Residual perfluorochemicals in the biochar from sewage sludge

Review of per- and poly-fluoroalkyl treatment in combustion-based thermal waste systems in the United States

Per- and poly-fluoroalkyl substances (PFAS) are a class of synthetic chemicals known for their widespread presence and environmental persistence. Carbon-fluorine (C-F) bonds are major components among PFAS and among the strongest organic bonds, thus destroying PFAS may present significant challenge. Thermal treatment such as incineration is an effective and approved method for destroying many halogenated organic chemicals. Here, we present the results of existing studies and testing at combustion-based thermal treatment facilities and summarize what is known regarding PFAS destruction and mineralization at such units. Available results suggest the temperature and residence times reached by some thermal treatment systems are generally favorable to the destruction of PFAS, but the possibility for PFAS or fluorinated organic byproducts to escape destruction and adequate mineralization and be released into the air cannot be ruled out. Few studies have been conducted at full-scale operating facilities, and none to date have attempted to characterize possible fluorinated organic products of incomplete combustion (PICs). Further, the ability of existing air pollution control (APC) systems, designed primarily for particulate and acid gas control, to reduce PFAS air emissions has not been determined. These data gaps remain primarily due to the previous lack of available methods to characterize PFAS destruction and PIC concentrations in facility air emissions. However, newly developed stack testing methods offer an improved understanding of the extent to which thermal waste treatment technologies successfully destroy and mineralize PFAS in these waste streams.

Biochar-based materials as remediation strategy in petroleum hydrocarbon-contaminated soil and water: Performances, mechanisms, and environmental impact

Petroleum contamination is considered as a major risk to the health of humans and environment. Biochars as low-cost and eco-friendly carbon materials, have been widely used for the removal of petroleum hydrocarbon in the environment. The purpose of this paper is to review the performance, mechanisms, and potential environmental toxicity of biochar, modified biochar and its integration use with other materials in petroleum contaminated soil and water. Specifically, the use of biochar in oil-contaminated water and soil as well as the factors that could influence the removal ability of biochar were systematically evaluated. In addition, the modification and integrated use of biochar for improving the removal efficiency were summarized from the aspects of sorption, biodegradation, chemical degradation, and reusability. Moreover, the functional impacts and associated ecotoxicity of pristine and modified biochars in various environments were demonstrated. Finally, some shortcoming of current approaches, and future research needs were provided for the future direction and challenges of modified biochar research. Overall, this paper gain insight into biochar application in petroleum remediation from the perspectives of performance enhancement and environmental sustainability.

Micropollutants in biochar produced from sewage sludge: A systematic review on the impact of pyrolysis operating conditions

Biochar obtained from sewage sludge serves as a valuable soil amendment in agriculture, enhancing soil properties by increasing the nutrient content, cation exchange capacity, water retention, and oxygen transmission. However, its utilisation is hampered by the presence of micropollutants such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated dibenzodioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), polychlorinated biphenyls (PCBs), and volatile organic compounds (VOCs). Previous studies indicate that the type and amount of micropollutants can be significantly adjusted by selecting the right process parameters. This literature review provides an overview of how (1) pyrolysis temperature, (2) carrier gas flow and type, (3) heating rate, and (4) residence time affect the concentration of micropollutants in biochar produced from sewage sludge. The micropollutants targeted are those listed by the European Biochar Certificate (EBC) and by the International Biochar Institution (IBI), including PAHs, PCDD/Fs, PCBs and VOCs. In addition, per- and poly-fluoroalkyl substances (PFAS) are also considered due to their presence in sewage sludge. The findings suggest that higher pyrolysis temperatures reduce micropollutant levels. Moreover, the injection of a carrier gas (N2 or CO2) during the pyrolysis and cooling processes effectively lowers PAHs and PCDD/Fs, by reducing the contact of biochar with oxygen, which is crucial in mitigating micropollutants. Nevertheless, limited available data impedes an assessment of the impact of these parameters on PFAS in biochar. In addition, further research is essential to understand the effects of carrier gas type, heating rate, and residence time in order to determine the optimal pyrolysis process parameters for generating clean biochar.

Ensuring safety standards in sewage sludge-derived biochar: Impact of pyrolysis process temperature and carrier gas on micropollutant removal

The application of sewage sludge to agricultural land is facing increasing restrictions due to concerns about various micropollutants, including polycyclic aromatic hydrocarbons (PAHs), dioxins and furans (PCDD/Fs), polychlorinated biphenyls (PCBs), per- and poly-fluoroalkyl substances (PFAS), and heavy metals (HMs). As an alternative approach to manage this residue, the use of pyrolysis, a process that transforms sludge into biochar, a carbon-rich solid material, is being explored. Despite the potential benefits of pyrolysis, there is limited data on its effectiveness in removing micropollutants and the potential presence of harmful elements in the resulting biochar. This study aims to evaluate the impact of the temperature and the use of a carrier gas (N2) during a two-stage pyrolysis and cooling on micropollutant removal. Pilot-scale tests showed that a higher temperature (650 °C) and the use of a carrier gas (0.4 L/min N2) during the pyrolysis and the cooling process led to a reduction of PAHs, PCDD/Fs, PCBs and PFAS below their detection limits. As such, the generated biochar aligns with the guidelines set by the International Biochar Initiative (IBI) and the European Biochar Certificate (EBC) for all micropollutants, except for zinc and copper. Additional investigation is required to determine whether the micropollutants undergo destruction or transition into other pyrolysis end-products, such as the gas or liquid phase.

Toxicity and Behavioral Effects of Amending Soils with Biochar on Red Imported Fire Ants, Solenopsis invicta

Solenopsis invicta, often known as the red imported fire ants (RIFAs), is a well-known global invasive ant species that can be found in agricultural, urban, and natural environments worldwide. Simultaneously, it also inhabits the soil. Biochar is generated by the pyrolysis of organic matter under high-temperature anoxic environments and widely used in agricultural ecosystems and soil amendment. However, to date, it remains unknown as to whether soil application of biochar has a negative effect on RIFAs. In our study, we investigated the toxicity and irritability effects of different amounts of biochar (0%, 1%, 2%, 5%, 10%, and 20%) introduced into the soil on red fire ants; upon comparison with the control soil (0% biochar), the application of 1%, 2%, and 5% biochar did not result in significantly different results. But the utilization of biochar at a concentration over 10% effectively repelled the RIFAs, resulting in their departure from the treated soils. High doses of biochar were able to cause death of red fire ants; the mortality rate of red fire ants reached 55.56% after 11 days of 20% biochar treatment. We also evaluated the effects of biochar on four behaviors of red fire ants, namely aggregation, walking, grasping, and attacking; 20% of the biochar treatment group reduced aggregation by 64.22% and this value was 55.22%, 68.44%, and 62.36% for walking, grasping, and attacking. Finally, we measured the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) enzyme activity and malondialdehyde (MDA) content in red fire ants; the results showed that the activities of the three enzymes increased with the increase in biochar addition, which indicated that a high dose of biochar induced oxidative stress in red fire ants. Our results indicate that biochar has the potential to cause toxicity and repel red imported fire ants (RIFAs) in a manner that is dependent on the concentration. We propose that biochar could be utilized in the control and manufacturing of baits for red fire ant management. This work establishes a foundation for the prevention and management of red fire ants and the logical utilization of biochar.

Smouldering to treat PFAS in sewage sludge

Wastewater treatment plants are accumulation points for per- and polyfluoroalkyl substances (PFAS), and are threfore important facilities for PFAS treatment. This study explored using smouldering combustion to treat PFAS in sewage sludge. Base case experiments at the laboratory scale (LAB) used dried sludge mixed with sand. High moisture content (MC) LAB tests, 75% MC sludge by mass, explored impacts of MC on treatment and supplemented with granular activated carbon (GAC) to achieve sufficient temperatures for thermal destruction of PFAS. Additional LAB tests explored using calcium oxide (CaO) to support fluorine mineralization. Further tests performed at an oil-drum scale (DRUM) assessed scale on PFAS removal. Pre-treatment sludge and post-treatment ash samples from all tests were analyzed for 12 PFAS (2C-8C). Additional emissions samples were collected from all LAB tests and analyzed for 12 PFAS and hydrogen fluoride. Smouldering removed all monitored PFAS from DRUM tests, and 4-8 carbon chain length PFAS from LAB tests. For base case tests, PFOS and PFOA were completely removed from sludge; however, high contents in the emissions (79-94% of total PFAS by mass) showed volatilization without degradation. Smouldering high MC sludge at ∼ 900 °C (30 g GAC/kg sand) improved PFAS degradation compared to treatment below 800 °C (<20 g GAC/kg sand). Addition of CaO before smouldering reduced PFAS content in emissions by 97-99% by mass; with minimal PFAS retained in the ash and minimal hydrofluoric acid (HF) production, as the fluorine from the PFAS was likely mineralized in the ash. Co-smouldering with CaO had dual benefits of removing PFAS while minimizing other hazardous emission by-products.

Pyrolysis-A tool in the wastewater solids handling portfolio, not a silver bullet: Benefits, drawbacks, and future directions

Pyrolysis is the process whereby carbonaceous materials, such as biosolids, are heated between 400°C and 900°C in the absence of oxygen. Three main products are generated: a solid product called biochar, a py-liquid that consists of aqueous phase and non-aqueous phase liquid, and py-gas. The biochar holds value as a beneficial soil amendment and sequesters carbon. The py-liquid is potentially hazardous and needs to be dealt with (including potentially reducing it on-site via catalysis or thermal oxidation). Py-gas can be used on-site for energy recovery. Pyrolysis has gained recent interest due to concern over per- and polyfluoroalkyl substances (PFAS) in biosolids. Although pyrolysis can remove PFAS from biosolids, it has been shown to produce PFAS that reside in py-liquid, and the fate in py-gas remains a knowledge gap. More research is needed to help close the PFAS and fluorine mass balance through pyrolysis influent and effluent products because pyrolysis alone does not destroy all PFAS. The moisture content of biosolids substantially affects the energy balance for pyrolysis. Utilities that already produce a dried biosolids product are in a better position to install pyrolysis. Pyrolysis has both defined benefits (solids reduction, PFAS removal from biosolids, and biochar production) as well as remaining questions (the fate of PFAS in py-gas and py-liquid, mass balance on nutrients, and py-liquid handling options) that will be answered through more pilot and full-scale demonstrations. Regulations and local policies (such as carbon sequestration credits) could affect pyrolysis implementation. Pyrolysis should be considered as an option in the biosolids stabilization toolbox with application being based on individual circumstances of a utility (e.g., energy, moisture content of biosolids, PFAS). PRACTITIONER POINTS: Pyrolysis has known benefits but limited full-scale operational data. Pyrolysis removes PFAS from biochar, but PFAS fate in gas phase is unknown. Moisture content of influent feed solids affects energy balance of pyrolysis. Policy on PFAS, carbon sequestration, or renewable energy could impact pyrolysis.

Biochar-mediated abiotic and biotic degradation of halogenated organic contaminants - A review

Prevailing global increases in population, urbanization, and agricultural production are causing increased pressures on water resources, especially as the use of chemicals in agriculture, industry, and medicine provide new challenges for water treatment and reuse. Organohalogen compounds are persistent contaminants that often evade current wastewater treatment technologies, resulting in their accumulation in the environment and posing a serious threat to ecosystem health. Recent advances in understanding pyrogenic carbons as electron shuttling and storing materials have exposed their potential for enhancing the dehalogenation and overall degradation of organohalide contaminants in soil, sediment, surface water, and wastewater systems. Biochar is a porous carbonaceous material produced during the thermochemical decomposition of biomass feedstock in the presence of little or no oxygen (pyrolysis). Interest in biochar for application towards environmental remediation is largely based on its three distinct benefits: I) carbon sequestration to offset greenhouse gas emissions, II) adsorption of (in-) organic contaminants and nutrients, and III) a strong electron exchange capacity. Due to the innate complexity of biochar materials, several electron transfer mechanisms exist by which biochar may mediate contaminant degradation. These electron transfer pathways include electron-accepting and donating cycles through redox-active functional groups and direct electron transfer via conductive carbon matrices. These mechanisms are responsible for biochar's participation in multiple redox-driven biogeochemical transformations with proven consequences for effective organohalogen remediation. This literature review summarizes the current knowledge on the mechanisms and processes through which biochar can directly or indirectly mediate the transformation of organohalogen compounds under various environmental conditions. Perspectives and research directions for future application of biochars for targeted remediation strategies are also discussed.

Pyrolysis processing of PFAS-impacted biosolids, a pilot study

Concentrations of per- and poly-fluoroalkyl substances (PFAS) present in wastewater treatment biosolids are a growing concern. Pyrolysis is a thermal treatment technology for biosolids that can produce a useful biochar product with reduced levels of PFAS and other contaminants. In August 2020, a limited-scope study investigated target PFAS removal of a commercial pyrolysis system processing biosolid with the analysis of 41 target PFAS compounds in biosolids and biochar performed by two independent laboratories. The concentrations of 21 detected target compounds in the input biosolids ranged between approximately 2 µg/kg and 85 µg/kg. No PFAS compounds were detected in the biochar. The PFAS concentrations in the biochar were assumed to equal the compounds' minimum detection limits (MDLs). The pyrolysis system's target PFAS removal efficiencies (REs) were estimated to range between >81.3% and >99.9% (mean >97.4%) with the lowest REs being associated with the lowest detected PFAS concentrations and the highest MDLs. No information on non-target PFAS compounds in influent or effluent media or products of incomplete combustion was considered. Selected gaseous emissions were measured by Fourier transform infrared spectroscopy and gas chromatography time-of-flight mass spectrometry to provide additional information on air emissions after process controls. This limited-scope study indicated that additional research to further understand this process is warranted.Implications: Development of alternative approaches to manage PFAS-impacted biosolids is of emerging international importance. A commercially operating biosolid pyrolysis process was shown to lower target PFAS levels in produced biochar. Additional research is warranted to understand all potential PFAS transformation emission routes and optimal air pollution emissions control strategies for this technology class.

Pyrolysis and gasification at water resource recovery facilities: Status of the industry

Wastewater treatment generates solids requiring subsequent processing. Costs and contaminant concerns (e.g., per- and polyfluoroalkyl substances [PFAS]) are challenging widely used landfilling and land application practices. These circumstances are partly driving the re-emergence of pyrolysis and gasification technologies along with beneficial reuse prospects of the char solid residual. Previously, technologies experienced operational challenges leading to revised configurations, such as directly coupling a thermal oxidizer to the reactor to destroy tar forming compounds. This paper provides an overview of pyrolysis and gasification technologies, characteristics of the char product, air emission considerations, and potential fate of PFAS and other pollutants through the systems. Results from a survey of viable suppliers illustrate differences in commercially available options. Additional research is required to validate performance over the long-term operation and confirm contaminant fate, which will help determine whether resurging interest in pyrolysis and gasification warrants widespread adoption. PRACTITIONER POINTS: Pyrolysis and gasification systems are re-emerging in the wastewater industry. Direct coupling of thermal oxidizers and other modifications offered by contemporary systems aim to overcome past failures. Process conditions when coupled with a thermal oxidizer will likely destroy most organic contaminants, including PFAS, but requires additional research. Three full-scale facilities recently operated, several in construction or design that will provide operating experience for widespread technology adoption consideration.

PFAS fate and destruction mechanisms during thermal treatment: a comprehensive review

Per- and polyfluoroalkyl substances (PFAS) are persistent chemicals and have been detected throughout the environment. Thermal treatment is the most common remediation approach for PFAS-contaminated solid wastes. Although various thermal treatment techniques have demonstrated the potential to destruct PFAS, the fate of PFAS, removal efficacy, potential emissions, and the formation of incomplete combustion products during thermal treatment are little known. This study provides a critical review on the behavior of PFAS based on different types of thermal treatment technologies with various PFAS-impacted environmental medias that include water, soil, sewage sludge, pure PFAS materials, and other PFAS-containing wastes. Different extents of PFAS thermal destruction are observed across various thermal treatment techniques and operating conditions. PFAS removal and destruction efficiencies rely heavily on PFAS structures, the complex combustion chemistry, the presence or absence of oxygen, temperature, and other operational conditions. This review also covers proposed PFAS thermal destruction mechanisms. Different thermal destruction mechanisms for perfluorooctanoic acid (PFOA) and perfluorooctanesulfonate (PFOS), and other PFAS are reviewed and compared. The majority of studies about PFAS thermal destruction mechanisms were focused on a specific list of PFAS and based mostly on the pyrolysis treatment. The basic pathway for PFAS destruction during pyrolysis is hydrodefluorination, which could be largely influenced by the alkaline condition. Future field-scale research that involves the characterization of PFAS destruction products and incomplete combustion products is needed to address public concerns and better emission control.

Molecular composition and biotoxicity effects of dissolved organic matters in sludge-based carbon: Effects of pyrolysis temperature

Sludge pyrolysis carbonization has shown potential to convert sludge biomass into multifunctional carbon materials. However, ecological risks of dissolved organic matters (DOMs) with obscure molecular characteristics retaining in sludge-based carbons (SBCs) have received little attention. This study investigated the impact of pyrolysis temperatures on the molecular conversion and biotoxicity effects of DOMs in SBCs. The results revealed that DOMs in SBCs_300-400 were mainly derived from depolymerization of biopolymers and the polycondensation and cyclization of small intermediate molecules, which mainly consisted of aromatic CHON compounds with 1-3 N atoms, featuring high unsaturation and molecular weights. High-temperature pyrolysis (500-800 °C) promoted the decomposition and ring-opening of aromatic CHON compounds into saturated aliphatic CHO compounds with 2-4 O atoms in SBCs_500-800. Noteworthily, SBCs_300-400-derived DOMs showed relatively strong biotoxicity on the growth and development of wild-type zebrafish embryos, pakchoi seeds, and Vibrio qinghaiensis Q67, which was significantly related to aromatic amines, phenols, and heterocyclic-N compounds in DOMs of SBCs_300-400. SBCs_500-800-derived DOMs were mainly straight-chain fatty acids and showed no observable acute biotoxicity. This study highlights the negative impact of DOMs in SBCs on the ecological environment, and provides the theoretical basis for controlling toxic byproducts in sludge pyrolysis process.

Electrochemical destruction and mobilization of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) in saturated soil

Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) have attracted attention due to their widespread distribution, recalcitrance, and substantial toxicity. In this work, high concentrations of PFOA and PFOS were degraded and mobilized through electrochemical treatments in a simulated source zone of saturated soil. Under a low constant voltage and direct current of 24 V and 467-690 mA, approximately 51.7% and 33% of PFOA and PFOS were degraded, respectively. Additionally, a total defluorination mass balance of 44.7% and 23% were detected for PFOA and PFOS, respectively, which indicates that the removal of PFOA and PFOS occurs through its destruction. Substantial electromigration causes the destruction and mobilization of solid PFOA and PFOS to shift into the water phase. Although electrochemical oxidation of PFAS (per- and polyfluoroalkyl substances) were previously reported and studied, this study is one of the few that focus on simultaneous desorption, mobilization, and destruction of PFAS in saturated soil containing a low-intensity electrical field.

Natural and engineered clays and clay minerals for the removal of poly- and perfluoroalkyl substances from water: State-of-the-art and future perspectives

Poly- and perfluoroalkyl substances (PFAS) present globally in drinking-, waste-, and groundwater sources are contaminants of emerging concern due to their long-term environmental persistence and toxicity to organisms, including humans. Here we review PFAS occurrence, behavior, and toxicity in various water sources, and critically discuss their removal via mineral adsorbents, including natural aluminosilicate clay minerals, oxidic clays (Al, Fe, and Si oxides), organoclay minerals, and clay-polymer and clay‑carbon (biochar and graphene oxide) composite materials. Among the many remediation technologies, such as reverse osmosis, adsorption, advanced oxidation and biologically active processes, adsorption is the most suitable for PFAS removal in aquatic systems. Treatment strategies using clay minerals and oxidic clays are inexpensive, eco-friendly, and efficient for bulk PFAS removal due to their high surface areas, porosity, and high loading capacity. A comparison of partition coefficient values calculated from extracted data in published literature indicate that organically-modified clay minerals are the best-performing adsorbent for PFAS removal. In this review, we scrutinize the corresponding plausible mechanisms, factors, and challenges affecting the PFAS removal processes, demonstrating that modified clay minerals (e.g., surfactant, amine), including some commercially available products (e.g., FLUORO-SORB®, RemBind®, matCARE™), show good efficacy in PFAS remediation in contaminated media under field conditions. Finally, we propose future research to focus on the challenges of using clay-based adsorbents for PFAS removal from contaminated water due to the regeneration and safe-disposal of spent clay adsorbents is still a major issue, whilst enhancing the PFAS removal efficiency should be an ongoing scientific effort.

Potential hazards of biochar: The negative environmental impacts of biochar applications

Biochar has been widely used as an environmentally friendly material for soil improvement and remediation, water pollution control, greenhouse gas emission reduction, and other purposes because of its characteristics such as a large surface area, porous structure, and abundant surface O-containing functional groups. However, some surface properties (i.e., (i) some surface properties (i.e., organic functional groups and inorganic components), (ii) changes in pH), and (iii) chemical reactions (e.g., aromatic C ring oxidation) that occur between biochar and the application environment may result in the release of harmful components. In this study, biochars with a potential risk to the environment were classified according to their harmful components, surface properties, structure, and particle size, and the potential negative environmental effects of these biochars and the mechanisms inducing these negative effects were reviewed. This article presents a comprehensive overview of the negative environmental impacts of biochar on soil, water, and atmospheric environments. It also summarizes various technical methods of environment-related risk detection and evaluation of biochar application, thereby providing a baseline reference and guiding significance for future biochar selection and toxicity detection, evaluation, and avoidance.

Preparation of ultrahigh-surface-area sludge biopolymers-based carbon using alkali treatment for organic matters recovery coupled to catalytic pyrolysis

In this work, we employed waste activated sludge (WAS) as carbon source to prepare ultrahigh specific surface area (SSA) biopolymers-based carbons (BBCs) through alkali (KOH) treatment coupled to pyrolysis strategy. Before the pyrolysis process, the involvement of KOH made a great recovery of soluble biopolymers from WAS, resulting in highly-efficient catalytic pyrolysis. The Brunner-Emmett-Teller and pore volume of BBCs prepared at 800°C (BBC₈₀₀) reached the maximum at 2633.89 m²·g^-1 and 2.919 m³·g^-1, respectively. X-ray photoelectron spectroscopy suggested that aromatic carbon in the form of C=C was the dominant fraction of C element in BBCs. The N element in BBCs were composed of pyrrolic nitrogen and pyridinic nitrogen at 700°C, while a new graphitic nitrogen appeared over 800°C. As a refractory pollutant of wastewater treatment plants, tetracycline (TC) was selected to evaluate adsorption performance of BBCs. The adsorption behavior of BBCs towards TC was conformed to the pseudo-second-order kinetic and the Langmuir models, signifying that chemisorption of monolayers was dominant in TC adsorption. The adsorption capacity of BBC₈₀₀ reached the maximum at 877.19 mg·g^-1 for 90 min at 298 K. Thermodynamic analysis indicated that the adsorption process was endothermic and spontaneous. Hydrogen bonding and π-π stacking interaction were mainly responsible for TC adsorption, and interfacial diffusion was the main rate-control step in adsorption process. The presence of soluble microbial products (SMPs) enhanced TC removal. This work provided a novel strategy to prepare bio-carbon with ultrahigh SSA using WAS for highly-efficient removal of organic pollutants.

Per- and polyfluoroalkyl substances thermal destruction at water resource recovery facilities: A state of the science review

Per- and polyfluoroalkyl substances (PFAS) are a recalcitrant group of chemicals and can be found throughout the environment. They often collect in wastewater systems with virtually no degradation prior to environmental discharge. Some PFAS partitions to solids captured in wastewater treatment which require further processing. Of all the commonly applied solids treatment technologies, incineration offers the only possibility to completely destroy PFAS. Little is known about the fate of PFAS through incineration, in particular, for the systems employed in water resource recovery facilities (WRRF). This review covers available research on the fate of PFAS through incineration systems with a focus on sewage sludge incinerators. This research indicates that at least some PFAS destruction will occur with incineration approaches used at WRRFs. Furthermore, PFAS in flue gas, ash, or water streams used for incinerator pollution control may be undetectable. Future research involving full-scale fate studies will provide insight on the efficacy of PFAS destruction through incineration and whether other compounds of concern are generated. PRACTITIONER POINTS: Thermal processing is the only commercial approach available to destroy PFAS. Thermal degradation conditions required for destruction of PFAS during incineration processes are discussed. Fate of PFAS through water resource recovery facility incineration technologies remains unclear. Other thermal technologies such as smoldering combustion, pyrolysis, gasification, and hydrothermal liquefaction provide promise but are in developmental phases.

Effect of soil organic matter on the plant uptake of perfluorooctanoic acid (PFOA) and perfluorooctanesulphonic acid (PFOS) in lettuce on granular activated carbon-applied soil

The presence of perfluorooctanoic acid (PFOA) and perfluorooctanesulphonic acid (PFOS) in crops is an important consideration for food safety. The soil organic matter (SOM) content may affect the adsorption potential of PFOA and PFOS in water and soil and their subsequent uptake in crops. To better understand these dynamics, the adsorption and uptake of PFOA and PFOS in lettuce were investigated using granular activated carbon (GAC)-treated soils with varying SOM content. The adsorption potential of GAC was investigated, with maximum adsorption capacities for PFOA and PFOS calculated to be 9.091 mg g-1 and 27.778 mg g-1, respectively. These values decreased to 5.208 mg g-1 and 17.241 mg g-1, respectively, after the addition of 0.04 wt% humic acid. The average plant uptake factor (PUF) in low and high perfluoroalkyl and polyfluoroalkyl acid (PFAA)-contaminated soils with 4.0 wt% SOM was restricted to 0.353 for PFOA and 0.108 for PFOS. The PUFs were approximately two times lower than those for soil with 2.6 wt% SOM. Addition of 1 wt% GAC to the soil successfully reduced the PUF by up to 99.4%, with values of 0.006 (PFOA) and 0.005 (PFOS) in 2.6 wt% SOM-treated soil and 0.079 (PFOA) and 0.023 (PFOS) in 4.0 wt% SOM-treated soil. Although the PUF in the GAC-treated soil was drastically decreased, the PUF of the soil with 4.0 wt% SOM was at least four times higher than that with 2.6 wt% SOM. Therefore, SOM content is an important consideration in the remediation of PFOA- and PFOS-contaminated farmland soil using carbonaceous adsorbent.

Conversion of sewage sludge into biochar: A potential resource in water and wastewater treatment

Production of biochar from sewage sludge (SS) is consistent with the goal of sustainable resource recovery and promotes a wastewater-based circular economy. Thermochemical conversion of SS to biochar resolves two major issues simultaneously as it minimizes the cost of disposal and acts as a resource to eliminate the toxic contaminants from water and wastewater. The reusability and ready availability of the biochar, irrespective of the season, makes it an economically viable material for wastewater treatment. In this review, explicit insights into the production, modification and usage of SS derived biochar are provided including (i) the production yield, (ii) characteristic features such as physical, chemical, electrochemical and morphological aspects, and (iii) impact on contaminant removal through adsorption, catalytic and electrochemical processes. Particular attention is given to the use of SS derived biochar as an adsorbent for contaminants present in wastewaters, the potential use of biochar as a catalyst and support material in advanced oxidation processes and the use of biochars as an electrode material. The effect of pyrolysis conditions and co-pyrolysis with other materials on biochar properties is explored and insight is provided into the toxicity of biochar components present at different process conditions.

THE DARK SIDE OF BLACK GOLD: Ecotoxicological aspects of biochar and biochar-amended soils

Biochar, a product of biomass pyrolysis, is characterized by significant surface area, porosity, high water holding capacity, and environmental persistence. It is perceived as a material that can counteract climate change due to its high carbon stability and is also considered suitable for soil amendment (fertility improvement, soil remediation). However, biochar can have a toxic effect on organisms as harmful substances may be present in it. This paper reviews the literature regarding the current knowledge of harmful substances in biochar and their potential negative impact on organisms from different trophic levels. The effects of biochar on the content and toxicity of harmful substances in biochar-amended soils are also reviewed. Application of biochar into soil does not usually have a toxic effect and very often stimulate plants, bacteria activity and invertebrates. The effect however is strictly determined by type of biochar (especially the feedstock used and pyrolysis temperature) as well as contaminants content. The pH, electrical conductivity, polycyclic aromatic hydrocarbons as well as heavy metals are the main factor usually responsible for biochar toxicity.

Environmental and dietary exposure of perfluorooctanoic acid and perfluorooctanesulfonic acid in the Nakdong River, Korea

This study performed the first environmental and dietary exposure assessment to explore plant uptake of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) from agricultural soil and irrigation water in the Nakdong River delta, South Korea. Annual average concentrations of total PFOA and PFOS ranged from 0.026 to 0.112 µg L^-1 (irrigation water), and from 0.818 to 1.364 µg kg^-1 (soil), respectively. PFOA and PFOS hotspots were identified downstream of the Nakdong River and were influenced by seasonal climatic variations. The observed average biennial concentration of the sum of PFOA and PFOS decreased in irrigation water, from 0.112 µg L^-1 in 2013 to 0.026 µg L^-1 in 2015, suggests that the 2013 Persistent Organic Pollutants Control Act may have helped to reduce levels of PFAS at this location. This study calculated some of the highest plant uptake factors reported to date, with values ranging from 0.962 in green onions to < 0.004 in plums. Leafy vegetables and rice are important components of the Korean diet; these groups had the largest contribution to the estimated dietary intake of PFOA and PFOS, which was calculated at 0.449 and 0.140 ng kg _bw ^-1 day^-1, respectively. This corresponded to 66.4% for PFOA and 7.9% for PFOS of the EFSA reference dose (RfD). The dietary intake of PFOA and PFOS from crops alone did not exceed the RfD. However, when the estimated daily intake (EDI) from other sources such as tap water, meat, fish, dairy, and beverages was included in the exposure risk assessment, both of the EDIs to PFOA and PFOS exceeded the RfDs, indicating that there may be a risk to human health. This study concludes that consumption of crops might, therefore, be a significant and underappreciated pathway for human exposure to PFAS.

Carbon dioxide as a carrier gas and mixed feedstock pyrolysis decreased toxicity of sewage sludge biochar

The common use of sewage sludge (SSL)-derived biochar can be limited due to contaminants present in SSL, which may affect SSL-derived biochar toxicity. We propose the reduction of SSL-derived biochar toxicity by it co-pyrolysis with biomass and in CO₂ atmosphere. Ecotoxicity of biochars produced at 500, 600, and 700 °C from SSL and SSL with the addition of willow (at a ratio of SSL:willow - 8:2 and 6:4, w/w) in an atmosphere of N₂ or CO₂ were investigated. The toxicity of aqueous extracts derived from the biochars (Lepidium sativum - Elongation test, Vibrio fischeri - Microtox) or solid-phase toxicity (Lepidium sativum - Phytotoxkit F, Folsomia candida - Collembolan test) was also studied. SSL-derived biochar produced at N₂ atmosphere usually was toxic for all tested organisms. Co-pyrolysis of mixed feedstock reduced the toxicity of the produced biochar. In the case of biochars produced from SSL and willow under N₂ atmosphere decrease in inhibition of F. candida reproduction (from 27 to 58%) or its stimulation (from 7 to 30%) in comparison to SSL alone derived biochar, was observed. Co-pyrolysis of SSL with willow significantly reduced the toxicity of extracts the SSL-derived biochar towards L. sativum. The aqueous extracts obtained from the biochars produced at temperatures of 500 and 600 °C with willow addition were also less toxic to V. fischeri than the biochars produced from SSL alone. The change of carrier gas from N₂ to CO₂, regardless of the feedstock used, in most cases reduced toxicity or positively affected the test organisms. This was probably caused by changes in the physicochemical properties and content of contaminants in the biochars produced in an atmosphere of CO₂, compared to N₂. An exception was root growth inhibition in the solid phase tests where no significant differences were found between biochars produced in N₂ and CO₂.

Biochar for Wastewater Treatment—Conversion Technologies and Applications

Biochar as a stable carbon-rich material shows incredible potential to handle water/wastewater contaminants. Its application is gaining increasing interest due to the availability of feedstock, the simplicity of the preparation methods, and their enhanced physico-chemical properties. The efficacy of biochar to remove organic and inorganic pollutants depends on its surface area, pore size distribution, surface functional groups, and the size of the molecules to be removed, while the physical architecture and surface properties of biochar depend on the nature of feedstock and the preparation method/conditions. For instance, pyrolysis at high temperatures generally produces hydrophobic biochars with higher surface area and micropore volume, allowing it to be more suitable for organic contaminants sorption, whereas biochars produced at low temperatures own smaller pore size, lower surface area, and higher oxygen-containing functional groups and are more suitable to remove inorganic contaminants. In the field of water/wastewater treatment, biochar can have extensive application prospects. Biochar have been widely used as an additive/support media during anaerobic digestion and as filter media for the removal of suspended matter, heavy metals and pathogens. Biochar was also tested for its efficiency as a support-based catalyst for the degradation of dyes and recalcitrant contaminants. The current review discusses on the different methods for biochar production and provides an overview of current applications of biochar in wastewater treatment.

Research progress on the removal of hazardous perfluorochemicals: A review

Perfluorinated substances are global and ubiquitous pollutants. The persistent organic pollution of perfluorochemicals (PFCs) have drawn attentions worldwide. In view of the current need for sustainable development, many researchers began to study the remediation techniques for PFCs. Due to its unique hydrophobic and oil-phobic characteristics, the requirements for the PFCs removal process are different, so that their remediation techniques are still under continuous exploration. Hence, this review summarized the removal behaviors of various PFCs on different materials which supply a good foundation for future investigations in this field. It is evident from previous literature that every remediation techniques for PFCs has its own advantages. Among various currently evaluated removal methods, adsorption seems to be one of the most commonly used and recognized techniques for PFCs pollution control. Other innovative and promising techniques, such as physical and/or chemical methods, have also been tested for their effectiveness in removing perfluorinated compounds.

Evaluation of sewage sludge incineration ash as a potential land reclamation material

This study evaluated the potential of utilising sewage sludge incineration ash as a land reclamation material. Toxicity assessment of the leachate of the ash was carried out for both terrestrial and marine organisms. Both the fruit fly Drosophila melanogaster and barnacle Amphibalanus amphitrite showed that both bottom and fly ash leached at liquid-to-solid (L/S) ratio 5 did not substantially affect viabilities. The leachate carried out at L/S 10 was compared to the European Waste Acceptance Criteria and the sewage sludge ashes could be classified as non-hazardous waste. The geotechnical properties of the sewage sludge ash were studied and compared to sand, a conventional land reclamation material, for further evaluation of its potential as a land reclamation material. It was found from direct shear test that both bottom and fly ashes displayed similar and comparable shear strength to that of typical compacted sandy soil based on the range of internal friction angle obtained. However, the consolidation profile of bottom ash was significantly different from sand, while that of fly ash was more similar to sand. Our study showed that the sewage sludge ash has the potential to be used as a land reclamation material.

Biochar for composting improvement and contaminants reduction. A review

Biochar is characterised by a large specific surface area, porosity, and a large amount of functional groups. All of those features cause that biochar can be a potentially good material in the optimisation of the process of composting and final compost quality. The objective of this study was to compile the current knowledge on the possibility of biochar application in the process of composting and on the effect of biochar on compost properties and on the content of contaminants in compost. The paper presents the effect of biochar on compost maturity indices, composting temperature and moisture, and also on the content and bioavailability of nutrients and of organic and inorganic contaminants. In the paper note is also taken of the effect of biochar added to composted material on plants, microorganisms and soil invertebrates.

Modified sequential extraction for biochar and petroleum coke: Metal release potential and its environmental implications

A modified Community Bureau of Reference (CBR) sequential extraction method was tested to assess the composition of untreated pyrogenic carbon (biochar) and oil sands petroleum coke. Wood biochar samples were found to contain lower concentrations of metals, but had higher fractions of easily mobilized alkaline earth and transition metals. Sewage sludge biochar was determined to be less recalcitrant and had higher total metal concentrations, with most of the metals found in the more resilient extraction fractions (oxidizable, residual). Petroleum coke was the most stable material, with a similar metal distribution pattern as the sewage sludge biochar. The applied sequential extraction method represents a suitable technique to recover metals from these materials, and is a valuable tool in understanding the metal retaining and leaching capability of various biochar types and carbonaceous petroleum coke samples.