Francesca Pagnanelli

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.

Olive mill solid residues as heavy metal sorbent material: a preliminary study.

Biosorption of heavy metals is an innovative and alternative technology to remove these pollutants from aqueous solutions using inactive and dead biomasses such as agricultural and industrial wastes, algae and bacteria. In this study olive mill solid residue was used as heavy metal adsorbent material for its wide availability as agricultural waste and also for its cellulosic matrix, rich of potential metal binding active sites. Preliminary studies concerned with the removal of different heavy metals (Hg, Pb, Cu, Zn and Cd), the effect of pre-treatments by water and n-hexane and the regeneration possibility. Olive mill solid residue resulted able to remove heavy metals from aqueous solutions with an affinity series reflecting the hydrolytic properties of the metallic ions, but also a particular affinity for copper. It can be supposed that biosorption phenomenon occur by a general ion exchange mechanism combined with a specific complexation reaction for copper ions. Water pre-treatment is sufficient to reduce COD release in the effluent according to the law limit, while n-hexane pre-treatment strongly reduces also the adsorption properties of this material. Experimental isotherms obtained under different operating conditions were fitted using a non linear regression method for the estimation of the Langmuir parameters. Moreover a simple Scatchard plot analysis was performed for a preliminary investigation of the active sites, showing the presence of two different site affinities depending on the metal concentration, according to the previous hypothesis of two kinds of uptake mechanisms for copper biosorption. Regeneration tests gave good results in terms of yield of regeneration and also concentration ratios.

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.

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.

Mechanisms of heavy-metal removal by activated sludge.

This paper investigates the chemical mechanisms operating in cadmium and lead removal by activated sludge in sequencing batch reactors. Selective extraction and acid digestion of sludge samples denoted that both Cd and Pb are mainly present as surface-bound metals. Characterisation of sludge samples by potentiometric titrations and IR spectra suggested that carboxylic and amino groups are the main active sites responsible for the binding properties of the biomass. Simulation of metal speciation implemented with complexation constants determined in biosorption tests, showed that cadmium predominates as biosorbed species, while lead was mainly removed by precipitation.

Chemical treatment of olive pomace : Effect on acid-basic properties and metal biosorption capacity

In this study, olive pomace, an agricultural waste that is very abundant in Mediterranean area, was modified by two chemical treatments in order to improve its biosorption capacity. Potentiometric titrations and IR analyses were used to characterise untreated olive pomace (OP), olive pomace treated by phosphoric acid (PAOP) and treated by hydrogen peroxide (HPOP). Acid-base properties of all investigated biosorbents were characterised by two main kinds of active sites, whose nature and concentration were determined by a mechanistic model assuming continuous distribution for the proton affinity constants. Titration modelling denoted that all investigated biosorbents (OP, PAOP and HPOP) were characterised by the same kinds of active sites (carboxylic and phenolic), but with different total concentrations with PAOP richer than OP and HPOP. Single metal equilibrium studies in batch reactors were carried out to determine the capacity of these sorbents for copper and cadmium ions at constant pH. Experimental data were analysed and compared using the Langmuir isotherm. The order of maximum uptake capacity of copper and cadmium ions on different biosorbents was PAOP>HPOP>OP. The maximum adsorption capacity of copper and cadmium, was obtained as 0.48 and 0.10 mmol/g, respectively, for PAOP. Metal biosorption tests in presence of Na(+) in solution were also carried out in order to evaluate the effect of chemical treatment on biomass selectivity. These data showed that PAOP is more selective for cadmium than the other sorbents, while similar selectivity was observed for copper.