DESCRIPTION OF ADSORBTION
Molecules from gas or liquid phase will be attached in a physical way to a surface, in this case the surface is from the Granular Active Carbon (GAC). The adsorption process takes place in three steps;
- Macro transport: The movement of organic material through the macro-pore system of the active carbon (macro-pore >50nm)
- Micro transport: The movement of organic material through the meso-pore and micro-pore system of the active carbon (micro-pore <2nm; meso-pore 2-50nm)
- Sorption: The physical attachment of organic material on the surface of active carbon in the meso-pores and micro-pores of the active carbon
The activity level of adsorption is based on the concentration of substance in the water, the temperature and the polarity of the substance. A polar substance (a substance which is good soluble in water) cannot or is badly removed by granular active carbon, a non-polar substance can be removed totally by granular active carbon. Every kind of carbon has its own adsorption and in the water treatment business this isotherm is definite by the function of Freundlich.
The function of Freundlich:
x/m = adsorbed substance per gram active carbon
Ce = concentration difference (between before and after)
Kf, n = specific constants
Factors that influence the performance of granular active carbon in water are:
- The type of compound to be removed. Compounds with high molecular weight and low solubility are better absorbed.
- The concentration of the compound to be removed. The higher the concentration, the higher the carbon consumption.
- Presence of other organic compounds which will compete for the available adsorption sites.
- The pH of the waste stream. For example, acidic compounds are better removed at lower pH.
1. - According to this we can classify some chemicals by their probability of being efficiently adsorbed by active carbon in water:
2.- Chemicals with high probability of being adsorbed by GAC:
Methyl Isobutyl Ketone
3.- Chemicals with moderate probability of being adsorbed by active carbon:
Methyl ethyl ketone
4.- Chemicals for which adsorption with active carbon is unlikely to be effective. However it may be viable in certain cases such as for low flow or concentrations:
Factors that influence the performance of active carbon in air:
- Type of compound to be removed: In general compounds with a high molecular weight, lower vapor pressure/higher boiling point and high refractive index are better adsorbed.
- Concentration: The higher the concentration, the higher the carbon consumption.
- Temperature: The lower the temperature, the better the adsorption capacity.
- Pressure: The higher the pressure, the better the adsorption capacity.
- Humidity: The lower the humidity, the better the adsorption capacity.