Removal of paracetamol from aqueous solution by activated carbon and silica. Experimental and computational study

The presence of pharmaceutical residues in the aquatic environment is a known problem worldwide. Paracetamol is widely used as an analgesic and antipyretic. Its high consumption implies a continuous discharge in aqueous environments through industrial and domestic wastewater that requires mitigation and remediation strategies. The aim of the present study was to analyse the removal of the paracetamol from aqueous solutions using the adsorption technique. For this, three commercial adsorbents with different textural properties were used: two activated carbons (CAT and CARBOPAL) and silica gel. A series of batch adsorption experiments were conducted at different values of pH (3.0, 7.0 and 10.5) and ionic strength (0.01, 0.5 and 1 M) to investigate the effects on the removal of paracetamol from the aqueous solution. In addition, we investigated the adsorption mechanism using the density functional theory. Adsorption was found to be higher in the acidic pH range, as varying pH showed significant influence on the surface charge of the adsorbents and degree of ionization of the paracetamol. Adsorption capacity of the adsorbents increased with an increase in the ionic strength of solution. At 25 °C, pH 3, ionic strength 1 M, 167 mg L^-1 of adsorbent and initial concentrations of paracetamol between 25 and 150 mg L^-1, the maximum adsorption capacity was 560 mg g^-1, 450 mg g^-1 and 95 mg g^-1, for CAT, CARBOPAL and silica respectively. The experimental kinetic data fitted well the pseudo-second order model and the equilibrium isotherm data the Langmuir model. The functional density theory methods provided atomistic details about paracetamol adsorbed on the surface of carbon and silica through molecular modeling.