Description of mutual interactions between silicon and phosphorus in Andisols by mathematical and mechanistic models Paula Cartes a,b,⇑ , Mara Cea a , Alejandra Jara a,b , Antonio Violante c , María de la Luz Mora a,b a Scientific and Technological Bioresource Nucleus, Universidad de La Frontera, Casilla 54-D, Temuco, Chile b Departamento de Ciencias Químicas y Recursos Naturales, Universidad de La Frontera, Casilla 54-D, Temuco, Chile c Dipartimento di Agraria, Università Degli Studi di Napoli Federico II, Naples, Italy highlights  Si and P sorption were studied on two Andisols with contrasting physico- chemical properties.  Mathematical and mechanistic models modeled Si and P sorption data.  Competition between Si and P was not symmetrical in binary systems. graphical abstract article info Article history: Received 24 October 2014 Received in revised form 27 January 2015 Accepted 25 February 2015 Available online 31 March 2015 Handling Editor: X. Cao Keywords: Andisol Phosphorus Silicon Sorption Freundlich equation Constant Capacitance Model abstract The Freundlich model and the Constant Capacitance Model (CCM) were used to describe silicon (Si) and phosphorus (P) sorption, both individually and for binary P–Si systems, on two Andisols with different chemical properties: Freire soil (FS) and Piedras Negras soil (PNS). Silicon sorption kinetics were examined through the Elovich equation, revealing that the initial sorption rate was 16 times greater in PNS. The Freundlich equation provides a good fit to the sorption data for both Andisols. When compared with FS, larger Si sorption capacity and lower Si affinity for the surface sites were observed in PNS; nevertheless, Si sorption decreased in both soils as P sorption increased. Slight reductions in P sorption capacity due to the presence of Si were found, whereas there was no apparent effect on P bonding intensity. The CCM was able to describe Si adsorption, and potentiometric titrations support that Si seems to be specifically sorbed mainly onto sites of negative charge. Comparable log K Si int values were obtained for both soils, indicating that Si was bound on similar sites. Phosphorus sorption was well described by the CCM, and log K P int denoted strong interactions of P with the surface sites. For binary systems, log K P int did not vary with increasing Si concentration; comparatively, log K Si int scarcely decreased with increasing P concentration in PNS, but a 28% reduction was found in FS at the highest initial P concentration. Ó 2015 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2015.02.059 0045-6535/Ó 2015 Elsevier Ltd. All rights reserved. Abbreviations: FS, Freire soil; PNS, Piedras Negras soil; CCM, Constant Capacitance Model. ⇑ Corresponding author at: Scientific and Technological Bioresource Nucleus, Universidad de La Frontera, Casilla 54-D, Temuco, Chile. E-mail address: paula.cartes@ufrontera.cl (P. Cartes). Chemosphere 131 (2015) 164–170 Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere