Hydrothermal Carbon as Reactive Fillers to Produce Sustainable Biocomposites with Aromatic Bio-Based Epoxy Resins

Sewage sludge hydrochar characterization and valorization via hydrothermal processing for 3D printing

Sewage sludge, a biosolid product of wastewater processing, is an often-overlooked source of rich organic waste. Hydrothermal processing has shown promise in converting sewage sludge into valorized materials with potential application in biofuels, asphalt binders, and bioplastics. Here we characterize the physicochemical properties of hydrochar, the carbonaceous solid phase product of hydrothermal processing, and investigate its use as bio-based filler in additive manufacturing. We find that the presence of metallic and metalloid dopants in sewage sludge, which are not typically found in biomass wastes, yields unusual results in organic material processing such as decreased graphitic ordering after thermal activation. We further find that addition of hydrochar generally decreases mechanical performance of additive manufacturing composites, however, some properties such as toughness can be recovered with nature-inspired architecting into gyroid microstructures. These findings demonstrate that more investigation is required to optimally valorize sewage sludge and similarly disordered waste streams.

Tailoring the porosity of chemically activated carbons derived from the HTC treatment of sewage sludge for the removal of pollutants from gaseous and aqueous phases

The management of sewage sludge is currently an open issue due to the large volume of waste to be treated and the necessity to avoid incineration or landfill disposal. Hydrothermal carbonization (HTC) has been recognized as a promising thermochemical technique to convert sewage sludge into value-added products. The hydrochar (HC) obtained can be suitable for environmental application as fuel, fertilizer, and sorbent. In this study, activated hydrochars (AHs) were prepared from sewage sludge through HTC followed by chemical activation with potassium hydroxide (KOH) and tested for the removal of pollutants in gaseous and aqueous environments, investigating carbon dioxide (CO2) and ciprofloxacin (CIP) adsorption capacity. The effects of activation temperature (550-750 °C) and KOH/HC impregnation ratio (1-3) on the produced AHs morphology and adsorption capacity were studied by Response Surface Methodology (RSM). The results of RSM analysis evidenced a maximum CO2 uptake of 71.47 mg/g for mild activation conditions (600-650 °C and KOH/HC = 1 ÷ 2), whereas the best CIP uptake of 628.61 mg/g was reached for the most severe conditions (750 °C, KOH/HC = 3). The prepared AHs were also applied for the removal of methylene blue (MB) from aqueous solutions, and the MB uptake results were used for estimating the specific surface area of AHs. High surface areas up to 1902.49 m2/g were obtained for the highest activation temperature and impregnation ratio investigated. Predictive models of CO2 and CIP uptake were developed by RSM analysis, and the optimum activation conditions for maximizing the adsorption performance together with high AH yield were identified: 586 °C and KOH/HC ratio = 1.34 for maximum yield (26.33 %) and CO2 uptake (67.31 mg/g); 715 °C and KOH/HC ratio = 1.78 for maximum yield (18.75 %) and CIP uptake (370.77 mg/g). The obtained results evidenced that chemical activation of previously HTC-treated sewage sludge is a promising way to convert waste into valuable low-cost adsorbents.

Natural Rubber Composites Using Hydrothermally Carbonized Hardwood Waste Biomass as a Partial Reinforcing Filler- Part I: Structure, Morphology, and Rheological Effects during Vulcanization

A new generation biomass-based filler for natural rubber, 'hydrochar' (HC), was obtained by hydrothermal carbonization of hardwood waste (sawdust). It was intended as a potential partial replacement for the traditional carbon black (CB) filler. The HC particles were found (TEM) to be much larger (and less regular) than CB: 0.5-3 µm vs. 30-60 nm, but the specific surface areas were relatively close to each other (HC: 21.4 m2/g vs. CB: 77.8 m2/g), indicating a considerable porosity of HC. The carbon content of HC was 71%, up from 46% in sawdust feed. FTIR and 13C-NMR analyses indicated that HC preserved its organic character, but it strongly differs from both lignin and cellulose. Experimental rubber nanocomposites were prepared, in which the content of the combined fillers was set at 50 phr (31 wt.%), while the HC/CB ratios were varied between 40/10 and 0/50. Morphology investigations proved a fairly even distribution of HC and CB, as well as the disappearance of bubbles after vulcanization. Vulcanization rheology tests demonstrated that the HC filler does not hinder the process, but it significantly influences vulcanization chemistry, canceling scorch time on one hand and slowing down the reaction on the other. Generally, the results suggest that rubber composites in which 10-20 phr of CB are replaced by HC might be promising materials. The use of HC in the rubber industry would represent a high-tonnage application for hardwood waste.

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.

A Sustainable Approach on Spruce Bark Waste Valorization through Hydrothermal Conversion

In the context of sustainable use of resources, hydrothermal conversion of biomass has received increased consideration. As well, the hydrochar (the solid C-rich phase that occurs after the process) has caused great interest. In this work, spruce bark (Picea abies) wastes were considered as feedstock and the influence of hydrothermal process parameters (temperature, reaction time, and biomass to water ratio) on the conversion degree has been studied. Using the response surface methodology and MiniTab software, the process parameters were set up and showed that temperature was the significant factor influencing the conversion, while residence time and the solid-to-liquid ratio had a low influence. Furthermore, the chemical (proximate and ultimate analysis), structural (Fourier-transform infrared spectroscopy, scanning electron microscopy) and thermal properties (thermogravimetric analysis) of feedstock and hydrochar were analyzed. Hydrochar obtained at 280 °C, 1 h processing time, and 1/5 solid-to-liquid ratio presented a hydrophobic character, numerous functional groups, a lower O and H content, and an improved C matter, as well as a good thermal stability. Alongside the structural features, these characteristics endorsed this waste-based product for applications other than those already known as a heat source.

Gap-Fillers for Wooden Artefacts Exposed Outdoors—A Review

Conservation of wooden artefacts that are exposed outdoors, mainly in open-air museums, is a very complex and difficult issue that aims to preserve both the integrity and aesthetics of valuable objects. Unceasingly subjected to several factors, such as alternating weather conditions and the activities of microorganisms, algae, and insects, they undergo continuous changes and inevitable deterioration. Their biological and physical degradation often results in the formation of gaps and cracks in the wooden tissue, which creates a need not only for wood consolidation, but also for using specialist materials to fill the holes and prevent further degradation of an object. To ensure effective protection for a wooden artefact, a filling material must both protect the wood against further degradation and adapt to changes in wood dimensions in response to humidity variations. A variety of substances, both organic and inorganic, have been used for conservation and gap filling in historic wooden objects over the years. The filling compounds typically consist of two components, of which one is a filler, and the second a binder. In the case of inorganic fillers, plaster has been traditionally used, while the most popular organic fillers were wood powder, wood shavings, and powdered cork. As with binders, mainly natural substances have been used, such as animal glues or waxes. Nowadays, however, due to the lower biodegradability and better physicochemical properties, synthetic materials are gaining popularity. This article discusses the types of filling compounds currently used for gap filling in wooden artefacts exposed outdoors, outlining their advantages and drawbacks, as well as future perspective compounds. It appears that particularly composite materials based on natural polymers deserve attention as promising filling materials due to their high elasticity, as well as similarity and good adhesion to the wooden surface. Their main shortcomings, such as susceptibility to biodegradation, could be eliminated by using some modern, bio-friendly preservatives, providing effective protection for historic wooden artefacts.