Dioxin-like polychlorinated biphenyls (dl-PCB) in hydrochars and biochars: Review of recent evidence, pollution levels, critical gaps, formation mechanisms and regulations

Contamination of chars with dioxin-like polychlorinated biphenyls (dl-PCB) significantly limits their use and hinders their deployment in the circular bioeconomy, specifically in applications that may lead to dietary exposure. Here, for the first time, we review the levels of contamination of chars produced from pyrolysis and hydrothermal carbonisation (HTC) with dl-PCB congeners. We conduct a detailed and critical examination of the role played by the processing parameters, such as temperature and residence time, and the reaction mechanisms, to detoxify the biomass under an oxygen-free atmosphere during its valorisation. The PCB-based toxicity of biomass depends mostly on the abundance of dl-PCB in the raw material, and on the dechlorination and other transformation processes that operate during the treatment. The key dechlorination steps make the toxicity of hydrochars pass through a maximum with increasing treatment time, whereas the toxicity of biochars in pyrolysis decreases monotonically. Pyrolysis displays more complex mechanistic pathways of volatilisation, dechlorination, degradation of PCB rings, minor de novo formation of dl-PCB in case of air leaks, and concentrating persistent organic pollutants (POP) in char matrices. In contrast, the mechanisms responsible for the evolution of toxicity in HTC processes comprise the dechlorination, possible chlorine position shift, and biomass densification. The kinetic model developed in this review affords insight into the evolution of the hydrochar toxicity that depends on process temperature and treatment time. The dl-PCB concentrations in treated biomass generally range from 1.06 ng WHO2005-TEQ (kg DM)-1 to 11.7 ng WHO2005-TEQ (kg DM)-1, whereas for biochars produced from contaminated sediments the toxicity varies from 0.00662 ng WHO1998-TEQ (kg DM)-1 to 1.42 ng WHO1998-TEQ (kg DM)-1. DM stands for dry matter, TEQ for toxic equivalency, and WHO1998 means the application of the toxic equivalency factors (TEF) set by the World Health Organization (WHO) in 1998 to calculate the TEQ. Finally, we identify the crucial gaps in the literature, review the regulations governing the use of biomass in feed and in the environment, and provide suggestions for future research. The findings in this article provide both the technical understanding of how to minimise the formation of dl-PCB in the production of chars and suggest modifications to the current guidelines. The latter will increase the consumer's trust in valorised biomass, leading to its wider acceptance in the circular bioeconomy as feed supplements and soil additives.

Study on the Effect of Hydrothermal Carbonization Parameters on Fuel Properties of Sewage Sludge Hydrochar

In the wake of economic and population growth, increased wastewater production poses a challenge related to sewage sludge treatment, which is problematic given its high moisture content, amount, and hazardous characteristics. This study focuses on the hydrothermal carbonization of sewage sludge to produce carbonous material-hydrochar, which may be an alternative to fossil fuels. The effect of process parameters, namely, temperature (180, 240, 300 °C) and duration time (30, 90, 180 min), on hydrochar properties (proximate and ultimate analysis, heating values) and process performance were studied. Obtained results indicate and confirm that hydrothermal carbonization, especially temperature increase, improves the fuel properties of carbonized sewage sludge. The highest low heating value was obtained for hydrochar derived at 300 °C in 180 min (~23 MJ × kg-1). The highest energy gain was noted for hydrochar derived at 240 °C in 180 min (~23%). As well as relatively high mass and energy yield in comparison to other hydrochars, these parameters are considered the most favorable for sewage sludge hydrothermal carbonization. However, high energy consumption (over 1300 kJ × g-1) suggests that more research on the process's economical efficacy is required.

Key Role of Corncob Based-Hydrochar (HC) in the Enhancement of Visible Light Photocatalytic Degradation of 2,4-Dichlorophenoxyacetic Acid Using a Derivative of ZnBi-Layered Double Hydroxides

A superior heterojunction of HC-ZnBi-LDO was synthesized in two steps, namely hydrothermal carbonization, followed by co-precipitation. The 2% HC-ZnBi-LDO heterojunction photocatalysts could degrade over 90.8% of 30 mg/L 2,4-dichlorophenoxyacetic acid (2,4-D) using 1.0 g/L of the catalyst after 135 min of visible light exposure at pH 4. The activity of 2% HC-ZnO-LDO was remarkably stable. Approximately 86.4-90.8% of 30 mg/L 2,4-D was degraded, and more than 79-86.4% of TOC was mineralized by 2% HC-ZnBi-LDO at pH 4 after 135 min of visible light exposure during four consecutive cycles. The rapid separation and migration of charge carriers at the interfaces between HC and ZnBi-LDO were achieved within 2% HC-ZnBi-LDO. Moreover, the electron acceptor characteristic of HC in 2% HC-ZnBi-LDO caused the recombination of charge carriers to decrease significantly, thus generating more reactive radicals, such as hydroxyl radicals (OH^●) and superoxide radicals (O₂^●-). These results demonstrate that the novel 2% HC-ZnBi-LDO is a superior photocatalyst for the remediation of hazardous organic pollutants.

Chicken Cartilage-Derived Carbon for Efficient Xylene Removal

Chicken cartilage was used for the first time as a raw material for the microwave-assisted synthesis of biochar and activated carbon. Various microwave absorbers, i.e., commercial active carbon, scrap tyres, silicon carbide, and chicken bone-derived biochar, as well as various microwave powers, were tested for their effect on the rate of pyrolysis and the type of products formed. Biochars synthesised under 400 W in the presence of scrap tyres and chicken bone-derived biochar were activated with KOH and K₂CO₃ with detergent to produce activated carbon with a highly developed porous structure that would be able to effectively adsorb xylene vapours. All carbons were thoroughly characterised (infrared spectroscopy, X-ray fluorescence spectrometry, nitrogen adsorption/desorption, Raman spectroscopy, proximate and ultimate analysis) and tested as xylene sorbents in dynamic systems. It was found that the activation causes an increase of up to 1042 m²·g^-1 in the specific surface area, which ensures the sorption capacity of xylene about 300 mg·g^-1. Studies of the composition of biogas emitted during pyrolysis revealed that particularly valuable gaseous products are formed when pyrolysis is carried out in the presence of silicon carbide as a microwave absorber.

A cradle to cradle approach towards remediation of uranium from water using carbonized arecanut husk fiber

Sustainable materials for remediation of pollutants from water is the need of the hour. In this study two carbonaceous adsorbents prepared through hydrothermal carbonisation and pyrolysis from arecanut husk fiber, an agricultural waste material were used for the adsorption of uranium from water. Batch adsorption data as interpreted using the Langmuir model showed adsorption capacities of 250 mg g-1 and 200 mg g-1 respectively at pH 6 for the hydrochar (AHFC) and the pyrochar (AHFT) exceeding that reported for most of the unmodified biochars. The adsorption followed pseudo-second order kinetics and was exothermic in nature. The high selectivity and excellent removal efficiencies on application to environmental ground water samples and good regeneration capacity make these sorbents promising eco-friendly materials for uranium remediation from water.