Per- and poly­fluo­ri­nated alkyl subs­tances (PFAS), also known as the Forever Chemi­cals, are a large chemical family of over 4,700 highly persis­tent chemi­cals that don’t occur in nature. PFAS are the most persis­tent synthetic chemi­cals to date. They hardly degrade in the natural envi­ron­ment and have been found in the blood and breast­milk of people and wild­life all round the world. PFAS tend to conta­mi­nate water, food chains, and soils at trace levels for generations.

Sources of PFAS conta­mi­na­tion include paper mills, land­fills, fire­fighting trai­ning faci­li­ties and fluo­ro­che­mical plants. After decades of use, PFAS are ubiqui­tous in soils, ground­water and surface water. This puts pres­sure on waste­water treat­ment plants to adequa­tely treat waste streams to prevent further spread of PFAS chemi­cals and increases pres­sure to reme­diate conta­mi­nated soils. This is where biochar comes into play.

Source: https://www.lemonde.fr/en/les-decodeurs/article/2023/02/23/forever-pollution-explore-the-map-of-europe-s-pfas-contamination_6016905_8.html

Scien­tific rese­arch has demons­trated that PFAS are elimi­nated by the process of pyro­lysis. Kundu et al. (2021) found that > 90 % of PFOS and PFOA in sewage sludge were destroyed in a pyro­lysis-combus­tion inte­grated process. Evidence from the US EPA Office of Rese­arch and Deve­lo­p­ment (2021) carried out on the US-based company Bioforcetech’s commer­ci­ally installed PYREG pyro­lysis plant shows that pyro­lysis at 600 °C for 10 minutes and combus­tion of pyro­lysis gases at 850 °C elimi­nate PFAS from sewage sludge. Biof­orce­tech (2021) has reported 38 PFAS compounds that were all kept at or removed to below detec­tion limit in the biochar in their pyro­lysis and pyro­lysis gas burning process. At the Fåre­vejle waste­water treat­ment plant in Denmark, sewage sludge pyro­lysis at a tempe­ra­ture of 650 °C and a resi­dence time of more than 3 minutes has showed to elimi­nate all 7 PFAS compounds previously detected in the feedstock.

In addi­tion to PFAS destruc­tion Biochar made from sludges, used as a sorbent, binds already exis­tent conta­mi­nants due to his high surface and proper­ties. What is a sorbent ? Previous studies have postu­lated that high surface area, poro­sity, and high carbon content are important for the sorp­tion of organic pollut­ants (Ahmad et al., 2014, Corne­lissen et al., 2005; Hale et al., 2016; Zimmerman et al., 2004 ). Nowa­days, acti­vated carbon (AC), gene­rally from fossil coal sources such as anthr­acite, is the most commonly used sorbent for soil reme­dia­tion due to its high poro­sity and high carbon content (Hage­mann et al., 2018). Biochar is an alter­na­tive to acti­vated carbon, which can be costly and chemical and energy inten­sive to produce (Ahmed et al., 2019). The main advan­tage of biochar over AC is its greater sustaina­bi­lity, as demons­trated by an endpoint life cycle analysis (Spar­revik et al., 2011) due to its poten­tial for carbon sequestra­tion (Smith, 2016) and reduced use of chemi­cals (Zheng et al., 2019). Biochar is often produced from wood-based sources (Hale et al., 2016). However, from a circular economy perspec­tive, it is at least as attrac­tive to use lightly conta­mi­nated waste such as sewage sludge as a substrate for the produc­tion of biochar sorb­ents. Pyro­lysis of sewage sludge to biochar is the possi­bi­lity of a more sustainable waste manage­ment alter­na­tive to land­fill or inci­ne­ra­tion, as it would remove many of the conta­mi­nants present in the sludge, inclu­ding much of the PFAS (Sajjadi et al., 2019), and produce a sorbent for PFAS.

Sewage sludge biochars as effec­tive PFAS-sorbents

In May 2023, there was now a ground­brea­king study showing that biochar from raw and digested sewage sludge can be used as an effec­tive sorbent for PFAS in most envi­ron­mental contexts, with similar or better effi­ci­en­cies than AC. This study is performed with biochar produced at Lindum AS (Drammen, Norway) by slow pyro­lysis at 700 °C and a resi­dence time of 20 minutes for WCBC and SSBC2 and 40 minutes for SSBC1 using Biogreen tech­no­logy. “High poro­sity in the right size range and carbon content were probably the main para­me­ters respon­sible for the high sorp­tion strength observed in the sludge-derived biochars, toge­ther with some possible influence of amine func­tional groups.” (Krahn, Corne­lissen et al. 2023).

Accor­ding to Prof. Cornelissen’s rese­arch team, further studies should examine a larger range of biochar samples prepared at diffe­rent pyro­lysis tempe­ra­tures to iden­tify the charac­te­ristics ideal for PFAS sorp­tion, such as surface area, pore volume, carbon content and mineral content (mainly Ca and Fe). Finally, studying the effect of acti­va­tion of sludge chars on sorp­tion strength could be useful for further impro­ving their sorp­tion properties.