Adsorption of perfluoroalkyl acids on granular activated carbon supported chitosan: Role of nanobubbles

Innovative techniques for combating a common enemy forever chemicals: A comprehensive approach to mitigating per- and polyfluoroalkyl substances (PFAS) contamination

The pervasive presence of per and polyfluoroalkyl substances (PFAS), commonly referred to as "forever chemicals," in water systems poses a significant threat to both the environment and public health. PFAS are persistent organic pollutants that are incredibly resistant to degradation and have a tendency to accumulate in the environment, resulting in long-term contamination issues. This comprehensive review delves into the primary impacts of PFAS on both the environment and human health while also delving into advanced techniques aimed at addressing these concerns. The focus is on exploring the efficacy, practicality, and sustainability of these methods. The review outlines several key methods, such as advanced oxidation processes, novel materials adsorption, bioremediation, membrane filtration, and in-situ chemical oxidation, and evaluates their effectiveness in addressing PFAS contamination. By conducting a comparative analysis of these techniques, the study aims to provide a thorough understanding of current PFAS remediation technologies, as well as offer insights into integrated approaches for managing these persistent pollutants effectively. While acknowledging the high efficiency of adsorption and membrane filtration in reducing persistent organic pollutants due to their relatively low cost, versatility, and wide applicability, the review suggests that the integration of these methods could result in an overall enhancement of removal performance. Additionally, the study emphasizes the need for researcher attention in key areas and underscores the necessity of collaboration between researchers, industry, and regulatory authorities to address this complex challenge.

Advanced technologies in water treatment: Chitosan and its modifications as effective agents in the adsorption of contaminants

Chitosan, derived from the abundant biopolymer chitin, has emerged as a promising option for water treatment due to its intrinsic bioavailability. This review emphasizes the notable characteristics of chitosan, which allow for various modifications, expanding its applications. The polymer's effectiveness in adsorbing contaminants, particularly in advanced water treatment technologies, is highlighted. The review underscores the potential of chitosan-based hybrid materials, including nanocomposites, hydrogels, membranes, films, sponges, nanoparticles, microspheres, and flakes, as innovative alternatives to traditional chemical-based adsorbents. The advantages of using these materials in wastewater treatment, especially in removing heavy metals, dyes, and emerging compounds, are explored. The study delves into the mechanisms involved in wastewater treatment with chitosan, emphasizing the interactions between the polymer and various contaminants. Additionally, the application of chitosan as a contaminant removal agent in a post-pandemic context is addressed, considering the challenges related to waste management and environmental preservation. The analysis highlights the potential contribution of chitosan in mitigating environmental impacts post-pandemic, offering practical solutions for treating contaminated effluents and promoting sustainability. The study addresses current obstacles and prospects for chitosan-based wastewater treatment, emphasizing its promising role in sustainable water management.

Activation of sawdust biochar with water and wastewater treatment residuals for sorption of perfluorooctanesulfonic acid in water

Recent research has found biochar to be a cost-effective adsorbent for removal of perfluoroalkyl substances in water. To promote cleaner production and sustainable waste management, this study explored the potential to produce activated biochars by co-pyrolyzing sawdust with iron-rich biosolids and polyaluminum sludge. The maximum capacity to adsorb perfluorooctanesulfonic acid (PFOS) reached 27.2 mg g-1 with biosolids-activated biochar and 19.2 mg g-1 with aluminum sludge-activated biochar, compared to 6.2 mg g-1 with sawdust biochar. The increased adsorption capacities were attributed to electrostatic interactions between the anionic PFOS and metal functionalities on the biochar surface. In contrast, hydrophobic interaction was the dominant adsorption mechanism of sawdust biochar. The presence of dissolved organic matter at 5-50 mg L-1 was found to inhibit adsorption of PFOS in water, while pH as low as 3.0 and sodium chloride concentrations up to 100 mM enhanced removal of PFOS by all the three adsorbents. In batch adsorption tests at environmentally relevant PFOS dosages and adsorbent dosage of 0.25 g L-1, the biosolids-sawdust biochar and Al sludge-sawdust biochar removed 71.4% and 66.9% of PFOS from drinking water and 77.9% and 87.9% of PFOS from filtrate of sludge digestate, respectively. The biosolids-sawdust biochar additionally removed Fe, although the Al sludge-sawdust biochar released Al into the alkaline drinking water and filtrate. Overall, this study proved co-pyrolyzing sawdust and Fe-rich biosolids to be an effective approach to activate sawdust biochar for enhanced removal of PFOS while recycling wastewater treatment residuals and sawdust.

Experimental and density functional theory investigation of surface-modified biopolymer for improved adsorption of mixtures of per- and polyfluoroalkyl substances in water

Glutaraldehyde (GTH) cross-linked chitosan (CTN) biopolymer-based and polyethyleneimine (PEI) functionalized (GTHCTNPEI) aerogels were proven promising for removing mixtures of long- and short chain per- and polyfluoroalkyl substances (PFAS) in water. In this study, to further improve the performance of the aerogel for short-chain PFAS and undecafluoro-2-methyl-3-oxahexanoic acid (GenX) removal, GTHCTNPEI aerogel chunks with an average size of 13.4 mm were turned into flakes with an average size of 9.1 mm. The GTHCTNPEI flakes achieved >99 % removal of all target PFAS, including long- and short-chain PFAS and >97 % for GenX after 10 h. In addition, the flakes can be regenerated and reused for at least four cycles. When added to tap water spiked with PFAS at initial concentrations of 30, 70, or 100 ng/L, the flakes removed almost 100 % of all tested PFAS. Mechanistic investigations using density functional theory (DFT) revealed strong stabilizing hydrophobic and electrostatic interactions between the aerogels and PFAS, with GTHCTNPEI to PFAS binding energies ranging between -24.0 - -30.1 kcal/mol for PFOA; -41.3 - -48.5 kcal/mol for PFOS; and -40.5 - -47.3 kcal/mol for PFBS. These results demonstrate the great potential of the flakes for removing PFAS from drinking water, surface water, and groundwater.

Surface modifications of biopolymers for removal of per- and polyfluoroalkyl substances from water: Current research and perspectives

Per- and polyfluoroalkyl substances (PFAS) are highly recalcitrant organic contaminants that have attracted ever-increasing attention from the general public, government agencies and scientific communities. To remove PFAS from water, especially the enormous volume of drinking water, stormwater, and groundwater, sorption is the most practical approach. Success of this approach demands green, renewable, and sustainable materials for capturing PFAS at ng/L or µg/L levels. To meet this demand, this manuscript critically reviewed sorbents developed from biopolymers, such as chitosan (CTN), alginate (ALG), and cellulose (CEL) covering the period from 2008 to 2023. The use of different cross-linkers for the surface modifications of biopolymers were described. The underlying removal mechanism of biosorbents for PFAS adsorption from molecular perspectives was discussed. Besides reviewing and comparing the performance of different bio-based sorbents with respect to environmental factors like pH, and sorption kinetics and capacity, strategies for modifying biosorbents for better performance were proposed. Additionally, approaches for regeneration and reuse of the biosorbents were discussed. This was followed by further discussion of challenges facing the development of biosorbents for PFAS removal.

Transdermal Sustained Release Properties and Anti-Photoaging Efficacy of Liposome-Thermosensitive Hydrogel System

Drug delivery systems have become a research priority in the biomedical field. The incorporation of liposomes to hydrogels further forms more robust multifunctional systems for more effective and sustained topical drug delivery. In this study, carboxymethyl-modified chitosan/hyaluronic acid (CMC/HA, CMH) thermosensitive hydrogel is developed for sustained transdermal delivery of liposomes. Hydrogels are crosslinked by hydrogen bonding, hydrophobic interaction and electrostatic interaction. The gel properties can be regulated by substitution degree (DS), and when DS = 18.20 ± 0.67% (CMH2), the gel temperature is 37.8 °C, allowing rapid gelation at body temperature (315 s). Moreover, CMH2 hydrogel has suitable spreadability (17.7-57.2 cm2 ), viscosity (2133.4 mPa s) and porous structure, which facilitated its adhesion and application on the skin and liposomes delivery. The hydrogel can retard the liposomes release, and the release rate of ascorbyl glucoside (AA2G) is 33.92-49.35% in 24 h. Hydrogel avoids the rapid clearance of liposomes from the skin and improved the skin retention, achieving the long-term release of bioactive components. Liposome-hydrogel system more efficiently promotes the anti-photoaging effect of AA2G on skin, reducing epidermal thickness, melanin deposition and lipid oxidative damage and increasing collagen density. Therefore, liposome-hydrogel systems are proposed as multifunctional delivery systems for sustained transdermal delivery.

PEI-modified chitosan/activated carbon composites for Cu(II) removal from simulated pyrophosphate plating rinsing wastewater

It is a challenging task to remove heavy metal ions efficiently from the wastewater containing high concentrations of complexants. In this work, a novel PEI-modified chitosan/activated carbon composite adsorbent (PCA) was prepared and applied to the removal of Cu(II) from pyrophosphate plating rinsing wastewater. The main species of Cu(II) in the pyrophosphate wastewater was [Cu(HP2O7)2]4- or [Cu(P2O7)2]6-, which were denoted as [Cu(II)-PP] anions. The maximum adsorption capacity of PCA for Cu(II) reached 1.41 mmol g-1 under the condition of pH = 8 and molar ratio of pyrophosphate to Cu(II) = 4:1. The adsorption kinetic behavior of Cu(II) on PCA followed the Elovich model best and PCA attained adsorption equilibrium within 36 h. The thermodynamic studies showed that the adsorption process of Cu(II) by PCA was endothermic and spontaneous. The PCA fixed bed column was used to remove Cu(II) from simulated pyrophosphate plating rinsing wastewater. After three consecutive adsorption-desorption cycles, the adsorption performance, hydraulic conductivity, and mechanical stability of PCA column did not decrease. The FTIR and XPS analysis results indicated that [Cu(II)-PP] anions can be adsorbed on PCA by electrostatic attraction with protonated amine groups or coordination with the amine groups of PCA via ligand substitution.

Nanobubbles in water and wastewater treatment systems: Small bubbles making big difference

Since the discovery of nanobubbles (NBs) in 1994, NBs have been attracting growing attention for their fascinating properties and have been studied for application in various environmental fields, including water and wastewater treatment. However, despite the intensive research efforts on NBs' fundamental properties, especially in the past five years, controversies and disagreements in the published literature have hindered their practical implementation. So far, reviews of NB research have mainly focused on NBs' role in specific treatment processes or general applications, highlighting proof-of-concept and success stories primarily at the laboratory scale. As such, there lacks a rigorous review that authenticates NBs' potential beyond the bench scale. This review aims to provide a comprehensive and up-to-date analysis of the recent progress in NB research in the field of water and wastewater treatment at different scales, along with identifying and discussing the challenges and prospects of the technology. Herein, we systematically analyze (1) the fundamental properties of NBs and their relevancy to water treatment processes, (2) recent advances in NB applications for various treatment processes beyond the lab scale, including over 20 pilot and full-scale case studies, (3) a preliminary economic consideration of NB-integrated treatment processes (the case of NB-flotation), and (4) existing controversies in NBs research and the outlook for future research. This review is organized with the aim to provide readers with a step-by-step understanding of the subject matter while highlighting key insights as well as knowledge gaps requiring research to advance the use of NBs in the wastewater treatment industry.

An overview on human exposure, toxicity, solid-phase microextraction and adsorptive removal of perfluoroalkyl carboxylic acids (PFCAs) from water matrices

Perfluoroalkyl carboxylic acids (PFCAs) are sub-class of perfluoroalkyl substances commonly detected in water matrices. They are persistent in the environment, hence highly toxic to living organisms. Their occurrence at trace amount, complex nature and prone to matrix interference make their extraction and detection a challenge. This study consolidates current advancements in solid-phase extraction (SPE) techniques for the trace-level analysis of PFCAs from water matrices. The advantages of the methods in terms of ease of applications, low-cost, robustness, low solvents consumption, high pre-concentration factors, better extraction efficiency, good selectivity and recovery of the analytes have been emphasized. The article also demonstrated effectiveness of some porous materials for the adsorptive removal of the PFCAs from the water matrices. Mechanisms of the SPE/adsorption techniques have been discussed. The success and limitations of the processes have been elucidated.

Contribution of air-water interface in removing PFAS from drinking water: Adsorption, stability, interaction and machine learning studies

As a class of synthetic persistent organic pollutants, contamination of Per-and poly-fluoroalkyl substances (PFAS) in drinking water has attracted widespread concern. Aeration has been confirmed to enhance the removal of PFAS in drinking water by activated carbon (AC). However, the contribution of the air-water interface in removing PFAS is not yet to be fully understood at the molecular level. In this work, molecular dynamics (MD) simulations were employed to investigate the role of nanobubble in removing PFAS in the aqueous environment. The result suggests that the free energies of the air-water interface are about 3-7 kcal mol^-1 lower than that of the bulk water region, indicating that the transformation of PFAS from the water phase into the air-water interface is favorable from the viewpoint of thermodynamics. The interface-water partition coefficients (P_sur/wat) of PFAS are in the order of PFOS > PFOA > PFHxS > PFBS. On the air-water-AC three-phase interface, PFBS can not only move along the interface region but also leave the interface region into water phase, while PFOS tended to move along the interface region until it was captured by AC. Finally, the ΔG_{water-interface} quantitative structure-activity relationships (QSAR) models were developed to predict the removal efficiencies of PFAS enhanced by aeration in aquatic systems. The proposed mechanism promotes the understanding of the contribution of air-water interface in removing PFAS from drinking water by activated carbon.

Adsorption behavior of long-chain perfluoroalkyl substances on hydrophobic surface: A combined molecular characterization and simulation study

Hydrophobic interaction is a prevalent sorption mechanism of poly- and perfluoroalkyl substances (PFAS) in natural and engineered environments. In this study, we combined quartz crystal microbalance with dissipation (QCM-D), atomic force microscope (AFM) with force mapping, and molecular dynamics (MD) simulation to probe the molecular behavior of PFAS at the hydrophobic interface. On a CH₃-terminated self-assembled monolayer (SAM), perfluorononanoic acid (PFNA) showed ∼2-fold higher adsorption than perfluorooctane sulfonate (PFOS) that has the same fluorocarbon tail length but a different head group. Kinetic modeling using the linearized Avrami model suggests that the PFNA/PFOS-surface interaction mechanisms can evolve over time. This is confirmed by AFM force-distance measurements, which shows that while the adsorbed PFNA/PFOS molecules mostly lay flat, a portion of them formed aggregates/hierarchical structures of 1-10 nm in size after lateral diffusion on surface. PFOS showed a higher affinity to aggregate than PFNA. Association with air nanobubbles is observed for PFOS but not PFNA. MD simulations further showed that PFNA has a greater tendency than PFOS to have its tail inserted into the hydrophobic SAM, which can enhance adsorption but limit lateral diffusion, consistent with the relative behavior of PFNA/PFOS in QCM and AFM experiments. This integrative QCM-AFM-MD study reveals that the interfacial behavior of PFAS molecules can be heterogeneous even on a relatively homogeneous surface.