External Mass Transfer Coefficient Research Articles

The effect of water temperature on the removal of 2-methylisoborneol and geosmin by preloaded granular activated carbon

Granular activated carbon (GAC) is widely used by drinking water treatment plants in the Great Lakes region to control 2-methylisoborneol (MIB) and geosmin associated with summertime algal blooms. Recently, however, taste and odour events are being detected in the winter, but there is limited information in the literature about the effect of temperature on GAC performance. In this study, batch isotherm experiments were conducted at 4 °C, 10 °C, and 20 °C to evaluate the temperature impact on adsorption thermodynamics. Pilot-scale column tests were then performed at the same temperatures to evaluate the temperature effect on overall removal, including both thermodynamics and kinetics. The pore and surface diffusion model (PSDM) was applied to fit the experimental data of the pilot-scale column tests, which allowed the kinetic parameters to be determined at each temperature. The isotherm results showed that water temperature did not have a significant effect on the equilibrium adsorption capacity (i.e., the thermodynamics) for MIB and geosmin under the conditions tested, but the pilot tests showed an increased removal of MIB and geosmin by 20–30% at 20 °C compared to the lower temperatures. This demonstrates that water temperature has more impact on kinetics than thermodynamics. Numerical simulations showed that the overall external mass transfer coefficient, Kf’, had the greatest impact on breakthrough. An exponential equation was proposed to correlate temperature and Kf’, and it could be incorporated into a site-specific PSDM to predict the removals of MIB and geosmin in GAC contactors at different temperatures. The model could then be used by a utility, for example, to predict the required increase in empty bed contact time needed to control taste and odour in the winter to the same extent as in the summer.

A mass transfer study considering intraparticle diffusion and axial dispersion for fixed-bed adsorption of crystal violet on pecan pericarp (Carya illinoensis)

This research was focused on the representation of the fixed bed adsorption of crystal violet dye (CV) on treated pecan pericarp (Carya illinoensis) using a mass transfer approach based on the intraparticle diffusion and axial dispersion. The operation was firstly evaluated in batch mode to determine the external mass transfer coefficients, the equilibrium relation, and the thermodynamic aspects. It was then studied the continuous operation. In this case, the breakthrough curves were modeled by a phenomenological approach, based on the combination of the fluid phase mass balance with intraparticle diffusion (FBID). The results were compared with the Thomas model, which not considers the axial dispersion. The external mass transfer was 4.39 × 10-3 cm s−1. The equilibrium relation was better represented by the Langmuir model. The adsorption process was endothermic, with a maximum adsorption capacity of 639.7 mg g−1. Concerning fixed-bed adsorption evaluation, Thomas and FBID models were able to describe the breakthrough tendencies. Nevertheless, the FBID was used to estimate the axial dispersion coefficient, which ranged from 0.79 × 10-6 to 7.81 × 10-6 cm2 s−1.

Investigation of external mass transfer in micropacked bed reactors

Abstract In this work, the hydrogenation of α-methylstyrene to cumene over Pd catalyst is adopted to determine the external mass transfer performance under the mass transfer limited regime in micro-packed bed reactors. The operating temperature, pressure, and solvent are first confirmed for this method. Then the effects of gas and liquid superficial velocities as well as particle size on the external overall volumetric mass transfer coefficient are investigated. The external overall volumetric mass transfer coefficient in micro-packed bed reactors can reach 0.15–2.76 s−1, which is about one to two orders of magnitude larger than those of conventional large trickle bed reactors. Furthermore, a correlation of the external overall volumetric mass transfer coefficient is proposed and the external mass transfer process analysis is carried out.

Nitrate removal from aqueous solutions by magnetic cationic hydrogel: Effect of electrostatic adsorption and mechanism

Excessive nitrate (NO3−) is among the most problematic surface water and groundwater pollutants. In this study, a type of magnetic cationic hydrogel (MCH) is employed for NO3− adsorption and well characterized herein. Its adsorption capacity is considerably pH-dependent and achieves the optimal adsorption (maximum NO3−-adsorption capacity is 95.88 ± 1.24 mg/g) when the pH level is 5.2–8.8. The fitting result using the homogeneous surface diffusion model indicates that the surface/film diffusion controls the adsorption rate, and NO3− approaches the center of MCH particles within 30 min. The diffusion coefficient (Ds) and external mass transfer coefficient (kF) in the liquid phase are 1.15 × 10−6 cm2/min and 4.5 × 10−6 cm/min, respectively. The MCH is employed to treat surface water that contains 10 mg/L of NO3−, and it is found that the optimal magnetic separation time is 1.6 min. The high-efficiency mass transfer and magnetic separation of MCH during the adsorption–regeneration process favors its application in surface water treatment. Furthermore, the study of the mechanism involved reveals that both –N+(CH3)3 groups and NO3− are convoluted in adsorption via electrostatic interactions. It is further found that ion exchange between NO3− and chlorine occurs.

Microbial growth rates and local external mass transfer coefficients in a porous bed biofilm system measured by 19 F magnetic resonance imaging of structure, oxygen concentration, and flow velocity.

19 F nuclear magnetic resonance (NMR) oximetry and 1 H NMR velocimetry were used to noninvasively map oxygen concentrations and hydrodynamics in space and time in a model packed bed biofilm system in the presence and absence of flow. The development of a local oxygen sink associated with a single gel bead inoculated with respiring Escherichia coli was analyzed with a phenomenological model to determine the specific growth rate of the bacteria in situ, returning a value (0.66 hr-1 ) that was close to that measured independently in planktonic culture (0.62 hr-1 ). The decay of oxygen concentration in and around the microbiologically active bead was delayed and slower in experiments conducted under continuous flow in comparison to no-flow experiments. Concentration boundary layer thicknesses were determined and Sherwood numbers calculated to quantify external mass transfer resistance. Boundary layers were thicker in no-flow experiments compared to experiments with flow. Whereas the oxygen concentration profile across a reactive biofilm particle was symmetric in no-flow experiments, it was asymmetric with respect to flow direction in flow experiments with Sherwood numbers on the leading edge (Sh = 7) being larger than the trailing edge (Sh = 3.5). The magnitude of the experimental Sh was comparable to values predicted by a variety of correlations. These spatially resolved measurements of oxygen distribution in a geometrically complex model reveal in innovative detail the local coupling between microbial growth, oxygen consumption, and external mass transfer.

New insights from modelling and estimation of mass transfer parameters in fixed-bed adsorption of Bisphenol A onto carbon materials

The removal of Bisphenol A, 2,2-bis (4-hydroxyphenyl) propane (BPA) in fixed-bed columns was investigated by breakthrough adsorption tests at different operation conditions and further prediction by a mathematical model to describe the adsorption-diffusion process onto two synthesized carbon porous materials. In this study, a xerogel (RFX) prepared by an optimized conventional sol-gel method and a lignin-based activated carbon (KLP) obtained via chemical activation were used in batch and fixed-bed adsorption experiments. The materials were fully characterized and their adsorptive properties were compared to those obtained with a commercial activated carbon (F400). RFX and KLP materials reached the equilibrium adsorption in only 24 h, whereas F400 activated carbon required 48 h. In addition, F400 and KLP adsorbents showed higher equilibrium adsorption capacity values (qe = 0.40 and 0.22 kg/kg, for F400 and KLP, respectively) than that obtained for the xerogel (qe = 0.08 kg/kg). Both synthesized carbon-adsorbents were studied in fixed-bed adsorption tests, exploring the effect of the operation conditions, e.g., initial BPA concentration (0.005–0.04 kg/m3), weight of adsorbent (0.01–0.05 g) and volumetric flow rate (0.2 to 1.0 mL/min), on the adsorption performance of the column. All the tested adsorption columns reached the equilibrium in a very short time, due to the efficient dimensionless of the bed. Additionally, the regeneration of the exhausted adsorbent was studied, achieving the total reuse of the solids after three consecutive cycles using methanol as regeneration agent. Finally, a mathematical model based on mass conservation equations was proposed, allowing to efficiently fit the experimental BPA breakthrough curves and estimate the external and adsorbed-phase mass transfer coefficients with a high accuracy.

Reduced order models with local property dependent transfer coefficients for real time simulations of monolith reactors

We present a novel and accurate reduced order model with local property dependent internal and external mass transfer coefficients for real time simulations of monolith reactors. We compare the accuracy of the present approach with other reduced order models in the literature and with exact (numerical) solution of the detailed washcoat diffusion model. We illustrate the application of the reduced order models with three examples: a four reaction model of a three-way catalytic converter (TWC) to determine the steady-state as well as transient behavior in real time; a simplified TWC model with oxygen storage and release, and the case of three reversible reactions with linear kinetics. We show that the use of local property dependent transfer coefficients in terms of the Jacobian of the reaction rate vector leads to the best accuracy followed by its diagonal approximation, followed by the asymptotic (slow reaction) approximation for the internal Sherwood number. It is shown that the proposed method has sufficient accuracy for most practical applications, especially in the washcoat diffusion controlled regime, while speeding up the calculations by orders of magnitude compared to detailed washcoat diffusion models.

Simple Methods for the Determination of Adsorption Rate and Equilibrium Parameters in the Recovery of Acetic Acid Using GAC and Synthetic Resins

Industrial acetic acid production based on anaerobic fermentation technology is not economically viable at present due to the high cost of product recovery by adsorption or extraction at neutral pH conditions. In this study, kinetics and equilibria of acetic acid sorption onto granular activated carbon and weak base resins were examined at pH < pKa of the acid. Comparative data on equilibrium sorption from a fermentation broth are also reported. It is observed that the weak base resins have higher adsorption capacities and selectivities for the uptake of organic acids from complex mixtures compared to activated carbon. A simple method to estimate intraparticle diffusivities from batch kinetic data using approximations to analytical solutions is developed. Methods for the accurate estimation of isotherm constants and external mass transfer coefficients are discussed and implemented. The estimated coefficients are shown to predict batch kinetic data well. This study will help in the screening of sorbents fo...

Brilliant red X-3B uptake by a novel polycyclodextrin-modified magnetic cationic hydrogel: Performance, kinetics and mechanism

A novel polycyclodextrin-modified magnetic cationic hydrogel (PCD-MCH) was developed and its performance, kinetics and mechanism for the removal of reactive brilliant red X-3B (X-3B) were studied. The results showed that the zeta-potential of PCD-MCH was 32.8 to 16.7mV at pH3.0–10.5. The maximum X-3B adsorption capacity of PCD-MCH was 2792.3mg/g. The adsorption kinetics could be well-described by the Weber–Morris model and the homogeneous surface diffusion model (HSDM). Diffusion stages corresponding to surface or film diffusion, intra-particle or wide mesopore diffusion, and narrow mesopore/micropore diffusion occurred at 0–120, 120–480 and 480–1200min, respectively. The latter two diffusion stages were rate-controlling for X-3B adsorption kinetics. At the initial X-3B concentration of 600mg/L, the diffusion coefficient (Ds) and external mass transfer coefficient in the liquid phase (kF) were 3×10−11cm2/min and 4.68×10−6cm/min, respectively. X-3B approaching the center of PCD-MCH particles could be observed at 360min. At the end of the third diffusion stage, the Cp at q/qe=0 was 45.20mg/L, which was close to the homogeneous Cp value of 46mg/L along the radius of PCD-MCH particles. At pH3.0–10.0, PCD-MCH showed stable X-3B adsorption capacities. After five regeneration-reuse cycles, the residual adsorption capacity of regenerated PCD-MCH was higher than 892.7mg/g. The corresponding adsorption mechanism was identified as involving electrostatic interactions, cyclodextrin cavities and hydrogen bonds, of which cyclodextrin cavities showed prominent capture performance towards dye molecules through the formation of inclusion complexes.

Insight into mass transfer during ultrasound-enhanced adsorption/desorption of blueberry anthocyanins on macroporous resins by numerical simulation considering ultrasonic influence on resin properties

The mass transfer mechanism about ultrasonic enhancement of blueberry anthocyanin adsorption and desorption on macroporous resins were investigated. Ultrasound treatment led to the decrease of resin size, whereas average pore diameter, specific surface area, total pore volume were less affected. Then, the pore volume and surface diffusion model considering ultrasound-induced changes of particle size and surface area was employed to model anthocyanin adsorption process numerically. Generally, sonication enhanced external mass transfer coefficient (kL) and surface diffusion coefficient (Ds) during adsorption. The values of kL and Ds under sonication at 279 W/L and 20 °C were 7.578 × 10−2 cm/s and 2.000 × 10−9 cm2/s, which were 2.0% and 140.1% higher than the kL and Ds values under orbital agitation at 20 °C. Accordingly, anthocyanins penetrated into the interior of macroporous resins faster under sonication. The overall intraparticle diffusion for anthocyanin adsorption was dominated by pore volume diffusion with the contribution percentage higher than 50%. Sonication enhanced the contribution of surface diffusion to the overall diffusion, which was consistent with the increase of Ds with ultrasound energy. Moreover, a general diffusion model also incorporating the variations of particle size and surface area was utilized to model anthocyanin desorption process. The overall anthocyanin diffusivity increased with both ultrasound energy level and temperature. Anthocyanins distributed more homogenously inside resins under sonication at 279 W/L than under other treatments. This work could be a reference for the valorization of anthocyanins in food industry, especially juice and wine industries.

Reactive adsorption of Safranin O: surface - pore diffusion modeling and degradation study.

Granular activated carbon was doped with iron (Fe-AC) and was used to study the removal of Safranin O (SO) using the Fe-AC/H2O2 system for reactive adsorption and Fe-AC for adsorption. Fe-AC and H2O2 doses were optimized to obtain maximum removal of SO. Maximum removal was found to be 96.1% after 5 h using 1.0 g/L Fe-AC and 5.0 mM hydrogen peroxide doses for 10 mg/L initial SO concentration. Kinetic study suggested the suitability of the pseudo-first-order model for reactive adsorption. The Langmuir isotherm explained well the sorption of SO onto Fe-AC. Parallel-pore-reactive adsorption model was applied and validated. By fitting the experimental data to the model, it is observed that the surface reaction rate coefficient, kr, was found to be five times that of the apparent rate constant, kapp. Parameters such as the external liquid film mass transfer coefficient, macro-pore and micro-pore diffusivities were estimated by regression analysis. Pore diffusion and surface reaction were found to be rate controlling for adsorption and reactive adsorption, respectively. An oxidative degradation of SO took place via hydroxylation and ring cleavage processes.

Intrinsic kinetics and external diffusion of catalytic steam gasification of fine coal char particles under pressurized and fluidized conditions

Catalytic steam gasification of fine coal char particles was carried out using a self-made laboratory reactor to determine the intrinsic kinetics and external diffusion under varying pressures (0.1–0.5 MPa) and superficial gas flow velocities (GFVs) of 13.8–68.8 cm∙s–1. In order to estimate the in-situ gas release rate at a low GFV, the transported effect of effluent gas on the temporal gasification rate pattern was simulated by the Fluent computation and verified experimentally. The external mass transfer coefficients (kmam) and the effectiveness factors were determined at lower GFVs, based on the intrinsic gasification rate obtained at a high GFV of 55.0 cm∙s–1. The kmam was found to be almost invariable in a wider carbon conversion of 0.2–0.7. The variations of kmam at a median carbon conversion with GFV, temperature and pressure were found to follow a modified Chilton-Colburn correlation: $$Sh = 0.311{\operatorname{Re} {2.83}}S{c^{\frac{1}{3}}}{(\frac{P}{{{P_0}}})^{ - 2.07}}$$ (0.04<Re<0.19), where P is total pressure and P0 is atmospheric pressure. An intrinsic kinetics/external diffusion integrating model could well describe the gasification rate as a function of GFV, temperature and pressure over a whole gasification process.

Efficient photoreduction of Cr(VI) on TiO2/functionalized activated carbon (TiO2/AC-AEMP): improved adsorption of Cr(VI) and induced transfer of electrons.

Slow and random transfer of pollutants and photo-induced carriers on photocatalysts causes loss of efficiency in photodegradation of contaminants. Enhancing and directing mass transfer of them are considered as two major methods for improving the photodegradation of pollutants over photocatalysts. Here in this work, we focused on the design of a novel photocatalyst which not only accelerated the transfer rate of Cr(VI) and electrons but also provided specific transfer routes for them. By careful characterizations, it is indicated that 2-((2-(2-aminoethylamino)ethylimino)methyl)phenol (AEMP) was covalently attached onto activated carbon (AC), which enhanced Cr(VI) transfer from bulk solution to AC through electrostatic or coordinative interactions. The external mass transfer coefficient (Kf) of Cr(VI) over TiO2/AC-AEMP was estimated as 1.75 × 10-6ms-1, which was ~ 12.79 and ~ 5.96 times that of TiO2 and TiO2/AC, respectively. Dense and homogeneous heterojunctions between AC and TiO2 were acquired synchronically by forming Ti-O-C linkages, which increased traveling of electrons from TiO2 to AC. Accordingly, Cr(VI) can capture photo-induced electrons on the surface of AC via concrete routes and then be reduced efficiently. The results showed that the photoreduction rate of Cr(VI) on TiO2/AC-AEMP reached to ~ 92.7%, and the overall photocatalytic activity of this well-designed TiO2/AC-AEMP has been enhanced significantly by 5.5 times compared to TiO2/AC. The enhanced photocatalytic activity of TiO2/AC-AEMP was mainly attributed to an improved synergetic process of mass transfer-induced adsorption-photoreduction by forming specific transfer routes for accelerative motion of Cr(VI) and electrons. This work provides a feasible strategy to improve the photoactivity of photocatalysts for the degradation of pollutants by effective mass transfer. Graphical abstract.

Release of pharmaceutical cocktails from small polymeric micelles

Here we model release of multidrug cocktails from small micelles. As most emerging anticancer agents remain sparingly soluble in water, they require carriers such as poly(ethylene glycol)-block-poly(D,L-lactic acid) (PEG-b-PLA) micelles to enhance their delivery. These micelles have been used as vectors for multidrug cocktails (e.g., the hydrophobic anticancer agents paclitaxel, 17-allylamino-17-demethoxygeldanamycin, and rapamycin). Empirically determined agent release takes hours and half-lives depend on oil-water partition coefficients, a representative thermodynamic parameter. This contrasts with the assertion that the cocktail release rate is diffusion controlled, with diffusion time scales less than a millisecond. Here we resolve this inconsistency in mechanism using a “lumped capacitance” method. The two parameter equation that results provides excellent agreement with experimental release profiles. Model time scales depend on the micelle radius, external mass transfer coefficient, and lumped partition coefficient. These findings enable tuning of release rates of multidrug mixtures from micelles, which will become increasingly important because many new drug candidates remain hydrophobic.

Comparative study of the batch adsorption kinetics and mass transfer in phenol-sand and phenol-clay adsorption systems

This study was aimed at evaluating the kinetics and mass transfer of phenol adsorption onto sand and clay sediments. The adsorption studies were carried out in batch mode. The results showed that the time taken to attain equilibrium in the adsorption of phenol onto sand and clay sediments are 21 and 27 h, respectively. At an equal given time, clay sediments relatively had higher adsorption of phenol than sand sediments, which can be ascribed to its large surface area and higher porosity. The kinetic data adjusted well to the pseudo-first-order and Elovich kinetic models; however, Elovich kinetic model best describe the phenol adsorption kinetics. Mass transfer models were applied to the adsorption data to evaluate the rate-limiting step in the adsorption of phenol onto the sand and clay sediments. The values of the external mass transfer coefficient and diffusion coefficient were estimated from the models. Biot number was determined using the estimated internal mass transfer (intraparticle diffusion) coefficient and the external mass transfer coefficient. From the applied models and the calculated Biot number, the results revealed that the mechanism of phenol adsorption onto sand and clay sediments was not only by intraparticle diffusion but also by film diffusion.

Visible-light photocatalytic degradation of acid violet 7 dye in a continuous annular reactor using ZnO/PPy photocatalyst: Synthesis, characterization, mass transfer effect evaluation and kinetic analysis

In this work, a composite of ZnO and polypyrrole (ZnO/PPy) was synthesized by polymerization method and characterized by X-ray diffraction (XRD), Ultraviolet–Visible with Near-infrared (UV–Vis NIR) spectroscopy and Fourier Transform Infrared (FT-IR) spectroscopy, both by diffuse reflectance mode. The ZnO/PPy composite was tested as photocatalyst for the degradation of acid violet 7 dye under visible light radiation in two systems: a) by using a batch reactor and b) in a continuous annular reactor. The hole formation in the valence band was evidenced using a hole scavenger test for ZnO/PPy composite and also the hydroxyl radicals formation capacity was determined. A three-resistance model of mass transfer (MT) was evaluated, it considers the traditional external mass transfer coefficient (She) to obtain an overall mass transfer coefficient (Shapp), the model is considered a full 3-D convection–diffusion–reaction applied to the photocatalyst of ZnO/PPy composite attached onto the interior wall of a quartz ring (annular space). The MT analysis showed in the continuous reactor that the degradation reaction was kinetically controlled. A Langmuir-Hinshelwood (L-H) model was analyzed according the fitting of the experimental data in the annular continuous photocatalytic reactor. The adsorption parameter obtained with L-H model was validated based on thermodynamic criteria for the changes in the standard enthalpy of adsorption (ΔHadso) and standard total entropy of adsorption (ΔSadso). The analysis of thermodynamic properties revealed that the adsorbed AV7 molecule on ZnO/PPy photocatalyst still presents mobility in order to react to the very short time OH radicals formed on surface.

Study on mass transfer of ethyl acetate in polymer adsorbent by experimental and theoretical breakthrough curves

Abstract A new polymeric adsorbent with highly hypercrosslinked structure was developed for the removal of VOCs from polluted air. The purpose of this work is to obtain the intraparticle mass transfer coefficient of the adsorbent particles. Adsorption experiments for obtaining breakthrough curves were carried out with a fixed bed system. A dynamic mathematical model for the fixed bed adsorption system was developed. By model fitting, the overall mass transfer coefficient was determined when the deviation error was minimum. Then, the intraparticle mass transfer coefficient of the adsorbent was determined when the external mass transfer resistance was eliminated at higher velocities. Furthermore, a linear relationship of the intraparticle mass transfer coefficient and equilibrium coefficient at lower inlet gas concentrations range was correlated. Moreover, an equation for predicting external mass transfer coefficient at low Reynolds number range at room temperature was obtained.

Determination of gas film mass transfer coefficient in a packed bed reactor for the catalytic combustion of hydrogen

The overall kinetics of catalytic combustion of hydrogen in air is decided by the contribution of the different resistances offered in individual steps such as external diffusion, internal diffusion and surface reaction. To estimate the contribution of external mass transfer resistance, empirical correlations in terms of dimensionless numbers viz., Sherwood number, particle Reynold's number, and Schmidt number are used in general. The Schmidt number for hydrogen in air is approximately 0.2 and most of the mass transfer correlations in packed bed reactors are valid for Schmidt number higher than 0.6, which corresponds to that of water vapour in the air. The external mass transfer correlations applicable for hydrogen-air system are scarce. In the present study, the external mass transfer effects were decoupled from internal diffusion and intrinsic reaction by conducting experiments with different particle Reynolds numbers in a packed bed batch reactor with complete recycle. Further, the system is theoretically modeled and the external film mass transfer coefficients were estimated at different particle Reynold's numbers using experimental data. Based on the experimental data, a new mass transfer correlation has been proposed to determine the gas film mass transfer coefficient of hydrogen in the air in packed bed reactors, in terms of conventional dimensionless numbers i.e., Sherwood number, Schmidt number, and particle Reynolds number. Further, the contribution of external mass transfer resistance on the overall reaction kinetics was also estimated.

Chemisorption and kinetic mechanisms of elemental mercury on immobilized V2O5/TiO2 at low temperatures.

To investigate the effect of low temperature and catalyst filling pattern on the adsorption of Hg° by DeNOx equipment, the chemisorption and kinetic mechanisms of Hg° adsorption on 5-30%V2O5/TiO2 immobilized on glass beads at 100-160 °C were investigated. The effects of the reaction temperature, influent Hg° concentration, and V2O5 doping amount on the adsorption efficiency and capacity for Hg° were explored. The active sites for Hg° adsorption were further identified. Additionally, the adsorption kinetics were modelled using the linear driving force approximation, Fick's diffusion model, and pseudo-second-order kinetic model. Finally, the influence of immobilization on the adsorption of Hg° was also investigated. Experimental results showed that the bridged oxygen atom of V-O-V played a key role in the adsorption of Hg°. The Hg° adsorption efficiencies decreased from >90% to 40% as the reaction temperature increased from 120 °C to 160 °C for 20%V2O5/TiO2, while the adsorptive capacities for Hg° were highly influenced by the influent Hg° concentration and V2O5 doping amount. 20%V2O5/TiO2 had the highest adsorptive capacity of 2547 μg Hg°/g V2O5/TiO2 at 160 °C. The kinetic results showed that the linear driving force approximation model fit the Hg° adsorption better than the other models. The diffusion resistance increased significantly for the immobilized catalysts because the external mass transfer coefficient decreased by more than 1200-fold.

External mass transport process during the adsorption of fluoride from aqueous solution by activated clay

The removal of fluoride ions from an aqueous solution using a modified sulfuric acid clay was studied in batch adsorption. The adsorption kinetics were studied to identify the retention mechanisms. For kinetics study, perfectly stirred batch experiments were carried out after the adjustment of the parameters influencing the system, such as the pH, the adsorbent dosage, the initial concentration of fluoride ions, and the stirring speed. It appears that the rate of fluoride ion removal (1) increased with the mass of adsorbent and the stirring speed, and (2) decreased with the initial fluoride ion concentration. The experimental results were adjusted according to kinetic equations representing four external transport models. The latter were used to calculate the external mass transfer coefficient, k f . The results showed that this coefficient is in the range of 10 −5 –10 −4 m s −1 . Nous avons étudié dans ce travail la cinétique d'adsorption en solution aqueuse des ions fluorure sur un matériau argileux modifié par l'acide sulfurique dans un adsorbeur en batch. L'influence de certains paramètres, comme le pH de la solution, la masse d'argile en solution, la concentration initiale des ions fluorure et la vitesse d'agitation, a été étudiée. Les résultats obtenus prouvent que : (i) la cinétique augmente avec la masse d'argile et la vitesse d'agitation et (ii) décroît avec la concentration initiale en ions fluorure. Quatre modèles de transport externe ont été étudiés et ont montré que l'ordre de grandeur du coefficient k f se trouve dans la gamme entre 10 −5 et 10 −4 m s −1 .