Regeneration of biochars (pristine and modified/engineered) and economic analysis of their use in the removal of per- and polyfluoroalkyl substances (PFAS) from water/wastewater

Highlights

	● Biochar application for PFAS sorptive removal from aqueous samples was reviewed.
	● Regeneration/reuse techniques for the spent biochars were summarized.
	● Economic analysis of biochar use in PFAS removal from water was presented.

Abstract

Currently, there is an increasing interest in developing efficient and cost-effective treatment technologies to remediate per- and polyfluoroalkyl substances (PFAS) in water. Biochars (pristine and modified/engineered) can be a good candidate among porous pyrogenic carbonaceous materials for the sorptive removal of PFAS from water/wastewater. There is a need to focus on developing efficient, environmentally friendly, and cost-effective techniques for desorbing PFAS from spent biochars (pristine and modified/engineered) to enable potential reuse or suitable disposal of these adsorbents, facilitating their future full-scale application in the water sector. This review article briefly compiles the state-of-the-art knowledge on the: (i) application of pristine and modified/engineered biochars for the sorptive removal of PFAS from aqueous samples; (ii) regeneration/reuse techniques for the spent biochars; and (iii) economic analysis of their use in PFAS removal from water/wastewater. Further investigations on (i) better modifying/engineering biochars to remove specially short-chain PFAS species in real environmental water samples due to challenging nature of their removal using conventional treatment technologies; (ii) feasible low-energy, environmentally friendly, and cost-effective strategies for regeneration/reuse of the spent biochars (pristine and modified/engineered) and management of their end-of-life; and (iii) large-scale and continuous column sorption operation for the real water/wastewater samples are still desirable to apply biochars for PFAS removal at full-scale in the future.

Graphical abstract

Keywords

Per- and polyfluoroalkyl substances / Biochar / Sorption / Water / Reuse / Cost

1 Introduction

Per- and polyfluoroalkyl substances (PFAS) are a large group of manufactured organofluorine chemical compounds (Buck et al., 2011; Lu et al., 2020; Vo et al., 2020; Lenka et al., 2021; Kancharla et al., 2022; Dey et al., 2024). Since the 1940s (US EPA, 2023a; 2023b; Vo et al., 2020), PFAS have been used in different industries and consumer products (e.g., textiles, paper, leather, food packaging, nonstick cookware, firefighting foams, cosmetics and personal care products, waterproof products, etc.), because of their useful physicochemical properties (Kotthoff et al. 2015; Lu et al., 2020; Gagliano et al., 2020; Ji et al., 2020; Vo et al., 2020; Lenka et al., 2021; US EPA, 2023a; 2023b). There are thousands of different PFAS (i.e., more than 4,000 PFAS (Vo et al., 2020; Lenka et al. 2021) or over 12,000 chemicals (Salvatore et al., 2022), some of which such as perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) have been widely utilized in various industrial sectors (Lu et al., 2020; US EPA, 2023a).

Since the report of PFAS in global wildlife (Giesy and Kannan, 2001) and human serum (Hansen et al., 2001), there has been an increasing interest in studies related to the sources, occurrences, environmental fate, transport, exposure and health effects, transformation, and approaches for the remediation of PFAS (Cousins, 2015; Lenka et al. 2021; Lyu et al., 2022; Fang et al., 2024). PFAS are found in water, soil (landfills, hazardous waste sites, etc.), sediment, air, and food (food packaging, fish caught from water contaminated by PFAS, etc.) as well as in materials available in homes or workplaces (carpets, clothing, cleaning products, nonstick cookware, paints, etc.) (US EPA, 2023a), which is now a significant public health concern. For instance, PFAS have been detected in the blood of most people in the United States. After the phaseout of the production and use of some PFAS (e.g., PFOA and PFOS) in the United States in 2002, PFAS levels in some blood serums of American have decreased over time (ATSDR, 2024).

Having potential to bioaccumulate in humans, wildlife, and the environment over time due to their persistent nature and/or strong C–F bonds (leading to resist to biodegradation), PFAS can cause health concerns (Vo et al., 2020; Militao et al., 2021; Dey et al., 2024). Scientific research studies are still ongoing to better understand health effects associated with exposure to different levels of PFAS (US EPA, 2023a and 2023b). However, some recent studies have reported that exposure to certain levels of PFAS may cause pregnancy-induced hypertension, children’s developmental effects or delays (low birthweight, accelerated puberty, etc.), high risk of some cancers (e.g., prostate, kidney, and testicular), high cholesterol levels and/or risk of obesity, thyroid disease, etc. (Lu et al., 2020; Gagliano et al., 2020; Vo et al., 2020; Lenka et al., 2021; US EPA, 2023a; 2023b).

Widespread occurrence of PFAS in the aquatic environment such as groundwater, surface water, drinking water, and wastewater has been observed (Gagliano et al., 2020; Vo et al., 2020; Lenka et al., 2021; Grunfeld et al., 2024). Non-regulatory concentrations of PFAS had been released by the United Sates Environmental Protection Agency (US EPA). However, these guidelines had been argued by some states to be insufficient for addressing the potential associated health risk, resulting in developing various PFAS threshold values different from the ones proposed by US EPA (Abunada et al., 2020). For instance, guidance values for PFOA and PFOS in groundwater, drinking water, and surface water/effluent in the United Sates are given in Tab.1. On April 10, 2024, the US EPA finalized the enforceable maximum contaminant levels (MCLs) for PFOA (4 ng/L), PFOS (4 ng/L), perfluorohexane sulfonate (PFHxS) (10 ng/L), perfluorononanoic acid (PFNA) (10 ng/L), and hexafluoropropylene oxide dimer acid (HFPO-DA and its ammonium salt are known as “GenX chemicals”) (10 ng/L) as well as PFAS mixtures containing two or more of PFHxS, PFNA, HFPO-DA, and perfluorobutane sulfonic acid (PFBS) (hazard index of 1) in drinking water. Also, the US EPA announced maximum contaminant level goals (health-based and non-enforceable) for PFOA (0 ng/L), PFOS (0 ng/L), PFHxS (10 ng/L), PFNA (10 ng/L), and HFPO-DA (10 ng/L) as well as PFAS mixtures containing two or more of PFHxS, PFNA, HFPO-DA, and PFBS (hazard index of 1) in drinking water (US EPA, 2024). This new regulation simply means that drinking water sources with the six PFAS concentrations above the enforceable MCLs need to be treated.

Tab.1 The guideline values for perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) in groundwater (GW), drinking water (DW), and surface water (SW)/effluent in the United Sates

Guideline/Standard	Year updated	Type	Promulgated rule	PFOA (ng/L)	PFOS (ng/L)
US EPA, Maximum contaminant level*	2024*	DW*	Yes*	4*	4*
US EPA, Maximum contaminant level goal*	2024*	DW*	No*	0*	0*
US EPA, Interim lifetime health advisory	2022	DW	No	0.004	0.02
US EPA, Regional screening level	2023	DW/GW	No	60	40
Alaska, Groundwater cleanup level	2016	GW	Yes	400	400
Alaska, Action level	2018	DW/GW/SW	No	70	70
California, Notification level	2022	DW	No	5	7
California, Reporting level	2022	DW	Yes	10	40
California, Public health goal	2024	DW	No	0.007	1
Colorado, Site-specific groundwater quality standard	2018	GW	Yes	70	70
Colorado, Translation levels	2020	GW/SW	Yes	70	70
Connecticut, Action level	2023	DW/GW	No	16	10
Connecticut, Additional polluting substance groundwater protection criteria	2018	GW	No	70	70
Delaware, Reporting level	2016	GW	No	70	70
Delaware, Screening level	2023	GW	Yes	6	4
Florida, Provisional groundwater target cleanup level	2020	GW	Other	70	70
Florida, Screening level	2020	SW	Other	500	10
Hawaii, Environmental action level	2021	GW	Other	40	40
Illinois, Health-based guidance level	2021	DW/GW	Other	2	14
Iowa, Statewide standards	2016	Protected GW	Yes	70	70
Maine, Remedial action guideline	2023	GW-Residential	Other	60	40
Maine, Interim DW standard	2021	DW	Other	20	20
Massachusetts, Drinking water values	2018/2019	DW	Other	20	20
Massachusetts, GW-1	2019	GW	Yes	20	20
Massachusetts, Maximum contaminant level	2020	DW	Yes	20	20
Michigan, Human noncancer value for surface drinking water	2023	SW	Yes	66	11
Michigan, Screening levels	2019	DW	No	9	8
Michigan, Maximum contaminant level/Generic cleanup criteria	2021	DW/GW	Yes	8	16
Minnesota, Health risk limit - subchronic, chronic	2023	DW/GW	Yes	35	300
Minnesota, Health-based value–subchronic, chronic	2024	DW/GW	No	0.24	2.3
Minnesota, Health-based value–cancer	2024	DW/GW	No	0.0079	7.6
Minnesota, Water quality standard	2023	SW-lake	Yes/No	25	0.05
Montana, Water quality standard	2019	GW	Yes	70	70
Nevada, Basic comparison level	2023	DW	No	100	70
New Hampshire, Ambient groundwater quality standard	2019	GW	Yes	12	15
New Hampshire, Maximum contaminant level	2020	DW	Yes	12	15
New Jersey, Maximum contaminant level	2020	DW	Yes	14	13
New Mexico, Preliminary screening levels	2022	DW	No	60.2	60
New York, Maximum contaminant level	2020	DW	Yes	10	10
Ohio, Action level	2022	DW	Other	70	70
Oregon, Health advisory level	2021	DW	No	30	30
Pennsylvania, Medium-specific concentration	2021	GW	Other	14	18
Pennsylvania, Maximum contaminant level	2023	DW	Yes	14	18
Rhode Island, Groundwater quality standard	2023	DW/GW	Yes	20	20
Rhode Island, Maximum contaminant level	2022	DW	Yes	20	20
Rhode Island, Action level	2023	SW	Yes	70	70
Texas, Tier 1 protective concentration level	2023	GW	Yes	290	560
Vermont, Maximum contaminant level	2020	DW/GW	Yes	20	20
Vermont, Lifetime health advisory	2018	DW/GW	Yes	20	20
Vermont, Groundwater enforcement standard	2019	GW	Yes	20	20
Vermont, Preventive action level	2019	GW	Yes	2	2
Washington, State action level	2022	GW	Yes	10	15
Washington, State action level	2022	DW	Yes	10	15
Wisconsin, Maximum contaminant level	2022	DW	Other	70	70

Note: *data was obtained from the US EPA (2024) and all other data was obtained from the ITRC (2023).

Surface water and groundwater are the major sources of drinking water in most countries; and the conventional treatment technologies (e.g., flocculation, sedimentation, and sand filtration) utilized in drinking water treatment plants are inefficient for PFAS removal (Vo et al., 2020; Militao et al., 2021). Currently, there is an increasing interest in developing suitable cost-effective treatment strategies to achieve PFAS remediation goal (Lu et al. 2020; Fang et al., 2024). Vo et al. (2020) reviewed the sources, occurrences, fate, and transformation of PFAS in water and wastewater as well as remediation technologies, including separation and concentration technologies (e.g., sorption and membrane process) and destruction technologies (e.g., advanced oxidation processes, plasma, thermal destruction, sonochemical degradation, and biodegradation). They concluded that it is needed to pay more attention to precursor and short-chain PFAS compounds. Also, a precise economic analysis of the PFAS remediation technology and management is required to utilize these technologies in the market/industry (Vo et al., 2020). Lu et al. (2020) reviewed treatment train studies and suggested that tandem combination of nanofiltration with electrochemical anodic oxidation and the parallel configuration of electro-Fenton and electrochemical anodic oxidation may be an optimal design for PFAS remediation (Lu et al., 2020). Jafarinejad (2024) reviewed PFAS removal in full-scale wastewater treatment plants (WWTPs) and presented three theoretical configurations (based on technologies such as granular activated carbon sorption, hybrid advanced oxidation processes, and membrane (nanofiltration or reverse osmosis)) for the wastewater processing train of modern WWTPs for PFAS removal.

Militao et al. (2021) summarized recent progress/developments of using natural and renewable material-based adsorbents (e.g., activated carbons and biochar from agro-wastes, polysaccharide-based materials) for PFAS removal from polluted water. Collivignarelli et al. (2023) reviewed the application of virgin adsorbent materials (e.g., granular activated carbon and powdered activated carbon, functional clays, metal-organic adsorbents, and functionalized organic polymers) and biochar (a residual product of other industrial processes) for the treatment of PFAS polluted wastewater. They concluded that conventional treatment units utilizing virgin adsorbent materials result in high sorption capacity as well as high costs, compared to the biochar-based solutions. Also, because the reactivation of PFAS-laden adsorbents is hardly achievable, the application of biochar may be cost-effective (Collivignarelli et al., 2023). Dey et al. (2024) reviewed carbon-based materials (e.g., activated carbon, biochar, carbon nanotubes, and graphene) for sorptive and photocatalytic treatment of PFAS-polluted water and discussed the challenges associated with the practical application of these materials (e.g., scalability). Behnami et al. (2024) reviewed biochar preparation conditions on PFAS sorption capacity and mechanisms governing PFAS sorption; and concluded that activated biochars show better PFAS sorption performance in comparison with pristine biochars. Gagliano et al. (2020) summarized the removal of long- and short-chain PFAS using sorption process and emphasized the regeneration challenges of PFAS-spent adsorbents. Among the sustainable carbonaceous materials (Vo et al., 2022), biochar (that can be produced by pyrolysis of a wide variety of biomass under oxygen-limiting conditions (Shakoor et al., 2021; Dey et al., 2024)) have various benefits for PFAS removal from water, including rich surface chemistry and functionality, tunable pore size and structure, and high specific surface area, which can be harnessed for PFAS sorption (Krebsbach et al., 2023a; 2023b). On the other hand, it is essential to address the adsorbent disposal/regeneration in economic analysis of PFAS removal technologies (Gagliano et al., 2020).

The overall objective of this review article is to address the following two questions concerning biochars regeneration and economic analysis of their use in removing PFAS from water/wastewater: (i) What are the advantages and disadvantages of each applied regeneration technique for the spent biochars in terms of simplicity, performance, and environmental effects? (ii) What are the unit price of biochar as well as the economic efficiency of its use in PFAS removal from water/wastewater? This review briefly summarizes the application of pristine and modified/engineered biochars for the sorptive removal of PFAS from aqueous solutions, regeneration/reuse techniques (e.g., chemical treatment, vacuum-ultraviolet (VUV)/sulfite reduction system, and thermal treatment) for the exhausted biochars, and economic analysis of their use in PFAS removal from water/wastewater.

2 Literature review of biochars (pristine and modified/engineered) for the sorptive removal of PFAS from aqueous samples

Biochar can be produced by pyrolysis of biomass (e.g., agricultural and forest residues, food wastes, manures, solid wastes, and sludge) under oxygen-limiting conditions (Shakoor et al., 2021; Dey et al., 2024). It is a highly porous carbon material with tailored properties that can be utilized as a green alternative to other adsorbents for the decontamination of water/wastewater (Dey et al., 2024). Generally, a wide variety of biochars with different physicochemical properties can be produced by different feedstocks and pyrolysis temperatures (Krebsbach et al., 2023a). Specifically, the biochar physiochemical properties rely on the type of feedstocks used for its production (Wang and Wang, 2019); and its surface functionality and characteristics (which are effective factors in biochar properties and potential for adsorbing contaminants) can significantly be affected by the pyrolysis temperature and process (Dey et al., 2024). The production of modified/engineered biochars is of interest to improve the characteristics (e.g., pore structure, specific surface area, and surface functional groups) of pristine/raw biochars for higher sorption capacity and enhanced contaminants removal potential (Shakoor et al., 2021). To prepare modified/engineered biochars, chemical modification (e.g., treatment with acid, alkaline, oxidizing agents, metal salts or metal oxides) is more widely used compared to physical modification (e.g., steam or gas purging) methods (Wang and Wang, 2019; Shakoor et al., 2021).

A summary of recent studies on the application of pristine/raw and modified/engineered biochars for the sorptive removal of long- and short-chain PFAS from aquatic environment and/or PFAS contaminated water/wastewater is shown in Tab.2. Tab.2 demonstrates that pristine biochars derived from different feedstocks (e.g., oak, maple, reed straw, sugarcane, corn, Douglas fir, eucalyptus, poplar, switchgrass, halophyte, cow bone, construction and demolition debris-wood, and biosolids) as well as modified/engineered biochars (e.g., magnesium chloride-treated biochar (Sörengård et al., 2020), Fe₃O₄-containing biochar (Rodrigo et al., 2022), post-pyrolysis air oxidation (PPAO)-treated biochar, poly(dimethyldiallylammonium) chloride (pDADMAC)-coated biochar (Wang et al., 2023a), biochar-polymer composite (BC-P(SB-co-AM)) (Deng et al., 2023), polypyrrole/biochar (PPy/BC) composites (Yu et al., 2023), biochar-alginate composite beads (Militao et al., 2023)) have been tested for the sorptive removal of PFAS from aqueous samples. All reported studies have been carried out at the laboratory-scale (batch and column tests). Also, most studies have evaluated PFAS removal from synthetic solutions (i.e., PFAS solutions prepared in ultrapure water); however, PFAS removal from PFAS-spiked groundwater (Liu et al., 2021), aqueous film-forming foams-impacted groundwater (Vo et al., 2022), brackish groundwater (Papes, 2022), river surface water (Yu et al., 2023), and landfill leachate (Cerlanek et al., 2024) has been reported as well.

Tab.2 Recent studies on the application of pristine and modified/engineered biochars for the sorptive remediation of PFAS from aqueous samples

Adsorbent	Target PFAS	Water matrix	Experimental mode	Major findings (sorption performance/capacity)	Reference
Biochar without biofilm	PFBS, PFHxS, PFOS, FOSA, PFBA, PFPeA, PFHxA, PFHpA, PFOA, PFNA, PFDA, PFUnDA, and PFDoDA	Synthetic solution	Column	The removal efficiency (20%–60%) and sorption capacity (0–88 ng ∑PFAS/g biochar) for short-chain PFCAs (C3–C6) and PFSA (C4)The removal efficiency (90%–99%) and the sorption capacity (73–168 ng ∑PFAS/g biochar) for C7–C11 PFCAs, C6, C8 PFSAs, and FOSA	Dalahmeh et al. (2019)
Hardwood (oak)-based biochar	PFOS and PFOA	Synthetic solution	Column	Up to 89% PFOA removal and up to 99% PFOS removal	Liu et al. (2019)
Magnesium chloride-treated biochar	PFBA, PFPeA, PFHxA, PFHpA, PFOA, PFNA, PFDA, PFUnDA, PFDoA, PFTeDA, PFHxDA, PFOcDA, PFBS, PFHxS, PFOS, FOSA, and FTSAs	Synthetic solution	Batch	PFAS sorption to magnesium chloride-treated biochar was 17–25-fold higher than to sand	Sörengård et al. (2020)
Hardwood (oak)-based biochar	PFOA, PFHpA, PFHxA, PFPeA, PFBA, PFOS, PFHpS, PFHxS, and PFBS	Synthetic solution	Batch	About 60% removal of PFOA, 94% removal of PFOS, 17% removal of PFHpA, 60%–70% removal of PFHpS, 30%–40% removal of PFHxS, and 20% removal of PFBS by the mixture of zero-valent iron and biochar (ZVI + BC)	Liu et al. (2020)
Softwood-derived biochar	PFOA, PFOS, PFBA, and PFBS	Synthetic solution	Batch	The maximum Langmuir sorption capacity for PFOA (52.08 ± 14.8 µmol/g), PFOS (70.42 ± 21.5 µmol/g), PFBA (48.31 ± 12.2 µmol/g), and PFBS (23.36 ± 7.4 µmol/g)	Zhang et al. (2021)
Activated spent coffee grounds biochar or SCGKOH (produced from a 1:1 mass ratio of pyrolyzed spent coffee grounds and potassium hydroxide)	PFOS	Synthetic solution	Batch	Sorption capacity of 43.4 mg/g	Steigerwald and Ray (2021)
Reed straw-derived biochar	PFBA, PFBS, PFHxA, PFHxS, PFOA, and PFOS	Synthetic solution and PFAS-spiked groundwater	Batch and column	Batch: 92%–96% removal efficiency for three short-chain PFAS (i.e., PFBA, PFBS, and PFHxA) Column: effective removal using reed straw-derived biochar-packed filter with the flow rate up to 45 mL/min	Liu et al. (2021)
Commercial Douglas fir biochar (BC) and Fe3O4-containing BC (Fe3O4/BC)	PFOS and PFOA	Synthetic solution	Batch	Sorption capacities of PFOS were 7–14.6 mg/g BC and 6.2–10.7 mg/g Fe3O4/BCSorption capacities of PFOA were 3.9–9 mg/g BC and 5.4–652 mg/g Fe3O4/BC	Rodrigo et al. (2022)
Sugarcane biochar	19 PFAS including 11 PFCAs (C3–C13) and 8 PFSAs (C4–C12)	PFAS-spiked Milli-Q water and aqueous film-forming foams-impacted groundwater	Column	1.3-fold higher sorption of PFSAs by biochar compared to PFCAs	Vo et al. (2022)
Non-modified biochars and engineered biochars using different feedstocks (switchgrass, water oak, and biosolid) and additives (FeCl3 and carbon nanotube)	PFOA	Synthetic solution	Batch	Sorption capacity of PFOA in the range of 39.54–469.65 μmol/g Better sorption capacities for PFOA using biosolid biochar and Fe-impregnated biochar	Wu et al. (2022)
Two halophyte biochars and cow bone biochar	PFOA, PFOS, and PFHxS	Brackish groundwater	Batch	All PFOS and PFHxS removal and 86% removal of PFOA by one of the halophyte biochars (at a dose of 1000 mg/L)	Papes (2022)
Commercial biochar and biochars from corn, Douglas fir, eucalyptus, poplar, and switchgrass	PFOS	Synthetic solution	Batch	Over 95% PFOS removal using Douglas fir biochar, poplar biochar, and commercial biochar	Krebsbach et al. (2023a)
Unmodified biochar and modified biochars (post-pyrolysis air oxidation-treated biochar and poly(dimethyldiallylammonium) chloride (pDADMAC)-coated biochar)	GenX, PFBA, PFPeA, PFHpA, PFOA, PFNA, PFDA, PFUnDA, PFBS, PFHxS, and PFOS	Synthetic solution	Batch	Coating with pDADMAC enhanced PFAS sorption by a factor of 10–3000	Wang et al. (2023a)
Commercial biochar (CB) and biosolids biochar (BB)	PFOA, PFOS, and PFHxS	Synthetic solution	Batch	CB outperformed BB with 88.06%–100% and 59.09–100% PFAS sorption in the single and multiple PFAS adsorption experiments, respectively	Nguyen et al. (2023)
Biochar and biochar-polymer composite (BC-P(SB-co-AM))	PFOS, PFOA, PFBA, and PFBS	Synthetic solution	Batch	The maximum sorption capacities of PFOS, PFOA, PFBS, and PFBA were 634, 536, 301, and 264 mg/g, respectively for BC-P(SB-co-AM)	Deng et al. (2023)
Polypyrrole/biochar (PPy/BC) composites	PFOS, PFOA, PFBA, and PFBS	Synthetic solution and river surface water	Batch	Sorption capacities of PFBA, PFBS, PFOA, and PFOS were 3.89, 1.53, 2.55, and 1.22 mmol/g, respectively.Above 95% removal of multiple PFAS from actual PFAS-contaminated surface water	Yu et al. (2023)
Biochar-alginate composite beads	PFOS and PFBS	Synthetic solution	Batch	Up to 99% removal efficiency of PFOS using 1.5 g/L of biochar-alginate composite beads in less than16 h	Militao et al. (2023)
Modified biochars prepared from different biomass materials (straw, wood chips, sludge, and chicken manure) and modification methods (e.g., acid (hydrochloric acid), alkali (sodium hydroxide), and oxidant (potassium permanganate) modifications)	PFOS and PFOA	Synthetic solution	Batch	Theoretical maximum sorption capacities of PFOS and PFOA were 72.17 and 45.88 mg/g, respectively for the acid-modified, 300 °C pyrolyzed, sludge-derived biochar	Zhang et al. (2023)
Raw softwood (mixed species) or hardwood (maple)-derived biochars and post-pyrolysis air oxidation (PPAO)-treated biochars	PFBA, PFPeA, PFHpA, PFOA, PFNA, PFDA, PFUnDA, PFBS, PFHxS, PFOS, and GenX	Synthetic solution	Batch	PPAO treatment can significantly enhance the sorption potential of biochars	Wang et al. (2023b)
Construction and demolition debris-wood-derived biochar	92 PFAS	Landfill leachate	Batch and column	Batch: achieving a maximum of 29% PFAS reduction compared to controlsColumn: Producing leachates with PFAS concentrations 50%–80% higher than those in control columns	Cerlanek et al. (2024)

Abbreviations: PFOA: Perfluorooctanoic acid; PFOS: Perfluorooctane sulfonate; PFBA: Perfluorobutanoic acid; PFBS: Perfluorobutane sulfonic acid; FOSA: Perfluorooctane sulfonamide; PFPeA: Perfluoropentanoic acid; PFHxA: Perfluorohexanoic acid; PFHxS: Perfluorohexane sulfonate; PFHpA: Perfluoroheptanoic acid; PFHpS: Perfluoroheptane sulfonic acid; PFNA: Perfluorononanoic acid; PFDA: Perfluorodecanoic acid; PFUnDA: Perfluoroundecanoic acid; PFDoDA: Perfluorododecanoic acid; PFTeDA: Perfluorotetradecanoic acid; PFHxDA: Perfluorohexadecanoic acid; PFOcDA: Perfluorooctadecanoic acid; GenX: Ammonium salt of hexafluoropropylene oxide dimer acid; PFCAs: Perfluoroalkyl carboxylic acids; PFSAs: Perfluorosulfonic acids; and FTSAs: 6:2 and 8:2 fluorotelomer sulfonic acids.

For example, Liu et al. (2021) reported 92%–96% removal efficiency for the three short-chain PFAS (i.e., perfluorobutanoic acid (PFBA), PFBS, and perfluorohexanoic acid (PFHxA)) (at an initial concentration of 1 μg/L) in batch tests using reed straw-derived biochar (pyrolyzed at 900 °C). Also, they observed effective removal of the mixture of six PFAS (PFBA, PFBS, PFHxA, PFHxS, PFOA, and PFOS) in the influent using reed straw-derived biochar-packed filter with the flow rate up to 45 mL/min (Liu et al., 2021). Krebsbach et al. (2023b) also studied the effects of environmental factors (e.g., salt and humic acid concentrations) on the sorption of PFAS from an artificial groundwater solution by Douglas fir (pyrolyzed at 900 °C) biochar and commercial biochar (i.e., Oregon Biochar Solutions); and reported positive and negative effects on PFAS sorption from salt and humic acid, respectively. Despite inhibited PFAS removal by both biochars from artificial groundwater solution, the commercial biochar can still remove over 70% of most studied PFAS from water. Future studies should focus on better modifying/engineering biochars to remove especially short-chain PFAS compounds in real environmental water samples, due to the challenging nature of their removal using conventional treatment technologies (Krebsbach et al., 2023b).

3 Regeneration of biochars (pristine and modified/engineered)

Generally, regeneration can play an important role in reducing the need for new adsorbent as well as issues associated with the disposal of spent adsorbent. However, regeneration/reuse of adsorbents saturated with PFAS is challenging (Gagliano et al., 2020; Deng et al., 2023). Thus, there is a need to focus on the regeneration and reusability of pristine and engineered/modified biochars. Specifically, studies related to developing efficient and cost-effective techniques for desorbing PFAS from spent biochars (pristine and modified/engineered) to enable potential reuse or suitable disposal of these adsorbents can help facilitate their full-scale application (Kancharla et al., 2022; Militao et al., 2023; Zhou et al., 2024).

Tab.3 summarizes the recent regeneration studies on pristine and modified/engineered biochars. Chemical treatment (using reagents such as methanol, ethanol, sodium chloride, sodium iodide, and sodium hydroxide) (Rodrigo et al., 2022; Steigerwald, 2022; Deng et al., 2023;Yu et al., 2023), VUV/sulfite reduction system (Deng et al., 2023), and thermal treatment (Deng et al., 2023) techniques have been reported for the regeneration/reuse of biochars (pristine and modified/engineered).

Tab.3 Recent regeneration studies on pristine and modified/engineered biochars

Adsorbents	Target PFAS	Regeneration technique	Regeneration results	Reference
Biochar (BC) and Fe3O4-containing BC (Fe3O4/BC)	PFOS and PFOA	Chemical regeneration using methanol	Better cyclic sorption-desorption for PFOS compared to PFOASimilar results in cyclic uptake-recovery tests with PFOS for BC and Fe3O4/BC despite slight capacity differences in desorptionBC and Fe3O4/BC can be utilized for several sorption cycles	Rodrigo et al. (2022)
Spent coffee grounds biochar (“SCGKOH”) and molecularly imprinted polymer coated SCGKOH biochar	PFOS, PFOA, and PFBS	Chemical regeneration using a 70% methanol, 1% sodium chloride solution	Successful regeneration of spent adsorbents	Steigerwald (2022)
Polypyrrole/biochar (PPy/BC) composites	PFOS, PFOA, PFBA, and PFBS	Chemical regeneration using different solvents (e.g., methanol, acetonitrile, methanol solution containing 1 mol/L sodium hydroxide, 70% methanol solution containing 1 mol/L sodium hydroxide, and single 1 mol/L sodium hydroxide solution)	Methanol as the optimal regeneration agentSuitable regeneration/reuse of spent PPy/BC composites at least five times	Yu et al. (2023)
Biochar and biochar-polymer composite (BC-P(SB-co-AM))	PFOS, PFOA, PFBA, and PFBS	Chemical regeneration using sodium chloride, sodium iodide, sodium hydroxide, and ethanol	Sorption efficiency following regeneration using sodium chloride, sodium iodide, sodium hydroxide, and ethanol were 1.3%–3.6%, 1.1%–2.2%, 3.8%–20.6%, and 11.2%–26.9%, respectively	Deng et al. (2023)
Biochar and biochar-polymer composite (BC-P(SB-co-AM))	PFOS, PFOA, PFBA, and PFBS	Vacuum-ultraviolet (VUV)/sulfite reduction system	Removal efficiency of PFOS (80.8%), PFOA (90.4%), PFBS (58.8%), and PFBA (70.6%) after the first regenerationRemoval efficiency of PFOS (56.2%), PFOA (55.7%), PFBS (29.6%), and PFBA (45.1%) after the third regenerationRemoval efficiency of PFOS (33.8%), PFOA (40.2%), PFBS (23.4%), and PFBA (31.8%) after the fourth regeneration	Deng et al. (2023)
Biochar and biochar-polymer composite (BC-P(SB-co-AM))	PFOS, PFOA, PFBA, and PFBS	Heat treatment (heating saturated adsorbent with PFAS in a regeneration solution at 50 °C for 12 h)	The removal efficiencies of PFOS, PFOA, PFBS, and PFBA were 78.3%, 82.2%, 65.8%, and 60.8%, respectively for the regenerated adsorbentThe removal efficiency of long-chain PFAS was greater than 60% after five cyclesThe removal efficiencies of PFOS, PFOA, PFBS, and PFBA were 96.2%, 94.3%, 90.8%, and 85.8%, respectively after first regeneration using heat treatment combined with sodium iodide and sodium hydroxide solution. Also, the removal efficiency was greater than 70% after five cycles	Deng et al. (2023)
Softwood (pyrolyzed at 600 °C)-post-pyrolysis air oxidation (PPAO)-treated biochar	PFBA, PFPeA, PFHpA, PFOA, PFNA, PFDA, PFUnDA, PFBS, PFHxS, PFOS, and GenX	Thermal treatment at 500 °C in air	Reactivated softwood (pyrolyzed at 600 °C)-PPAO-treated biochar showed greater PFAS KD values compared to that of the original	Wang et al. (2023b)

3.1 Chemical regeneration

Generally, chemical treatment is the most common method to regenerate saturated adsorbents including biochar and other adsorbents (Deng et al., 2023), which can usually be applied in situ at full-scale treatment plants. However, the associated environmental impacts and costs should be considered (Gagliano et al., 2020). Particularly, chemical solvents with stronger affinity toward PFAS such as ethanol, methanol, and acetonitrile are commonly used to extract sorbed PFAS from spent biochars for the regeneration and reuse, these used chemical solvents need to be carefully handled and treated from environmental safety perspective.

Rodrigo et al. (2022) used methanol to regenerate biochar and Fe₃O₄-containing biochar (Fe₃O₄/BC) and reported better cyclic sorption-desorption for PFOS, compared to PFOA. Also, similar outcomes in cyclic uptake-recovery tests with PFOS for biochar and Fe₃O₄/BC were reported, despite slight capacity differences in desorption behaviors. They concluded that these adsorbents can be utilized for several sorption cycles. Steigerwald (2022) also reported successful regeneration of spent adsorbents (spent coffee grounds biochar (“SCGKOH”) and molecularly imprinted polymer coated SCGKOH biochar) using a solution (70% methanol, 1% sodium chloride). Yu et al. (2023) used chemical treatment based on different solvents (e.g., methanol, acetonitrile, methanol solution containing 1 mol/L sodium hydroxide, 70% methanol solution containing 1 mol/L sodium hydroxide, and single 1 mol/L sodium hydroxide solution) to regenerate the spent PPy/BC composites. They reported methanol as the optimal regeneration agent for suitable regeneration/reuse of spent PPy/BC composites for at least five cycles. Deng et al. (2023) applied chemical regeneration using sodium chloride, sodium iodide, sodium hydroxide, and ethanol for regeneration of BC-P(SB-co-AM), while reporting poor sorption efficiency after regeneration using sodium chloride (1.3%–3.6%), sodium iodide (1.1%–2.2%), sodium hydroxide (3.8%–20.6%,), and ethanol (11.2%–26.9%) (Deng et al., 2023).

3.2 Vacuum-ultraviolet (VUV)/sulfite reduction system

After biochar sorption, VUV/sulfite reduction system can be applied to break down the C–F bond to accomplish PFAS degradation and defluorination (Gao et al., 2021; Liu et al., 2022; Deng et al., 2023; He et al., 2024) for regeneration of adsorbents. However, this method has some weaknesses. In reality, it can also act on the adsorbents, leading to their structural destruction and changes in their physical and chemical properties; thereby affecting their sorption performance after regeneration (Deng et al., 2023). For example, Deng et al. (2023) reported the removal efficiency of PFOS (80.8%), PFOA (90.4%), PFBS (58.8%), and PFBA (70.6%) after the first regeneration of BC-P(SB-co-AM) using VUV/sulfite reduction system. However, the sorption efficiencies during the third and fourth cycles showed significant decrease. Specifically, the removal efficiencies of PFOS (56.2%), PFOA (55.7%), PFBS (29.6%), and PFBA (45.1%) in the third regeneration were higher than those in the fourth regeneration, e.g., PFOS (33.8%), PFOA (40.2%), PFBS (23.4%), and PFBA (31.8%) (Deng et al., 2023).

3.3 Thermal regeneration

Thermal treatment of saturated adsorbents with PFAS (using temperatures in the range of 800–1000 °C in a furnace) can destruct the adsorbed PFAS; however, it also consumes high energy. Furthermore, the sorption behavior and capacity of the regenerated adsorbents can be affected due to changes in their physical and chemical properties because of exposure to high temperatures (Siriwardena et al., 2021; Deng et al., 2023).

Deng et al. (2023) used heat treatment (i.e., heating saturated adsorbent with PFAS in a regeneration solution at 50 °C for 12 h) to regenerate BC-P(SB-co-AM). The PFOS, PFOA, PFBS, and PFBA removal efficiencies were reported at 78.3%, 82.2%, 65.8%, and 60.8%, respectively, for the regenerated adsorbent. The removal efficiency of long-chain PFAS was greater than 60% after five cycles. The regeneration performance of the adsorbent could be improved using thermal treatment combined with sodium iodide and sodium hydroxide solution which after first regeneration, the removal efficiencies of PFOS, PFOA, PFBS, and PFBA were reported at 96.2%, 94.3%, 90.8%, and 85.8%, respectively. Also, the removal efficiency was greater than 70% after five cycles (Deng et al., 2023). Wang et al. (2023b) reported thermal reactivation of the saturated biochar with PFAS at a moderate temperature (500 °C) in N₂ or air. After thermal reactivation of softwood (pyrolyzed at 600 °C)-post-pyrolysis air oxidation (PPAO)-treated biochar in air, it showed greater solid-water distribution ratio/coefficient (K_D) values of PFAS compared to that of the original biochar. It is worth mentioning that thermal treatment can be utilized to reactivate biochar, albeit at the cost of mass loss. However, it is needed to pay attention to the possible hazardous volatile species generated during treatment (e.g., smaller nonpolar fluorinated compounds and reactive F species including HF) and consider control actions/techniques for this issue (Wang et al. 2023b). Emissions of reactive F species could be remarkably decreased in the presence of kaolinite (Alinezhad et al., 2022; Wang et al., 2023b) or lime (Wang et al., 2013; 2023b). There is still a need to focus on the feasible low-energy, environmentally friendly, and cost-effective strategies for regeneration/reuse of the spent biochars (Militao et al., 2021; 2023).

4 Economic analysis

Biochar (pristine or modified/engineered) can be a good candidate among porous pyrogenic carbonaceous materials for PFAS removal from water/wastewater due to its cost-effectiveness and the ready availability of feedstocks (Wang et al., 2023a) such as naturally available/renewable materials and waste biomass (e.g., agricultural, forested, and animal waste) (Yu et al., 2023). It is obvious that the circular economy and net-zero goals of the water industry can be benefited from the regeneration and reuse of bio-solids biochar (Nguyen et al., 2023).

The cost of biochar can vary in different countries which processing needs, pyrolysis conditions, reactor and local precursor availability, recycling, and lifetime issues (Mohan et al., 2014), and labor cost (Wu et al., 2022) can affect the production cost. According to Wu et al. (2022), most studies have reported a lower unit price for biochar close to 0.05 $/kg, which might not be feasible in terms of industrial production (Wu et al., 2022). For instance, Vo et al. (2022) reported that the estimated cost of biochar ranged 246 $/t (Ahmad et al., 2014; Vo et al., 2022) to 500 $/t (Mohan et al., 2014; Vo et al., 2022); whereas the commercial price of activated carbon was 1,500 $/t (Ahmad et al., 2014; Vo et al., 2022). However, Alhashimi and Aktas (2017) reviewed commercial sources (excluding literature reported values as low as 0.05 $/kgforbiochar)toevaluatetheunitpriceofbiocharandactivatedcarbon;andreported0.8–18$/kg and 0.3–22 $/kg for biochar and activated carbon, respectively (Alhashimi and Aktas, 2017). A report by the International Biochar Initiative (IBI) stated that the average wholesale and retail prices for pure biochar in 2014 based on 56 pure biochar products on the marketplace were 2.06 and 3.08 $/kg, respectively (Jirka and Tomlinson, 2015).

Limited information is available on the economic evaluation/assessment of biochar application in PFAS removal from water/wastewater. Thus, there is a need to focus on this field. In an economic assessment of engineered biochar (i.e., Fe-impregnated biochar) application in PFOA removal, Wu et al. (2022) considered that engineered biochar/activated carbon can be utilized at the end of treatment units of a medium-sized wastewater treatment plant (with 361 million L/d and PFOA concentration up to 138.7 ng/L) to remove PFOA from the effluent. Saving up to $3.19 (82.2%) per day using engineered biochar instead of activated carbon and engineered biochar sorption capacity of 469.65 μmol/g was reported (Wu et al., 2022). Studies on the techno-economic assessment (TEA) of biochar use for PFAS removal in real large-scale cases are still of interest.

5 Conclusions and future perspectives

This review article explored the application of pristine and modified/engineered biochars for the sorptive removal of PFAS from aqueous samples, regeneration/reuse techniques for the exhausted biochars, and economic analysis of their use in PFAS removal from water/wastewater. Major findings of this review include:

• PFAS remediation technologies include separation and concentration technologies (e.g., sorption and membrane process) and destruction technologies (e.g., advanced oxidation and reduction processes, plasma, thermal destruction, sonochemical degradation, and biodegradation).

• In addition to the pyrolysis temperature and process, the biochar physiochemical properties (which are important for biochar performance) can rely on the type of feedstocks used for its production. Pristine biochars derived from different feedstocks (e.g., oak, maple, reed straw, sugarcane, corn, Douglas fir, eucalyptus, poplar, switchgrass, halophyte, cow bone, and biosolids) as wells as modified/engineered biochars (e.g., magnesium chloride-treated biochar, Fe₃O₄-containing biochar, PPAO-treated biochar, pDADMAC-coated biochar, BC-P(SB-co-AM), PPy/BC composites, biochar-alginate composite beads) have been tested for the sorptive removal of PFAS from aqueous samples.

• Most reported studies were carried out at the laboratory-scale (batch and column tests). Also, most studies only evaluated PFAS removal from synthetic solutions; however, PFAS removal from PFAS-spiked groundwater, aqueous film forming foam (AFFF)-impacted groundwater, brackish groundwater, river surface water, and landfill leachate has been reported.

• More focus should be paid to the short-chain PFAS as well as the effects of environmental factors on the sorption in the sorptive removal of PFAS using biochars from water/wastewater.

• Chemical treatment (using reagents such as methanol, ethanol, sodium chloride, sodium iodide, and sodium hydroxide), VUV/sulfite reduction system, and thermal treatment techniques may be utilized for the regeneration/reuse of biochars (pristine and modified/engineered). Chemical treatment is the common method to regenerate saturated adsorbents.

Mechanistic understanding of PFAS sorption by biochars needs to be further developed. For example, hydrophobic interaction, electrostatic attraction, hydrogen bond, and other interactions are reported to control the sorption behaviors of different PFAS by biochars. However, it remained unclear which interactions predominate PFAS sorption, especially with respect to the diverse PFAS family with different hydrophobicity, carbon chain length, headgroups, and others. An improved understanding of the sorption mechanisms against different PFAS compounds will provide guidance for the development and optimization of engineered biochars for enhanced PFAS sorption. Further investigations on better modifying/engineering biochars to remove specially short-chain PFAS compounds in real environmental water samples due to the challenging nature of their removal using treatment technologies (Krebsbach et al., 2023a; 2023b), feasible low-energy, environmentally friendly, and cost-effective strategies for regeneration/reuse of the spent biochars (pristine and modified/engineered) and management of their end-of-life, large-scale and continuous column sorption operation for the real water/wastewater samples (Militao et al., 2021, Militao et al., 2023) are still desirable to apply biochars for PFAS removal at full-scale in the future.

References

[1] Abunada Z, Alazaiza M Y D, Bashir M J K. (2020). An overview of per- and polyfluoroalkyl substances (PFAS) in the environment: source, fate, risk and regulations. Water, 12(12): 3590

[2] ATSDR (2024). PFAS in the U.S. Population. Atlanta: Agency for Toxic Substances and Disease Registry

[3] Ahmad M, Rajapaksha A U, Lim J E, Zhang M, Bolan N, Mohan D, Vithanage M, Lee S S, Ok Y S. (2014). Biochar as a sorbent for contaminant management in soil and water: a review. Chemosphere, 99: 19–33

[4] Alhashimi H A, Aktas C B. (2017). Life cycle environmental and economic performance of biochar compared with activated carbon: a meta-analysis. Resources, Conservation and Recycling, 118: 13–26

[5] Alinezhad A, Sasi P C, Zhang P, Yao B, Kubátová A, Golovko S A, Golovko M Y, Xiao F. (2022). An investigation of thermal air degradation and pyrolysis of per- and polyfluoroalkyl substances and aqueous film-forming foams in soil. ACS ES&T Engineering, 2(2): 198–209

[6] Behnami A, Pourakbar M, Ayyar A S R, Lee J W, Gagnon G, Benis K Z. (2024). Treatment of aqueous per- and poly-fluoroalkyl substances: a review of biochar adsorbent preparation methods. Chemosphere, 357: 142088

[7] Buck R C, Franklin J, Berger U, Conder J M, Cousins I T, De Voogt P, Jensen A A, Kannan K, Mabury S A, van Leeuwen S P J. (2011). Perfluoroalkyl and polyfluoroalkyl substances in the environment: terminology, classification, and origins. Integrated Environmental Assessment and Management, 7(4): 513–541

[8] Cerlanek A, Liu L, Robey N, Timshina A S, Bowden J A, Townsend T G. (2024). Assessing construction and demolition wood-derived biochar for in-situ per- and polyfluoroalkyl substance (PFAS) removal from landfill leachate. Waste Management, 174: 382–389

[9] Collivignarelli M C, Bellazzi S, Caccamo F M, Calatroni S, Milanese C, Baldi M, Abbà A, Sorlini S, Bertanza G. (2023). Removal of per- and polyfluoroalkyl substances by adsorption on innovative adsorbent materials. Sustainability, 15(17): 13056

[10] Cousins I T. (2015). Per- and polyfluoroalkyl substances in materials, humans and the environment. Chemosphere, 129: 1–3

[11] Dalahmeh S S, Alziq N, Ahrens L. (2019). Potential of biochar filters for onsite wastewater treatment: effects of active and inactive biofilms on adsorption of per- and polyfluoroalkyl substances in laboratory column experiments. Environmental Pollution, 247: 155–164

[12] Deng J, Han J, Hou C, Zhang Y, Fang Y, Du W X, Li M, Yuan Y, Tang C, Hu X. (2023). Efficient removal of per- and polyfluoroalkyl substances from biochar composites: cyclic adsorption and spent regenerant degradation. Chemosphere, 341: 140051

[13] Dey D, Shafi T, Chowdhury S, Dubey B K, Sen R. (2024). Progress and perspectives on carbon-based materials for adsorptive removal and photocatalytic degradation of perfluoroalkyl and polyfluoroalkyl substances (PFAS). Chemosphere, 351: 141164

[14] Fang J, Li S, Qiu R, Zhang WX. (2024). “Forever chemicals”: a sticky environmental problem. Frontiers of Environmental Science & Engineering, 18: 131

[15] Gagliano E, Sgroi M, Falciglia P P, Vagliasindi F G A, Roccaro P. (2020). Removal of poly- and perfluoroalkyl substances (PFAS) from water by adsorption: role of PFAS chain length, effect of organic matter and challenges in adsorbent regeneration. Water Research, 171: 115381

[16] Gao P, Cui J, Deng Y. (2021). Direct regeneration of ion exchange resins with sulfate radical-based advanced oxidation for enabling a cyclic adsorption-regeneration treatment approach to aqueous perfluorooctanoic acid (PFOA). Chemical Engineering Journal, 405: 126698

[17] Giesy J P, Kannan K. (2001). Global distribution of perfluorooctane sulfonate in wildlife. Environmental Science & Technology, 35(7): 1339–1342

[18] Grunfeld D A, Gilbert D, Hou J, Jones A M, Lee M J, Kibbey T C G, O’Carroll D M. (2024). Underestimated burden of per-and polyfluoroalkyl substances in global surface waters and groundwaters. Nature Geoscience, 17: 340–346

[19] Hansen K J, Clemen L A, Ellefson M E, Johnson H O. (2001). Compound-specific, quantitative characterization of organic: fluorochemicals in biological matrices. Environmental Science & Technology, 35(4): 766–770

[20] He J, Boersma M, Song Z, Krebsbach S, Fan D, Duin E C, Wang D. (2024). Biochar and surfactant synergistically enhanced PFAS destruction in UV/sulfite system at neutral pH. Chemosphere, 353: 141562

[21] ITRC (2023). PFAS－Per- and Polyfluoroalkyl Substances, PFAS Fact Sheets. Washington, DC: Interstate Technology and Regulatory Council

[22] Jafarinejad S. (2024). Some possible process configurations for modern wastewater treatment plants for per- and polyfluoroalkyl substances (PFASs) removal. Sustainability, 16(18): 8109

[23] Ji B, Kang P, Wei T, Zhao Y. (2020). Challenges of aqueous per- and polyfluoroalkyl substances (PFASs) and their foreseeable removal strategies. Chemosphere, 250: 126316

[24] JirkaS, Tomlinson T (2015). State of the Biochar Industry 2014: a Survey of Commercial Activity in the Biochar Sector. New York: International Biochar Initiative (IBI)

[25] Kancharla S, Alexandridis P, Tsianou M. (2022). Sequestration of per- and polyfluoroalkyl substances (PFAS) by adsorption: surfactant and surface aspects. Current Opinion in Colloid & Interface Science, 58: 101571

[26] Kotthoff M, Müller J, Jürling H, Schlummer M, Fiedler D. (2015). Perfluoroalkyl and polyfluoroalkyl substances in consumer products. Environmental Science and Pollution Research International, 22(19): 14546–14559

[27] Krebsbach S, He J, Adhikari S, Olshansky Y, Feyzbar F, Davis L C, Oh T S, Wang D. (2023a). Mechanistic understanding of perfluorooctane sulfonate (PFOS) sorption by biochars. Chemosphere, 330: 138661

[28] Krebsbach S, He J, Oh T S, Wang D. (2023b). Effects of environmental factors on the sorption of per- and polyfluoroalkyl substances by biochars. ACS ES&T Water, 3(10): 3437–3446

[29] Lenka S P, Kah M, Padhye L P. (2021). A review of the occurrence, transformation, and removal of poly- and perfluoroalkyl substances (PFAS) in wastewater treatment plants. Water Research, 199: 117187

[30] Liu N, Wu C, Lyu G, Li M. (2021). Efficient adsorptive removal of short-chain perfluoroalkyl acids using reed straw-derived biochar (RESCA). Science of the Total Environment, 798: 149191

[31] Liu Y Y, Blowes D W, Ptacek C J, Groza L G. (2019). Removal of pharmaceutical compounds, artificial sweeteners, and perfluoroalkyl substances from water using a passive treatment system containing zero-valent iron and biochar. Science of the Total Environment, 691: 165–177

[32] Liu Y Y, Ptacek C J, Baldwin R J, Cooper J M, Blowes D W. (2020). Application of zero-valent iron coupled with biochar for removal of perfluoroalkyl carboxylic and sulfonic acids from water under ambient environmental conditions. Science of the Total Environment, 719: 137372

[33] Liu Z, Chen Z, Gao J, Yu Y, Men Y, Gu C, Liu J. (2022). Accelerated degradation of perfluorosulfonates and perfluorocarboxylates by UV/sulfite + iodide: reaction mechanisms and system efficiencies. Environmental Science & Technology, 56(6): 3699–3709

[34] Lu D, Sha S, Luo J, Huang Z, Jackie X Z. (2020). Treatment train approaches for the remediation of per- and polyfluoroalkyl substances (PFAS): a critical review. Journal of Hazardous Materials, 386: 121963

[35] LyuX, XiaoF, ShenC, Chen J, ParkCM, SunY, FluryM, WangD (20222022. Per- and polyfluoroalkyl substances (PFAS) in subsurface environments: occurrence, fate, transport, and research prospect. Reviews of Geophysics, 60(3), e2021RG000765 doi:10.1029/2021RG000765

[36] Militao I M, Roddick F, Fan L, Zepeda L C, Parthasarathy R, Bergamasco R. (2023). PFAS removal from water by adsorption with alginate-encapsulated plant albumin and rice straw-derived biochar. Journal of Water Process Engineering, 53: 103616

[37] Militao I M, Roddick F A, Bergamasco R, Fan L. (2021). Removing PFAS from aquatic systems using natural and renewable material-based adsorbents: a review. Journal of Environmental Chemical Engineering, 9(4): 105271

[38] Mohan D, Sarswat A, Ok Y S, Pittman C U Jr. (2014). Organic and inorganic contaminants removal from water with biochar, a renewable, low cost and sustainable adsorbent: a critical review. Bioresource Technology, 160: 191–202

[39] Nguyen M D, Sivaram A K, Megharaj M, Webb L, Adhikari S, Thomas M, Surapaneni A, Moon E M, Milne N A. (2023). Investigation on removal of perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS), perfluorohexane sulfonate (PFHxS) using water treatment sludge and biochar. Chemosphere, 338: 139412

[40] PapesB (20222022. Per- and Polyfluoroalkyl Substances Mitigation from Contaminated Groundwater Using Halophyte and Cow Bone Biochars. Thesis for the Master’s Degree. Houghton: Michigan Technological University

[41] Rodrigo P M, Navarathna C, Pham M T H, McClain S J, Stokes S, Zhang X, Perez F, Gunatilake S R, Karunanayake A G, Anderson R. . (2022). Batch and fixed bed sorption of low to moderate concentrations of aqueous per- and poly-fluoroalkyl substances (PFAS) on Douglas fir biochar and its Fe₃O₄ hybrids. Chemosphere, 308: 136155

[42] Salvatore D, Mok K, Garrett K K, Poudrier G, Brown P, Birnbaum L S, Goldenman G, Miller M F, Patton S, Poehlein M. . (2022). Presumptive contamination: a new approach to PFAS contamination based on likely sources. Environmental Science & Technology Letters, 9(11): 983–990

[43] Shakoor M B, Ye Z L, Chen S. (2021). Engineered biochars for recovering phosphate and ammonium from wastewater: a review. Science of the Total Environment, 779: 146240

[44] Siriwardena D P, James R, Dasu K, Thorn J, Iery R D, Pala F, Schumitz D, Eastwood S, Burkitt N. (2021). Regeneration of per- and polyfluoroalkyl substance-laden granular activated carbon using a solvent based technology. Journal of Environmental Management, 289: 112439

[45] Sörengård M, Östblom E, Köhler S, Ahrens L. (2020). Adsorption behavior of per- and polyfluoralkyl substances (PFASs) to 44 inorganic and organic sorbents and use of dyes as proxies for PFAS sorption. Journal of Environmental Chemical Engineering, 8(3): 103744

[46] SteigerwaldJ M (20222022. Development of a Novel Biochar-Molecularly Imprinted Polymer Composite for Targeted Adsorption of Perfluoroalkyl Substances in Water Treatment Applications. Thesis for the Master’s Degree. Seattle: University of Washington

[47] Steigerwald J M, Ray J R. (2021). Adsorption behavior of perfluorooctanesulfonate (PFOS) onto activated spent coffee grounds biochar in synthetic wastewater effluent. Journal of Hazardous Materials Letters, 2: 100025

[48] US EPA (2023a). Our Current Understanding of the Human Health and Environmental Risks of PFAS. Washington, DC: United States Environmental Protection Agency

[49] US EPA (2023b). PFAS Explained. Washington, DC: United States Environmental Protection Agency.

[50] US EPA (2024). Per- and Polyfluoroalkyl Substances (PFAS), Final PFAS National Primary Drinking Water Regulation. Washington, DC: United States Environmental Protection Agency

[51] Vo H N P, Ngo H H, Guo W, Nguyen T M H, Li J, Liang H, Deng L, Chen Z, Nguyen T A H. (2020). Poly- and perfluoroalkyl substances in water and wastewater: a comprehensive review from sources to remediation. Journal of Water Process Engineering, 36: 101393

[52] Vo H N P, Nguyen T M H, Ngo H H, Guo W, Shukla P. (2022). Biochar sorption of perfluoroalkyl substances (PFASs) in aqueous film-forming foams-impacted groundwater: effects of PFASs properties and groundwater chemistry. Chemosphere, 286: 131622

[53] Wang F, Shih K, Lu X, Liu C. (2013). Mineralization behavior of fluorine in perfluorooctanesulfonate (PFOS) during thermal treatment of lime-conditioned sludge. Environmental Science & Technology, 47(6): 2621–2627

[54] Wang J, Wang S. (2019). Preparation, modification and environmental application of biochar: a review. Journal of Cleaner Production, 227: 1002–1022

[55] Wang Z, Alinezhad A, Nason S, Xiao F, Pignatello J J. (2023a). Enhancement of per- and polyfluoroalkyl substances removal from water by pyrogenic carbons: tailoring carbon surface chemistry and pore properties. Water Research, 229: 119467

[56] Wang Z, Alinezhad A, Sun R, Xiao F, Pignatello J J. (2023b). Pre- and postapplication thermal treatment strategies for sorption enhancement and reactivation of biochars for removal of per- and polyfluoroalkyl substances from water. ACS ES&T Engineering, 3(2): 193–200

[57] Wu Y, Qi L, Chen G. (2022). A mechanical investigation of perfluorooctane acid adsorption by engineered biochar. Journal of Cleaner Production, 340: 130742

[58] Yu H, Chen H, Zhang P, Yao Y, Zhao L, Zhu L, Sun H. (2023). In situ self-sacrificial synthesis of polypyrrole/biochar composites for efficiently removing short- and long-chain perfluoroalkyl acid from contaminated water. Journal of Environmental Management, 344: 118745

[59] Zhang D Q, He Q, Wang M, Zhang W, Liang Y. (2021). Sorption of perfluoroalkylated substances (PFASs) onto granular activated carbon and biochar. Environmental Technology, 42(12): 1798–1809

[60] Zhang Y, Tan X, Lu R, Tang Y, Qie H, Huang Z, Zhao J, Cui J, Yang W, Lin A. (2023). Enhanced removal of polyfluoroalkyl substances by simple modified biochar: adsorption performance and theoretical calculation. ACS ES&T Water, 3(3): 817–826

[61] Zhou T, Li X, Liu H, Dong S, Zhang Z, Wang Z, Li J, Nghiem L D, Khan S J, Wang Q. (2024). Occurrence, fate, and remediation for per-and polyfluoroalkyl substances (PFAS) in sewage sludge: a comprehensive review. Journal of Hazardous Materials, 466: 133637

Acknowledgements

All authors acknowledge support from the United States Environmental Protection Agency (No. SU840873). In addition, S. J. acknowledges support from the Procter & Gamble (P&G) (No. DA24-00647) and the 2024–2025 Henry C. McBay Faculty Research Fellowship from UNCF.

Conflict of Interests

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

RIGHTS & PERMISSIONS

2025 Higher Education Press 2025