Cadmium removal from aqueous solutions by biosorbents: study of the operating conditions

The increasing presence of cadmium in the environment is mainly due to its use in electroplating, paint pigments, plastics, alloy preparation, mining, ceramics and silver–cadmium batteries. The chronic exposure to Cd can lead to damages to humans both directly or indirectly throughout the food chain, thus this metal and its compounds are considered as carcinogens. The most common methods used to remove heavy metals from wastewater are ion-exchange, reverse osmosis and chemical precipitation, but all of them are too expensive. In recent years, a number of studies were carried out to search for alternative, less expensive sorbents such as living or dead organisms. Our studies on Cd removal, which are reviewed in this paper, focused on the application of living microorganisms ubiquitous in wastewaters, specifically Sphaerotilus natans and Zoogloea ramigera, or dead microbial biomass of Arthrospira (Spirulina) platensis or even cellulosic biomass such as Agave americana fibers. Under the best operating conditions, Cd biosorption by dry biomass of A. platensis was very effective ranging from 149 to 357 mg g−1. As far as living biomass is concerned, Z. ramigera and S. natans were able to remove up to 91 and 93% of initial Cd concentration, respectively, whilst fibers of A. americana were able to remove only up to 12.5 mg g-1 of Cd. FT-IR spectroscopy as well as scanning electron and metallographic microscopy allowed us to get information on the mechanisms involved in Cd biosorption. Finally, Cd bioaccumulation was experimentally determined in living cells of A. platensis (maximum specific biosorption capacity of 2.60 mg g-1), and citotoxicity of extracts from contaminated cells evaluated as EC50 against L929 mouse fibroblasts.