Biowaste raw materials were used for biochar preparation through pyrolysis at 850 °C under a limited oxygen atmosphere. Raw materials and the corresponding biochar samples were characterized by XRD, FTIR, SEM, TGA, N₂-sorption, pH-equilibrium, and ash content measurements. These samples were evaluated as sustainable sorbents for use in methylene blue (MB) removal from artificial fresh water. All biochar samples exhibited high specific surface areas (367–870 m²·g⁻¹), low crystallinity, and low population of functional groups (C–O–C, –COOH, –N–O, –N–H, and –OH) on their surfaces. They were mainly micro-porous materials with a significant fraction of pores in the meso-porous range. The specific surface area of the latter pores proved very important for the physical adsorption of MB from aqueous solution. Although the raw materials exhibited low MB sorption capacity, ranging from 29 to 54 mg·g⁻¹, the corresponding biochar samples exhibited important MB sorption efficiency ranging from 58 to 370 mg·g⁻¹. Among the biochar samples studied, those produced from coffee residues proved most promising for MB removal from water solution (sorption capacity: 280–370 mg·g⁻¹), addressing the United Nations Sustainability Development Goal (SDG) 6: Clean Water and Sanitation by improving the index related to anthropogenic wastewater that has received treatment.

This study aims to utilize biochars derived from residual biomass as supports for Ni-based catalysts. For the preparation of the biochars, byproducts of agro-industrial activities were used, such as espresso coffee residue (C) and rice husks (R). Sufficient quantities of the respective biochars (BioC and BioR) were prepared via pyrolysis at 850 °C of the aforementioned materials under limited oxygen conditions. The biochars were further treated with hot water (WBioC, WBioR), H₃PO₄ or H₂SO₄ solution (BioC-P, BioC-S, BioR-P, BioR-S), and NaOH solution (BioC-A and BioR-A), and the obtained solids were characterized using various physicochemical techniques. The biochars produced were microporous with high surface areas (367–938 m²g⁻¹). The most promising biochars were selected as supports for the preparation of nickel catalysts (10 wt.% Ni) with high Ni dispersion (mean crystal size: 8.2–9.8 nm) and suitable acidity. The catalysts were evaluated in a high-pressure semi-batch reactor for the transformation of waste cooking oil (WCO) into green diesel. The 10Ni_BioC-P catalyst exhibited the best performance, resulting in a complete conversion of the WCO but a low hydrocarbon yield (7.5%). Yield improvement was achieved by promoting this catalyst with molybdenum. The addition of Mo increased the hydrocarbon yield by almost three times (19.5%).

Nickel-based catalysts promoted by Mo-oxo species have been proven to be quite promising for transformation of triglycerides into green diesel. The selection of the support for such catalysts is crucial. In the present study, pyrolyzed rice husk subjected to acid and/or alkaline post-treatment was studied as support for the aforementioned catalysts. Biochar produced by slow pyrolysis of raw material at 850 °C for 5 h under limited-oxygen conditions exhibited the most promising textural characteristics. These were substantially improved by alkaline post-treatment. Thus, the corresponding MoNi catalysts proved the most efficient for upgrading used cooking oil (UCO) to green diesel via a solvent-free hydrotreatment process, performed in a semi-batch reactor (at 310 °C, 40 bar, 100 mL/min H₂, 100 mL UCO and 1 g of catalyst). The enhanced catalytic performance (complete conversion of UCO and 32 wt.% green diesel yield) of MoNi catalysts supported on biochar subjected at least to an alkaline post-treatment step has been attributed to their high SSA, suitable pore size distribution (enhanced mesoporosity), and acidity (enhanced population of acid sites with moderate strength), as well as to the high dispersion of the active phase.

Biochars are currently proposed as soil amendments or sorbent materials. There is an extensive scientific literature that deals with biochars originating from different raw materials. However, a holistic physicochemical characterization with simple analytical techniques is needed to provide insights on the characteristics of the biochars produced from malt spent rootlets (MSRs) and how they vary using different pyrolysis temperatures. This way, their properties can be fully understood, and they can be used for commercial purposes more effectively. Initially, the texture of the biochars were visualized by SEM and was quantified by the adsorption/desorption of nitrogen and the Brunauer, Emmett, and Teller (BET) equation. Additionally, the moisture content, the ash content and the pH of each sample were measured. Furthermore, the electrical conductivity of each sample was measured. Different techniques were used to determine the properties of carbon and of the surface functional groups (Total Carbon, XRD, ATR-FTIR) and leachable organic matter. Also, sorption of the methylene blue dye solution has been studied, which is an indication of mesopores for each biochar. Molasses number was also determined, as this is an indicator of macropores. Finally, the chlorine removal rate was determined for each type of biochar. The experiments marked that the change in mass of biochars has stopped after three hours at 50 °C in the drying oven. The measured moisture content ranged from 6 to 11%. The specific surface area of our materials, calculated through the BET equation, for low temperature biochars (e.g., 28 m²/g, at 350 °C), is much lower than that of high temperature pyrolyzed biochar (e.g., 286 m²/g, at 850 °C). The pH value ranged from 7 to 10. The electrical conductivity values of samples ranged from 800 μS/cm to 2.55 mS/cm, and these decreased during the measurement after the second wash with deionized water. Crystallinity increased with increasing pyrolysis temperature whereas the number of functional groups decreased. MSR biochars produced at temperatures equal or higher than 750 °C demonstrate different characteristics to the ones produced at lower temperatures.

Annually, large amounts of rice husk (>80 million tonnes) are produced around the world and high quantities of them are produced by developing countries. Determination of chemical and physical properties of new materials produced from the activation of rice husk biochars such as density, ash, surface area, surface topography, surface functional groups and suspension pH could affect sorption capacity and catalyst properties. In this study, biochars (RH), produced from raw rice husk (RAW-RH) at 850oC through pyrolysis process, were activated with H2O (W-RH), H2SO4 (RH-S), H3PO4 (RH-P), and NaOH (RH-ALK) and their physicochemical parameters were analysed.  This study aims to characterize the surface area and the porosity, the suspension pH, and the functional groups observed on the surface of RAW-RH, RH, W-RH, RH-S, RH-P, and RH-ALK. The determination of the surface area, the pore volume and the average pore size was performed by using gas (N2) adsorption−desorption with the Micromeritics TriStar 3000 Analyzer system using the Brunauer, Emmett, and Teller (BET) equation. Before analysis, the RAW-RH was degassed at 60oC under mild nitrogen flow for 1 h and RHs were degassed at 300oC under mild nitrogen flow for 1 h. The results were recorded by Micromeritics TriStar 3000 software. The t-plot for the RHs disclosed that the activation with NaOH leads to high specific surface (938 m2/g) and micropore area (588 m2/g) compared to other RH samples (367-386 m2/g). Simultaneously, the activation of RH increases the pore size of biochar and the highest pore size was observed for the RH-ALK (51 Å) compared to activated RH-S and RH-P (48 Å). This can be attributed to the silica removal from RAW-RH by alkaline solution that creates a mesoporous material which is more suitable as a catalyst support for bulky reactants. RAW-RH presents various peaks on its surface corresponding to different functional groups. During the high pyrolysis temperature (850oC) most of these peaks either disappear or begin to diminish and only few new peaks appear. Furthermore, RAW-RH has more acidic behavior than the other samples (pH 6.0). For the activated RH, RH-S and RH-P are slightly acidic materials (pH~6.2) and RH-ALK and W-RH are both slightly alkaline (pH~8.5). To conclude, activation with alkali solution creates a more desirable sorbent material since it increases the surface area by 260% compared to activation with acidic solutions which increases the surface area by ~5%. AcknowledgmentWe acknowledge support of this work by the project “Research Infrastructure on Food Bioprocessing Development and Innovation Exploitation – Food Innovation RI” (MIS 5027222), which is implemented under the Action “Reinforcement of the Research and Innovation Infrastructure”, funded by the Operational Programme "Competitiveness, Entrepreneurship and Innovation" (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund).

Biochar considered a carbonaceous material obtained from thermal treatment of unwanted biomass under oxygen limited conditions. This paper aims to offer an insight to further understand the effect of pyrolysis time on biochar physicochemical characteristics and sorption capacity for the removal of a model dye compound such as Methylene Blue (MB) from aqueous solutions. Biochars were produced from pyrolyzing untreated rice husk (RH) at 850oC for 1 (RH1), 2 (RH2), 4 (RH4), and 6 (RH6) h. Biochar yield, BET surface areas and pH values were monitored with pyrolysis time. Sorption experiments were performed with 3 mg of each biochar (sorbent) added in 20 mL of ΜΒ solution at a concentration of 20 mg/L. Biochar yield was reduced as the pyrolysis time increased. The highest biochar yield was 36% (RH1) and the lowest was observed for RH6 (26%). After 1 h of pyrolysis, the biochar yield is linearly decreased by 2% per h of extra pyrolysis. Biochar surface properties constitutes an important parameter for biochar applications such as catalysts supports or sorbents for water treatment. Based on the results, pyrolysis time is significant for these properties. Hence, the increase of pyrolysis time corresponds to an increase of pore volume and pore size. The t-plot disclose that the biochar pore volume increased from 0.15 to 0.28 cm3/g as the pyrolysis time also increased from 1 to 6 h. Apart of the significant changes of biochar porosity, a noteworthy increase of specific surface area (SSA) was not observed. The SSAs of the tested biochars were 280, 354, 393, and 386 m2/g for RH1, RH2, RH4, and RH6, respectively. Biochar produced from RH is alkaline in nature and as the pyrolysis time extended from 1 to 6 h the pH value was reduced, possibly due to the increase of ash content. More specifically, the pH values ranged from 10.3 (RH1) to 9.5 (RH6). At 24 h, the sorption capacities of RH1, RH2, RH4, and RH6 biochars were 6, 22, 32, and 38 mg/g, while at 8 days, that they reached equilibrium, the sorption capacities were increased to 16, 58, 125, and 127 mg/g, respectively. The sorption experiments disclosed the vital role of pyrolysis time on the sorption of MB. The RH biochars demonstrated different removal abilities which significantly increased as the pyrolysis time also increased from 1 to 4 h. RH4 and RH6 exhibited similar removal capacities, suggesting that 4 h pyrolysis time of untreated RH is enough time for yielding an optimum sorbent. ACKNOWLEDGMENT We acknowledge support of this work by the project “Research Infrastructure on Food Bioprocessing Development and Innovation Exploitation – Food Innovation RI” (MIS 5027222), which is implemented under the Action “Reinforcement of the Research and Innovation Infrastructure”, funded by the Operational Programme "Competitiveness, Entrepreneurship and Innovation" (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund).