The biosorption mechanisms of different heavy metallic cations (Cd, Ni, Pb) to active chemical groups on the cell wall matrix of the nonliving brown marine macroalga, Sargassum vulgaris in its natural form, were examined by the following instrumental and chemical techniques: Fourier-transform infrared (FTIR) analysis, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and extraction of alginic acid and sulfated polysaccharides, which act as metal-binding moieties present in cell wall. From the different techniques used and the known chemical composition of the algal cell wall, it was observed that biosorption of the metallic cations to the algal cell wall component was a surface process. The binding capacities of the different metal cations were between 1 and 1.2 mmol metal/g on a dry weight basis. The main chemical groups involved in the metallic cation biosorption were apparently carboxyl, amino, sulfhydryl, and sulfonate. These groups were part of the algal cell wall structural polymers, namely, polysaccharides (alginic acid, sulfated polysaccharides), proteins, and peptidoglycans. The main cadmium cation sequestration mechanism by the algal biomass was apparently chelation, while the nickel cation sequestration mechanism was mainly ion exchange. Lead cations exhibit higher affinity to the algal biomass, and their binding mechanism included a combination of ion exchange, chelation, and reduction reactions, accompanied by metallic lead precipitation on the cell wall matrix. During the ion exchange process, calcium, magnesium, hydrogen cations, and probably other cations (sodium and potassium) in the algal cell wall matrix were replaced by the tested heavy metals.

Experimental studies showed that brown marine algae, Sargassum vulgaris and Padina pavonia, can be used to develop an efficient biosorbent for heavy metal removal from aqueous solutions. Sargassum vulgaris exhibited high uptake capacities for cadmium (0.9 to 1.1 mmol Cd/gr) and nickel (0.85 to 1 mmol Ni/gr) that are higher than those of other types of biomass and powdered activated carbon, while P. pavonia showed a broader range of nickel and cadmium uptake capacities (0.7 to 1 mmol Ni/gr and 0.8 to 1.1 mmol Cd/gr). The metal adsorption and desorption processes were rapid, with 70% of the sorption and desorption completed within 10 minutes. The equilibrium data for both algae fit well to Langmuir and Freundlich isotherm models. More than 90% desorption of adsorbed metals from the algae was achieved by hydrochloric acid and ethylenediaminetetraacetic acid (1:1 molar ratio). After eight to nine adsorption and desorption cycles, S. vulgaris showed a 15 to 35% decrease in metal uptake capacities; P. pavonia showed a higher decrease of 50 to 60%. Water Environ. Res., 75, 246 (2003). IntroductionThe contamination of water by toxic heavy metal ions is a worldwide environmental problem. Precipitation, filtration, ion exchange, membrane separation, and other techniques usually achieve removal of heavy metals. However, in many situations these processes do not work efficiently or fail to lower the metal concentration below the regulatory standards. Heavy metal precipitation produces intractable sludge that must then be treated and disposed, often at a high cost. In addition, many metal-bearing waste streams contain substances such as organic matter, alkaline earth metals, and others that may decrease the removal capability of the metal cations (Brauckmann, 1990;Eilbeck and Mattock, 1987).Biosorption of heavy metals from aqueous solutions is a relatively new wastewater treatment technology (Volesky, 1990). Adsorbent materials (biosorbents) that are derived from a suitable biomass can be used for the effective removal and recovery of heavy metal ions from wastewater streams. Biosorption of metals involves several mechanisms that differ qualitatively and quantitatively according to the species used and the origin of the biomass and its processing procedure. Metal sequestration during biosorption follows complex mechanisms, primarily ion exchange, chelation, adsorption by physical forces, and ion entrapment in intra interfibrillar capillaries and spaces of the structural polysaccharide network. The following chemical groups could attract and sequester the metals in biomass: acetamido, amino, amido, sulfhydryl, sulfate, and carboxyl (Ashkenazy et al.