Mechanism of adsorption of different humic acid fractions on mesoporous activated carbons with basic surface characteristics Anna Kolodziej • Marta Fuentes • Roberto Baigorri • Ewa Lorenc-Grabowska • Jose M. Garcı ´a-Mina • Philippe Burg • Gra _ zyna Gryglewicz Received: 26 September 2013 / Revised: 12 February 2014 / Accepted: 1 April 2014 Ó Springer Science+Business Media New York 2014 Abstract The effects of the humic acid (HA) nature and the activated carbon (AC) surface chemistry on the effec- tiveness of HA removal were investigated. Brown (BHA) and gray (GHA) humic acid fractions of different structure and physicochemical properties were tested in the adsorp- tion process using mesoporous ACs. The modification of chemical structure and surface properties of AC was achieved by ammonization (AC/N) and hydrogen treatment (AC/H). Both approaches led to a decrease in the oxygen content followed by an increase in the carbon basicity, maintaining the porous texture of AC nearly unaltered. Over twice higher removal degree of BHA and GHA was observed for the modified ACs. The kinetics of adsorption of HA fractions have been discussed using the pseudo- second-order model and the intraparticle diffusion model. All ACs showed a higher adsorption capacity toward BHA compared to GHA, which is mainly attributed to the lower molecular weight of BHA. The shape of the equilibrium isotherms indicates a strong competition between water and HA molecules for adsorption sites of the carbon surface. Keywords Humic acid Activated carbon Surface chemistry Adsorption List of symbols t Time (min) q t Amount of HA adsorbed by carbon at time t (mg/g) q e exp Amount of HA adsorbed at equilibrium - experimental data (mg/g) q e,c Amount of HA adsorbed at equilibrium - calculated data (mg/g) k ads The first-order rate constant of adsorption (1/min) k 2ads The second-order rate constant of adsorption [g/(mg min)] k i IPD rate constant [mg/(g min 0.5 )] C Constant for a given experiment (mg/g) 1 Introduction The humic acid (HA) constitutes the major fraction of natural organic matter in water supplies, decreasing con- siderably water quality. It contains both hydrophilic and hydrophobic moieties with functional groups such as car- boxylic, phenolic, carbonyl and hydroxyl groups (Steven- son 1994). The various fractions of humic substances are known to show different interactions with the adsorbent due to their polydispersive nature. The molecular frac- tionation of HA carried out according to the different sol- ubility of the molecules under specific conditions of pH A. Kolodziej E. Lorenc-Grabowska (&) G. Gryglewicz Department of Polymer and Carbonaceous Materials, Faculty of Chemistry, Wroclaw University of Technology, ul. Gdan ´ska 7/9, 50-344 Wroclaw, Poland e-mail: ewa.lorenc-grabowska@pwr.wroc.pl M. Fuentes J. M. Garcı ´a-Mina Department of Chemistry and Soil Chemistry, University of Navarra, 31080 Pamplona, Spain R. Baigorri J. M. Garcı ´a-Mina CIPAV-Roullier Group, Inabonos, Poligono Arazuri-Orcoyen, 31160 Orcoyen, Spain P. Burg Laboratory of Chemistry and Environmental Methodologies, Paul-Verlaine University Metz, Technopole Metz 2000, 1Bd Arago, 57078 Metz Cedex, France 123 Adsorption DOI 10.1007/s10450-014-9610-3