By George Alther, Dr. Henry Nowicki, Wayne Schuliger
When granular activated carbon is used to remove BTEX from water, a detrimental phenomenon can occur. Depending on the relative concentrations of the various compounds, the molecules with lower solubility, such as toluene and xylene, can desorb benzene, which has a higher solubility. The amount of benzene can be higher in the effluent than in the influent, thereby causing the water to become hazardous again. This phenomenon is called "Roll-Off" or "Roll-Over".
Definition:
"Roll-Off" or "Roll-Over" is defined as the elution of adsorbate exhibiting concentrations greater than that in the influent, caused by desorption of fast-moving bands by components in the slow moving bands.
Roll-Over of Fast-Moving Bands, Description:
The extent of Roll-Over (Roll-Off) is determined from the multi-component adsorption isotherms. The desorption is due to differences in the adsorbate capacities on the carbon in the slower and faster moving adsorption bands.
Every component of the influent mixture will experience Roll-Over, except the most slowly moving component. The fastest moving band will experience Roll-Over from every other component band in the column and is strictly a thermodynamic phenomenon. Its extent is independent of the adsorption kinetics. The kinetics only change the rate the Roll-Over occurs, but not the extent or amount of component Roll-Over.
Example: BTX concentration at a given point in time in the:
| Inflow | Outflow |
|---|
| Benzene | 20 ppm | 25 ppm |
| Toluene | 9 ppm | N/D |
| Xylene | 7.1 ppm | N/D |
This phenomenon does not occur with Oilsorb because of the partition-desorption mechanism vs. adsorption-desorption mechanism for carbon.
To evaluate data of a lab study about possible desorption of benzene from organoclay, check this lab report.
Visit Biomin's website for additional technical information.
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