Pseudo-second Order Rate Equation Research Articles

Cadmium (II) Adsorption from Aqueous Solutions Using Onion Skins

The potential of onion skins for removal of aqueous Cd(II) was investigated. Onion skin powder was chemically modified using thioglycolic acid to develop a suitable, low-cost, and efficient adsorbent for the removal of Cd(II) from aqueous solutions. Influences of temperature, contact time, initial concentration of Cd(II), adsorbent dosage, and pH on the removal of Cd(II) were probed. Optimal adsorption conditions were found at pH 5 and 4, and at 60- and 30-min equilibrium time for the modified and native onion skins, respectively. The equilibrium process was well described by the Freundlich isotherm model. The maximum Cd(II) adsorption capacities, from the Langmuir model, are 17.86 mg/g (modified) and 21.28 mg/g (native). The adsorption process followed the mechanism of physisorption. Pseudo second-order rate equation fitted the kinetic data better than the pseudo first-order rate equation for the two adsorbents. Thermodynamic parameters, such as standard free energy change (ΔG°), standard enthalpy change (ΔH°), and standard entropy change (ΔS°), were calculated for adsorption experimental studies. The results showed that the adsorption of Cd(II) on native/unmodified and modified onion skins was a feasible process and exothermic under the studied conditions. The Cd(II) adsorbed was efficiently desorbed from adsorbent using 0.3 M HCl.

Simplified Batch and Fixed-Bed Design System for Efficient and Sustainable Fluoride Removal from Water Using Slow Pyrolyzed Okra Stem and Black Gram Straw Biochars.

Okra stem biochar (OSBC) and black gram straw biochar (BGSBC) were prepared by slow pyrolysis at 500 and 600 °C, respectively. OSBC and BGSBC were characterized using SBET, Fourier transform infrared, X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy, SEM–energy dispersive X-ray, and energy dispersive X-ray fluorescence. High carbon contents (dry basis) of 66.2 and 67.3% were recorded in OSBC and BGSBC, respectively. The OSBC surface area (23.52 m2/g) was higher than BGSBC (9.27 m2/g). The developed biochars successfully remediate fluoride contaminated water. Fluoride sorption experiments were accomplished at 25, 35, and 45 °C. Biochar-fluoride adsorption equilibrium data were fitted to Langmuir, Freundlich, Sips, Temkin, Koble–Corrigan, Radke and Prausnitz, Redlich–Peterson, and Toth isotherm models. The sorption dynamic data was better fitted to the pseudo-second order rate equation versus the pseudo-first order rate equation. The Langmuir sorption capacities of QOSBC0 = 20 mg/g and QBGSBC0 = 16 mg/g were obtained. Biochar fixed-bed dynamic studies were accomplished to ascertain the design parameters for developing an efficient and sustainable fluoride water treatment system. A column capacity of 6.0 mg/g for OSBC was achieved. OSBC and BGSBC satisfactorily remediated fluoride from contaminated ground water and may be considered as a sustainable solution for drinking water purification.

Adsorption kinetics at solid/aqueous solution interfaces: On the boundaries of the pseudo-second order rate equation

The empirical pseudo-second order rate equation has become the equation of choice to account for adsorption kinetics at solid/aqueous solution interfaces. However, its validity boundaries still remain faint. Here, they are set on the basis of the predictions of the well-established Langmuir model, cast in terms of the relevant kinetic variables; i.e., the initial concentrations of the adsorbing species and of the free surface sites. It is shown that the pseudo-second order rate equation becomes valid when the adsorption equilibrium constant is large (larger than ca. 5×105 M-1), the initial concentrations of the reactants are nearly equal, and they surpass a certain critical value. For systems characterized by typical adsorption equilibrium constants, the pseudo-second order model seldom applies within the usually explored experimental conditions. This latter finding alerts on the significance of the reported rate constants that derive from the lax use of the pseudo-second order rate equation.

Recovery of Chromium(VI) Oxyanions from Aqueous Solution Using Cu(OH)2 and CuO Embedded Chitosan Adsorbents

Owing to its availability, low cost and strong reducibility capacity, chitosan can be widely used for Cr(VI) recovery, in this study the removal of Cr(VI) from aqueous solution was investigated using composite adsorbents derived from chitosan (obtained from shrimp peels) and Cu(OH)2 or CuO (prepared by alkaline precipitation of Cu(II) ions) to manufacture ChiCu(OH)2 and ChiCuO, respectively. The adsorbents were characterized by using SEM, BET surface area and zeta potential measurements. Adsorption uptake was highly dependent on pH, temperature, Cr(VI) concentration and sorbent mass: The optimum pH for Cr(VI) removal being close to 4. The adsorption isotherm can be described by Langmuir adsorption isotherm while the uptake kinetics was highly fitted with the pseudo-second order rate equation. The effect of temperature was also evaluated, verifying the endothermic nature of the adsorption process. Chromium desorption was performed using alkaline saline solution (20 g L−1 NaCl with the pH set to 12 with NaOH). The recycling of the sorbent was tested, maintaining a removal efficiency and a metal recovery over 90% for three successive adsorption/desorption cycles. The results showed that, the fabricated nanocomposites are promising adsorbents for Cr(VI) from aqueous solution.

Ni-Fe-layered double hydroxide/N-doped graphene oxide nanocomposite for the highly efficient removal of Pb(II) and Cd(II) ions from water

Heavy metal removal from industrial wastewater has remained a tremendous challenge as the water pollution caused by them is posing a serious ecological threat. Several nanostructured materials have been developed for effective scavenging of water-borne metal ions, including high surface area metal oxides, mesoporous silica and carbon based materials. The oxide mediated interaction between the metal ions and the adsorbent leads to efficient loading of these toxic metal ions. Therefore, in order to design a highly efficient adsorbent for trapping toxic lead and cadmium ions from wastewater, a nanocomposite of Ni-Fe layered double hydroxide with N-doped graphene oxide have been synthesized via co-precipitation technique at ambient conditions and its metal adsorption characteristics have been further investigated under varying conditions such as pH, concentration, adsorbent loading and the time of contact. This novel nanocomposite has been found to exhibit Langmuir type adsorption of the metal ions indicating chemical nature of their adsorption on the surface of the nanocomposite and significantly higher adsorption efficiency as compared to the individual constituents. The adsorption process was found to follow a pseudo second-order rate equation with a rate constant of 0.025 g mg−1min−1. The nanocomposite was found to have a very high loading capacity of ∼1000 mg/g for both lead and cadmium ions respectively. It also works well over a wide pH range (5–11) and the adsorbent can be easily regenerated after use. Thus, this material holds strong potential for applications in the treatment of heavy metal intoxicated industrial wastewater.

Waste sludge derived adsorbents for arsenate removal from water

Aqueous arsenate [As(V)] was removed using an aluminum-based adsorbent (ABA) and coal mine drainage sludge coated polyurethane (CMDS-PU) prepared using alum and coal mine sludge, respectively. Their As(V) removal efficiencies were compared with each other and granular ferric hydroxide (GFH). The mineralogy and surface chemistry of materials were determined using wavelength dispersive X-ray fluorescence (WD XRF) and Fourier transform infrared spectroscopy (FTIR), respectively. The angle-resolved X-ray photoelectron spectroscopy (AR-XPS) studies confirmed As(V) retention on the adsorbent surfaces. The adsorption kinetics data were fitted to pseudo second-order rate equation. The faster As(V) uptake kinetics of GFH and ABA (GFH > ABA > CMDS-PU) were attributed to their large pore volume and mesoporous nature. Langmuir adsorption capacities of 22, 31 and 10 mg/g, were achieved for GFH, ABA and CMDS-PU, respectively. As(V) adsorption on GFH, ABA and CMDS-PU was endothermic. GFH and ABA were efficient over a wide pH range (3–10). In column studies, GFH, ABA, and CMDS-PU successfully treated 23625, 842, and 158 bed volumes (BVs) and 2094, 6400, and 17 BVs of As(V)-contaminated water with 9.5 and 27 EBCT, respectively (at pH = 6.0, Asi = 600 μg/L). The GFH and ABA have a potential to be used at large-scale aqueous phase As(V) remediation.

Ultrasonic-assisted synthesis of polyacrylamide/bentonite hydrogel nanocomposite for the sequestration of lead and cadmium from aqueous phase: Equilibrium, kinetics and thermodynamic studies

Clay-hydrogel nanocomposites are suitable material for mitigating the pollution/environmental impact because of their high adsorption capacity. In this study, the synthesis of polyacrylamide/bentonite hydrogel nanocomposite was assisted by ultrasound through successful incorporation of nanobentonite as filler and cross-linker into polyacrylamide framework. The adsorbent was characterized by FTIR, XRD, BET, SEM-EDX, and TEM in order to observe structural changes and sorption interactions. The effect of adsorbent dose, contact time, initial metal ion concentration and pH on the sequestration of Pb2+ and Cd2+ was analyzed. The adsorbent removed more than 95% Pb2+ and Cd2+ within first 20 min, which corresponds to relatively high pseudo-first order rate constant, k1 (0.240 for Pb2+ and 0.253 1/min for Cd2+) and pseudo-second order rate constant, k2 (0.031 for Pb2+ and 0.033 g/mg/min for Cd2+). The isotherm and kinetics modeling data were best described by Freundlich isotherm over the entire concentration range and pseudo-second order rate equation, respectively. The thermodynamic studies implied spontaneous and endothermic nature of adsorption process. The maximum adsorption capacity (138.33 for Pb2+ and 200.41 mg/g for Cd2+) determined using Langmuir model along with a good regeneration potential depicts that polyacrylamide/bentonite hydrogel nanocomposite could be used effectively for Pb2+ and Cd2+ uptake from aqueous solution.

Self-assembled GO incorporated CMC and Chitosan-based nanocomposites in the removal of cationic dyes

The purpose of the present study is to synthesise a self assembled nanocomposite by incorporating GO nanosheets into the polymer network of carboxy methyl cellulose and chitosan without using any initiator and cross-linker system. By incorporation of GO nanosheets, a great enhancement in various properties likes surface roughness, percentage swelling (from 2347% to 4633.8%) and adsorbent properties. The nanocomposite samples were investigated for removal of various kinds of dyes from waste water sources and were found highly selective for different cationic dyes. The adsorbent showed 97.9% removal of BG in 7 h whereas 100% removal of MB in just 1.5 h. The adsorption kinetics statistics fitted well in pseudo-second order rate equation. The correlation values and favourable RL of adsorption data suggested better fit for Langmuir adsorption for both the dyes.

Sequestration of U(VI) from aqueous phase using diethylenetriamine anchored polystyrene-divinylbenzene copolymer

Abstract Diethylenetriamine (DETA) moiety was chemically anchored on polystyrene-divinylbenzene (PS-DVB) co-polymer, and the resultant resin (PS-DETA) was studied for the extraction of uranium from aqueous solution. The extraction behavior of U(VI) was studied in PS-DETA a function of duration of equilibration, pH of the feed solution, concentration of U(VI), CO 3 2 − , Fe2＋, Co 2 + and Ni 2 + ions. The rate of extraction of uranium in PS-DETA was fitted into pseudo-first order and pseudo-second order rate equations. The extraction isotherm of uranium was fitted to Langmuir adsorption model. Recycling option of the resin PS-DETA was studied by subjecting the resin to repeated extraction and back-extraction of uranium. The performance of PS-DETA under the dynamic condition was evaluated by following the breakthrough curve of uranium. The extracted uranium was quantitatively recovered by using dilute nitric acid.

Biosorption of Direct Fast Scarlet 4BS from aqueous solution using the green-tide-causing marine algae Enteromorpha prolifera.

Enteromorpha prolifera, the marine algae frequently causing green tide in the littoral areas in recent years, was investigated as a feasible biosorbent to remove azo dye Direct Fast Scarlet 4BS (DFS-4BS) from aqueous solution. The FTIR spectra of the acid-treated Enteromorpha prolifera (ATEP) biomass before and after dye sorption were used to identify the main functionalities involved in the biosorption process. Results of batch experiments showed that the adsorption capacity of ATEP was dependent on solution pH, ATEP dosage and temperature. The adsorption kinetics followed the pseudo-second order rate equation, while the adsorption isotherm could be well described by the Langmuir model with maximum sorption capacity of 318.87mg·g-1, demonstrating the excellent performance of ATEP as a biosorbent to remove DFS-4BS from aqueous solution. The negative values of free energy change (ΔG°) and positive value of enthalpy change (ΔH°) confirmed that the sorption of DFS-4BS onto ATEP was spontaneous and endothermic in the temperature range of 303-333K. The isosteric heat of adsorption increased with the increase of surface loading, suggesting that the ATEP biomass had heterogeneous surface and lateral interactions might exist among adsorbed molecules. According to the sorption results and the FTIR spectra, it was deduced that the adsorption mechanism involved hydrogen bonding, electrostatic attraction and bonding, and hydrophobic and van der Waals interaction.

Applications of Fe3O4@AC nanoparticles for dye removal from simulated wastewater

This study deals with the removal of cationic dyes from the simulated wastewater using Fe3O4 nanoparticles loaded activated carbon. Fe3O4@AC nanoparticles were synthesised using co-precipitation methods. The Fe3O4@AC nanoparticles (nps) were characterised using different techniques and data revealed that the synthesised nanoparticles were 6-16 nm in diameter. pHpzc of Fe3O4@AC nanoparticles was 7.8. BET surface area of Fe3O4@AC nps was found to be 129.6 m2/g by single point method and 1061.9 m2/g by multipoint method. Adsorption experiments were performed to optimize the effect of process conditions such as pH of solution, nanoparticles dose, temperature, concentration of dye and contact time on contaminant removal. The maximum uptake capacity of Fe3O4@AC was found to be 138 and 166.6 mg/g for methylene blue and brilliant green dyes, respectively. In order to assess dye adsorption behaviour, adsorption isotherm models viz., Langmuir, Freundlich and Temkin were applied to the data. Langmuir isotherm best fitted [R2 = 0.993 (MB) and R2 = 0.920 (BG)] to the experimental data of both the dyes. Further, Pseudo-second order rate equation fitted better to the experimental data. Reuse potential of the nanoparticles was also investigated for the removal of both the dyes and it is inferred from the data that the synthesised nanoadsorbent has promising reuse potential, therefore can be used for several cycles.

Mechanism of As(V) removal by green synthesized iron nanoparticles.

The application of green synthesized iron nanoparticles (nFe) for the removal of arsenic (As) from contaminated sites has often been proposed as one of the most promising remediation methods. In this work, TEM analysis showed that As(V) was uniformly adsorbed on the surfaces of nFe, while FTIR analysis confirmed that adsorption was mainly via an FeOAs bond, and XPS analysis indicated that only As(V) was adsorbed. Hence, the removal mechanism proposed for As(V) is based on initially nFe reacting with As(V) to form a monodentate chelating ligand and subsequently a bidentate binuclear complex. The initially high surface area (51.14 m2 g-1) of nFe was decreased by half (26.03 m2 g-1) after As(V) adsorption, where the materials high As(V) adsorption capacity(21.59 mg g-1) was attributed to unique properties derived from the green synthesis. Adsorption isotherm modeling indicated that As(V) adsorption by nFe best fit the Langmuir equation (RL2 = 0.9903), and thus suggested chemisorption was occurring. The adsorption also fitted the pseudo second-order kinetic rate equation well, which confirmed that adsorption was via chemisorption. Overall, the green synthesis of nFe exhibited great application potential for As(V) removal due to high As(V) adsorption capacity and simplicity of synthesis.

Kinetics of neptunium sorption and desorption in the presence of aluminum (hydr)oxide minerals: Evidence for multi-step desorption at low pH

Kinetics analyses of sorption and desorption provide important insight into reaction mechanisms occurring at the mineral-water interface. They are also needed to determine when equilibrium is achieved, identify intermediate chemical species, and inform models describing neptunium mobility. Neptunium sorption to and desorption from four different aluminum (hydr)oxides - bayerite (α-Al(OH)3), gibbsite (γ-Al(OH)3), corundum (α-Al2O3), and γ-alumina (γ-Al2O3) - were investigated as a function of mineral concentration (5 – 170 m2 L−1), neptunium concentration (10−9 – 10−7 M), and pH (5.5 – 10.5). Neptunium sorption was characterized by a two-step reaction with an initial fast sorption step occurring within minutes followed by a slower equilibrium process, which was attributed to initial sorption of neptunium to a small number of strong sorption sites followed by sorption of neptunium to a larger number of weak sorption sites. The kinetics data were modeled using the linear and non-linear forms of the pseudo-first and pseudo-second order rate equations and the goodness of fit parameters were compared. Non-linear pseudo-second order rate constants described neptunium sorption to aluminum (hydr)oxides most accurately and were used to determine the reaction orders with respect to mineral concentration and [H+]. Neptunium desorption experiments demonstrated that the desorption mechanism changed as a function of pH and that the forward and reverse reactions were not equivalent. At pH ≥ 7.5, desorption reached steady-state within an hour and was accurately described by the non-linear pseudo-second order rate equations. A desorption plateau was observed at pH 5.5 that could not be described by either pseudo-first or –second order kinetics, suggesting the possibility of a multi-step desorption reaction. The comparatively slow desorption kinetics observed here suggests that sorbed neptunium could be slowly released back into the aqueous phase and act as a continuous source of contamination to the environment.

Removal of drug oxcarbazepine from wastewater at 3D porous NiFe2O4 nanoparticles

The adsorption behavior of an anticonvulsant drug oxcarbazepine from aqueous solution onto 3D porous NiFe2O4 was studied at 30, 40, and 50 °C. The optimum conditions for the adsorption of the drug oxycarbazepine have been found as 0.144 mg/mL drug concentration, pH 2.5, and 6 mg 3D porous NiFe2O4 adsorbent dose. The estimated values for ΔG° were −5.10 × 103 onto 3D porous NiFe2O4 adsorbent dose at 203 K (30 °C), indicate toward a feasible and spontaneous process. Adsorption isotherms were interpreted from various isotherm models like Langmuir, Freundlich, Dubinin–Radushkevich and Tempkin and their characteristic parameters were evaluated and compared. The adsorption energy, E and adsorption capacity (Q0) for adsorption on 3D porous NiFe2O4 were estimated using Dubinin-Radushkevich (DR) equation. The mass transfer property of the sorption process was studied using series of rate equations like pseudo first-order, pseudo second-order rate equation, Intra-particle diffusion and Liquid film diffusion model. The adsorption performance was investigated thermodynamically under batch equilibrium conditions at 303, 313 and 323 K. The optimum temperature of adsorption is 303 K.

Chemical pretreatments of Trapa bispinosa's peel (TBP) biosorbent to enhance adsorption capacity for Pb(ll)

AbstractIn this study,Trapa bispinosa'speel (TBP) biomass is exploited as an effective, low cost and new adsorbent to remove Pb(II) from aqueous solution. TBP is pretreated and modified with HNO3, HClO4and H2O2to enhance the Pb(II) removal and it is perceived that chemical modifications enhance the adsorption capacity of TBP. The adsorption behavior of Pb(II) is studied under different conditions, including pH (3-6), TBP dose (0.050.8 g), stirring speed (100-200 rpm), initial Pb(II) ion concentration (25-400 mg L-1) and contact time (0-1440 min). Kinetic study reveals sorption is fast in first 15 to 30 min achieving equilibrium in 60 min with qmax(mg g-1) are 77.09, 105.40 and 123.82 for NT-TBP, NA-TBP and HCA-TBP respectively. The Langmuir model successfully defines the sorption data having higher R2and good agreement between theoretical and experimental uptake capacity of Pb(II). The kinetic study exhibits that the pseudo-second order rate equation is better portrayed sorption process. TBP modified with HClO4shows the highest metal uptake in comparison to HNO3, H2O2modified TBP and native TBP.

Antimony Removal from Water by a Chitosan-Iron(III)[ChiFer(III)] Biocomposite.

The presence of antimony(III) in water represents a worldwide concern, mainly due to its high toxicity and carcinogenicity potential. It can be separated from water by the use of sustainable biopolymers such as chitosan or its derivatives. The present study applied chitosan modified with iron(III) beads to Sb(III) removal from aqueous solutions. The resulting material performed with a high adsorption capacity of 98.68 mg/g. Material characterization consisted of Raman spectroscopy (RS), X-ray diffraction (XRD), scanning electron microscope observations (SEM-EDX), Fourier transform infrared spectroscopy (FTIR) and point of zero charge (pHpzc). The adsorption study included pH study, effect of initial concentration, kinetics, ion effect, and reusability assessment. The RS, XRD, and FTIR results indicated that the main functional groups in the composite were related to hydroxyl and amino groups, and iron oxyhydroxide species of α-FeO(OH). The pHpzc was found to be 7.41. The best adsorption efficiency was set at pH 6. The equilibrium isotherms were better fitted with a non-linear Langmuir model, and the kinetics data were fitted with a pseudo-second order rate equation. The incorporation of iron into the chitosan matrix improved the Sb(III) uptake by 47.9%, compared with neat chitosan (CS). The material did not exhibit an impact in its performance in the presence of other ions, and it could be reused for up to three adsorption–desorption cycles.

Sorption and Desorption Studies of Pb(II) and Ni(II) from Aqueous Solutions by a New Composite Based on Alginate and Magadiite Materials.

A new composite material based on alginate and magadiite/Di-(2-ethylhexyl) phosphoric acid (CAM-D2EHPA) was successfully prepared by previous impregnation of layered magadiite with D2EHPA extractant, and then immobilized into the alginate matrix. Air dried beads of CAM-D2EHPA were characterized by FTIR and SEM–EDX techniques. The sorbent was used for the separation of lead and nickel from nitrate solutions; the main parameters of sorption such as contact time, pH of the solution, and initial metal concentration were studied. The beads recovered 94% of Pb(II) and 65% of Ni(II) at pH 4 from dilute solutions containing 10 mg L−1 of metal (sorbent dosage, S.D. 1 g L−1). The equilibrium data gave a better fit using the Langmuir model, and kinetic profiles were fitted using a pseudo-second order rate equation. The maximum sorption capacities obtained (at pH 4) were 197 mg g−1 and 44 mg g−1 for lead and nickel, respectively. The regeneration of the sorbent was efficiently carried out with a dilute solution of HNO3 (0.5 M). The composite material was reused in 10 sorption–elution cycles with no significant differences on sorption uptake. A study with synthetic effluents containing an equimolar concentration of both metals indicated a better selectivity towards lead ions.

Amidoxime functionalization of a poly(acrylonitrile)/silica composite for the sorption of Ga(III) – Application to the treatment of Bayer liquor

The copolymerization of styrene with divinylbenzene (DVB) in the presence of silica particles produces a commercial silica-supported polymer (SiO2-P) that was chemically reacted with acrylonitrile to synthesize silica-supported polyacrylonitrile (PAN/SiO2). PAN/SiO2 was functionalized by grafting amidoxime moieties to produce PAO/SiO2 sorbent (silica-supported polyacrylamidoxime). The sorbent (PAO/SiO2) was characterized by XPS, TGA, FTIR, pHpzc, elemental and SEM-EDX analyses and tested for Ga(III) recovery from aqueous solutions. The sorption performance was investigated through the study of pH effect, sorption isotherms at different temperatures, and uptake kinetics under slightly acidic conditions (around pH 4) and at pH close to 13.7 (similar to Bayer liquor). The equilibrium was reached within 60 min; the kinetic profiles can be fitted by the pseudo-second order rate equation (PSORE) and the resistance to intraparticle diffusion equation (RIDE). The maximum sorption capacity reaches 1.34 mmol Ga g−1 at 25 °C (1.76 mmol Ga g−1 at 55 °C): the sorption process is endothermic for solutions prepared by alkaline dissolving of Ga2O3. The sorption capacities are higher for solutions prepared with Ga(NO3)3 salt: the maximum sorption capacity increased up to 2.1–2.6 mmol Ga g−1 (from 25 to 55 °C). Langmuir and Sips equations were used for fitting sorption isotherms. Metal desorption and sorbent recycling were performed using 1.5 M HCl solution. Forty-five minutes were sufficient for achieving complete desorption of Ga(III). Sorption and desorption performances remain stable for a minimum of 5 cycles. Sorbent selectivity for gallium recovery was demonstrated in the presence of an excess of competitor cations. The sorbent was successfully applied for Ga(III) recovery from Bayer liquor at high pH values (around 13): combining relatively high efficiency and selectivity compared with LSC-600 amidoxime resin.

Pollutant Removal from Wastewater at Different Stages of the Tanning Process by Electrocoagulation

The removal of chemical oxygen demand (COD), total organic carbon (TOC), turbidity, and chromium content from tannery wastewater at different stages of the process was experimentally investigated using electrocoagulation (EC) with iron and aluminium electrodes. In the EC of the beamhouse wastewater (S1), the effects of initial pH and current density were analyzed and electrical energy consumption was determined. The COD and TOC in the solution were effectively removed, with an initial pH 7.0, using either metallic electrode. With a current density of 28 mA/cm2 for an electrolysis procedure of 60 minutes, the removal efficiency of COD and TOC was 72% and 57% with aluminium electrodes and 69% and 60% with iron electrodes, respectively. The minimum energy consumption for the highest COD and TOC removal was 0.37 and 0.69 kWh/m3 when employing iron or aluminium electrodes, respectively. At the optimal conditions, removal efficiencies close to 100% for turbidity and chromium content for wastewaters S1-beamhouse, S2-tanning, S3-retanning, and S4-a mixture 1 : 1 : 1 (v/v/v) were achieved. Results show that a pseudosecond-order rate equation provides a good correlation for the removal rate of the parameters. Finally, the results indicate that for tannery wastewater, the EC process does not depend noticeably on the electrode material, but that the stage of the tanning process of wastewater sample has the principal effect on treatment efficiency.

Evaluation of torrefied poplar-biomass as a low-cost sorbent for lead and terbium removal from aqueous solutions and energy co-generation

Two low-cost renewable poplar-based materials were manufactured in this work for energy production and as sorbents for lead and terbium removal from aqueous effluents. Torrefaction was used as a pretreatment process for conditioning the raw biomass. Two different operating conditions were used in the multiple-hearth furnace of the torrefaction pilot-plant: i) 250 °C; ii) 280 °C, with residence times of 75 min and 60 min, respectively. The raw and torrefied biomasses have been characterized using SEM-EDX, FTIR, TGA, XRD and elemental analyses (C, H, N, S, O); an increase of the torrefaction severity, results in an increase of the carbon/oxygen ratio and in a greater mass loss (21% at 250 °C, and 53% at 280 °C). The torrefaction had a positive impact on the sorption of metals, it allowed the increase of lignin content of the manufactured materials, and it allowed the storage of the sorbents for longer time with reduced moisture content. The equilibrium studies were performed in batch system and the experimental data were described with the Sips equation. The maximum sorption capacity was found as 30 mg g−1 for lead and 9.4 mg g−1 for terbium (at pH 4). The kinetic profiles were fitted using the pseudo-second order rate equation. The regeneration of the sorbent was demonstrated by three sorption-desorption cycles using dilute HNO3 solution (0.1 M) as eluent for metal recovery.