The conversion of food waste and sludge into biogas via anaerobic digestion technology is gaining attention in recent years, which plays a significant role in waste valorization into bioenergy and promotes environmental sustainability. Biochar is a carbonaceous material produced via thermochemical conversion of biomass waste, and tailoring biochar for diverse environmental applications adheres to the principle of circular economy. The emerging application of biochar as an additive in the anaerobic digestion of food waste and sludge has been intensively investigated in the last few years. However, a comprehensive understanding of multifunctional roles of biochar and its mechanisms in the production of biogas via miscellaneous/complex anaerobic digestion process is yet to be attained. This review scrutinizes the key roles of biochar as an additive and emphasizes the influences of biochar characteristics on the anaerobic digestion processes and their capability to address the foremost challenges. This review also evaluates the techno-economic and environmental impacts of biochar synthesis and its emerging application for biogas production via anaerobic digestion to make the integrated process more economical and environmentally sustainable, and identifies challenges and prospects for future studies. (C) 2021 Elsevier Ltd. All rights reserved.

Weathering of microplastics (MPs, < 5 mm) in terrestrial and aquatic environments affects MP transport and distribution. This paper first summarizes the sources of MPs, including refuse in landfills, biowastes, plastic films, and wastewater discharge. Once MPs enter water and soil, they undergo different weathering processes. MPs can be converted into small molecules (e.g., oligomers and monomers), and may be completely mineralized under the action of free radicals or microorganisms. The rate and extent of weathering of MPs depend on their physicochemical properties and environmental conditions of the media to which they are exposed. In general, water dissipates heat better, and has a lower temperature, than land; thus, the weathering rate of MPs in the aquatic environment is slower than in the terrestrial environment. These weathering processes increase oxygen-containing functional groups and the specific surface area of MPs, which influence the sorption and aggregation that occur between weathered MPs and their co-existing constituents. More studies are needed to investigate the various weathering processes of diverse MPs under natural field conditions in soils, sediments, and aquatic environments, to understand the impact of weathered MPs in the environment. (c) 2021 Elsevier Ltd. All rights reserved.

Carbon dioxide (CO2) is the main anthropogenic greenhouse gas contributing to global warming, causing tremendous impacts on the global ecosystem. Fossil fuel combustion is the main anthropogenic source of CO2 emissions. Biochar, a porous carbonaceous material produced through the thermochemical conversion of organic materials in oxygen-depleted conditions, is emerging as a cost-effective green sorbent to maintain environmental quality by capturing CO2. Currently, the modification of biochar using different physico-chemical processes, as well as the synthesis of biochar composites to enhance the contaminant sorption capacity, has drawn significant interest from the scientific community, which could also be used for capturing CO2. This review summarizes and evaluates the potential of using pristine and engineered biochar as CO2 capturing media, as well as the factors influencing the CO2 adsorption capacity of biochar and issues related to the synthesis of biochar-based CO2 adsorbents. The CO2 adsorption capacity of biochar is greatly governed by physico-chemical properties of biochar such as specific surface area, microporosity, ammaticity, hydrophobicity and the presence of basic functional groups which are influenced by feedstock type and production conditions of biochar. Micropore area (R-2 = 0.9032, n = 32) and micropore volume (R-2 = 0.8793, n = 32) showed a significant positive relationship with CO2 adsorption capacity of biochar. These properties of biochar are closely related to the type of feedstock and the thermochemical conditions of biochar production. Engineered biochar significantly increases CO2 adsorption capacity of pristine biochar due to modification of surface properties. Despite the progress in biochar development, further studies should be conducted to develop cost-effective, sustainable biochar-based composites for use in large-scale CO2 capture.

Hydrogen sourced from energy recovery processes and conversion of waste materials is a method of providing both a clean fuel and a sustainable waste management alternative to landfill and incineration. The question is whether waste-to-hydrogen can become part of the zero-carbon future energy mix and serve as one of the cleaner hydrogen sources which is economically viable and environmentally friendly. This work critically assessed the potential of waste as a source of hydrogen production via various thermochemical (gasification and pyrolysis) and biochemical (fermentation and photolysis) processes. Research has shown hydrogen production yields of 33.6 mol/kg and hydrogen concentrations of 82% from mixed waste feedstock gasification. Biochemical methods such as fermentation can produce hydrogen up to 418.6 mL/g. Factors including feedstock quality, process requirements and technology availability were reviewed to guide technology selection and system design. Current technology status and bottlenecks were discussed to shape future development priorities. These bottlenecks include expensive production and operation processes, heterogeneous feedstock, low process efficiencies, inadequate management and logistics, and lack of policy support. Improvements to hydrogen yields and production rates are related to feedstock processing and advanced energy efficiency processes such as torrefaction of feedstock which has shown thermal efficiency of gasification up to 4 MJ/kg. This will affect the economic feasibility and concerns around required improvements to bring the costs down to allow waste to viewed as a serious competitor for hydrogen production. Recommendations were also made for financially competitive waste-to-hydrogen development to be part of a combined solution for future energy needs.

A large amount of plastic waste released into natural waters and their demonstrated toxicity have made the transformation of microplastics (MPs; < 5 mm) and nanoplastics (NPs; < 100 nm) an emerging environmental concern. Aggregation is one of the most important environmental behaviors of MPs, especially in aquatic en-vironments, which determines the mobility, distribution and bioavailability of MPs. In this paper, the sources and inputs of MPs in aquatic environments were first summarized followed by the analytical methods for investigating MP aggregation, including the sampling, visualization, and quantification procedures of MP' particle sizes. We critically evaluated the sampling methods that still remains a methodological gap. Identification and quantification of MPs were mostly carried out by visual, spectroscopic and spectrometric techniques, and modeling analysis. Important factors affecting MP aggregation in natural waters and environmental implications of the aggregation process were also reviewed. Finally, recommendations for future research were discussed, including (1) conducting more field studies; (2) using MPs in laboratory works representing those in the en-vironment; and (3) standardizing methods of identification and quantification. The review gives a comprehensive overview of current knowledge for MP aggregation in natural waters, identifies knowledge gaps, and provides suggestions for future research.

Biochar application has multiple benefits for soil fertility improvement and climate change mitigation. Biochar can act as a source of nutrients and sequester carbon (C) in the soil. The nutrient release capacity of biochar once applied to the soil varies with the composition of the biochar, which is a function of the feedstock type and pyrolysis condition used for biochar production. Biochar has a crucial influence on soil C mineralization, including its positive or negative priming of microorganisms involved in soil C cycling. However, in various cases, biochar application to the soil may cause negative effects in the soil and the wider environment. For instance, biochar may suppress soil nutrient availability and crop productivity due to the reduction in plant nutrient uptake or reduction in soil C mineralization. Biochar application may also negatively affect environmental quality and human health because of harmful compounds such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated dibenzodioxins, and dibenzofurans (PCDD/DF). In this review, we discuss the linkage between biochar composition and function, evaluate the role biochar plays in soil fertility improvement and C sequestration, and discuss regulations and concerns regarding biochar's negative environmental impact. We also summarize advancements in biochar production technologies and discuss future challenges and priorities in biochar research.