Adsorption of uranium from crude phosphoric acid using activated carbon

The adsorption of uranium from crude phosphoric acid has been investigated using conventional activated carbons. It was found that treatment with nitric acid oxidized the surface of activated carbon and significantly increased the adsorption capacity for uranium in acidic solutions. The parameters that affect the uranium(VI) adsorption, such as contact time, solution pH, initial uranium(VI) concentration, and temperature, have been investigated. Equilibrium data were fitted to a simplified Langmuir and Freundlich isotherms for the oxidized samples which indicate that the uranium adsorption onto the activated carbon fitted well with Langmuir isotherm than Freundlich isotherm. Equilibrium studies evaluate the theoretical capacity of activated carbon to be 45.24 g kg^?1.     

Polyaniline impregnated activated carbon fabric: Adsorption and protection studies against nerve agent sarin

The synergistic effect of polyaniline (PANI) was explored in detail by conducting experiments with liquid and vapour sarin (GB) exposure tests. For that, different samples were prepared and breakthrough behaviour, liquid sarin elution profile and dynamic adsorption of vapour were studied at a different doping level of PANI and activated carbon fabric (PANI-ACF) was optimized to achieve optimum breakthrough behaviour. The results reveal that PANI-ACF of a surface density of 1.2–1.27 g/cm² has a high surface area (914–980 m²/g) and micropore volume (0.20–0.22 (mL/g). Specific breakthrough volume (BTV), breakthrough time (BTT) and average breakthrough concentration rate were determined and found to be 243 L/g, 220–350 min and 0.004–0.0014 μg h^?1cm^?2 respectively which is much better than only ACF. Hence, PANI reinforced ACF applies to selective adsorption of nerve agent sarin and consecutively in chemical nerve agent protection.     

Kinetics of salicylic acid adsorption on activated carbon

The adsorption and desorption of salicylic acid from water solutions was investigated in HPLC microcolumns packed with activated carbon. The adsorption isotherm was obtained by the step-up frontal analysis method in a concentration range of 0-400 mg/L and was well fitted with the Langmuir equation. The investigation of rate aspects of salicylic acid adsorption was based on adsorption/desorption column experiments where different inlet concentrations of salicylic acid were applied in the adsorption phase and desorption was conducted with pure water. The concentration profiles of individual adsorption/desorption cycles data were fitted using several single-parameter models of the fixed-bed adsorption to assess the influence of different phenomena on the column behavior. It was found that the effects of axial dispersion and extraparticle mass transfer were negligible. A rate-determining factor of fixed-bed column dynamics was the kinetics of pore surface adsorption. A bimodal kinetic model reflecting the heterogeneous character of adsorbent pores was verified by a simultaneous fit of the column outlet concentration in four adsorption/desorption cycles. The fitted parameters were the fraction of mesopores and the adsorption rate constants in micropores and mesopores, respectively. It was shown that the former rate constant was an intrinsic one whereas the latter one was an apparent value due to the effects of pore blocking and diffusional hindrances in the micropores.     

Selective fast pyrolysis of biomass impregnated with ZnCl2: Furfural production together with acetic acid and activated carbon as by-products

Fast pyrolysis of biomass materials impregnated with ZnCl₂ offered a promising way to obtain a liquid product rich in furfural (FF) and acetic acid (AA), and the pyrolytic solids could be used as the precursors to prepare activated carbons (ACs). In this study, a lab-scale fast pyrolysis set was designed and used for the quantitative production of the three chemicals. The maximum FF was produced from the corncob impregnated with at least 15 wt% ZnCl₂ and at the pyrolysis temperature around 340 °C, with the yield of more than 8 wt% compared with only 0.49 wt% from the raw corncob. Meanwhile, AA of around 4 wt% could be obtained. The content of the FF and AA was over 50 wt% and 25 wt% on the water-free basis of the pyrolytic liquids. In addition, ACs were prepared from the pyrolytic solids, and they exhibited similar properties as those prepared from direct activation of ZnCl₂-impregnated biomass materials.     

Kinetics of the gas-phase chlorination of activated carbon with carbon tetrachloride

The chlorination of activated charcoals KAU and SCN with CCl₄ vapor in the temperature range 300–600 °C and also the thermal stability of the added chlorine has been studied. It is shown that chlorination with CCl₄ vapor led to the addition of up to 22 mass % (6.2 mmol/g) of chlorine to the surface layer of the carbon. The added chlorine is desorbed from the surface with formation of HCl, while at temperatures above 500 °C desorption of chlorine goes practically to completion. The effective rate constants and the energies of activation of the processes of chlorination and desorption have been found.     

Quantum-chemical studies of dimethylformamide 1 : 1 complexes with phosphoric acid

The structures of two phosphoric acid conformations, dimethylformamide (DMFA), four protonated DMFA forms, and nine DMFA-H₃PO₄ complexes in which the proton acceptor is a oxygen or nitrogen atom of the DMFA molecule are optimized by DFT/B3LYP using the 6-31++G(d, p) basis set. The structural changes in DMFA that occur upon its protonation are discussed. The stabilization energy and transferred charge values upon the formation of a hydrogen bond are calculated for all of the studied complexes by means of NBO analysis. The potential energy surface is scanned to study the possibility of proton transfer.     

Comparative study of acid yellow 119 adsorption onto activated carbon prepared from lemon wood and ZnO nanoparticles loaded on activated carbon

Activated carbon from lemon wood (AC) and ZnO nanoparticles loaded on activated carbon (ZnO‐NP‐AC) were prepared and their efficiency for effective acid yellow 199 (AY 199) removal under various operational conditions was investigated. The dependence of removal efficiency on variables such as AY 199 concentration, amount of adsorbent and contact time was optimized using response surface methodology and Design‐Expert. ZnO nanoparticles and ZnO‐NP‐AC were studied using various techniques such as scanning electron microscopy, X‐ray diffraction and energy‐dispersive X‐ray analysis. The optimum pH was studied using one‐at‐a‐time method to achieve maximum dye removal percentage. Small amounts of the proposed adsorbents (0.025 and 0.025 g) were sufficient for successful removal of AY 199 in short times (4.0 and 4.0 min) with high adsorption capacity (85.51 and 116.29 mg g^?1 for AC and ZnO‐NPs‐AC, respectively). Fitting the empirical equilibrium data to several conventional isotherm models at optimum conditions indicated the appropriateness of the Langmuir model with high correlation coefficient (0.999 and 0.978 for AC and ZnO‐NPs‐AC, respectively) for representation and explanation of experimental data. Kinetics evaluation of experiments at various time intervals revealed that adsorption processes can be well predicted and fitted by pseudo‐second‐order and Elovich models. This study revealed that the combination of ZnO nanoparticles and AC following simple loading led to significant improvement in the removal process in short adsorption time which was enhanced by mixing the media via sonication.     

Conversion of teak wood waste into microwave-irradiated activated carbon for cationic methylene blue dye removal: Optimization and batch studies

The objective of this study is to find optimum preparation conditions in converting teak wood waste into activated carbon (TWAC) and to evaluate its performance in adsorbing cationic dye of methylene blue (MB). TWAC was produced via physiochemical activation (potassium hydroxide, KOH chemical treatment, and carbon dioxide, CO₂ gasification) and heated through microwave irradiation technique. With the aid of response surface methodology (RSM), optimized TWAC was successfully synthesized at radiation power, radiation time, and impregnation ratio (IR) of 366 W, 5.30 min, and 1.15 g/g, respectively. These preparation conditions produced TWAC with MB adsorption uptakes of 66.69 mg/g and a yield of 38.23%. Characteristics of TWAC in terms of BET surface area, mesopores surface area, total pore volume, and average pore diameter were determined to be 1345.25 m²/g, 878.63 m²/g, 0.6140 cm³/g, and 2.85 nm, respectively. Isotherm studies divulged that the MB-TWAC adsorption system followed the Langmuir model with a maximum monolayer adsorption capacity of 567.52 mg/g. In terms of kinetic studies, this adsorption system fit pseudo-second order model the best whereas Boyd plot confirmed that the adsorption process was controlled by the film diffusion mechanism. Thermodynamic parameters of enthalpy change, ΔH°, entropy change, ΔS°, Gibbs free energy, ΔG° and Arrhenius activation energy, E_a were calculated to be ?4.06 kJ/mol, 0.06 kJ/mol.K, –22.69 kJ/mol and 16.03 kJ/mol, respectively. The activation and microwave heating methods employed succeeded to produce TWAC with excellent adsorption performance in removing MB dye. TWAC was also successfully regenerated for 5 cycles via microwave heating technique.     

Understanding the pyrolysis of CCA-treated wood: Part II. Effect of phosphoric acid

The pyrolysis process of chromated copper arsenate-treated wood impregnated with phosphoric acid (2–6 wt.%) has been investigated using a Kugelrohr short-path distillation apparatus. The presence of phosphoric acid is shown to change the overall pyrolysis kinetics and yields of products resulting in a decrease in the amount of volatile products and an increase in the char formation. The yield of levoglucosan was found to decrease in the presence of relatively higher concentrations of phosphoric acid, while the yield of levoglucosenone increased. The pyrolysis of chromated copper arsenate-treated wood in the presence of phosphoric acid gave rise to 22% (on water-free tar basis) levoglucosenone. Such data demonstrates the potential for a new pathway toward the rational utilization of chromated copper arsenate-treated wood waste.     

Pyrolysis of various biomass residues and char utilization for the production of activated carbons

Biomass samples (almond shell, walnut shell, almond tree pruning and olive stone) were subjected to thermoanalytical investigation to evaluate their thermal behaviour and its correlation with their lignocellulosic composition. Then, the pyrolysis process of these materials was studied in terms of the energy content of the phases generated (gas and liquid). Finally, the feasibility of obtaining effective adsorbents from the char generated was studied. With this aim, the char was used to prepare activated carbons (ACs) by steam gasification at fixed activation temperature and time, identical for the four chars. The differences found in the porosity development of the activated carbons were related to the lignocellulosic composition of the raw material. It is shown that the four biomass residues used are versatile precursors that allow the preparation of adsorbent materials with different textural characteristics.     

Adsorption of cationic methylene blue dye using microwave-assisted activated carbon derived from acacia wood: Optimization and batch studies

This study assesses the performance of optimized acacia wood-based activated carbon (AWAC) as an adsorbent for methylene blue (MB) dye removal in aqueous solution. AWAC was prepared via a physicochemical activation process that consists of potassium hydroxide (KOH) treatment, followed by carbon dioxide (CO₂) gasification under microwave heating. By using response surface methodology (RSM), the optimum preparation conditions of radiation power, radiation time, and KOH-impregnation ratio (IR) were determined to be 360 W, 4.50 min, and 0.90 g/g respectively, which resulted in 81.20 mg/g of MB dye removal and 27.96% of AWAC’s yield. Radiation power and IR had a major effect on MB dye removal while radiation power and radiation time caused the greatest impact on AWAC’s yield. BET surface area, mesopore surface area, and pore volume of optimized AWAC were found to be 1045.56 m²/g, 689.77 m²/g, and 0.54 cm³/g, respectively. Adsorption of MB onto AWAC followed Langmuir and pseudo-second order for isotherm and kinetic studies respectively, with a Langmuir monolayer adsorption capacity of 338.29 mg/g. Mechanism studies revealed that the adsorption process was controlled by film diffusion mechanism and indicated to be thermodynamically exothermic in nature.     

Preparation and characterization of activated carbon fibers from liquefied wood

Wooden activated carbon fibers (WACFs) were prepared from phenolated Chinese fir (Cunninghamia lanceolata) using CO₂ activation; microstructure characterization, the adsorption capacity, BET-specific surface area, and pore distribution of WACFs were investigated by SEM and X-ray analysis. Results showed that WACFs have a smooth surface and round or elliptical cross-section. The (002) crystal plane diffraction peak of the WACFs was obviously heightened, also showing an apparent (100) diffraction peak. With increased activation temperature, the value of d ₍₀₀₂₎ gradually decreased, whereas the values of the crystallite sizes L _a and L _c initially decreased and then increased. The L _c/d ₍₀₀₂₎ and g values corresponding to the degree of change in the graphitization structure increased. WACFs mainly have micropores as well as a few macro- and mesopores. The micropore diameter of WACFs has a narrow range (0.3–0.5 nm). With increased activation temperature, the single-point surface area, Brunauer-Emmett-Teller surface area, micropore area, single-point total pore volume, and micropore volume of WACFs increased, while the pore diameter decreased. At 900 °C, the iodine adsorption and yield rate of WACFs were 779.22 mg/g and 51.48 %, respectively.     

Effect of some ammonium salts on thermal decomposition of impregnated wood and properties of activated carbons

Methods of chemical, thermal, IR spectral, and X-ray phase analysis were used to study the effect of ammonium additives NH₄Cl + NH₄NO₃ introduced into a phosphorus-nitrogen formulation on the thermal decomposition of impregnated wood in the temperature range 20–700°C and on adsorption characteristics of the resulting activated carbon.     

Effects of phosphoric acid as additive in the preparation of activated carbon fibers from poly(p-phenylene benzobisoxazole) by carbon dioxide activation

Poly(p-phenylene benzobisoxazole) (PBO) was impregnated with small amounts of H₃PO₄, and the effects of this additive on the porosity and other characteristics of chars and activated carbon fibers (ACFs) derived from this polymer were investigated. To this end, PBO-AS impregnated with 5, 10 or 15 wt.% H₃PO₄ was pyrolyzed at 850 °C, and the resulting chars were physically activated with carbon dioxide at 800 °C to different burn-off (BO) degrees. Thermal analysis techniques only detected minor effects of H₃PO₄ on PBO pyrolysis. The char yield and char reactivity towards CO₂ increased following PBO-AS impregnation with H₃PO₄. Structural (X-ray diffraction), porous textural (CO₂ adsorption) and surface chemical (temperature-programmed desorption, X-ray photoelectron spectroscopy) characterizations of the pyrolysis chars indicated that the increase in char reactivity is probably associated with a higher content of oxygenated functionalities. Following CO₂ activation, the surface area and pore volume of the obtained ACFs chiefly depended on the BO degree, but impregnation with H₃PO₄ restricted the pore size to the micropore and narrow mesopore range, thus producing adsorbents with a slightly narrower pore size distribution than in the absence of H₃PO₄. The results are compared with those previously obtained under equivalent conditions with other high-crystallinity polymers as precursors for ACFs.     

Semi-global mechanisms for the oxidation of diammonium phosphate impregnated wood

Thermogravimetric measurements in air of fir wood impregnated with diammonium phosphate in the concentration range 2-20% and heating rates between 5 and 20 K/min show the existence of three sequential stages: wood decomposition, induction and char oxidation. The additive causes an anticipation in the decomposition stage up to about 100 K, a reduction in the maximum rate up to a factor of three and an increase in the charred solid residue from about 30 to 60%. The total duration of the induction and oxidation stages is also increased. The experimental curves are interpreted by either a three-step mechanism (one reaction for each stage) or a seven-step mechanism (three reactions for decomposition and two reactions for the induction and oxidation stages). The estimated kinetic parameters are independent from the heating rate but vary with the diammonium phosphate concentration. However, for both mechanisms, the activation energies of the various steps remain practically constant except for the decomposition of the cellulose component or the decomposition step, depending on the complexity of the mechanism.     

Radiotracer investigation of phosphoric acid and phosphatic fertilizers production process

In the phosphoric acid production process, the time a particle spends inside the chemical reactor (residence time) is of paramount importance to process engineers. Residence time distribution (RTD) gives information on the efficiency of the chemical reactor, on the efficiency of the process, and also the availabilities of the reactive volume for the reaction (active volume vs. dead volume). Traditionally, chemical engineers used chemical tracer to determine the RTD. However, first disadvantage is that the chemical tracer could not allow an online diagnosis: the samples containing chemical tracer have to go to a lab for analysis, second disadvantage is that the chemical tracer is less sensitive than radioactive ones because of its adsorption onto strata or its retention in rocks. Consequently, chemical tracer results are not always precise and cannot convincingly explain the multiple flow-path model. Radioactive tracers are the only tracers capable of measuring the active RTD with high degree of precision and give information on the internal recirculation rate. In this work, we will describe the application of radiotracer method for RTD measurement in the phosphoric acid production process and give results and discussion of each case encountered.     

Recycling of carbon fibre reinforced polymeric waste for the production of activated carbon fibres

Composite waste composed of carbon fibres and polybenzoxazines resin has been pyrolysed in a fixed bed reactor at temperatures of 350, 400, 450, 500 and 700 °C. Solid residues of between 70 and 83.6 wt%, liquid yields 14 and 24.6 wt% and gas yields 0.7 and 3.8 wt% were obtained depending on pyrolysis temperature. The derived pyrolysis liquids contained aniline in high concentration together with oxygenated and nitrogenated aromatic compounds. The pyrolysis gases consisted mainly of CO₂, CO, CH₄, H₂ and other hydrocarbons. The carbon fibres used in the composite waste were separated from the char of the solid residue via oxidation of the char at two different temperatures and investigated for their mechanical strength properties. The carbon fibres recovered from the sample pyrolysed at 500 °C and oxidised at 500 °C exhibited mechanical properties which were 90% of that of the original virgin carbon fibres. Steam activation of the recovered carbon fibres was carried out at 850 °C at different times of activation. The effect of activation time on BET surface area, activated carbon fibres yield, porosity and the morphological structure of activated carbon fibres was evaluated. A maximum BET surface area of over 800 m² g⁻¹ was obtained for the activated carbon fibres produced at 850 °C for 5 h of activation. Nitrogen adsorption-desorption isotherms showed that the adsorption capacity increased as the activation time increased up to 5 h of activation and then after that decreased.     

Interaction of activated carbon electrodes with sulfuric acid solutions

Impedance and potentiodynamic studies have been carried out to investigate reversible processes on the surface of electrodes based on activated carbon (AC). Relationships have been found between the reversible capacity and true resistance, on the one hand, and the potential, on the other. Stoichiometric characteristics of AC charging in sulfuric acid have been obtained. A mechanism of the reversible surface process is suggested. It is concluded that Faraday pseudocapacity makes a considerable contribution to charging of AC electrolytes. The amount of gas liberated in the course of AC wetting in electrolyte has been measured. It is assumed that the amount of liberated gas depends on the amount of active oxygen involved in the current-forming reaction. Based on the data obtained, a redox surface reaction is substantiated in general form.     

Flame retarding of wood by impregnation with boric acid - Pyrolysis products and char oxidation rates

The pyrolysis of fir wood impregnated with boric acid (0-5.4%) has been investigated for heating temperatures of 650 and 800 K by examining the yields of char, water, permanent gases (CO₂, CO, CH₄) and total organic products (together with 32 compounds). The yields of the last product class continuously decrease to the advantage of char and water, but the most significant modifications occur for acid contents below 2%. The formation of levoglucosan (with 2-acetylfuran, 5-methyl-2-furaldehyde and other minor species) first and levoglucosenone (with 2-furaldehyde) afterwards is favoured, whereas other compounds generated from the holocellulosic (hydroxyacetaldehyde, hydroxypropanone, acetic acid and minor carbohydrates) and lignin (phenols, cresols) fractions generally decline. Conversion times become longer and volatilization rates are reduced. The oxidation characteristics of char have been studied by means of thermogravimetric analysis and interpreted according to a three-step reaction mechanism. The boric acid treatment lowers the activation energy and reaction order of the most important step (145 versus 226 kJ/mol and 1.2 versus 0.86, respectively) which also shows lower rates and is slightly delayed.