Biosolids – treated sewage sludge from wastewater treatment plants – are considered a more sustainable alternative to chemical fertilizer. But they’re also a major source of microplastics – which represents something of a regulatory blindspot. Limits focus on “visible” debris above 2 mm, but the particles most likely to slip through monitoring – and into soils – are far smaller. There’s an analytical gap too: smaller microplastics are notoriously difficult to characterize – especially in messy biosolid matrices.
Now, a team working with biosolids from large-scale Swedish facilities has shown how a smarter cleanup workflow, paired with sub-micron optical photothermal infrared (O-PTIR) spectroscopy, can push reliable chemical identification into the low-micron range – and do it faster.
The researchers’ suspicion that conventional approaches were missing many microplastics present in biosolids was simple: most prior studies chemically characterize only relatively large particles. “In our study, we used a cutoff of 5 µm for optical characterization under the microscope and down to 1 µm for chemical characterization using O-PTIR,” says Crislaine Bertoldi, lead author and postdoctoral researcher at the Centre for Environmental and Climate Science (CEC), Lund University, Sweden.
Sample prep turned out to be the make-or-break step. Fenton oxidation plus a cellulase digestion delivered the cleanest filters, removing ~97 percent of the total sample mass, and improving downstream identification without significantly changing overall microplastic counts. “Pretreatment is the most critical step because it removes impurities that would otherwise complicate microplastic characterization,” Bertoldi explains. “This is particularly important when targeting smaller particles, which can be obscured by or embedded among larger impurity particles. While particles can sometimes still be visually detected under an optical microscope despite partial coverage by impurities, chemical identification is much more sensitive. In chemical analyses, residual impurities can prevent accurate determination of the polymer composition of potential microplastics.”
O-PTIR – first commercialized in 2018 – then did what µ-FTIR and Raman often struggle to do in messy environmental matrices: deliver clean, high-resolution spectra for very small targets. “The main advantage of O-PTIR is its higher spatial resolution,” says Bertoldi. “Because complex samples are expected to contain very small particles, techniques such as FTIR are often insufficient for their characterization. In addition, O-PTIR is not affected by light diffraction effects typical of FTIR in reflection mode, making it easier to obtain high-quality spectra.” In the study, O-PTIR enabled identification of fine fibers just a couple of microns wide and small particles around ~5 µm.
Finally, the team tested a time-saving “helical” counting pattern that inspects 56 percent of the filter area. The approach produced counts comparable to total counting, effectively halving the microscopy workload. “It is less time-consuming than total counting, requiring roughly half the analysis time. This allows for faster characterization, which is particularly important for large-scale monitoring studies,” Bertoldi says.
“This study helps move the field forward by showing that alternative filter inspection approaches can significantly reduce analysis time,” Bertoldi says. “It also demonstrates that O-PTIR is a new spectroscopic technique capable of overcoming key limitations of conventional Raman and FTIR methods, particularly in terms of spatial resolution, analytical simplicity, and faster data acquisition for chemical identification.”
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