Pore size distributions derived from adsorption isotherms, immersion calorimetry, and isosteric heats: A comparative study

We compare the pore size distribution of a well-characterized activated carbon derived from model-dependent, adsorption integral equation (AIE) methods with those from model-independent, immersion calorimetry and isosteric heat analyses. The AIE approach applied to nitrogen gave a mean pore width of 0.57 nm; the CO2 distribution exhibited wider dispersion. Spherical model application to CO2 and diffusion limitations for nitrogen and argon were proposed as primary reasons for inconsistency. Immersion enthalpy revealed a sharp decrease in available area equivalent to a cut-off due to molecular exclusion when the accessible surface was assessed against probe kinetic diameter. Mean pore width was identified as 0.58 ± 0.02 nm, endorsing the underlying assumptions for the nitrogen-based AIE approach. A comparison of the zero-coverage isosteric heat of adsorption for various non-polar adsorptives by the porous test sample was compared with the same adsorptives in contact with a non-porous reference adsorbent, leading to an energy ratio or adsorption enhancement factor. A linear relationship between the energy ratio and probe kinetic diameter indicated a primary pore size at 0.59 nm. The advantage of this enthalpy, model-independent methods over AIE were due to no assumptions regarding probe molecular shape, and no assumptions for pore shape and/or connectivity.

Post-combustion CO2 adsorption on activated carbons with different textural properties

Fixed bed CO2 adsorption tests were carried out in model flue-gas streams onto two commercial activated carbons, namely Filtrasorb 400 and Nuchar RGC30, at 303 K, 323 K and 353 K. Thermodynamic adsorption results highlighted that the presence of a narrower micropore size distribution with a prevailing contribution of very small pore diameters, observed for Filtrasorb 400, is a key factor in determining a higher CO2 capture capacity, mostly at low temperature. These experimental evidences were also corroborated by the higher value of the isosteric heat derived for Filtrasorb 400, testifying stronger interactions with CO2 molecules with respect to Nuchar RGC30. Dynamic adsorption results on the investigated sorbents highlighted the important role played by both a greater contribution of mesopores and the presence of wider micropores for Nuchar RGC30 in establishing faster capture kinetics with respect to Filtrasorb 400, in particular at 303 K. Furthermore, the modeling analysis of 15% CO2 breakthrough curves allowed identifying intraparticle diffusion as the rate-determining step of the process.

Physico-chemical characterization of metal-doped bone chars and their adsorption behavior for water defluoridation

New bone chars for fluoride adsorption from drinking water have been synthetized via metallic doping using aluminum and iron salts. A detailed statistical analysis of the metal doping process using the signal-to-noise ratios from Taguchi's experimental designs and its impact on the fluoride adsorption properties of modified bone chars have been performed. The best conditions, including the proper metallic salt, for metal doping were identified to improve the fluoride uptakes of modified bone chars. Results showed that the fluoride adsorption properties of bone chars can be enhanced up to 600% using aluminum sulfate for the surface modification. This aluminum-based adsorbent showed an adsorption capacity of 31 mg/g, which outperformed the fluoride uptakes reported for several adsorbents. Surface interactions involved in the defluoridation process were established using FTIR, DRX and XPS analysis. Defluoridation using the metal-doped bone chars occurred via an ion exchange process between fluoride ions and the hydroxyl groups on the adsorbent surface, whereas the Al(OH)xFy, FexFy, and CaF2 interactions could play also an important role in the removal process. These metal-doped adsorbents anticipate a promising behavior in water treatment, especially in developing countries where the efficiency – cost tradeoff is crucial for implementing new defluoridation technologies.

Assessment of naproxen adsorption on bone char in aqueous solutions using batch and fixed-bed processes

An integrated analysis of naproxen adsorption on bone char in batch and packed-bed column conditions has been performed. Kinetic, thermodynamic and breakthrough parameters have been calculated using adsorption models and artificial neural networks. Results show that naproxen removal using bone char in batch conditions is a feasible and effective process, which could involve electrostatic and non-electrostatic interactions depending mainly on pH conditions. However, the application of packed-bed column for naproxen adsorption on bone char is not effective for the treatment of diluted solutions due to the low degree of adsorbent utilization (below 4%) at tested operating conditions. The proposed mechanism for naproxen removal using bone char could include a complexation process via phosphate and naproxen, hydrogen bonding and the possibility of hydrophobic interactions via π–π electron. This study highlights the relevance of performing an integrated analysis of adsorbent effectiveness in batch and dynamic conditions to establish the best process configuration for the removal of emerging water pollutants such as pharmaceuticals.

Removal of paracetamol on biomass-derived activated carbon: Modeling the fixed bed breakthrough curves using batch adsorption experiments

The remediation of paracetamol (PA), an emerging contaminant frequently found in wastewater treatment plants, has been studied in the low concentration range (0.3–10 mg L−1) using as adsorbent a biomass-derived activated carbon. PA uptake of up to 100 mg g−1 over the activated carbon has been obtained, with the adsorption isotherms being fairly explained by the Langmuir model. The application of Reichemberg and the Vermeulen equations to the batch kinetics experiments allowed estimating homogeneous and heterogeneous diffusion coefficients, reflecting the dependence of diffusion with the surface coverage of PA. A series of rapid small-scale column tests were carried out to determine the breakthrough curves under different operational conditions (temperature, PA concentration, flow rate, bed length). The suitability of the proposed adsorbent for the remediation of PA in fixed-bed adsorption was proven by the high PA adsorption capacity along with the fast adsorption and the reduced height of the mass transfer zone of the columns. We have demonstrated that, thanks to the use of the heterogeneous diffusion coefficient, the proposed mathematical approach for the numerical solution to the mass balance of the column provides a reliable description of the breakthrough profiles and the design parameters, being much more accurate than models based in the classical linear driving force.

Preparation of graphene oxide decorated Fe3O4@SiO2 nanocomposites with superior adsorption capacity and SERS detection for organic dyes

The fast detection and removal of organic dyes from contaminated water has become an urgent environmental issue due to their high toxicity, chemical stability, and low biodegradability. In this paper, we have developed graphene oxide decorated Fe3O4@SiO2 (Fe3O4@SiO2-GO) as a novel adsorbent aiming at the rapid adsorption and trace analysis of organic dyes followed by surface enhanced Raman scattering (SERS). The structure and morphology of the nanocomposites were characterized by transmission electron microscopy (TEM), Fourier infrared spectrometry (FT-IR), X-ray diffraction (XRD), and vibrating sample magnetometer (VSM). The obtained nanocomposites were used to adsorb methylene blue (MB) in aqueous solution based on π-π stacking interaction and electrostatic attraction between MB and GO, and the adsorption behaviors of MB were investigated. Moreover, the obtained nanocomposites with adsorbed dyes were separated from the solution and loaded with silver nanoparticles for SERS detection. These nanocomposites showed superior SERS sensitivity and the lowest detectable concentration was 1.0 × 10-7 M.

Investigation of chitosan adsorption onto cotton fabric with atmospheric helium/oxygen plasma pre-treatment

In this study, the effects of helium or a helium/oxygen mixture atmospheric pressure plasma treatment on the adsorption of chitosan onto the cotton fabric were investigated. Fabrics were treated with plasma prior to a chitosan finishing process, whereby fabrics were surface coated using a pad/dry/cure method. Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, surface energy analyser and contact angle measurements were used to investigate the changes on the cotton surface. Furthermore, antimicrobial activity of the cotton fabric was evaluated. The results showed that plasma pre-treatment enhanced the chitosan adsorption to the cotton surface through physical bonding and there was weak evidence of chemical bonding interactions. A combination of plasma and chitosan treatment did not show any significant differences on the antimicrobial properties compared to chitosan only treated fabric. Plasma treatment changed the fibres physically and enhanced the surface energy and thickness of chitosan distributed on the fibres.

Removal of copper ions from aqueous solution by adsorption onto novel polyelectrolyte film-coated nanofibrous silk fibroin non-wovens

In this approach, polyelectrolyte film-coated nanofibrous silk fibroin (SF) nonwovens were prepared from the alternate deposition of positively charged polyethylenimine (PEI) and negatively charged SF using electrostatic layer-by-layer (LBL) self-assembled technology. The composite membranes were characterized by scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectrometer. The SF-PEI multilayer-assembled nanofibers (less than five layers) were fine and uniform with the fiber diameter from 400 nm to 600 nm, and had very large surface area and high porosity (more than 70%). The amino groups of PEI were proved to be deposited onto SF nonwovens, which granted the coated nonwovens with potential applicability for copper ions adsorption. The PEI films coated SF substrate showed much higher copper ions adsorption capacity than that of ethanol treated SF nanofibers. Adding the number of PEI coated could enhance the Cu²⁺ adsorption capacity significantly. The maximum milligrams per gram of copper ions adsorbed reached 59.7 mg/g when the SF substrate was coated with 5 bilayers of SF-PEI. However, the copper ions adsorption capacity had no obvious change as the number of PEI continued to increase. These results suggest potential for PEL film-coated nanofibrous nonwovens as a new adsorbent for metal ions.

Enhanced homogeneity and interfacial compatibility in melt-extruded cellulose nano-fibers reinforced polyethylene via surface adsorption of poly(ethylene glycol)-block-poly(ethylene) amphiphiles

In this paper, we demonstrate that an amphiphilic block copolymer such as polyethylene glycol-b-polyethylene can be used as both dispersing and interfacial compatibilizing agent for the melt compounding of LLDPE with cellulose nano-fibers. A simple and effective spray drying methodology was first used for the first time for the preparation of a powdered cellulose nano-fibers extrusion feedstock. Surface adsorption of the amphiphilic PEG-b-PE was carried out directly in solution during this process. These various dry cellulosic feedstock were subsequently combined with LLDPE via extrusion to produce a range of nano-composites. The collective outcomes of this research are several folds. Firstly we show that presence of surface adsorbed PEG-b-PE effectively hindered the aggregation of the cellulose nano-fibers during the extrusion, affording clear homogenous materials with minimum aggregation even at the highest loading of cellulose nano-fibers (∼23 vol.%). Secondly, the tailored LLDPE/cellulose interface arising from intra- and inter-molecular hydrogen and Van der Waals bonds yielded significant levels of mechanical improvements in terms of storage and tensile modulus. We believe this study provides a simple technological template to produce high quality and performant polyolefins cellulose-based nano-composites.

Liquid phase adsorption kinetics and equilibrium of toluene by novel modified-diatomite

The adsorption equilibria of toluene from aqueous solutions on natural and modified diatomite were examined at different operation parameters such as pH, contact time, initial toluene concentration was evaluated and optimum experimental conditions were identified. The surface area and morphology of the nanoparticles were characterized by SEM, BET, XRD, FTIR and EDX analysis. It was found that in order to obtain the highest possible removal of toluene, the experiments can be carried out at pH 6, temperature 25°C, an agitation speed of 200 rpm, an initial toluene concentration of 150 mg/L, a centrifugal rate of 4000 rpm, adsorbent dosage = 0.1 g and a process time of 90 min. The results of this work show that the maximum percentage removal of toluene from aqueous solution in the optimum conditions for NONMD was 96.91% (145.36 mg/g). Furthermore, under same conditions, the maximum adsorption of natural diatomite was 71.45% (107.18 mg/g). Both adsorption kinetic and isotherm experiments were carried out. The experimental data showed that the adsorption follows the Langmuir model and Freundlich model on natural and modified diatomite respectively. The kinetics results were found to conform well to pseudo-second order kinetics model with good correlation. Thus, this study demonstrated that the modified diatomite could be used as potential adsorbent for removal of toluene from aqueous solution.

Use of optical interferometry to measure gold nanoparticle adsorption on silica

Adsorption of metal nanoparticles is at the heart of many chemical and biosensor techniques, but there are few approaches that can provide quantitative characterisation of individual nanoparticle films fabricated at different times and/or under different conditions. Using synthesised gold nanoparticles (Au NPs) as a model, the nanoparticle films were investigated using an optical interferometry technique known as fringes of equal chromatic order (FECO), which was further systematically validated against both in situ quartz crystal microbalance (QCM) and ex situ atomic force microscopy (AFM) measurements. The results indicate that the FECO wavelengths has a quantifiable red shift with increasing particle densities, making it possible to quantify the degree of surface coverage via the analysis of the fringe shift at a fixed fringe order. Moreover, the calculated formula between the FECO shifts and the surface coverage allows quantitative analysis of the whole adsorption kinetics investigated. Particularly, the as-proposed FECO technique can successfully monitor the Au NP adsorption in situ, which could be a new versatile technology platform for “online” monitoring method, for example in biosensor applications using Au NP-tagged analytes.

Non-covalent adsorption of amino acid analogues on noble-metal nanoparticles: influence of edges and vertices

The operation of many nanostructured biomolecular sensors and catalysts critically hinges on the manipulation of non-covalent adsorption of biomolecules on unfunctionalised noble-metal nanoparticles (NMNPs). Molecular-level structural details of the aqueous biomolecule/NMNP interface are pivotal to the successful realisation of these technologies, but such experimental data are currently scarce and challenging to obtain. Molecular simulations can generate these details, but are limited by the assumption of non-preferential adsorption to NMNP features. Here, via first principles calculations using a vdW-DF functional, and based on nanoscale sized NMNPs, we demonstrate that adsorption preferences to NP features vary with adsorbate chemistry. These results show a clear distinction between hydrocarbons, that prefer adsorption to facets over edges/vertices, over heteroatomic molecules that favour adsorption onto vertices over facets. Our data indicate the inability of widely used force-fields to correctly capture the adsorption of biomolecules onto NMNP surfaces under aqueous conditions. Our findings introduce a rational basis for the development of new force-fields that will reliably capture these phenomena.

Molecule-induced conformational change in boron nitride nanosheets with enhanced surface adsorption

Surface interaction is extremely important to both fundamental research and practical application. Physisorption can induce shape and structural distortion (i.e., conformational changes) in macromolecular and biomolecular adsorbates, but such phenomena have rarely been observed on adsorbents. Here, it is demonstrated theoretically and experimentally that atomically thin boron nitride (BN) nanosheets as an adsorbent experience conformational changes upon surface adsorption of molecules, increasing adsorption energy and efficiency. The study not only provides new perspectives on the strong adsorption capability of BN nanosheets and many other two-dimensional (2D) nanomaterials but also opens up possibilities for many novel applications. For example, it is demonstrated that BN nanosheets with the same surface area as bulk hexagonal BN particles are more effective in purification and sensing.