F. Vegliò

Biosorption of heavy metals by Sphaerotilus natans: an equilibrium study at different pH and biomass concentrations

Abstract In the present work a lyophilised cell suspension of Spherotilus natans was studied as biosorbent material for cadmium and copper removal from aqueous solutions. The biomass was firstly characterised by potentiometric titration and evaluation of the major ionic content. The experimental data highlight that the biomass cell wall contains two main acidic groups with a total amount of 5 meq/g. Equilibrium biosorption trials of cadmium and copper were carried out to investigate the effects of two important experimental factors, pH and biomass concentration. As expected, both cadmium and copper biosorption extent was repressed by pH decrease. The effect of the biomass concentration changes both with the equilibrium pH value and the kind of metal adsorbed. In the case of copper biosorption at pH>5, the increase of biomass concentration causes a diminution of the maximum specific metal uptake due probably to cell aggregation phenomena; whereas at acidic pH values the previous trend is inverted perhaps because of the effect of partial hydrolysis of the bacterial cell wall constituents. A different behaviour was observed for cadmium biosorption in relation to the microbial cell concentration: the effect of biomass concentration is less evident and opposite with respect to copper at pH 6 and 3 and no definitive explanation was found for this case. Equilibrium modelling was performed for both metals by using the most used equations reported in the literature. A comparison of the biosorption characteristic of S. natans has been also performed with respect to some results reported by other researchers.

pH-related equilibria models for biosorption in single metal systems

Biosorption is an innovative technology used to remove heavy metals from aqueous solutions. A wider application of this alternative process is strictly related to the understanding of the chemico-physical aspects involved, in order to optimize the operative conditions. Mechanistic models are the most useful tools used for understanding purposes, even if the empirical models are still widely applied for their simplicity. In this paper, two original models were used to represent the experimental data of copper and cadmium biosorption onto Sphaerotilus natans in different operative conditions of pH and biomass concentration. Both models can represent the effect of pH onto biosorption performances using two different approaches. The first model is empirical, based on the observation that the maximal specific metal uptake courses vs. pH presents a logistic pattern. The second model is based on the non-competitive mechanism between heavy metals and H+ protons. Both models can represent adequately the experimental data, but the non-competitive model also gives a realistic description of the mechanism operating in the system according to a preliminary biomass characterization.

Metal speciation and pH effect on Pb, Cu, Zn and Cd biosorption onto Sphaerotilus natans: Langmuir-type empirical model

Biosorption data of lead, copper, zinc and cadmium onto Sphaerotilus natans at different equilibrium pH (3-5 units) were here reported and analysed. Experimental results outlined the positive effect of pH increase on pollutant uptake and also the biomass affinity series (Pb > Cu > Zn > Cd) reflecting the hydrolytic properties of metals. An original empirical model was proposed to represent the effect of pH on heavy metal biosorption inserting q(max) vs. pH empirical functions into the classical Langmuir isotherm.

Equilibrium biosorption studies in single and multi-metal systems

Different models were used in order to describe equilibrium data obtained by heavy metal biosorption tests. Arthrobacter sp. biomass was suspended in aqueous solution and tested as biosorbent material for copper and cadmium ions. The effects of pH on biosorption in single metal systems (copper and cadmium) and on the selectivity of biomass in two metal systems (copper–cadmium) was studied. Biosorption trials performed for each single metal (in the range 0–2 mmol/l) at different equilibrium pHs (3–6 units) showed a general positive effect of pH increase on metal specific uptake. Experimental data obtained for each metal were fitted using adsorption models including Langmuir, Freundlich and Redlich–Peterson isotherms. Biosorption tests were also carried out using binary solutions of copper and cadmium at different equilibrium pH to evaluate biomass selectivity. Modified Langmuir and Freundlich models were used to fit these equilibrium data. The biomass tested was more selective for copper ions and this selectivity changes with equilibrium pH. In both single and two metal systems, a simple procedure of model discrimination was performed to establish which of the tested models better represents the experimental behaviour.

Bioleaching of zinc and aluminium from industrial waste sludges by means of Thiobacillus ferrooxidans.

Biological solubilisation of heavy metals contained in two different kinds of industrial wastes was performed in batches employing a strain of Thiobacillus ferroxidans. The wastes tested were: a dust coming from the iron-manganese alloy production in an electric furnace (sludge 1) and a sludge coming from a process treatment plant of aluminium anodic oxidation (sludge 2). The experimental results pointed out the ability of the used strain to maintain the environment, that initially has a pH about 8, at strongly acid conditions (pH 2.5-3.5), producing sulphuric acid that is the chemical agent responsible for the metals solubilisation. At wastes initial concentration of 1%, the percentage of solubilised metals was 76 and 78% for the wastes 1 and 2, respectively, but the lag phase was considerably longer for sludge 2 than for sludge 1, indicating a different affinity of microorganisms for the solid phase. Increasing the initial slurry concentration, the percentage of removed metal reached 72-73% for the sludge 1, while in case of sludge 2, the total amount of solubilized metal progressively decreased. Two kinetic models are proposed to describe the trends of metals solubilization curves.

Multi-metallic modelling for biosorption of binary systems

In this paper a specially propagated biomass of Sphaerotilus natans was tested as adsorbent for binary solutions of Cu-Cd, Cu-Pb and Cu--Zn at different equilibrium pH. The experimental results outline the buffering effect of H+ at low pH. which masks the competition among metals. In each binary system the biomass affinity follows the acidic properties of the heavy metals probably due to an ionic exchange mechanism operating among active sites and metals in solution. The experimental results were fitted according to an empirical approach with growing complexity that outlines the inadequacy of the predictive models and the non-ideal interactions among metals.

Shrinking core model with variable activation energy: a kinetic model of manganiferous ore leaching with sulphuric acid and lactose

A kinetic model of manganiferous ore dissolution varying sulphuric acid and lactose as reducing agent was developed here. The Shrinking Core Model (SCM) was modified to take into account both the variable reagent concentrations and the reaction rate change with respect to the manganese dioxide conversion: this last aspect is due to several oxidation reactions of the carbohydrates in a hot sulphuric acid medium. The effects of three factors were investigated for three ore size fractions: temperature, lactose and sulphuric acid concentrations. The application of the kinetic model to different size ore fractions with and without the particle size effect shows that the best results in data fitting were gained by applying the model separately to each particle size fraction. The influence of the particle size in the kinetic model is discussed with the support of SEM analysis and BET surface measurements before and after leaching.

Biosorption of copper by Sphaerotilus natans immobilised in polysulfone matrix: Equilibrium and kinetic analysis

Abstract Copper biosorption by Sphaerotilus natans immobilised in polysulfone matrices has been studied. Firstly, a rough characterisation of biosorbent beads has been performed, and operating conditions for beads preparation aimed at biosorption have been optimised. Then, the equilibrium of the process was studied in order to determine the effect of pH and biomass concentration inside beads; experimental data were successfully fitted by the Langmuir equation, and the highest value for loading was 5.4 mg/g estimated at pH 5.5 and 0.18 g of lyophilised biomass per gram of beads. Biosorption kinetics has also been studied, and an original kinetic model was developed which is able to correlate experimental data. This model was developed from the Shrinking Core Model, considering a variable copper diffusion coefficient dependent on the process conversion. The estimated values for copper diffusion coefficient were obviously lower than copper diffusivity in water, and they depend on biomass concentration inside beads. Beads regeneration was studied using EDTA, HCl and CaCl 2 . Satisfactory biosorption performances were observed also after 10 sorption/desorption cycles, with CaCl 2 as regeneration solution. All the results confirmed the technical feasibility of the biosorption process by a polysulfone-entrapped biomass even though biosorption efficiency should be improved.

Reductive leaching of manganiferous ores by glucose and H2SO4: effect of alcohols

Abstract The effect of alcohols on the dissolution of manganese, calcium, and iron from manganiferous ore is reported. The extractive process was studied in sulphuric acid solution by using glucose as reducing agent. The alcohols were employed in order to evaluate their effect on the leaching performance with and without glucose as reducing agent. Three different alcohols MeOH, EtOH, and n-BuOH, were tested in order to investigate the influence of the organic chain length on the metal dissolution during the leaching process. Two different test sequences were followed. The first one was based on the leaching of manganiferous ore by using glucose as reducing agent and a mixture of alcohol/H2O/H2SO4 as reaction media. In the second one, leaching was carried out by using glucose in the H2O/H2SO4 reaction media and after leaching, the aqueous solution was separated from the solid by filtration. Successively, alcohols were added to the leach liquor. The experimental results of Mn, Ca, and Fe dissolution obtained by using the latter procedure were compared with those obtained in the former one. In both the experimental sequences, methyl alcohol gave better results as compared to ethyl and n-butyl alcohol. MeOH showed a notable negative effect on calcium dissolution, thereby decreasing its concentration with respect to manganese and iron extraction. The effect was larger in the runs performed by alcohol addition to the leach liquor than that obtained in the mixed solvent leaching tests. The decrease in calcium dissolution caused by the presence of alcohol has been analysed and modelled.

Process flow-sheet for gold and antimony recovery from stibnite

Abstract Recovery of gold from refractory ores requires a pretreatment to liberate the gold particles from the host mineral. In particular, in the case of stibnite (Sb 2 S 3 ), the antimony (Sb) forms stable compounds with sodium cyanide (NaCN) during the cyanidation process; as a consequence, cyanide consumption increases. Pretreatment is usually an oxidation step. As an alternative, chemical leaching can be applied to liberate the gold particles from the sulfur matrix. The aim of the present investigation was to ascertain — at laboratory scale — the best conditions for alkaline leaching of a refractory gold-bearing Sb 2 S 3 (13.25% Sb 2 S 3 ; 30 g t −1 Au) coming from South America. The solutions were constituted by sodium sulfide (Na 2 S) and sodium hydroxide (NaOH). Main parameters studied were: Na 2 S concentration, NaOH concentration, pulp density and temperature. After leaching, antimony has been recovered by electrodeposition, in order to increase the economical convenience of the subsequent gold extraction. Antimony recovery has been about 70% Sb for suitable conditions of leaching and electrowinning. Metallic antimony with high purity was obtained. After the study of the leaching parameters, the influence of the pretreatment on the cyanidation process has been evaluated. It was revealed that the chemical pretreatment improves the gold extraction yield and favours a low consumption of reagents: after cyanidation low recovery has been obtained without pretreatment (about 30% Au), while a high gold recovery was achieved in the case of the pretreated samples (about 75% Au), considering also the subsequent steps of carbon concentration–purification and electrowinning. In conclusion, experimental results have shown the technical feasibility of the alkaline leaching pretreatment prior to the conventional cyanidation; moreover, a complete process flow-sheet with low environmental impact, considering technical and economical factors, is proposed.