5 Adsorption and adsorption based sampling

Dr. Geeta

Introduction

Adsorption in wider sense defined as either fluid-fluid interface (liquid-gas, liquid-liquid) or a solid–fluid interface (solid-gas, solid–liquid). It is the increase in concentration of target compound on the porous solid surface, selected as adsorbent. Adsorption may also be defined as either partitioning of chemical species between bulk phase and interface or accumulation of substances near the interface.

When a solid surface is exposed to liquid or gas, the molecules from solution or gas phase concentrate on the surface, this phenomenon is called adsorption. For example activated carbon is the most commonly used adsorbent in the treatment of waste water from various industries. Activated carbon used in adsorption process is extremely porous whose internal surface area is very large which attract and hold organic molecule, certain metals and organic molecule. Adsorption occurs because of low solubility of contaminant in the waste and greater affinity toward carbon.

12.1 Application of Adsorption: Adsorption technique is used in separation, cleaning of liquid and gaseous effluents from industry, medicine and environmental protection. Some others applications include air separation by fractionated vacuum swig adsorption, food discoloring in sugar production, purification of pharmaceuticals products, separation of different types of hydrocarbons from oil mixtures by selective surface flow adsorbent membranes, used in effluent gas monitoring, gas /solid chromatography, drying of gases (for example control of CO2 and volatile organic compounds in instrumentation),used in catalytic processes in chemical reaction, treatment of industrial off-gases, decrease of NOx emission from vehicles exhaust, removal of odorous substances from landfill gas, recovery of solvent vapors and volatile organic compounds.

Adsorption has found numerous applications in the field of petrochemical, chemical, biochemical and in environmental industries due to the separation and purification of mixture of fluids. Adsorption is a simple method for the removal of pollutant (up to 99%). Adsorption is helpful in removing impurity and minor amount of pollutant as the adsorbent have limited adsorption capacity. Adsorption is effective when a contaminants ranges from 1.0 ppb to 1000 ppm.

The removal of low concentration of gases and vapors from the exhaust stream on the porous solid material is a practical example of adsorption. Proper selection of suitable adsorbent and contact time between solid and vapors make the adsorbent successful with high removal efficiencies.

12.2 Mechanism of adsorption: Adsorption occurs in three steps. First the contaminant diffuse from the gas stream to the surface of the adsorbent, then the molecule of contaminant migrate from external surface to the macro pores and micro pores and finally the molecules adhere to the pore surface (physical bonding between molecules and adsorbent surface) as shown in figure 1.

12.3 Adsorption forces: The process of adsorption is classified into two categories depending upon attraction:-

1. Physical or physiosorption: When the force of attraction between adsobate and adsorbent are weak Vander Waal force then it is called physiosorption. These are weak bonding which does not involve a sharing and transfer of electron. It is an exothermic process where heat of adsorption is higher than the heat of vaporization of the adsorbed materials. Multiple molecular layers can be form in this process. These forces are present in all state of matter i.e. gas, liquid and solid.

     Table 1.Comparsion between Physical and chemical adsorption

2. Chemical or chemisorptions: When of force of attraction between adsorbate and adsorbent are chemical bond then it is called chemisorptions. It is not easily reversible. The oxidation of SO2 to SO3 on activated carbon is an example of chemisorptions. It is limited to monolayer formation (See above Table 1.)

The rate of adsorption and desorption is controlled by diffusion through the network of pores.

12.4 Adsorption Isotherms: The capacity of an adsorbent to adsorb a particular adsorbate is directly proportional to the molecular weight and inversely proportional to the vapor pressure of adsorbate. The best mathematical models for describing adsorption equilibrium are Freundlich and Langmuir isotherm.

Freundlich adsorption isotherm: Freundlich in 1909 give an empirical equation known as freundlich adsorption isotherms which representing the isothermal variation of adsorption of quantity of gas adsorbed on unit mass of solid adsorbent with pressure.

Where x is the amount of adsorbate, m is the weight of the adsorbent, P is pressure, k and n are constant and their value are depends upon adsorbate and adsorbent at a particular temperature.

At high pressure 1/n=0 so adsorption is independent of pressure but at high pressure adsorption is depend upon pressure. So this isotherm is failed at high pressure.

Langmuir isotherm: Langmuir in 1916 proposed an isotherm which is based upon assumption that dynamic equilibrium is exist between adsorbed gaseous molecule and free gaseous molecules

Where A(g) is unabsorbed gaseous molecules(s) is unoccupied metal surface and AB is adsorbed gaseous molecule.

Langmuir then derived an equation which explained the relationship between the number of active site of the surface undergoing adsorption and pressure and the equation is known as Langmuir equation.

Where ɵ is number of site of the surface, P is pressure, K is equilibrium constant. The main limitation of this equation is that it is valid only at low pressure.

12.5 Commonly used Adsorbent: Adsorbents are natural or synthetic material of amorphous or microcrystalline structure. The most commonly used adsorbents are activated carbon, molecular sieves, zeolites, alumina silica gel, ion exchange resins, polymeric adsorbents and activated alumina. Activated carbon is most commonly used adsorbent.

a) Activated carbon: The term activated means the adsorbent material that internal and external surface area is increased by special treatment processes. Any carbonaceous material can be converted to activated carbon such as coconut shell, wood coal, lignin, petroleum coke and bones. Most commonly used carbon source in industry is bituminous coal. The manufacturing of activated carbon is done by first dehydrating and carbonizing (heating in the absence of oxygen) the carbonaceous raw material. The carbon tetrachloride number and iodine number are used to measure the adsorption capacity.

b) Activated Alumina: It is manufactured by heating alumina trihyrdrate at 400oC in airstream. It is commonly used in drying of gas stream, support material in catalytic reactions and in solvent recovery.

c) Silica Gel: It is prepared by treating sodium silicate with strong acid, followed by washing to remove excess electrolyte. The washed solid is then dried and activated by heating. There are three main type of silica gel range in density from 0.15 to 0.70g/L. It is used to remove moisture from exhaust streams. Silica gels are ineffective at temperature above 260oC.

d) Zeolites (molecular sieves): They have a crystalline structure and uniform diameter pores. It is used to remove moisture from exhaust streams, remove nitrogen oxides compounds from air pollution and in separation of hydrocarbon in refining process.

e) Synthetic polymers: Polymeric adsorbent are made by cross linking long chain polymer having variety of functional groups. They have very high adsorption capacity for selective organic compounds like ketones, aldehyes and reactive monomers. They can be rapidly desorbed in hot air, steam, microwaves and hot nitrogen.

12.6 Adsorption based sampling: Adsorption is used for monitoring the air quality by means of collecting and concentrating the contaminants for analysis. There are large number of hazards chemical vapors and gases present in the environment. A special type of sorbents is required for sampling of air to check the air quality. In the 1970’s , the united states National Institute for Occupational Safety and Health(NIOSH) developed a method of sampling by sorption and concentration. Air is drawn through a sample tube containing a sorbent material which trap the contaminant for later desorption and analysis. This method has advantage that lightweight pumps and sample tube are cheap and user acceptable.

12.6.1 Grab sampling: It is collection of small but a representative air sample using an evaporated flask or an inflatable bag. Grab sampling is usually be taken at a particular location and time within a few seconds up to a minutes using an adsorbent cartridge. It is suitable in place where electricity and laboratory facilities are lacking.

12.6.2 Extractive isokinetic sampling: To make the gas sample representative of whole stream than it should have density and concentration equivalent to the average value of main stream. It is done for monitoring particles and particles bound materials and aerosols must be on an isokinetic basis. It means that the removal of sample gas from the flow of waste gas must be at the same speed as the flow of waste gas at the measurement point to avoid the sedimentation when taking the sample. So streamlined nozzle and probe should be chosen having small size (between 3 and 6mm).

Extractive sampling for gas measurements: It can be taken in the form of grid measurement or spot measurements.

12.6.3 Accumulative sampling (Absorption): Inorganic gaseous SO2, SO3, Chlorine and fluorine compounds and basic nitrogen compounds are collected by means of accumulation in liquid phase (absorption).The sample gas is extracted from the waste gas using suction pipe. The suction pipe must be made up material which does not interact with sample gas.

After sampling the absorption solution are analyzed in the laboratory using different method such as titration, ion chromatography, potentiometric titration, analysis with ions and photometric determination. The concentration of pollutant can be measured by gravimetric method in this measure the weight of filter in case of high volume sampler before and after a known volume of air has been sampled. Concentration of particulate pollutant is measured in µg/m3.

Other analytical technique used are colrimetry, conductometry, chemiluminescence, flame ionization, gas chromatography, IR etc.

12.6.4 Gas adsorption equipment

There are two types of adsorption equipment are used

a) Thin bed absorber: It consists of thin layer or bed of adsorbent like activated alumina. When gaseous streams enter it rapidly adsorbed on the soild.It is used when low quantities of pollutants are measured.

b) Deep bed adsorber: It has depth of 2.5-4 cm and activated charcoal is used as adsorbent. It is used when gas streams is heavily polluted.

Different types of sorbent used in air sampling: Chemical reagent impregnated onto solid supports will collect 100% of the passing contaminants until saturation. The most commonly used sorbents is active carbon derived from variety of source material like wood, petroleum, nutshell, coal, silica gel, alumina, carbon molecular sieves, zeolite and porous polymer like chromosorb, porapak, amberlite and tenax. A single sorbent is not be able to collect all the compounds so there is a need of characterization and evaluation of new sorbents.

12.7 Method of selection of sorbent: There are mainly three parameter on the basis of which sorbent quality has been checked.

1. Sorbent capacity: Capacity of sorbent to collect the measurable quantity of contaminant. It should collect reasonable volume of air which is at least one liter.

2. Desorption efficiency: It is the ability of sorbent to tightly hold the adsorbed molecules during sampling and release them during analysis.

3. Sample storage stability: Analytical facility is not available at the site of collection. Sample has to undergo to another place by transport. Many sorbents surface catalyzed the reaction on the surface such as breakdown, polymerization, chemisorptions and other chemical reaction with the sample result in loss of sample.

The selection of good sorbent is done by primary screening on these three factors. Sampling of polar organic compound like ketones can be done on anasorb 747 but not on coconut charcoal. The most common gaseous air pollutants are SOx, NOx, O3, H2S, HCL, hydrocarbon and others vapors. There are numerous methods available for their collection. The sampling method based upon adsorption techniques are following:

Adsorption on solid: Hydrocarbons and other higher boiling pollutant are best adsorbed on the surface of solids such as activated carbons, activated alumina and silica gel. The rate of adsorption is depends upon the surface area, concentration of materials to be adsorbed, temperature and pressure. Low temperature and high concentration of materials favors the rate of adsorption. The sample air is passed through the packed column containing fine solid adsorbents. The column is maintained at below room temperature of in dry ice bath for better result.

Adsorption of NOx: Mixture of nitrogen oxides contain NO, which is first oxidized to NO2 and then adsorbed to suitable adsorbents such as silica gel, activated carbon. The adsorbent is the heated to remove NO2 and reused in the manufacturing of nitric acid.

a) Adsorption on silica gel: Dry silica gels have no adsorption capacity for NO. It helps to catalyze the oxidation of NO to NO2. Moist silica gel has no effect on oxidation. Silica gel has high capacity to absorb moisture so care should be taken while selecting the type of silica gel.

b) Adsorption on activated carbon: It adsorbs NOx efficiently only when fresh bed is used.Repated regeneration reduced the adsorption capacity of activated carbon.

c) Adsorption on zeolite: It can remove NOx up to 200ppm.The zeolite bed can easily be regenerated by hot air or steam.

These adsorbents are used for Sox and other pollutant also. The operating cost is less when compared with catalytic reduction process.

12.8 The two commonly used ambient air sampler are:

Dust–fall sampler: Pollutant are generally produce from natural and anthropogenic source having smaller size (below 10 micron) and larger size (above than 10 micron) paricles.The larger size particle are collected in outdoor environment by dust fall sampler It is made up of polyethylene, glass or stainless steel. It is constructed from a plastic material of 22 to 33 cm high and 10 to 15 cm in diameter at the base with a slight inward taper from top to bottom. It can be placed 30m part from the industry when there is study on the effect of industry. In the large sampling area, it is advisable to employ one dust fall for every 15 square kilometers.

High volume sampler: The pollutants containing smaller size particles are collected by high volume sampler. It consists of a shelter and filter assembly. The shelter allows an unrestricted access of ambient air from all direction using a suction blower of known capacity. The air is passed through the glass fiber filter at a flow rate of 1 to 1.7 m3/minute. It is an adequate method in determination of total suspended particles present in the ambient air. The particles size more than 10µ size is separated out from the air before going to the filter.

Adsorption based samplers: POCIS (Adsorbebts+PES(Polethersulfone) membrane) and Chemcatche (Empore disk + PES membrane).

Summary:

• Adsorption is a separation process where adsorbate is concentrated on the porous surface of the adsorbents.
• The adsorption may be physiosorption or chemisorptions depending upon the type of forces exist.
• Adsorption method is used in the monitoring air quality by collecting and concentrating contaminants for analysis.
• There are large number of sorbents which are being used in sampling the air for hazardous gases (NOx, SOx etc) and vapors.
• Most commonly used adsorbents are activated carbon, silica gel and zeolite.Sampling can also be done by ambient air sampler such as dust fall and high volume sampler.

    Glossary

Adsorption– to gather (a gas, liquid, or dissolved substance) on a surface in a condensed layer:

Ambient Air– The air occurring at a particular time and place outside of structures. Often used interchangeably with “outdoor air.”

Carbonaceous material-Any solid material (mixture or compound) other than an inorganic carbonate which contains carbon or carbon containing compounds such as coke or wood.

Exothermic– noting or pertaining to a chemical change that is accompanied by a liberation of heat

Partitioning -distribution of a solute, S, between two immiscible solvents (such as aqueous and organic phases) is an equilibrium condition that is described by the following equation:

Saq <=> Sorg

The equilibrium constant for this equilibrium condition is:

K = [Sorg] / [Saq]

where [Sorg] and [Saq] are the solute concentrations in the organic and aqueous phases, respectively, and the equilibrium constant, K, is called the partition or distribution coefficient.

Vander Waals force– a weak short range attractive force between atoms or molecules caused by their dipole moments

Sample-A portion of the material which represents the essential characteristics of the bulk.

References

• Habuda-stanic M, Ravancic ME and Flanagan A(2014) A review on adsorption of fluoride from aqueous solution, Materials : 6317-6366
• Martin Harper, Methods of characterizing sorbents for air sampling purposes. Fundamental of adsorption. Proc. IVth int. conf. on fundamental of adsorption Kyoto, 1992.
• Ralph T Yang, Adsorbents: Fundamentals and application, John Wiley and sons, 2003.