Green and facile approach for enhancing the inherent magnetic properties of carbon nanotubes for water treatment applications

Green Synthesis of Carbon Nanoparticles (CNPs) from Biomass for Biomedical Applications

In this review, we summarize recent work on the "green synthesis" of carbon nanoparticles (CNPs) and their application with a focus on biomedical applications. Recent developments in the green synthesis of carbon nanoparticles, from renewable precursors and their application for environmental, energy-storage and medicinal applications are discussed. CNPs, especially carbon nanotubes (CNTs), carbon quantum dots (CQDs) and graphene, have demonstrated utility as high-density energy storage media, environmental remediation materials and in biomedical applications. Conventional fabrication of CNPs can entail the use of toxic catalysts; therefore, we discuss low-toxicity manufacturing as well as sustainable and environmentally friendly methodology with a focus on utilizing readily available biomass as the precursor for generating CNPs.

Why reuse spent adsorbents? The latest challenges and limitations

Despite the abundance of published reviews over the last few years, the inconsistent data representation in regards to the use of adsorbents in each work, renders the task of comparing them challenging. Disposing the adsorbent may have adverse environmental impact, which should be mitigated through regeneration and reuse processes, such as desorption. This review discusses how the importance of desorption and regeneration equates that of the adsorption stage, and presents various regeneration methods as well as the influencing parameters, advantages, and disadvantages thereof. For the purposes of this work, the adsorbents have been categorized into four groups: (i) graphene, (ii) carbon nanotubes, (iii) activated carbon compounds and (iv) clays and polymer adsorbents as representatives in order to further study their desorption and regeneration abilities, using a variety of desorption media/eluants. The process conditions, such as pH, dose required, concentration, adsorption ability and the cost of the adsorbents were examined for further analysis. The recovery efficiency and ability to get reused through the desorption process was also evaluated. The highest adsorption capacity was observed for graphene-based adsorbents reaching between 108 and >480 mg/g, and for activated carbon materials ranging from 34 to >384 mg/g, whereas carbon nanotubes and polymer-based adsorbents indicated rather low and greatly varying adsorption capacities, between 1 and >138 mg/g and between 7 and >57 mg/g, respectively. Most of the reviewed cases appear to fit the pseudo-second order (PSO) kinetic model. These materials have demonstrated a removal effectiveness between 71% and 99%. Overall, all the aforementioned adsorbents share the advantage of being highly reusable.

Decorating graphene oxide with zeolitic imidazolate framework (ZIF-8) and pseudo-boehmite offers ultra-high adsorption capacity of diclofenac in hospital effluents

This study reports on an easy and scalable synthesis method of a novel magnetic nanocomposite (GO/ZIF-8/γ-AlOOH) based on graphene oxide (GO) nanosheets decorated with zeolitic imidazolate framework-8 (ZIF-8), pseudo-boehmite (γ-AlOOH), and iron oxide (Fe₃O₄) nanoparticles by combining solvothermal and solid-state dispersion (SSD) methods. The nanocomposite was successfully applied to remove of diclofenac sodium (DCF) - a widely used pharmaceutical - from water. Response Surface Methodology (RSM) was used to optimize the adsorption process and assess the interactions among the influencing factors on DCF removal efficiency; including contact time, adsorbent dosage, initial pH, solution temperature, and DCF concentration. Adsorption isotherm results showed a good fitting with the Langmuir isotherm model with an exceptional adsorption capacity value of 2594 mg g^-1 at 30 °C, which was highly superior to the previously reported adsorbents. In addition, kinetic and thermodynamic investigations further illustrated that the adsorption process was fast (equilibrium time = 50 min) and endothermic. The regeneration of GO/ZIF-8/γ-AlOOH nanocomposite using acetic acid solution (10% v/v) after a simple magnetic separation was confirmed in five consecutive cycles, which eliminate the usage of organic solvents. The nanocomposite has also shown a superior performance in treating a simulated hospital effluent that contained various pharmaceuticals as well as other organic, and inorganic constituents.

Photocatalytic degradation of organic micropollutants: Inhibition mechanisms by different fractions of natural organic matter

Natural organic matter (NOM) can inhibit the photocatalytic degradation of organic micropollutants (OMPs) through inner filter effect, reactive oxygen species (ROS) scavenging, and competitive adsorption. However, previous studies have focused solely on the bulk properties of NOM and our understanding of the inhibition mechanism by NOM fractions during photocatalytic degradation of OMP is still fragmentary. In this study, five well-characterized different NOM samples (i.e., secondary treated wastewater, river water, and three standard NOM surrogates) were used to elucidate the inhibition mechanisms during photocatalytic degradation of carbamazepine (a model OMP) using TiO₂ and its composites with carbon nanotubes (CNT-TiO₂) under UVC and solar-light irradiation. The results indicated that terrestrially derived NOM with high aromaticity, a low oxygen/carbon atom ratio, and large molecular weight is the major fraction that participates in ROS scavenging, competitive adsorption, and inner filter effect. Furthermore, the modeling analysis suggested that inner filter effect due to NOM and ROS scavenging was the most influential inhibitory mechanism. In the case of secondary treated wastewater, the presence of high concentrations of inorganic species (e.g., PO₄^3-, Cl^-, and NO₃^-) together with NOM significantly reduced the photocatalytic degradation of carbamazepine. Overall, the methods and the results of this study provide a comprehensive understanding of the effects of NOM fractions on photocatalysis and highlight the need to further consider the interplay between NOM and background inorganic constituents in photocatalytic degradation of OMP.

Disinfection mechanism of E. coli by CNT-TiO2 composites: Photocatalytic inactivation vs. physical separation

Magnetic carbon nanotube (MCNT) composites with titanium dioxide (TiO₂) have an enhanced photocatalytic disinfection efficiency (i.e. higher disinfection rate) and better applicability (i.e. solar light applicability and catalyst separation using its magnetic property) than bare TiO₂ and/or MCNT. However, the role and mechanism of MCNT in the disinfection process are still unclear. Therefore, this study aimed at investigating the disinfection mechanism of Escherichia coli using MCNT-TiO₂ nanocomposites under various conditions (i.e. the presence and absence of light and reactive oxygen species scavengers, and different MCNT-TiO₂ ratio) and photocatalytic disinfection models. The results showed that (i) MCNT and its nanocomposites with TiO₂ had much higher disinfection efficiencies than bare TiO₂, (ii) the physical bacterial capture was the dominant disinfection mechanism, (iii) the higher disinfection rate was found at an optimum MCNT:TiO₂ ratio of 5:1 under the tested experimental conditions, (iv) hydroxyl radical (OH) was the influencing reactive oxygen species on the photocatalytic disinfection using MCNT-TiO₂, and (v) good correlation between experimental parameters (i.e. carbon contents, surface area and concentration of MCNT-TiO₂) and the contribution rate of physical and photocatalysis reactions. The finding from this study and the methods proposed herein are essential for understanding the photocatalytic disinfection processes using TiO₂ and its carbonaceous nanocomposites, which can promote the application of photocatalytic disinfection process.

Cationic polymer for selective removal of GenX and short-chain PFAS from surface waters and wastewaters at ng/L levels

The emerging classes of perfluorinated alkyl substances (PFAS) (e.g., Perfluorobutanoic acid (PFBA), perfluorobutane sulfonic acid (PFBS), GenX, ADONA, and F-53B) are persistent and recalcitrant to removal by conventional treatment techniques. Herein, we report on poly (N-[3-(dimethylamino)propyl]acrylamide, methyl chloride quaternary, DMAPAA-Q) hydrogel matrix as an effective sorbent for sequestering PFAS from different water matrices. The selective removal of 16 PFAS from different classes using DMAPAA-Q polymer was confirmed in surface waters and treated wastewater at environmentally relevant concentration (i.e., <1000 ng/L). The results showed fast removal kinetics with equilibrium time of 60-120 min and a higher removal of sulfonated than carboxylic PFAS, regardless of their chain lengths. These observations were in agreement with adsorption energy calculations of short- and long-chain PFAS on poly DMAPAA-Q hydrogel using density functional theory (DFT). No desorption was observed when the experimental time was extended to 24 h, which gives an added advantage of poly DMAPAA-Q hydrogel over previously reported adsorbents in the literature. In addition, the removal efficiency was not affected under a varying pH range of 4-10. The impact of background anions on PFAS removal by poly DMAPAA-Q hydrogel was tested and found to follow an order of SO₄^2- > Cl^- > NO₃^-. The performance of poly DMAPAA-Q hydrogel was maintained in six consecutive adsorption/regeneration cycles to remove PFAS. The unique fast kinetics and high adsorption activity of poly DMAPAA-Q hydrogel towards PFAS exhibits a great potential for being a promising material for PFAS control.

Photodegradation of pharmaceuticals and personal care products in water treatment using carbonaceous-TiO2 composites: A critical review of recent literature

The high concentrations of pharmaceuticals and personal care products (PPCP) that found in water in many locations are of concern. Among the available water treatment methods, heterogeneous photocatalysis using TiO₂ is an emerging and viable technology to overcome the occurrence of PPCP in natural and waste water. The combination of carbonaceous materials (e.g., activated carbon, carbon nanotubes and graphene nanosheets) with TiO₂, a recent development, gives significantly improved performance. In this article, we present a critical review of the development and fabrication of carbonaceous-TiO₂ and its application to PPCP removal including its influence on water chemistry, and the relevant operational parameters. Finally, we present an analysis of current priorities in the ongoing research and development of carbonaceous-TiO₂ for the photodegradation of PPCP.

Natural organic matter undergoes different molecular sieving by adsorption on activated carbon and carbon nanotubes

We have comprehensively compared the molecular sieving of natural organic matter (NOM) by adsorption on activated carbon (AC) and multi-walled carbon nanotubes (CNT) using different types of NOM. All water samples were characterized using UV-visible and fluorescence spectroscopies as well as high-performance size-exclusion chromatography (HPSEC) before and after adsorption. Adsorption isotherm results fitted well with Freundlich model (R² = 0.95-0.99) and the model parameters indicated higher adsorption of NOM on CNT than AC. Fluorescence index (FI) and freshness index (BIX) showed preferential adsorption of microbial derived and fresh NOM on AC, whereas, terrestrial derived and decomposed NOM were preferentially adsorbed on CNT. Further, HPSEC revealed that AC adsorbed NOM fractions with small molecular weight (MW) (<0.4 kDa) faster than the fractions with higher MW. In contrast, CNT adsorbed NOM fractions characterized by high MW (>1 kDa) while the smallest fraction (<0.4 kDa) was not adsorbed, possibly due to its hydrophilic character. Our results also demonstrated a good correlation between FI and average MW of NOM (R² > 0.93). These findings illustrate the influence of the adsorbent's type and characteristics (i.e., porosity and pore size distribution) on the preferential adsorption of different NOM fractions.