Pretreatment, modification and applications of sewage sludge-derived biochar for resource recovery- A review

A critical review on emerging industrial applications of chars from thermal treatment of biosolids

Thermochemical treatment is rapidly emerging as an alternative method for the management of stabilised sewage sludges (biosolids) to effectively reduce waste volume, degrade contaminants, and generate valuable products, particularly biochar and hydrochar. Biosolids-derived char has a relatively high concentration of heavy metals compared with agricultural chars but is still applied to land due to its beneficial properties and ability to retain metals. However, non-agricultural applications can provide additional economic and environmental benefits, promote sustainability and support a circular economy. This review identifies extensive non-agricultural opportunity for biosolids biochar, including adsorption, catalysis, energy storage systems, biological process enhancement, and as additives for rubber compounding and construction. Biosolids chars have received limited attention vs agricultural char, and we draw on both areas of literature, as well as evaluating differences between agricultural and biosolids chars. A key opportunity for biosolids biochar in comparison with other materials and agricultural chars is its sustainable and low-cost nature, relatively high metals content, improving catalyst properties, and ability to modify in various stages to tune it to specific applications. The specific opportunities for hydrochar have only received limited attention. Research needs to include better understanding of the benefits and limitations for specific applications, as well as adjacent drivers, including society, regulation, and market and economics.

Advances in sustainable nano-biochar: precursors, synthesis methods and applications

Nano-biochar, characterized by its environmentally friendly nature and unique nanostructure, offers a promising avenue for sustainable carbon materials. With its small particle size, large specific surface area, abundant functional groups and tunable pore structure, nano-biochar stands out due to its distinct physical and chemical properties compared to conventional biochar. This paper aims to provide an in-depth exploration of nano-biochar, covering its sources, transformation mechanisms, properties, applications, and areas requiring further research. The discussion begins with an overview of biomass sources for nano-biochar production and the conversion processes involved. Subsequently, primary synthesis methods and strategies for functionalization enhancement are examined. Furthermore, the applications of nano-biochar in catalysis, energy storage, and pollutant adsorption and degradation are explored and enhanced in various fields.

Biochar derived from alkali-treated sludge residue regulates anaerobic digestion: Enhancement performance and potential mechanisms

Biochar produced from bio-wastes has been widely used to promote the performance of anaerobic digestion. Waste activated sludge (WAS) is considered as a kind of popular precursor for biochar preparation, but the abundant resources in WAS were neglected previously. In this study, the roles of biochar prepared from raw, pretreated, and fermented sludge on anaerobic digestion were investigated. That is, parts of carbon sources and nutrients like polysaccharides, proteins, and phosphorus were firstly recovered after sludge pretreatment or fermentation, and then the sludge residuals were used as raw material to prepare biochar. The methane yield improved by 22.1% with adding the biochar (AK-BC) prepared by sludge residual obtained from alkaline pretreatment. Mechanism study suggested that the characteristics of AK-BC like specific surface area and defect levels were updated. Then, the conversion performance of intermediate metabolites and electro-activities of extracellular polymeric substances were up-regulated. As a result, the activity of electron transfer was increased with the presence of AK-BC, with increase ratio of 21.4%. In addition, the electroactive microorganisms like Anaerolineaceae and Methanosaeta were enriched with the presence of AK-BC, and the potential direct interspecies electron transfer was possibly established. Moreover, both aceticlastic and CO2-reducing methanogenesis pathways were improved by up-regulating related enzymes. Therefore, the proposed strategy can not only obtain preferred biochar but also recover abundant resources like carbon source, nutrients, and bioenergy.

Innovative treatment of industrial effluents through combining ferric iron and attapulgite application

The escalation of industrial activities has escalated the production of pharmaceutical and dyeing effluents, raising significant environmental issues. In this investigation, a hybrid approach of Fenton-like reactions and adsorption was used for deep treatment of these effluents, focusing on effects of variables like hydrogen peroxide concentration, catalyst type, pH, reaction duration, temperature, and adsorbent quantity on treatment effectiveness, and the efficacy of acid-modified attapulgite (AMATP) and ferric iron (Fe(III))-loaded AMATP (Fe(III)-AMATP) was examined. Optimal operational conditions were determined, and the possibility of reusing the catalysts was explored. Employing Fe3O4 as a heterogeneous catalyst and AMATP for adsorption, CODCr was reduced by 78.38-79.14%, total nitrogen by 71.53-77.43%, and phosphorus by 97.74-98.10% in pharmaceutical effluents. Similarly, for dyeing effluents, Fe(III)-AMATP achieved 79.87-80.94% CODCr, 68.59-70.93% total nitrogen, and 79.31-83.33% phosphorus reduction. Regeneration experiments revealed that Fe3O4 maintained 59.48% efficiency over three cycles, and Fe(III)-AMATP maintained 62.47% efficiency over four cycles. This work offers an economical, hybrid approach for effective pharmaceutical and dyeing effluent treatment, with broad application potential.

Comparison of methods for activating sesame stalk lignin biochar for removing benzo[a]pyrene from sesame oil

In this study, three activators and two activation methods were employed to activate sesame lignin-based biochar. The biochar samples were comprehensively characterized, their abilities to adsorb benzo[a]pyrene (BaP) from sesame oil were assessed, and the mechanism was analyzed. The results showed that the biochar obtained by one-step activation was more effective in removing BaP from sesame oil than the biochar produced by two-step activation. Among them, the biochar generated by one-step activation with ZnCl2 as the activator had the largest specific surface area (1068.8776 m3/g), and the richest mesoporous structure (0.7891 m3/g); it removed 90.53 % of BaP from sesame oil. BaP was mainly adsorbed by the mesopores of biochar. Mechanistically, pore-filling, π-π conjugations, hydrogen bonding, and n-π interactions were involved. The adsorption was spontaneous and heat-absorbing. In conclusion, the preparation of sesame lignin biochar using one-step activation with ZnCl2 as the activator was found to be the best for removing BaP from sesame oil. This biochar may be an economical adsorbent for the industrial removal of BaP from sesame oil.

Hydrogen from sewage sludge: Production methods, influencing factors, challenges, and prospects

The rising global population and rapid industrialization have frequently resulted in a significant escalation in energy requirements. Hydrogen, renowned for its eco-friendly and renewable characteristics, has garnered substantial interest as a fuel alternative to address the energy needs currently fulfilled by fossil fuels. Embracing such energy substitutes holds pivotal importance in advancing environmental sustainability, aiding in the reduction of greenhouse gas emissions - the primary catalysts of global warming and climate fluctuations. This study elucidates recent trends in sewage sludge (SS)-derived hydrogen through diverse production pathways and critically evaluates the impact of varying parameters on hydrogen yield. Furthermore, a detailed analysis of the breakdown of the hydrogen generation process from SS is provided, along with an assessment of its economic dimensions. The review culminates by illuminating key obstacles in the adoption of this innovative technology, accompanied by practical recommendations to surmount these challenges. This comprehensive analysis is expected to attract considerable interest from stakeholders within the hydrogen production domain, fostering substantial engagement.

Synergistic effects of adsorption and chemical reduction towards the effective Cr(VI) removal in the presence of the sulfur-doped biochar material

In this study, the anaerobic sludge withdrawn from thickener in a sewage treatment plant served as the precursor for sludge-based biochar fabrication, which was further modified via sulfur (S) heteroatom doping (i.e., S-BC). The S atom doping resulted in the adjustment of the physicochemical properties towards the carbon material, endowment of abundant functional groups on biochar surface, and increasing the binding sites between biochar and Cr(VI). Compared to the primary biochar (i.e., biochar without heteroatomic doping, named BC), S-BC exhibited a rough surface and possessed remarkable advantages in ash content, specific surface area, and pore volume. The existence of graphene carbon crystal structure for S-BC was confirmed through S-BC by XRD and FTIR analysis. The studies of adsorption kinetics and isotherms showed that pseudo-second-order kinetics and the Langmuir model more fitted the Cr(VI) removal behavior in the presence of S-BC. Therefore, the chemisorption and monolayer adsorption were the primary mechanisms involved in the Cr(VI) removal process. Additionally, XPS analysis results illustrated the aqueous Cr(VI) was efficiently eliminated through the synergistic effect of chemisorption and reduction to Cr(III) in the presence of S-BC. Moreover, S-BC could still achieve the Cr(VI) eliminating efficiency of 85.31% undergoing five cycles with unchanged functional group and crystal structure via FTIR and XRD analysis. Thus, the results of this study may shed light on a new approach for simultaneous economical sludge disposal and the sustainable remediation of the Cr(VI)-contaminated wastewater.

Efficient adsorption of Cu2+ and Cd2+ from groundwater by MgO-modified sludge biochar in single and binary systems

In this study, MgO-modified sludge biochar (1MBC) prepared from sewage sludge was successfully used as an efficient adsorbent to remove heavy metals from groundwater. The adsorption performance and mechanism of 1MBC on Cu2+ and Cd2+ were investigated in single and binary systems, and the contribution of different mechanisms was quantified. Adsorption kinetics and isotherms analysis revealed that the adsorption processes of Cu2+ and Cd2+ by 1MBC followed the pseudo-second-order kinetic and Langmuir isotherm model in both systems, indicating that Cu2+ and Cd2+ were mainly controlled by chemisorption, and their theoretical maximum adsorption capacities were 240.36 and 219.06 mg·g-1, respectively. The results of the binary system showed that due to the competitive adsorption, the adsorption capacity of 1MBC for both heavy metals was lower than that of the single system, and the selective adsorption of Cu2+ was higher. The influencing variable experiments revealed that the adsorption of Cu2+ and Cd2+ by 1MBC had a wide pH adaption range and strong anti-interference ability to coexisting organics and ions. The adsorption mechanisms involved ion exchange (Cu: 47.39%, Cd: 53.17%), mineral precipitation (Cu: 35.31%, Cd: 24.18%), functional group complexation (Cu: 10.44%, Cd: 14.53%), and other possible mechanisms (Cu: 6.87%, Cd: 8.12%). Furthermore, 1MBC demonstrated excellent regeneration potential after five cycle times. Overall, the results have significant reference value for the practical application of removing heavy metals.

Use of biochar and a post-coagulation effluent as an adsorbent of malachite green, beneficial bacteria carrier, and seedling substrate for plants belonging to the poaceae family

Wastewater treatment plants produce solid and semi-solid sludge, which treatment minimises secondary environmental pollution because of wastewater treatment and obtaining new bioproducts. For this reason, in this paper, the co-pyrolysis of biogenic biomasses recovered from a biological reactor with immobilised fungal and bacterial biomass and a tertiary reactor with Chlorella sp. used for dye-contaminated wastewater treatment was carried out. Biogenic biomasses mixed with pine bark allowed the production and characterisation of two types of biochar. The raw material and biochar were on the "in vitro" germination of Lolium sp. seeds, followed by adsorption studies for malachite green (MG) dye using the raw material and the biochar. Results showed that using 60 mg L-1 of a cationic coagulant at pH 6.5 allowed for the recovery of more than 90% of the microalgae after 50 min of processing. Two biochar resulted: BC300, at pH 5.08 ± 0.08 and BC500, at pH 6.78 ± 0.01. The raw material and both biochars were co-inoculated with growth-promoting bacteria; their viabilities ranged from 1.7 × 106 ± 1.0 × 101 to 7.5 × 108 ± 6.0 × 102 CFU g-1 for total heterotrophic, nitrogen-fixing and phosphate-solubilising bacteria. Re-use tests on Lolium sp. seed germination showed that with the post-coagulation effluent, the germination was 100%, while with the biochar, with and without beneficial bacteria, the germination was 98 and 99%, respectively. Finally, BC500 adsorbed the highest percentage of malachite green at pH 4.0, obtaining qecal values of 0.5249 mg g-1 (R2: 0.9875) with the pseudo-second-order model.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03766-x.

Efficacy and mechanisms of δ-MnO2 modified biochar with enhanced porous structure for uranium(VI) separation from wastewater

Even though uranium (U) is considered to be an essential strategic resource with vital significance to nuclear power development and climate change mitigation, U exposure to human and ecological environment has received growing concerns due to its both highly chemically toxic and radioactively hazardous property. In this study, a composite (M-BC) based on Ficus macrocarpa (banyan tree) aerial roots biochar (BC) modified by δ-MnO2 was designed to separate U(VI) from synthetic wastewater. The results showed that the separation capacity of M-BC was 61.53 mg/g under the solid - liquid ratio of 1 g/L, which was significantly higher than that of BC (12.39 mg/g). The separation behavior of U(VI) both by BC and M-BC fitted well with Freundlich isothermal models, indicating multilayer adsorption occurring on heterogeneous surfaces. The reaction process was consistent with the pseudo-second-order kinetic model and the main rate-limiting step was particle diffusion process. It is worthy to note that the removal of U(VI) by M-BC was maintained at 94.56% even after five cycles, indicating excellent reusability and promising application potential. Multiple characterization techniques (e.g. Scanning Electron Microscope-Energy Dispersive Spectrometer (SEM-EDS), Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), Brunauer-Emmett-Teller (BET) and X-ray Photoelectron Spectroscopy (XPS)) uncovered that U(VI) complexation with oxygen-containing functional groups (e.g. O-CO and Mn-O) and cation exchange with protonated ≡MnOH were the dominant mechanisms for U(VI) removal. Application in real uranium wastewater treatment showed that 96% removal of U was achieved by M-BC and more than 92% of co-existing (potentially) toxic metals such as Tl, Co, Pb, Cu and Zn were simultaneously removed. The work verified a feasible candidate of banyan tree aerial roots biowaste based δ-MnO2-modified porous BC composites for efficient separation of U(VI) from uranium wastewater, which are beneficial to help address the dilemma between sustainability of nuclear power and subsequent hazard elimination.

Life Cycle Assessment of Raw and Fe-Modified Biochars: Contributing to Circular Economy

Biochar is a carbonaceous material, which can be decorated with metals, that has been garnering attention to be used in the treatment of water due to its contribution to waste management and circular economy. This study presents the life cycle assessment (LCA) regarding the generation of Pinus patula raw biochar and its modification with iron (Fe-modified biochar). SimaPro 9.3.0.3 software was used to simulate the environmental impacts of both carbonaceous materials. The potential environmental effects obtained from the production of Pinus patula raw biochar were mainly ascribed to the source of energy utilized during this process. The potential impacts demonstrated that the generation of gases and polycyclic aromatic hydrocarbons are the main concern. In the case of Fe-modified biochar, the potential environmental effects differed only in the stage of the biomass modification with the metal. These effects are associated with the extraction of Fe and the generation of wastewater. These findings provide an insight into the environmental effects linked to the production of raw and Fe-modified biochar. However, further LCA research should be performed concerning other materials and compounds than can be generated during the biomass thermochemical conversion.

Transformation of Zn and Cr during co-combustion of sewage sludge and coals: influence of coal and steam

Combustion experiments of sewage sludge (SS) blended with low-rank coal were conducted through a drop tube furnace (DTF) to explore the effects of low-rank coal type, blending ratio, and steam on the transformation of Zn and Cr. The results showed that the retention rates of Zn and Cr in ash increased from 24.35% and 71.49% for sludge combustion alone to 53.77% and 117.49%, respectively, for coal blended to SS with a mass ratio of 7:3. The greater the proportion of low-rank coal in the fuel, the greater the residual rate of heavy metals in the ash. Meanwhile, rapid diffusion of vapor occupied adsorption sites on metal mineral surfaces, reducing the retention of Zn and Cr in the co-combustion ash. The leaching toxicity analysis of ash showed that the co-combustion ash of SS with coal was free from leaching toxicity hazards in simulated scenarios. The extraction rate of Zn in co-combustion ash increased from 90.72% with hydrothermal acid leaching to 95.46% with microwave-assisted in 2 mol/L H2SO4 extract. The extraction rate of Cr in hydrothermal acid leaching was 62.80%, which was much higher than that in microwave-assisted extraction (31.76%).

Nano-biochar: recent progress, challenges, and opportunities for sustainable environmental remediation

Biochar is a carbonaceous by-product of lignocellulosic biomass developed by various thermochemical processes. Biochar can be transformed into "nano-biochar" by size reduction to nano-meters level. Nano-biochar presents remarkable physico-chemical behavior in comparison to macro-biochar including; higher stability, unique nanostructure, higher catalytic ability, larger specific surface area, higher porosity, improved surface functionality, and surface active sites. Nano-biochar efficiently regulates the transport and absorption of vital micro-and macro-nutrients, in addition to toxic contaminants (heavy metals, pesticides, antibiotics). However an extensive understanding of the recent nano-biochar studies is essential for large scale implementations, including development, physico-chemical properties and targeted use. Nano-biochar toxicity on different organisms and its in-direct effect on humans is an important issue of concern and needs to be extensively evaluated for large scale applications. This review provides a detailed insight on nanobiochar research for (1) development methodologies, (2) compositions and properties, (3) characterization methods, (4) potentiality as emerging sorbent, photocatalyst, enzyme carrier for environmental application, and (5) environmental concerns.

Lipid extraction in the primary sludge generated from urban wastewater treatment: characteristics and seasonal composition analysis

A seasonal study of the lipid composition of a primary sludge (dry and dewatered base) obtained from an urban wastewater treatment plant located in Aguascalientes (Mexico) is reported. This study assessed the variability in sludge composition to establish its potential as a raw material for biodiesel production. Lipid recovery was achieved by extraction using two solvents. Hexane was employed for lipid extraction from dry sludge, whereas hexane and ethyl butyrate were used for comparison with dewatered sludge. The formation (%) of fatty acid methyl esters (biodiesel) was determined using extracted lipids. The extraction results from the dry sludge showed 14 and 6% of recovered lipids and their conversion to biodiesel, respectively. For the dewatered sludge, the lipid recovery and biodiesel formation were 17.4 and 60% using hexane, and 23 and 77% for ethyl butyrate, respectively, on a dry basis. Statistical data indicated that lipid recovery depended on the physicochemical characteristics of sewage sludge, which were related to seasonal changes, population activities, and changes in plant configuration, among other factors. These variables must be considered in the design of large-scale extraction equipment for the application and commercial exploitation of biomass waste in biofuel production.

Highly-efficient degradation of organic pollutants by oxalic acid modified sludge biochar: Mechanism and pathways

The application of sludge biochar (SC) materials as efficient catalysts for organic pollutants mineralization via advanced oxidation process meets the good strategy of "make waste profitable". The catalytic oxidations of methyl orange (MO) and pyrene by oxalic acid modified sludge biochar (SC-OA) with and without H₂O₂ were carried out. The analysis of Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), electronic paramagnetic resonance spectrometer (EPR) and free radical quenching experiment were performed and the definite relationships between persistent free radicals (PFRs) type and specific reactive oxygen species (ROS) were made clear. It is suggested for the first time that carbon-centered type PFRs in SC-OA without H₂O₂ could form O₂•^- and •OH from COOH groups, while oxygen-centered type PFRs induced H₂O₂ to produce •OH. The degradation intermediates of MO and pyrene were identified and the transformation pathways were proposed. SC-OA, possessing good sustainable utilization and clean catalytic property, is expected to be popularized and applied in the mineralization of organic pollutants, especially in the in-situ remediation of contaminated soil where is no continuous supply of H₂O₂.

Co-pyrolysis technology for enhancing the functionality of sewage sludge biochar and immobilizing heavy metals

Sewage sludge (SS) is a frequent and challenging issue for countries with big populations, due to its massive output, significant hazard potential, and challenging resource utilization. Pyrolysis can simultaneously realize the reduction, harmlessness and recycling of SS. Co-pyrolysis offers a wide range of potential in terms of increasing product quality and immobilizing heavy metals (HMs), thanks to its capacity to use additives to address the mismatch between SS characteristics and pyrolysis. High-value utilization potential of SS biochar is the key to evaluating the advancement of treatment technology. A further requirement for using biochar resources is the immobilization and bioavailability reduction of HMs. Due to the catalytic and synergistic effects in the co-pyrolysis process, co-pyrolysis SS biochar exhibits enhanced functionality and has been applied in soil improvement, pollutant adsorption and catalytic reactions. This review focuses on the research progress of different additives in improving the functionality of biochar and influencing the behavior of HMs. The key limitation and challenges in SS co-pyrolysis are then discussed. Future research prospects are detailed from seven perspectives, including pyrolysis process optimization, co-pyrolysis additive selection, catalytic mechanism research of process and product, biochar performance improvement and application field expansion, cooperative immobilization of HMs, and life cycle assessment. This review will offer recommendations and direction for future research paths, while also assist pertinent researchers in swiftly understanding the current state of SS pyrolysis research field.

Remediation competence of nanoparticles amalgamated biochar (nanobiochar/nanocomposite) on pollutants: A review

Advanced biochar blended nanoparticles substances, such as nano biochar or nanocomposites, have provided long-term solutions to a wide range of modern-day problems. Biochar blended nano-composites can be created to create better composite materials that combine the benefits of biochar and nanoparticles. Such materials have been typically improved with active functional groups, porous structure, active surface area, catalytic deterioration ability, as well as easy recovery or separation of pollutants. Such biochar-basednanocomposites have good adsorption properties for a variety of pollutants in various form of polluted medium (soil and water contamination). Catalytic nanoparticle encapsulated biochar, can perform concurrently the adsorption (by biochar) as well as catalytic degradation (nanoparticles) functions for pollutants removal from polluted sites. In this review, the advanced and practically feasible techniques involved in the biochar blended nanoparticles-based nanocomposites have been discussed with environmental applications. Furthermore, the mechanisms involved in this composite material in remediation, as well as the advantages and disadvantages of biochar blended nanoparticles-based nanocomposites, were discussed, and future directions for study in this field were suggested.

Mesoporous carbon material prepared from sewage sludge hydrochar using Pluronic F127 as template for efficient removal of phenolic compounds: Experimental study and mechanism interpretation via advanced statistical physics model

Mesoporous carbon material (MCM) with rich ether surface group was prepared from sewage sludge hydrochar using Pluronic F127 as template under pyrolysis activation, which provided an energy-efficient method to promote the resource utilization of sewage sludge as adsorbents for phenols removal from water. The MCM possessed high surface area (549 m²/g), abundant mesopores (average width 3.81 nm) and well-developed graphite structure. Acidic conditions and low temperatures favored the adsorption of phenolic compounds. The quick adsorption process of reaching over 85% of the capacity in the first 10 min and intraparticle diffusion as primary rate-limiting step were observed for all phenolic compounds. Advanced statistical physics analysis was used successfully to interpret the adsorption mechanism of phenols onto MCM and revealed a multi-molecular monolayer adsorption process primarily through negative charge-assisted hydrogen bond interaction where the ether functional group contributed to the predominant active sites. The adsorption capacity of phenolic compounds depended upon the number of molecules adsorbed per ether active site and the available density of ether bond group on the surface of MCM. 2,4,6-trichlorophenol showed a highest adsorption priority to occupy the limited ether active sites and its adsorption capacity reached 0.49 mmol/g, while p-nitrophenol exhibited a maximum number of molecules adsorbed on the single ether active site, showing an adsorption capacity of 0.42 mmol/g. The synergistic effect of multi-interactions mechanisms resulted in phenolic compounds removal with adsorption energies lower than 30 kJ/mol. This prepared MCM adsorbent is promising for application in treatment of water polluted by phenols.

Co-hydrothermal carbonization of organic solid wastes to hydrochar as potential fuel: A review

The organic solid waste (OSW) is a potential resource that loses its original value in people's daily production process. It can be used for secondary energy utilization through hydrothermal technology, which is similar to artificially simulating the natural coalification process. Co-hydrothermal carbonization (co-HTC) is a promising thermochemical conversion pathway, and advanced mechanisms can eliminate the drawbacks of single-feedstock hydrothermal carbonization (HTC). The preparation and production process of hydrochar can solve the problems of energy crisis and environmental pollution. This paper comprehensively reviews the key mechanisms of co-HTC to prepare solid fuels, and reviews the development process and practical application of hydrothermal technology. To begin with, the physical and chemical properties and combustion performance of co-hydrochar depend on the production method, process parameters, and selection of raw materials. The co-hydrochar usually has a higher HHV and a low atomic ratio of H/C and O/C, which improves combustion performance. Subsequently, the transformation path of the hydrothermal process of lignocellulosic and protein OSW was comprehensively expounded, and the reaction mechanism of the co-HTC of the two OSWs was effectively proposed. The effect of the ratio of different raw materials on the synergistic effect of co-HTC was also analyzed. Furthermore, the typical advantages and disadvantages of environmental safety, technical economy, and practical application in the co-HTC process are expounded. All in all, this review provides some foundations and new directions for the co-HTC of OSWs to prepare potential fuel. In addition, several prospects for the development and integrated application of co-HTC are presented in the future.

Feasibility of Biochar Derived from Sewage Sludge to Promote Sustainable Agriculture and Mitigate GHG Emissions-A Review

Sewage sludge (SS) has been connected to a variety of global environmental problems. Assessing the risk of various disposal techniques can be quite useful in recommending appropriate management. The preparation of sewage sludge biochar (SSB) and its impacts on soil characteristics, plant health, nutrient leaching, and greenhouse gas emissions (GHGs) are critically reviewed in this study. Comparing the features of SSB obtained at various pyrolysis temperatures revealed changes in its elemental content. Lower hydrogen/carbon ratios in SSB generated at higher pyrolysis temperatures point to the existence of more aromatic carbon molecules. Additionally, the preparation of SSB has an increased ash content, a lower yield, and a higher surface area as a result of the rise in pyrolysis temperature. The worldwide potential of SS output and CO₂-equivalent emissions in 2050 were predicted as factors of global population and common disposal management in order to create a futuristic strategy and cope with the quantity of abundant global SS. According to estimations, the worldwide SS output and associated CO₂-eq emissions were around 115 million tons dry solid (Mt DS) and 14,139 teragrams (Tg), respectively, in 2020. This quantity will rise to about 138 Mt DS sewage sludge and 16985 Tg CO₂-eq emissions in 2050, a 20% increase. In this regard, developing and populous countries may support economic growth by utilizing low-cost methods for producing biochar and employing it in local agriculture. To completely comprehend the benefits and drawbacks of SSB as a soil supplement, further study on long-term field applications of SSB is required.

Sewage sludge treatment technology under the requirement of carbon neutrality: Recent progress and perspectives

In the context of climate policies that advocate carbon neutrality, carbon emission reduction provides a new restriction in evaluating the waste activated sludge (WAS) treatment technologies and procedures. This review provides an overview of current researches and development efforts in WAS treatment, focusing on the dual attributes of WAS as contaminants and resources. Firstly, the improved technical requirements posed by heavy metals, micro(nano) plastics, or other emerging plastics in WAS are studied. Furthermore, in terms of carbon emission reduction, the applications and limitations of widely deployed WAS treatment technologies are discussed. Based on carbon neutrality requirements, the anaerobic co-digestion and co-pyrolysis technologies are comprehensively discussed from the views of pollutants removing efficiencies, enhancement methods, carbon emissions, and resource recovery. Finally, a workable new route for WAS treatment is proposed for future technological advancement and engineering innovation.

Assessing the influence of sewage sludge and derived-biochar in immobilization and transformation of heavy metals in polluted soil: Impact on intracellular free radical formation in maize

As one of the most common ways to get rid of municipal waste, landfill leachate, waste with complicated compositions and high levels of contaminants, has become a significant threat to the world's environment. Here, the impact of sewage sludge (SS) and derived-biochar (SSB) amendments on the immobilization and potential mobility of heavy metals in a contaminated soil-plant system was investigated. The sequential fractionation findings showed that using SS-2%, SSB-2%, and SSBC-1% reduced the potential mobility of heavy metals while increasing the residual fraction in polluted soils. The translocation and bioconcentration factors showed that heavy metals were slightly transferred into shoots from roots and lowered accumulation in roots from contaminated soils. Fourier transform infrared (FTIR) and X-ray photoelectron spectrum (XPS) comprehensive characterization results indicated the significant role of applied amendments for heavy metals transformation from the exchangeable-soluble fractions to the least available form by lowering their mobility to confirm the adsorption-based complexes, which results in the surface adsorption of heavy metals with functional groups. The electron paramagnetic resonance (EPR) results indicated the dominance of reactive oxygen species (ROS) in the intracellular formation of hydroxyl radicals (•OH) in maize plant roots and shoots. ROS (•OH) generation plays a critical influence in the interaction between the physiological processes of plants and heavy metals. Moreover, all the amendments increased maize growth and biomass production. Our study suggests that alone and combined application of SS and SSB have great potential to remediate heavy metals contaminated soil for environmental sustainability.

A sustainable reuse strategy of converting waste activated sludge into biochar for contaminants removal from water: Modifications, applications and perspectives

Conversion of sewage sludge to biochar for contaminants removal from water achieves the dual purpose of solid waste reuse and pollution elimination, in line with the concept of circular economy and carbon neutrality. However, the current understanding of sludge-derived biochar (SDB) for wastewater treatment is still limited, with a lack of summary regarding the effect of modification on the mechanism of SDB adsorption/catalytic removal aqueous contaminants. To advance knowledge in this aspect, this paper systematically reviews the recent studies on the use of (modified) SDB as adsorbents and in persulfate-based advanced oxidation processes (PS-AOPs) as catalysts for the contaminants removal from water over the past five years. Unmodified SDB not only exhibits stronger cation exchange and surface precipitation for heavy metals due to its nitrogen/mineral-rich properties, but also can provide abundant catalytic active sites for PS. An emphatic summary of how certain adsorption removal mechanisms of SDB or its catalytic performance in PS-AOPs can be enhanced by targeted regulation/modification such as increasing the specific surface area, functional groups, graphitization degree, N-doping or transition metal loading is presented. The interference of inorganic ions/natural organic matter is one of the unavoidable challenges that SDB is used for adsorption/catalytic removal of contaminants in real wastewater. Finally, this paper presents the future perspectives of SDB in the field of wastewater treatment. This review can contribute forefront knowledge and new ideas for advancing sludge treatment toward sustainable green circular economy.

The GHG mitigation opportunity of sludge management in China

Greenhouse gas (GHG) mitigation in wastewater treatment sector is indispensable in China's carbon neutral target. As an important component of wastewater system, sludge generation is rapidly increased with the acceleration of urbanization in China. It is crucial to investigate the carbon footprint of various sludge management strategies and quantify the potential optimization of GHG reduction effect at national scale. Therefore, this study conducted a comprehensive analysis of sludge distribution and GHG profiles of various sludge systems. The overall dry sludge generation in China is 12.15 Mt, with spatial resolution at city level. Different sludge treatment options were categorized into four types: energy recovery, nutrient recovery (e.g. phosphorus and nitrogen), material valorisation (e.g. brick, biochar) and conventional disposal. With various sludge treatment options, the GHG profile of annual sludge management in China ranges from -35.86 Mt/year to 57.11 Mt/year. The best GHG mitigation can be achieved through energy recovery by co-incineration system and the greatest reduction opportunity is concentrated in highly urbanized regions, such as Yangtze River Delta, Pearl River Delta, and Beijing-Tianjin-Hebei urban agglomerations.

Phosphorus release from hydrothermally carbonized digested sewage sludge using organic acids

Hydrothermal carbonization (HTC) is a treatment technique with great potential for sanitizing digested sewage sludge (SS) and converting it into valuable products. In particular, phosphorus (P) recovery from hydrothermally carbonized SS has attracted special attention in recent years. This work aims to examine the leaching efficiency of P and the consequent release of metals and heavy metals from SS hydrochars (at 180, 215 and 250 °C) using organic acids (oxalate and citrate) over a range of pH values (0-4) and extraction times (5 min-24 h). Both organic acids triggered P extraction efficiencies exceeding 75 % at the lowest pH, but only oxalate reached a nearly complete P release from hydrochars at pH > 0 and for all carbonization temperatures. Low HTC temperature (180 °C) and short extraction time (5 min) were the optimal conditions treatment for P recovery when reacted in oxalate solutions of maximal pH buffering capacity (pH = 1.4). However, oxalate leaching also transferred metals/heavy metals into the P-leachate, with the exception of Ca being retained in the solid residue from HTC as Ca-oxalate precipitate. Different characterization methods confirmed the presence of this precipitate, and provided information about the surface and morphological changes of the SS hydrochars following acid treatment. The results suggest that HTC not only a promising technique to sanitize and reduce the volume of SS, but also an efficient means for P recovery using oxalic acid, thus contributing to the circular economy of P.

Trivalent antimony removal using carbonaceous nanomaterial loaded with zero-valent bimetal (iron/copper) and their effect on seed growth

As rapid industrial and social growth, antimony mines are the overexploited, leading to the accumulation of trivalent antimony in the aquatic environment near smelters, which harm human health. To eradicate trivalent antimony from water, an innovative nanomaterial in the form of sludge biochar loaded with zero-valent bimetal was synthesized using a liquid-phase reduction method. The adsorption performance of the nanomaterial for trivalent antimony was investigated based on a series of adsorption experiments using sludge biochar, nano zero-valent iron biochar, and nano zero-valent bimetal biochar. The results showed that the optimal adsorption performance of the three nanomaterials for trivalent antimony, considering the economic practicability, was highlighted at solution pH of 3 and 0.05 g of nanomaterial. Additionally, the maximum adsorption capacity of sludge biochar, nano zero-valent iron biochar, and nano zero-valent bimetal biochar is 3.89 mg g^-1 at 35 °C, 32.01 mg g^-1 at 25 °C, 50.96 mg g^-1 at 25 °C, respectively. The adsorption process of sludge biochar is endothermic, resulting in an increase in the adsorption capacity with increasing temperature, whereas the exothermic reaction contributes to decrease in the adsorption capacity at increasing temperature for the other two carbon nanomaterials. The inhibitory effect of coexisting ions was in the order: Al³⁺ > NH₄⁺ > Na⁺ > K⁺; CO₃^2- > CH₃COO^- > H₂PO₄^- > S^2-. Additionally, nanomaterials promoted seed germination and growth. Investigation of the adsorption mechanism using X-ray photoelectron spectroscopy showed that trivalent antimony was oxidised to pentavalent antimony, and Fe(III) was reduced to Fe(II). The formed primary battery formed by copper ions and iron acclerated electron transfer and improved the adsorption rate. This implied that trivalent antimony could be removed through the synergistic action of the adsorption behaviour and redox reaction. Therefore, the biochar loaded with the zero-valent bimetal serves as a pathway for eradicating trivalent antimony.

Mechanistic insights of removing pollutant in adsorption and advanced oxidation processes by sludge biochar

With the accelerated industrialization, more and more sewage sludge (SS) needs to be treated properly. The conversion of sludge into harmless biochar material with dual utilization value of adsorption and catalysis by pyrolysis is in line with the concept of sustainable development. However, the reaction mechanisms of pristine sludge biochar (SDBC) and its composites (SDBCs) in adsorption, persulfate (PS), and Fenton-like advanced oxidation processes (AOPs) are very closely related to its adsorption performance and catalytic efficiency. In this paper, from the application mechanisms of SDBC in adsorption and AOPs, we review in detail the common methods for synthesizing SDBC and their characteristics. We discuss the synthesis techniques that affect the structural, chemical, and catalytic properties of SDBC, including gasification, pyrolysis, and hydrothermal carbonation (HTC). The pyrolysis temperature, environmental factors, and sludge characteristics have important effects on the properties of SDBC, leading to different mechanisms in adsorption and catalytic processes. Furthermore, this paper systematically generalizes the mechanisms of SDBCs in adsorption, where π-π interactions and electrostatic attractions are the main adsorption mechanisms. Then, activation mechanisms of SDBCs in PS and Fenton-like AOPs systems are discussed, including free radical pathways and non-free radical pathways. Finally, we present several challenges and perspectives for the application of SDBC and SDBCs in the field of adsorption, PS, and Fenton-like AOPs from the mechanistic point of views.

Sustainable biochar-based soil fertilizers and amendments as a new trend in biochar research

Today's world is struggling with many environmental problems. Due to the ever-growing size of the population, it is necessary to produce more and more food. The consequence of such a large demand for food is excessive fertilization of soils, often in an uncontrolled manner. The paper presents an overview of the different types of biochar (BC) fertilizers obtained by: coating BCs with a protective layer, coating commercial fertilizers with a layer of BCs, or mixing BCs with commercial fertilizers. Although the use of these new types of fertilizers has a positive effect on soil properties and crop yields, the production and use of "simple" inorganic fertilizers are dominant. The solution to starting the change of this trend may be the use of BC-compost systems as an effective soil amendment, due to the fact that composts are still the most frequently used products by farmers. The review summarized two types of BC-compost soil amendments: BC mixed with ready-made compost and BC co-composted with compost raw material. These types of soil amendments contribute to a significant reduction in the consumption of commercial inorganic fertilizers, and thus less pollution of the natural environment, while allowing for a high yield of safe food.

Long-Term, Simultaneous Impact of Antimicrobials on the Efficiency of Anaerobic Digestion of Sewage Sludge and Changes in the Microbial Community

The aim of this study was to evaluate the influence of simultaneous, long-term exposure to increasing concentrations of three classes of antimicrobials (β-lactams, fluoroquinolones and nitroimidazoles) on: (1) the efficiency of anaerobic digestion of sewage sludge, (2) qualitative and quantitative changes in microbial consortia that participate in methane fermentation, and (3) fate of antibiotic resistance genes (ARGs). Long-term supplementation of sewage sludge with a combination of metronidazole, amoxicillin and ciprofloxacin applied at different doses did not induce significant changes in process parameters, including the concentrations of volatile fatty acids (VFAs), or the total abundance of ARGs. Exposure to antibiotics significantly decreased methane production and modified microbial composition. The sequencing analysis revealed that the abundance of OTUs characteristic of Archaea was not correlated with the biogas production efficiency. The study also demonstrated that the hydrogen-dependent pathway of methylotrophic methanogenesis could significantly contribute to the stability of anaerobic digestion in the presence of antimicrobials. The greatest changes in microbial biodiversity were noted in substrate samples exposed to the highest dose of the tested antibiotics, relative to control. The widespread use of antimicrobials increases antibiotic concentrations in sewage sludge, which may decrease the efficiency of anaerobic digestion, and contribute to the spread of antibiotic resistance (AR).

Removal of pharmaceuticals from water using sewage sludge-derived biochar: A review

In recent years, considerable attention has been paid to the beneficial utilization of sewage sludge to reduce the risks associated with sludge disposal. Besides other applications of sludge, biochar produced from sludge has also been employed for the elimination of various pollutants from water. This review critically evaluates the recent progress in applications of sludge-based biochar for the adsorption of pharmaceuticals from water. The synthesis techniques of biochar production from sludge and their effects on physicochemical characteristics of produced biochar are discussed. The removal of various pharmaceuticals by sludge-based biochar are described in detail, with the emphasis on the adsorption mechanism and their reusability potential. It is evident from the literature that sludge-based biochar has demonstrated excellent potential for the adsorption of numerous pharmaceuticals from the aqueous phase. The major hurdles and issues related to the synthesis of sludge-based biochar and applications are highlighted, with reference to the adsorption of pharmaceuticals. Finally, a roadmap is suggested along with future research directions to ensure the sustainable production of biochar from sludge and its applications in water treatment.

Multiple chemical modifications and Cd2+ adsorption characteristics of sludge-based activated carbon

The multiple chemical modifications were carried out to achieve N-doping and pore-making to modify sludge-based activated carbon (SACU–PF′). SACU–PF′ possessed abundant functional groups and high adsorption capacity of Cd2+.