Calcium interference with continuous biosorption of zinc by Sargassum sp. (Phaeophyceae) in tubular laboratory reactors Francisca Pess^ oa de Franc ßa a , Ana Paula Mora Tavares a , Antonio CarlosAugusto da Costa b, * a Universidade Federal do Rio de Janeiro, Escola de Qu  ımica, Departamento de Engenharia Bioqu  ımica, Ilha do Fund~ ao, Rio de Janeiro, RJ 21949-900, Brazil b Universidade do Estado do Rio de Janeiro, Instituto de Qu  ımica, Departamento de Tecnologia de Processos Bioqu  ımicos, Rua S~ ao Francisco Xavier 524, Rio de Janeiro, RJ 20550-013, Brazil Received 26 June 2001; received in revised form 10 October 2001; accepted 14 October 2001 Abstract The zinc biosorptive capacity of the brown seaweed Sargassum sp. (Phaeophyceae) was studied in the presence or absence of competing calcium ions, using a continuous system with tubular fixed-bed reactors. In order to detect the effect of calcium on zinc biosorption, a 130 mg/l zinc solution was used, and calcium was added at 50–340 mg/l. The potential zinc biosorptive capacity of the biomass was markedly influenced by the presence of ionic calcium. Zinc sorption decreased with increasing calcium concentrations, as expressed by zinc uptake rates. Calcium was effectively recovered only during the initial stages of the process, as expressed by the decrease in its uptake rates. Calcium uptake rates were also much higher than zinc uptake rates, indicating that calcium was preferentially recovered when compared to zinc. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Biosorption; Calcium; Environment; Sargassum sp.; Seaweed; Zinc 1. Introduction Biological materials provide an attractive solution for the removal of heavy metals from aqueous streams when compared to conventional chemical treatments, mainly due to their reduced cost. Classical processing of solu- tions containing heavy metals includes precipitation with lime, ion-exchange, activated carbon adsorption, electrodialysis and reverse osmosis. In many cases, classical strategies are not completely efficient, as heavy metal levels are not sufficiently reduced to meet the limiting concentrations established by local legislation (V  ılchez et al., 1997). Brown seaweed biosorption of metallic elements has been investigated under batch conditions using Ascophyllum nodosum (de Carvalho et al., 1995) and Sargassum fluitans (Schiewer and Volesky, 1995; Leusch et al., 1996). Brown seaweeds constitute potential biomaterials for the recovery of heavy metals, mainly due to its polysaccharide content. Carboxyl groups from alginates are the main sites of heavy metal biosorptive sorption in Sargassum sp., even though they are not the exclusive ones. In an extensive screening test that compared the biosorptive perfor- mance of different types of seaweed (brown, red and green), Sargassum sp. proved to be the best accumulator of heavy metals, due to its high biosorption capacity, low cost and low equilibrium concentrations (da Costa and de Franc ßa, 1996). There is a specific interest on certain types of sea- weeds, such as Sargassum, Ecklonia, Ascophylum, Gracilaria and Padina, mainly due to their availability in the ocean and also due to their potential to accumulate heavy metal elements in their structures (Leusch et al., 1995). Most brown algae are found in cold seawater, being structurally complex, with some species presenting air flotation bubbles. Their cell walls consist of poly- saccharides, proteins and lipids, both rich in functional groups able to interact with heavy metal ions (Remacle, 1990; Kiefer et al., 1997). Some workers have been in- vestigating the interference of metallic ions on heavy metal biosorption in batch systems (de Carvalho et al., 1995; Figueira et al., 1997; Schiewer and Volesky, 1997). Only a few studies describe the application of con- tinuous systems for heavy metal biosorptive sorption Bioresource Technology 83 (2002) 159–163 * Corresponding author. Fax: +55-21-2587-7206. E-mail address: acosta@uerj.br (A. Carlos Augusto da Costa). 0960-8524/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII:S0960-8524(01)00198-5