Insight into the adsorption kinetics models for the removal of contaminants from aqueous solutions

Adsorption Behavior of Pb(II), Cd(II), and Zn(II) onto Agave Bagasse, Characterization, and Mechanism

Biosorption is an alternative procedure to remove metal ions from aqueous media using agricultural waste. In this work, the adsorption capacity and removal efficiency of agave bagasse (AB) toward Pb(II), Cd(II), and Zn(II) were analyzed. Parameters such as equilibrium pH, particle size, AB dosage, time, and initial metal ion concentration were discussed. The results showed that pH 5.5, 0.4 g (<250 μm), and only 15 min of contact assured conditions for maximum adsorption capacity. The kinetic studies were fitted to the pseudo-second-order model, whereas the isotherms showed good agreement with the Langmuir model. AB has a higher affinity for Pb(II) over Cd(II) and Zn(II), and the maximum adsorption capacities were 93.14, 28.50, and 24.66 mg g^-1, respectively. The results of the characterization evidenced two adsorption mechanisms. Scanning electron microscopy and X-ray diffraction displayed adsorption via the ion exchange mechanism by releasing Ca(II). The ¹³C cross-polarization mode with magic-angle spinning nuclear magnetic resonance analysis demonstrated a complexation mechanism by cellulose, hemicellulose, and lignin groups with Pb(II) and Cd(II), whereas the complexation is mainly observed by cellulose groups for Zn(II). AB is a good alternative for the removal of metals without prior thermal or chemical treatment, with rapid kinetics, suitable adsorption capacity, and high removal efficiency contributing to waste management.

Development of electrospun lignin nanofibers for the adsorption of pharmaceutical contaminants in wastewater

Emerging contaminants present a challenge for water preservation, threatening humans' health and all ecosystems. They consist of a variety of molecules ranging from pharmaceutical and personal care products to pesticides and endocrine disruptors detectable in wastewater, sewage effluent, surface water, drinking water, and ground waters at trace level concentrations (e.g., ng/L, μg/L). Conventional wastewater treatment plants (WWTPs) possess low efficiency to remove them. Therefore, new technologies capable of removing such residues are needed. Lignin recognized as a renewable and abundant biopolymer is transformed through electrospinning into an anionic nanofibrous nonwoven adsorbent to extract those contaminants and dispose them safely. Electrospinning allows the manufacture of fibers at the micro- or nanoscale under the influence of an electric current. In this study, nanofibers of alkali lignin and a co-polymer, poly(vinyl alcohol), were developed and tested on the adsorption of a pharmaceutical contaminant (fluoxetine) in an aqueous solution. Results showed that the lignin nanofibers, of 156 nm in diameter, adsorbed 70% of fluoxetine in solution which corresponds to 32 ppm of contaminants removed in water.

Evaluation of phosphate adsorption on zirconium/magnesium-modified bentonite

In this work, a zirconium/magnesium-modified bentonite (ZrMgBT) was prepared and characterized by SEM, EDS, XRD and pH_PZC. The performance and mechanism of phosphate adsorption onto ZrMgBT was evaluated in detail using batch experiments and ³¹P NMR. Results showed the adsorption isotherm data were well described by the Langmuir, Freundlich and Dubinin-Radushkevich models, and the kinetic data fitted better to the pseudo-second-order kinetic model than the pseudo-first-order kinetic model. The phosphate adsorption capacity of ZrMgBT was slightly affected by the presence of Na⁺, K⁺, Cl^-, [Formula: see text] and [Formula: see text], but it was enhanced by coexisting Mg²⁺ and [Formula: see text]. The mechanism for phosphate adsorption onto ZrMgBT at pH 7 was mainly the complexation reaction between phosphate and zirconium. In addition, ZrMgBT exhibited more excellent adherence to phosphate than zirconium-modified bentonite (ZrBT). Especially, the maximum monolayer phosphate adsorption capacity for ZrMgBT at pH 7 and 0.5 g/L of adsorbent dosage calculated based on the Langmuir isotherm model (13.0 mg P/g) was 67.5% higher than that for ZrBT. The higher phosphate adsorption capacity for ZrMgBT than ZrBT could be attributed to the higher specific surface area as well as higher Mg²⁺ releasing ability of the former. The enhancement of phosphate adsorption by the release of Mg²⁺ from ZrMgBT could be mainly due to the formation of [Formula: see text] in the solution firstly and then the adsorption of [Formula: see text] on ZrMgBT forming ≡Zr(OPO₃H)Mg on the ZrMgBT surface. In general, we conclude that ZrMgBT is a more promising adsorbent for phosphate removal from aqueous solution than ZrBT.

Removal of Antibiotics from Water by Polymer of Intrinsic Microporosity: Isotherms, Kinetics, Thermodynamics, and Adsorption Mechanism

Traces of antibiotics within domestic and industrial effluents have toxic impact on human health as well as surrounding flora and fauna. Potential increase in antibiotic resistance of microorganisms is likely to rise due to the incomplete removal of antibiotics by traditional wastewater processing, methods such as membrane filtration and biological treatment. In this study, we investigated a novel class of material termed Polymer of Intrinsic Microporosity (PIM) that is based on amorphous microporous organic materials for the application of antibiotic removal form aqueous environments. The adsorption of four commonly used antibiotics (doxycycline, ciprofloxacin, penicillin G, and amoxicillin) was evaluated and found that at least 80% of the initial concentrations was eliminated under the optimized conditions. Langmuir and Freundlich models were then employed to correlate the equilibria data; the Freundlich model fit well the data in all cases. For kinetic data, pseudo-first and second order models were examined. Pseudo-second order model fit well the kinetic data and allowed the calculation of the adsorption rate constants. Thermodynamic parameters were obtained by conducting the adsorption studies at varied reaction temperatures. Surface potential, adsorption at various solution pHs, thermogravimetric analysis (TGA), Infrared spectroscopy (IR), and surface area experiments were conducted to draw possible adsorption mechanisms. The removal of antibiotics from water by PIM-1 is likely to be governed by both surface and pore-filling adsorption and could be facilitated by electrostatic interactions between the aromatic rings and charged functional groups as well as hydrogen bond formation between the adsorbent and adsorbate. Our work shows that the application of such novel microporous material could contribute to the removal of such challenging and persistent contaminants from wastewater with further optimizations of large-scale adsorption processes.

Food Waste Materials as Low-Cost Adsorbents for the Removal of Volatile Organic Compounds from Wastewater

The aim of this work was to study the potential of food waste materials (banana peel, potato peel, apple peel, lemon peel, coffee waste, decaf coffee waste, grape waste, and carob peel) as low-cost adsorbents for the removal of aliphatic and aromatic volatile organic compounds (VOCs) from wastewater. The ability of examined food waste materials to adsorb VOCs from synthetic multi-component standard solutions was evaluated and the examined food waste materials showed high removal efficiency. Performances of coffee waste, grape waste, and lemon peel were detailed by using Trichloroethylene and p-Xylene in mono-component standard solutions. The adsorption capacity of the three selected food wastes was determined by using linear Langmuir and Freundlich isotherm models. Two errors functions, average percentage error (APE) and the chi-square test (χ²), were used for isotherm optimization prediction. Freundlich isotherm well described the adsorption of VOCs on the considered materials. According to the obtained results, a multilayer, physical, and cooperative adsorption process was hypothesized, particularly evident when the VOCs’ concentrations are high. This was confirmed by the high adsorption efficiency percentages (E% > 80%) of VOCs from a real polluted matrix (urban solid waste leachate), containing high concentrations of total organic content.

Insights into the isotherm and kinetic models for the coadsorption of pharmaceuticals in the absence and presence of metal ions: A review

Pharmaceuticals are a wide class of emerging pollutants due to their continuous and the increasing consumption of users. These pollutants are usually found in the real environment as mixtures alone or with metal ions. Thus, the migration risk increases, which complicates the removal of pharmaceuticals because of the combined and synergistic effects. The focus of treatment of pharmaceutical mixtures and their coexistence with metals is of considerable importance. For this purpose, adsorption has been efficiently applied to several studies for the treatment of such complex systems. In this article, the coadsorption behavior of pharmaceuticals in the absence and existence of metals on several adsorbents has been reviewed. The adsorption isotherms and kinetics of these two systems have been analyzed using different models and discussed. Important challenges and promising routes are suggested for the future development of the coadsorption of the studied systems. This article provides an overview on the most utilized and effective adsorbents, widely studied adsorbates, best applied isotherm and kinetic models, and competitive effect in coadsorption of pharmaceuticals, both with and without metals.

Microbe Encapsulation for Selective Rare-Earth Recovery from Electronic Waste Leachates

Rare earth elements (REEs) are indispensable components of many green technologies and of increasing demand globally. However, refining REEs from raw materials using current technologies is energy intensive and enviromentally damaging. Here, we describe the development of a novel biosorption-based flow-through process for selective REE recovery from electronic wastes. An Escherichia coli strain previously engineered to display lanthanide-binding tags on the cell surface was encapsulated within a permeable polyethylene glycol diacrylate (PEGDA) hydrogel at high cell density using an emulsion process. This microbe bead adsorbent contained a homogenous distribution of cells whose surface functional groups remained accessible and effective for selective REE adsorption. The microbe beads were packed into fixed-bed columns, and breakthrough experiments demonstrated effective Nd extraction at a flow velocity of up to 3 m/h at pH 4-6. The microbe bead columns were stable for reuse, retaining 85% of the adsorption capacity after nine consecutive adsorption/desorption cycles. A bench-scale breakthrough curve with a NdFeB magnet leachate revealed a two-bed volume increase in breakthrough points for REEs compared to non-REE impurities and 97% REE purity of the adsorbed fraction upon breakthrough. These results demonstrate that the microbe beads are capable of repeatedly separating REEs from non-REE metals in a column system, paving the way for a biomass-based REE recovery system.

Biochar synthesis from sweet lime peel for hexavalent chromium remediation from aqueous solution

Sweet lime (Citrus limetta) peel biochar was obtained by slow pyrolysis of raw biomass at 450 °C with 5 °C/min heating rate. Proximate and ultimate analysis, physico-chemical characterization of the biochar was done. Batch adsorption experiments for Cr(VI) removal were performed with varying pH, biochar dose, contact time and initial Cr(VI) concentrations. It took 8-24 h to reach the equilibrium at 30 °C for varying Cr(VI) concentrations. The biochar was found to possess higher adsorption capacity (100 mg/g) than the adsorbents reported in several previous studies. Langmuir adsorption isotherm and pseudo second order model best explained the experimental data, suggesting monolayer adsorption as the dominant mechanism. Chemical interaction, ion exchange of solute and sorbate ions and physical adsorption also contributed into Cr(VI) adsorption process. Further, Cr(VI) adsorption was found to be a multistep process. The findings suggested that sweet lime peel biochar can be utilized as a low cost and efficient alternative for Cr(VI) removal, which could be useful for aqueous solutions, as well as to promote overall protection against soil and water degradation and pollution.

Effect of the Surface Charge on the Adsorption Capacity of Chromium(VI) of Iron Oxide Magnetic Nanoparticles Prepared by Microwave-Assisted Synthesis

Solid phase extraction using magnetic nanoparticles has represented a leap forward in terms of the improvement of water quality, preventing the contamination of industrial effluents from discharge in a more efficient and affordable way. In the present work, superparamagnetic iron oxide nanoparticles (MNP) with different surface charges are tested as nanosorbents for the removal of chromium(VI) in aqueous solution. Uniform magnetic nanoparticles (~12 nm) were synthesized by a microwave polyol-mediated method, and tetraethyl orthosilicate (TEOS) and (3-aminopropyl) triethoxysilane (APTES) were grafted onto their surface, providing a variation in the surface charge. The adsorptive process of chromium was evaluated as a function of the pH, the initial concentration of chromium and contact time. Kinetic studies were best described by a pseudo-second order model in all cases. TEOS@MNP barely removed the chromium from the media, while non-grafted particles and APTES@TEOS@MNP followed the Langmuir model, with maximum adsorption capacities of 15 and 35 mgCr/g, respectively. The chromium adsorption capacities abruptly increased when the surface became positively charged as the species coexisting at the experimental pH are negatively charged. Furthermore, these particles have proven to be highly efficient in water remediation due their 100% reusability after more than six consecutive adsorption/desorption cycles.

Electrochemical method to study the environmental behavior of Glyphosate on volcanic soils: Proposal of adsorption-desorption and transport mechanisms

Glyphosate is used extensively worldwide, but current evidence suggests detrimental effects on the environment, pollinators, and human health. Glyphosate adsorption kinetics and adsorption/desorption were studied through batch sorption experiments in ten typical volcanic ash-derived soils from Andisol and Ultisol orders. Two kinetic models were used to fit the experimental data: i. Models that allowed establishment of principally kinetic parameters and modeling of the adsorption process, and ii. Models described solute transport mechanisms commonly used for remediation purposes. Adsorption kinetic data were best fitted by the pseudo-second-order kinetic model and Two-Site Nonequilibrium model. These models suggest that mechanisms are complex due to rapid surface adsorption in ultisols with mass transfer controlling adsorption kinetics across the boundary layer, as indicated by the highhand lowt_1/2values. High intraparticle diffusion into macropores and micropores was observed for Andisols. The Freundlich model accurately represented adsorption equilibrium data in all cases (R² > 0.9580) with comparatively higher adsorption capacity on Andisols. K_f values (2.50-52.28 μg^1-1/n mL^1/n g^-1) and hysteresis were significant in all studied soils. Taken together, these data suggest that Glyphosate may be adsorbed more on Andisol soils in comparison to Ultisols.

Adsorption Characteristics and Transport Behavior of Cr(VI) in Shallow Aquifers Surrounding a Chromium Ore Processing Residue (COPR) Dumpsite

This study explored the stratigraphic distribution and soil/shallow aquifer characteristics surrounding a chromium ore processing residue (COPR) dumpsite at a former chemical factory in China. Total Cr levels in top soils (5–10 cm) nearby the COPR dumpsite were in the range of 8571.4–10711.4 mg/kg. Shallow aquifers (1–6 m) nearby the COPR dumpsite showed a maximum total Cr level of 9756.7 mg/kg. The concentrations of Cr(VI) in groundwater nearby the COPR dumpsite were 766.9–1347.5 mg/L. These results display that the top soils, shallow aquifers, and groundwater of the study site are severely polluted by Cr(VI). Then, three aquifers (silt, clay, and silty clay), respectively, collected from the depth of 1.4–2.4 m, 2.4–4.8 m, and 4.8–11.00 m were first used to evaluate the adsorption characteristics and transport behavior of Cr(VI) in shallow aquifers by both batch and column experiments. The adsorption of Cr(VI) on tested aquifers was well described by pseudo-second-order equation and Freundlich model. The adsorption capacities of Cr(VI) on three aquifers followed the order: clay > silty clay > silt. The kinetics proved that Cr(VI) is not easily adsorbed by the aquifer mediums but transports with groundwater. Thermodynamics indicated that Cr(VI) adsorption on tested aquifers was feasible, spontaneous, and endothermic. Cr(VI) adsorption on tested aquifers decreased with increasing pH. Furthermore, the transport of Cr(VI) in adsorption columns followed the sequence of clay < silty clay < silt. Desorption column experiments infer that the Cr(VI) adsorbed on aquifers will desorb and release into groundwater in the case of rainwater leaching. Therefore, a proper treatment of the COPR and a comprehensive management of soils are vital to prevent groundwater pollution.

Development of novel sericin and alginate-based biosorbents for precious metal removal from wastewater

In this study, two novel low water-soluble sericin and alginate-based biosorbents were successfully developed for precious metal removal from wastewater: sericin and alginate particles chemically crosslinked by proantocyanidins (SAPAs) and sericin, alginate and polyvinyl alcohol particles (SAPVA). The proportions of proantocynidins (PAs) or polyvinyl alcohol (PVA) added to sericin (2.5% w/v) and alginate (2.0% w/v) blend were 0.5, 1.5, 2.5 and 3.5% w/v. Among these concentrations, particles produced with 0.5% w/v of PVA or 2.5% w/v of PAs presented the lowest water solubility percentages (3.74 ± 0.05 and 3.56 ± 0.21%, respectively) and the following metallic affinity order: AuCl₄^- > PdCl₄^2- > PtCl₆^2- > Ag⁺. Then, gold biosorption kinetics by SAPAs was evaluated at three gold initial concentrations (72.88, 187.12, and 273.79 mg/L), and its performance was compared to activated carbon adsorbent uptake. The data modeling revealed that the process follows pseudo-first-order kinetics and is mainly controlled by external diffusion. SAPAs before and after gold biosorption (SAPAs-gold) were characterized by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, optical microscopy, helium pycnometry, mercury porosimetry, N₂ physisorption, and Fourier-transform infrared spectroscopy.

Adsorptive removal of azithromycin from aqueous solutions using raw and saponin-modified nano diatomite

This study aims to investigate the performance and mechanism of raw (R-ND) and saponin-modified nano diatomite (M-ND) in the removal of azithromycin from aqueous solutions. Adsorbent characterization was performed using X-ray fluorescence, Brunauer-Emmett-Teller (BET), scanning electron spectroscopy, dynamic light scattering and energy-dispersive X-ray analyses. It was shown that the specific surface area of R-ND was 119.5 m²/g, 14-fold higher than that for raw diatomite, and for M-ND it was 90.1 m²/g. Various adsorption conditions, i.e. adsorbent dosage, pH, initial concentration and contact time were investigated. According to the results, despite reducing the specific surface area by 25%, modification of nano diatomite by saponin markedly enhanced its performance in the removal of azithromycin. The maximum adsorption capacity of R-ND and M-ND in the removal of azithromycin was 68 and 91.7 mg/g, respectively. Fourier transform infrared spectroscopy results revealed that azithromycin was adsorbed by O-H groups on the diatomite surface. Weber-Morris intra-particle diffusion (IPD) model suggested that while IPD is not the rate-controlling step in high concentrations of azithromycin, it is the only step that controls the rate of adsorption in low concentrations. In comparison to R-ND, M-ND showed a higher efficiency in the removal of azithromycin and, therefore, it can be used as a promising low-cost adsorbent to remove azithromycin from aqueous solutions.

Enhanced Adsorptive Properties and Pseudocapacitance of Flexible Polyaniline-Activated Carbon Cloth Composites Synthesized Electrochemically in a Filter-Press Cell

Electrochemical polymerization is known to be a suitable route to obtain conducting polymer-carbon composites uniformly covering the carbon support. In this work, we report the application of a filter-press electrochemical cell to polymerize polyaniline (PAni) on the surface of large-sized activated carbon cloth (ACC) by simple galvanostatic electropolymerization of an aniline-containing H₂SO₄ electrolyte. Flexible composites with different PAni loadings were synthesized by controlling the treatment time and characterized by means of Scanning Electron microscopy (SEM), X-Ray Photoelectron Spectroscopy (XPS), physical adsorption of gases, thermogravimetric analysis (TGA), cyclic voltammetry and direct current (DC) conductivity measurements. PAni grows first as a thin film mostly deposited inside ACC micro- and mesoporosity. At prolonged electropolymerization time, the amount of deposited PAni rises sharply to form a brittle and porous, thick coating of nanofibrous or nanowire-shaped structures. Composites with low-loading PAni thin films show enhanced specific capacitance, lower sheet resistance and faster adsorption kinetics of Acid Red 27. Instead, thick nanofibrous coatings have a deleterious effect, which is attributed to a dramatic decrease in the specific surface area caused by strong pore blockage and to the occurrence of contact electrical resistance. Our results demonstrate that mass-production restrictions often claimed for electropolymerization can be easily overcome.

Removal and extraction efficiency of Quaternary ammonium herbicides paraquat (PQ) from aqueous solution by ketoenol-pyrazole receptor functionalized silica hybrid adsorbent (SiNPz)

Pesticides and herbicides have been used extensively in agricultural practices to control pests and increase crop yields. Paraquat (PQT²⁺, 1,1-dimethyl-4,4-dipyridinium chloride) is one of the herbicide that belois classified as bipyridines and is used over the world. The objective of this study is to use ketoenol–pyrazole receptor functionalized silica hybrid as adsorbent for removal PQT²⁺ from aqueous solution. The adsorbent was synthesized, and characterized using scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), Thermal analysis and other techniques. Different experimental parameters such as the effect of the amount of adsorbent, solution pH and temperatures and contact times were studied. Pseudo-order kinetics models were studied, and our data followed a pseudo second order. Experimental data were analyzed for both Langmuir and Freundlich models and the data fitted well with the Langmuir isotherm model. To understand the mechanism of adsorption, thermodynamic parameters like standard enthalpy, standard Gibbs free energy, and standard entropy were studied. The study indicated that the process is spontaneous, exothermic in nature and follow physisorption mechanisms. The novelty of this study showed surface of pyrazol-enol-imine-substituted silica (SiNPz) has the ability to highlight the surface designed for efficient removal of PQT²⁺, from aqueous solutions more than other studies. The study also showed that ketoenol–pyrazole receptor can be regenerated in five cycles using HNO₃ without affecting its adsorption capacity.

Techno-economic estimation of electroplating wastewater treatment using zero-valent iron nanoparticles: batch optimization, continuous feed, and scaling up studies

Electroplating manufacturing processes release industrial effluents that comprise severe levels of heavy metals into the environment. This study investigated the utilization of nanoparticles of zero-valent iron (nZVI) for the treatment of electroplating wastewater industry containing multiple heavy metal ions. In batch experiments using Cu²⁺ as a single solute, the optimum operating condition was pH 7.3, nZVI dosage 1.6 g/L, time 36 min, temperature 30 °C, and agitation speed 180 rpm, achieving almost 100% Cu²⁺ removal efficiency. The adsorption mechanisms were illustrated using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and Fourier transform infrared (FTIR) spectrometer. Moreover, continuous-feed experiments were performed to treat real electroplating wastewater industry via adsorption and sedimentation processes. The system attained removal efficiencies of 91.3% total suspended solids (TSS), 68.3% chemical oxygen demand (COD), 94.2% nitrogen (N), 98.5% phosphorus (P), 66.7% Cr⁶⁺, 91.5% Pb²⁺, 83.3% Ag⁺, 80.8% Cu²⁺, 17.4% Ni²⁺, 47.1% Mn²⁺, 54.6% Zn²⁺, 94.7% Fe³⁺, 100.0% Al³⁺, and 42.1% Co²⁺. The removal mechanisms included reduction of Meⁿ⁺ to Me^(n-x)+/Me⁰ by the Fe⁰ core, adsorption to the oxide shell as Me(OH)_x and Me-Fe-OOH, oxidation of Meⁿ⁺ to Me^(n+z)+, specific surface bonding, and sequential steps of electron transfer and precipitation. The total cost, including amortized and operating expenses for scaling up the adsorption system, was 4.45$ per m³ of electroplating wastewater. Graphical abstract.

Unusual behavior of MgFe2O4 during regeneration: desorption versus specific adsorption

The reusability of spent adsorbents is the most important characteristic for their practical application. The process of MgFe₂O₄ regeneration after methylene blue (MB) adsorption was studied. The effect of the nature (HCl, HNO_3, and MgCl₂) and the concentration (10^-3-10^-1 M) of regeneration agents was established. All the regeneration agents at 10^-3 and 10^-2 M had high efficiency and adsorption capacity recovery reached 80-90%, whereas for 10^-1 M concentration the adsorption efficiency was in the range of 4.5-36.2%. It was shown that the concentration of desorbed MB was much less than what had been previously adsorbed and did not correlate with regeneration efficiency. The unusual behavior of MgFe₂O₄ during regeneration could be due to different mechanisms of regeneration by OH₃ ⁺ and Mg²⁺ ions: (i) for acidic regeneration the main process was the non-specific adsorption of OH₃ ⁺ ions in a diffusion layer and the substitution of adsorbed MB due to electrostatic forces; (ii) in the case of Mg²⁺ as a regeneration agent, there was specific adsorption due to the completion of a crystal lattice of MgFe₂O₄ nanoparticles by Mg²⁺ ions (according to the rules of Fayans-Pannet) with the formation of new Mg-OH adsorption sites and the super-equivalent adsorption of Mg²⁺ ions (according to DLVO (Derjaguin, Landau, Verwey, and Overbeek) theory) accompanied by a recharge of the MgFe₂O₄ surface. These phenomena of MgFe₂O₄ regeneration using Mg²⁺ ions must be taken into account in the theory and practice of adsorption.

Use of zeolites for macronutrients removal from wastewater

Natural or synthetic zeolites have unique physical, chemical and structural properties that predetermine their use in many processes, including wastewater treatment. This study presents the results of our preliminary research in the field of nitrogen and phosphorus removal using adsorption and adsorptive ozonation with natural and modified zeolites. Iron-modified zeolite was the most efficient for the removal of ammonium nitrogen by adsorption. Phosphorus removal efficiency using adsorption was relatively low and natural zeolite was not suitable for the phosphorus removal at all. Ozone had no significant impact on the removal efficiency. Regeneration of loaded zeolites with ozone has also been studied. This method was partly efficient but it needs be further examined.

Activated carbons derived from hydrothermal impregnation of sucrose with phosphoric acid: remarkable adsorbents for sulfamethoxazole removal

A series of activated carbons with surface areas of 925-1929 m2 g-1 were synthesized by in situ hydrothermal impregnation of sucrose with H3PO4 and subsequent calcination at 500-900 °C. The prepared various types of activated carbons were utilized for the removal of sulfamethoxazole (SMX) from its solution by adsorption, and the effects of contact time, adsorbent dosage, initial concentration, adsorption temperature and pH on SMX adsorption were studied. The pseudo-second-order and the intra-particle diffusion model were used to analyze the adsorption kinetic data. The adsorption isotherm studies showed that the activated carbon prepared at 900 °C (C-900) showed the highest Langmuir maximum adsorption capacity of 808.7 mg g-1 among them, much higher than that of C-500 (274.0 mg g-1). Adsorption thermodynamic results showed that the adsorption of SMX was a spontaneous exothermic process, with a standard enthalpy change of -6.59 kJ mol-1 and a standard entropy change of 47.7 J mol-1 K-1. It was deduced that hydrophobic, electron donor-acceptor and electrostatic interactions were involved in the adsorption mechanism. Finally, regeneration experiments showed that more than 90% of the adsorption capacity could be recovered after four cycles through ethanol washing. Considering the remarkable and regenerable adsorption ability as well as the economic and environmental merits, these activated carbons are considered as promising candidates for potential practical applications in adsorptive removal of SMX.

Optimizing the removal of strontium and cesium ions from binary solutions on magnetic nano-zeolite using response surface methodology (RSM) and artificial neural network (ANN)

The feasibility of using magnetic nano-zeolite (MNZ) to remove cesium and strontium from their binary corrosive solutions was investigated by considering the multi-variant/multi-objective nature of the process. RSM (Response Surface Methodology) and ANN (Artificial Neural Network) were used to model and optimize the removal system and assess sensitive parameters that can affect the process reliability. MNZ is characterized by its high surface area and cation exchange capacity and possesses good regeneration behavior for both elements using citric acid. Its stability is comparable to other sorbents in acidic media and the stability increases in alkaline media, where dissolution rate follow first order reaction on heterogeneous sites. MNZ removes both contaminants simultaneously with small tendency toward Cs, where MNZ is suggested for application in pre-treatment of highly contaminated alkaline solutions. The percentage removal, decontamination factors, and separation factors have different dependency on the effluent/process conditions; this dependency is the same for both contaminants. Sorption kinetics is initially controlled by external mass transfer through the boundaries then intra-particle diffusion dominates the reactions. The process sensitivity to pH changes is attributed to changes in structural elements -species distribution at the solid/aqueous interface. Cs⁺ and Sr⁺² are exchanged with Na⁺ and H⁺, regardless the effluent pH value, and with Al and Fe cations at specific pH. Isosteric heat of sorption calculations indicated that the total heat needed to complete the reaction was considerably reduced by operating the process at optimized temperature.

Functionalized graphene nanosheets as absorbent for copper (II) removal from water

Functionalized graphene nanosheets (FGNs) with high surface area and various functional groups were prepared by oxidation method. The characteristics of FGNs were studied by nitrogen adsorption using the Brunauer-Emmett-Teller (BET) method, Fourier transform infrared spectroscopy (FTIR), transmission electron microscope (TEM), scanning electron microscope (SEM), energy dispersion spectrum (EDS), and atomic force microscopy (AFM). The specific surface area of obtained FGNs was measured as 834.06 m² g^-1, which was 20-40% higher than graphene nanosheets (GNs) before oxidation. An abundance of oxygen-containing functional groups, such as carboxyl, hydroxyl and epoxy groups, was grafted on the edge and surface of GNs. Moreover, FGNs demonstrated excellent adsorption and desorption performance when used as absorbent to remove Cu (II) from aqueous solution. The removal percentage could reach 96% within 1 h and remain 72% after 5 adsorption-desorption cycles. Adsorption process and mechanism were elucidated by kinetics models and isotherm models. The results showed FGNs has a great potential to be an adsorbent for removal copper ions from water.

A hollow mesoporous carbon from metal-organic framework for robust adsorbability of ibuprofen drug in water

Herein, we described a tunable method for synthesis of novel hollow mesoporous carbon (MPC) via direct pyrolysis (800^oC) of MIL-53 (Fe) as a self-sacrificed template. The structural characterization revealed a hollow, amorphous, defective and mesoporous MPC along with high surface area (approx. 200 m² g^-1). For the experiments of ibuprofen adsorption onto MPC, effects of contact time, MPC dosage, ionic strength, concentration and temperature were systematically investigated. The optimal conditions consisted of pH = 3, concentration 10 mg l^-1 and dose of 0.1 g l^-1 for the highest ibuprofen removal efficiency up to 88.3% after 4 h. Moreover, adsorption behaviour, whereby chemisorption and monolayer controlled the uptake of ibuprofen over MPC, were assumed. Adsorption mechanisms including H-bonding, π-π interaction, metal-oxygen, electrostatic attraction were rigorously proposed. In comparison to several studies, the MPC nanocomposite in this work obtained the outstanding maximum adsorption capacity (206.5 mg g^-1) and good reusability (5 cycles); thus, it can be used as a feasible alternative for decontamination of ibuprofen anti-inflammatory drug from water.

Variable diffusivity homogeneous surface diffusion model and analysis of merits and fallacies of simplified adsorption kinetics equations

Adsorption and ion exchange phenomena are encountered in several separation processes, which in turn, are of vital importance across various industries. Although the literature on adsorption kinetics modeling is rich, the majority of the models employed are empirical, based on chemical reaction kinetics or oversimplified versions of diffusion models. In this paper, the fifteen most popular simplified adsorption kinetics equations are presented and discussed. A new versatile variable-diffusivity two-phase homogeneous diffusion model is presented and used to evaluate the analytical adsorption models. Aspects of ion exchange kinetics are also addressed.

The Impact of Temperature on the Removal of Inorganic Contaminants Typical of Urban Stormwater

Appropriate management of urban stormwater requires consideration of both water quantity, resulting from flood control requirements, and water quality, being a consequence of contaminant distribution via runoff water. This article focuses on the impact of temperature on the efficiency of stormwater treatment processes in permeable infiltration systems. Studies of the removal capacity of activated carbon, diatomite, halloysite, limestone sand and zeolite for select heavy metals (Cu and Zn) and biogenes (NH4-N and PO4-P) were performed in batch conditions at 3, 6, 10, 15, 22, 30 and 40 °C at low initial concentrations, and maximum sorption capacities determined at 3, 10, 22 and 40 °C. A decrease in temperature to 3 °C reduced the maximum sorption capacities (Qmax) of the applied materials in the range of 10% for diatomite uptake of PO4-P, to 46% for halloysite uptake of Cu. Only the value of Qmax for halloysite, limestone sand and diatomite for NH4-N uptake decreased slightly with temperature increase. A positive correlation was also observed for the equilibrium sorption (Qe) of Cu and Zn for analyses performed at low concentrations (with the exception of Zn sorption on limestone sand). In turn, for biogenes a rising trend was observed only in the range of 3 °C to 22 °C, whereas further temperature increase caused a decrease of Qe. Temperature had the largest influence on the removal of copper and the smallest on the removal of phosphates. It was also observed that the impact of temperature on the process of phosphate removal on all materials and ammonium ions on all materials, with the exception of zeolite, was negligible.

Algal sorbent derived from Sargassum horneri for adsorption of cesium and strontium ions: equilibrium, kinetics, and mass transfer

An algal sorbent derived from Sargassum horneri was prepared and used to adsorb cesium and strontium ions from aqueous solution. The phenomenological mathematical models associated to the predicted equilibrium isotherms were developed to determine the rate-limiting steps of the adsorption process. The maximum adsorption capacity of cesium ion and strontium ion was calculated to be 0.358 and 1.72 mmol g-1, respectively. The adsorption kinetics followed to the pseudo-second-order equation. It was found that adsorption of cesium or strontium ions onto the active sites of the biosorbent was the rate-limiting step. In addition, the external mass transfer and the internal mass transfer cannot be neglected for the adsorption of strontium ion based on the error analysis. The functional groups relevant to the adsorption were carboxyl and sulfate groups.

Competition in chromate adsorption onto micro-sized granular ferric hydroxide

Hexavalent chromium is highly toxic and elaborate technology is necessary for ensured removal during drinking water production. The present study aimed at estimating the potential of a micro-sized iron hydroxide (μGFH] adsorbent for chromate removal in competition to ions presents in drinking water. Freundlich and Langmuir models were applied to describe the adsorption behaviour. The results show a high dependency on the pH value with increasing adsorption for decreasing pH values. The adsorption capacity in deionized water (DI) at pH 7 was 5.8 mg/g Cr(VI) while it decreased to 1.9 mg/g Cr(VI) in Berlin drinking water (DW) at initial concentrations of 1.2 mg/L. Desorption experiments showed reversible adsorption indicating ion exchange and outer sphere complexes as main removal mechanisms. Competing ions present in DW were tested for interfering effects on chromate adsorption. Bicarbonate was identified as main inhibitor of chromate adsorption. Sulfate, silicate and phosphate also decreased chromate loadings, while calcium enhanced chromate adsorption. Adsorption kinetics were highly dependent on particle size and adsorbent dose. Adsorption equilibrium was reached after 60 min for particles smaller than 63 μm, while 240 min were required for particles from 125 μm to 300 μm. Adsorption kinetics in single solute systems could be modelled using the homogeneous surface diffusion model (HSDM) with a surface diffusion coefficient of 4∙10 ^-14 m²/s. Competitive adsorption could be modelled using simple equations dependent on time, adsorption capacity and concentrations only.

Urtica dioica leaves-calcium alginate as a natural, low cost and very effective bioadsorbent beads in elimination of dyes from aqueous medium: Equilibrium isotherms and thermodynamic studies

A very effective and low cost bioadsorbents derived from Urtica dioica leaf powder (UDL) and composite beads UDL encapsulated with calcium alginate (UDL/A) were used in removal of cationic dye crystal violet (CV) from aqueous medium. The new adsorbents were characterized by Fourier transform infrared spectroscopy (FTIR) and isoelectric points (pHpzc). Adsorption was studied in batch system according to initial CV concentration (30-150 mg/L), contact time, pH (2-11), temperature (10 to 40 °C) and adsorbent mass (10 to 200 mg). UDL and UDL/A beads showed maximum CV adsorption of 137.8 and 121.8 mg/g for 150 mg/L initial CV dye concentration, pH 6.5, 1 g/L adsorbent dose, temperature of 23 ± 1 °C and contact time of 6 h. Analysis of kinetics data shows that adsorption of CV onto adsorbents was well described by pseudo-second order model. The equilibrium adsorption data fitted the Langmuir isotherm well with a maximum adsorption capacity of 1790 and 1107 mg/g respectively for UDL and UDL/A beads. Results of thermodynamic studies showed negative values of ΔG°; positive values of ΔH° 10.30 and 6.54 kJ/mol for CV onto UDL and UDL/A beads respectively. Adsorption processes are spontaneous, physical and endothermic in nature.