Stabilization of PFAS-contaminated soil with activated biochar

Erlend Sørmo,Ludovica Silvani,Nora Bjerkli,Nikolas Hagemann,Andrew R Zimmerman,Sarah E Hale,Caroline B Hansen,Thomas Hartnik,Gerard Cornelissen

doi:10.1016/j.scitotenv.2020.144034

Abstract

Biochars are considered potential sustainable sorbents to reduce the leaching of per- and polyfluoroalkyl substances (PFAS) from contaminated soils. However, biochar characteristics must probably be optimized to achieve useful sorption capacity. In the present work, eight waste timber biochars were produced, including biochars activated to different degrees, at different temperatures, and using both steam and CO2. In laboratory batch experiments, the eight biochars were amended to soil samples from two different horizons, with low and high total organic carbon (TOC, 1.6% and 34.2%, respectively), of a heavily PFAS-contaminated soil (1200–3800 μg kg−1 PFAStot), at varying doses (0, 0.1, 0.5, 1.0 and 5.0%). With a 5% amendment to the low-TOC soil, all eight biochars resulted in strongly reduced leachate PFAS concentrations (by 98–100%). At the same amendment dose in the high-TOC soil, leachate concentration reductions were more modest (23–100%). This was likely due to a strong PFAS-sorption to the high-TOC soil itself, as well as biochar pore clogging in the presence of abundant organic matter, resulting in fewer sorption sites available to PFAS. Reduction in PFAS leaching was proportional to the degree of activation and activation temperature. Thus, lower amendment doses of activated biochars were needed to reduce PFAS leaching to the same level as with the non-activated biochar. Activation however, came at a tradeoff with biochar yield. Furthermore, the adsorption ability of these biochars increased proportionally with PFAS-fluorocarbon chain length, demonstrating the role of hydrophobic interactions in reduction of PFAS leaching. Development of internal surface area and porosity was proposed as the main factor causing the improved performance of activated biochars. This study shows that woody residues such as waste timber can be used to produce effective sorbents for the remediation of PFAS-contaminated soil. It also highlights the desirability of sorbate and matrix-specific optimization of biochar production.

Highlights

Per- and polyfluoroalkyl substances (PFAS) are found in soils all over the world (Rankin et al, 2016)
This leachability is illustrated by low partitioning coefficients between soil organic carbon (OC) and water (KOC) reported for perfluorooctanesulfonic acid (PFOS, 103.0±0.7 L kg−1) and perfluorooctanoic acid (PFOA, 102.1±1.0 L kg−1) in soils and sediments (Zareitalabad et al, 2013), compared to the higher KOC of strongly sorbing compounds such as polycyclic aromatic hydrocarbons (PAHs), e.g. phenanthrene (104.37±0.17 L kg−1) or pyrene (105.11±0.15 L kg−1) (Chiou et al, 1998)
The total carbon content was about 90% for all the biochars produced, and there was no significant correlation between biochar carbon content and degree of activation (0–1.25) or activation temperature (800–900 °C)

Summary

Introduction

Per- and polyfluoroalkyl substances (PFAS) are found in soils all over the world (Rankin et al, 2016). This is a direct result of five decades of widespread production, use, and subsequent release of PFAS from local point sources such as fluorochemical plants (Jin et al, 2015), firefighting training facilities (Banzhaf et al, 2017), metal and paper industry (Clara et al, 2008), landfills (Lang et al, 2017), and waste water treatment plants, which usually fail to effectively eliminate PFAS (Gallen et al, 2018). Various PFAS readily leach from soils to groundwater and surface water (Banzhaf et al, 2017). The problematic properties and ubiquitous presence of PFAS in soils has prompted a need for effective remediation techniques (Mahinroosta and Senevirathna, 2020)

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Conclusion