Recontamination of water with activated carbon; OILSORB to the rescue.
George Alther, Dr. Henry Novicki, Wayne Schuliger.
When granular activated carbon is used to remove BTEX from water, a detrimental phenomenon can occur. The molecules with lower solubility, such as toluene and xylene, can kick off benzene, which has a higher solubility. The amount of benzene is now higher in the effluent than in the influent, 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 the influent, caused by desorption of fast-moving bands by components in the slow moving bands.
The desorption is due to differences in the adsorbate capacities on the carbon in the slower and faster moving adsorption bands.
Roll-Over of Fast-Moving Bands, Description:
The extent of Roll-Over (Roll-Off) is determined from the multi-component adsorption isotherms.
Every component of the influent mixture will experience Roll-Over, except the most slowly moving component.
The fastest moving band will experience Roll-Off from every other component band in the column.
The Roll-Over 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 in the inflow is: | Outflow:
Benzene: 20 ppm | 25 ppm
Toluene: 9 ppm | N/D
Xylene: 7.1 ppm | N/D
Discharge limits:
Benzene: 74 ppb
Xylene: 134 ppb
This phenomenon does not occur with Oilsorb because:
This is consistent with the partition -desorption mechanism v.s. adsorption-desorptiom mechanism for carbon. A experiment has been conducted by Dr, James Smith, University of Virginia, to proof that “Roll-Off or Roll – Over does “not occur with organoclay”. Therefore it is advisable to pre-polish a water stream containing BTEX with OILSORB, followed by activated carbon. Other data to be found in the web site, Products, EC-199 show that organoclay works as well, or better, than activated carbon for the removal of xylene and toluene.
