Susmita Sen Gupta

Kinetics of adsorption of metal ions on inorganic materials: A review

It is necessary to establish the rate law of adsorbate-adsorbent interactions to understand the mechanism by which the solute accumulates on the surface of a solid and gets adsorbed to the surface. A number of theoretical models and equations are available for the purpose and the best fit of the experimental data to any of these models is interpreted as giving the appropriate kinetics for the adsorption process. There is a spate of publications during the last few years on adsorption of various metals and other contaminants on conventional and non-conventional adsorbents, and many have tried to work out the kinetics. This has resulted from the wide interest generated on using adsorption as a practical method for treating contaminated water. In this review, an attempt has been made to discuss the kinetics of adsorption of metal ions on inorganic solids on the basis of published reports. A variety of materials like clays and clay minerals, zeolites, silica gel, soil, activated alumina, inorganic polymer, inorganic oxides, fly ash, etc. have been considered as the adsorbents and cations and anions of As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Se, and Zn as adsorbate have been covered in this review. The majority of the interactions have been divided into either pseudo first order or second order kinetics on the basis of the best fit obtained by various groups of workers, although second order kinetics has been found to be the most predominant one. The discussion under each category is carried out with respect to each type of metal ion separately. Application of models as given by the Elovich equation, intra-particle diffusion and liquid film diffusion has also been shown by many authors and these have also been reviewed. The time taken for attaining equilibrium in each case has been considered as a significant parameter and is discussed almost in all the cases. The values of the kinetic rate coefficients indicate the speed at which the metal ions adsorb on the materials and these are discussed in all available cases. The review aims to give a comprehensive picture on the studies of kinetics of adsorption during the last few years.

Adsorption of heavy metals on kaolinite and montmorillonite: a review

The process of adsorption is considered to be one of the best water treatment technologies around the world. Different heavy metals, due to their toxic and hazardous nature, are possibly the most widespread groundwater contaminants imposing a serious threat to human health. In this review, an attempt has been made to discuss the use of two common clay materials, namely kaolinite and montmorillonite, along with their modified forms for heavy metal removal on the basis of published reports (2008 onwards). The modifications of clays have been attempted by the process of pillaring, intercalation, acid/base activation, functionalization, etc. The adsorption of toxic metals, viz., As, Cd, Cr, Co, Cu, Fe, Pb, Mn, Ni, Zn, etc., has been studied predominantly. Montmorillonite and its modified forms have much higher metal adsorption capacity compared to that of kaolinite as well as modified-kaolinite. The modification often boosted the adsorption capacities of the clays, however, reverse trends are also reported in some cases.

Adsorption of metal ions by clays and inorganic solids

This review deals with adsorption of metal ions, particularly those considered as hazardous, on clays and some inorganic solids and covers the publication years 2000–2013 describing and quantifying the use of isotherms to obtain the adsorption capacities of the solids. The inorganic solids in the review include clays and clay minerals, their modified forms (obtained by treatment with acids and alkalis, organic functionalization, etc.), zeolites, silica gel, soil, river sediment, activated alumina, inorganic polymers, red mud, inorganic oxides, fly ash, etc. The use of two parameter and three parameter linear isotherms are only discussed with a view to obtain quantitative description of adsorptive accumulation and separation of metal ions from aqueous solution on the solids. The extensively used isotherms are those of Freundlich, Langmuir, Dubinin–Radushkevich and Redlich–Peterson. How these isotherms are being used to obtain the adsorption capacities along with their interpretations form the bulk of the review. The review is divided into sections, each describing the use of a particular isotherm. The metal cations receiving immense importance in adsorption studies are As(III)/As(V), Cd(II), Cr(III)/Cr(VI), Cu(II), Co(II), Pb(II), Ni(II), Hg(II), and Zn(II), and the review covers both the cationic and the anionic metal ions. A few other cations such as Mn(II), Fe(III), Se(V), which have not received much attention, are also covered.

Adsorption of Crystal violet on raw and acid-treated montmorillonite, K10, in aqueous suspension.

Crystal violet is used as a dye in cotton and silk textiles, paints and printing ink. The dye is hazardous and exposure to it may cause permanent injury to the cornea and conjunctiva including permanent blindness, and in severe cases, may lead to respiratory and kidney failure. The present work describes removal of Crystal violet from aqueous solution by adsorption on raw and acid-treated montmorillonite, K10. The clay mineral was treated with 0.25 and 0.50 M sulfuric acid and the resulting materials were characterized by XRD, zeta potential, SEM, FTIR, cation exchange capacity, BET surface area and pore volume measurements. The influences of pH, interaction time, adsorbent amount, and temperature on adsorption were monitored and explained on the basis of physico-chemical characteristics of the materials. Basic pH generally favors adsorption but considerable removal was possible even under neutral conditions. Adsorption was very rapid and equilibrium could be attained in 180 min. The kinetics conformed to second order model. Langmuir monolayer adsorption capacity of raw montmorillonite K10 was 370.37 mg g(-1) whereas 0.25 M and 0.50 M acid treated montmorillonite K10 had capacities of 384.62 and 400.0 mg g(-1) respectively at 303 K. Adsorption was exothermic and decreased in the temperature range of 293-323 K. Thermodynamically, the process was spontaneous with Gibbs energy decreasing with rise in temperature. The results suggest that montmorillonite K10 and its acid treated forms would be suitable for removing Crystal violet from aqueous solution.

Adsorption of Co(II) from Aqueous Medium on Natural and Acid Activated Kaolinite and Montmorillonite

Abstract Hazardous metal cations enter water through the natural geochemical route or from the industrial wastes. Their separation and removal can be achieved by adsorptive accumulation of the cations on a suitable adsorbent. In the present work, toxic Co(II) ions are removed from water by accumulating them on the surface of clay minerals. Clay adsorbents are obtained from kaolinite, montmorillonite, and their acid activated forms, and are characterized with the measurement of XRD patterns, specific surface area, and cation exchange capacity. The adsorption experiments are carried out in a batch process in environments of different pH, initial Co(II) concentration, amount of clay, interaction time, and temperature. Adsorption of Co(II) on the clays increases continuously from pH 1.0 to 8.0 after which adsorption could not be carried out due to the decreasing solubility of Co(II). Under appropriate conditions, the adsorption of Co(II) is very fast at low coverage approaching equilibrium within 240 min and the interactions are best described by second order kinetics. Langmuir monolayer capacity has been computed in the range of 11.2 to 29.7 mg/g and Co(II) accumulation has the order of acid‐activated montmorillonite>montmorillonite>acid activated kaolinite>kaolinite. Adsorption of Co(II) on kaolinite and acid‐activated kaolinite is endothermic driven by entropy increase but the same process follows exothermically on montmorillonite and acid‐activated montmorillonite supported by entropy decrease. In both cases, spontaneous adsorptive accumulation is ensured by favorable Gibbs energy decrease. It is found that acid activation enhances the adsorption capacity of kaolinite and montmorillonite.

Uptake of Ni(II) Ions from Aqueous Solution by Kaolinite and Montmorillonite: Influence of Acid Activation of the Clays

Abstract Kaolinite and montmorillonite were treated with 0.25 M H2SO4 and the acid activated clays along with the parent clays were tested for their uptake capacity for Ni(II) ions from aqueous solution. The batch adsorption experiments were conducted under a set of variables (concentration of Ni(II) ion, amount of clay, pH, time and temperature of interaction). Increasing pH favored Ni(II) uptake till the ions were precipitated as the insoluble hydroxides at pH > 8.0. The uptake was rapid up to 40 min and equilibrium was obtained within 180 min. The kinetics of the process was evaluated by subjecting the results to a number of models like the pseudo-first order, second order, Elovich equation, liquid film diffusion, and intra-particle diffusion and it was found that the data more closely resembled a second order process. The experimental data conformed to both Langmuir and Freundlich isotherms showing that the interactions were mostly chemical in nature. The clays had reasonable monolayer adsorption capacity of 10.4, 11.9, 28.4, and 29.5 mg g−1 for kaolinite, acid activated kaolinite, montmorillonite, and acid-activated montmorillonite respectively. Montmorillonite had much better adsorption capacity than kaolinite and the acid activation boosted the adsorption capacity of both kaolinite and montmorillonite. The interactions were exothermic in nature, accompanied by decrease in both entropy and Gibbs energy. The results have established good potentiality for kaolinite, montmorillonite and their acid-activated forms to take up and separate Ni(II) from aqueous medium through adsorption-mediated immobilization.

Kinetics, equilibrium isotherms and thermodynamics of adsorption of Congo red onto natural and acid-treated kaolinite and montmorillonite

AbstractNaturally occurring kaolinite and montmorillonite were treated with 0.25 and 0.50 M H2SO4 and the modified clays along with the parent clays were used as adsorbents for the dye, Congo red in water. Adsorption was carried out in batch process with pH, dye concentration, amount of clay mineral, interaction time and temperature as the experimental variables. Adsorption was fast attaining equilibrium within 120 min and conforming to second-order kinetics. The Langmuir monolayer capacity for kaolinite and 0.25 M and 0.50 M acid-treated kaolinite varied from 0.024 to 0.033 mmol g−1, 0.029 to 0.035 mmol g−1, 0.030 to 0.036 mmol g−1, respectively, in the temperature range of 293–323 K. The corresponding values for the montmorillonites were 0.223–0.243 mmol g−1, 0.227–0.249 mmol g−1 and 0.231–0.256 mmol g−1. Thermodynamic studies indicated that the dye molecules were held to the clay mineral surface by weak bonds and the whole process was endothermic near ambient temperature. The acid-treated clay minerals...