In recent years, biochar has emerged as a remarkable biosourced material for addressing global environmental, agricultural, biomedical, and energy challenges. However, the performances of biochar rest in part on finely tuning its surface chemical properties, intended to obtain specific functionalities. In this review, we tackle the surface treatment of biochar with silane and other coupling agents such as diazonium salts, titanates, ionic/non-ionic surfactants, as well as nitrogen-containing (macro)molecules. We summarize the recent progress achieved mostly in the last five years and correlate the nature and extent of functionalization to eye-catchy end applications of the surface-engineered biochar.

The aim of the present work is to valorise the brewing industry’s waste, i.e., brewer’s spent grain (BSG), into functional biocarbon for environmental catalysis applications. In this context, cost-effective and environmentally friendly biochar support coated with in-situ-generated Ag-Cu nanocrystals, was developed via the wet impregnation of BSG biomass powder with copper (II) nitrate trihydrate and silver nitrate aqueous solution prior to pyrolysis at moderate temperature (500 °C). Small-size homogenously distributed Ag-Cu nanocrystals (≤80 nm) on the surface of the biochar (Biochar@Ag-Cu) were observed by field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). Elemental compositions were determined by X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray analysis (EDX). The crystalline nature of the nanoparticles was confirmed by X-ray powder diffraction (XRD). Information about the thermal stability of the materials and quality were obtained by thermogravimetric analysis (TGA) and Raman, respectively. The potentiality of the Biochar@Ag-Cu catalyst in the field of pollutant removal is demonstrated by taking methyl orange and methylene blue as model dyes. A kinetics study was performed and analyzed by UV–vis spectroscopy. Its highly active catalytic nature is proved by the complete mineralization of the methyl orange dye (100%) through oxidative degradation. The reusability of the catalyst has shown 96% removal efficiency after 3 cycles. The linear plot of −Ln (C_A/C₀) vs. time (R² = 0.9892) reveals that the mineralization of the methyl orange dye follows pseudo-first-order kinetics (k = 0.603 × 10⁻² min⁻¹). A methyl orange + methylene blue dye mixture degradation study has revealed the faster kinetics of the present catalyst towards methylene blue degradation. The current study suggests that BSG Biochar@Ag-Cu can be a potential candidate in contribution towards SDG 6.

The present work focuses on valorizing the various biochar supports of nano-catalysts and investigates the effect of the type of the initial biomass on the deposition and salient physico-chemical features of the zinc oxide (ZnO) nanoparticles. In this regard, we have used four different biomasses, namely, sugarcane bagasse (Saccharum officinarum), algae (Phaeophyta), mandarin orange peels (Citrus reticulata), and China rose petals (Rosa chinensis) as sources of biochar. Their wet impregnation with zinc acetate was followed by pyrolysis at 500 °C. It led to biochar (nicknamed “sweety,” “salty,” “sour,” and “romantic” biochar, respectively) loaded with very well dispersed, 20–360 nm-sized (mostly) ZnO nanoparticles. Interestingly, depending on the type of biomass used, the size, shape (quartz-like, semi-spherical, spherical, semi-cauliflower, needle or rod-like), and degree of crystallinity of ZnO nanoparticles (hexagonal crystal system) vary, in spite of all other synthesis parameters being similar. Nanoparticle-induced graphitization and crystallization of biochar have been also observed by Raman spectroscopy. The malachite green dye mineralization efficiency in the presence of H₂O₂ and different biochar-supported ZnO nanocatalysts was 15.1%, 46.3%, 99.9%, 67.9%, and 66.4% for H₂O₂, china rose petals, algae, mandarin peels, and sugarcane bagasse-supported ZnO catalyst in the presence of H₂O₂, respectively. Malachite green removal fits in very well with a pseudo-first-order kinetic model with R ² = 0.9701 (at algae biochar-ZnO). 6.6 times enhancement in the mineralization efficiency is observed as compared to just H₂O₂. The recyclability test of algae biochar impregnated with ZnO after 5 cycles indicates the mineralization efficiency levels up to 81.4%. Beyond these scientific results, this work is based on the principle of biomass waste valorization for sustainable development and circular economy, on the one hand, and addresses the UN Sustainable Development Goals 6, 13, and 14, on the other hand. It is also very clear that biochar is the new romance in the field of materials science and for sustainable future. Graphical Abstract

The present work is based on the principle of biomass waste valorization. Brewer’s spent grains (BSG) come from breweries as by-products. Their huge amount of production on an industrial scale should focus our attention on their valorization, which creates challenges as well as opportunities. One way to valorize BSG by-products is to convert them into biochar, a functional material with multiple potential applications. With an emphasis on sustainable development and the circular economy, in this work, we focused on a comparative study of the different mechanical processes of BSG grinding and their effect on the resulting biochar formed after pyrolysis. Home appliances such as blenders, coffee mills, and mortar and pestles were used for this purpose. FESEM images confirmed the successful creation of five different morphologies from the same BSG under the same pyrolysis conditions. Interestingly, a novel Chinese tea leaf egg-like biochar was also formed. It was found that a series of physical pretreatments of the biomass resulted in the reduced roughness of the biochar surface, i.e., they became smoother, thus negatively affecting the quality of the biochar. XRD revealed that the biomass physical treatments were also reflected in the crystallinity of some biochar. Via a Raman study, we witnessed the effect of mechanical pressure on the biomass for affecting the biochar features through pressure-induced modifications of the biomass’s internal structure. This induced enhanced biochar graphitization. This is a good example of the role of mechanochemistry. DSC revealed the thermochemical transformation of the five samples to be exothermic reactions. This study opens up an interesting possibility for the synthesis of biochar with controlled morphology, crystallinity, degree of graphitization, and heat capacity.