The influence of activated carbon surface chemical composition on the adsorption of acetaminophen (paracetamol) in vitro - The temperature dependence of adsorption at the neutral pH

The in vitro adsorption and desorption of acetaminophen from water solution on four activated carbons at three temperatures (300, 310 and 320 K) and at the neutral pH (7) were investigated. The carbons were characterized using the low temperature nitrogen adsorption, the mercury porosimetry, Bachmann's method, Boehm's method as well as the water immersion calorimetry. As an initial adsorbent, the de-ashed-commercial, 'non-modified' carbon D43/1 (Carbo-Tech, Essen, Germany) was applied. To change the chemical composition of its surface, concentrated nitric and sulfuric acids as well as gaseous ammonia were applied as chemical modificators. The acetaminophen adsorption and desorption isotherms on the 'non-modified' as well as on the chemically modified carbons were measured, together with the enthalpy of immersion in paracetamol solution. It is shown that, generally, for all the investigated carbons, acetaminophen adsorption increases with temperature. A slightly marked hysteresis on adsorption-desorption isotherms was observed at higher adsorption values. Among the applied procedures of the changing of carbon surface chemical composition, the modification with fuming sulphuric acid leads to the increase in paracetamol adsorption, whilst the opposite effect is observed for the carbon modified with concentrated nitric acid. The modification in the stream of ammonia practically does not change the adsorption properties towards paracetamol. The changes in the adsorption properties of carbons after modification are analysed using isotherms, adsorbability, relative enthalpy of displacement as well as the values of the integral enthalpy of adsorption. To calculate this enthalpy the solubility of acetaminophen in the investigated range of temperatures was determined, and the enthalpy of solution at infinite dilution was calculated using Abraham's method. (C) 2000 Elsevier Science B.V. All rights reserved.