Microalgae Recovery from Water for Biofuel Production Using CO2-Switchable Crystalline Nanocellulose

Computational Insights into a CO2-Responsive Emulsion Prepared Using the Superamphiphile Assembled by Electrostatic Interactions

The superamphiphiles exhibit broad prospects for fabricating stimuli-responsive emulsions. Because the superamphiphiles are assembled via noncovalent interactions, they have the advantage of fast response and high efficiency. Recently, a series of switchable emulsions using CO2-responsive superamphiphiles have been developed, which extends the applications of CO2-responsive materials in widespread field. However, there is still a lack of fundamental understanding on the switching mechanism related to the assembled structure of superamphiphiles at the oil-water interface. We employed molecular dynamics (MD) simulations to investigate the reversible emulsification/demulsification process of a responsive emulsion system stabilized by a recently developed responsive superamphiphile (BTOA), which consists of oleic acid (OA) and cationic amine (named 1,3-bis(aminopropyl)tetramethyldisiloxane, BT). The simulation results present the morphologies in both the emulsion and demulsification states. It is found that the ionized OA- and the protonated BT+ together form an adsorption layer at the oil-water interface. The hydrophobic parts of BT+ are inserted into the adsorption layer, and the two amine groups contact the water phase. This adsorption layer reduces the interfacial tension and stabilizes the emulsion. After the bubbling of CO2, the surfactants were fully protonated to OA and BT2+. Because of the changes in the molecular polarity, OA and BT2+ entered the oil and water phases, respectively, resulting in demulsification. The structural and dynamical properties were analyzed to reveal the different intermolecular interactions that were responsible for the reversible reversibility of the emulsion. The observations are considered to be complementary to experimental studies and are expected to provide deeper insights into studies on developing responsive materials via supramolecular assemblies.

Effect of free ammonia shock on Chlorella sp. in wastewater: Concentration-dependent activity response and enhanced settleability

The unstable microbial activity and unsatisfactory settling performance impede the development and implementation of microalgal wastewater treatment, especially in high-ammonium wastewater in the presence of free ammonia (FA). The shock of FA due to the nutrient fluctuation in wastewater was demonstrated as the primary stress factor suppressing microalgal activities. Recent study has clearly revealed the inhibition mechanism of FA at a specific high level (110.97 mg/L) by inhibiting the genetic information processing, photosynthesis, and nutrient metabolism. However, the effects of various FA shock concentrations on microalgal activities and settling performance remain unknown, limiting the wastewater bioremediation efficiencies improvement and the process development. Herein, a concentration-dependent shock FA (that was employed on microalgae during their exponential growth stages) effect on microalgal growth and photosynthesis was observed. Results showed that the studied five FA shock concentrations ranging from 25 to 125 mg/L significantly inhibited biomass production by 14.7-57.0%, but sharp reductions in photosynthesis with the 36.0-49.0% decreased Fv/Fm values were only observed when FA concentration was above 75.0 mg/L. On the other hand, FA shock enhanced microalgal settling efficiency by 12.8-fold, which was believed to be due to the stimulated intra- and extracellular protein contents and thereby the enhanced extracellular polymer substances (EPS) secretion. Specifically, FA shock induced 40.2 ± 2.3% higher cellular protein content at the cost of the decreased carbohydrates (22.6 ± 1.3%) and fatty acid (39.0 ± 0.8%) contents, further improving the protein secretion by 1.21-fold and the EPS production by 40.2 ± 2.3%. These FA shock-induced variations in intra- and extracellular biomolecules were supported by the up-regulated protein processing and export at the assistance of excessive energy generated from fatty acid degradation and carbohydrates consumption. In addition, FA shock significantly decreased the biomass nutritional value as indicated by the 1.86-fold lower essential amino acid score and nearly 50% reduced essential to non-essential amino acids ratio, while slightly decreased the biodiesel quality. This study is expected to enrich the knowledge of microalgal activities and settling performance in response to fluctuant ammonium concentrations in wastewater and to promote the development of microalgal wastewater treatment.

Instant Inhibition and Subsequent Self-Adaptation of Chlorella sp. Toward Free Ammonia Shock in Wastewater: Physiological and Genetic Responses

Free ammonia (FA) has been recently demonstrated as the primary stress factor suppressing microalgal activities in high-ammonium wastewater. However, its inhibition mechanism and microalgal self-adaptive regulations remain unknown. This study revealed an initial inhibition and subsequent self-adaptation of a wastewater-indigenous Chlorella sp. exposed to FA shock. Mutual physiological and transcriptome analysis indicated that genetic information processing, photosynthesis, and nutrient metabolism were the most influenced metabolic processes. Specifically, for the inhibition behavior, DNA damage was indicated by the significantly up-regulated related genes, leading to the activation of cell cycle checkpoints, programmed apoptosis, and suppressed microalgal growth; FA shock inhibited the photosynthetic activities including both light and dark reactions and photoprotection through non-photochemical quenching; ammonium uptake was also suppressed with the inhibited glutamine synthetase/glutamine α-oxoglutarate aminotransferase cycle and the inactivated glutamate dehydrogenase pathway. With respect to microalgal self-adaptation, DNA damage possibly enhanced overall cell viability through reprogramming the cell fate; recovered nutrient uptake provided substances for self-adaptation activities including amino acid biosynthesis, energy production and storage, and genetic information processing; elevated light reactions further promoted self-adaptation through photodamage repair, photoprotection, and antioxidant systems. These findings enrich our knowledge of microalgal molecular responses to FA shock, facilitating the development of engineering optimization strategies for the microalgal wastewater bioremediation system.

Nanocellulose for Sustainable Water Purification

Nanocelluloses (NC) are nature-based sustainable biomaterials, which not only possess cellulosic properties but also have the important hallmarks of nanomaterials, such as large surface area, versatile reactive sites or functionalities, and scaffolding stability to host inorganic nanoparticles. This class of nanomaterials offers new opportunities for a broad spectrum of applications for clean water production that were once thought impractical. This Review covers substantial discussions based on evaluative judgments of the recent literature and technical advancements in the fields of coagulation/flocculation, adsorption, photocatalysis, and membrane filtration for water decontamination through proper understanding of fundamental knowledge of NC, such as purity, crystallinity, surface chemistry and charge, suspension rheology, morphology, mechanical properties, and film stability. To supplement these, discussions on low-cost and scalable NC extraction, new characterizations including solution small-angle X-ray scattering evaluation, and structure-property relationships of NC are also reviewed. Identifying knowledge gaps and drawing perspectives could generate guidance to overcome uncertainties associated with the adaptation of NC-enabled water purification technologies. Furthermore, the topics of simultaneous removal of multipollutants disposal and proper handling of post/spent NC are discussed. We believe NC-enabled remediation nanomaterials can be integrated into a broad range of water treatments, greatly improving the cost-effectiveness and sustainability of water purification.

Evaluation of renewable pH-responsive starch-based flocculant on treating and recycling of highly saline textile effluents

Herein, we report a novel renewable pH-responsive starch-based flocculant (CIAT-ST) via etherifying 2-chloro-4,6-isopropylamino-[1,3,5]-triazine (CIAT) onto the starch backbones for decontamination and reuse of highly saline effluents. The obtained CIAT-ST shows a unique pH-sensibility and reversibility in response to a subtle pH change due to a pH-controllable surface charge density of polymer chains. The level of residual CIAT-ST in the solution can be facilely monitored by using UV-vis detection. The dye flocculation performance of CIAT-ST was evaluated by using a batch experiment. The results exhibited that the dye removal was highly dependent on the solution pH (optimal pH was 2), the flocculation equilibrium can be achieved within 5 min, and the maximum flocculation capacity of CIAT-ST for K-2BP and KN-B5 were calculated to be 2452.6 ± 23.9 and 792.7 ± 14.1 mg/g, respectively. The multiple flocculation mechanisms, including charge neutralization, bridging and charge patching, may participate in the flocculation process. Adjustment in pH-mediated hydrophilicity-hydrophobicity switch of flocculant facilitates readily recovery and then sequentially reused three times while retaining satisfying flocculation efficiency. A significant contribution was also confirmed that the highly saline effluents after flocculation and sedimentation were reused in three successive dyeing processes without sacrificing fabric quality (ΔE* < 1) due to relatively low polymer residuals, and the efficiency of salt reuse for consecutive regeneration processes could be achieved above 85%. The present work could provide alternative thoughts for the reutilization of spent flocculant and clarified saline wastewater, which is also an efficient and sustainable strategy for textile wastewater management.

Emerging Developments Regarding Nanocellulose-Based Membrane Filtration Material against Microbes

The wide availability and diversity of dangerous microbes poses a considerable problem for health professionals and in the development of new healthcare products. Numerous studies have been conducted to develop membrane filters that have antibacterial properties to solve this problem. Without proper protective filter equipment, healthcare providers, essential workers, and the general public are exposed to the risk of infection. A combination of nanotechnology and biosorption is expected to offer a new and greener approach to improve the usefulness of polysaccharides as an advanced membrane filtration material. Nanocellulose is among the emerging materials of this century and several studies have proven its use in filtering microbes. Its high specific surface area enables the adsorption of various microbial species, and its innate porosity can separate various molecules and retain microbial objects. Besides this, the presence of an abundant OH groups in nanocellulose grants its unique surface modification, which can increase its filtration efficiency through the formation of affinity interactions toward microbes. In this review, an update of the most relevant uses of nanocellulose as a new class of membrane filters against microbes is outlined. Key advancements in surface modifications of nanocellulose to enhance its rejection mechanism are also critically discussed. To the best of our knowledge, this is the first review focusing on the development of nanocellulose as a membrane filter against microbes.

Enhanced Secretions of Algal Cell-Adhesion Molecules and Metal Ion-Binding Exoproteins Promote Self-Flocculation of Chlorella sp. Cultivated in Municipal Wastewater

The mechanism of self-flocculation remains unclear, partially impeding its efficiency enhancement and commercial application of microalgae-based municipal wastewater (MW) bioremediation technology. This study revealed the contributions of exoproteins [PN, proteins in extracellular polymeric substances (EPS)] to the separation of indigenous microalgae from treated MW. Compared to the low light intensity group, the high light intensity (HL) group produced Chlorella sp. with 4.3-fold higher self-flocculation efficiencies (SE). This was attributed to the enriched biological functions and positional rearrangement of increased PN within 2.9-fold higher EPS. Specifically, a total of 75 PN was over-expressed in the HL group among the 129 PN identified through label-free proteomics. The algal cell-adhesion molecules (Algal-CAMs) and metal-ion-binding PN were demonstrated as two dominant contributors promoting cell adhesion and bridging, through function prediction based on the contained domains. The modeled 3D structure showed that Algal-CAMs presented less hydrophilic α-helix abundance and were distributed in the outermost position of the EPS matrix, further facilitating microalgal separation. Moreover, the 10.1% lower hydrophily degree value, negative interfacial free energy (-19.5 mJ/m²), and 6.8-fold lower energy barrier between cells also supported the observed higher SE. This finding is expected to further fill the knowledge gap of the role of PN in microalgal self-flocculation and promote the development of biomass recovery from the microalgae-wastewater system.

Glycine betaine grafted nanocellulose as an effective and bio-based cationic nanocellulose flocculant for wastewater treatment and microalgal harvesting

Flocculation is a widely used technology in industry including for wastewater treatment and microalgae harvesting. To increase the sustainability of wastewater treatment, and to avoid contamination of the harvested microalgal biomass, there is a need for bio-based flocculants to replace synthetic polymer flocculants or metal salt coagulants. We developed the first cellulose nanocrystalline flocculant with a grafted cationic point charge, i.e. glycine betaine (i.e. N,N,N-trimethylglycine) grafted cellulose nanocrystals (CNCs) effective for the flocculation of kaolin (a model system for wastewater treatment), the freshwater microalgae Chlorella vulgaris, and the marine microalgae Nannochloropsis oculata. We successfully grafted glycine betaine onto CNCs using a one-pot reaction using a tosyl chloride activated esterification reaction with a degree of substitution ranging from 0.078 ± 0.003 to 0.152 ± 0.002. The degree of substitution is controlled by the reaction conditions. Flocculation of kaolin (0.5 g L^-1) required a dose of 2 mg L^-1, a comparable dose to commercial polyacrylamide-based flocculants. Flocculation was also successful for freshwater as well as marine microalgae (biomass concentration about 300 mg L^-1 dry matter), although the flocculation efficiency of the latter remained below 80%. The dose to induce flocculation (DS = 0.152 ± 0.002) was 20 mg L^-1 for the freshwater Chlorella vulgaris and 46 mg L^-1 for the marine Nannochloropsis oculata, comparable to other bio-based flocculants such as chitosan or TanFloc.

Separation of Boron from Geothermal Waters with Membrane System

This study presents the separation and recovery of boron from geothermal waters with a polymeric membrane system and suggests a transport mechanism. The optimum relative parameters of the transport were examined. The recovery value of boron was 60.46% by using polymeric membrane system from prepared aquatic solution to the acceptor phase. The membrane's capacity and selectivity of the transport process were examined. Kinetics values were calculated for each transport parameter. The optimum kinetic values were 1.4785 × 10^-6 (s^-1), 7.3273 × 10^-8 (m/s), 13.5691 × 10^-8 (mol/m².s), 5.8174 × 10^-12 (m²/s) for constant rate, permeability coefficient, flux, and diffusion coefficient, respectively. Boron was transported selectively and successfully from geothermal waters in the presence of other metal cations with 59.85% recovery value. This study indicates the application of real samples in polymeric membrane systems, which are very practical, economic, and easy to use for large-scale applications. The chemical and physical properties of polymer inclusion membranes (PIMs) offer the opportunity to be specially designed for specific applications.

Use of static magnetic fields to increase CO2 biofixation by the microalga Chlorella fusca

This study aimed to use different conditions of magnetic field (MF) application during Chlorella fusca cultivation and evaluate CO₂ biofixation by the microalga through growth kinetics in addition to the biomass composition. For this purpose, we tested different MF intensities applied for 1 h d^-1 and for 24 h. Cultures exposed to the MF for 1 h d^-1 (in both intensities) had greater biomass concentrations (1.42 g L^-1) and 34% more productivity in the same time as the control assay. The biofixation rate increased by 50% with 60 mT for 1 h d^-1, and the protein content was enhanced by 30 mT (56.21% w w^-1). This study was the first to consider the MF effect on CO₂ biofixation. MF applied for 1 h d^-1 proved to be an efficient alternative method to increase the CO₂ biofixation and growth of C. fusca besides to be an inexpensive and nontoxic method.

Determination of Microalgal Lipid Content and Fatty Acid for Biofuel Production

Biofuels produced from microalgal biomass have received growing worldwide recognition as promising alternatives to conventional petroleum-derived fuels. Among the processes involved, the downstream refinement process for the extraction of lipids from biomass greatly influences the sustainability and efficiency of the entire biofuel system. This review summarizes and compares the current techniques for the extraction and measurement of microalgal lipids, including the gravimetric methods using organic solvents, CO₂-based solvents, ionic liquids and switchable solvents, Nile red lipid visualization method, sulfo-phospho-vanillin method, and the thin-layer chromatography method. Each method has its own competitive advantages and disadvantages. For example, the organic solvents-based gravimetric method is mostly used and frequently employed as a reference standard to validate other methods, but it requires large amounts of samples and is time-consuming and expensive to recover solvents also with low selectivity towards desired products. The pretreatment approaches which aimed to disrupt cells and support subsequent lipid extraction through bead beating, microwave, ultrasonication, chemical methods, and enzymatic disruption are also introduced. Moreover, the principles and procedures for the production and quantification of fatty acids are finally described in detail, involving the preparation of fatty acid methyl esters and their quantification and composition analysis by gas chromatography.

Comparison of Four Quantitative Techniques for Monitoring Microalgae Disruption by Low-Frequency Ultrasound and Acoustic Energy Efficiency

Ultrasound has been regarded as an environmental friendly technology to utilize microalgae biomass and control algal blooms. In this study, four quantitative techniques, including cell counting, optical density of algal suspension, pigments release, and protein release, were performed on three species of microalgae ( M. aeruginosa, C. pyrenoidosa, and C. reinhardtii) to develop effective techniques for rapid monitoring of cell disruption and to optimize the acoustic energy efficiency. Results showed optical density of algal suspensions was not an optimal indicator to monitor cell disruption, although it is a common technique for determining cell concentration in microbial cultures. Instead, an accurate and reliable technique was to determine the release of intracellular pigments (absorbance peaks of supernatant) for indicating cell rupture. The protein released during sonication could also be a useful indicator if it is the component of interest. A fitted power functional model showed a strong relationship between cell disruption and energy consumption ( R² > 0.87). This model could provide an effective approach to directly compare the energy efficiency of ultrasound in different systems or with varying microalgae species. This study provides valuable information for microalgae utilization and the treatment of algal blooms by ultrasound, so as to achieve energy conservation and environmental safety.