Dr. MIKE MORRIS

University College Cork

PASSIVE SORBENTS FOR ORGANIC ACID REMOVAL

ADDITIONAL NOTES

Pollutants can be extracted by catalytic decomposition or sorption (adsorption or absorption). Catalytic combustion does not involve burning, it is simple and inexpensive. For example, formaldehyde is known to decompose with lanthanide oxides at 200 - 300 °C.

ADSORBENTS
Sorbents for adsorption are inexpensive, have a high surface area, can normally be regenerated, and require non-toxic, inexpensive housing. However, it should be noted that there is no point in using a bed of sorbent as the kinetic energy of gases is not sufficient to force the gas into the sorbent unless there is a flow (positive pressure). Surface areas of adsorbents are typically 100 m² g^-1. Typically, only 10 g max of sorbent is required. For example, if the adsorbing molecule has a surface area of 2 * 10^-20 m², then the sorption capacity per gram is 5 * 10²¹ molecules. For acetic acid therefore, the maximum sorption capacity per gram is 0.5 g.

Alumina : £30/kg. Surface area is typically 155 m² g^-1. It is acidic or can be bought in the activated basic form (although this form can not be regenerated) therefore will be ineffective for acid pollutants. It is macroporous therefore the mass transport is limited and it only uses a fraction of its surface area. Sorption capacity is < 0.01g, and slow.

Silica : SiO² : £100 / kg. Surface area is about 20 m² g^-1. Weakly basic. It has very little porosity and so mass transport is limited. The sorption capacity < 0.01g, and slow.

Silica Gel : £20/kg. Surface area is typically 800 m² g^-1. Weakly acidic but will be okay for acid pollutants due to the H-bonding. It has a very high surface area of 500 m² g^-1, BUT, the acid may be displaced by water. The sorption capacity is 0.15 g, but it can be up to 0.5 g. It is macroporous and so allows little mass transport.

Zeolites
Mixture of silica and alumina in a cage structure + basic solids. Its biggest use is as detergents and catalysts. It is microporous and has a pore size of 2 - 15 * 10^-10 m. They are crystalline materials. The surface area to nitrogen is as high as 1000 m² g^-1, and to acids is potentially 10 m² g^-1. The problem is the pore size (2-15 Å). The area is not open as in other porous materials therefore a pressure is needed to drive the gas through it. The sorption capacity is potentially > 0.25g, but you may need to divide this by 100 if no pressure is used to force the gas into the zeolite structure. Overall, zeolites are expensive and are not great sorbents as their sorption capacities are low.

MCM Silica
Surface area is approx. 1500 m² g^-1. Have pores between 20 -100 *10^-10 m. They are made by the reaction: silica gel + surfactant molecules (with amine heads) --> form regular (isotropic) micelles ---> silica condenses around micelles ---> form hexagonal or cubic arrays.
Left with silica with lined up holes therefore the pores are huge (50 Å). This means that the whole silica structure is very available for molecules. The high surface area means that adsorption is rapid. The sorption capacity > 0.5 g. The cost is approx. £200 /kg. There is no requirement for high pressure because of the open structure. Basic groups and boron are added to improve its resistance to water which can cause the pores to collapse. It is commonly used in salmon farms to remove formic acid.

ABSORBENTS
These react with the sorbing molecules to give a new compound, for example, zinc reacts with hydrogen sulphide to produce zinc sulphide. The reaction time from the surface of the sorbent to the bulk is critical, it has to be very rapid. To find a good absorbent for acetic and formic acid we require basic materials for example, lanthanide oxides (Which are not radioactive !!).
Lanthanide oxides are very basic materials, as are their hydroxides and aqueous ions.

La(OH)₃ + 3 CH₃COOH ---> La(CH₃COO)₃ + 3 H₂O

La₂O₃ + 6 CH₃COOH ---> 2 La(CH₃COO)₃ + 3 H₂O

La (H₂O)₆ ³⁺ + 3 CH₃COOH ---> La (H₂O) 3 (CH₃COO)₃ + 3 H₃O+

The final reaction has the added bonus of producing an acid, so litmus could be added as a dye to determine when the sorbent is spent. Theoretically the sorption capacity of La₂O₃ for acetic acid is 5.47 g. But is the reaction fast enough ?? With formic acid no formate is formed on the sorbent, instead it is decomposed to carbon dioxide and we form LaCO₃ and LaOH.

Conclusion :
There are a host of suitable materials. The best ones are not the conventional ones. One idea might be to wrap a light bulb with catalyst. The light would provide sufficient heat and air circulation. Or use a small turbine at the end of a tube. Remember if you are going to use sorbents they may cause more harm than good if you place them in the wrong part of the cabinet.