Bio-aging of Polyethylene And Its Impact on Sorption of Organics

Introduction Microplastics (MPs) are plastic debris resulting from plastic waste fragmentation or direct emissions of micro- and nano-sized plastics from a variety of sources. Recently, MPs were added to the list of global threats because they are ubiquitous and may pose a risk to the food chain as a result of their interaction with organic contaminants (OCs) and heavy metals (Y. Wang et al., 2022; Zhou et al., 2022). Hence, understanding the interaction mechanisms of OCs with MPs is essential for evaluating the impact of MPs on the environment. Wastewater treatment plants receive a high quantity of MPs from a variety of sources. These MPs accumulate in sludge and this, together with the sorption potential for OCs, some of them endocrine disruptors (EDCs), on MPs cause a special concern for land application of biosolids. Furthermore, MPs also undergo aging in treatment processes and in environment, which may in turn affect their interaction with OCs. MPs subjected to UV aging have been widely studied in the literature, however, their biological aging of them are not equally investigated (Zafar et al., 2022). Especially bioaging during sludge digestion which is applied right before land application was not investigated. Additionally studies indicated that MPs not only simply bio-age during digestion but also inhibit the microbial community and may lower methane production levels when present in high doses which also creates additional concern for the environment as the doses of MPs keep increasing (Wei et al., 2019). This study aims to investigate the bioaging potential of polyethylene (PE) MPs in mesophilic anaerobic digesters operated as BMP reactors. Study also examines the comparative sorption of two OCs, one of them an EDC, on pristine (PE0) and bio-aged polyethylene (PE1) collected from anaerobic digesters. For this purpose, biotic and abiotic anaerobic reactors (BMP test bottles) containing a high dose of PE MPs (200 mg/g TS) were operated together with reactors containing no MPs. Following digestion bio-aged and pristine MPs are characterized using FTIR and SEM to compare the changes in PE's physical and chemical properties. Sorption of two OCs, namely 2,3,6-trichlorophenol and triclosan (an EDC) are investigated. Methane production is monitored in biotic and abiotic reactors to see the effect of MPs on digestion process to check whether there is inhibition on the microbial community. Materials and Methods Three sets of reactors were designed: biotic (B), abiotic (A), and seed control (S). BMP reactors were set-up using waste activated sludge (WAS) and digested sludge (seed) collected from a conventional WWTP with a capacity of 765,000 m3/d (4 millon design population equivalent). In reactors F/M ratio was 1, TS was adjusted to 2% and VS/TS ratio was 0.59. Details of reactor sets including the number of replicates are explained in Table 1. Biogas volumes in reactors were measured using a water displacement set-up. A gas chromatography (GC) device equipped with a thermal conductivity detector (TCD) was used to measure the biogas methane content. When the reactors are terminated full analysis of parameters shown in Table 2 will be conducted. PE used in reactors had particle size range of 425-500 µm; which were sieved, washed, and ultrasonicated before added to the reactors. The initial characteristics of PE were such that the density was 0.935 g/cm3, the crystallinity (by DSC) was 49.4% and melting temperature (Tm) was 126 °C. Sorption tests with pristine PE (PE0) showed a maximum sorption capacity of 1.0 mg/g for 2,3,6-trichlorophenol (TCP) and 6.9 mg/g for triclosan (TCS). Bioaging may affect sorption onto MPs favorably or unfavorably, depending on the type of changes that happen on PE (e.g., oxidation, peeling, etc.) affecting sorption mechanisms. Results and Discussion Table 2 demonstrates the initial reactor characteristics. The initial VS/TS ratio is higher for biotic and abiotic reactors, on the other hand it is lower in seed control, indicating that seed is more stable as expected. pH values of the samples are as expected as well; with seed having a little higher pH due to being digested compared to the sample. The COD value in abiotic reactors showed to have a wider range and higher values than any other set due to the effect of HgCl2, added to suppress the microorganisms and inhibit biotic activity. On the other hand, seed control showed the lowest COD content because it is stable and there is no WAS in the reactor so total organic content is almost half of the other reactors. The cumulative biogas volume was measured for 45 days (data not shown). Biotic reactors with no MPs addition showed higher biogas production than those with MPs, which, could be due to inhibition caused by high quantity of MPs. Reactors kept producing biogas but at lower amounts as time went on. Seed control reactors produced much smaller amount of biogas. Figure 1 shows the cumulative methane production in mL up to 45 days. Corresponding to increasing biogas volume, methane content also increased within all the biotic reactors. Similar to biogas production, methane production slowed down with time. B0 reactors showed the highest amount of methane, suggesting inhibition effect of MPs. S0 reactors produced much smaller amount of methane since they already reached a stable state before reactor set-up. Abiotic reactors produced no gas, with some days having negligibly small amount of gas production. Interestingly, in the reactors that contain MPs, although methane volume, parallel to the biogas volume, was less than those that did not contain MPs, the percent methane in the biogas was not necessarily less. With our experience we know that biogas and methane production decrease as the organic content in sludge is consumed; which corresponds to a time long before 60 days. Operating reactors for 60 days is with the expectation that 60 days is long enough to see some effect of bio-aging on the plastics. At the reactor termination, full characterization of the reactors will be done. MPs will be manually sieved and picked up from sludge then washed and left to dry. In an earlier trial to analyse the recovery of MPs from the sludge samples, it was found that 84% recovery by wt was achieved with MPs in the size range used here. Recovered MPs will be used for further analysis where, a sample of the abiotic-aged and biotic-aged MPs will be sent to FTIR analysis and SEM imaging. At the same time sorption tests will be conducted on the obtained MPs, for sorption potential of TCP and TCS as model OCs. Based on the level of aging and preliminary results, new reactors will be set for further studies. Results of sorption studies with bio-aged MPs will be included in the full-text manuscript. Conclusion Presence of PE in the digesters affected biogas production negatively, showing discernible difference when compared to the control reactors. Interestingly, in the reactors that contain MPs, although methane volume, in parallel with the biogas volume, was less than those that did not contain MPs, the percent methane in the biogas was not necessarily less. Experimental results currently indicate that the biogas volume in biotic reactors started decreasing each day, demonstrating that sludge is stabilized and the experiment is coming to an end. Once the reactors are taken down, the plastics will be isolated from the reactor content, examined for bio-aging and will be used for sorption of TCP and TCS. Acknowledgements This study was supported by TUBITAK Project No: 220N044.