An Insight into Valorization of Lignocellulosic Biomass by Optimization with the Combination of Hydrothermal (HT) and Biological Techniques: A Review

Biomass valorization plays a significant role in the production of biofuels and various value-added biochemicals, in addition to lowering greenhouse gas emissions. In terms of biorefining methods, hydrothermal (HT) and biological techniques have demonstrated the capability of valorizing biomass raw materials to yield value added end-products. An inter-disciplinary bio-economical approach is capable of optimizing biomass’s total potential in terms of environmental perspective and circular bioeconomy standpoint. The aim of this review is to provide an in-depth overview of combinatorial HT and biological techniques to maximize biomass value, which includes biological valorization following HT pretreatment and HT valorization of lignocellulosic substrates emanating from biocatalytic hydrolysis/anaerobic digestion and/or pretreated food waste for the ultimate yield of biogas/biochar and biocrude. In this study, we discuss recent advances regarding HT and biological treatment conditions, synergies between the two technologies, and optimal performance. Additionally, energy balances and economic feasibility assessments of alternative integrated solutions reported in previous studies are compared. Furthermore, we conclude by discussing the challenges and opportunities involved in integrating HT and biologicals methods toward complete biomass utilization.

Evaluating the Ability of Bone Char/nTiO2 Composite and UV Radiation for Simultaneous Oxidation and Adsorption of Arsenite

The reuse of waste materials for water treatment purposes is an important approach for promoting the circular economy and achieving effective environmental remediation. This study examined the use of bone char/titanium dioxide nanoparticles (BC/nTiO2) composite and UV for As(III) and As(V) removal from water. The composite was produced via two ways: addition of nTiO2 to bone char during and after pyrolysis. In comparison to the uncoated bone char pyrolyzed at 900 °C (BC900), nTiO2 deposition onto bone char led to a decrease in the specific surface area and pore volume from 69 to 38 m2/g and 0.23 to 0.16 cm3/g, respectively. However, the pore size slightly increased from 14 to 17 nm upon the addition of nTiO2. The composite prepared during pyrolysis (BC/nTiO2)P had better As removal than that prepared after pyrolysis with the aid of ultrasound (BC/nTiO2)US (57.3% vs. 24.8%). The composite (BC/nTiO2)P had higher arsenate oxidation than (BC/nTiO2)US by about 3.5 times. Arsenite oxidation and consequent adsorption with UV power of 4, 8 and 12 W was examined and benchmarked against the composite with visible light and BC alone. The highest UV power was found to be the most effective treatment with adsorption capacity of 281 µg/g followed by BC alone (196 µg/g). This suggests that the effect of surface area and pore volume loss due to nTiO2 deposition can only be compensated by applying a high level of UV power.

Efficient Extraction of the RuBisCO Enzyme from Spinach Leaves Using Aqueous Solutions of Biocompatible Ionic Liquids

Ribulose-1,5-biphosphate carboxylase/oxygenase (RuBisCO) is the most abundant protein on the planet, being present in plants, algae and various species of bacteria, with application in the pharmaceutical, chemical, cosmetic and food industries. However, current extraction methods of RuBisCO do not allow high yields of extraction. Therefore, the development of an efficient and selective RuBisCOs’ extraction method is required. In this work, aqueous solutions of biocompatible ionic liquids (ILs), i.e., ILs derived from choline and analogues of glycine-betaine, were applied in the RuBisCO’s extraction from spinach leaves. Three commercial imidazolium-based ILs were also investigated for comparison purposes. To optimize RuBisCO’s extraction conditions, response surface methodology was applied. Under optimum extraction conditions, extraction yields of 10.92 and 10.57 mg of RuBisCO/g of biomass were obtained with the ILs cholinium acetate ([Ch][Ac]) and cholinium chloride ([Ch]Cl), respectively. Circular dichroism (CD) spectroscopy results show that the secondary structure of RuBisCO is better preserved in the IL solutions when compared to the commonly used extraction solvent. The obtained results indicate that cholinium-based ILs are a promising and viable alternative for the extraction of RuBisCO from vegetable biomass.

Coumarin 153 Dynamics in Ethylammonium Nitrate: The Effects of Dilution with Methanol

Magic angle intensity decay and dynamic fluorescence anisotropy measurements were made on the binary solvent system composed of ethylammonium nitrate ([N2,0,0,0+][NO3−], EAN) + methanol (MeOH) across the complete EAN mole fraction range (xIL = 0–1) using the neutral dipolar solute coumarin 153 (C153) at 295 K. Stokes–Einstein–Debye (SED) hydrodynamic theory was used as a model framework to assess the C153 rotational reorientation dynamics. Departure from stick SED prediction was observed (in contrast to literature reports that used cationic or anionic dyes) and indicated a significant influence of domain nanoheterogeneity on probe dynamics. Steady-state spectroscopy indicated minimal changes in spectral peak and width with mole fraction, except at xIL = 0.3 where absorption widths decreased by ~170 cm−1, signaling that C153 sensed a change in solution heterogeneity. Magic angle intensity decays corroborated the steady-state observation and the excited-state lifetimes showed a marked change from xIL = 0.2–0.4 where EAN-EAN interactions became notably more significant. C153 average rotation times (⟨τrot⟩) showed significant solvent decoupling with increased EAN. The rotational data were fit to a power law dependence, ⟨τrot⟩ ∝ (ηT)p, where p = 0.82, demonstrating the presence of dynamic heterogeneity in the EAN/MeOH solutions. With increased EAN, rotation times showed that the heterogeneity became increasingly more significant since the rotation times systematically decreased away from the hydrodynamic stick limit.

Chlorophylls Extraction from Spinach Leaves Using Aqueous Solutions of Surface-Active Ionic Liquids

Chlorophylls and their derivatives have been extensively studied due to their unique and valuable properties, including their anti-mutagenic and anti-carcinogenic features. Nevertheless, high-purity-level chlorophylls extracted from natural sources are quite expensive because the methods used for their extraction have low selectivity and result in low yields. This study aimed to develop a “greener” and cost-effective technology for the extraction of chlorophylls from biomass using aqueous solutions of ionic liquids (ILs). Several aqueous solutions of ILs, with hydrotropic and surface-active effects were evaluated, demonstrating that aqueous solutions of surface-active ILs are enhanced solvents for the extraction of chlorophylls from spinach leaves. Operating conditions, such as the IL concentration and solid–liquid ratio, were optimized by a response surface methodology. Outstanding extraction yields (0.104 and 0.022 wt.% for chlorophyll a and b, respectively, obtained simultaneously) and selectivity (chlorophyll a/b ratio of 4.79) were obtained with aqueous solutions of hexadecylpyridinium chloride ([C16py]Cl) at moderate conditions of temperature and time. These extraction yields are similar to those obtained with pure ethanol. However, the chlorophyll a/b ratio achieved with the IL aqueous solution is higher than with pure ethanol (3.92), reinforcing the higher selectivity afforded by IL aqueous solutions as viable replacements to volatile organic compounds and allowing the obtainment of more pure compounds. Finally, the recovery and reuse of the solvent were evaluated by using a back-extraction step of chlorophylls using ethyl acetate. The results disclosed here bring new perspectives into the design of new approaches for the selective extraction of chlorophylls from biomass using aqueous solutions of surface-active ILs.

Hydrocarbon Compatible SOFC Anode Catalysts and Their Syntheses: A Review

With the fast depleting rate of fossil fuels, the whole world is looking for promising energy sources for the future, and fuel cells are perceived as futuristic energy sources. Out of the different varieties of fuel cells, solid oxide fuel cells (SOFCs) are promising due to their unique multi-fuel operating capability without the need for an external reformer. Nonetheless, the state-of-the-art anode material Ni–YSZ undergoes carburization in presence of hydrocarbons (HCs), resulting in performance degradation. Several strategies have been explored by researchers to overcome the issue of carburization of the anode. The important strategies include reducing SOFC operating temperature, adjustment of steam: carbon ratio, and use of alternate anode catalysts. Among these, the use of alternate anodes is a promising strategy. Apart from the carburization issue, the anode can also undergo sulfur poisoning. The present review discusses carburization and sulfur poisoning issues and the different strategies that can be adopted for tackling them. The quintessence of this review is to provide greater insight into the various developments in hydrocarbon compatible anode catalysts and into the synthesis routes employed for the synthesis of hydrocarbon compatible anodes.

Technospheric Mining of Mine Wastes: A Review of Applications and Challenges

The concept of mining or extracting valuable metals and minerals from technospheric stocks is referred to as technospheric mining. As potential secondary sources of valuable materials, mining these technospheric stocks can offer solutions to minimise the waste for final disposal and augment metals’ or minerals’ supply, and to abate environmental legacies brought by minerals’ extraction. Indeed, waste streams produced by the mining and mineral processing industry can cause long-term negative environmental legacies if not managed properly. There are thus strong incentives/drivers for the mining industry to recover and repurpose mine and mineral wastes since they contain valuable metals and materials that can generate different applications and new products. In this paper, technospheric mining of mine wastes and its application are reviewed, and the challenges that technospheric mining is facing as a newly suggested concept are presented. Unification of standards and policies on mine wastes and tailings as part of governance, along with the importance of research and development, data management, and effective communication between the industry and academia, are identified as necessary to progress technospheric mining to the next level. This review attempts to link technospheric mining to the promotion of environmental sustainability practices in the mining industry by incorporating green technology, sustainable chemistry, and eco-efficiency. We argue that developing environmentally friendly processes and green technology can ensure positive legacies from the mining industry. By presenting specific examples of the mine wastes, we show how the valuable metals or minerals they contain can be recovered using various metallurgical and mineral processing techniques to close the loop on waste in favour of a circular economy.

Synthesis of Silica-Based Materials Using Bio-Residues through the Sol-Gel Technique

This study focused on the use of citrus bio-waste and obtention of silica-based materials through the sol-gel technique for promoting a greener and more sustainable catalysis. The sol-gel method is a versatile synthesis route characterized by the low temperatures the materials are synthesized in, which allows the incorporation of organic components. This method is carried out by acid or alkali hydrolysis combined with bio-waste, such as orange and lemon peels, generated as co-products in the food processing industry. The main objective was to obtain silica-based materials from the precursor TEOS with different catalysts—acetic, citric and hydro-chloric acids and ammonium hydroxide—adding different percentages of lemon and orange peels in order to find the influence of bio-waste on acids/alkali precursor hydrolysis. This was to partially replace these catalysts for orange or lemon peels. The solids obtained were characterized with different techniques, such as SEM, FT₋IR, potentiometric titration and XRD. SEM images were compared with pure silica obtained to contrast the morphology of the acidic and alkali hydrolysis. However, until now, few attempts have been made to highlight the renewability of reagents used in the synthesis or to incorporate bio-based catalytic processes on larger scales.

A New Method for Solid Acid Catalyst Evaluation for Cellulose Hydrolysis

A systematic and structure-agnostic method for identifying heterogeneous activity of solid acids for catalyzing cellulose hydrolysis is presented. The basis of the method is preparation of a supernatant liquid by exposing the solid acid to reaction conditions and subsequent use of the supernatant liquid as a cellulose hydrolysis catalyst to determine the effects of in situ generated homogeneous acid species. The method was applied to representative solid acid catalysts, including polymer-based, carbonaceous, inorganic, and bifunctional materials. In all cases, supernatant liquids produced from these catalysts exhibited catalytic activity for cellulose hydrolysis. Direct comparison of the activity of the solid acid catalysts and their supernatants could not provide unambiguous detection of heterogeneous catalysis. A reaction pathway kinetic model was used to evaluate potential false-negative interpretation of the supernatant liquid test and to differentiate heterogeneous from homogeneous effects on cellulose hydrolysis. Lastly, differences in the supernatant liquids obtained in the presence and absence of cellulose were evaluated to understand possibility of false-positive interpretation, using structural evidence from the used catalysts to gain a fresh understanding of reactant–catalyst interactions. While many solid acid catalysts have been proposed for cellulose hydrolysis, to our knowledge, this is the first effort to attempt to differentiate the effects of heterogeneous and homogeneous activities. The resulting supernatant liquid method should be used in all future attempts to design and develop solid acids for cellulose hydrolysis.

Use of Pyrolyzed Soybean Hulls as Fillers in Polypropylene and Linear Low Density Polyethylene

In the competitive market of plastic fillers, inexpensive and reliable materials are always sought after. Using a method of thermal conversion called pyrolysis, a potential contender was created from a plant biomass known as soybean hulls (SBH). SBH are a byproduct of the soybean farming industry and represent an abundant and inexpensive feedstock. The thermal conversion of SBH material gives rise to a lightweight carbon-rich filler called pyrolyzed soybean hulls (PSBH). We created two separate lots, lots A and B, with lot A corresponding to SBH pyrolyzed at 450 °C (PSBH-A) and lot B corresponding to SBH pyrolyzed at 500 °C (PSBH-B). Both lots of PSBH were also milled to reduce their particle size and tested against the as-received PSBH fillers. These milled materials were designated as ground soybean hulls (GSBH). Two different polyolefins, linear low-density polyethylene (LLDPE) and polypropylene (PP), were used for this study. The PSBH fillers were added to the polyolefins in weight percentages of 10%, 20%, 30%, 40%, and 50%, with the resulting plastic/PSBH composites being tested for their mechanical, thermal, and water absorption properties. In general, the addition of filler increased the maximum stress of the LLDPE/PSBH composites while reducing maximum stress of the PP/PSBH composites. The strain at maximum stress was reduced with increasing amounts of the PSBH filler for all composites. The modulus of elasticity generally increased with increasing filler amount. For thermal properties, the addition of the PSBH filler increased the heat distortion temperature, increased the thermal decomposition temperature, and reduced the heat of fusion of the composites compared to the neat polyolefins. The liquid absorption and thickness swelling in the materials were small overall but did increase with increasing amounts of the PSBH filler and with the time spent submerged in liquid. Milling the PSBH material into GSBH generally had small effects on the various tested material properties and led to easier mixing and a smoother finish on the surface of processed samples. The differences observed between lot A and lot B composites were often small or even negligible.