Luigi Toro

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.

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.

Biotreatment and bioassessment of heavy metal removal by sulphate reducing bacteria in fixed bed reactors.

In this work a batch-optimised mixture (w/w %: 6% leaves, 9% compost, 3% Fe(0), 30% silica sand, 30% perlite, 22% limestone) was investigated in a continuous fixed bed column reactor for the treatment of synthetic acid-mine drainage (AMD). A column reactor was inoculated with sulphate-reducing bacteria and fed with a solution containing sulphate and heavy metals (As(V), Cd, Cr(VI), Cu and Zn). At steady state, sulphate abatement was 50+/-10%, while metals were totally removed. A degradation rate constant (k) of 0.015+/-0.001h(-1) for sulphate removal was determined from column data by assuming a first order degradation rate. Reduction of AMD toxicity was assessed by using the nematode Caenorhabditis elegans as a test organism. A lethality assay was performed with the toxicants before and after the treatment, showing that only 5% of the animals were still alive after 48h in presence of the contaminants, while the percentage increased to 73% when the nematodes were exposed to the solution eluted from the column.

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.

Acid leaching process by using glucose as reducing agent: A comparison among the efficiency of different kinds of manganiferous ores

Abstract The experimental results of a leaching process of different manganiferous ores are here reported. The leaching treatment showed a satisfactory selectivity of manganese extraction in comparison of other metals such as Al, Si and Na. XRD analyses have been done on the ores before and after the leaching treatment. The experimental results showed a good technical feasibility of the process which can use alternative feedstocks (containing carbohydrates) to treat different manganese ores.