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The monumental challenge associated with food waste management has emphasized the dire need of upcycling it into useful materials, including ultraporous adsorbent. Among various technologies of maximizing porosity of such waste-derived porous sorbents, potassium hydroxide (KOH) activation of food waste hydrochar has emerged to be a prominent one. There are two different ways to synthesize ultraporous adsorbent, namely, direct chemical activation (DCA) and char impregnation (CI). This study aims in investigating the environmental impact comparison of DCA and CI using life cycle assessment (LCA). The results demonstrate that CI processes in an environmentally sound way for synthesizing ultraporous carbons from food waste, where freshwater ecotoxicity (57.2%) plays the major contributing role in environmental impact category, primarily due to acid neutralization in the mixer unit of the CI technique of activation. In addition, the dryer unit in the CI process, which is powered by natural gas combustion, was responsible for climate change impact category. Therefore, as an alternative, employment of renewable solar energy (from solar thermal power plant) was also investigated, and results highlighted the possibility of achieving reduced climate change and acidification potential. 

Hydrothermal carbonization (HTC) is a prominent thermochemical technology that can convert high-moisture waste into a valuable product (called hydrochar) at a relatively mild treatment condition (180–260 °C and 2–10 MPa). With rapidly growing research on HTC and hydrochar in recent years, review articles addressing the current and future direction of this research are scarce. Hence, this article aims to review various emerging applications of hydrochars, e.g., from solid fuel to soil amendment, from electron storage to hydrogen storage, from dye adsorption, toxin adsorption, heavy metal adsorption to nutrient recovery, and from carbon capture to carbon sequestration, etc. This article further provides an insight in the hydrochar’s working mechanism for various applications and how the applications can be improved through chemical modification of the hydrochar. Finally, new perspectives with appropriate recommendations have been made to further unveil potential applications and its improvement through hydrochar and its modified version. 

Hydrogen (H2) is largely regarded as a potential cost-efficient clean fuel primarily due to its beneficial properties, such as its high energy content and sustainability. With the rising demand for H2 in the past decades and its favorable characteristics as an energy carrier, the escalating USA consumption of pure H2 can be projected to reach 63 million tons by 2050. Despite the tremendous potential of H2 generation and its widespread application, transportation and storage of H2 have remained the major challenges of a sustainable H2 economy. Various efforts have been undertaken by storing H2 in activated carbons, metal organic frameworks (MOFs), covalent organic frameworks (COFs), etc. Recently, the literature has been stressing the need to develop biomass-based activated carbons as an effective H2 storage material, as these are inexpensive adsorbents with tunable chemical, mechanical, and morphological properties. This article reviews the current research trends and perspectives on the role of various properties of biomass-based activated carbons on its H2 uptake capacity. The critical aspects of the governing factors of H2 storage, namely, the surface morphology (specific surface area, pore volume, and pore size distribution), surface functionality (heteroatom and functional groups), physical condition of H2 storage (temperature and pressure), and thermodynamic properties (heat of adsorption and desorption), are discussed. A comprehensive survey of the literature showed that an “ideal” biomass-based activated carbon sorbent with a micropore size typically below 10 Å, micropore volume greater than 1.5 cm3/g, and high surface area of 4000 m2/g or more may help in substantial gravimetric H2 uptake of >10 wt% at cryogenic conditions (−196 °C), as smaller pores benefit by stronger physisorption due to the high heat of adsorption.