Removal cadmium ions from aqueous solution by biosorbent - immobilized aquatic weed M. spicatum

The lack of clean water and waste generation are problems that exist in many parts all over the world. Different industries: electroplating, smelting and alloying could discharge cadmium pollutants into water body. Cadmium as toxic metal must be properly removed from wastewaters because it may lead to: kidney damage, respiratory insufficiency, hypertension, cancer, gastrointestinal disorder and osteoporosis.

Globally depletion of natural and nonrenewable resources has induced need for application of renewable sources in environment protection practices; such as renewable biomass. Biosorption has emerged as an economically feasible alternative for the environment cleanup using naturally occurring, abundant, waste biomass. Application of biosorbents in the form of powder have difficulties associated with their powder properties, indicating need for suitable form with adequate mechanical strength (e.g. beads) for easily manipulation and further use in flow systems.

The waste biomass which is tested as biosorbent of cadmium is cosmopolitan invasive aquatic weed - waste Myriophyllum spicatum immobilized as beads in natural polymer matrix-alginate – (MsA-B). M.spicatum grows on five continents and based on its widely negative effects on the environment in the whole world, this weed has been categorized as Category 1 Weed. M.spicatum used for preparation of beads originates from Sava Lake (Belgrade, Serbia) where it is regularly harvested by public company "Ada Ciganlija”. Ratio M. spicatum and alginate in beads is 2:1.

This biosorbent (MsA-B) was characterized by Scanning-Electron-Microscopy-EnergyDispersive-X-Ray-Spectroscopy (SEM-EDX). EDX spectra showed: Ca, Mg, Na, K and Cl. Large amount of calcium originate from the water environment in which this aquatic weed lives. The surface of (MsA-B) is different from the surface of aquatic weed because the alginate covered the characteristic rectangular openings. The beads are not spherical, but more ovoid shape. The surface is wrinkled and clearly layered. After Cd(II) sorption, reduced peaks of Ca as well as a new peak of Cd(II) are observed

The effect of the Cd(II) concentration on adsorption was studied at pH 5.0 in the concentration range from 20 to 750 mg/L as batch biosorption tests with 2 g/L MsA-B. Experimental results were fitted by six isotherm models: Langmuir, Freundlich, Sips, Redlich and Peterson, Toth, Temkin. After biosorption experiments, beads didn’t change its shape, size or colour. They have been easily separated from the solution by decanting. Comparison of models was made on the basis of the coefficients of determination R² and comparison value of biosorption capacities experimented and calculated by models. Order of best fit isotherm models: RP>L>S>Th>T>F. MsA-B showed high capacity for Cd(II) removal q=82,5 mg/g which is 33% higher capacity then capacity of dry biomass M.spicatum. During the biosorption experiments with MsA-B, changes of pH were insignificant. That is very important because there is no need to control pH, no demand for chemicals which greatly simplifies the process and makes it cheaper. Easy application and manipulation, much easier than other biosorbents simplify further investigations and applications of this aquatic weed beads – MsA-B in industrial wastewaters and also in the next step of the biosorption processes: flow systems and reactors.

Acknowledgement
These results are part of the projects supported by the Ministry of Education and Science of the Republic of Serbia, TR 31003. Jelena Milojković is grateful to the public company "Ada Ciganlija" (Belgrade, Serbia) for providing samples of aquatic weed M. spicatum.