A review on hydrothermal carbonization of potential biomass wastes, characterization and environmental applications of hydrochar, and biorefinery perspectives of the process

It is imperative to search for appropriate processes to convert wastes into energy, chemicals, and materials to establish a circular bio-economy toward sustainable development. Concerning waste biomass valorization, hydrothermal carbonization (HTC) is a promising route given its advantages over other thermochemical processes. From that perspective, this article reviewed the HTC of potential biomass wastes, the characterization and environmental utilization of hydrochar, and the biorefinery potential of this process. Crop and forestry residues and sewage sludge are two categories of biomass wastes (lignocellulosic and non-lignocellulosic, respectively) readily available for HTC or even co-hydrothermal carbonization (Co-HTC). The temperature, reaction time, and solid-to-liquid ratio utilized in HTC/Co-HTC of those biomass wastes were reported to range from 140 to 370 °C, 0.05 to 48 h, and 1/47 to 1/1, respectively, providing hydrochar yields of up to 94 % according to the process conditions. Hydrochar characterization by different techniques to determine its physicochemical properties is crucial to defining the best applications for this material. In the environmental field, hydrochar might be suitable for removing pollutants from aqueous systems, ameliorating soils, adsorbing atmospheric pollutants, working as an energy carrier, and performing carbon sequestration. But this material could also be employed in other areas (e.g., catalysis). Regarding the effluent from HTC/Co-HTC, this byproduct has the potential for serving as feedstock in other processes, such as anaerobic digestion and microalgae cultivation. These opportunities have aroused the industry interest in HTC since 2010, and the number of industrial-scale HTC plants and patent document applications has increased. The hydrochar patents are concentrated in China (77.6 %), the United States (10.6 %), the Republic of Korea (3.5 %), and Germany (3.5 %). Therefore, considering the possibilities of converting their product (hydrochar) and byproduct (effluent) into energy, chemicals, and materials, HTC or Co-HTC could work as the first step of a biorefinery. And this approach would completely agree with circular bioeconomy principles.

Effect of sequential acid-alkaline treatment on physical and chemical characteristics of lignin and cellulose from pine (Pinus spp.) residual sawdust

Timber industry generates large amounts of residues such as sawdust. Softwoods have a significant economic value for timber production and the Pinus genus is widely utilized. Thus, the aim of this work was to study the hemicellulose extraction and lignin recovery from pine (Pinus spp.) residual sawdust (PRS) by sequential acid-alkaline treatment, generating a cellulose-rich solid fraction. The hemicellulose removed was 87.11% (wt·wt^-1) after dilute acid treatment at 130 °C, 4.5% (wt·wt^-1) of H₂SO₄ for 20 min at 120 rpm. Three temperatures were evaluated for recovering the lignin and the highest yield, 93.97% (wt·wt^-1), was achieved at 170 °C, 10% (wt·wt^-1) of NaOH for 90 min at 120 rpm. Lignin was characterized by Fourier-transform infrared spectroscopy, nuclear magnetic resonance and thermogravimetry. The resulting cellulose-rich fraction exhibited polymorphic transformation. The results demonstrated that PRS is a promising lignocellulosic residue whose lignin and carbohydrates can be readily obtained.

Biochar and hydrochar in the context of anaerobic digestion for a circular approach: An overview

Biochar and hydrochar are carbonaceous materials with valuable applications. They can be synthesized from a wide range of organic wastes, including digestate. Digestate is the byproduct of anaerobic digestion (AD), which is performed for bioenergy (biogas) production from organic residues. Through a thermochemical process, such as pyrolysis, gasification, and hydrothermal carbonization - HTC, digestate can be converted into biochar or hydrochar. The addition of either biochar or hydrochar in AD has been reported to improve biochemical reactions and microbial growth, increasing the buffer capacity, and facilitating direct interspecies electrons transfer (DIET), resulting in higher methane (CH₄) yields. Both biochar and hydrochar can adsorb undesired compounds present in biogas, such as carbon dioxide (CO₂), hydrogen sulfide (H₂S), ammonia (NH₃), and even siloxanes. However, an integrated understanding of biochar and hydrochar produced from digestate through their return to the AD process, as additives or as adsorbents for biogas purification, is yet to be attained to close the material flow loop in a circular economy model. Therefore, this overview aimed at addressing the integration of biochar and hydrochar production from digestate, their utilization as additives and effects on AD, and their potential to adsorb biogas contaminants. This integration is supported by life cycle assessment (LCA) studies, showing positive results when combining AD and the aforementioned thermochemical processes, although more LCA is still necessary. Techno-economic assessment (TEA) studies of the processes considered are also presented, and despite an expanding market of biochar and hydrochar, further TEA is required to verify the profitability of the proposed integration, given the specificities of each process design. Overall, the synthesis of biochar and hydrochar from digestate can contribute to improving the AD process, establishing a cyclic process that is in agreement with the circular economy concept.

Co-hydrothermal carbonization of pine residual sawdust and non-dewatered sewage sludge - effect of reaction conditions on hydrochar characteristics

Waste valorization is mandatory to develop and consolidate a circular bioeconomy. It is necessary to search for appropriate processes to add value to different wastes by utilizing them as feedstocks to provide energy, chemicals, and materials. For instance, hydrothermal carbonization (HTC) is an alternative thermochemical process that has been suggested for waste valorization aiming at hydrochar production. Thus, this study proposed the Co-HTC of pine residual sawdust (PRS) with non-dewatered sewage sludge (SS) - two wastes largely produced in sawmills and wastewater treatment plants, respectively - without adding extra water. The influence of temperature (180, 215, and 250 °C), reaction time (1, 2, and 3 h), and PRS/SS mass ratio (1/30, 1/20, and 1/10) on the yield and characteristics of the hydrochar were evaluated. The hydrochars obtained at 250 °C had the best coalification degree, showing the highest fuel ratio, high heating value (HHV), surface area, and N, P, and K retention, although presenting the lowest yields. Conversely, hydrochar functional groups were generally reduced by increasing Co-HTC temperatures. Regarding the Co-HTC effluent, it presented acidic pH (3.66-4.39) and high COD values (6.2-17.3 g·L^-1). In general, this new approach could be a promising alternative to conventional HTC, in which a high amount of extra water is required. Besides, the Co-HTC process can be an option for managing lignocellulosic wastes and sewage sludges while producing hydrochar. This carbonaceous material has the potential for several applications, and its production is a step towards a circular bioeconomy.

A review of plastic debris in the South American Atlantic Ocean coast - Distribution, characteristics, policies and legal aspects

The presence of plastics in the oceans has already become a pervasive phenomenon. Marine pollution by plastics surpasses the status of an emerging threat to become a well-established environmental problem, boosting research on this topic. However, despite many studies on the main seas and oceans, it is necessary to compile information on the South American Atlantic Ocean Coast to identify the lack of research and expand knowledge on marine plastic pollution in this region. Accordingly, this paper conducted an in-depth review of monitoring methods, sampling, and identification of macroplastics and microplastics (MPs) in water, sediments, and biota, including information on legal requirements from different countries as well as non-governmental initiatives. Brazil was the country with the highest number of published papers, followed by Argentina. MPs accounted for 75 % of the papers selected, with blue microfibers being the most common morphology, whereas PE and PP were the most abundant polymers. Also, a lack of standardization in the methodologies used was identified; however, the sites with the highest concentrations of MPs were the Bahía Blanca Estuary (Argentina), Guanabara Bay (Brazil), and Todos os Santos Bay (Brazil), regardless of the method applied. Regarding legislation, Uruguay and Argentina have the most advanced policies in the region against marine plastic pollution due to their emphasis on the life cycle and the national ban on certain single-use plastics. Therefore, considering its content, this expert review can be useful to assist researchers dealing with plastic pollution along the South American Atlantic Ocean Coast.

Hydrochar from sawdust and sewage sludge - a potential media for retaining heavy metals in sustainable drainage systems (SuDS)

Waste valorization is an essential aspect of sustainable development. From this perspective, co-hydrothermal carbonization (Co-HTC) is a promising thermochemical process for converting organic waste into hydrochar. Hydrochar is a solid material whose physicochemical properties could make it suitable for adsorbing pollutants such as heavy metals. Accordingly, this work evaluated the hydrochar from Co-HTC of sawdust and non-dewatered sewage sludge as a potential adsorbent of heavy metals at low concentrations. In the context of sustainable drainage systems (SuDS), it is notable that heavy metals are present at very low but still potentially harmful concentrations, which presents a potential opportunity for the application of hydrochar. Thus, three hydrochars (H-180, H-215, and H-250), produced by Co-HTC at 180, 215, and 250 °C, were tested herein for their ability to retain lead (Pb2+). The H-180 presented better performance than other hydrochars (H-215 and H-250), suggesting that chemisorption could be the main adsorption mechanism. Interestingly, the presence of other cationic heavy metals (Cu2+, Zn2+, Cd2+, Cr6+, and Ni2+) did not hinder the Pb2+ adsorption, for which the removal efficiency remained close to 100%. In fact, in such a multi-metal system, hydrochar can be suitable for capturing both lead and cadmium. Therefore, the hydrochar from Co-HTC of sawdust and non-dewatered sewage sludge can be useful for removing heavy metals at low concentrations, such as those found in urban runoff waters. Although further studies are required, these findings suggest hydrochar as a potential material for application in SuDS.