water and sodium hypochlorite solution (12.5% Cl2 by weight) were added to the vials to achieve the desired concentrations, while the solution pH was kept in the range of 6e7 (California State Water Resources Control Board, 1981; USEPA, 1999). Vials were closed with a lid and incubated at the conditions specified. Upon incubation, microplastics were rinsed with DI water, dried and stored in clean 20-ml glass scintillation vials prior to analysis. 2.3. Chlorination chemistry and dosage Both virgin plastics and environmentally weathered plastics were exposed to a Cl2 concentration of 5 mg/L for 30 min at a CT value of 150 mg min/L (WWTP conditions) (USEPA, 1999) followed
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by the common dechlorination procedure (Procedure, 2004) employing sodium thiosulfate as well as Cl2 concentration of 2.5 mg/L for 30 min at a CT value of 75 mg min/L (drinking water conditions) (World Health Organization, 2004) both at room temperatures. Two additional sets of experiments at extreme conditions which are much higher in concentration and contact time than those extant in WWTPs, including: one set of plastics exposed to Cl2 concentrations of 25,000 mg/L for 1 day equivalent to a CT value of 25 g d/L incubated at ambient temperature, and a second set to test even harsher conditions using Cl2 concentrations of 5.5 104 mg/L for 1e9 days, equivalent to CT values of 55e500 g d/ L at 65 C; additional information is provided in the supporting information (SI). Environmentally weathered plastics sourced from an Asian beach, available only for HDPE and PP, were analyzed according to the same protocol detailed above. The pH range of 6e7 was maintained for all of the experiments to ensure the maximum availability of hypochlorous acid. H2O þ NaOCl / HOCl þ NaOH
(1)
HOCl þ HCl )/ Cl2 þ H2O
(2)
2 NaOCl þ H2SO4 / Na2SO4 þ 2 HOCl
(3)
2.4. Concentration-time (CT) values Effective CT values were calculated using CT ¼ C T
(4)
Where C is the concentration of chlorine mg/L and T is the time in minutes (mg min/L).
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before and after chlorine exposure. Given the widespread use of both plastic polymers and chlorine disinfection, one would expect conventional polymers to be fairly resistant to oxidative attack during sterilization. Indeed, no changes visible to the naked eye were apparent in plastics exposed to chlorine at CT values of 75 mg min/L and 150 mg min/L. However, use of Raman spectroscopy revealed that plastics are by no means completely resistant to chlorine attack. Whereas the susceptibility of plastics to chemical degradation varied by polymer type, all three materials investigated showed some characteristic alterations, detectable as changes in Raman intensity, complete disappearance of Raman peaks (correlating to the breaking of a chemical bonds) and emergence of new Raman peaks indicative of the formation of new chemical bonds particularly under very aggressive chlorine dosage regimes not extant during water treatment. 3.1. HDPE degradation The chemical structure of HDPE was not altered at dosages reflective of drinking water treatment and wastewater treatment. However, notable peak shifts and new peaks were observed at escalated dosages of 25 g d/L (Fig. 1). Notable Raman peaks for HDPE included 1064 cm1 (CeCeC asymmetrical chain), 1130 cm1 (CeCeC symmetrical chain), 1295 cm1 (CH2 twist), and 1416 cm1 (CH2 bend) (Coluthup et al., 1975; Larkin, 2011; Pigeon et al., 1991), which shifted at a higher chlorine dose, hinting at a force of compression on the peaks due to intense chlorination (Eichhorn et al., 2001). No significant shifts were observed at 75 and 150 mg min/L which signified that chlorine doses used during drinking water and wastewater treatment are not strong enough to cause chemical changes in HDPE microplastics. The method detection limit (MDL) for each new bond formed was calculated as three times the average signal to noise ratio of the 0 mg min/L HDPE spectra for the three replicates. Introduction of a new chlorinecarbon bond (CleCH2eCeH) is evidenced by the peak at 678 cm1 in Fig. 1A.
2.5. Dechlorination dosage 3.2. Polypropylene degradation Dechlorination, or quenching of free chlorine, was carried out using powdered sodium thiosulfate (Na2S2O3 at a dose of 12 mg/L) immediately after the chlorination reaction was concluded. Exposed microplastics then were removed, rinsed with DI water, air dried, and analyzed using Raman spectroscopy. 2.6. Raman spectroscopy Micro-Raman imaging was carried out using a Renishaw InVia confocal microscope and Raman spectrometer (Renishaw InVia, London, UK) at magnifications of 5x, 20x, 50x and 100x and a numerical aperture of 0.75. Samples were analyzed using a 15-mW laser of a 488-nm wavelength at 5e10% laser intensity with exposure times of 10e15 s. Silicon (111) wafer material served as the calibration standard in all experiments. Focusing was achieved incrementally using the 5x, 20x and finally using 50x lenses. Obtained spectra were exported to Origin Pro data analysis software for data processing. The baseline was corrected using the baseline correction feature in Origin Pro, and peak heights were normalized to 1.0 to study relative changes in peak height ratios. Absolute intensity plots were obtained as an average of multiple readings of Raman intensity plotted against the time (n ¼ 3). 3. Results Chemical changes in the microplastics were investigated with Raman spectroscopy by comparing obtained spectra from polymers
Polypropylene (PP) is used in many commercial, scientific, and industrial applications. The widespread use of PP motivated its selection for studying its potential for oxidative degradation during chlorination. In contrast to HDPE, PP was found to be essentially resistant to chlorination. No new peaks or disappearance of bonds were observed in the Raman spectra of PP even at very high chlorine dosages and extended exposure times (see Fig. 2). The minor changes in peak intensity observed in these extreme conditions can be considered as being negligible. 3.3. Polystyrene degradation Polystyrene is widely used in manufacturing of foams, coffee cup lids, restaurant to-go containers, and floatation materials at docks because of its low cost of production and vast utility. The twin peaks at 400 cm1 and 445 cm1 were observed to widen at moderate CT values (75 and 150 mg min/L), eventually giving rise to a new peak at 348 cm1 in extreme chlorination conditions. Widening of a peak is synonymous with the partial degradation of a bond, in this case, as a result of oxidative stress. In extreme conditions, a peak shift was detected at the higher wave numbers, indicating a compression action (Eichhorn et al., 2001) on the bond corresponding to 445 cm1. Shifting of the aliphatic CeH backbone from 2901 cm1 to 2940 cm1 also was observed (Fig. 3B) at the highest chlorine dosage and a widening of peaks at the more modest chlorination dosages, indicating polymer degradation. The
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Fig. 1. Raman shifts observed for high density polyethylene (HDPE) as a result of chlorination. (A) In the spectral range of 600-800 cm-1, formation of a new peak at 678 cm-1 and peak shifts were observed only at high chlorine doses of 25 g d/L. (B) Furthermore, a 20% decrease in CH2 stretch intensity was observed upon high chlorine dose, while no significant effects were observed at CT values of 75 and 150 mg min/L.
Fig. 2. Raman shifts observed for polypropylene (PP) because of chlorination. (A) The spectral range of 250 to 1500 cm-1. highlighting the bonds at 801 cm-1 (C-C-C stretch) and 1454 cm-1 (CH2 bend), is shown. (B) The spectral range of 2700 to 3000 cm-1, highlighting bonds at 2839, 2851 and 2873 cm-1, is shown.
Fig. 3. Raman shifts observed for polystyrene (PS) as a result of chlorination. (A) Detected changes included the formation of peak at 348 cm-1 and a peak shift at 440 cm-1 within a spectral range of 300-700 cm-1. (B) A widening observed at the backbone peak at 2901 cm-1 (aliphatic C-H group was notable even at low chlorine doses of 75 and 150 mg min/L.
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detected shift gradually progressed to 2940 cm1 over time. This shift of the backbone bond towards a higher wavenumber signifies the force of compression on the backbone bond towards a state of higher energy (Eichhorn et al., 2001). Additionally, a study done on chlorine degradation of PS and poly (styrene-b-butadiene-b-styrene), FTIR analysis revealed the presence of chlorine at both the aliphatic and aromatic positions of PS (Zebger et al., 2003). A direct oxidative attack on the backbone of a polymer was possible because of the absence of both plasticizers and annealing of the plastics before chlorination. Thus, the presence of plastic additives and thermal treatments can play a major role in altering the plastic structure even with a strong oxidant like chlorine. 3.4. Differential scanning calorimetry (DSC) Differential scanning calorimetry is a useful technique for assessing the physical characteristics of polymers. The DSC analysis of plastics exposed to extreme chlorination conditions yielded a significant change, indicated by altered melting point characteristics of stressed plastics (see Supporting Information). No such changes were expected to occur under the softer disinfection techniques that lacked such strong structural changes, as determined previously by Raman spectroscopy. 3.5. Dechlorination Dechlorination is practiced primarily in the treatment of wastewater to remove residual disinfectant prior to reclaiming of treated effluent via discharge to surface waters. Exposure of the three polymers to the one dechlorination agent primarily used in wastewater treatment, powdered sodium thiosulfate (Na2S2O3) at a dose of 12 mg/L, did not result in any noticeable changes as determined by Raman spectrometry.
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and UV) used at treatment plants may yield different effects on polymer structure and properties. Additionally, this study focused exclusively on polymers in isolation, leaving uncertain any potential interactions of chlorine with plasticizers that may be formulated into plastic polymers (Hahladakis et al., 2018; Mitroka et al., 2013). 4. Conclusions This research demonstrates that commercial plastic polymers, which are in widespread use globally, have the potential to be structurally and physically altered when contacted with chlorine as a sterilization agent. Favorable findings were that all plastics exhibited an appreciable degree or even a very strong degree of resistance to oxidative attack by chlorine. Specifically, for HDPE and PP it was found that disinfection conditions prevailing at drinking water and wastewater treatment plants are not aggressive enough to lead to detectable and important changes in these polymeric materials. However, PS was an exception to this conclusion, as this polymer exhibited detectable changes in structure even at CT values as low as 75 and 150 mg min/L. From an ecotoxicological perspective, the discovery of the formation of new chlorine-carbon bonds during chlorination of HDPE under extreme dosages was the most noteworthy finding. Whereas the harsh conditions tested here will be rarely met in typical industrial settings, collected data suggest that chlorination of polymers does have the potential to introduce new structural features of elevated ecotoxicological and human health concern, specifically, the de novo formation of chlorine-carbon bonds, as observed here in HDPE microplastics. Overall, the data collected in this work may inform a better assessment of types and magnitudes of risks posed by wastewaterborne and sewage sludge-borne microplastics destined, respectively, for discharge into receiving surface waters and for application on land.
3.6. Chlorination of environmentally weathered plastics Funding Microplastics in water and wastewater may have diverse origins and potentially may have been subjected to weathering in one or multiple environmental settings. When environmentally weathered plastics made from HDPE and PP were exposed to chlorination and dechlorination agents, the resultant spectra yielded no observable changes different from what was observed with virgin materials (see SI). However, the spectra of environmental MPs in general displayed more baseline variation, potentially indicating rougher surface characteristics that caused the Raman laser focal point to deviate, producing more noisy signals.
This project was supported in part by award LTR 05/01/12 of the Virginia G. Piper Charitable Trust. The content is solely the responsibility of the authors and does not necessarily represent the official views of the sponsor. Conflicts of interest The authors declare no known or perceived conflict of interest. Acknowledgements
3.7. Study limitations This study yielded new data and insights into the behavior of commercial polymers during chlorination, but it also had some limitations. All experiments were carried out with reagent grade water. In real-world chlorination applications, the presence of other pollutants, microorganisms and biofilms may alter the extent of structural changes observable due to competitive reactions and chlorine quenching. Thus, the effective dosage of chlorine available for interaction with drinking water- and particularly wastewaterborne microplastics may be smaller than the dosages used here. Therefore, the results reported are conservative and represent a worst-case scenario. As mentioned previously, the extreme doses of 2.5 104 mg/L at 24-h contact time and 5.5 104 mg/L at 1e9 days contact time are not representative of the conditions prevalent in wastewater or drinking water treatment plants. This study focused on the effects of NaClO on three plastic types based on their abundance. Other disinfectants (Cl2 gas, ozone, chlorine dioxide
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