Financial support: Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES).

Keywords: chromium removal, heavy metal, Langmuir model.

Mining activities, agricultural run off, industrial and domestic effluents are mainly responsible for the increase of the metallic species released into the environment.

Contrary to toxic organics, that in many cases can be degraded, the metallic species that are released into the environment tend to persist indefinitely, accumulating in living tissues throughout the food chain.

A complete understanding about noxious effects caused by the release of toxic metals into the environment and the emergence of more severe environmental protection laws, have encouraged studies about removal/recovery of heavy metals from aqueous solutions using biosorption.

Conventional methods as precipitation, oxidation/reduction, ion exchange, filtration, membranes and evaporation are extremely expensive or inefficient for metal removal from diluted solutions containing from 1 to 100 mg/L of dissolved metal. In this context, the biosorption process has been recently evaluated (Volesky, 1990).

Although biosorption is promising, its mechanism is not well elucidated. This knowledge is essential for understanding the process and it serves as a basis for quantitative stoichiometric considerations, which are fundamental for mathematical modeling and scale-up (Volesky, 1986).

There are potent biosorbents easily available in all the three groups: algae, fungi and bacteria. A source of low cost biomass produced in great quantities, are marine macroalgae.

Studies about the technological aspects of the metal removal by algae are scarce (Volesky and Holan, 1995). In this sense, the aim of this work is to determine the potential of chromium uptake, a highly toxic metal present in several industrial effluents, by the inactive biomass of the marine alga Sargassum sp., abundant on Brazilian coast.

Experiments were carried out to determine the contact time required for equilibrium sorption and evaluate the influence of biosorbent size, pH and temperature on chromium biosorption by brown marine alga Sargassum sp.

Figure 1 presents the results obtained with the milled biomass at two different initial concentrations of chromium. A contact time of 6 hours was enough for the system to reach the equilibrium.

Influence of biosorbent size on chromium biosorption

The influence of biosorbent size on chromium biosorption can be evaluated from Figure 2. The experimental results indicate that the biosorbent size did not influence the capacity and rate of chromium biosorption.

 Effect of pH

The effect of pH on metal biosorption have been studied by many researches, and the results demonstrated the increasing cation uptake with increasing pH values, as fungi biomass (Tsezos and Volesky, 1981; Guibal et al. 1992) as algae biomass (Darnall et al. 1986; Kuyucak and Volesky, 1989; Aksu and Kutsal, 1991; Garnham et al. 1993; Holan et al. 1993; Holan and Volesky, 1994; Kratochvil et al. 1998).

Figure 3 shows the effect of the pH on the biosorption capacity of the marine alga Sargassum sp. at different temperatures. As shown, pH is an important parameter for the sorption process, especially in the temperature range from 30ºC to 40ºC. The chromium biosorption capacity was at all higher at pH 4.0 (at pH 5.0 a chromium precipitate was observed).

 Effect of temperature

Temperature has not been  studied as a relevant variable in biosorption experiments. The tests are usually performed at approximately 25-30ºC. However, Tsezos and Volesky, 1981; Kuyucak and Volesky, 1989; and Aksu and Kutsal, 1991, reported a slight increase in cation uptake by seaweed in the range of 4 to 55ºC.

The effect of temperature on chromium biosorption by Sargassum sp. was not as pronounced as the effect of pH. This fact can be observed on Figure 4.

• The biomass of the marine alga Sargassum sp. demonstrated a good capacity of chromium biosorption, highlighting its potential for effluent treatment processes;
• The kinetics of chromium biosorption by inactive biomass of the marine alga Sargassum sp. was fast, reaching 60% of the total biosorption capacity in 10 minutes;
• The biosorbent size had no influence on chromium biosorption rate;
• pH had a strong effect on chromium biosorption capacity;
• The capacity of chromium biosorption by biomass increased with pH and was higher at pH 4.0;
• Within the range tested, the effect of temperature on the chromium biosorption capacity was mild.

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