Kinetics of adsorption of metal ions on inorganic materials: A review
Graphical Abstract
The review discusses the kinetics of adsorption of metal ions on inorganic solids based on publications of last ten years. Materials like clays and clay minerals, zeolites, silica gel, soil, activated alumina, inorganic polymer, inorganic oxides, fly ash, etc. have been used as sorbents for As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Se, and Zn. The majority of the interactions have been reported as following either pseudo first order or second order kinetics. Application of Elovich equation, intra-particle diffusion and liquid film diffusion kinetics has also been reviewed. The kinetic rate coefficients indicate the speed at which the metal ions are taken up by the materials.
Second order plots for Cr(VI) adsorbed on natural and modified kaolinite (experimental conditions: adsorbent 2 g L− 1, Cr(VI) 50 mg L− 1, pH 4.6, and temperature 303 K).
Research Highlights
► Kinetics of adsorption of metal ions on inorganic solids done during last ten years is reviewed. ► Clays, zeolites, silica gel, alumina, oxides, fly ash, etc., are considered as sorbents. ► Most interactions are reported as following pseudo first order or second order kinetics. ► Application of Elovich, intra-particle and liquid film diffusion models are also reviewed. ► The rate coefficients for sorption of metal ions on various materials are given and discussed.
Introduction
The rates at which metal ions are transferred from the bulk solution to the adsorbent surface and are accumulated there determine the kinetics of adsorption and hence, the efficiency of the adsorption process. The study of kinetics provides an insight into the possible mechanism of adsorption along with the reaction pathways. The residence time of a solute on the adsorbent surface is important in determining whether the process will go to completion or not and also to estimate the total uptake. These are important parameters in designing an actual treatment plant for removing different contaminants from water. It is often very difficult to arrive at an unambiguous rate law, which requires precise knowledge of all the molecular details of the adsorbate–adsorbent interactions, including the energy requirements and stereochemical considerations and also the elementary steps that lead to the adsorption of the solute following a particular mechanism. The process becomes much more complicated when it involves a porous solid with pore diffusion playing an important role. Evaluation of adsorption rate processes yield valuable information about the interactions and have therefore attracted the interests of almost all involved in experimenting with adsorption on solid surfaces from the liquid phase.
It has been universally recognized that adsorption of a species on a solid surface follows three steps, viz., (i) transport of the adsorbate (ions in case of solutions) from the bulk to the external surface of the adsorbent, (ii) passage through the liquid film attached to the solid surface, and (iii) interactions with the surface atoms of the solid leading to chemisorption (strong adsorbate–adsorbent interactions equivalent to covalent bond formation) or weak adsorption (weak adsorbate–adsorbent interactions, very similar to van der Waals forces), in the latter case, desorption may be the ultimate result. In case of porous solids, after passing through the liquid film attached to the external surface, the adsorbate slowly diffuses into the pores and get trapped (adsorbed). It is easily recognized that any of the above steps may be the slowest step determining the overall rate of the interactions and hence the kinetics of the adsorption process. If the step (i) is the slowest, the adsorption will be a transport-limited process (a physical process) and the actual interactions with the solid surface may not be important in determining the adsorption efficiency of the solid. When the step (ii) is the rate determining slowest step, the physical process of diffusion through the liquid film influences the outcome of the process and the efficiency of the solid as an adsorbent can hardly be improved. Only when the step (iii) is the slowest, the adsorption is controlled by a chemical process and the efficiency of the adsorbent can be influenced by suitably controlling the interactions. Usually, the step (i) is the rate-limiting step in systems which are characterized by poor mixing, dilute concentration of adsorbate, small particle size of the adsorbent, etc. In contrast, when dealing with a porous adsorbent, the pore diffusion becomes important when the adsorbate is present in higher concentration, the adsorbent is made of large particles and good mixing is ensured [1], [2], [3].
An adsorbent material must have high internal volume accessible to the components being removed from the solvent. Surface area, particularly the internal surface area, pore size distribution and the nature of the pores markedly influence the type of adsorption processes. It is also important that the adsorbent has good mechanical properties such as strength and resistance to destruction and the adsorbent particles are of appropriate size and form. The chemical properties of the adsorbent, namely, degree of ionization at the surface, types of functional groups present, and the degree to which these properties change in contact with the solution are important considerations in determining the adsorption capacity of a solid. The presence of active functional groups on the adsorbent surface allows chemical interactions that usually produce effects different from and less reversible than physical adsorption.
The range of materials chosen as sorbents for treating water contaminated with heavy metals has been truly limitless. While the initial and the continuing trend has been the use of inorganic materials like the clays and the oxides for the purpose, many workers have now turned their attention towards naturally available biomass as the alternative. The emphasis in this review is to consider the rate processes on inorganic materials leading to adsorption of the toxic metal cations and anions and while details have been discussed later, examples of inorganic materials include hydrous ferric oxide [4], simple iron oxide [5], modified Fe3O4 [6], modified layered double hydroxide [7], modified SBA-15 [8], and bagasse fly ash [9] from a few recent works.
A careful search of the leading literature resources yields an equally impressive number of bio-materials being experimented as heavy metal scavengers. A few interesting examples have been the use of peat for Cu(II) [10] and Pb(II) [11], cladophora crispate for Cu(II) [12], aeromonas caviae for Cd(II) [13], green alga spirogyra for Cu(II) [14] and Pb(II) [15], orange waste for Cd(II) [16], cyanobacterium for Cr(VI) [17], alligator weed for Cr(VI) [18], coffee waste for Cr(VI) [19], oedogonium species for Cd(II) [20], Cr(VI) [21] and Ni(II) [22], rice bran for Zn(II) [23], crab shell for As(V) [24], etc.
Several reviews have appeared on water treatment through sorption. Use of chitin and chitosan to remove metal ions from wastewater has been reviewed [25] with particular emphasis on equilibrium studies of sorption capacities and kinetics. Applications of second-order kinetic models to large varieties of adsorption systems were reviewed by Ho [26]. The feasibility of using kaolinite and montmorillonite clay minerals as adsorbents for removal of toxic heavy metals has been reviewed [27]. A good number of works were reported where the modifications of these natural clays were done to carry the adsorption of metals from aqueous solutions. The equilibrium and kinetic studies of heavy metal adsorption on biosorbents, published between 1999 and early 2008, has also been reviewed [28] in which the pseudo-first and -second order equations were considered as the most notable models for describing kinetics. Recently, Gupta et al. [29] have comprehensively reviewed the use of low-cost adsorbents in wastewater treatment. The adsorbents reviewed include alumina, silica gel, zeolite, resin, activated carbon, natural materials like wood, peat, coal, lignite, etc.; industrial/agricultural/domestic wastes or byproducts such as slag, sludge, fly ash, bagasse fly ash, red mud, etc.; and various synthesized products.
The present work gives an overview of the approaches followed by different groups of workers since 2000 in trying to understand the rate processes for adsorption of heavy metals on inorganic adsorbents. The inorganic materials considered were mainly clays and clay minerals, zeolites, silica gel, soil, river sediment, activated alumina, inorganic polymer, red mud, inorganic oxides (viz., hydrous zirconium oxide, titanium oxide, stannic oxide, ferric oxide, etc.), fly ash, etc. Many authors have chemically modified these substances and used the modified materials successfully as adsorbents. To maintain largely ‘inorganic’ nature of the adsorbents, any material of plant or animal origin is excluded from this review.
Section snippets
Kinetics of adsorption: theoretical basis
Kinetics is the study of the rates of chemical processes to understand the factors that influence the rates. Study of chemical kinetics includes careful monitoring of the experimental conditions which influence the speed of a chemical reaction and hence, help attain equilibrium in a reasonable length of time. Such studies yield information about the possible mechanism of adsorption and the different transition states on the way to the formation of the final adsorbate–adsorbent complex and help
Experimental insight into kinetics of adsorption
Adsorption mechanisms depend on the characteristics of the adsorbate and adsorbent, adsorbate–adsorbent interactions and the system conditions like pH, temperature, etc. The interactions may also involve the solvent molecules and the attractive forces may be of different nature. Such forces usually act in concert, but one particular type may be more dominant in a particular situation. The differential distribution of the solute molecules or ions between the liquid and the solid phases results
Conclusions
From the large number of recent works reviewed here, it is observed that the mechanism and kinetics of adsorption of metal cations and anions on inorganic adsorbents depend on the chemical nature of the materials and the experimental conditions, viz., ion concentration, adsorbent amount, pH and temperature of the medium. The pseudo first order model has been almost universally tested, but the validity has remained doubtful in many cases because of large discrepancy between the experimental and
Acknowledgements
The authors are grateful to the University Grants Commission, India for sponsoring a substantial part of this work. The authors are also thankful to the reviewers for very useful comments and suggestions.
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