Characteristics of Adsorbents

The effectiveness of the treatment of toxic and heavy metal-bearing water and wastewater through the process of adsorption is solely dependent upon the physical and chemical characteristics/properties of the adsorbents. The particle size, surface area (external/internal), porosity, pore size distribution, pore volume, pore structure, bulk density including the hydrophobic and hydrophilic behavior, chemical and thermal stability, surface morphology, presence of functional groups on the surface, and surface chemistry are some of the physical and chemical characteristics of the adsorbents that are reported to affect the adsorption type. The nature of the properties of the adsorbents forms the basis of their categorization into physical and chemical characteristics (Worch 2012a; Wang, Shi, Wang, et al. 2020).

Physical Characteristics

The morphological features of the adsorbent constitute the physical characteristics such as surface area, particle size, pore size, and density. The porosity of any solid is the part of the space available in the total volume of the material. Generally, the more porous the material, the more will be its adsorption capacity (Worch 2012a).

The external surface area is supposed to influence the mass transfer rate during the adsorption process. For porous adsorbents, the external mass transfer occurs via the formation of hydrodynamic sheath around the adsorbent, whereas internal mass transfer happens by way of intra-particle diffusion processes. The internal surface area is considered as the most important parameter of any adsorbent as the adsorption capacity of the adsorbents totally depends upon it. Sometimes, it exceeds several fold the external surface area of the porous adsorbents. This is the reason that led research groups worldwide to give emphasis on the synthesis of materials with extremely large internal surface areas.

The materials possessing large internal surface areas are endowed with a plethora of pores of variable shapes and sizes that form the basis of their classification into macropores, mesopores, and micropores based on their sizes. Macropores and mesopores play a significant role in the mass transfer of adsorbate into the internal part of the adsorbent, and the volume possessed by the micropores aids in the determination of size of the internal surface and, in turn, the adsorption capacity of the adsorbent. Higher micropore volume exhibits the larger quantity of the material that will be adsorbed. However, in the case of minute pores and large adsorbate molecules, the size exclusion limits the extent of adsorption.

The particle size is of extreme importance in the case where nanomaterials are used as adsorbents. The unique features that endow them as ideal adsorbents are all size-dependent, particularly large surface area and surface area-to-volume ratio leading to their improved adsorption capacity. The high surface area provides a greater number of active sites for interaction with a diverse class of contaminants. The particles having their sizes in the nanoscale range are highly reactive and possess catalytic potential, enabling them as a better alternative than conventional materials (Worch 2012a; Hua et al. 2012; Ali 2012).

Chemical Characteristics

The chemical composition, existence of diverse functional groups on the surface-active sites of the adsorbent and their interaction with a wide range of contaminants, and surface chemistry constitute the chemical characteristics of the adsorbent.

The interaction between the adsorbate and the adsorbent is largely affected by the surface chemistry of the adsorbent, especially for the case where adsorption of ions occurs onto oxidic surfaces (Worch 2012a). In this perspective, the pHpzc of the adsorbent assists in the comprehension of the influence of pH on the overall adsorption of charged species on their surfaces. The point of zero charge (pHpzc) evaluates the point at which any adsorbent is electrically neutral in nature and is referred to as a region in the pH scale where the surface bears a net zero charge due to the equal sum of positive and negative charges on the surface of the adsorbent. When the pH of the medium is lower than the pHpzc of the adsorbent - the surface becomes positively charged and the medium becomes acidic due to donation of protons more than the hydroxide groups by the acidic water (Roushani et al. 2017). Thus, attraction of negatively charged species becomes feasible toward positive adsorbent surface. On the contrary, at a pH value higher than the pHpzc, the surface bears negative charges elicited by the attraction of cations and vice versa. The point of zero charge (pHpzc) is the characteristic feature of adsorbents. Generally, the electrostatic attraction/repulsion forces strongly affect the adsorption of charged adsorbate molecules on the charged adsorbent surfaces (Worch 2012a; Hua et al. 2012; Ali 2012).

In recent years, considerable effort has been made in the modification of adsorbent surfaces or functionalization of adsorbents in order to overcome their inherent limitations and enhance their low adsorption capacities and selectivity of both organic and inorganic moieties from water. The adsorbent surfaces are being modified with a variety of different compounds to alter their wide range of surface characteristics such as stability, functionality, reactivity, biocompatibility, dispensability, and inertness in intolerable environments (Manyangadze et al. 2020). Surface modification or functionalization has been carried out with a multiplicity of agents; for example, some carbon materials are activated by H2SO4, H,PO4, ZnCl2, H2O2, KOH, etc., and certain ligands are also used, for example, surfactants, polymers, dendrimers, small molecules, and biomolecules. Surface modification also aids in controlling the aggregation of nanoparticles, which is their perennial property, assists in their easy separation when the surface is magnetically functionalized, results in high density of reactive sites, and assures high and rapid adsorption rates. Oftentimes, it is witnessed that the degree of modification/functionalization of adsorbent surface leads to a decrease in specific surface area but eventually results in an increased adsorption capacity (Yang, Wan, et al. 2019; Worch 2012a).

Adsorption

Adsorption is a surface phenomenon defined as the accumulation of any substance from the bulk to the surface of any material in contact. This accumulation results from the surface energy generated by the imbalanced forces of attraction experienced by the molecules of any substance over the material surface (Dqbrowski 2001).

The surface-active materials, i.e. the materials capable of adhering other substance onto their surfaces, are called adsorbents, and the materials that are accumulated on the adsorbent surfaces are referred to as adsorbates. The strength with which the adsorbate molecules are adhered or attached on the surface of adsorbents dictates the nature of adsorption as physical or chemical (Worch 2012a).

Physical Adsorption

Physical adsorption or physisorption is an adsorption process in which the forces operating amid adsorbate and adsorbent are weak intermolecular forces (van der Waals forces) of attraction. This is characterized by the formation of multiple layers of adsorbed molecules over the adsorbent surface via van der Waals forces, electrostatic interaction, hydrogen bonds, weak covalent bonding, or dipole-dipole interaction (Lima et al. 2015).

2.2.3.1.1 Features of Physisorption

The salient features of physisorption are as follows (Grassi et al. 2012):

  • Energetics and kinetics: Due to weak van der Waals force of attraction, physisorption is described by lower enthalpy values (i.e. 20-80 kJ/mol) and is generally an exothermic process. The activation energy is also low and the sorption process is reversible in nature (Piccin et al. 2017).
  • Temperature: Because of the exothermic nature of physisorption, adsorption declines with an increase in temperature.
  • Specificity: The presence of universal van der Waals forces of attraction makes the adsorbent surface indifferent for any adsorbate; i.e. physical adsorption is inexplicit in nature.
  • Surface area of adsorbent: Generally, the surface area of the adsorbent increases the magnitude of physical adsorption. Accordingly, finely divided porous substances, metals, etc., are supposed to be good candidates as adsorbents.

Chemical Adsorption

Chemical adsorption or chemisorption is an adsorption process in which the operational force amid the adsorbate-adsorbent is a valence force similar to that involved in the formation of any chemical compound. Due to the involvement of valence bonds, the adsorbed molecules of the adsorbate get linked to the adsorbent surface, usually reside in certain adsorption sites available on the surface, and eventually lead to the formation of a unilayer (monolayer) of chemisorbed molecules (Lima et al. 2015; Crini and Badot 2008).

2.2.3.2.1 Features of Chemisorption

The salient features of chemisorption are as follows (Grassi et al. 2012):

  • Energetics and kinetics: In chemisorption, bond between the adsorbed molecules of the adsorbate and the adsorbent surface is characterized by higher values of enthalpy (80-450 kJ/mol). The magnitude of energy associated with chemisorption is of the same order as the energy change in a chemical reaction between a solid and a fluid. Therefore, chemisorption may be of exothermic or endothermic nature and the magnitude may vary from very small to very large. The fundamental steps in chemisorption involve activation energy (also called activated adsorption), so chemisorption possesses high activation energy and takes place steadily. The chemisorption process is practically irreversible in nature (Piccin et al. 2017).
  • Temperature: Chemisorption involves higher kinetic energy barrier, so it does not favor lower temperature scale. Thus, resembling any other chemical reaction, chemisorption increases with increasing temperature up to a certain value and then shows downfall.
  • Specificity: Chemical adsorption occurs only when there is any possibility of chemical interaction within the molecules of the adsorbate and the adsorbent surface for the formation of chemical bond. Consequently, chemisorption is highly specific in nature.
  • Surface area of the adsorbent: Similar to physisorption, the extent of chemisorption also escalates with an increase in the surface area of the adsorbent.
 
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