Biosorption of heavy metal ions from aqueous solutions by nonliving water hyacinth roots

Abstract

The nonliving dried water hyacinth (Eichhornia crassipes (Mart.) Solms) roots were investigated for biosorption of cadmium, lead, mercury and other heavy metal ions in aqueous solutions. Dried water hyacinth roots, when suspended in solution, strongly and rapidly sorb several different metal ions. Factors that affect metal uptake, such as reaction time, pH of the system, initial concentration of metal ions, temperature, presence of other metal ions and biomass concentration, were investigated. Metal ions, including Cd2+ and Pb2+, were bound to the root in a very pH-dependent manner, with the binding increasing between pH 2 and 3.5. However, the binding of Hg2+ was relatively independent of pH. For the metal ions (Hg, Pb and Cd) high levels of removal were found. This effect is consistent across a range of initial concentrations, even down to low levels. The generalized Freundlich and Langmuir isotherm equations were adapted to the sorption of mercury, lead and cadmium as a function of initial concentration at different conditions. The relative affinity of the roots for the metal ions was found to be: mercury > lead > cadmium. The maximum sorptions of the roots were about 33 and 86 mg/g for cadmium and lead, respectively. Only 15-20 minutes were required for the process of uptake onto the biomass to occur. The batch sorption kinetics of the uptake of cadmium and lead can be described by a first-order reversible reaction model. The rate limiting step for the sorption appears to be film diffusion. The roots can be regenerated after the metal ions are stripped. The desorption of the metals from the water hyacinth roots were examined using H2S04, HN03, EDTA, NaCl, CaCl2 and A1C13 solutions. The solution of CaCl2 in acidic medium appeared to be the best eluant capable of desorbing 100 pet of the sequestered Cd and Pb at pH 3, while the best result for Hg desorption reached only 76 percent. The kinetics of the metal stripping process were quite rapid. The biosorption process was found to be spontaneous and endothermic. Successful desorption of Cd and Pb from the biomass by acidic CaCl2 solutions revealed that the metal uptake phenomenon is reversible, implying physical sorption of Cd and Pb. Chemical and instrumental analyses including atomic adsorption, electron microscopy, infrared (IR) spectroscopy and X-ray energy dispersion analyses provided supporting evidence that the biosorption mechanism involves predominantly ion exchange. The different behaviors of sorption and desorption of Hg were observed. On the basis of high binding capacity, low cost, and low technology, the water hyacinth roots are considered to have potential as a sorbent for future industrial use.