Relación Escuela-Familia: elementos claves

There are many factors affecting adsorption and the effectiveness of the adsorption process depends on them. They include solubility of solute, degree of ionization, contact time, particle size, pH, concentration, dosage, etc

i. Solubility of solute (adsorbate) in liquid (wastewater):

Substances slightly soluble in water will be easily removed from water than substances with high solubility. Also, non-polar substances will be easily removed than polar substances since the latter have greater affinity for water.

ii. Degree of Ionization of the adsorbate molecule:

More highly ionized molecules are adsorbed to smaller degree than neutral molecules.

This is because it takes greater amount of energy to separate the ions from their molecules

 Contact time:

This is the adsorption time. That is, the duration the adsorbate and the adsorbent are mixed together. Sufficient contact time is required to reach adsorption equilibrium and to maximize adsorption efficiency. It was found out that as the contact time between the adsorbent and the adsorbate increases, the percentage removal of the adsorbate increases and also its residual concentration decreases. These happen until equilibrium adsorption is reached where further increase in time no longer has an effect on the percentage removal (Ehssan and Yehia, 2012).

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iii. Particle size:

The size of the adsorbent particle also affects the adsorption capacity as it relates to the surface area. The smaller the particle size, the greater the surface area and this will result to increase in the adsorption capacity. Smaller particle sizes reduce internal diffusion and mass transfer limitation to the penetration of the adsorbate inside the adsorbent (i.e., equilibrium is more easily achieved and nearly full adsorption capability can be attained).

However, wastewater drop across columns packed with powdered material is too high to be used in packed beds. Large surface area is necessary to facilitate the adsorption process by providing adsorption sites on the appropriate channels to transport the adsorbate. In an ideal adsorption situation--where all other conditions (such as pore size, surface chemistry and adsorbent-adsorbate interactions) are optimal for contaminant removal- the surface area will serve as the limiting factor for the adsorption process. The smaller the particle size of the adsorbent for a given mass, the more surface area is available and as a consequence the greater the number of binding sites available, hence the greater the adsorption capacity (Yuh-Shan et al, 2001).

iv. pH

The initial pH of the adsorbate solution significantly affects the adsorption efficiency.

The pH of the solution can be reduced by adding drops of an acid such as HCl, H₂SO₄ or it can be increased by adding drops of an alkali such as NaOH. The effect of pH affects the surface charge of the adsorbent, degree of ionization and speciation of the adsorbate species (Afshin et al, 2012). Most previous works showed that increase in the pH (from 2 upwards) of the adsorbate solution increases the adsorption efficiency until a pH of about 5-7 where it remains constant and in some cases, will start to decrease again. This may be because, at very low pH, the negative charges on the adsorbent will be occupied by H⁺ ions which will inhibit the approach of positively charged adsorbate ions; but as the pH is increased, the adsorbent surface becomes more negatively charged thereby supporting more adsorbate ion adsorption by electrostatic attraction due to columbic forces. At

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higher pH of beyond 7, there is the formation of anionic hydroxides complexes which decreases the concentration of the adsorbate ions, thereby resulting in decrease of the adsorption capacities (Njoku et al, 2011; Djebbar et al, 2012; Afshin et al, 2012).

 Initial concentration:

This is the initial amount of the adsorbate in the solution and it has an effect on the amount of the adsorbate removed from the solution. An increase in the initial adsorbate concentration increases the adsorption capacity but not necessarily the adsorption efficiency. The increase is because of the increase in the concentration gradient which acts as a driving force for the adsorption process. But at high concentration, the active sites on the adsorbent become easily saturated and subsequent increase in the concentration does not affect the adsorption capacity. This is because the available sites of adsorption have become fewer (Njoku et al, 2011; Ehssan and Yehia, 2012).

v. Adsorbent dosage:

The amount or mass of the adsorbent has an effect on the adsorption capacity. This also relates to the adsorbent (solid)/adsorbate (liquid) ratio. Results show that as the amount of the adsorbent increases, the removal efficiency increases up to the optimum dosage beyond which the removal does not increase again. This may be because larger amounts of adsorbent mean more availability of large surface area or a larger number of adsorption sites, and therefore higher capacity for adsorption (Ehssan and Yehia, 2012;

Omar, 2012; Afshin et al, 2012).

vi. Temperature:

From previous works, temperature has some effect on the adsorption capacity.

Adsorption is normally carried out at room temperature or even below it. A low temperature favours the adsorption capacity while temperature will reduce the adsorption capacity. This is because an elevation temperature increases the escaping tendency of the adsorbate from the surface (Yuh-Shan et al, 2001).

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vii. Solvent:

The solvent of the adsorbate has an important effect since it competes with the adsorbent surface in attracting the solute. Thus, adsorption of an organic solute out of an organic solvent is much less than its adsorption out of an aqueous solution. But in most cases of interest, the solvent would be water (Cooney, 1998).

viii. Surface area of adsorbent:

This is one of the main factors that determine the efficiency of the adsorption process.

Larger surface area implies a greater adsorption capacity while lower surface area will imply a lower adsorption capacity.

ix. Number of carbon atoms.

For substances in the same homologous series a large number of carbon atoms is generally associated with a lower polarity and hence a greater potential for being adsorbed (e.g., the degree of adsorption in gases in the sequence formic-acetic- propronic-butyric acid).

x. Size of molecules with respect to size of the pores:

Larger molecules may be too large to enter small pores. This may reduce adsorption independently of other causes.