Kinetic, equilibrium and thermodynamic studies of cadmium (II) adsorption by modified agricultural wastes

Agricultural wastes have great potential for the removal of heavy metal ions from aqueous solution. The contamination of water by toxic heavy metals is a worldwide environmental problem. Unlike organic pollutants, the majority of which are susceptible to biological degradation, heavy metals do not degrade into harmless end products. Discharges containing cadmium, in particular, are strictly controlled because of the highly toxic nature of this element and its tendency to accumulate in the tissues of living organisms. This work aims to develop inexpensive, highly available, effective metal ion adsorbents from natural wastes as alternatives to existing commercial adsorbents. In particular, Tamrix articulata wastes were modified chemically by esterification with maleic acid to yield a carboxyl-rich adsorbent. The adsorption behavior of treated Tamrix articulata wastes toward cadmium ions in aqueous solutions in a batch system has been studied as a function of equilibration time, adsorbent dose, temperature and pH. Results showed that the maximum adsorption capacity was 195.5 mg/g in a pH 4 solution at 30 °C with a contact time of 120 min, an initial concentration of 400 mg/L and an adsorbent dose of 0.3 g/L. The kinetic data were analyzed using pseudo-first-order and pseudo-second-order kinetic models. It was shown that the adsorption of cadmium could be described by a pseudo-second-order equation. The experimental data were also analyzed using the Langmuir and Freundlich models of adsorption. Thermodynamic parameters such as ΔG°, ΔH° and ΔS° have been evaluated and it has been found that the sorption process was spontaneous and exothermic in nature. From all of our data, we conclude that the treated Tamrix articulata wastes investigated in this study showed good potential for cadmium removal from aqueous solutions.     

Kinetic and thermodynamic aspects of U(VI) and Th(IV) sorption on a zeolitic volcanic tuff

Summary The sorption behavior of U(VI) and Th(IV) from simulated waste solutions on a zeolitic volcanic tuff from Nereju (Vrancea, Romania) has been studied in the absence of the ionic competition as a function of contact time, radioactive ions concentration, temperature and pH using a batch mode technique. The effect of the above-mentioned parameters on the sorption efficiency was discussed. The apparent thermodynamic parameters of the sorption of uranium and thorium onto the considered volcanic tuff were calculated, showing that the process is endothermic and higher temperatures favor the sorption process.     

Structural characteristics of modified activated carbons and adsorption of explosives

Several series of activated carbons prepared by catalytic and noncatalytic gasification and subsequent deposition of pyrocarbon by pyrolysis of methylene chloride or n-amyl alcohol were studied by FTIR, chromatography, and adsorption methods using nitrogen and probe organics (explosives). The relationships between the textural characteristics of carbon samples and the recovery rates (eta) of explosives on solid-phase extraction (SPE) using different solvents for their elution after adsorption were analyzed using experimental and quantum chemical calculation results. The eta values for nitrate esters, cyclic nitroamines, and nitroaromatics only partially correlate with different adsorbent parameters (characterizing microporosity, mesoporosity, pore size distributions, etc.), polarity of eluting solvents, or characteristics of probe molecules, since there are many factors strongly affecting the recovery rates. Some of the synthesized carbons provide higher eta values than those for such commercial adsorbents as Hypercarb and Envicarb.     

Study on adsorption of Th(IV) using surface modified dibenzoylmethane molecular imprinted polymer

The adsorption of Th(IV) was studied using a novel dibenzoylmethane molecular imprinted polymers, which was prepared using acryloyl-β-cyclodextrin as a monomer on surface modified functional silica gel. X-ray photoelectron spectroscopy and FTIR were employed to confirm the reliability of the synthetic polymer. Scanning electron microscope was used to analyze the surface properties of the experimental materials. UV-spectrophotometer was employed to investigate the adsorption property and the concentration of Th(IV). Adsorption kinetics and adsorption isotherm were carried out. In pH 3.5, the adsorption equilibrium could reach a balance in 45 min, the resultant adsorbing capacity was 30.8 mg g^?1, and the remove ratio of Th(IV) was 88.1 %.     

Naphthalene adsorption on activated carbons using solvents of different polarity

The hydrophobic-hydrophilic character of a series of microporous activated carbons was explored as a key factor in competitive adsorption of a non-polar compound from liquid phase. The selectivity of the carbon surface towards naphthalene was explored by performing the adsorption isotherms in water, cyclohexane and heptane. Solvent polarity and adsorbent hydrophobic character were found to strongly influence the adsorption capacity of naphthalene. In aqueous media, despite the non-polar character of the adsorbate, surface acidity lowered adsorption capacity. This is attributed to the competition of water from the adsorption sites, via H-bonding with surface functionalities and the formation of hydration clusters that reduce the accessibility and affinity of naphthalene to the inner pore structure. In organic media the uptake decreased due to competition of the hydrophobic solvent for the active sites of the carbon and to solvation effects. This competitive effect of the solvent is minimized in oxidized carbons as opposed to the trend obtained in aqueous solutions. The results confirmed that although adsorption of naphthalene strongly depends on the narrow microporosity of the adsorbent, competitive adsorption of the solvent for the active sites becomes important.     

Equilibrium,kinetic and thermodynamic studies of copper adsorption onto poly(n-vinylpyrrolidone) modified clay

We demonstrated in this study the removal of copper ions from aqueous solutions by a new and cost efficiency adsorbent based on poly(N-vinylpyrrolidone) modified sodium bentonite. The X-ray diffraction analysis confirmed that the poly(N-vinylpyrrolidone) is well intercalated in the clay inter-sheets. Adsorption of copper was analyzed by the atomic absorption spectroscopy technical.

Quantitative removal of copper in aqueous solution and the optimum conditions in batch experimental set-up were attained by following searching effects such as contact time, initial metal concentration, pH and temperature.

The elimination of this pollutant by this a new composite reached 3.27 mg.g^?1, which is more important than the amount of copper adsorbed by the non-modified clay (1.5 mg.g^?1). The experimental results showed that the equilibrium and shaking time was attained within 05 min. The better retention capacity of copper was obtained at pH between 4 and 5.8.

Equilibrium data were well fitted with the Langmuir, Freundlich and Elovich models.

Also, the pseudo first order and pseudo-second-order were also applied. The experimental data follow well the pseudo-second-order kinetics. The thermodynamic of this new adsorbent of copper shows spontaneous and exothermic process with a negative value of ΔS.     

Effect of reduction treatment on copper modified activated carbons on NO(x) adsorption at room temperature

Activated carbon was impregnated with copper salt and then exposed to reductive environment using hydrazine hydrate or heat treatment under nitrogen at 925 °C. On the obtained samples, adsorption of NO(2) was carried out at dynamic conditions at ambient temperature. The adsorbents before and after exposure to nitrogen dioxide were characterized by X-ray diffraction (XRD), thermal analysis, scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM-EDX), X-ray photoelectron spectroscopy (XPS), N(2)-sorption at -196 °C, and potentiometric titration. Copper loading improved the adsorption capacity of NO(2) as well as the retention of NO formed in the process of NO(2) reduction on the carbon surface. That improvement is linked to the presence of copper metal and its high dispersion on the surface. Even though both reduction methods lead to the reduction of copper, different reactions with the carbon surface take place. Heat treatment results in a significant percentage of metallic copper and a reduction of oxygen functional groups of the carbon matrix, whereas hydrazine, besides reduction of copper, leads to an incorporation of nitrogen. The results suggest that NO(2) mainly is converted to copper nitrates although the possibility to its reduction to N(2) is not ruled out. A high capacity on hydrazine treated samples is linked to the high dispersion of metallic copper on the surface of this carbon.     

Study of chromium(VI) adsorption onto modified activated carbons with respect to analytical application

Two different types of modification of activated carbon, by treatment with concentrated solution of HNO₃ and outgassing treatment at high temperature, were studied in order to obtain the most effective adsorption of chromium(VI) ions from water solution. The basic parameters affecting the adsorption capacity of Cr(VI) ions on modified activated carbons were studied in details and the effect of modifications of activated carbons has been determined by studying the initial runs of adsorption isotherms. The obtained Cr(VI) adsorption isotherms were well fitted in the Freundlich equation. The reduction of Cr(VI) to Cr(III) and further ion exchange mechanism of adsorption onto oxidizing activated carbon and surface precipitation to Cr(OH)₃ in case of outgassing activated carbon were found as the main adsorption mechanisms of Cr(VI) ions onto modified activated carbons. Presence of chlorides and nitrates in studied adsorption system strongly decreased the adsorption ability of Cr(VI) onto outgassing activated carbon and mechanism of this behavior is proposed.     

Calorimetric evaluation of activated carbons modified for phenol and 2,4-dinitrophenol adsorption

Two commercial activated carbons with differences in their superficial chemistry, one granular and the other pelletised, were modified for use in phenol and 2,4-dinitrophenol adsorption. In this paper, changes to the activated carbon surface will be evaluated from their immersion calorimetry in water and benzene, and they will then be compared with Area BET, chemical parameters, micropore size distributions and hydrophobicity factors of the modified activated carbons. The activated carbons were modified using 60 % solutions of phosphoric acid (H₃PO₄), nitric acid (HNO₃), zinc chloride (ZnCl₂) and potassium hydroxide (KOH); the activated carbon/solution ratio was 1:3 and impregnation was conducted 291 K for a period of 72 h before samples were washed until a constant pH was obtained. Water immersion calorimetry showed that the best results were obtained from activated carbons modified with nitric acid, which increased from ?10.6 to ?29.8 J g^?1 for modified granular activated carbon, and ?30.9 to ?129.3 J g^?1 for pelletised activated carbon. Additionally, they showed the best results in phenol and 2.4-dititrophenol adsorption. Those results indicate that impregnation with nitric acid under the employed conditions could generate a greater presence of oxygenated groups on their surface, which favours hydrogen bond formation and the increased adsorption of polar compounds. It should also be noted that immersion enthalpy in benzene for modified activated carbon with nitric acid is the method with the lowest value, which is consistent with the increased presence of polar groups on its surface. Regarding hydrophobicity factors, it was observed that granular carbons modified with nitric acid and potassium hydroxide have the lowest ratios, indicating greater interaction with water.     

Kinetic and isothermal studies of lead ion adsorption onto palygorskite clay

The use of a natural palygorskite clay for the removal of Pb(II) from aqueous solutions for different contact times, pHs of suspension, and amounts and particle sizes of palygorskite clay were investigated. The variations of the pH value of Pb(II) solutions on natural palygorskite in the adsorption process were determined. Batch adsorption kinetic experiments revealed that the adsorption of Pb(II) onto palygorskite clay involved fast and slow processes. It was found that the adsorption mechanisms in the lead/palygorskite system follow pseudo-second-order kinetics with a significant contribution from film diffusion. SEM observations demonstrated that an important interaction at the lead-granule interface occurred during the adsorption process. The adsorption isotherms were described by means of the Langmuir and Freundlich isotherms and the Langmuir model represents the adsorption process better than the Freundlich model. The maximum adsorption capacity of Pb(II) onto natural palygorskite was 104.28 mg g(-1).     

Pore structure and surface properties of chemically modified activated carbons for adsorption mechanism and rate of Cr(VI)

Effects of hydrochloric acid and sodium hydroxide treatments of activated carbons (ACs) on chromium(VI) reduction were studied. The surface properties were determined by pH, acid-base values, FT-IR, and X-ray photoelectron spectrometer (XPS). And the porous structure of the activated carbons was characterized by adsorption of N(2)/77 K. The Cr(VI) adsorption experiments were carried out to analyze the influence of porous texture and surface properties changed by the chemical surface treatments of ACs on adsorption rate with carbon-solution contact time. From the experimental results, it was observed that the extent of adsorption and reduction processes depends on both microporous structure and functional groups. And the adsorption of Cr(VI) ion was more effective in the case of acidic treatment on activated carbons, resulting from the increases of acid value (or acidic functional group) of activated carbon surfaces. However, basic treatment on activated carbons was not significantly effective on the adsorption of Cr(VI) ion, probably due to the effects of the decrease of specific surface area and basic Cr(VI) in nature.     

Synthesis, characterization, and application of titanate nanotubes for Th(IV) adsorption

Titanate nanotubes (TNTs) have been synthesized by a hydrothermal method using rutile TiO₂ powder as titanium source. The determination of the structure and morphology was characterized by XRD, FTIR, SEM and TEM. The results indicate that the TNTs successfully synthesized under hydrothermal conditions of 150 °C. The adsorption of Th(IV) on TNTs was studied as a function of contact time, pH values, ionic strength, initial Th(IV) concentration and temperature under ambient conditions by using batch technique. The results indicate that adsorption of Th(IV) on TNTs is strongly dependent on pH values, but weakly dependent on ionic strength; Adsorption kinetics was better described by the pseudo-second-order model. The adsorption isotherms are simulated by Langmuir and Freundlich models well. ΔG°, ΔH° and ΔS° free energy were calculated from experimental data, The results indicate that the adsorption of Th(IV) on TNTs is an endothermic and a spontaneous process, and increases with increasing temperature. The adsorption of Th(IV) on TNTs is mainly dominated by chemical sorption or surface complexation.     

基于ZIF-9的多孔碳对次甲基绿的吸附动力学研究

以含钴的类沸石咪唑酯骨架结构材料ZIF-9为模板,在氩气中进行碳化,以及在碳化时加入硫粉,分别得到含钴和钴硫双掺杂的多孔碳材料Co/C-700和CoS/C-700.对合成材料的结构和组成进行了PXRD、Raman光谱、SEM和氮气吸附脱附的表征,结果显示:合成材料保持了模板的骨架结构,比表面积分别为270.59和108.78 m²·g^-1,是微孔介孔碳纳米复合材料.Co/C-700和CoS/C-700对染料次甲基绿的最大吸附量分别为58.31和57.90 mg·g^-1,吸附符合准二级动力学模型、孔扩散模型和Langmuir模型.分离、再生以及循环使用4次之后的Co/C-700对次甲基绿的吸附率仅下降了4%.     

Removal of copper from aqueous solutions by adsorption on activated carbons

The adsorption isotherms of Cu(II) ions from aqueous solutions in the concentration range 40–1000 mg l⁻¹ on two samples of granulated and two samples of activated carbon fibres containing varying amounts of associated oxygen have been reported. The adsorption isotherms are type I of BET classification showing initially a rapid adsorption tending to be asymptotic at higher concentrations. The amounts of oxygen associated with the carbon surface has been enhanced by oxidation with nitric acid and ammonium persulphate in the solution phase and with oxygen gas at 350°C and decreased by degassing of the oxidized carbon samples at 400, 650 and 950°C. The adsorption of Cu(II) ions increases on oxidation and decreases on degassing. The increase in adsorption on oxidation depends on the nature of the oxidative treatment while the decrease in adsorption on degassing depends on the temperature of degassing. This has been attributed to the increase in the carbon–oxygen acidic surface groups on oxidation and their decrease on degassing. Suitable mechanisms consistent with the results have been proposed.     

Kinetic studies on the reduction of a nickel (III) oxime-imine complex by sulphur(IV) and selenium(IV)

The kinetics of the reduction of [Ni^III(L¹)]²⁺ (where HL¹ = 15-amino-3-methyl-4,7,10,13-tetraazapentadec-3-en-2-one oxime) by sulphur(IV) and selenium (IV) over the regions pH 2.50–8.02 and 2.01–4.00 respectively have been investigated at 30°C. Attempts were made to evaluate the reactivity of all the reacting species, of sulphur(IV) and selenium(IV) by considering suitable pH ranges. The oxidation of SC₂·H₂O and HSO ₃ ⁻ is proposed to proceed through the formation of a hydrogen-bonded adduct. The reaction with SO ₃ ²⁻ seems to follow a direct outer-sphere route which is well supported by Marcus crossrelation calculation. The oxidation of HSeO _l3 ⁻ is ≈ 10³ times slower than that of H₂SeO₃. The kinetic data indicate that the oxidation of sulphur(IV) by [Ni^IIIL¹)]²⁺ is much more favourable as compared to the corresponding oxidation of selenium(IV).     

Kinetic and thermodynamic studies of cesium(I) sorption on hydrous silica

Summary Batch sorption experiments of cesium, Cs⁺, on SiO₂ ^. xH₂O (silica gel) have been conducted with variable times of equilibration, amounts of silica gel (0.10-1.00 g), cesium concentrations (5.00 ^. 10^-5-2.40 ^. 10^-3M), ionic strengths (0.20-1.40M NaClO₄), pH (2.50-7.70), and temperatures (273-333 K). The diffusion coefficient of Cs⁺ ion was calculated to be (9.19±0.86) ^. 10^-11 m^{2 .} s^-1 under particle diffusion-controlled conditions. The sorption rate was (3.94±0.65) ^. 10^-3 s^-1 at 298 K, pH 7.70±0.05 in 0.20M NaClO₄. The sorption data fits the Freundlich, Langmuir and Dubinin-Radushkevich (D-R) isotherms. Cesium sorption on 0.20 g silica gel decreased with ionic strength from (40.42±0.34)% in 0.20M NaClO₄ to (6.35±0.40)% in 1.40M NaClO₄, at pH_(initial) 8.20±0.05. A gradual decrease in pH with increased ionic strength is consistent with a cation-exchange mechanism. Sorption of Cs⁺ on silica gel decreased with increased temperature, indicating an exothermic enthalpy. The presence of anions such as fluoride, carbonate, phosphate and oxalate in the aqueous medium did not influence the cesium sorption profile.