Heavy metals are one of the most important sources of pollution and exist in aqueous waste streams of many industries. Copper, one such heavy metal is widely employed in electrical, electroplating industries and in brass production. The presence of copper usually causes serious toxicological concerns; it is known to deposit in brain, liver and myocardium. Therefore it is almost necessary to remove copper from industrial effluents that is reflected in a tightening and enforcement of environmental regulations.

Conventional methods for removing copper such as chemical precipitation, adsorption and membrane techniques are either insufficient or too costly to implement. Therefore recent research attention has focused on developing a new technology for treatment of copper-bearing effluents.

Bioremediation, an innovative technology utilizing biological organisms to clean the environment, has gained importance due to its simplicity, effectiveness and environmental friendliness. Biosorption is a one such technique for treatment of heavy metals. Biosorption is a process that utilizes inactive (dead) biomass to sequester heavy metals and is particularly useful for polishing step in waste-water treatment. Bacteria, fungi, algae and crab shells are some of the biosorbents widely employed for removal of heavy metals.

Marine algae (popularly known as Seaweeds) are widely distributed throughout the world. The presence of functional groups such as carboxyl, sulfate, amino and hydroxyl groups in the algal cell wall makes it an ideal biosorbent for heavy metal removal. Brown algae (Phaeophyta), Green algae (Chlorophyta) and Red algae (Rhodophyta) are the few important divisions of algae. When comparing these divisions on metal sequestration characteristics, Phaeophyta comprises of alginate and Chlorophyta comprises of xylans and mannans in their respective cell walls. Rhodophyta comprises galactans in addition to xylans.

Ulva reticulata, a marine green alga is the focus of the present study to remove copper from aqueous solution. Biosorption is usually influenced by several factors such as pH, initial metal concentration and contact time.

Discussion

pH is one of the most important environmental factors influencing not only the site dissociation, but also the solution chemistry of the heavy metals. Algal biomass usually comprises of binding sites, which is previously occupied by light metal ions such as Na⁺, K⁺, Ca²⁺ and Mg²⁺. A suitable pH condition is therefore required to exchange the light metal ions with metal ions of interest. At very low pH conditions, excess of H⁺ ions compete with Cu²⁺ ions in occupying the sites. At very high pH conditions, excess of OH^- ions combines with Cu²⁺ to form precipitates. Both the cases usually decrease copper biosorption. Therefore an optimum pH is adequate to achieve maximum biosorption.

The equilibrium of the biosorption process is often described by fitting the experimental points with models. Langmuir and Freundlich isotherm models are widely used for fitting the biosorption data. Even though these models are not capable of describing mechanisms of metal uptake, they are simple and capable of reflecting sorption isotherm.

The contact time required to attain equilibrium metal concentration in a biosorption process is very important to design water treatment units. Solution pH and initial metal concentration usually influence the biosorption contact time. Since continuous systems are usually operated at high flow rates, it is always desirable to have minimum contact time as it also allow large quantity of wastewater be treated.

In order to reduce the process cost it is always desirable to reuse the biosorbent. An eluting agent must be carefully selected on the basis of the mechanism of metal binding to the biomass. It must also be cheap, non-polluting and non-damaging to the biomass. Mineral acids such as HCl, H₂SO₄ and HNO₃ are good eluting agents for most of the biosorbents. But they often damage the biomass and make it unsuitable for subsequent sorption cycle. Since ion-exchange appear as a major metal binding mechanism of marine algae, CaCl₂ may serve as a good eluting agent. The Ca²⁺ ions from CaCl₂ get exchanged with Cu²⁺ ions to occupy the binding sites. The sorption performance of the biosorbent in the subsequent cycles should be unaltered to be regarded as an effective biosorbent.

Concluding remarks

Biosorption thus offer many advantages over conventional methods including low operating cost and high efficiency. In an attempt to identify a new biosorbent for copper removal, Ulva reticulata performed effectively as supported by the results obtained through experiments. At pH 5.5, U. reticulata exhibited a maximum copper uptake of 74.63 mg/g. The solution of 0.1 M CaCl₂ in HCl at pH 3 exhibited an elution efficiency of 90.25% and also caused least damage to the biosorbent.