Biochar sorption of perfluoroalkyl substances (PFASs) in aqueous film-forming foams-impacted groundwater: Effects of PFASs properties and groundwater chemistry

Highlights

• Sorption of PFSAs to biochar was 1.3-fold higher than that of PFCAs.

• Log K_d of long chain PFASs ranged from 0.77 to 4.63

• Log K_d of short chain PFASs was less than 0.68

• DOM influenced PFASs sorption higher than salinity, pH and SUVA.

• DOM formed complexes (e.g., monodentate) provide more sorption sites for PFASs.

Abstract

The widespread use of per- and polyfluoroalkyl substances (PFASs)-related products such as aqueous film-forming foams (AFFF) has led to increasing contamination of groundwater systems. The concentration of PFASs in AFFF-impacted groundwater can be several orders of magnitude higher than the drinking water standard. There is a need for a sustainable and effective sorbent to remove PFASs from groundwater. This work aims to investigate the sorption of PFASs in groundwater by biochar column. The specific objectives are to understand the influences of PFASs properties and groundwater chemistry to PFASs sorption by biochar. The PFASs-spiked Milli-Q water (including 19 PFASs) and four aqueous film-forming foams (AFFF)-impacted groundwater were used. The partitioning coefficients (log K_d) of long chain PFASs ranged from 0.77 to 4.63 while for short chain PFASs they remained below 0.68. For long chain PFASs (C ≥ 7), log K_d increased by 0.5 and 0.8 for each CF₂ moiety of PFCAs and PFSAs, respectively. Dissolved organic matter (DOM) was the most influential factor in PFASs sorption over pH, salinity, and specific ultraviolet absorbance (SUVA). DOM contained hydrophobic compounds and metal ions which can form DOM-PFASs complexes to provide more sorption sites for PFASs. The finding is useful for executing PFASs remediation by biochar filtration column, especially legacy long chain PFASs, for groundwater remediation.

Introduction

Aqueous film-forming foams (AFFF) are surfactant products comprising of per- and polyfluoroalkyl substances (PFASs). They are used for extinguishing flammable liquid fuel fires in training and fire-fighting activities at airports and military bases. PFASs seep into groundwater through the soil layer and then in some cases entering the drinking water reservoirs (Baduel et al., 2017; Zhang et al., 2019c). A variety of PFASs has been detected in AFFF-impacted groundwater throughout the world (Dauchy et al., 2019; Sammut et al., 2019; Xu et al., 2021). The concentration of perfluoroalkyl carboxylates (PFCAs) and perfluoroalkyl sulfonates (PFSAs) in groundwater near the firefighting training grounds has been found several orders of magnitude above the drinking water standard of Australia (0.08 μg/L) (Bräunig et al., 2017; AECOM, 2016). Exposure to the AFFF-impacted groundwater, has led to the accumulation of certain types of PFASs within humans. Recent reports have linked PFASs with thyroid hormone disruption, low activity sperm, diabetes and cancer, especially in the blood serum of firefighters and local people who are frequently or acutely exposed to PFASs (Barton et al., 2020; Donat-Vargas et al., 2019a, 2019b; Lin et al., 2019; Rotander et al., 2015).

A sustainable method is needed for removing PFASs from groundwater. Anion-exchange resins, polymers and a range of synthetic materials are widely used for the removal of pollutants by adsorption, however they are costly and not a viable solution for developing countries (Lu et al., 2020). Biochar is a sustainable carbonaceous sorbent that can be produced locally using agriculture and timber waste, compared to traditional activated carbons which are generally produced from fossil fuels (Sørmo et al., 2021). Biochar exhibits comparable PFASs sorption to traditional activated carbon sorbents, and offers substantial benefits such as sustainable values and being an environmentally-friendly alternative (Silvani et al., 2019; Zhang et al., 2019b). Biochar has been applied for a wide range of environmental applications such as groundwater and soil remediation (e.g., pesticide, heavy metals) and amendment (Singh et al., 2014; Xiao et al., 2017). An agriculture waste-based biochar has been chosen to investigate the sorption of PFASs in AFFF-impacted groundwater.

PFASs co-exist in groundwater with other contaminants. The behavior and mechanism of PFASs sorption mainly rely on the characteristics of sorbent, the properties of PFASs, and the matrices of groundwater. Perfluoroalkyl acids (PFAAs) comprise of PFSAs and PFCAs and are commonly detected in groundwater (Bräunig et al., 2017; Yong et al., 2021). Studies that cover mechanistic sorption of PFAAs in groundwater are still lacking and the differences of PFCAs and PFSAs in terms of sorption onto biochar sorbents have not been adequately documented (Park et al., 2020; Sørmo et al., 2021). The effect of functional groups and carbon chain length on the sorption of PFASs to biochar needs further elaboration, particularly in the context of groundwater matrices.

Some studies that have reported PFASs sorption in groundwater covered sorption to different biochars and a semi pilot test of the process of remediating groundwater by biochar (Kundu et al., 2021; Xiao et al., 2017). However, a study of real-world PFASs-impacted groundwater is lacking because actual groundwater contains several co-contaminants which may influence the effectiveness of the process such as pH, salinity, specific ultraviolet absorbance (SUVA) and dissolved organic matter (DOM). These factors are inter-related, co-exist in groundwater and affect PFASs sorption together in a complex way. For instance, the change of pH in groundwater partly depends on the deprotonation of organic matter surface charge and ions (i.e., Ca²⁺, Na⁺). Most of the published studies investigated the effects of those factors without considering their co-presence in groundwater but using PFASs-spiked MQ water with either pH, salinity, synthetic DOM (i.e., fulvic and humic) and DOM-free solutions (Gagliano et al., 2020; Jeon et al., 2011; Nguyen et al., 2020; Yu et al., 2012).

Whether salinity influences PFASs sorption onto carbonaceous materials remains a topic of controversy, potentially due to the complexity of groundwater matrices (Dontsova and Bigham, 2005; Du et al., 2015; Wu et al., 2020). The molecular structure of hydrophobic DOM is similar to that of PFASs to a certain extent, due to the net negative charge and molecular weight (200–1000 Da) (Kothawala et al., 2017). The aromatic DOM contains a hydrophobic backbone like PFASs. These features result in two contradictory influences of DOM on PFASs: competition with PFASs for sorbent sites and providing additional PFAS binding sites on their own (Kothawala et al., 2017; McCleaf et al., 2017; Wu et al., 2020; Yu et al., 2012). The competition or mutual support of DOM for PFASs sorption strongly depends on the concentration and composition of DOM in groundwater (i.e. hydrophobicity). Groundwater may consist of various organic compounds such as hydrophilic acids, proteins, phenolic groups, amino acids and Fe/Al oxides. DOM is able to form complexes with PFASs either by electrostatic interaction or cation bridging with multivalent ions such as Ca²⁺, Fe³⁺ and Al³⁺ (Gagliano et al., 2020). Overall, these factors (i.e., pH, salinity, SUVA and DOM) have posed challenges in interpreting the effects of groundwater chemistry on PFASs sorption. It is necessary to understand the effects of PFASs properties and groundwater chemistry on PFASs sorption to assess the biochar column for practical application.

Therefore, this work aims to investigate the effects of PFASs properties and groundwater chemistry on PFASs sorption in a rapid small-scale column testing (RSSCT) biochar column. The specific objectives were to (i) determine the influences of PFASs functional groups and carbon chain length of PFCAs and PFSAs, and (ii) study the synergistic/competitive effects of pH, salinity, SUVA and DOM from different AFFF-impacted groundwater sources on PFASs sorption. This study provides important insight into the sorption of PFASs in a biochar column. The sorption behaviour of PFSAs and PFCAs subgroups in a sophisticated groundwater matrix were explained in detail.