Heavy metals

Although widely used, the term heavy metal includes more than one definition, changing according to the branch of science where it is included. From the toxicological point of view, toxicologists will present heavy metals as chemical elements with a toxic effect on superior mammals and other forms of animal and microbial life. From the agroindustrial point of view, it can be presented as the chemical elements that are toxic to plant cultures, or chemical elements that decrease agricultural productivity, due to its presence at high concentrations in soil.

Chemists define heavy metal as the chemical elements in the periodic table whose density is higher than 6 g cm^-3. No matter how it is defined, those elements are metal ions, generally associated to polluting processes, as well as essential elements for living organisms, if present at very low concentrations. Thus, toxic metal seems to be a feasible alternative for the term heavy metal; however, this term can be used for non essential elements, such as Pb, Cd, Hg, As, U. Those elements are found in several effluents originating from batteries, electronic materials, plastics industries, as well as the electroplating industry. The uncontrolled discharge of those effluents in soils and water bodies, highly contaminates environmental sites. As well, burning residues from industrial activities may cause partial volatilization of those elements, contaminating atmosphere.

Alternative technologies, based on the use of biological materials to concentrate toxic metals are currently under development, with the use of microbial species, as well as giant seaweed plants collected as waste material from the beaches in tropical countries. However, to be used as good sorbent materials, physico-chemical parameters of those biomaterials must be evaluated, in order to check the feasibility of using them as effective heavy metal biosorbents.

The mechanisms involved in the capture of heavy metal ions will depend on the biomaterial used, the charge of the metal ion, the genus of the seaweed and the chemical species of the element in aqueous solution.

Toxic metals in solution are not necessarily present as free metal ions. Some other ions, called ligands, are capable of interacting with metal ions, forming complexes. The knowledge of this interaction between free metal ions and ligands is the principle of the development of biosorption processes. The same interaction is expected to happen between the free metal ions and the ligands present in the surface of the selected biomass, for instance, polysaccharides from seaweeds.

The parameters that determine the interaction between heavy metal ions and seaweeds are influenced by the activity of this ion in solution, that, as well, is also affected by temperature, pH, diffusion, and other physico-chemical parameters. Those are selected and important parameters, because they will define if the ligands will remain in molecular or ionic form.

Simple ligands, such as sulphate, chloride and nitrate may present distinct affinities for a specific metal ion; thus, the knowledge about the chemical composition of the biomass to be used, as well as their intrinsic chemical properties, is a useful information for the success of the development of a proper biosorbent material.

Biosorption

The tremendous diversity of microbial, plant, animal and seaweed structures implies on the existence of a wide range of biomaterials to be tested; as well, those distinct chemical structures introduces several mechanisms of metal uptake from solutions. Those varieties of mechanisms are not completely understood: some of them are metabolically mediated and some mechanisms are completely independent from the metabolism of the biomass. Beyond this fact, if one considers the site for metal deposition on the surface (or in the interior) of a biomass, a wide range of mechanisms can be detected, such as simple accumulation, extra cellular precipitation, adsorption, volatilization and intracellular accumulation.

The transport of the metal through the cell membrane and the intracellular accumulation are metabolism dependent mechanisms, and, in those specific cases the process is not immediate; an elapsed time is needed for the transport of the toxic ion to take place.

In the cases where physico-chemical interaction occurs between the toxic metal and the surface polysaccharides of the biomass, ion-exchange, complexation and adsorption take place, and the phenomenon is not metabolism dependent. The surface of the seaweeds, for example, is constituted of polysaccharides and proteins that provide a wide range of ligands for heavy metal ions. Differently from metabolism dependent mechanisms, these processes are rapid and reversible.

Seaweeds as biosorbent materials for toxic metal ions

Seaweeds can be found in large quantities around the world. They can be easily harvested from the oceans in some places of the world, specially tropical countries and the Caribbean region. On the other hand, they can also be easily cultivated in laboratory to serve as a source of food or polysaccharides. This outstanding natural occurrence stimulates the development of biotechnologies with the use of seaweeds for the uptake of heavy metals from industrial effluents or to decontaminate polluted areas through bioremediation. It is important to mention that seaweeds can be also used as biological indicators of contamination, and this property is also based on their ability to concentrate heavy metals due to their chemical composition.

There is a specific interest in some algal genera, such as Sargassum, Ecklonia, Ascophylum, Gracilaria and Padina, mainly due to their wide availability in the oceans and also due to their chemical composition, that favours the interaction with heavy metal elements. Most brown seaweeds can be found in cold waters, and due to their presence in cold waters, a high content of polysaccharides is synthesized (alginate, fucoidin and cellulose).

Copper is a nutrient that is needed for the growth of seaweeds at extremely low concentrations; at moderate and high concentrations, it acts as an algaecide. Copper can be found in its free state or complexed with organic samples in the seawater. Cupric ion is its most toxic form, depending on the pH and the concentration of the ligand.

Although already studied for some authors, the biosorption of copper by brown seaweeds still faces technological gaps. Sargassum, Padina, Codium, Colpomenia, Ulva and several other seaweeds are being investigated as potential biosorbers for heavy metals. Inside each of those genera, several species might exist, and inside each species several varieties might also exist.

Irrespective of the biomass (seaweed, microbe, waste material, plant) the investigation of physico-chemical parameters that govern the process constitute a central focus for the comprehension of the mechanism involved in the uptake. The comprehension of the mechanism involved in the uptake and the knowledge of the optimum biosorption conditions will dictate, for each biomass, the technical and economic feasibility of use in a metal treatment plant.