A Virtuous Cycle of Phytoremediation, Pyrolysis and Biochar towards Safe PFAS Levels in Soil and Food
Creators: name: Arp, Hans Peter H.
Publisher: name: Zenodo
Year: 2025

description: By Hans Peter H. Arp, Nathalie Briels, Thomas D. Bucheli, Nicolas Estoppey, Andrea Gredelj, Nikolas Hageman, Sylvain Lerch, Simon Lotz, Daniel Rasse, Hans Peter Schmidt, Erlend Sørmo and Gerard Cornelissen

 

 

 

Remediation of PFAS-impacted agricultural soil with destructive methods (soil washing, excavation, incineration, chemical oxidation) is challenging because of the diffuse character of the pollution [1]. Furthermore, these destructive approaches will impair soil ecosystem services and cause carbon emissions [1]. In-situ methods such as phytoremediation [2] and sorbent amendment [3] are less intrusive, more cost-effective, and align with the concept of nature-based solutions [1,2]. Biochar from biomass combustion in the absence of air (“pyrolysis”), has been proposed as a sustainable sorbent material, due to its benefits in terms of carbon sequestration [4], nutrient recycling / waste handling [1, 5], energy generation [1] and contaminant immobilization [3]. This is particularly the case if the biochar is made from contaminated biosolids, as pyrolysis can mineralize the PFAS [5, 6]. Amendment with 1% sludge biochar or (activated) high-temperature (> 800 °C) wood biochar reduced PFOS leaching from contaminated soil by up to 92-99% [7, 8], with notably better effectiveness for long-, than for short-chain (C4 and C5) PFAS (40-70% [7]).

 

A high potential for terrestrial and aquatic uptake of PFAS has been shown for ultra-short chain and short-chain perfluoroalkyl acids (PFAA) [2]. Most studies were performed with crop species and rarely tested in field conditions [2, 9]. The main challenges for phytoremediation include not only high variabilities between the uptake potential of different PFAS (chain length, charge), but also high variations between plant species and field conditions, making it hard to predict removal efficiencies [1, 9]. Disposal of / Pyrolyzing the contaminated biomass alleviates the constraints of biomass disposal.

 

 

With the suggested solution, we create a virtuous cycle when combining phytoremediation to accumulate short-chain PFAS, the destroying the accumulated PFAS by pyrolytic treatment , and applying the resulting biochar to immobilize long-chain PFAS (Fig. 1). The proposed cycle makes use of the high phytoextraction potential for (ultra-) short-chain PFAS that sorb less strongly to biochar and solves issue of the contaminated biomass. We further suggest incorporating a small portion of biochar into ruminant feed to reduce PFAS uptake and resulting contamination in milk and meat. 

 

For optimizing results on the biochar side, it is likely necessary to pyrolyse this biomass into biochar at temperatures > 800 °C or potentially lower temperatures for mineralizing PFAS if the appropriate agents are added [10]. However, biochar from woody or grassy materials produced at lower temperatures possesses pores too small [< 2 nm [6, 11, 14]] to sorb PFAS molecules with molecular diameters > 1.5 nm [11]. 

 

Roughly 5 tonnes ha-1 of grass per year could be turned into 1 tonne biochar to be applied on one hectare per year of biochar. Acquiring enough biochar to amend the top 20 cm of a soil (ρ=1.3 g cm-3) with 1% biochar would then take 35 y. Using alternative feedstocks such as biosolids [10] or reeds could shorten this. Assuming a biochar price of 300 € tonne-1, the cost would be around 8000 € ha-1 plus that of the incorporation into the soil, far below that of more intrusive methods [1].

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