Differential pulse polarography: a method for the direct study of biosorption of metal ions by live bacteria from mixed metal solutions

The technique of differential pulse polarography is shown here to be applicable to the monitoring directly the biosorption of metal ions from solution by live bacteria from mixed metal solutions. Biosorption of Cd(II), Zn(II) and Ni(II) by P. cepacia was followed using data obtained at the potential which is characteristic of the metal ion in the absence and presence of cells. Hepes buffer (pH 7.4, 50 mM) was used as a supporting electrolyte in the polarographic chamber and metal ion peaks in the presence of cells of lower amplitude were obtained due to metal-binding by the cells. Well defined polarographic peaks were obtained in experiments involving mixtures of metal ions of Cd(II)-Zn(II), Cu(II)-Zn(II), Cu(II)-Cd(II) and Cd(II)-Ni(II). Biosorption of Cd(II), Zn(II) increased with solution pH. The method was also tested as a rapid technique for assessing removal of metal ions by live bacteria and the ability of the polarographic technique in measuring biosorption of metal ions from mixed metal solutions is demonstrated. Cu(II) was preferentially bound and removal of metals was in the order Cu(II) > Ni(II) > Zn(II), Cd(II) by intact cells of P. cepacia. This revised version was published online in August 2006 with corrections to the Cover Date.     

Comparison of the Heavy Metal Biosorption Capacity of Active,Heat‐Inactivated and NaOH‐Treated Phanerochaete chrysosporium Biosorbents

Three different kinds of Phanerochaete chrysosporium (NaOH‐treated, heat‐inactivated and active) biosorbent were used for the removal of Cd(II) and Hg(II) ions from aquatic systems. The biosorption of Cd(II) and Hg(II) ions on three different forms of Phanerochaete chrysosporium was studied in aqueous solutions in the concentration range of 50–700 mg/L. Maximum biosorption capacities of NaOH‐treated, heat‐inactivated and active Phanerochaete chrysosporium biomass were found to be 148.37 mg/g, 78.68 mg/g and 68.56 mg/g for Cd(II) as well as 224.67 mg/g, 122.37 mg/g and 88.26 mg/g for Hg(II), respectively. For Cd(II) and Hg(II) ions, the order of affinity of the biosorbents was arranged as NaOH‐treated > heat‐inactivated > active. The order of the amount of metal ions adsorbed was established as Hg(II) > Cd(II) on a weight basis, and as Cd(II) > Hg(II) on a molar basis. Biosorption equilibriums were established in about 60 min. The effect of the pH was also investigated, and maximum rates of biosorption of metal ions on the three different forms of Phanerochaete chrysosporium were observed at pH 6.0. The reusability experiments and synthetic wastewater studies were carried out with the most effective form, i.e., the NaOH‐treated Phanerochaete chrysosporium biomass. It was observed that the biosorbent could be regenerated using 10 mM HCl solution, with a recovery of up to 98%, and it could be reused in five biosorption‐desorption cycles without any considerable loss in biosorption capacity. The alkali‐treated Phanerochaete chrysosporium removed 73% of Cd(II) and 81% of Hg(II) ions from synthetic wastewater.     

Biosorption of Heavy Metals from Wastewater by Biosolids

In a study where the removal of heavy metals from wastewater is the primary aim, the biosorption of heavy metals onto biosolids prepared as Pseudomonas aeruginosa immobilized onto granular activated carbon was investigated in batch and column systems. In the batch system, adsorption equilibriums of heavy metals were reached between 20 and 50 min, and the optimal dosage of biosolids was 0.3 g/L. The biosorption efficiencies were 84, 80, 79, 59 and 42 % for Cr(VI), Ni(II), Cu(II), Zn(II) and Cd(II) ions, respectively. The rate constants of biosorption and pore diffusion of heavy metals were 0.013–0.089 min^–1 and 0.026–0.690 min^–0.5. In the column systems, the biosorption efficiencies for all heavy metals increased up to 81–100 %. The affinity of biosorption for various metal ions towards biosolids was decreased in the order: Cr = Ni > Cu > Zn > Cd.     

Biosorption of cobalt by fungi from serpentine soil of Andaman

Fungi belonging to Aspergillus, Mortierella, Paecilomyces, Penicillium, Pythium, Rhizopus and Trichoderma, isolated from serpentine soil of Andaman (India) were screened for cobalt-resistance. Eleven out of total 38 isolated fungi which tolerated > 6.0 mM Co(II) were evaluated for cobalt biosorption using dried mycelial biomass. Maximum Co(II)-loading (1036.5 microM/g, 60 min) was achieved with Mortierella SPS 403 biomass, which removed almost 50% of 4.0 mM cobalt from the aqueous solution. Co(II)-sorption kinetics of Mortierella SPS 403 biomass was fast and appreciable quantities of metal [562.5 microM/g] was adsorbed during first 10 min of incubation. The metal biosorption capacity of the isolate was accelerated with increasing cobalt concentration, while it was reverse with increase of initial biomass. The optimum pH and temperature for Co(II) removal were 7.0 and 30 degrees C, respectively. However, Co(II)-uptake was inhibited in presence of other metals (Pb, Cd, Cu, Ni, Cr and Zn). Freundlich adsorption isotherm appropriately describes Mortierella SPS 403 biomass as an efficient Co(II)-biosorbent.     

Concurrent sorption of Zn(II), Cu(II) and Co(II) by Oscillatoria angustissima as a function of pH in binary and ternary metal solutions

This paper reports biosorption of Zn(II), Cu(II) and Co(II) onto O. angustissima biomass from single, binary and ternary metal solutions, as a function of pH and metal concentrations via Central Composite Design generated by statistical software package Design Expert 6.0. The experimental design revealed that metal interactions could be best studied at lower pH range i.e. 4.0-5.0, which facilitates adequate availability of all the metal ions. The sorption capacities for single metal decreased in the order Zn(II)>Co(II)>Cu(II). In absence of any interfering metals, at pH 4.0 and an initial metal concentration of 0.5 mM in the solution, the adsorption capacities were 0.33 mmol/g Zn(II), 0.26 mmol/g Co(II) and 0.12 mmol/g Cu(II). In a binary system, copper inhibited both Zn(II) and Co(II) sorption but the extent of inhibition of former was greater than the latter; sorption values being 0.14 mmol/g Zn(II) and 0.27 mmol/g Co(II) at initial Zn(II) and Co(II) concentration of 1.5 mM each, pH 4.0 and 1mM Cu(II) as the interfering metal. Zn(II) and Co(II) were equally antagonistic to each others sorption; Zn(II) and Co(II) sorption being 0.23 and 0.24 mmol/g, respectively, at initial metal concentration of 1.5 mM each, pH 4.0 and 1mM interfering metal concentration. In contrast, Cu(II) sorption remained almost unaffected at lower concentrations of the competing metals. Thus, in binary system inhibition dominance observed was Cu(II)>Zn(II), Cu(II)>Co(II) and Zn(II) approximately Co(II), due to this the biosorbent exhibited net preference/affinity for Cu(II) sorption over Zn(II) or Co(II). Hence, the affinity series showed a trend of Cu(II)>Co(II)>Zn(II). In a ternary system, increasing Co(II) concentration exhibited protection against the inhibitory effect of Cu(II) on Zn(II) sorption. On the other hand, the inhibitory effect of Zn(II) and Cu(II) on Co(II) sorption was additive. The model equation for metal interactions was found to be valid within the design space.     

Biotechnological potential of Microcystis sp. in Cu, Zn and Cd biosorption from single and multimetallic systems

This paper provides information on biosorption of Cu, Zn and Cd by Microcystis sp. in single, bi and trimetallic combination. Highest biosorption of Cu followed by Zn and Cd in single as well as in mixtures containing two or three metals was noticed. The order of inhibition of Cu, Zn and Cd biosorption in bi and trimetallic combinations was suggestive of screening or competition for the binding sites on the cell surface. This observation was reconfirmed by Freundlich adsorption isotherm. K_f values were maximum for Cu (K_f=45.18), followed by Zn (K_f=16.71), and Cd (K_f=15.63) in single metallic system. The K_f values for each test metal was reduced in solution containing more than one metal. Further, the reduction in biosorption of each metal ion due to presence of other metal ion was of greater magnitude at relatively higher concentrations of interfering metal ion. The biosorption of Cu at saturation was less affected when secondary metal (Cd or Zn) was added in the medium. Above results suggest that Microcystis holds great potential for metal biosorption from mixture.     

Biosorption of heavy metals by lyophilized cells of <Emphasis Type="Italic">Pseudomonas stutzeri</Emphasis>

Biosorptive capacity of Pb(II), Cd(II) and Cu(II) by lyophilized cells of Pseudomonas stutzeri was investigated based on Langmuir and Freundlich isotherms. Biosorptive capacity for Pb(II), Cd(II) and Cu(II) decreased with an increase of metal concentration, reaching 142, 43.5 and 36.2 mg/g at initial concentration of 300 mg/l, respectively. Biosorption capacity for metal ions increased with increasing pH. The optimum pH for biosorption rate of Cd(II) and Cu(II) were 5.0, and 6.0 for Pb(II) biosorption. The experimental data showed a better fit with the Langmuir model over the Freundlich model for metal ions throughout the range of initial concentrations. The maximum sorptive capacity (q _max) obtained from the Langmuir equation for Pb(II), Cd(II) and Cu(II) were 153.3 (r ²  = 0.998), 43.86 (r ²  = 0.995), and 33.16 (r ²  = 0.997) for metal ions, respectively. The selectivity order for metal ions towards the biomass of P. stutzeri was Pb(II) > Cd(II) > Cu(II) for a given initial metal ions concentration. The interactions between heavy metals and functional groups on the cell wall surface of bacterial biomass were confirmed by FTIR analysis. The results of this study indicate the possible removal of heavy metals from the environment by using lyophilized cells of P. stutzeri.     

Biosorption of copper by marine algae Gelidium and algal composite material in a packed bed column

Marine algae Gelidium and algal composite material were investigated for the continuous removal of Cu(II) from aqueous solution in a packed bed column. The biosorption behaviour was studied during one sorption–desorption cycle of Cu(II) in the flow through column fed with 50 and 25 mg l⁻¹ of Cu(II) in aqueous solution, at pH 5.3, leading to a maximum uptake capacity of ≈13 and 3 mg g⁻¹, respectively, for algae Gelidium and composite material. The breakthrough time decreases as the inlet copper concentration increases, for the same flow rate. The pH of the effluent decreases over the breakthrough time of copper ions, which indicates that ion exchange is one of the mechanisms involved in the biosorption process. Temperature has little influence on the metal uptake capacity and the increase of the ionic strength reduces the sorption capacity, decreasing the breakthrough time. Desorption using 0.1 M HNO₃ solution was 100% effective. After two consecutive sorption–desorption cycles no changes in the uptake capacity of the composite material were observed. A mass transfer model including film and intraparticle resistances, and the equilibrium relationship, for adsorption and desorption, was successfully applied for the simulation of the biosorption column performance.     

Biosorption of heavy metals by the exopolysaccharide produced by <Emphasis Type="Italic">Paenibacillus jamilae</Emphasis>

The biosorption of several toxic heavy metals (Pb, Cd, Co, Ni, Zn and Cu) by the exopolysaccharide (EPS) produced by Paenibacillus jamilae, a potential biosorbent for metal remediation and recovery was studied. Firstly, the biochemical composition of this bacterial polymer was determined. Glucose was the most abundant neutral sugar, followed by galactose, rhamnose, fucose and mannose. The polymer presented a high content of uronic acids (28.29%), which may serve as binding sites for divalent cations. The presence of carboxylic groups was also detected by infrared spectroscopy. The EPS presented an interesting affinity for Pb in comparison with the other five metals. Lead biosorption (303.03 mg g⁻¹) was tenfold higher (in terms of mg of metal adsorbed per gram of EPS) than the biosorption of the rest of metals. Biosorption kinetics, the effect of pH and the effect of competitive biosorption were determined. Finally, we found that the EPS was able to precipitate Fe(III), but the EPS-metal precipitate did not form with Fe(II), Pb(II), Cd(II), Co(II), Ni(II), Cu(II) and Zn(II).     

Investigation of pH-dependent DNA-metal ion interactions by surface plasmon resonance

Ni(II) and Zn(II) M-DNA formation and denaturation of double-stranded DNA (dsDNA) by Cd(2+) were monitored by surface plasmon resonance (SPR). When exposed to immobilized 30 bp 50% GC dsDNA, Zn(2+) and Ni(2+) were found to give signals indicative of a conformational change at pH 8.5 but not 7.5, while Mg(2+) and Ca(2+) caused small changes at both pHs. The concentrations that gave 50% of the maximum responses were 0.06 and 0.50 mM for Zn(2+) and Ni(2+), respectively. At pH 8.5, Cd(2+) denatured over 40% of the dsDNA, while other metals denatured less than 5% of the DNA. Smaller pH-dependent signals were induced by Zn(2+), Ni(2+) or Cd(2+) with 50% GC single-stranded DNA (ssDNA), and with a homopolymer of d(T)30. Homopolymers d(A)30 and d(C)30 showed small signals that were largely independent of pH in the presence of Zn(2+) or Ni(2+).     

Biosorption of nickel and cadmium by metal resistant bacterial isolates from agricultural soil irrigated with industrial wastewater

Agricultural soil irrigated with industrial wastewater (more than two decades) analysed for heavy metals revealed high levels of Fe, Cr, Cu, Zn, Ni and Cd. Out of a total of 40 bacterial isolates obtained from these soils, 17 belonged to the family enterobacteriaceae and 10 were Pseudomonas spp. A maximum MIC of 200 for Cd, 400 for Zn and Cu, 800 for Ni, and 1600 microg/ml for Pb was observed. Biosorption of Ni and Cd studies over a range of metal ion concentrations with Escherichia coli WS11 both in single and bi-metal systems showed that the adsorption of Cd and Ni was dependent on the concentrations and followed the Freundlich adsorption isotherm. The biosorption of Ni increased from 6.96 to 55.31 mg/g of cells, and Cd from 4.96 to 45.37 mg/g of cells at a concentration ranging from 50 to 400 microg/ml after 2h of incubation in a single metal solution. A further increase in incubation time had no significant effect on the biosorption of metals.     

Localization effect on the metal biosorption capability of recombinant mammalian and fish metallothioneins in Escherichia coli

In this study, we examined the expression of mammalian and fish metallothioneins (MTs) in Escherichia coli as a strategy to enhance metal biosorption efficiency of bacterial biosorbents for lead (Pb), copper (Cu), cadmium (Cd), and zinc (Zn). In addition, MT proteins were expressed in either the cytoplasmic or periplasmic compartment of host cells to explore the localization effect on metal biosorption. The results showed that MT expression led to a significant increase (5-210%) in overall biosorption efficiency (eta(ads)), especially for biosorption of Cd. The MT-driven improvement in metal biosorption relied more on the increase in the biosorption rates (r(2), a kinetic property) than on the equilibrium biosorption capacities (q(max), a thermodynamic property), despite a 10-45% and 30-80% increase in q(max) of Cd and Zn, respectively. Periplasmic expression of MTs appeared to be more effective in facilitating the metal-binding ability than the cytoplasmlic MT expression. Notably, disparity of the impacts on biosorption ability was observed for the origin of MT proteins, as human MT (MT1A) was the most effective biosorption stimulator compared to MTs originating from mouse (MT1) and fish (OmMT). Moreover, the overall biosorption efficiency (eta(ads)) of the MT-expressing recombinant biosorbents was found to be adsorbate-dependent: the eta(ads) values decreased in the order of Cd > Cu > Zn > Pb.     

Three high-resolution crystal structures of cadmium-substituted carboxypeptidase A provide insight into the enzymatic function

Three high-resolution crystal structures of Cd(II)-substituted carboxypeptidase A (CPA) have been determined by X-ray diffraction from crystals prepared in three different buffer systems to assess the effect of pH and ionic strength on the Cd(II) coordination geometry. All crystallize in the space group P2(1) with identical cell dimensions. Cd-CPA(7.5): Cd(II)-substituted CPA prepared at pH 7.5 with [Cl(-)]=2 mM determined to 1.70 A resolution ( R=17.4% and R(free)=19.8%); Cd-CPA(5.5): Cd(II)-substituted CPA prepared at pH 5.5 with [Cl(-)]=2 mM to 2.00 A resolution ( R=16.1% and R(free)=18.6%); Cd-CPA(7.5)-Cl: Cd(II)-substituted CPA prepared at pH 7.5 with [Cl(-)]=250 mM to 1.76 A resolution ( R=16.7% and R(free)=17.8%). No noticeable structural changes were observed between the three structures. Two water molecules coordinate to Cd(II), in contrast to the single water molecule coordinating to Zn(II) in the Zn-CPA structure. No binding sites for anions could be identified, even in the structure with a high concentration of chloride ions. It is suggested that the anion inhibition is due to weak outer-sphere association of Cl(-) ions at several binding sites, shielding the strong positive charge distribution at the surface of the protein near the active site. Based on structural data and a sequence alignment of 18 non-redundant carboxypeptidases, a more elaborate version of the earlier reaction model is proposed that also addresses the transport of water to and from the active site. Conserved residues whose function was not addressed previously delineate the proposed pathways used in the transport of water during catalysis.     

Cadmium biosorption on Spirulina platensis biomass

Dry biomass of Spirulina platensis re-hydrated for 48 h was employed as a biosorbent in tests of cadmium(II) removal from water. Various concentrations of biomass (from 1 to 4 g l⁻¹) and metal (from 100 to 800 mg l⁻¹) were tested. Low biomass levels (X_o  2 g l⁻¹) ensured metal removal up to 98% only at Cd₀= 100 and 200 mg l⁻¹, while X_o  2.0 g l⁻¹ were needed at Cd₀ = 400 mg l⁻¹ to achieve satisfactory results. Whereas X_o = 4.0 g l⁻¹ was effective to remove up to Cd₀ = 500 mg l⁻¹, a further increase in metal concentration (Cd₀ = 600 and 800 mg l⁻¹) led to progressive worsening of the system performance. At a given biomass levels, the kinetics of the process was better at low Cd²⁺ concentrations, while, raising the adsorbent level from 1.0 to 2.0 g l⁻¹ and then to 4.0 g l⁻¹, the rate constant of biosorption increased by about one order of magnitude in both cases and the adsorption capacity of the system progressively decreased from 357 to 149 mg g⁻¹.     

Using biosorption to enrich the biomass of <Emphasis Type="Italic">Chlorella vulgaris </Emphasis>with microelements to be used as mineral feed supplement

The paper discusses biosorption of Cr(III), Cu(II), Mn(II), Zn(II) and Co(II) to the biomass of Chlorella vulgaris, to produce a biologically bound, concentrated form of microelements. The kinetics of biosorption was described with a pseudo-second order equation and equilibrium with the Langmuir isotherm. The mechanism of biosorption was identified as cation-exchange with alkaline metals. Cation-exchange capacity was evaluated as 4.07 meq g⁻¹. The effect of operation conditions, pH and temperature, on biosorption performance was investigated and the best operation conditions for biosorption were selected (pH 5, temperature 25 °C). The maximum sorption capacity of microelements was determined in single-metal system at pH 5 and 25 °C: Zn(II) 3.30 meq g⁻¹, Cu(II) 1.77 meq g⁻¹, Co(II) 1.75 meq g⁻¹, Cr(III) 1.74 meq g⁻¹, Mn(II) 0.764 meq g⁻¹. Biosorption experiments were also carried out in multi-metal system. The biomass of C. vulgaris enriched with microelements via the process of biosorption in both single- and multi-metal system was discussed in terms of preparation of feed supplement for laying hens and piglets. The experiments showed that 1 kg of conventional feed for laying hens can be supplemented with 0.20 g of the biomass enriched with microelements and for piglets with 0.15 g of the preparation.     

Experimental and theoretical approaches for Cd(II) biosorption from aqueous solution using Oryza sativa biomass

Biomass of Oryza sativa (OS) was tested for the removal of Cd(II) ions from synthetic and real wastewater samples. Batch experiments were conducted to investigate the effects of operating parameters on Cd(II) biosorption. Fourier transform infrared spectroscopy, scanning electron microscopy, and energy-dispersive x-ray spectroscopy were used to examine the surface characteristics of the Cd(II)-loaded biomass. The maximum removal efficiency of Cd(II) was 89.4% at optimum pH 6.0, biosorbent dose 10.0 g L^?1, initial Cd(II) 50 mg L^?1, and biosorbent particle size 0.5 mm. The applicability of Langmuir and Freundlich isotherms to the sorbent system implied the existence of both monolayer and heterogeneous surface conditions. Kinetic studies revealed that the adsorption process of Cd(II) followed the pseudo-second-order model (r2: 0.99). On the theoretical side, an adaptive neuro-fuzzy inference system (ANFIS) was applied to select the operating parameter that mostly influences the Cd(II) biosorption process. Results from ANFIS indicated that pH was the most influential parameter affecting Cd(II) removal efficiency, indicating that the biomass of OS was strongly pH sensitive. Finally, the biomass was confirmed to adsorb Cd(II) from real wastewater samples with removal efficiency close to 100%. However, feasibility studies of such systems on a large-scale application remain to be investigated.     

Biosorption of cadmium(II), lead (II) and copper(II) with the filamentous fungus Phanerochaete chrysosporium

The biosorption from artificial wastewaters of heavy metals (Cd(II), Pb(II) and Cu(II)) onto the dry fungal biomass of Phanerochaete chryosporium was studied in the concentration range of 5-500 mg l(-1). The maximum absorption of different heavy metal ions on the fungal biomass was obtained at pH 6.0 and the biosorption equilibrium was established after about 6 h. The experimental biosorption data for Cd(II), Pb(II) and Cu(II) ions were in good agreement with those calculated by the Langmuir model.     

Removal of cadmium(II) ion from aqueous system by dry biomass, immobilized live and heat-inactivated Oscillatoria sp. H1 isolated from freshwater (Mogan Lake)

Oscillatoria sp. H1 (Cyanobacteria, microalgae) isolated from Mogan Lake was used for the removal of cadmium ions from aqueous solutions as its dry biomass, alive and heat-inactivated immobilized form on Ca-alginate. Particularly, the effect of physicochemical parameters like pH, initial concentration and contact time were investigated. The sorption of Cd(II) ions on the sorbent used was examined for the cadmium concentrations within the range of 25-250 mg/L. The biosorption of Cd(II) increased as the initial concentration of Cd(II) ions increased in the medium up to 100 mg/L. Maximum biosorption capacities for plain alginate beads, dry biomass, immobilized live Oscillatoria sp. H1 and immobilized heat-inactivated Oscillatoria sp. H1 were 21.2, 30.1, 32.2 and 27.5 mg/g, respectively. Biosorption equilibrium was established in about 1 h for the biosorption processes. The biosorption was well described by Langmuir and Freundlich adsorption isotherms. Maximum adsorption was observed at pH 6.0. The alginate-algae beads could be regenerated using 50 mL of 0.1 mol/L HCl solution with about 85% recovery.     

Cadmium and zinc removal from aqueous solutions by Bacillus jeotgali: pH, salinity and temperature effects

Effects of pH, salinity and temperature on biosorption of Cd and Zn by bacteria Bacillus jeotgali strain U3 were evaluated in batch experiments. Traditional and Subsequent Addition Methods (SAM) were used to carry out the bioassays. Sorption of metals was higher when pH or temperature was increased, or when salinity was reduced. The Langmuir isotherm better fit the biosorption data for Cd, while the Freundlich model fitted better for Zn biosorption. A comparison with similar biosorbents suggested that Bacillus jeotgali strain U3 could be considered a good biosorbent for Cd and Zn recovery.     

Zn(II)- and Cu(II)-induced non-fibrillar aggregates of amyloid-β (1–42) peptide are transformed to amyloid fibrils, both spontaneously and under the influence of metal chelators

Aggregation of amyloid-β (Aβ) peptides is a central phenomenon in Alzheimer's disease. Zn(II) and Cu(II) have profound effects on Aβ aggregation; however, their impact on amyloidogenesis is unclear. Here we show that Zn(II) and Cu(II) inhibit Aβ₄₂ fibrillization and initiate formation of non-fibrillar Aβ₄₂ aggregates, and that the inhibitory effect of Zn(II) (IC₅₀ = 1.8 μmol/L) is three times stronger than that of Cu(II). Medium and high-affinity metal chelators including metallothioneins prevented metal-induced Aβ₄₂ aggregation. Moreover, their addition to preformed aggregates initiated fast Aβ₄₂ fibrillization. Upon prolonged incubation the metal-induced aggregates also transformed spontaneously into fibrils, that appear to represent the most stable state of Aβ₄₂. H13A and H14A mutations in Aβ₄₂ reduced the inhibitory effect of metal ions, whereas an H6A mutation had no significant impact. We suggest that metal binding by H13 and H14 prevents the formation of a cross-β core structure within region 10–23 of the amyloid fibril. Cu(II)-Aβ₄₂ aggregates were neurotoxic to neurons in vitro only in the presence of ascorbate, whereas monomers and Zn(II)-Aβ₄₂ aggregates were non-toxic. Disturbed metal homeostasis in the vicinity of zinc-enriched neurons might pre-dispose formation of metal-induced Aβ aggregates, subsequent fibrillization of which can lead to amyloid formation. The molecular background underlying metal-chelating therapies for Alzheimer's disease is discussed in this light.