Colloidal Activated Carbon

"Soluble" Activated Carbon  photos: R.Köhler / UFZ

Activated carbon (AC) is the most widely used sorbent in environmental technology. It has only one disadvantage for in-situ use: For an in-situ sorption barrier huge amounts of sediment have to be dug away.Objective of our studies is the development of a technological concept which is based on the injection of colloidal activated carbon (AC) suspensions, the homogeneous distribution of the AC particles with the groundwater flow over several metres in length and finally their immobilization as sorption zone within the aquifer sediment. In order to achieve this, knowledge of the transport behaviour of AC colloids in the aquifer has to be gained. The innovative idea of this concept is the utilization of AC in finely ground form. In aqueous solution, the substance receives a new property: The AC becomes "quasi soluble". From the literature it is known that the mobility of colloids in the aquifer depends on the size and density of the colloids, the net surface charge of the colloids and the mineral grains, the pH value and the ionic strength as well as the flow velocity of the groundwater. Results of column experiments showed that native activated-carbon colloids have a sufficient mobility only at high pH values (pH higher than 10) and low ionic strength of the eluent. The mobile fraction in this case was about 90% of the injected amount of AC. However, such high pH values are not relevant for a technical application. In order to achieve an increase in the mobility of the AC colloids in the neutral pH range, various concepts were applied:

1) oxidative functionalization
2) impregnation of the AC colloids with polyanionic substrates or anionic surfactants and
3) impregnation of the AC colloids with coal-derived humic substances.

By means of all three methods a high mobility of the pre-treated AC colloids in the neutral pH range could be obtained. However, the loading of the AC colloids with humic substances can be evaluated as being the most suitable method. In contrast to the oxidative pre-treatment, the impregnation methods have only a marginal effect on the sorption capacity of the AC. Compared with anionic surfactants, humic substances have the advantage of being natural, non-toxic additives that are more suited for a practical application.