This study investigated the impact of commonly used treatment chemicals on the morphology and molecular structure of microfibers (MFs) and microplastic films (MPFs) to determine whether significant changes could occur during wastewater treatment. MFs and MPFs were exposed to sodium hypochlorite (NaOCl), hydrogen peroxide (H2O2), calcium hydroxide (Ca(OH)2, pH 11), sodium hydroxide (NaOH, pH11), and hydrochloric acid (HCl, pH 3) at typical doses and exposure times used at wastewater treatment plants. Scanning electron microscopy (SEM) analysis and attenuated total reflectance-Fourier-transform infrared (ATR-FTIR) were used to examine any morphological or chemical changes after the treatment. Morphological changes were observed in the form of cracks, and increased roughness was revealed in the SEM and 3-D surface images. The results showed that MFs were more resistant to surface degradation than MPFs. Moreover, intensity peaks of ATR-FTIR revealed some partial dislodgement of the bonds in both MFs and MPFs after chemical treatment, but the overall polymer structure remained intact. The changes that occur on the surface of MFs and MPFs during chemical treatment can impact their fate, removal, and transportation behavior both at the treatment plant and after discharge to the environment.
We investigated the impact of treatment chemicals on the morphology and molecular structure of microfibers (MFs) and microplastic films (MPFs).
MFs and MPFs were exposed to NaOCl, H2O2, Ca(OH)2, NaOH and HCl.
SEM and ATR-FTIR were used to examine the changes.
Results showed some degradation of MFs and MPFs at low concentrations and exposure times, which may impact their fate, removal, and transport.
Plastics are omnipresent in today's world, and plastic pollution is one of the renowned anthropogenic legacies. Plastic debris has been researched for many years, but it is only recently that microplastics (MPs) have gained attention due to their bioavailability (Laskar & Kumar 2019). Plastics can obstruct the airways and feeding pathways, cause blockage of internal organs, and cause gastrointestinal problems for marine, aquatic, and terrestrial animals. MPs are in high quantities in the environment and, as a result, are in food, air, and drinking water (Nikiema & Asiedu 2022). MPs are plastic polymers with a size <5 mm and can further be grouped into primary and secondary MPs (Jiang et al. 2021). Manufactured in sizes <5 mm, such as microbeads in personal care products, are called primary plastics, and those that are degraded or fragmented to <5 mm in the environment by mechanical or photo-oxidative processes are considered secondary MPs (Adams et al. 2021; Prajapati et al. 2021). Microfibers (MFs) are thread-like fragments of various textiles with a length-to-diameter ratio greater than three and a length between 100 μm and 5 mm (Belzagui & Gutiérrez-Bouzán 2022). The most prevalent synthetic MFs that are found in the environment include nylon (polyamide), polyester (PE), polyethylene terephthalate (PET), polypropylene (PP), and acrylic (polyacrylonitrile) (Sait et al. 2021). MFs are released in surprisingly large quantities from textiles and laundry practices and ultimately end up in wastewater treatment facilities (Ramasamy et al. 2022).
MPs and MFs remain in the environment for many years and degrade into smaller fragments through environmental processes (Mishra et al. 2022a). Previous research reported the degradation of MPs and MFs in the presence of mechanical stress, UV irradiation, and temperature changes (Ding et al. 2020; Sørensen et al. 2021; Corcoran 2022). Ariza-Tarazona et al. (2020) studied the reduction of HDPE MPs using carbon and nitrogen-doped TiO2 by photocatalysis and observed the effects of pH and temperature on photocatalysis. The study showed that low pH values with low temperature favor the photocatalysis of HPDE MPs. Chowdhury et al. (2022) studied the degradation of MPs by mineral acids such as HNO3, H2SO4, and HCl under varying contact time and temperatures and reported significant degradation using scanning electron microscopy (SEM) and attenuated total reflectance-Fourier-transform infrared (ATR-FTIR). MPs became more reactive to oxidation reactions and had increased adsorption capacity at low pH values compared to higher pH (Neghlani et al. 2011). UV irradiation was ineffective in degrading microplastic films (MPFs) at typical UV doses used at treatment plants, but longer irradiation times and higher doses resulted in measurable physical and chemical changes (Cai et al. 2018; Almomani et al. 2019; Ranjan & Goel 2019). Lee et al. (2020) explored the degradation of polyamide 66 (PA66) MFs with UV irradiation and TiO2 and observed a 97% mass loss within 48 h of irradiation. Li et al. (2019) observed significant morphological and molecular changes in polyether sulfone (PES) membrane after exposure to hydrogen peroxide (H2O2) and sodium hypochlorite (NaOCl) for 100 h, where NaOCl resulted in more severe fouling compared to H2O2.
Rodríguez-narvaez et al. (2021) reviewed several engineering technologies for separating and degrading MPs in water, including photocatalysis, Fenton-based, flocculation, chemical or biological digestion, advanced oxidation process, and wet oxidation for MP degradation. Their study concluded that the utilization of the mentioned processes showed concerns about an increase in nanoparticles, long treatment times, and high cost for the feasibility of utilization on a larger scale. They also highlighted the minimal number of studies available on the topic, particularly on the degradation of MPs through biological, chemical, and advanced treatment processes used at treatment plants. The existing studies primarily have focused on separation processes, such as sedimentation, filtration, and membrane processes. There are significant knowledge gaps on the physical, chemical, and morphological changes that occur when MPs go through biological and chemical treatment processes. Furthermore, the main goal of previous studies was to remove MPs from wastewater via separation processes or through chemical degradation.
In addition to the removal of MPs during wastewater treatment processes, it is crucial to study the changes that occur in their structure, surface, size, shape, and morphology since they play a significant role in determining the fate and transport of MPs both at the wastewater treatment plants and later in the environment after effluent discharges. For example, increased surface roughness can impact biofilm formation on MPs and change the density, which in turn would affect their settling and transport behavior. Almomani et al. (2019) reported that roughening the surface of HDPE increased the surface roughness, contact angle, hydrophobicity, biofilm attachment and growth, percentage of live cells, and the performance of an attached-growth sludge treatment process. An increase in the density of MPs and MFs due to biofilm formation can affect the coagulation and flocculation-related separation processes (Rodríguez-narvaez et al. 2021). Due to the increased surface area by biofilm formation, the metal adsorption capacity of MPs increases (Schmitt-Jansen 2017; Bhagwat et al. 2021; Mishra et al. 2022b), which may have significant environmental consequences. Biofilm development attracts invertebrates and algae, further altering the density and transport of MPs in surface waters.
Several harsh chemicals are used at treatment plants for wastewater treatment, including sodium hypochlorite (NaOCl) for disinfection, hydrogen peroxide (H2O2) for advanced oxidation, calcium hydroxide (Ca(OH)2) for phosphorus precipitation, flocculation, and lime treatment, and sodium hydroxide (NaOH) and hydrochloric acid (HCl) for pH adjustment (Metcalf &Eddy and AECOM 2012). This study aimed to investigate the impact of treatment chemicals used for domestic wastewater and sludge treatment on the morphology and molecular structure of MPFs and MFs and determine whether significant changes can occur at typical chemical doses and exposure times used at wastewater treatment plants using advanced microscopy and analytical methods. This is a significant knowledge gap and there are only a few studies on MPFs and none on MFs in this area. If such changes occur at relatively low chemical concentrations and exposure times, further studies would be needed to understand the mechanisms and full implications for wastewater treatment plants and discharges to water environments.
MATERIALS AND METHODS
Chemicals and materials
All chemicals utilized in this study were of analytical grade. Commercially available sodium hypochlorite (NaOCl; 6% w/v), hydrogen peroxide (H2O2; 35% w/v) from Caledon Laboratory Chemicals (Ontario, Canada), calcium hydroxide (Ca(OH)2) from Fisher Scientific, sodium hydroxide (NaOH) solution and HCl from Sigma-Aldrich Chemical Co. were used for the experiment. Distilled water was used to prepare solutions. Microfibre samples were shaved from synthetic clothing and household items, and the MPF was procured from plastic bags by cutting them down into pieces. For further analysis, the vacuum filter apparatus was used to separate the MFs and MPF from the solutions. A 0.45 μm membrane filter of 45 mm diameter from Fisher Scientific was utilized for vacuum filtration.
MP treatment with chemicals
The MFs and MPFs were exposed to various chemicals for 24 h to record morphological or chemical changes. NaOCl, H2O2, Ca(OH)2, NaOH, and HCl were selected for use in this study. In water treatment facilities, NaOCl is used for disinfection, H2O2 is utilized in advanced oxidation processes, Ca(OH)2 is used as a wastewater coagulant and precipitant and for lime treatment of sludge, and NaOH and HCl are used for pH adjustment. A working solution of NaOCl with 50 mg/L of chlorine and a working solution of 15% H2O2 were prepared. Furthermore, working solutions of Ca(OH)2 and NaOH were also prepared in distilled water until pH 11 was reached, whereas the HCl solution was prepared until pH 3 was achieved. 100 ml of each working solution was taken in 150 ml beakers. Each solution was spiked with either 10 mg of MFs or 10 mg of MPFs and was kept for 24 h. Afterward, SEM and ATR-FTIR analyses were performed to observe any change or degradation in the MFs and MPFs. Also, a similar experiment was performed for 30 min to observe any possible change in the MFs and MPFs samples in a shorter time using ATR-FTIR analysis. 30 min and 24 h were selected as representative exposure times during wastewater treatment (Metcalf &Eddy and AECOM 2012).
Characterization of sample MFs and MPFs
Morphological changes in the sample surface were observed utilizing a scanning electron microscope. MFs and MPFs samples were mounted on SEM holders utilizing carbon conductive tape, then coated with a thin conductive layer of Au (∼10 nm) using Quorum Q150T-ES sputter coater imaged using TESCAN Vega-11 XMU SEM at 10KV acceleration voltage with both secondary electron detector and backscattered electron detector. Also, the roughness characteristics were analyzed by the SEM analysis tool. Thermo Scientific Nicolet iS5 FTIR Spectrometer equipped with an iD7 Diamond ATR was utilized to monitor any chemical or molecular level change. The ATR allowed the infrared spectra of the samples to be obtained simply by pressing the MFs and MPFs against a transparent crystal. ATR-FTIR was used in this study to characterize the MFs and MPFs because of its well-established performance and non-destructive nature compared to destructive methods such as thermal degradation methods such as Pyr-GC-MS (Ivleva 2021; Liu et al. 2022). Also, FTIR is preferred over Raman Spectroscopy when the size of the fibers or MPs is larger than 10 μm, which was the case in this study.
RESULTS AND DISCUSSION
Morphology and surface properties of MFs and MPFs before the treatment
Morphology and surface properties of MFs and MPFs after the treatment
Treatment with NaOCl
Treatment with H2O2
Treatment with Ca(OH)2 at pH 11
Treatment with NaOH at pH 11
Treatment with HCl at pH 3
Implications for wastewater treatment plants
MPs is a relatively new area of research and only in recent years has attention been paid to the fate and removal of MPs at wastewater treatment plants. Wastewater treatment plants are a pathway for MPs and wastewater contains high concentrations of all sorts of MPs. MFs and MPFs are understudied compared to MPs and macroplastics, especially in the context of wastewater treatment. The available research on the impact of wastewater treatment processes has mainly focused on the removal performance of treatment processes for MPs, and this study is unique in that it investigated the impact of commonly used wastewater treatment processes on the morphology and molecular structure of MFs and MPFs to determine whether significant changes could occur during wastewater treatment. The results show that MFs and MPFs exposed to wastewater chemicals (NaOCl, H2O2, Ca(OH)2, NaOH, and HCl) at typical doses and exposure times applied at wastewater treatment plants cause changes in the structure of plastics detectable with SEM, ATR-FTIR, and 3-D surface images. Morphological changes were observed on the surfaces of MFs and MPFs, resulting in increased roughness and cracks. Some partial dislodgement of the bonds in both MFs and MPFs after chemical treatment could also be detected using ATR-FTIR, but the overall polymer structure remained intact. Higher doses and exposure times may lead to more damage to the polymer structure and possibly disintegration. The changes that occur on the surface of MFs and MPFs during chemical treatment can impact their fate, removal, and transportation behavior both at the wastewater treatment plant and after wastewater effluents are discharged into the environment. Changes in the surface roughness, for example, can increase biofilm attachment and growth and change the weight and density of plastic particles and impact their settling, dispersion, and transport. In addition, changes in biofilm structure would also impact the microbial population and may assist with harboring pathogens and other microorganisms of concern for the environment or public health. Finally, damage to the polymeric structure, even at microscopic levels, can result in the leaching of resins and other harmful chemicals, potentially impacting biological wastewater treatment processes and aquatic ecosystems.
This study studied the impact of chemicals used at wastewater treatment plants on MPFs and MFs to understand whether significant changes in their morphology and molecular structure could occur at typical doses and exposure times used during wastewater treatment. Morphological and chemical changes in MPs were analyzed after 24 h of exposure to typical chemicals used for water and wastewater treatment. Also, a 30-min exposure experiment was run to investigate if any degradation or molecular change occurred in a shorter time. SEM analysis revealed that MFs and MPFs underwent morphological changes in the form of cracks when treated with NaOCl and H2O2. Moreover, 3-D analysis by SEM showed that the surface roughness of MPFs increased after the treatment, and MFs were more resistant to changes in surface roughness. Furthermore, in the case of Ca(OH)2 (pH 11), NaOH (pH 11), and HCl (pH 3) exposure to the samples, no such change in the morphology occurred except in the case of HCl where due to low pH, some cracks on the surface, as well as increased roughness, was observed on the MPFs. Yet, MFs remained unaltered by the pH change. ATR-FTIR analysis observed some partial dislodgement and stretching of the bonds after the treatment, but the overall polymer structure in both cases for all chemical applications did not change. Overall, the study results showed some degradation of MFs and MPFs at relatively low chemical concentrations and exposure times representative of those employed at wastewater treatment plants, which may impact the fate, removal, and transport of MFs and MPFs both during treatment and after effluent discharges to the environment.
The authors thank the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Jarislowsky Foundation for the funding and resources provided for this research.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.