Ion Exchange Modeling of the Competitive Adsorption of Cu(II) and Pb(II) Using Chemically Modified Solid Waste Coffee

Abstract

The presence of potentially toxic metals such as Cu(II) and Pb(II) in aquifers and industrial effluents represents a serious health problem due to their high toxicity, non-biodegradability, and ability to bioaccumulate. In this study, the removal of these pollutants individually and as a binary mixture has been studied, using solid coffee waste modified with 0.6\xa0M citric acid as the adsorbent, and a mathematical model based on the ion exchange mechanism was implemented to elucidate the adsorption equilibrium. The characterization of modified coffee waste showed a pH value at the point of zero charge of 2.97 and a high concentration of carboxylic groups, which are susceptible to ion exchange. Furthermore, the quantification of interchangeable ions confirmed that the main mechanism of adsorption is the ion exchange of metal ions with the protons present on the adsorbent’s surface. The experimental data of the individual and binary adsorption equilibrium using a model based on a phenomenological approach was analyzed. The phenomenological model was compared with the Freundlich and Langmuir empirical solid-liquid adsorption models. The results showed that the adsorption capacities of Cu(II) and Pb(II) individually were 1.46 and 1.18\xa0meq/g, and in a binary mixture were 1.43 and 1.24\xa0meq/g, respectively, at pH 5 and 30\xa0°C. In addition, the separation coefficients from ion exchange model revealed the predominance of protons as an exchangeable ion, which is in accordance with the experimental evidence. Finally, the correlation coefficient showed that the proposed model predicts accurately the adsorption equilibrium.