Biosorption of cadmium ions by Actinomycetes and separation by ¯otation M.I. Kefala, A.I. Zouboulis*, K.A. Matis Division of Chemical Technology, Department of Chemistry, Aristotle University, GR-54006 Thessaloniki, Greece Received 27 April 1998; revised 16 September 1998; accepted 16 September 1998 Abstract Among the most important parameters which have to be examined, when treated waste waters are going to be recycled, is their content of toxic metals, due to ever decreasing metal disposal limits. For this reason, the examination of eective and innovate waste water treatment methods becomes an important need. The removal of cadmium, a toxic metal of high environmental priority due to its toxicity, from dilute aqueous solutions has been studied in the present work, applying microorganisms and using living, as well as non-living bacterial biomass of two specially isolated Actinomycetes strains, AK61 and JL322. The main parameters in¯u- encing this treatment process, namely contact time, pH of the solution, temperature and toxic metal and biomass concentrations have been examined and Langmuir isotherms have been depicted. Dispersed-air ¯otation was applied as the subsequent separation method for harvesting the suspended metal-laden microorganisms, following the biosorption of cadmium. The investigated para- meters (in batch mode, laboratory scale) were in this case the dispersion pH, the ¯otation time, the air ¯owrate and the surfactant concentration. Electrokinetic measurements have been also performed for the biomass dispersions, under similar conditions with biosorption, providing useful information for the process mechanisms. Applying the optimum de®ned conditions, over 95% removal of cadmium has been achieved in one stage and simultaneously, quantitative separation of the used (metal-loaded) biomass has been obtained. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Actinomycetes; Biosorption; Bioaccumulation; Biomass; Cadmium; Flotation; Zeta-potential 1. Introduction The content of residual toxic metals in treated waste waters, being in direct relation with the eciency of the applied metal removal process, highly in¯uences the reuse options of them. Several methods have been applied for their ®nal (tertiary) treatment (Patterson, 1985), such as adsorption using activated carbon or other appropriate sorbents (alumina, zeolites, etc.), post-precipitation, ion exchange, etc., in order to remove the greatest possible degree of toxic metals resi- dual concentrations and therefore, to enhance the recy- cling characteristics of treated waste waters (EPA, 1992). Nevertheless, certain drawbacks of these methods have been noticed, such as high cost, low eciency, operational complexity, and others diculties (Patter- son, 1985). Hence, there is an important necessity for novel removal methods of toxic metals (Horan, 1997), such as those currently introduced (advanced oxidation, biological removal of metals, etc.). The use of biological materials, including living and non-living microorganisms, to remove and possibly recover toxic or precious metals from industrial waste waters, has gained important credibility during recent years, because of the good performance and low cost of this sorbent material. The natural anity of biological compounds for metallic elements could contribute to economically purifying metal-loaded waste water, a fact which has been already proved in many cases and by many researchers (see for example Volesky, 1990; Wilde and Benemann, 1993). However, despite the available (extensive) literature on metal-microbial interactions, relatively little information exists (Rayner and Sadler, 1989), correlating metal uptake with the speciation of the metal in dilute aqueous solutions, which in turn is depending on the physicochemical conditions (pH, temperature, etc.) of the solution (Remacle, 1992). Although many dierent mechanisms may take place simultaneously during bioaccumulation (the term is gen- erally applied, when living microorganisms are being 0269-7491/99/$Ðsee front matter # 1999 Elsevier Science Ltd.. All rights reserved PII: S0269-7491(98)00178-X ENVIRONMENTAL POLLUTION Environmental Pollution 104 (1999) 283±293