Stefano Ubaldini

Recovery of valuable metals from electronic and galvanic industrial wastes by leaching and electrowinning

In the present paper, a study on laboratory scale to perform a treatment for valuable metals recovery from electronic and galvanic industrial wastes, is reported. The characterisation of the waste, performed by XRD, SEM, EDX and chemical analysis, showed a high metals content in the sludge, such as Cu, Ni, Mn, Pb, Sn, W. A leaching process, coupled by electrowinning, is then proposed in order to reduce the volume of the waste material and to recover selectively valuable metals, such as Cu and Ni. During the leaching step, carried out by using H(2)SO(4), several factors were investigated (acid concentration, temperature and time of treatment). The leached liquor has been successfully treated with an electrowinning process, to recover copper and nickel. The copper and nickel depositions, were performed in acid and alkaline conditions, respectively. The Faraday yield was of about 95%. The energy consumption was 2.13 and 4.43 kWh per kg of copper and nickel recovered, respectively. At the end of the process, about 94-99% of the initial content of Cu and Ni was recovered at the cathode. The experimental results obtained, showed the technical feasibility of the process.

Auto- and heterotrophic acidophilic bacteria enhance the bioremediation efficiency of sediments contaminated by heavy metals

This study deals with bioremediation treatments of dredged sediments contaminated by heavy metals based on the bioaugmentation of different bacterial strains. The efficiency of the following bacterial consortia was compared: (i) acidophilic chemoautotrophic, Fe/S-oxidising bacteria, (ii) acidophilic heterotrophic bacteria able to reduce Fe/Mn fraction, co-respiring oxygen and ferric iron and (iii) the chemoautotrophic and heterotrophic bacteria reported above, pooled together, as it was hypothesised that the two strains could cooperate through a mutual substrate supply. The effect of the bioremediation treatment based on the bioaugmentation of Fe/S-oxidising strains alone was similar to the one based only on Fe-reducing bacteria, and resulted in heavy-metal extraction yields typically ranging from 40% to 50%. The efficiency of the process based only upon autotrophic bacteria was limited by sulphur availability. However, when the treatment was based on the addition of Fe-reducing bacteria and the Fe/S oxidizing bacteria together, their growth rates and efficiency in mobilising heavy metals increased significantly, reaching extraction yields >90% for Cu, Cd, Hg and Zn. The additional advantage of the new bioaugmentation approach proposed here is that it is independent from the availability of sulphur. These results open new perspectives for the bioremediation technology for the removal of heavy metals from highly contaminated sediments.

EMPIRICAL MODELS FOR OXYGEN MASS TRANSFER : A COMPARISON BETWEEN SHAKE FLASK AND LAB-SCALE FERMENTOR AND APPLICATION TO MANGANIFEROUS ORE BIOLEACHING

Abstract An investigation on oxygen mass transfer in shake flasks and in a lab-scale bioreactor is reported. An analysis of the main factors influencing oxygen mass transfer in shake flasks has been carried out using full and fractional factorial designs. The factors investigated were: mixing conditions (rpm); temperature (°C); weight of closure (g); liquid hold-up (ml); and geometry of shake flasks. The ANOVA (analysis of variance) showed the importance of the investigated factors on the oxygen mass transfer coefficient. An empirical model with dimensionless parameters has been proposed in order to predict the oxygen mass transfer in several experimental conditions. An investigation has also been performed on the oxygen mass transfer in a lab-scale bioreactor, considering stirring, air flow rate and temperature as factors. Parameters of an usual empirical model for oxygen transfer coefficient have been estimated. The relationships proposed have been obtained in a wide range of experimental conditions, and they can be used both to determine if oxygen limiting conditions occur during biological tests in the shake-flask scale or in the lab-scale bioreactor, and in the phase of translation of experimental data to a bigger scale. Using this model the bioleaching of manganiferous minerals by heterotrophic microorganisms is judged to take place just in microaerobic conditions, i.e. in oxygen limiting conditions.

Bioremediation of Dredged Sediments Polluted by Heavy Metals

The present work deals with a bioremediation study of a heavy-metal polluted harbour sediment, obtained from the Italian Adriatic Coast. Bioleaching of the sediment sample was performed with a mixed culture of acidophilic, chemi-autotrophic Fe/S oxidising bacteria. The effect of an anaerobic biostimulation pre-treatment on the extent of Cd, Cu, Zn, Ni, Pb, Hg, As, Cr extraction by bioleaching was evaluated. The biostimulation pre-treatment was intended to stimulate autochthonous sulfate reducing strains, to enhance the sulfide fraction in the sediment, to favour subsequent activity of reduced-sulfur-oxidizing bacteria in the subsequent bioaugmentation (bioleaching). The effect of the duration of anaerobic pre-treatment (21 and 30 days) in the presence and absence of 1% glucose was tested. The results obtained showed that the activity of the reducedsulfur- oxidising strains was significantly enhanced after an anaerobic pre-treatment of the sediments and showed real promise for the application of bioleaching for metal polluted sediments.

Study of Precipitating Methods for Elimination of Heavy Metals from Acid Mine Drainage

Abstract The submitted paper deals with the study of combination of chemical and biological-chemical methods for the heavy metals elimination from the acid mine drainage. The experiments were carried out at the laboratory scale using a synthetic solution with similar properties to the real sample of acid mine drainage, originating from the zinc mine located in Tùnel Kingsmill outflow of the Rio Yaulì (district of Yauli - Perù). The successive repetition of the metal precipitations as hydroxides (chemical method) and sulphides (biological-chemical method) at the various acid mine drainage pH was the basis of the examined processes. For the hydrogen sulphide production the sulphate-reducing bacteria of genus Desulfovibrio were used. Results confirmed the precipitation of Fe, As, Al and Mn in the hydroxide form; Cu and Zn in the form of sulphides. For the selective removal of Cu, Zn, Al and Mn the excellent results were received. Selective precipitation of Fe and As was not so successful since the co-precipitation of Fe and As was later determined as a main mechanism of the precipitate forming.