Liquid–Solid Mass Transfer in Adsorption Systems—An Overlooked Resistance?

Refractory COD removal from bio-treated paper wastewater using powdered activated coke adsorption technology with ozonation regeneration: Performance and molecular insights

The present study employed powdered activated coke (PAC) for the adsorptive removal of refractory COD from the bio-treated paper wastewater (BTPW). The adsorption reached equilibrium after 3 h, resulting in a decrease in the COD concentration from 98.9 mg L-1 in BTPW to 42.6 mg L-1 when utilizing a PAC dosage of 5 g L-1. The dominant fractions of dissolved organic matter in BTPW were hydrophilic acids (HIA), hydrophilic neutrals (HIN), and hydrophobic acids (HOA), accounting for 48.8%, 34.2%, and 17.0% of the total dissolved organic carbon, respectively. Three fractions were all predominantly composed of humic/fulvic acid-like substances, while the HOA fraction exhibited highest susceptibility to adsorption by PAC, followed by the HIA and HIN fractions. FT-ICR MS data revealed PAC preferentially adsorbed the unsaturated and oxygen-rich substances containing more carboxyl groups. Additionally, the spent PAC was regenerated through ozonation and subsequently utilized in the adsorption cycles. The regeneration was successfully conducted under an ozone concentration of 1 mg L-1 for a duration of 10 min, and the regeneration efficiency remained about 87.0% even after undergoing five-cycle of adsorption-regeneration. The findings of this study demonstrate that PAC adsorption is a viable and efficacious treatment technology for efficiently removing refractory COD from BTPW.

Efficient removal of organophosphate insecticide employing magnetic chitosan-derivatives

Herein, this work reports an efficient acephate adsorption using chitosan (CS) incorporating varying amounts of magnetite. A co-precipitation methodology was employed for the functionalization of chitosan with iron nanoparticles, using Fe2+ as the sole iron source and with a low energy requirement. The adsorbents were characterized by FTIR, XRD, VSM, and nitrogen porosimetry techniques. The CS∙Fe3O4 1:1 NPs showed the highest acephate removal percentage (74.96 %) at pH 9 and ambient temperatures. The adsorption process exhibited high dependencies on pH, adsorbent dosage, initial concentration of adsorbate, and ionic strength. Sips and pseudo-second-order kinetics models best adjusted the experimental data, suggesting that the process occurs on a heterogeneous surface. Thermodynamic evaluation showed that the adsorption was exothermic, favorable, and predominately through chemical interactions. Finally, the CS∙Fe3O4 showed no significant decrease after several cycles of adsorption/desorption, avoiding centrifugation-filtration steps.

Ionic liquids functionalized chitosan: An effective, rapid and green adsorbent for gold recovery

Thiosulfate has been considered as a more environmentally-friendly alternative to cyanide salts for the extraction of gold from gold ores and the development of affordable, green and efficient adsorbents for the isolation of gold-thiosulfate complex (Au(S2O3)23-) from the leaching solution remains a significant challenge. To address this issue, chitosan, a natural macromolecule, was selected as a carrier and chemically modified with ionic liquids. The ionic liquids modified chitosan showed greater adsorption capacity towards Au(S2O3)23- compared with pristine chitosan. The adsorption of Au(S2O3)23- on ionic liquid modified chitosan followed Freundlich isotherm and pseudo-second order kinetic models, involving an anion-exchange mechanism with liquid film diffusion as the rate-limiting step. The chitosan modified with butylimidazolium-based ionic liquid had an adsorption capacity of 5.0 mg g-1 for gold (10 mg L-1 of gold, pH 6, 2 g L-1 of adsorbent dosage), outperforming other reported adsorbents. The ionic liquid modified chitosan showed a high adsorption efficiency of up to 96.7 % for Au(S2O3)23- in an actual thiosulfate leaching solution with a desorption efficiency of 98.4 %, suggesting that the ionic liquid modified chitosan has the potential to be a eco-friendly, biocompatible and effective adsorbent for the recovery of Au(S2O3)23-.

Near Equilibration Times in the Diffusion-Controlled Adsorption

A model of the Frumkin isotherm following adsorption on the surface of the rotating disk is developed with the purpose to calculate the time that is needed for the establishment of the near equilibrium state between the amount of adsorbate and the bulk concentration of surfactant. The "near" is quantified as 99% of the proper equilibrium surface coverage. The near equilibration time depends on the bulk concentration and on the coefficient of molecular attraction in the adsorbed layer. In the vicinity of the inflection point of the isotherm, this time exhibits sharp spike. It is caused by the diminished flux that increases surface coverage very slowly but continues for a long time. Eventually, both the flux and the surface coverage increase with time drastically under the influence of the exponential part of the Frumkin isotherm.

Use of waste marble powder for the synthesis of novel calcium-rich biochar: Characterization and application for phosphorus recovery in continuous stirring tank reactors

This study investigates-for the first time-the synthesis of a novel Ca-rich biochar (N-Ca-B) and its potential use for phosphorus (P) recovery from both synthetic solutions (SS) and treated urban wastewater (TUW) in a continuous stirring tank reactor (CSTR) mode. The novel biochar was synthesized by pyrolysis at 900 °C of a mixture composed of three different materials: animal biomass (poultry manure; PM), lignocellulosic waste (date palm fronds; DPFs), and abundant mineral waste (waste marble powder; WMP). Characterization of N-Ca-B showed that it has good textural properties: well-developed porosity, and high specific surface area. Furthermore, high calcium hydroxide (Ca(OH)2) and calcium oxides (CaO) nanoparticle loads were observed on the biochar surface. The dynamic CSTR assays indicated that the P recovery efficiency mainly depended on the biochar mass, P influent concentration, and, especially, the Ca content of the feeding solution. Owing to its richness in Ca cations, TUW exhibited the highest adsorbed P amount (109.2 mg g-1), i.e., about 14% larger than the SS. P recovery occurs through precipitation as hydroxyapatite, surface complexation, and electrostatic interactions with positively charged biochar particles. In real-world scenarios, CSTR systems can be applied as a tertiary treatment step in existing wastewater treatment plants (WWTPs). Decanted P-loaded biochar can be used in agriculture as a slow-release fertilizer instead of commercial products.

Sustainable use of low-cost adsorbents prepared from waste fruit peels for the removal of selected reactive and basic dyes found in wastewaters

Agricultural wastes are potential sustainable adsorbents since they are available in large quantities, are low-cost, and may require little or no treatment, in some cases. In this study, several fruit peels, such as banana, orange, and pomegranate, were collected from local markets and prepared by a simple and eco-friendly method and used as natural adsorbents for the removal of both anionic (Reactive Red 120 (RR120), Reactive Black 5 (RB5), Remazol Brilliant Blue R (RBBR)) and cationic Methylene Blue (MB) dyes found in wastewaters. Many industries, such as leather and textiles, can release huge amounts of synthetic dyes into the wastewater during dyeing processes. These are one of the most important pollutants of water pollution as they cause enormous damage to the water body and also affect the health of organisms due to their toxicity and carcinogenicity. The search for a sustainable and at the same time efficient material for the removal of a wide variety of dyes is the innovation of this work. These peels were prepared by washing, drying, grinding, and finally sieving, under natural sustainable conditions. Porosometry (BET analysis), FTIR, SEM/EDS, and XRD techniques were used to characterize the fruit peels before and after the adsorption process. Factors affecting the adsorption of dyes (adsorbent dosage, pH solution, initial concentration of dyes, contact time, and temperature) were investigated. According to the results, in terms of the effectiveness of fruit peels as (natural) adsorbent materials, for anionic dyes, 5.0-6.0 g/L of banana or orange dry peels was sufficient to remove near or even more than 90% anionic dyes at pH 2.0, and 4.0 g/L was sufficient to remove 98% of cationic MB dye at pH 9.0. Similar amount of pomegranate peels had lower efficiency for anionic dyes (50-70%), while cationic MB was still efficiently removed (98%) at pH 9.0. Moreover, the adsorption process in all cases was found to better fit to pseudo-second-order model, in comparison to pseudo-first-order model. According to isotherms, Freundlich model fitted better in some cases to the equilibrium data, while the Langmuir model in others. Finally, this study demonstrates the viability of reusing the banana, orange, and pomegranate peel adsorbents for eight, four, and five cycles, showing a gradual reduction of around 50% of their effectiveness.

Novel Machine Learning-Based Method for Estimation of the Surface Area of Porous Silica Particles

This work reports a novel and quick method to estimate the surface area of porous materials. Conventionally, surface area measurement requires the BET method/N2 adsorption experiment which is time-consuming. In this work, we developed a method based on machine learning (ML) and the adsorption of a conductive dye on porous materials. The rate and quantity of dye adsorption, which is characterized by dynamic measurement of conductivity, provide an indirect measure of surface area and zeta potential. An ML-based soft sensor is developed to relate the measured conductivity profiles with surface area and zeta potential. A phenomenological model on dye adsorption is also developed, validated, and used to augment experimental data for training the soft sensor. The developed method was tested for porous silica particles with a range of surface areas (250-1100 m2/g) and zeta potential (-17 mV: -29 mV). The developed soft sensor was able to estimate the surface area and zeta potential quite well. The developed approach and method reduce overall measurement time for surface area from several hours to a few minutes. The method can potentially be implemented in continuous plants producing porous materials like silica.

Ni(II) and Pb(II) Removal Using Bacterial Cellulose Membranes

Bacterial cellulose (BC) is a highly crystalline nanosized material with a high number of active groups. This study focuses on the synthesis of BC membranes through fermentation, their characterization and application to remove Ni(II) and Pb(II) from wastewater by adsorption under different conditions. Four-day-grown BC membranes form three-dimensional nanofibril networks with a pH of 6.3 and a high cationic demand (52.5 μeq·g-1). The pseudo-second-order kinetic model and the Sips isotherm model best describe the adsorption of both metals. The intraparticle diffusion model of Ni(II) revealed a three-step mechanism of adsorption-plateau-adsorption, while Pb(II) adsorption followed a typical reducing-slope trend up to saturation. The highest removal of Ni(II) and Pb(II) was obtained at pH 4 with a BC dosage of 400 mg·L-1. The maximum adsorption capacities were 28.18 mg·g-1 and 8.49 mg·g-1 for Ni(II) and Pb(II), respectively, involving the total coverage of the material active sites. Thermodynamically, Ni(II) adsorption was exothermic, and Pb(II) was endothermic. The obtained values of sorption heat, activation and Gibbs' energy depicted a physisorption process. Ni(II) removal mechanism was ruled by crystallization on the metals adsorbed on the BC active groups, while Pb(II) was driven by the adsorption process, as shown by TEM images of the spent material.

An Exploratory Study of Hydrochar as a Matrix for Biotechnological Applications

This paper explores the potentialities of hydrochar in protein separation and enzyme immobilization for non-energy biorefinery applications of hydrothermal carbonization. An innovative experimental procedure monitors soluble protein-hydrochar interactions and enzymatic reactions in a continuously stirred tank reactor. The hydrochar comes from hydrothermal carbonization of silver fir (200 °C, 30 min, 1/7 solid/water ratio) and standard activation (KOH, oven, 600 °C). Bovine serum albumin, a non-active, globular protein, was adsorbed at ≤3300 mg/g. Sip's isotherms fitted data well (R2 = 0.99999). The immobilization used a commercial β-glucosidase, which catalyzes the hydrolysis of cellobiose to glucose, a bottleneck of the cellulose to fermentable sugar bioconversion network due to the fast enzyme deactivation. The hydrochar adsorbed ≤26 w/w% of enzyme. The heterogeneous biocatalyst operational stability was 24 times that of the soluble one. The results encourage further investigations and foreshadow process schemes coupling hydrothermal carbonization and industrial bioconversions.

The Adsorption Efficiency of Regenerable Chitosan-TiO2 Composite Films in Removing 2,4-Dinitrophenol from Water

In this work, the great performance of chitosan-based films blended with TiO₂ (CH/TiO₂) is presented to adsorb the hazardous pollutant 2,4-dinitrophenol (DNP) from water. The DNP was successfully removed, with a high adsorption %: CH/TiO₂ exhibited a maximum adsorption capacity of 900 mg/g. For pursuing the proposed aim, UV-Vis spectroscopy was considered a powerful tool for monitoring the presence of DNP in purposely contaminated water. Swelling measurements were employed to infer more information about the interactions between chitosan and DNP, demonstrating the presence of electrostatic forces, deeply investigated by performing adsorption measurements by changing DNP solutions' ionic strength and pH values. The thermodynamics, adsorption isotherms, and kinetics were also studied, suggesting the DNP adsorption's heterogeneous character onto chitosan films. The applicability of pseudo-first- and pseudo-second-order kinetic equations confirmed the finding, further detailed by the Weber-Morris model. Finally, the adsorbent regeneration was exploited, and the possibility of inducing DNP desorption was investigated. For this purpose, suitable experiments were conducted using a saline solution that induced the DNP release, favoring the adsorbent reuse. In particular, 10 adsorption/desorption cycles were performed, evidencing the great ability of this material that does not lose its efficiency. As an alternative approach, the pollutant photodegradation by using Advanced Oxidation Processes, allowed by the presence of TiO₂, was preliminary investigated, opening a novel horizon in the use of chitosan-based materials for environmental applications.

Comparative effect of mesoporous carbon doping on the adsorption of pharmaceutical drugs in water: Theoretical calculations and mechanism study

In this study, mesoporous doped-carbons were synthesized from sucrose, a natural source, boric acid and cyanamide as precursors, generating B- or N-doped carbon. These materials were characterized by FTIR, XRD, TGA, Raman, SEM, TEM, BET, and XPS, confirming the preparation of a tridimensional doped porous structure. B-MPC and N-MPC showed a high surface specific area above 1000 m²/g. The effect of B and N doping on mesoporous carbon was evaluated on the adsorption of emerging pollutants from water. Diclofenac sodium and paracetamol were used in adsorption assays, reaching removal capacities of 78 and 101 mg.g^-1, respectively. Kinetic and isothermal studies indicate the chemical nature of adsorption controlled by external and intraparticle diffusion and multilayer formation due to strong adsorbent/adsorbate interactions. DFT-based calculations and adsorption assays infer that the main attractive forces are hydrogen bonds and Lewis acid-base interactions.

Ozone β-Cyclodextrin Inclusion Complex Characterization and Application in the Remediation of Total Petroleum Hydrocarbons

Green remediation is essential in the current practice of water resources management. In this study, a series of ozone β-cyclodextrin (O3-βCD) inclusion complexes were prepared under a selected range of different ozone concentrations, β-CD concentrations, and solution pHs to test their ozone release rates and efficiencies in the treatment of total petroleum hydrocarbons (TPH) in water. The main objectives of this study are to characterize the O3-βCD system, mathematically model its ozone release rate, and test its capability in the degradation of pollutants. From the results, it was found that by defining a set of dimensionless parameters, including β-CD to ozone molar ratio and various degrees of ozone saturation, the steady-state conditions in the O3-βCD system can be represented by a newly developed dimensionless plot. In an optimal condition, the dissolved ozone release rate of 6.8 × 10−5 mM/min can be achieved in the O3-βCD system. A mathematical model was successfully developed to estimate the ozone release rate. In the TPH removal experiments, the effects of β-CD to ozone molar ratio and ozone dosage on the removal efficiency were rigorously examined. Overall, an optimal TPH removal of nearly 90% can be achieved in the treatment of 50 mg/L of TPH in water using this inclusion complex reagent.

Magnetic biocomposite based on peanut husk for adsorption of hexavalent chromium, Congo red and phosphate from solution: Characterization, kinetics, equilibrium, mechanism and antibacterial studies

A biocomposite (PN-Fe₃O₄-PEI) was synthesized via the chemical modification of peanut husk (a low-cost adsorbent) with Fe₃O₄ particles and polyethyleneimine under benign environmental conditions. The modification agents used in this study were observed to overcome the challenges associated with the use pristine peanut husk with a concomitant enhancement in its efficiency as an adsorbent. Results from the characterization studies employed in this study confirmed PN-Fe₃O₄-PEI to be a crystalline magnetic adsorbent with a mesoporous structure. The adsorption property of the developed material (PN-Fe₃O₄-PEI) for wastewater treatment was investigated using Chromium (VI), Phosphates (PO₄^3-) and Congo red (CR) as model pollutants. Using the batch method, PN-Fe₃O₄-PEI exhibited a maximum monolayer adsorption capacity of 58.4, 13.5 and 71.3 mg g^-1 for Cr(VI), PO₄^3- (as P g L^-1) and CR, respectively and was dependent on temperature and initial adsorbate concentration. Kinetic studies revealed that the Elovich equation, the pseudo-second order kinetic model and double constant equation well described the uptake of Cr(VI), PO₄^3- and CR onto PN-Fe₃O₄-PEI, respectively. These results may confirm the uptake of these pollutants to be mainly driven by chemical forces. In addition, PN-Fe₃O₄-PEI was observed to be efficient for the decontamination of the studied pollutants in real water samples as well as exhibit antibacterial properties towards the growth of S. aureus. These properties of PN-Fe₃O₄-PEI with its other excellent features such as high stability in solution, good regeneration properties and its facile retrieval from the solution using a magnet promote its suitability for practical wastewater treatment.

Numerical investigation of ibuprofen removal from pharmaceutical wastewater using adsorption process

In the present study, a mathematical modelling was developed to investigate ibuprofen adsorption from pharmaceutical wastewater into activated carbon and sonicated activated carbon. The developed model was dissolved based on the finite element method. Effect of different operating parameters including particle porosity and diameter as well as ibuprofen diffusion coefficient in solution on the amount of ibuprofen adsorption at different time point and position in the particle were evaluated. It was found good agreement between experimental values and modelling results in terms of ibuprofen adsorption as a function time. The 84.5% and 92.5% of maximum adsorption was achieved for the AC and SAC at the centre of particle after 150 min. Increasing the particle porosity and ibuprofen diffusion coefficient was improved the ibuprofen adsorption into the adsorbent. However, the particle diameter had negative impact on the system performance. There was a decrease in solute adsorption from 84.10 to 7.30 mg/g and from 106 to 15.73 mg/g for the AC and SAC respectively with increasing the particle radius from 173 to 500 µm. Finally, it was concluded that the particle specifications play important role in the adsorption process as it was observed considerable change in the amount of adsorption at different positions in the particle with changing the particle specifications.