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Third-generation biofuels that are derived from microalgal biomass have gained momentum as a way forward in the sustainable production of biodiesel. Such efforts are propelled by the intention to reduce our dependence on fossil fuels as the primary source of energy. Accordingly, growing microalgal biomass in the form of suspended cultivation has been a conventional technique for the past few decades. To overcome the inevitable harvesting shortcomings arising from the excessive energy and time needed to separate the planktonic microalgal cells from water medium, researchers have started to explore attached microalgal cultivation systems. This cultivation mode permits the ease of harvesting mature microalgal biomass, circumventing the need to employ complex harvesting techniques to single out the cells, and is economically attractive. However, the main bottleneck associated with attached microalgal growth is low biomass production due to the difficulties the microalgal cells have in forming attachment and populating thereafter. In this regard, the current review encompasses the novel techniques adopted to promote attached microalgal growth. The physicochemical effects such as the pH of the culture medium, hydrophobicity, as well as the substratum surface properties and abiotic factors that can determine the fate of exponential growth of attached microalgal cells, are critically reviewed. This review aims to unveil the benefits of an attached microalgal cultivation system as a promising harvesting technique to produce sustainable biodiesel for lasting applications.

Solvothermal treatment using glycerol effectively removes chlorine from polyvinyl chloride (PVC). Additive-free PVC was dechlorinated by treatment with glycerol in an autoclave at 200–240 °C. Liquid glycerol was coexistent with a red powder after the reaction at 200 °C. At reaction temperatures of 220 and 240 °C, the liquid glycerol disappeared, and carbonization proceeded to form a black powder. In this reaction, the glycerol was directly converted into glycidol. Epichlorohydrin was also produced from the glycerol via 1,3-dichloro-2-propanol as an intermediate. The PVC was converted into the polyethylene oxide structure, following the generation of the polyene structure by zip-elimination. Finally, the converted PVC was resinified by an epoxidation reaction with the glycidol and epichlorohydrin. Thus, glycerol acted as a useful reaction medium for this reaction system. The analysis of the decomposition ratio and dechlorination yield showed that the PVC was converted into solid products in high yields via chlorine release, and 30% of the glycerol was solidified by resinification at 240 °C. The effects of the additives on dechlorination were investigated using commercially available PVC sheets. Elemental analysis showed that the calorific value of the solid product dechlorinated at 240 °C was found to be higher than that of standard bituminous coal.

Attached microalgal growth of Chlorella vulgaris on palm decanter cake (PDC) under irradiation with various visible monochromatic and polychromatic spectra to produce biodiesel was studied in this work. The results demonstrated that the white spectrum cultivation exhibited the highest microalgal density of 1.13 g/g along with 1.213 g/L day of microalgal productivity. Correspondingly, the biodiesel obtained was comprised mainly of C16 and C18 fatty acids, possessing a high cetane number and oxidation stability from the high saturated fatty acid content (70.38%), which was appealing in terms of most biodiesel production requirements. Nevertheless, the highest lipid content (14.341%) and lipid productivity (93.428 mg/L per day) were discovered with green spectrum cultivation. Blue and white spectra led to similar protein contents (34%) as well as carbohydrate contents (61%), corroborating PDC as a feasible carbon and nutrient source for growing microalgae. Lastly, the energy feasibilities of growing the attached microalgae under visible spectra were investigated, with the highest net energy ratio (NER) of 0.302 found for the yellow spectrum. This value outweighed that in many other works which have used suspended growth systems to produce microalgal fuel feedstock. The microalgal growth attached to PDC is deemed to be a suitable alternative cultivation mode for producing sustainable microalgal feedstock for the biofuel industry.

The potentiality of a microalgal-bacterial culture system was explored in bioremediating wastewater while generating biomass for biodiesel production. A pre-determined optimal activated sludge and microalgal ratio was adopted and cultivation performance was evaluated in both synthetic and municipal wastewater media for nitrogen removal along with biomass and lipid generation for biodiesel production. The microalgal-bacterial consortium grown in the municipal wastewater medium produced higher biomass and lipid yields than those in the synthetic wastewater medium. The presence of trace elements in the municipal wastewater medium, e.g., iron and copper, contributed to the upsurge of biomass, thereby leading to higher lipid productivity. Both the microbial cultures in the synthetic and municipal wastewater media demonstrated similar total nitrogen removal efficiencies above 97%. However, the nitrification and assimilation rates were relatively higher for the microbial culture in the municipal wastewater medium, corresponding to the higher microbial biomass growth. Accordingly, the feasibility of the microalgal-bacterial consortium for bioremediating real municipal wastewaters was attested in this study by virtue of higher biomass and lipid production. The assessment of fatty acid methyl esters (FAME) composition showed the mixed microbial biomasses comprised 80–93% C16 to C18 FAME species, signifying efficient fuel combustion properties for quality biodiesel requirements.

The catalytic potential of biochar, a new carbonaceous material with robust adsorption capabilities, is being recognized by environmental applications more and more. This thorough analysis explores the core ideas behind the processes that produce persistent free radicals (PFRs) during biomass pyrolysis as well as the different detection techniques used to find PFRs. A thorough analysis shows how certain pyrolysis parameters, such as temperature (between 300 and 600 °C) and heating rate (between 5 and 50 °C/min), in addition to the degradation of lignin (which makes up 15–35% of biomass) and other biomass components, affect the synthesis of PFR in biochar. The article provides a thorough examination of many PFR detection techniques, such as electron paramagnetic resonance (EPR) spectroscopy, which has shown 70% detection effectiveness. Furthermore, it investigates the biochar-PFRs’ catalytic potential in environmental remediation, emphasizing how well they work in redox systems, sophisticated oxidation processes, and photocatalytic applications. With research demonstrating up to 80% degradation of pollutants and a 90% decrease of metal ions, the review highlights the usefulness of biochar-PFRs in pollution management and water treatment. The assessment also discusses important future directions and difficulties, highlighting the need for ongoing developments to fully use PFRs for environmentally friendly solutions. Understanding the redox behavior of metal ions in biochar-PFR systems and scaling up applications for wider effects are two of the main issues that have been highlighted. Graphical Abstract