Effectiveness of a new green technology for metal removal from contaminated water

Water pollution by heavy metals is a matter of growing concern due to their potential toxicity for biota. The development of new and cost-effective remediation strategies is a priority in this field. For this purpose, a green technology, consisting of organic waste made up of a composite of water hyacinth dry matter and sodium alginate in microsphere form, was tested for its potential to adsorb lead and cadmium in river water samples. The water hyacinth-sodium alginate composite molecular conformation was analysed first in order to obtain the potential energy surfaces. A high dipole moment, useful for the adsorption of heavy metals, was found. Accordingly, the cross-linked microspheres were prepared by combining water hyacinth dry matter and sodium alginate in a 1:1 ratio. Their metal adsorption and any effect on the natural microbial river community were verified with laboratory microcosm experiments lasting 11 days, using river water spiked with lead and cadmium (1 mg/L each). For this purpose, chemical (metal concentrations) and microbial (microbial abundance, viability and taxonomic composition) analyses were performed. Moreover, the ecotoxicity (with Vibrio fischeri ISO 11348-3:2007) of water samples from microcosms in the presence/absence of the microspheres and metals was assessed. Effective concentrations (EC20 and EC50) of Pb and Cd were also determined. Finally, electron microscopy analyses were performed with a Field Emission Scanning Electron Microscope (FE-SEM) to visualize the metal adsorption capacity of the microspheres and investigate the metal distribution on the spheres (adsorbed on the surface or inside). The overall results showed that the microspheres were able to remove high heavy metal concentrations (about 1 mg/L) from river water in a short time (at 96 h just 1.3% and 5.8% of the Pb and Cd initial concentrations were detected) and no ecotoxicological effects were recorded. Moreover, the microspheres had a positive effect on the microbial community by promoting an increase in live cell numbers, probably through the release of organic carbon. Further analyses (at about four months) showed that the microspheres were able to keep metals adsorbed for a long time. These results show that the green technology proposed is an effective remediation method for Pb and Cd removal from river water and is very promising, not least because of its low cost.