ORIGINAL PAPER Modeling of trivalent chromium speciation in binding sites of marine macroalgae Sargassum Cymosum F. B. de Souza • S. M. A. Guelli Ulson de Souza • A. A. Ulson de Souza • Carina A. E. Costa • Cida ´lia M. S. Botelho • Vı ´tor J. P. Vilar • Rui A. R. Boaventura Received: 6 June 2012 / Accepted: 12 December 2012 / Published online: 10 January 2013 Ó Springer-Verlag Berlin Heidelberg 2013 Abstract In this study, the marine macroalgae Sargassum cymosum was used for the purification of waters contami- nated with trivalent chromium. FTIR analysis revealed a high heterogeneity of the biosorbent surface, as indicated by the different absorption peaks. Biomass titration revealed two main functional groups, carboxylic and hydroxyl. The pK 1,H value and the number of carboxylic groups were estimated as 3.05 ± 0.01 and 1.90 ± 0.01 mmol g -1 , respectively. An equilibrium model considering the metal speciation in aqueous solution was able to predict the experimental data at different pH values. Adsorption of chromium increases significantly with an increase of the solution pH. Furthermore, the speciation of the binding sites as a function of the solution pH was predicted, showing that Cr(OH) 2? has a higher affinity than Cr 3? to the binding sites. A mass transfer model considering an intraparticle diffusion resistance was able to predict the kinetic data, showing that Cr 3? diffuses faster that CrOH 2? . Keywords Biosorption  Trivalent chromium  Speciation  Binding sites  Modeling Introduction The increase of heavy metal pollution is a problem of great concern worldwide since this group of pollutants may have potentially toxic and carcinogenic effects on human health and living organisms (Prabhakaran et al. 2009; Silva et al. 2012). Chromium is one of the most toxic contaminants found in polluted soils and surface/ground waters. The extensive use of this metal leads to its release into the environment through several pollution sources originating from anthropogenic activities such as electroplating, tan- ning, water cooling, pulp production, metal finishing, tex- tile plants, and ore and petroleum refining processes (Park et al. 2011). Chromium can be present in different oxida- tion states (e.g., divalent, trivalent, pentavalent, and hexa- valent) and each species exhibits different properties of toxicity, mobility, and concentration in aquatic systems (Espinoza-Quin ˜ones et al. 2010). However, only hexava- lent chromium (Cr(VI)) and trivalent chromium (Cr(III)) can be found as stable forms in natural environments. Cr(VI) is the most abundant in natural aquifers and Cr(III) prevails in municipal wastewaters rich in organics (Pra- bhakaran et al. 2009). Chromium in the trivalent form and at low doses is an essential element for humans. Also, in general, it is only toxic to plants in very high concentra- tions and is less toxic, or nontoxic, to animals, although long-term exposure to high levels of this metallic species may cause poisoning symptoms. In contrast, Cr(VI) is known to be very toxic to both plants and animals, because it is a strong oxidizing agent and a potential carcinogen (Espinoza-Quin ˜ones et al. 2010). As a result of the dif- ferences in the physical and chemical properties of these two forms of Cr, the United States Environmental Protec- tion Agency (USEPA) has classified this metal as a human carcinogen and, consequently, has defined limits for its F. B. de Souza  S. M. A. G. U. de Souza  A. A. U. de Souza EQA-LABMASSA-Mass Transfer Laboratory (Laborato ´rio de Transfere ˆncia de Massa), Federal University of Santa Catarina State (UFSC), Floriano ´polis, Brazil e-mail: selene@enq.ufsc.br C. A. E. Costa  C. M. S. Botelho  V. J. P. Vilar (&)  R. A. R. Boaventura Laboratory of Separation and Reaction Engineering (LSRE), Associate Laboratory LSRE/LCM, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal e-mail: vilar@fe.up.pt 123 Clean Techn Environ Policy (2013) 15:987–997 DOI 10.1007/s10098-012-0573-3