Exclusion of Salt Solutions from Activated Carbon Pores and the Relationship to Contact Angle on Graphite

Abstract

The exclusion of salts from the pores of activated carbon granules is demonstrated for the first time for the salts K2CO3, K2HPO4 and K3PO4. In soaking experiments, the concentrations of solutions of K2CO3, K2HPO4, and K3PO4 inside the pores are less than in the bulk solutions. This phenomenon is observed by measuring the bulk concentration, which increases upon addition of carbon. This suggests that these salts are repelled by the carbon surface. In incipient wetness experiments, the three aforementioned salts do not enter the carbon at concentrations greater than ∼1 M. The contact angles of these three salt solutions on highly oriented pyrolitic graphite and polished resin-impregnated graphite surfaces rise with concentration and approach 90°, which suggests that these solutions would not enter activated carbon pores. By contrast, soaking, incipient wetness, and contact angle measurements for salts like ZnCl2 and Zn(CH3COO)2 show preferential adsorption by soaking (the salt is depleted from the solution), high imbibing limits (maximum incipient wetness volumes) at high solution concentrations, and a reduction of contact angle with increase in concentration. It is found that the behavior of these solutions on graphite surfaces approximates the solution behavior on an activated carbon that is relatively low in acidic surface functional groups.