Biosolids are byproducts of wastewater treatment. They are obtained through diversified processes including dehydration and stabilization. Biosolids have historically been used in agriculture, but more and more end-uses are now considered in engineering fields for example.

The risks associated with biosolids reuse

Biosolids are known to contain heavy metals (HMs) and persistent organic pollutants (POPs), which have to be analyzed before reuse to achieve regulatory requirements. Indeed, using polluted biosolids means spreading the pollutants in the environment, with associated environmental and health risks.

Currently, two to three different methods are used: inductively coupled plasma (ICP) is necessary for the analysis of heavy metals; liquid or gas chromatographies (LC/GC) coupled to mass spectrometry (MS) are necessary for the analysis or persistent organic materials. In fact, these are the techniques we used for our study at the beginning but they are demanding in terms of sample amounts and analysis time. Moreover, they use high amounts of solvents, among which some are toxics. Heavy metals analysis also necessitates high temperatures, which might be risky to human health.

The bright ideas

In our laboratory we routinely use mass spectrometry imaging (MSI) for the analysis of biological tissues in order to spatially localize compounds of interest in intact tissues. This spatial localization is due to biological reactions that happen in living organisms.

Our boss had the surprising idea to use MSI to analyze biosolids, which finally was a very bright idea! Indeed, we discovered that specific spatial distributions occur in biosolids and these distributions are linked to the physico-chemical properties of the pollutants detected. The first problem we encountered was the fact that biosolids would not stick to the glass slide usually used for MSI. So, the next clever idea was to use a copper tape, already described to be useful for fragile plant tissues, and indeed it worked to keep biosolids attached!

And here we go, searching for POPs and HMs in biosolids, not knowing what would come out of it. The results were finally so amazing that the method itself was presented as a standalone manuscript, which was published in Nature Communications in July 2023.

Outcomes of the study

An all-in-one methodology is now available using matrix assisted laser desorption ionization (MALDI) mass spectrometry imaging (MSI). Small amounts of sample are necessary, small volumes of solvents are used and the sample preparation time is dramatically reduced. Moreover, a single instrument is necessary for the analysis, which reduces the costs.
The method can be applied to any non-cohesive material thanks to the use of a conductive copper tape to keep the samples attached to the glass slide used for MALDI-MSI.