* Corresponding author

Financial support: CNPq (Conselho Nacional do Desenvolvimento Científico e Tecnológico), CAPES and UERJ (Universidade do Estado do Rio de Janeiro).

The Chemistry of brown seaweeds - Cell wall and metal ligands

Brown seaweeds (Phaeophyceae) constitute an algal group containing the characteristic pigment fucoxantine, responsible for their brown color. Floating masses of Sargassum constitute the Sargasso Sea, famous in science and fiction and a very common type of seaweed in the Brazilian Coast. All Sargassum species contain floating bubbles, responsible for their decreased density, thus contributing for their presence in the marine environment.

Quantitatively, the most abundant polysaccharide in the cell wall of brown seaweeds is alginic acid. Alginic acid is a polymer constituted by two uronic acids (b -1,4-D-mannuronic-M and a -1,4-L guluronic-G), with molar ratio between the acids from 0.25 to 2.5. Alginic acid is present in the seaweeds usually as calcium, magnesium and sodium salts, mainly in the cell wall. It is a structural polysaccharide with strong ion-exchange properties.

The toxic effect of zinc

Bioaccumulation corresponds to the amount of metal accumulated by a seaweed, directly from water or through its diet. A metal is considered deleterious if bioaccumulation factors exceed limiting values. The bioaccumulation of essential metals constitutes a natural process required by living organisms, depending on factors such as total amount of metal and bioavailability in the medium. Essential metals present toxicity levels both due to their lack of excess in solution. Usually they become toxic when their concentrations exceed those limits needed for the correct metabolism of cells and plants.

Most zinc produced in the world come from zinc sulfide ores, extensively used in galvanization processes. Beyond these applications zinc is used in the cosmetic, talc, paints and rubber industries, as well as in fertilizing industries. Zinc is probably the less toxic heavy metal, essential to most living organisms; however, its deficiency causes increasing lack of susceptibility to infections.

According to Brazilian FEEMA, discharge limits for zinc is 1.0 mg/l, and according to FDA human needs of zinc are around 15 mg/day. However, increased incorporation of zinc causes a decrease in the high density cholesterol and adverse reactions in the immune system.

Biosorption

Biosorption is a general term that describes the removal of metals from a solution by biological materials. Most works in the literature, however, emphasizes the use of microbial cells to accumulate metals. Several mechanisms are involved in the uptake of metals, mainly adsorption, precipitation, complexation, transport and ion-exchange.

Large volumes of industrial effluents contaminated with heavy metals are treated by conventional techniques. However, those techniques are usually expensive of not completely efficient in the treatment. Thus, biosorption can be an attractive alternative for the final treatment of industrial solutions, complementing conventional techniques.

Interference between metals

In the last few years the uptake of a specific metal from a solution is being studied. However, little is known about the interaction of metals present in the same solution. Usually, industrial effluents will contain more than one metal, so, the behavior of the biomass in relation to complex solutions must be investigated.

A combination of metals can produce three different behaviors: synergism (the effect produced by the mixture increases individual effects produced by the metals), antagonism (the effect produced by the mixture decreases individual effects produced by the metals) and no interaction (the effect produced by the mixture is the same as that produced by the individual metals). Interactive effects of a mixture of metals for an aquatic organism (such as a seaweed) are extremely complex, being affect by environmental parameters such as pH and temperature, as well as algae species, the type of combination of metals, as well as their concentration in solution.

Conventional technologies versus biosorption of metals

Biosorption offers some advantages in relation to conventional technologies. Conventional evaporation and reverse osmosis are not suitable for the treatment of industrial solutions containing metals at low concentrations. The use of membranes present the following disadvantages: ion concentrations must be low; there is a great energy consumption; the retention of metals is not efficient; the resistance of the membrane is dependent on pH and is also amenable to microbial degradation. Precipitation requires huge amount of chemical reagents and the residual concentration of metals is not suitable for discharge in aqueous streams. Ion-exchange resins present low resistance to thermal and osmotic shocks being also amenable to chemical degradation. Based on these assumptions biosorption seems to be a competitive technology, being, however, its state-of-the-art still a developing technology.

Final considerations

Biosorption, as a part of an integrated wastewater treatment facility, is suitable for a broad range of heavy metal containing solutions. If the cost and application of biosorbents are competitive they would represent a suitable alternative for the detoxification of industrial effluents. Especially Sargassum sp. proved to be an alternative for the uptake of heavy metals, sequestering metals through adsorption and ion-exchange mechanisms. Results indicated that during zinc biosorption, alkaline and alkaline-earth elements were released into solution, a fact confirmed by the changes in the X-ray spectrum of virgin and zinc-loaded biomasses.