Journal of Hazardous Materials 180 (2010) 289–296 Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat Effect of sorption kinetics on nickel toxicity in methanogenic granular sludge Jan Bartacek a,b,∗ , Fernando G. Fermoso a , Alba Beas Catena a , Piet N.L. Lens a,b a Sub-Department of Environmental Technology, Wageningen University, “Biotechnion”-Bomenweg 2, P.O. Box 8129, 6700 EV Wageningen, The Netherlands b Department of Environmental Resources, UNESCO-IHE, P.O. Box 3015, 2601 DA Delft, The Netherlands article info Article history: Received 10 November 2009 Received in revised form 19 February 2010 Accepted 7 April 2010 Available online 14 May 2010 Keywords: Nickel toxicity Metal speciation Anaerobic granular sludge Donnan membrane technique Intra-particle diffusion abstract This study investigates the effect of nickel speciation and its equilibrium kinetics on the nickel toxicity to methylotrophic methanogenic activity. Toxicity tests were done with anaerobic granular sludge in three different media containing variable concentrations of complexing ligands. A correlation between nickel toxicity and the free nickel concentration failed, because not the equilibrium conditions, but the kinetics of the speciation processes taking place in the medium (precipitation, sorption, liquid speciation, etc.) determine nickel bio-uptake and its toxic effect. The latter was confirmed with an F-test (p-value always lower than 0.1). It was shown that the biological activity (methane production) took place within 3–20 days upon the start of methanogenic experiments, i.e. prior the chemical–physical equilibrium of nickel speciation was established in the methanogenic medium (10–20 days). The process of nickel sorption in the methanogenic granular sludge was limited by intra-particle diffusion and the experimental data fitted to the Weber–Morris sorption model. The other sorption kinetic models applied (pseudo-first order sorption kinetics, pseudo-second order sorption kinetics and first order reversible reaction kinetics) did not fit the experimental data satisfactorily. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The widely used models for metal toxicity such as the Free Ion Activity Model (FIAM) or the Biotic Ligand Model (BLM) are based on the assumption of chemical–physical equilibrium in the medium. If the equilibrium state is established, toxicity of a given metal can be estimated based on the concentration of any of the metal species present in the medium [1]. Because the free metal ion is always present in the aqueous solution, analytical tech- niques have intensively focused on the quantification of the free ion concentration [1–3]. However, several interfering phenomena have been recognized, such as the presence of competing met- als [1], accumulation of dissolved organic matter (DOM) on the surface of living cells [4], interaction of complexed metals with the cell membrane [5] and limitation of the bio-uptake by metal transport and dissociation [6]. Most importantly, the assump- tion of the chemical–physical equilibrium is only fulfilled when the physical–chemical processes (sorption, precipitation, chemical speciation, etc.) are slower than the metal transport over the cell membrane [1,7]. In the case that the physical–chemical processes become the rate-limiting step for the bio-uptake, predicting metal ∗ Corresponding author at: Department of Environmental Resources, UNESCO- IHE, P.O. Box 3015, 2601 DA Delft, The Netherlands. Tel.: +31 152 151 880; fax: +31 152 122 921. E-mail address: j.bartacek@unesco-ihe.org (J. Bartacek). toxicity based on the free ion concentration at steady state fails [7]. The interaction between heavy metals and anaerobic granular sludge has been widely studied describing the sorption capacity of the sludge [8], kinetics of the sorption process [9] and the speciation of metals inside the methanogenic granules – solid state speciation [8,10]. However, the implications of these interactions, especially of the speciation equilibrium kinetics, for heavy metal toxicity have not yet been studied in detail. This study aims to evaluate the effect of nickel liquid phase speciation and its kinetics on nickel toxicity to methylotrophic methanogenesis by anaerobic granular sludge. This was done con- sidering also the crucial effect of nickel sorption kinetics. 2. Materials and methods 2.1. Medium composition Three different media were prepared with different concen- trations of EDTA and phosphates in order to vary the speciation of nickel: medium I contained a high concentration of phosphate and no EDTA, medium II contained both EDTA and phosphate and medium III contained no EDTA and a minimum concentration of phosphate (Table 1). Nickel chloride was used as the nickel source in media I and III. The nickel-EDTA complex ([NiEDTA] 2- ) was pre- pared from NiCl 2 and Na 2 H 2 EDTA in molar ratio 1:1.5 and supplied into medium II. 0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.04.029