Monitoring microplastics in wastewater treatment plants by exploiting the innovative Laser Direct InfraRed (LDIR) chemical imaging technique

MPs (i.e. plastic particles in the size range 1 µm – 5 mm of primary or secondary origin [1]) became a contaminant of increasing concern [2]. The role of wastewater treatment plants (WWTPs) as potential sources of microplastic pollution in aquatic ecosystems was pointed out in the literature [3,4]. To date, the lack of reference methodologies represents a bottleneck for MPs monitoring purposes, thus highlighting the need for effective detection, quantification, and characterization methods also in view of a regulatory framework development. In this perspective, this work aims to propose a robust method for monitoring MPs in WWTPs through the innovative Laser Direct InfraRed (LDIR) chemical imaging technique. Since the LDIR technique is still poor investigated for its potential in detecting MPs in wastewaters, this work firstly aimed at fine-tuning both experimental and analytical procedures. Various pre-treatment methods were preliminary compared, leading to the following optimized protocol: (i) sample concentration by vacuum-filtration and subsequent recovery of the material retained on the filter via backwashing with ultrapure water; (ii) chemical digestion via Fenton reaction to remove organics [5]; (iii) density separation in 30 % (w/w) ZnCl2 aqueous solution to remove inorganics (optional); (iv) MPs recovery by vacuum-filtration and backwashing of the filter with ethanol. The dispersions of MPs in ethanol thus obtained were analysed by Agilent 8700 LDIR. Chemical identification of MPs was carried out via real-time matching of the acquired IR spectra with the integrated libraries. Particles recognized as 'cellulosic' were included in the MPs counting since LDIR workflow associates to this polymeric class both cellulose of natural origin and chemically modified/semi-synthetic cellulose (e.g. artificial textile fibers). Size and morphology characterization of the detected particles was performed based on the dimensional and geometric parameters given by the LDIR analysis (i.e. width, height, equivalent diameter, aspect ratio, circularity, etc.): MPs were hence classified into fibers, spheres, pellets and fragments, and a characteristic dimension was assigned to each of them (Table 1). A large WWTP located in Tuscany (Italy) treating both municipal and industrial wastewaters (the latter partially collected to the plant by a separate sewer system) was selected as case-study to apply the above-described methodology. 24-hour composite samples were collected by autosamplers (200 mL sampled each 30 minutes) in the following sections: influent from separate industrial sewer (INSeparate industrial sewer), inlet after union of flows from combined and separate sewers (IN) and effluent (OUT). Preliminary tests were carried out to identify the minimum sample volume to be processed and hence characterized by LDIR as above. Blank control samples were processed to quantify the potential microplastic cross-contamination deriving from both sampling and pre-treatment phases (and hence to subtract it from the MPs concentration in the experimental samples). The results related to the application of the fine-tuned methodologies to the selected case-study are reported below. Preliminary analyses indicated that the MPs concentrations in all treated samples from the monitored WWTP decreased with increasing the processed volume up to a achieve an almost constant value for the 1 L-treated aliquots. For summary constrains, only data related to 1 L-aliquots, judged as sufficiently representative, are hence presented. Considering an average MPs recovery rate of 96% for the applied pre-treatment, the overall concentrations of MPs estimated from the LDIR analysis were about 2.24·105 and 2.03·104 items/L for IN and OUT, respectively (8.08·104 and 4.06·103 items/L for IN and OUT, respectively, without including cellulosic materials in the particle counting). According to these results, it is possible to predict a MPs removal efficiency of about 91% along the water line of the monitored WWTP (that increased up to around 95% if cellulosic particles are not considered), in line with literature data [4]. MPs concentrations (including cellulosic particles) in the range 1.67·105 – 2.26·105 items/L were observed for INSeparate industrial sewer, thus suggesting that a large fraction of MPs entering the WWTP derived from the industrial discharges. The physical and chemical characterization of the detected MPs is summarized in Figure 1. Particles of various origin (mainly in the form of fragments and pellets lower than 200 µm in size and fibers of 100–200 µm) were found in the influent, with a predominance of cellulose-derived materials (64%) followed by polytetrafluoroethylene (PTFE), polyethylene terephthalate (PET), polyurethane (PU) and acrylonitrile butadiene styrene (ABS). These findings agreed with literature data: the lowest removal efficiencies are usually reported for the smallest plastic particles (<190 µm) and fibers that, thanks to their morphology, can more easily by-pass various process units in WWTPs [4]. In summary, coherent results were obtained for the monitored WWTP in terms of both estimated removal efficiency and type of plastic particles detected, which were consistent with the nature of treated wastewater. Further methodological insights related to both non-described results and ongoing activities will be presented at the Conference.