Bio- and hydrochars from rice straw and pig manure: Inter-comparison

The boom era of emerging contaminants: A review of remediating agricultural soils by biochar

Emerging contaminants (ECs) are widely sourced persistent pollutants that pose a significant threat to the environment and human health. Their footprint spans global ecosystems, making their remediation highly challenging. In recent years, a significant amount of literature has focused on the use of biochar for remediation of heavy metals and organic pollutants in soil and water environments. However, the use of biochar for the remediation of ECs in agricultural soils has not received as much attention, and as a result, there are limited reviews available on this topic. Thus, this review aims to provide an overview of the primary types, sources, and hazards of ECs in farmland, as well as the structure, functions, and preparation types of biochar. Furthermore, this paper emphasizes the importance and prospects of three remediation strategies for ECs in cropland: (i) employing activated, modified, and composite biochar for remediation, which exhibit superior pollutant removal compared to pure biochar; (ii) exploring the potential synergistic efficiency between biochar and compost, enhancing their effectiveness in soil improvement and pollution remediation; (iii) utilizing biochar as a shelter and nutrient source for microorganisms in biochar-mediated microbial remediation, positively impacting soil properties and microbial community structure. Given the increasing global prevalence of ECs, the remediation strategies provided in this paper aim to serve as a valuable reference for future remediation of ECs-contaminated agricultural lands.

Prediction model for biochar energy potential based on biomass properties and pyrolysis conditions derived from rough set machine learning

Biochar is a high-carbon-content organic compound that has potential applications in the field of energy storage and conversion. It can be produced from a variety of biomass feedstocks such as plant-based, animal-based, and municipal waste at different pyrolysis conditions. However, it is difficult to produce biochar on a large scale if the relationship between the type of biomass, operating conditions, and biochar properties is not understood well. Hence, the use of machine learning-based data analysis is necessary to find the relationship between biochar production parameters and feedstock properties with biochar energy properties. In this work, a rough set-based machine learning (RSML) approach has been applied to generate decision rules and classify biochar properties. The conditional attributes were biomass properties (volatile matter, fixed carbon, ash content, carbon, hydrogen, nitrogen, and oxygen) and pyrolysis conditions (operating temperature, heating rate residence time), while the decision attributes considered were yield, carbon content, and higher heating values. The rules generated were tested against a set of validation data and evaluated for their scientific coherency. Based on the decision rules generated, biomass with ash content of 11-14 wt%, volatile matter of 60-62 wt% and carbon content of 42-45.3 wt% can generate biochar with promising yield, carbon content and higher heating value via a pyrolysis process at an operating temperature of 425°C-475°C. This work provided the optimal biomass feedstock properties and pyrolysis conditions for biochar production with high mass and energy yield.

Towards Negative Emissions: Hydrothermal Carbonization of Biomass for Sustainable Carbon Materials

The contemporary production of carbon materials heavily relies on fossil fuels, contributing significantly to the greenhouse effect. Biomass is a carbon-neutral resource whose organic carbon is formed from atmospheric CO2. Employing biomass as a precursor for synthetic carbon materials can fix atmospheric CO2 into solid materials, achieving negative carbon emissions. Hydrothermal carbonization (HTC) presents an attractive method for converting biomass into carbon materials, by which biomass can be transformed into materials with favorable properties in a distinct hydrothermal environment, and these carbon materials have made extensive progress in many fields. However, the HTC of biomass is a complex and interdisciplinary problem, involving simultaneously the physical properties of the underlying biomass and sub/supercritical water, the chemical mechanisms of hydrothermal synthesis, diverse applications of resulting carbon materials, and the sustainability of the entire technological routes. This review starts with the analysis of biomass composition and distinctive characteristics of the hydrothermal environment. Then, the factors influencing the HTC of biomass, the reaction mechanism, and the properties of resulting carbon materials are discussed in depth, especially the different formation mechanisms of primary and secondary hydrochars. Furthermore, the application and sustainability of biomass-derived carbon materials are summarized, and some insights into future directions are provided.

Fe3O4/biochar modified with molecularly imprinted polymer as efficient persulfate activator for salicylic acid removal from wastewater: Performance and specific recognition mechanism

In this study, a novel Fe3O4-based biochar coupled surface-imprinted polymer was constructed via simple hydrothermal route for salicylic acid recognition and degradation in advanced oxidation processes. The material exhibited excellent adsorption capability, up to 118.23 mg g-1, and efficient degradation performance, 87.44% removal rate within 240 min, based on integrating the advantages of both huge specific surface area as well as abundant functional groups from biochars and specific recognition sites from imprinted cavities. Moreover, high selectivity coefficient (11.67) showed stable recognition in single and binary systems. SO4•- and •OH were confirmed as reactive oxygen species in catalytic reaction according to quenching experiments and EPR analysis. The degradation mechanism and pathway were unraveled by DFT calculations and LC-MS. Furthermore, the results of toxicity evaluation, stability and reusability demonstrated application potential in the field of water environment restoration. This study confirmed that molecular imprinting provided a promising solution to targeted removal of emerging environmental pollutants by degrading after the enrichment of pollutants to the composites surface.

Biochar enhanced the stability of toluene removal in extracted groundwater amended with nitrate under microaerobic conditions

Groundwater pollution caused by the leakage of petroleum and various fuel oils is becoming a serious environmental problem. In this study, carbon-based materials including biochar and hydrochar were applied to investigate the effects of additives on the toluene removal in the extracted groundwater under microaerobic condition with the addition of nitrate. Biochar and hydrochar could adsorb toluene, and thus enhance the toluene removal in the system. The toluene removal efficiency was 8.2-8.9 mg/(g·h) at the beginning, and then decreased with time in the control and the hydrochar treatment, while it remained the stable values in the biochar treatment, owing to the fact that biochar could reduce the NO3--N loss by partial denitrification. Moreover, biochar could prompt the growth of toluene-degrading bacteria including Thauera, Rhodococcus, Ideonella and Denitratisoma, which had the capability of denitrification. However, hydrochar could stimulated the growth of denitrifiers without toluene-degrading capacity including Candidatus Competibacter and Ferrovibrio, which might play a key role in the partial denitrification of the system. The findings are helpful for developing remediation techniques of contaminated groundwater.

Migration of phosphorus in pig manure during pyrolysis process and slow-release mechanism of biochar in hydroponic application

Pyrolysis is an effective method for treating of livestock and poultry manure developed in recent years. It can completely decompose pathogens and antibiotics, stabilize heavy metals, and enrich phosphorus (P) in biochar. To elucidate the P migration mechanism under different pig manure pyrolysis temperatures, sequential fractionation, solution 31P nuclear magnetic resonance, X-ray photoelectron spectroscopy, X-ray diffraction, and K-edge X-ray absorption near-edge structure techniques were used to analyze the P species in pig manure biochar (PMB). The results indicated that most of the organic P in the pig manure was converted to inorganic P during pyrolysis. Moreover, the transformation to different P groups pathways was clarified. The phase transition from amorphous to crystalline calcium phosphate was promoted when the temperature was above 600 °C. The content of P extracted by hydrochloric acid, which was the long-term available P for plant uptake, increased significantly. PMB pyrolyzed at 600 °C can be used as a highly effective substitute for P source. It provides the necessary P species (e.g. water-soluble P.) and metal elements for the growth of water spinach plants, and which are slow-release comparing with the Hogland nutrient solution.

Hydrothermal carbonization of coking sludge: Formation mechanism and fuel characteristic of hydrochar

In this study, the chemical structures, fuel characteristic, and formation mechanism of hydrochar during hydrothermal carbonization (HTC) at 150-270 °C for 0-120 min were investigated using coking sludge (CS) as the feedstock. The results showed that the yield decreased from 96.86 to 60.98%, whereas the carbonization rate increased from 6.74 to 93.41% at 270 °C. More stable structures with aromatic and N-heterocycles rings were formed through hydrolysis and polymerization. The H/C and O/C ratio decreased from 1.75 to 0.60 to 1.04 and 0.09, and the combustion stability index (Hf) decreased from 0.86 to 0.60 °C.103, and the flammability index (S) increased from 24.16 to 26.42 %/(min2 °C3) 10-8, indicating an improvement of fuel performance. A kinetic model to describe the conversion of organic components of CS was developed to elucidate the formation mechanism of hydrochar combined with the change of water-soluble intermediates (SM). The solid-solid conversion reaction of protein and humus components was the predominant hydrochar formation pathway, with an activation energy (Ea) of 26.06 kJ/mol. The polymerization of aromatic compounds slightly participated in the hydrochar formation, with an Ea of 86.12 kJ/mol. The water-soluble intermediates mostly transformed into inorganic substances (IS) through decarboxylation, deamination, or decomposition reaction, with an Ea of 5.73 kJ/mol. This study provided insights for understanding the formation of hydrochar from CS through HTC, which is vital for controlling the polymerization of intermediates and solid-solid conversion to enhance the carbonization efficiency.

Hydrothermal carbonization of food waste: Process parameters optimization and biomethane potential evaluation of process water

Food waste (FW) is one of the major biomasses produced in large quantities in urban areas, which contributes to more than one-third of global greenhouse gas emissions. FW must be properly managed to minimize its environmental consequences. Hydrothermal carbonization (HTC) of FW is a promising technology compared to conventional methods. The objective of the present study is to maximize the mass yield (MY), higher heating value (HHV) and energy yield (EY) of FW by optimizing the operational variables of HTC process. Additionally, process water generated during HTC of FW under optimal conditions was evaluated for methane yield using anaerobic digestion. To optimize the HTC process, three operational variables, including solid-to-liquid (S/L) ratio, temperature, and reaction time, were manipulated using response surface methodology (RSM). According to RSM studies, the optimum operating conditions are 198.5 °C for 150 min with a 0.2 S/L ratio, resulting in MY, HHV and EY as 62.5%, 21.24 MJ/kg and 81.71%, respectively. Proximate and elemental analysis for the hydrochars synthesized at various operating conditions reveals that the temperature and reaction time have a significant impact on fixed carbon and carbon percentage. The anaerobic digestion results showed that the combination of process water and hydrochar, yielded a maximum cumulative methane production of 298.5 ± 16.34 mL/g COD. To mimic methane production, the modified Gompertz model was utilized. Thus, this finding contributes towards the commercialization of the HTC process to produce solid fuel (hydrochar) and provides a way to find an alternative energy source that enhances the HTC process and tackles the problem of process water disposal.

Ba-modified peanut shell biochar (PSB): preparation and adsorption of Pb(II) from water

The impact of Ba-modified peanut shell biochar (Ba-PSB) on Pb(II) removal was studied and BaCl2 was used as a modifier. It was shown that the PSB obtained at 750 °C had the best adsorption effect, and the Ba-PSB had a larger specific surface area and a good adsorption effect on Pb(II). At pH = 5, concentration was 400 mg/L, time was 14 h, and temperature was 55 °C, the loading amount of black peanut shell biochar (BPSB), red peanut shell biochar (RPSB), Ba-BPSB, and Ba-RPSB reached 128.050, 98.217, 379.330, and 364.910 mg/g, respectively. In addition, based on the non-linear fitting, it was found that the quasi-second-order kinetic model, and isothermal model could be applied to describe Pb(II) adsorption on PSB and Ba-PSB. The adsorption behavior of PSB unmodified and modified was a spontaneous process. Moreover, chemical modification of BPSB, RPSB, Ba-BPSB, and Ba-RPSB for hindering of -COOH and -OH groups revealed 81.81, 77.08, 86.90, and 83.65% removal of Pb(II), respectively, which was due to the participation of -COOH, while 17.61, 21.70, 12.77, and 15.06% was from -OH group, respectively. The increase of cation strength (Na+, K+, Ca2+, and Mg2+) will reduce the adsorption capacity of PSB for Pb(II).

2-Methyl-4-chlorophenoxyacetic acid (MCPA) sorption and desorption as a function of biochar properties and pyrolysis temperature

2-Methyl-4-chlorophenoxyacetic acid (MCPA) is a highly mobile herbicide that is frequently detected in global potable water sources. One potential mitigation strategy is the sorption on biochar to limit harm to unidentified targets. However, irreversible sorption could restrict bioefficacy thereby compromising its usefulness as a vital crop herbicide. This research evaluated the effect of pyrolysis temperatures (350, 500 and 800°C) on three feedstocks; poultry manure, rice hulls and wood pellets, particularly to examine effects on the magnitude and reversibility of MCPA sorption. Sorption increased with pyrolysis temperature from 350 to 800°C. Sorption and desorption coefficients were strongly corelated with each other (R2 = 0.99; P < .05). Poultry manure and rice hulls pyrolyzed at 800°C exhibited irreversible sorption while for wood pellets at 800°C desorption was concentration dependent. At higher concentrations some desorption was observed (36% at 50 ppm) but was reduced at lower concentrations (1-3% at < 5 ppm). Desorption decreased with increasing pyrolysis temperature. Sorption data were analyzed with Langmuir, Freundlich, Dubinin-Radushkevich and Temkin isotherm models. Freundlich isotherms were better predictors of MCPA sorption (R2 ranging from 0.78 to 0.99). Poultry manure and rice hulls when pyrolyzed at higher temperatures (500 and 800°C) could be used for remediation efforts (such as spills or water filtration), due to the lack of desorption observed. On the other hand, un-pyrolyzed feedstocks or biochars created at 350°C could perform superior for direct field applications to limit indirect losses including runoff and leaching, since these materials also possess the ability to release MCPA subsequently to potentially allow herbicidal action.

Energy recovery from agro-forest wastes through hydrothermal carbonization coupled with hydrothermal Co-gasification: Effects of succinic acid on hydrochars and H2 production

Aiming to upgrade agro-forest wastes into value-added solid and gaseous fuels in the present investigation, hydrothermal carbonization (HTC) of spruce (SP), canola hull (CH), and canola meal (CM) was optimized in terms of operating conditions, maximizing the higher heating value of hydrochars. The optimal operating conditions were achieved at HTC temperature, reaction time, and solid-to-liquid ratio of 260 ^°C, 60 min, and 0.2 g mL^-1, respectively. At the optimum condition, succinic acid (0.05-0.1 M) was used as HTC reaction medium to investigate the effects of acidic medium on the fuel characteristics of hydrochars. The succinic acid assisted HTC was found to eliminate ash-forming minerals e.g., K, Mg, and Ca from hydrochar backbones. The calorific values, H/C and O/C atomic ratios of hydrochars were in the range of 27.6-29.8 MJ kg^-1, 0.8-1.1, and 0.1-0.2, respectively, indicating the biomass upgrading into coal-like solid fuels. Finally, hydrothermal gasification of hydrochars with their corresponding HTC aqueous phase (HTC-AP) was assessed. Gasification of CM resulted in a relatively high H₂ yield of 4.9-5.5 mol kg^-1 followed by that for SP with 4.0-4.6 mol H₂ per kg of hydrochars. Results suggest that hydrochars and HTC-AP have a great potential for H₂ production via hydrothermal co-gasification, while suggesting HTC-AP reuse.

Effects of physical aging processes on the bioavailability of heavy metals in contaminated site soil amended with chicken manure and wheat straw biochars

The physicochemical properties of biochars undergo slow changes in soils due to the natural aging processes, which influences their interaction with heavy metals. The effects of aging on immobilization of co-existing heavy metals in contaminated soils amended with fecal and plant biochars possessing contrasting properties remain unclear. This study investigated the effects of wet-dry and freeze-thaw aging on the bioavailability (extractable by 0.01 M CaCl₂) and chemical fractionation of Cd and Pb in a contaminated site soil amended with 2.5% (w/w) chicken manure (CM) biochar and wheat straw (WS) biochar. Compared to that in the unamended soil, the contents of bioavailable Cd and Pb in CM biochar-amended soil decreased by 18.0% and 30.8%, respectively, after 60 wet-dry cycles, and by 16.9% and 52.5%, respectively, after 60 freeze-thaw cycles. CM biochar, which contained significant levels of phosphates and carbonates, effectively reduced the bioavailability of Cd and Pb and transformed them from the labile chemical fractions to the more stable ones in the soil during the accelerated aging processes, mainly through precipitation and complexation. In contrast, WS biochar failed to immobilize Cd in the co-contaminated soil in both aging regimes, and was only effective at immobilizing Pb under freeze-thaw aging. The changes in the immobilization of co-existing Cd and Pb in the contaminated soil resulted from aging-induced increase in oxygenated functional groups on biochar surface, destruction of the biochar's porous structure, and release of dissolved organic carbon from the aged biochar and soil. These findings could help guide the selection of suitable biochars for simultaneous immobilization of multiple heavy metals in co-contaminated soil under changing environmental conditions (e.g., rainfall, and freezing and thawing of soils).

Catalytic modification of corn straw facilitates the remediation of Cd contaminated water and soil

Turning postharvest residue into high-value-added products is crucial for agricultural waste management and environmental remediation. In this proof-of-concept study, nanosized Pt/TiO₂ was used as a model catalyst to modify corn straw (CS) materials through a simple low-temperature oxidation process. This method was demonstrated to be self-sustainable, waste-free, and with high yields. At an optimal temperature of 220 °C, O₂ treatment with 1 wt% Pt/TiO₂ greatly changed ultra-micropore and mesopore structures, dissolved organic carbon, aromatic contents and surface oxygen (O)-containing functional groups in CS products. This treatment resulted in an approximately 5-fold increase of cadmium (Cd) adsorption from aqueous solution and immobilization rate of 43.1% at 7d for bioavailable Cd in soil. Spectroscopic and linear regression analysis demonstrated that both acidic and basic functional groups in CS contributed to Cd adsorption, suggesting chemical adsorption. According to the d-band theory, the unexpected role of catalysts in CS modification could be associated with dissociative adsorption of molecular O₂ on the Pt surface. These results provide insights for the development of economic and sustainable technologies to reutilize agricultural waste biomass for water and soil remediation.

A review on hydrothermal carbonization of potential biomass wastes, characterization and environmental applications of hydrochar, and biorefinery perspectives of the process

It is imperative to search for appropriate processes to convert wastes into energy, chemicals, and materials to establish a circular bio-economy toward sustainable development. Concerning waste biomass valorization, hydrothermal carbonization (HTC) is a promising route given its advantages over other thermochemical processes. From that perspective, this article reviewed the HTC of potential biomass wastes, the characterization and environmental utilization of hydrochar, and the biorefinery potential of this process. Crop and forestry residues and sewage sludge are two categories of biomass wastes (lignocellulosic and non-lignocellulosic, respectively) readily available for HTC or even co-hydrothermal carbonization (Co-HTC). The temperature, reaction time, and solid-to-liquid ratio utilized in HTC/Co-HTC of those biomass wastes were reported to range from 140 to 370 °C, 0.05 to 48 h, and 1/47 to 1/1, respectively, providing hydrochar yields of up to 94 % according to the process conditions. Hydrochar characterization by different techniques to determine its physicochemical properties is crucial to defining the best applications for this material. In the environmental field, hydrochar might be suitable for removing pollutants from aqueous systems, ameliorating soils, adsorbing atmospheric pollutants, working as an energy carrier, and performing carbon sequestration. But this material could also be employed in other areas (e.g., catalysis). Regarding the effluent from HTC/Co-HTC, this byproduct has the potential for serving as feedstock in other processes, such as anaerobic digestion and microalgae cultivation. These opportunities have aroused the industry interest in HTC since 2010, and the number of industrial-scale HTC plants and patent document applications has increased. The hydrochar patents are concentrated in China (77.6 %), the United States (10.6 %), the Republic of Korea (3.5 %), and Germany (3.5 %). Therefore, considering the possibilities of converting their product (hydrochar) and byproduct (effluent) into energy, chemicals, and materials, HTC or Co-HTC could work as the first step of a biorefinery. And this approach would completely agree with circular bioeconomy principles.

Comparison of cadmium adsorption by hydrochar and pyrochar derived from Napier grass

Biochar (e.g. pyrochar and hydrochar) is considered a promising adsorbent for Cd removal from aqueous solution. Considering the vastly different physicochemical properties between pyrochar and hydrochar, the Cd²⁺ sorption capacity and mechanisms of pyrochars and hydrochars should be comparatively determined to guide the production and application of biochar. In this study, the hydrochars and pyrochars were prepared from Napier grass by hydrothermal carbonization (200 and 240 °C) and pyrolysis (300 and 500 °C), respectively, and the physicochemical properties and Cd²⁺ sorption performances of biochars were systematically determined. The pyrochars had higher pH and ash content as well as better stability, while the hydrochars showed more oxygen-containing functional groups (OFGs) and greater energy density. The pseudo second order kinetic model best fitted the Cd²⁺ sorption kinetics data of biochars, and the isotherm data of pyrochar and hydrochar were well described by Langmuir and Freundlich models, respectively. In comparison with hydrochar, the pyrochar exhibited better Cd²⁺ sorption capacity (up to 71.47 mg/g). With increasing production temperature, the Cd²⁺ sorption capacity of pyrochar elevated, while the reduction was found for hydrochar. The mineral interaction, complexation with surface OFGs, and coordination with π electron were considered the main mechanisms of Cd²⁺ removal by biochars. The minerals interaction and the complexation with OFGs was the dominant mechanism of Cd²⁺ removal by pyrochars and hydrochars, respectively. Therefore, the preparation technique and temperature have significant impacts on the sorption capacity and mechanisms of biochar, and pyrochar has better potential for Cd²⁺ removal than the congenetic hydrochar.

An environmental assessment for municipal organic waste and sludge treated by hydrothermal carbonization

Climate change is the world's greatest challenge today, the reason why it is urgent to optimize industrial processes and find new renewable energy sources. Hydrothermal carbonization (HTC) is one of the Waste-to-Energy technologies with greater projections due to its operative advantages. However, for its large-scale implementation, there are challenges related to the variability of the composition of the waste biomass and the seasonal and geographical availability. This research applied the Life Cycle Analysis methodology to evaluate the environmental impacts caused by three biomasses blends as raw material in the HTC process at laboratory scale. The blends analyzed considered different organic fractions of municipal solid waste (food and pruning) and sewage sludge. The results showed that blend 1 had a lower environmental impact for the case of production in the experimental laboratory level, compared with blends 2 and 3. This is mainly due to its greater calorific value and mass yield, which allows obtaining more hydrochar compared with the other blends, increasing the energy efficiency of the process. Also, between 87.94% and 98.00% of the energy reduction is required to obtain neutral impacts regarding the energy requirements in the experimental laboratory level scenario and the Chilean energy matrix. The processing of blends in HTC has excellent potential in a context where municipal solid wastes have been disposed in sanitary landfills or dumps, as in most emerging countries. Since this study incorporated data from the literature, future studies should perform an elemental analysis to provide experimental and differentiated data.

Evaluation of Poultry Manure: Combination of Phosphorus Recovery and Activated Carbon Production

Intensive growth of poultry production leads to generation of a large-scale accumulation of wastes, which is a critical concern for poultry farming. An environmentally friendly and effective solution is still being sought for sustainable management of poultry manure. In this study, evaluation of poultry manure both as a carbon source for production of solid fuels and activated carbon and as a phosphorus source has been investigated. The study focuses on the following: (1) biochar and hydrochar production under different process conditions for production of carbon-rich fuel from poultry manure; (2) phosphorus recovery by acid leaching-alkali precipitation from manure ash, biochar, and hydrochar; and (3) activated carbon production from acid-leached hydrochar and biochar. The results reveal that production of biochar and hydrochar is not a promising method for upgrading laying hen manure into an energy-dense solid fuel. Phosphorus in ash and chars was recovered as amorphous calcium phosphate with yields of 57.3-48.5% by acid leaching-alkali precipitation. Untreated and acid-leached chars were subjected to a chemical activation process with KOH and ZnCl₂ to produce activated carbon. Due to the catalytic effect of inorganics in chars, the KOH activation resulted in a very low yield of activated carbon. The surface areas of activated carbons prepared using ZnCl₂ were comparable to activated carbons derived from typical biomass using ZnCl₂.

Biochar and hydrochar in the context of anaerobic digestion for a circular approach: An overview

Biochar and hydrochar are carbonaceous materials with valuable applications. They can be synthesized from a wide range of organic wastes, including digestate. Digestate is the byproduct of anaerobic digestion (AD), which is performed for bioenergy (biogas) production from organic residues. Through a thermochemical process, such as pyrolysis, gasification, and hydrothermal carbonization - HTC, digestate can be converted into biochar or hydrochar. The addition of either biochar or hydrochar in AD has been reported to improve biochemical reactions and microbial growth, increasing the buffer capacity, and facilitating direct interspecies electrons transfer (DIET), resulting in higher methane (CH₄) yields. Both biochar and hydrochar can adsorb undesired compounds present in biogas, such as carbon dioxide (CO₂), hydrogen sulfide (H₂S), ammonia (NH₃), and even siloxanes. However, an integrated understanding of biochar and hydrochar produced from digestate through their return to the AD process, as additives or as adsorbents for biogas purification, is yet to be attained to close the material flow loop in a circular economy model. Therefore, this overview aimed at addressing the integration of biochar and hydrochar production from digestate, their utilization as additives and effects on AD, and their potential to adsorb biogas contaminants. This integration is supported by life cycle assessment (LCA) studies, showing positive results when combining AD and the aforementioned thermochemical processes, although more LCA is still necessary. Techno-economic assessment (TEA) studies of the processes considered are also presented, and despite an expanding market of biochar and hydrochar, further TEA is required to verify the profitability of the proposed integration, given the specificities of each process design. Overall, the synthesis of biochar and hydrochar from digestate can contribute to improving the AD process, establishing a cyclic process that is in agreement with the circular economy concept.

The Effects of Rabbit Manure-Derived Biochar on Soil Health and Quality Attributes of Two Mine Tailings

Biochar amendment is becoming a promising technology for mining soil restoration. The addition of biochar can improve soil microbiological parameters related to soil quality, such as enzyme activities. The aim of the present research was to evaluate the effect of rabbit manure (RM) and two rabbit manure biochars prepared at two pyrolysis temperatures (300 and 600 °C) on the biochemical properties of two mining soils in the Portman area (Spain) in the presence or absence of vegetation. Soils were amended with the RM, the two biochars and a mixture of the rabbit manure and biochars (50/50 w/w) at a rate of 10% in a mesocosms experiment to study the changes in dehydrogenase, phosphomonoesterase, β-glucosidase activities, geometric mean of enzyme activities (GMea) and soil microbial biomass (SMB). Changes in individual enzyme activities were not always consistent. However, when using the GMea as a measure of soil quality, our results showed an increase in the GMea (217–360 times) after the addition of rabbit manure to mining soils, while this increase was from 81–270 times following the addition of rabbit manure with biochar prepared at 300 °C. Therefore, the use of biochar prepared at low temperatures could be a promising direction for the improvement of soil quality and soil carbon sequestration.

Review on upgrading organic waste to value-added carbon materials for energy and environmental applications

Value-added materials such as biochar and activated carbon that are produced using thermo-chemical conversion of organic waste have gained an emerging interest for the application in the fields of energy and environment because of their low cost and unique physico-chemical properties. Organic waste-derived materials have multifunctional abilities in the field of environment for capturing greenhouse gases and remediation of contaminated soil and water as well as in the field of energy storage and conversion. This review critically evaluates and discusses the current thermo-chemical approaches for upgrading organic waste to value-added carbon materials, performance enhancement of these materials via activation and/or surface modification, and recent research findings related to energy and environmental applications. Moreover, this review provides detailed guidelines for preparing high-performance organic waste-derived materials and insights for their potential applications. Key challenges associated with the sustainable management of organic waste for ecological and socio-economic benefits and potential solutions are also discussed.

Co-hydrothermal carbonization of swine manure and cellulose: Influence of mutual interaction of intermediates on properties of the products

Co-hydrothermal carbonization (HTC) of livestock manure and biomass might improve the fuel properties of the hydrochar due to the high reactivity of the biomass-derived intermediates with the abundant oxygen-containing functionalities. However, the complicated compositions make it difficult to explicit the specific roles of the individual components of biomass played in the co-HTC process. In this study, cellulose was used for co-HTC with swine manure to investigate the influence on the properties of the hydrochar. The yield of hydrochar obtained from co-HTC reduced gradually with the cellulose proportion increased, and the solid yield was lower than the theoretical value. This was because the cellulose-derived intermediates favored the stability of the fragments from hydrolysis of swine manure. The increased temperature resulted in the reduction of the hydrochar yield whereas the prolonged time enhanced the formation of solid product. The interaction of the co-HTC intermediates facilitated the formation of O-containing species, thus making the solid more oxygen- and hydrogen-rich with a higher volatility. In addition, the co-HTC affected the evolution of functionalities like -OH and CO during the thermal treatment of the hydrochar and altered its morphology by stuffing the pores from swine manure-derived solid with the microspheres from HTC of cellulose. The interaction of the varied intermediates also impacted the formation of amines, ketones, carboxylic acids, esters, aromatics and the polymeric products in distinct ways.

Synthesis of hydrochar supported zero-valent iron composites through hydrothermal carbonization of granatum and zero-valent iron: potential applications for Pb2+ removal

In the present investigation, a one-step synthesis of hydrochar (HC) supported zero-valent iron (ZVI) was performed through hydrothermal carbonization (HTC) of granatum and ZVI. According to XRD, XPS, and FTIR data, ZVI was evenly distributed on the surface of the hydrochar. In addition, the external ZVI oxide layer and the functional groups present in the hydrochar remained on the surface of the HC/ZVI composites after HTC treatment. The surface area of the HC/ZVI composites was between 31.11 and 44.16 m²/g. These numbers were higher than those obtained for hydrochar (20.36 m²/g) and ZVI (12.14 m²/g) separately. The Pb²⁺ adsorption capacity of hydrochar and ZVI was 28.64 and 192.44 mg/g, respectively (25 °C, pH = 6.05, Pb²⁺ concentration of 200 mg/L with 0.05 g HC and 0.01 g ZVI). In addition, the adsorption capacity of the composites was between 49.63 and 88.09 mg/g. The data obtained for Pb²⁺ removal by the samples used in this experiments fitted well the pseudo-second-order kinetics and Langmuir isotherm models. Therefore, hydrochar may represent a promising supporting material for the synthesis of ZVI composites.

The inhibiting effects of biochar-derived organic materials on rice production

The effects of PBC and HBC on rice production, NUE and corresponding mechanisms were examined. Six treatments, P05, P30, H05, H30 (P: PBC; H: HBC; 05 and 30 represented the application rate of 0.5 and 3.0% w/w), CKU (urea application without char) and CK (no application of char and urea), were set up. Results showed that P05, P30 and H05 increased grain yield by 1.8-7.3% (P > 0.05), whereas H30 reduced grain yield by 60.4% (P < 0.05), compared to CKU. Meanwhile, HI under P05, P30 and H05 increased by 3.4-3.6%, while H30 decreased by 9.1% (P < 0.05). NUE and NAE showed similar trends with rice yield. By investigation, the excessive introduction of BDOM plays a crucial role in the reduction of rice production and NUE under higher HBC application. GC-MS/MS analysis showed that the soluble BDOM of HBC and PBC was quite different, and compounds such as 2,6-dimethoxyphenol might stress rice growth. ESI-FT-ICR-MS analysis showed that the BDOM of HBC contained a certain quantity of aromatic compounds, which may also stress rice growth. Overall, HBC pretreatment should be conducted, and the application rate should be strictly controlled before its agricultural application.

Improving nutrients removal and energy recovery from wastes using hydrochar

Hydrothermal carbonization (HTC) is an eco-friendly, flexible and efficient way to valorise wet solid wastes, producing a carbon-rich material named as hydrochar. Considerable efforts have been devoted to studying the feasibility of using hydrochar in waste management to achieve the goal of circular economy. However, a comprehensive evaluation of the impacts of hydrochar on energy recovery from anaerobic digestion (AD), nutrient reclamation, and wastewater treatment is currently lacking. To understand the influence of hydrochar type on its application, this review will firstly introduce the mechanisms and biomass treatment for hydrochar preparation. Most recent studies regarding the improvement of methane (CH₄) and volatile fatty acids (VFAs) production after dosing hydrochar in anaerobic digesters are quantitatively summarized and deeply discussed. The potential of using various hydrochar as slow-fertilizer to support the growth of plants are analysed by providing quantitative data. The usage of hydrochar in remediating pollutants from wastewater as effective adsorbent is also evaluated. Based on the review, we also address the challenges and demonstrate the opportunities for the future application of hydrochar in waste management. Conclusively, this review will not only provide a systematic understanding of the up-to-date developments of improving the nutrients removal and energy recovery from wastes by using hydrochar but also several new directions for the application of hydrochar in the future.

P-enriched hydrochar for soil remediation: Synthesis, characterization, and lead stabilization

One-step synthesis of multifunctional materials using biomass waste for environmental remediation is a current research hotspot. In this study, a novel P-enriched hydrochar was obtained by co-hydrothermal treatment of biomass (bamboo or hickory) with concentrated H₃PO₄ (biomass: H₃PO₄ = 1:4) at 200 °C for 7 h. The characteristics of the P-enriched hydrochar were determined and its effect on the stabilization of Pb in soils was investigated. Compared to pristine hydrochar, the weight yield of the P-enriched hydrochar was greater (by over 2 times). This was due to the enrichment of P (over 20% by weight), as the C, N, and H weight content was reduced. Moreover, the aromaticity, thermal stability, and surface functionality of P-enriched hydrochar were all higher than that of pristine hydrochar. Addition of the pristine hydrochar to a simulated 1300 mg·kg^-1 Pb-contaminated soil at 3% (w/w) resulted in a 20%-40% reduction in leached Pb only after 4 weeks, compared to the control without hydrochar amendment. However, addition of the P-enriched hydrochar to the spiked Pb-contaminated soil reduced Pb leaching by about 60% after only 1 week and about 90% after 3 weeks. Besides, using a real Pb-contaminated soil (149,000 mg·kg^-1 Pb), P-enriched hydrochar addition at 5% (w/w) resulted in a 100% decrease in Pb leaching in the first week and maintained leached Pb levels at <2 mg L^-1, meeting U.S.-E.P.A. standards. Thus, P-enriched hydrochar stabilized Pb in both simulated and real Pb-contaminated soil quickly and efficiently. Hence, the potential of one-step co-hydrothermal carbonization of biomass with H₃PO₄ to produce a novel and sustainable P-enriched hydrochar with properties suitable for environmental remediation of cationic metals.

Speciation of Main Nutrients (N/P/K) in Hydrochars Produced from the Hydrothermal Carbonization of Swine Manure under Different Reaction Temperatures

Hydrothermal carbonization (HTC) has been proved to be a promising technology for swine manure (SM) treatment. Currently, there is a lack of systematic understanding of the transformation characteristics of nutrient speciation in the HTC of SM. In this study, the speciation of the main nutrients (N/P/K) in SM-derived hydrochar produced at different reaction temperatures (200-280 °C) was investigated. The recovery of P (61.0-67.1%) in hydrochars was significantly higher than that of N (23.0-39.8%) and K (25.5-30.0%), and the increase in reaction temperature promoted the recovery of P and reduced the recovery of N. After the HTC treatment, the percentage of soluble/available P was reduced from 61.6% in raw SM to 4.0-23.9% in hydrochars, while that of moderately labile/slow-release P was improved from 29.2% in raw SM feedstock to 65.5-82.7%. An obvious reduction was also found in the amounts of available N (from 51.3% in raw SM feedstock to 33.0-40.5% in hydrochars). The percentages of slow-release N and residual N in hydrochars produced at 240 °C reached the maximum and minimum values (46.4% and 18.9%), respectively. A total of 49.5-58.3% of K retained in hydrochars was residual (invalid) potassium. From the perspective of the mobility and availability of N, P and K only, it was suggested that the HTC of SM should be carried out at 220-240 °C. Compared with the original SM, it is safer and more effective to use the SM-derived hydrochar as an organic fertilizer.

One-Pot Synthesis of Nano CuO-ZnO Modified Hydrochar Derived from Chitosan and Starch for the H2S Conversion

A novel kind of hydrochar adsorbent, modified by CuO-ZnO and derived from chitosan or starch, was synthesized for H2S adsorption. The prepared adsorbent was characterized by BET, XRD, EDX, SEM, and XPS. The results showed that the modified hydrochar contained many amino groups as functional groups, and the nanometer metal oxide particles had good dispersion on the surface of the hydrochar. The maximum sulfur capacity reached 28.06 mg/g-adsorbent under the optimized conditions. The amine group significantly reduced the activation energy between H2S and CuO-ZnO conducive to the rapid diffusion of H2S among the lattices. Simultaneously, cationic polyacrylamide as a steric stabilizer could change the formation process of CuO and ZnO nanoparticles, which made the particle size smaller, enabling them to react with H2S sufficiently easily. This modified hydrochar derived from both chitosan and starch could be a promising adsorbent for H2S removal.

Integrated thermochemical conversion process for valorizing mixed agricultural and dairy waste to nutrient-enriched biochars and biofuels

Hydrothermal carbonization (HTC) and pyrolysis are two promising thermochemical conversion strategies to valorize agricultural wastes, yet neither process can be implemented alone to sustainably upgrade both wet and dry feedstocks. HTC is ideal for wet feedstocks, such as manure, but its solid hydrochars suffer from low surface area and stability. Pyrolysis is well suited to dry agricultural residues, but pyrolysis biochars have low nutrient contents and bio-oils are often highly oxygenated. We propose an integrated process that co-pyrolyzes a nutrient-rich cow manure hydrochar with raw agricultural residues, which effectively reduces the environmental impact of these wastes while producing value-added bioproducts. Biochars produced from the proposed process are more suitable for soil amendments due to their enhancement in bioavailable nutrients and surface area than the manure hydrochars and raw biomass. Co-pyrolysis of blends enriched with cow manure yield oils higher in alkanes and alkenes with fewer oxygenated compounds.

Influence of process parameters on hydrothermal modification of soybean residue: Insight into the nutrient, solid biofuel, and thermal properties of hydrochars

Soybean (SB) solid residue after oil extraction was investigated in a hydrothermal modification process to provide an eco-friendly solution to SB solid waste disposal for an actual environmental management effort. SB hydrochars (HCs) were derived either by conventional heating hydrothermal treatment (HTT) under intense conditions (200, 250, and 300 °C for 2 h) or by microwave-assisted hydrothermal treatment (MHTT) under mild conditions (160, 190, and 220 °C for 1 h). Physicochemical properties of SB HCs and the transformation of nitrogen (N) and phosphorus (P) functionalities during HTT and MHTT were characterized using several tools. Ultimate and XPS analyses elucidated N transformation, e.g., 5.51 wt % N of raw SB residue decreased to 3.48 and 3.51 wt % after HTT and MHTT, respectively. The P bioavailability of raw SB (3.46 mg/g) was improved after HTT (26.7 mg/g) and MHTT (10.9 mg/g), depicting the practical application of HCs for soil amendment. Atomic H/C and O/C ratios of SB HCs decreased as treatment temperature increased. HCs showed credible higher heating value (HHV; 22.3-25.5 MJ/kg for HTT and 20.5-22.1 MJ/kg for MHTT), higher than various low-rank coals. Besides, energy densification and fuel ratio improved in intense conditions. The thermogravimetric analysis showed HCs possessed better thermal stability. The improved performance of SB HCs indicated that HTT and MHTT provided a green environmental route of SB waste management, valorization, and utilization.

Adsorption of sulfonamides on biochars derived from waste residues and its mechanism

Waste residues have been prepared as biochar (BC) adsorbents to remove sulfonamides (SAs) at low cost, but the mechanisms of the differences in the SA adsorption performance of different BCs are not clear. Thus, the adsorption characteristics of two SAs (sulfadiazine and sulfathiazole) on three BCs derived from waste residues (sewage sludge (SB), pig manure (PB), and rice straw (RB)) were investigated. The results showed that the adsorption mechanism was chemisorption and RB was the preferred BC under the different tested conditions (pH, Ca²⁺, and humic acid), followed by PB and SB. To interpret the phenomena, FTIR, XRD, and XPS analyses were performed and results indicated that SB had the lowest C content, and there was a very significant difference in the concentrations of the two O functional groups (C˭O and C‒O) for PB and RB (P < 0.01). Density functional theory calculations revealed that the mechanisms of SA adsorption onto BCs were mainly through π-π electron donor acceptor interactions and H bonds. There was no significant difference in the π interactions between the SAs-BC containing C‒O (BC(OH)) and the SAs-BC containing C˭O (BC(C˭O)), whereas the H bond strength of SAs-BC(OH) was much stronger than that of SAs-BC(C˭O).

Hydrochar obtained with by-products from the sugarcane industry: Molecular features and effects of extracts on maize seed germination

Sugarcane bagasse, vinasse and a mixture of sugarcane bagasse and vinasse were hydrothermally carbonized (HTC), with and without the addition of phosphoric acid, in order to propose new applications of sucroenergetic industry by-products on soil. Detailed information on the composition and properties of hydrochars has been obtained through elemental composition, thermogravimetric analysis, nuclear magnetic resonance and, thermochemolysis GC-MS. The soluble acidic fraction from the hydrochar samples were applied to maize seeds to evaluate the agronomic potential as biostimulants and relate the molecular features with maize seed germination. The HTC treatment converted polysaccharide-based biomasses into hydrochars with hydrophobic characteristics (C-Aryl and C-Akyl). Furthermore, the addition of phosphoric acid further increased the overall hydrophobicity and shifted the thermal degradation of the hydrochars to higher temperatures. Biomass influenced the hydrochars that formed, in which the molecular features of sugarcane bagasse determined the formation of more polar hydrochar, due to the preservation of lignin and phenolic components. Meanwhile, the HTC of vinasse resulted in a more hydrophobic product with an enrichment of condensed and recalcitrant organic fractions. The germination assay showed that polar structures of bagasse may play a role in improving the maize seeds germination rate (increase of ~11%), while the hydrophobic domains showed negative effects. The responses obtained in germination seems to be related to the molecular characteristics that organic extracts can present in solution.

A state-of-the-art review of biowaste biorefinery

Biorefineries provide a platform for different industries to produce multiple bio-products enhancing the economic value of the system. The production of these biorefineries has led to an increase in the generation of biowaste. To minimize the risk of environmental pollution, numerous studies have focused on a variety of strategies to mitigate these concerns reflected in the vast amount of literature written on this topic. This paper aims to systematically analyze and review the enormous body of scientific literature in the biowaste and biorefinery field for establishing an understanding and providing a direction for future works. A bibliometric analysis is first performed using the CorTexT Manager platform on a corpus of 1488 articles written on the topic of biowaste. Popular and emerging topics are determined using a terms extraction algorithm. A contingency matrix is then created to study the correlation of scientific journals and key topics from this field. Then, the connection and evolution of these terms were analyzed using network mapping, to determine relationships among key terms and analyze notable trends in this research field. Finally, a critical review of articles was presented across three main categories of biowaste management such as mitigation, sustainable utilization, and cleaner disposal from the perspective of the biorefinery concept. Operational and technological challenges are identified for the integration of anaerobic digestion in biorefineries, especially in developing nations. Moreover, logistical challenges in the biorefinery supply-chain are established based on the economics and collection aspect of handling biowaste.

Removal of aqueous Cr(VI) by Zn- and Al-modified hydrochar

Pristine hydrochar is a carbonaceous material that can sorb hexavalent chromium (Cr(VI)), a kind of toxic pollutants and difficult to removal, from aqueous solution but its capacity is limited. With the goal of improving this ability, two modified hydrochars were produced by co-hydrothermal carbonization (200 °C, 7h) of bamboo sawdust with zinc chloride (ZnCl₂) or aluminum chloride (AlCl₃). Compared to the pristine hydrochar, the ZnCl₂-and AlCl₃-modified hydrochars were more fully carbonized (higher C content and lower H/C) and had higher surface area (increased by 26 and 4.3 times, respectively) and larger pore volume (increased by 43 and 5.5 times, respectively). Due to these improved properties, the Cr(VI) maximum adsorption capacity (modeled via Langmuir isotherms) of ZnCl₂-and AlCl₃-modified hydrochar increased by 3.4 and 2.8 times, respectively. In addition, Cr(VI) adsorption kinetic of modified hydrochar was well fitted by the pseudo-second-order model. Cr sorption capacity increased at low pH and ion strengths, suggesting the potential roles of electrostatic interaction and ion exchange mechanisms. These results indicate that hydrochars modified by ZnCl₂ and AlCl₃ treatment are promising in environmental applications that require Cr(VI) removal.

Remediation of mining soils by combining Brassica napus growth and amendment with chars from manure waste

Mining activities lead to important physical, chemical and biological effects on soil properties, generating severe impacts in the establishment and maintenance of vegetation. Assisted phytoremediation can be considered an environmentally friendly approach for soil remediation. In this study, two mining soils (PORT and GAM) were treated with 10%, by mass, of the following amendments: manure biochars prepared at 450 °C (BMW450) and 600 °C (BMW600), hydrochars prepared by hydrothermal carbonization (HTC) of manure at 190 °C (HWM190) and 240 °C (HMW240) and manure waste (MW). Brassica napus was used as a phytoextraction species. After 45 days of plant growth, soil samples were widely characterized, including microbial biomass carbon, enzymatic activity and metal content. In addition, plant biomass production, bioconcentration factor, translocation factor and metal uptake were determined. Experimental results showed that addition of biochars improved the As uptake by Brassica napus in both soils but just in the roots increasing bioconcentration factor between 22.1 and 39.5% for GAM soil and between 28.6 and 53.4% for PORT soil. Brassica napus cannot be considered as Zn accumulator in GAM soil samples and in the case of PORT samples, only the addition of BMW600 and HMW240 enhanced the phytoextraction process of Zn on the roots. Soil enzyme activity improved in hydrochar amended soils.

Effects of modification and magnetization of rice straw derived biochar on adsorption of tetracycline from water

Rice straw derived biochar shows low-cost superiority as a potential adsorbent in tetracycline (TC) removal, but limited by its poor adsorption capacity and N, P leaking risk. Herein, an alkali-acid combined and magnetization method was proposed for its modification. The sorption kinetic and isotherm data showed modification enhanced the performance for tetracycline removal with adsorption capacity up to 98.33 mg·g^-1. The strong adsorption mechanisms were dominated by hydrogen bonding and pore-filling effect due to the increase of specific surface area and pore volume. Furthermore, the effect of pH was insignificant over a pH range from 3 to 10. The strong competition between ionic and TC was identified, where Ca²⁺ and PO₄^3- markedly inhibited the sorption. The enhanced TC adsorption, strong N and P removal, easy magnetic recovery, and good reusability in water samples entrusted it with good potential for wastewater treatment and rice straw resource disposal.

The comparison of dissolved organic matter in hydrochars and biochars from pig manure

Dissolved organic matter (DOM) has an important effect on soil fertility, activity of microorganisms and transport of contaminants. In this study, DOM released by the hydrochar and biochar prepared under various conditions from pig manure, was assessed using a combination of UV-Visible spectroscopy, fluorescence excitation-emission (EEM) spectrophotometry and ¹H-nuclear magnetic resonance (¹H NMR). The dissolved organic carbon (DOC) extracted from the hydrochar and biochar ranged from 3.34-11.96% and 0.38-0.48%, respectively, and the highest DOM was released by HCK0.5 (180 °C and 0.5% KOH). The aliphatic compounds were most common in DOM which mainly included three humic acid-like and one protein-like substance. The hydrochar-DOM had a larger molecular weight and lower aromaticity than biochar-DOM, but the effect of temperature on the DOM characteristics of hydrochar and biochar was opposite. The acidic treatment increased the content of functional groups containing oxygen and nitrogen in hydrochar-DOM, and alkaline treatment increased the content of aliphatic compounds. The results obtained are beneficial to select carbonation process and guide the rational application of hydrochar and biochar from pig manure in soil remediation field.

A critical review of different factors governing the fate of pesticides in soil under biochar application

Pesticides are extensively used in the modern agricultural system. The inefficient and extensive use of pesticides during the last 5 to 6 decades inadvertently led to serious deterioration of environmental quality with health risk to living organisms, including humans. It is important to use some environmentally-friendly and sustainable approaches to remediate, restore and maintain soil quality. Biochar has gained considerable attention globally as a promising soil amendment because it has the ability to adsorb and as such minimize the bioavailability of pesticides in soils. This review emphasizes the recent trends and implications of biochar in pesticide-contaminated soils, as well as highlights need of the pesticides use and associated environmental issues in context of the biochar application. The overarching aim of this review is to signify the role of biochar on primary processes such as effect of biochar on the persistence, mineralization, leaching and efficacy of pesticides in soil. Notably, the effects of biochar on pesticide adsorption-desorption, degradation and bioavailability under various operating/production conditions are critically discussed. This review delineates the indirect impact of biochar on pesticides persistence in soils and proposes key recommendations for future research which are essential for the remediation and restoration of pesticides-impacted soils.

Potential of siltstone and its composites with biochar and magnetite nanoparticles for the removal of cadmium from contaminated aqueous solutions: Batch and column scale studies

The present study is the first attempt to evaluate the pilot and batch scale adsorption potential of siltstone (SS) and its nanocomposites with biochar (EDB/SS), magnetite nanoparticles (MNPs/SS) and MNPs/EDB/SS for Cd removal from contaminated water. The SS, EDB/SS, MNPs/SS and MNPs/EDB/SS were characterized with FTIR, XRD, BET, SEM, TEM, TGA and point of zero charge (PZC). The effects of adsorbent dosage, contact time, initial Cd concentration, pH and presence of competing ions were evaluated on the Cd removal and its adsorption. The order for Cd removal was: MNPs/EDB/SS > MNPs/SS > EDB/SS > SS (95.86-99.72% > 93.10-98.5% > 89.66.98-98.40% > 74.90-90%). Column scale experiments yielded maximum retention (95%) of Cd even after 2 h of injection at 100 mg Cd/L. The exhausted SS, EDB/SS, MNPs/SS and MNPs/EDB/SS were reused without losing significant adsorption potential. Similarly, maximum Cd adsorption (117.38 mg/g) was obtained with MNPs/EDB/SS at dose 1.0 g/L. The results revealed that coexisting cations reduced the Cd removal due to competition with Cd ions. The experimental results were better explained with Freundlich isotherm model and pseudo 2nd order kinetic models. The results revealed that SS and its composites can be used efficiently for the removal of Cd from contaminated water.

Improved lead removal from aqueous solution using novel porous bentonite - and calcite-biochar composite

Biochar-mineral (bentonite/calcite) composite (BC-CM) prepared at different temperatures were tested under varied conditions for effective removal of lead (Pb) from aqueous solution. With increasing pyrolysis temperature, increased surface area, pore volume, bentonite decomposition and less or no decomposition of calcite occurred. Bentonite-biochar (BCS) and calcite-biochar (CCS) prepared at 700 °C were found most suitable for efficient removal of Pb (99.9%). Bentonite and calcite acted as catalyst and contributed to changes in yield, pH, texture, functional groups, minerals and carbonization that facilitated efficient Pb removal by BCS 700 and CCS 700. Pb concentration, pH, dose of BCS and CCS, and contact time were further optimized using response surface methodology (RSM) for maximizing removal percentage (R%) of Pb and adsorption capacity (qt). Both BCS 700 and CCS 700 showed similar effects (positive/negative) of factors on R% and qt. Under optimized conditions, 0.21 g of BCS 700 effectively removed 99.2% of 431 mg/L in 3.6 h at solution pH of 4.2, while 0.07 g CCS 700 removed 97.06% of 232 mg/L in 3.5 h at 5.5 pH. Removal of Pb onto both BCS and CCS was by monolayer adsorption with maximum adsorption capacity of 500 mg/g. Rapid Pb removal was observed within 2 h of contact time (CCS 700 > BCS 700) and equilibrium was achieved within 10 h. BCS 700 followed first order and CCS 700 followed second order kinetic model. Electrostatic attraction between Pb ions and mineral groups present in BCS 700 and CCS 700 also played important role in Pb removal. This study clearly demonstrated that composite of biochar with bentonite or calcite under optimized conditions significantly improved Pb removal and adsorption capacity that can be further utilized for larger scale applications.

Zirconia-Assisted Pyrolysis of Coffee Waste in CO2 Environment for the Simultaneous Production of Fuel Gas and Composite Adsorbent

This work newly employed monoclinic zirconia (ZrO₂) as a promoter to improve CO₂ pyrolysis of coffee waste (CW). The CO₂ pyrolysis of CW presented the high level of CO production (14.3 mol%) during two stages of non-isothermal (280 to 700 °C) and isothermal pyrolysis (kept at 700 °C). At the same condition, the incorporation of ZrO₂ improved the CO generation up to about twice that of CW (29.5 mol%) by possibly inducing more conversion of pyrolytic oil into gas. The characterization results exhibited that ZrO₂-impregnated biochar (ZrB) possessed the distinctive surface morphology that highly graphitic- and porous carbon layers were covered by ZrO₂ nanoparticle clusters. In a series of adsorption experiments, ZrB composite showed pH-dependent As(V) adsorption and pH neutralization ability. The adsorption proceeded relatively rapid with 95% removal during 120 min in the early stage, followed by 5% removal in the remaining 240 min. The maximum adsorption capacity was found to be 25.2 mg g^-1 at final pH 8. The reusability and stability of ZrB were demonstrated in the 6 consecutive cycles of adsorption/desorption. As a result, ZrO₂-assisted CO₂ pyrolysis can potentially produce fuel gas with high CO fraction and composite adsorbent suitable for As(V) removal in acidic wastewater.

Activation or sequestration of heavy metals during hydrothermal process of swine manure: Interactions among metal species and particulates

Whether the heavy metals in solid biomass is activated or sequestrated during hydrothermal process (HTP) is still debated. Herein, the speciation of light and heavy metals during HTP of swine manure (SM) was investigated to reveal the interactions among these metal species and specific particulates. With increasing temperature, most of exchangeable species and that bound to carbonates were released to liquid phase via ion exchange and acid dissolution. Dissociation of Fe-Mn oxides rarely happened in spite of anoxic atmosphere formed during HTP. Substantial decomposition of lignocelluloses hardly caused significant liberation of fraction bound to organics. Instead, a part of fraction in liquid phase was re-captured by new oxygen-containing functional groups on solid product surface to form fraction bound to organics. Donpeacorite, butschliite and iwakiite were formed as primary minerals, resulting in increase of residual fraction of all metals except for K and Mg at 250 °C. In summary, Cu, Zn and Pb species evolution was affected by speciation of K, Ca, Mg, Fe and Mn significantly. Cu, Zn, Pb, Fe, Mn and Ca were sequestrated whereas K and Mg were activated with enhancing temperature during HTP in terms of their mobility factors.

Changes of nutrients and potentially toxic elements during hydrothermal carbonization of pig manure

The effects of reaction temperature, residence time, sulfuric acid and potassium hydroxide on the total concentration and speciation of N and P, potentially toxic elements (salts and metal elements) of pig manure during its hydrothermal carbonization (HTC) were investigated. Concentrations of Cl, K, Na and Mg in the hydrochars were much lower but total N, P and nitrate-nitrogen (NO₃^--N) contents were significantly higher than in untreated pig manure. The acid-extractable fractions of Cu and Zn in hydrochars were 0.03-0.63 and 0.17-0.66 times lower than those in pig manure and decreased significantly with increasing reaction temperature. The addition of sulfuric acid (H2SO4) or potassium hydroxide (KOH) in HTC reduced the contents of P, Ca, Mg, Cl and heavy metal elements (HMEs) in hydrochars, and the removal rates of Cu and Zn were up to 55% and 59%, respectively. Overall, the rapid treatment of pig manure by HTC reduced the harm of salts and HMEs, and effectively recovered the nutrients in pig manure. The HTC under alkaline conditions was desirable for optimizing the main elemental composition of the hydrochars.

Hydrothermal carbonization of different wetland biomass wastes: Phosphorus reclamation and hydrochar production

Hydrothermal carbonization (HTC) has drawn increasing interest for the disposal of solid wastes with a high moisture content, while minimal attention has been paid to HTC treatment of wetland plants and the corresponding phosphorus (P) transformation. In order to evaluate its feasibility for wetland plants treatment, hydrochars from different wetland plants were produced at different temperatures (200 °C, 220 °C, 240 °C, and 260 °C) and characterized, and the transformation of P was investigated. In comparison with wetland plant derived biochars, the derived hydrochars had a moderate pH (5.0-7.7), more oxygen-containing groups, and higher energy density (18.0-27.1 MJ kg^-1). These properties were affected by hydrothermal temperature and feedstock choice. In contrast to high water-soluble P in biomass (71.0-73.2% of total P), more recalcitrant P species formed in hydrochars, implying that HTC treatment could achieve P immobilization and reduce P leaching loss. Nuclear magnetic resonance (NMR) results indicated that monoester-P and soluble orthophosphate were transformed to insoluble orthophosphate during the HTC treatment. Therefore, HTC is a promising treatment technique for wetland plants to produce valuable char with P reclamation.

Environment-enhancing process for algal wastewater treatment, heavy metal control and hydrothermal biofuel production: A critical review

Coupling algae growth on wastewater with hydrothermal liquefaction (HTL) is regarded as an environment-enhancing pathway for wastewater management, biomass amplification, sustainable energy generation and value-added products generation. Through this integrated pathway, microalgae can not only recover nitrogen and phosphorus, but also absorb heavy metals from the wastewater. The migration and transformation of heavy metals need to be specifically assessed and considered due to the environmental concerns associated with metal toxicity. This work reviewed recent advances with respect to bioremediation mechanisms. Particular emphasis was placed on the heavy metal migration, transformation, and the key factors involved in algal wastewater treatment and biomass conversion. Additionally, the challenges of coupling algae wastewater treatment, hydrothermal conversion, and heavy metal control were addressed. Finally, a paradigm involving enhanced algal wastewater treatment and bioenergy production for field application was proposed.

Comparison of hydrochar- and pyrochar-based solid acid catalysts from cornstalk: Physiochemical properties, catalytic activity and deactivation behavior

Biochar made from biowaste provides renewable carbon precursors for catalysts preparation. Here, solid acid catalysts were prepared through functionalizing biochars produced via hydrothermal and pyrolytic carbonization of cornstalk with -SO₃H groups. Hydrochar-based catalysts (HAC) and pyrochar-based catalysts (PAC) exhibited significantly different physiochemical properties, catalytic activities and deactivation behaviors. The test of catalytic effects on cellulose degradation uncovered that HAC had a higher density of -SO₃H but lower surface special area than PAC. Specifically, PAC prepared at 400 °C resulted in the maximum increase of cellulose conversion by 16.00-50.50%. In comparison, the highest yields of glucose (11.14%) and 5-hydroxymethylfurfural (29.54%) were achieved catalyzed via HAC prepared at 240 °C. The results of catalyst deactivation behavior further revealed that used catalysts had an obvious reduction of -SO₃H density. Interestingly, used HAC-240 catalysts showed similar patterns of weight loss to fresh ones due to its high stability.