Cobalt Salts Research Articles

Co9S8/NC@FeCoS2/NC composites with hollow yolk shell structure as the counter electrode for Pt-free dye-sensitized solar cells

Carbon-based materials derived from metal-organic framework compounds have tunable chemical composition, morphology, and electrochemically active surface combined with good electrical conductivity and stability. In this work, Fe(OH)3 grows on a zeolitic imidazolate framework, 2-Methylimidazole cobalt salt (ZIF-67), through oxidation and deposition. The Co9S8/NC@FeCoS2/NC composites with hollow yolk shell structure (HYS-Co9S8/NC@FeCoS2/NC) is prepared through solvothermal sulfidation combined with high-temperature carbonization using ZIF-67@Fe(OH)3 as the precursor. A dye-sensitized solar cell assembled with HYS-Co9S8/NC@FeCoS2/NC as a counter electrode achieves a photovoltaic conversion efficiency of 7.07% (PCEPt = 7.36%). The reason for this phenomenon is that the hollow yolk shell structure of Co9S8/NC@FeCoS2/NC material has a high specific surface area, which affords a good deal of active sites and promotes the contact between electrolyte and catalytic sites. The Fe-Co bimetal center synergies with each other to catalyze the reduction of iodide ion, showing a high catalytic activity. Besides, the material has good electrochemical stability in iodide-based electrolysis.

Electrochemical co-deposition of cobalt and graphene, produced from recycled polypropylene, on TiO2 nanotube as a new catalyst for photoelectrochemical water splitting

In this study, to improve the photoelectrochemical properties of TiO2 nanotubes (TNT), the hybrid TNT-graphene-Co(OH)2 was prepared using electrodeposition of graphene and cobalt salt. The employed graphene was obtained from the waste polypropylene using chemical vapor deposition method. The hybrid material showed high thermal stability and promising visible light absorption. Moreover, its XRD pattern and Raman spectrum are in accordance with the precursors. The photoelectrochemical performance of this hybrid material was investigated using linear sweep voltammetry, transient open circuit potential, and chronoamperometry. Because of the existence of graphene, the prepared hybrid material exhibited excellent photoelectrochemical properties and the results were more appropriate than those of TNT and cobalt deposited TNT. Among the various samples with different amounts of graphene, the sample with the highest graphene content showed the highest photoelectrochemical abilities. This improvement is mainly related to the visible-light harvesting ability of graphene and facile recombination of photogenerated electron-hole pairs due to the synergistic effect of the involved materials.

Effect of various transition metal ions on mordanting stage of yak wool bleaching process

Yak wool is a smooth, warm, and durable natural protein-structured fiber that could compete with cashmere and other high-end protein-structured fibers on the market. However, it suffers from drawing consumers’ attention due to the lack of color due to the shortfall of the yak wool bleaching technology. Herein, we studied the applicability of various transition metals, i.e., copper (II), cobalt (II), iron (II), and nickel (II) salts, as a mordanting reagent based on their effect on the hydrogen peroxide decomposition reaction and the morphological and mechanical properties of the bleached yak wool with the presence of these transition metal. Our study suggested that the iron (II) ion was the most efficient reagent for the mordant bleaching since it provided less fiber damage, relatively high strength, and elongation to the bleached yak wool with good whiteness, while the Cu (II) was the least favorable agent for the yak wool bleaching process.

Cobalt salts in feed mixes for bull-calves

Cobalt salts in feed mixes for bull-calves

Micro-Structural Design of CoFe2O4/SWCNTs Composites for Enhanced Electromagnetic Properties

In order to prepare microwave-absorbing materials with low density and high wave absorption performance, CoFe2O4/SWCNTs composites with well-designed necklace-like structures were successfully prepared in this paper by a simple solvothermal method. CoFe2O4/SWCNTs composites with different cobalt salt contents were synthesized by adjusting the experimental parameters. The results of the relative complex permeability and relative permittivity of the samples, which were investigated by vector network analysis in the frequency range of 2 to 18 GHz, are collected to support the study of the microwave absorption characteristics of the samples. Different microsphere densities and different cobalt salt contents have obvious differences in the electromagnetic absorption properties of the composites. When the additions of FeCl3·6H2O, Co(Ac)2·4H2O, and NH4Ac were 0.432, 0.200, and 0.400 g, respectively, the best reflection loss reached −42.07 dB, and the effective absorption frequency (RL < −10 dB) ranges from 3.2 to 18 GHz. Therefore, this is a preparation strategy of CoFe2O4/SWCNTs composites with necklace structure, which has the advantages of simple process, environmental friendliness, low cost, and high stability. The unique necklace-like structure design makes the carbon nanotubes partially exposed, which is more beneficial to achieve good impedance matching and giving the CoFe2O4/SWCNTs composite excellent electromagnetic loss capability.

Carbonylative Ring Expansion of Epoxides to β‐Lactones Using Inorganic Salt as Catalytic Species Precursor

AbstractTransition‐metal‐catalyzed carbonylative transformation is a well‐established and indispensable tool in organic chemistry, partially due to that CO is a cheap and readily available C1‐building block. The β‐lactone products via carbonylative ring expansion of epoxide are particularly attractive, because they feature broad utility in organic and polymer chemistry. One of the major challenges for this promising reaction is the usage of toxic, moderately air sensitive, unstable and pyrophoric low‐valent cobalt catalysts. Inorganic salts feature high abundance in nature, and the generation of catalytic species from in situ reduction of divalent cobalt(II) salt has received intense attention, which avoids the unnecessary storage and transport of sensitive low‐valent cobalt compounds. Herein, we have employed this strategy for the selectively carbonylative ring expansion of epoxides under an extremely mild condition, e. g. 40 °C and 1 atm CO pressure. This method eliminates the usage of extremely sensitive cobalt carbonyl catalyst. In addition, this reaction is tolerant to a variety of epoxides, and up to 15 examples of β‐lactones with excellent functionality have been isolated in moderate to excellent yields and selectivity.

Cobalt and Nickel Free Disorder Rock Salt Cathodes – Recent Developments

Disordered Li-excess rock salt (DRX) cathodes are a promising class of high capacity (and voltage) cathodes for next-generation Li-ion batteries. Compared to conventional Li-ion cathodes which are generally restricted to Co/Ni-based compositions with specific cation ordering, DRX materials have a wide compositional design space based on earth abundant metals such as Mn, Ti, Mo, Al, Zr, V, and Nb. Furthermore, Li-rich oxyfluoride DRX cathodes have demonstrated specific energies up to 1,000 Wh/kg which exceeds that of state-of-the-art layered LiNixMnyCo1-x-yO2 (NMC) cathodes (~700 Wh/kg). Despite such scientific advancements, widespread adoption of DRX cathodes has been hindered by several technical challenges. First, most of the high fluorine content DRX compositions are synthesized using mechano-chemical synthesis that has inherent limitation in scalability and maintaining a uniform particle-size and morphology. Further, DRX compositions have about 3-4 order lower electronic conductivity compared to layered cathodes requiring a high carbon loading in the electrode (up to 25%) compromising energy density. The focus of this talk will be on developing alternate synthesis method for Li-Mn rich DRX compositions. Specifically solid-state and sol-gel synthesis methods to produce DRX cathodes with the nominal composition Li1.2Mn0.4+xTi0.4-xO2-xFx (x = 0-0.3). Detailed advanced characterization (i.e., neutron total scattering and electron microscopy) and modelling efforts (i.e., Reverse Monte Carlo simulations) to understand how SRO impacts cathode performance will be presented. Different choice of precursors based on thermodynamic and kinetic consideration that minimizes formation of LiF impurity phases will be proposed.AcknowledgmentThis work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Vehicle Technologies Office, under the Applied Battery Materials Program, of the U.S. Department of Energy under Contract No. DE-AC05-00OR22725

In-depth investigations of size and occupancies in cobalt ferrite nanoparticles by joint Rietveld refinements of X-ray and neutron powder diffraction data.

Powder X-ray diffraction (PXRD) and neutron powder diffraction (NPD) have been used to investigate the crystal structure of CoFe2O4 nanoparticles prepared via different hydro-thermal synthesis routes, with particular attention given to accurately determining the spinel inversion degrees. The study is divided into four parts. In the first part, the investigations focus on the influence of using different diffraction pattern combinations (NPD, Cu-source PXRD and Co-source PXRD) for the structural modelling. It is found that combining PXRD data from a Co source with NPD data offers a robust structural model. The second part of the study evaluates the reproducibility of the employed multipattern Rietveld refinement procedure using different data sets collected on the same sample, as well as on equivalently prepared samples. The refinement procedure gives reproducible results and reveals that the synthesis method is likewise reproducible since only minor differences are noted between the samples. The third part focuses on the structural consequences of (i) the employed heating rate (achieved using three different hydro-thermal reactor types) and (ii) changing the cobalt salt in the precursors [aqueous salt solutions of Co(CH3COOH)2, Co(NO3)2 and CoCl2] in the synthesis. It is found that increasing the heating rate causes a change in the crystal structure (unit cell and crystallite sizes) while the Co/Fe occupancy and magnetic parameters remain similar in all cases. Also, changing the type of cobalt salt does not alter the final crystal/magnetic structure of the CoFe2O4 nanoparticles. The last part of this study is a consideration of the chemicals and parameters used in the synthesis of the different samples. All the presented samples exhibit a similar crystal and magnetic structure, with only minor deviations. It is also evident that the refinement method used played a key role in the description of the sample.

Dehydrogenative Synthesis of Quinolines and Quinazolines via Ligand-Free Cobalt-Catalyzed Cyclization of 2-Aminoaryl Alcohols with Ketones or Nitriles

We present a convenient and efficient protocol to synthesize quinolines and quinazolines in one pot under mild conditions. A variety of substituted quinolines were synthesized in good to excellent yields (up to 97% yield) from the dehydrogenative cyclizations of 2-aminoaryl alcohols and ketones catalyzed by readily available Co(OAc)2·4H2O. This cobalt catalytic system also showed high activity in the reactions of 2-aminobenzyl alcohols with nitriles, affording various quinazoline derivatives (up to 95% yield). The present protocol offers an environmentally benign approach for the synthesis of N-heterocycles by employing an earth-abundant cobalt salt under ligand-free conditions.

Co nanoparticles encapsulated in nitrogen-doped nanocarbon derived from cobalt-modified covalent organic framework as peroxymonosulfate activator for sulfamerazine degradation

In this work, new covalent organic framework (COF) materials with high crystallinity and large specific surface areas were used as cobalt-loaded supports. Cobalt nanoparticle-embedded nitrogen-doped carbon porous catalysts (Co@COF) were obtained by high-temperature pyrolysis of the COF materials and cobalt salts. With multiple active sites (Co0, pyridine N, and graphitic N), Co@COF exhibited superior catalytic properties for peroxymonosulfate (PMS) activation toward the degradation of sulfamerazine (SMZ). The efficiency of SMZ degradation reached 92.4 % within 10 min and the total organic carbon (TOC) removal rate reached 70.3 % within 30 min. Using radical quenching experiments and electron paramagnetic resonance (EPR) analysis, sulfate radicals (SO4•−), hydroxyl radicals (•OH), and singlet oxygen (1O2) were identified in this system, while 1O2 was the dominant active species for SMZ removal. The degradation intermediates of SMZ in Co@COF/PMS were monitored by high performance liquid chromatography–time of flight mass spectrometry (HPLC–TOFMS).

Redox Conversion of Cobalt(II)‐Diselenide to Cobalt(III)‐Selenolate Compounds: Comparison with Their Sulfur Analogs

AbstractThe synthesis of the selenium‐based ligand L1SeSeL1 (2,2’‐diselanediylbis(N,N‐bis(pyridin‐2‐ylmethyl)ethan‐1‐amine) is described along with its reactivity with cobalt(II) salts. The cobalt(II)‐diselenide complex [Co2(L1SeSeL1)Cl4] was obtained in good yield, and its spectroscopic properties closely resemble that of its sulfur analog. Reaction of L1SeSeL1 with Co(II) thiocyanate results in the formation of the cobalt(III) compound [Co(L1Se)(NCS)2], similar to reaction of L1SSL1. The redox‐conversion reactions from the Co(II)‐diselenide compound [Co(L1SeSeL1)Cl4] using external triggers such as removal of the halide ions or the addition of the strong‐field ligand 8‐quinolinolate resulted in good yields of the Co(III)‐selenolate complexes [Co(L1Se)(MeCN)2](SbF6)2 and [Co(L1Se)(quin)]Cl. Our computational studies show that the ligand‐field splitting energy of the selenium compounds is smaller than their sulfur analogs, indicating that redox‐conversion of cobalt(II)‐diselenide to cobalt(III)‐selenolate complexes may be more arduous than that for the related sulfur compounds.

Facile fabrication of carbon nanotubes-encapsulated cobalt (nickel) salt nanocomposites and their highly efficient catalysis in the thermal degradation of ammonium perchlorate and hexogen

For reducing particle aggregation and improving catalytic activities of metal salts in composite solid propellants, ten cobalt and nickel compounds were encapsulated into the channels of oxidized carbon nanotubes (CNTs) by ultrasonication, which was proven by TEM, SEM, XRD, Raman, XPS, etc. The catalytic performances of the nanocomposites for ammonium perchlorate (AP) and hexogen (RDX) thermal degradation were evaluated by the DSC technique and the results revealed that the composites exhibit significant effects on the thermal decomposition of AP and RDX. Among them, 5 wt. % Co(NO3)2@CNTs(D1) as the catalyst increased the released heat of AP from 746.53 J/g to 2489.26 J/g, higher in catalytic efficiency than CoO@CNTs(D1) (1791.29 J/g), and advanced the peak temperature of AP at the high-temperature decomposition stage from 406.6°C to 312.4°C; while 1 wt. % Ni(NO3)2@CNTs(D1) as an additive enhanced the pyrolytic heat of RDX approximately three times and was more active than that of NiO@CNTs(D1). Kinetic studies showed that the activation energies of AP and RDX were sharply decreased with Co(NO3)2@CNTs(D1) and Ni(NO3)2@CNTs(D1) as additives. In addition, the thermal decomposition mechanism of RDX was studied by TG-FTIR-MS and it was found that the as-prepared nanocomposites promoted the cleavage of the C-N bonds of RDX and the generation of N2O gas.

Structural investigations and reactivity of cobalt(II)–disulfide compounds

Controlling the bio-inspired redox conversion of Co(II)–disulfide compounds to their related Co(III)–thiolate complexes is a perplexing task as the factors triggering this reaction are not fully understood. Three disulfide ligands 2,2′–disulfanediylbis(N,N-bis((3,5–dimethyl-1H–pyrazol–1–yl)methyl)ethan-1-amine) (LmpzSSLmpz), 3,3′-disulfanedylbis(N,N-bis(pyridin-2-ylmethyl)propan-1-amine) (LpSSLp), and 2,2′-disulfanediylbis(N,N-bis(6-methylpyridin-2–ylmethyl)ethan-1–amine) (L3SSL3) with different chain lengths and pyrazole or pyridine groups were reacted with cobalt(II) salts and the resulting complexes were studied for their potential to form Co(III)–thiolate complexes. Crystal structures of [Co2(LmpzSSLmpz)(Br)4] [1Br], [Co2(LmpzSSLmpz)(NCS)4] [1NCS], [Co2(LpSSLp)(NCS)4] [2NCS], and [Co2(L3SSL3)(Cl)4] [3Cl] show that generally the disulfide sulfur donors do not coordinate to the cobalt(II) centers, resulting in dinuclear structures containing two 5-coordinate cobalt centers. However, a unique asymmetric structure is found in [1Br] in which one of the sulfur atoms coordinates to one of the cobalt ions. Thiocyanate–induced Co(III)–thiolate formation in the presence of either ligand LmpzSSLmpz or LpSSLp is evidently unsuccessful, as Co(II)–disulfide complexes were obtained. The reaction of the Co(II)–disulfide complexes with 8-quinolinol suggest that redox conversion to Co(III)–thiolate complexes may take place, but the reactions are not clean.

Electroreduction of oxygen on iron- and cobalt-containing nitrogen-doped carbon catalysts prepared from the rapeseed press cake

• Nitrogen, Co- and/or Fe-doped nanocarbons prepared from rapeseed press cake. • ORR activity of Fe-containing catalysts is close to that of Pt/C in 0.1 M KOH. • Inhibition of ORR by CN − ions shows the existence of MN x centres. • Catalysts show high methanol tolerance and good stability. • In AEMFC , P max = 131 mW cm −2 achieved with bimetallic catalyst. Transition metal-containing nitrogen-doped carbon catalysts for the electrochemical oxygen reduction reaction (ORR) were prepared from inexpensive biomass - rapeseed press cake; cobalt and/or iron salts and dicyandiamide were employed as metal and nitrogen sources. After acid treatment, the catalysts showed excellent electrocatalytic ORR activity in rotating disk electrode experiments in 0.1 M KOH solution, comparable to that of Pt/C (20 wt%) catalyst; good tolerance to methanol and high stability in short-time tests. The bimetallic catalyst displayed moderate performance as cathode material in anion exchange membrane fuel cell (AEMFC) test, by reaching the peak power density of 131 mW cm −2 .

Hollow CoO Nanoparticles Embedded in N‐doped Mesoporous Graphene for Efficient Oxygen Reduction Reaction

AbstractDevelopment of reactive and stable non‐preciously metallic catalysts toward oxygen reduction reaction is a fascinating research field for fuel cells. Herein, a hybrid electrocatalyst consisted of hollow CoO nanoparticles embedded in N‐doped mesoporous graphene (CoO@N−G) has been successfully prepared via carbonizing the precursor which is prepared from graphene oxide and N, N′‐Bis(salicylidene) ethylenediamine cobalt salt in the presence of hydrazine hydrate. The formation of hollow CoO nanoparticles is generally attributed to the nanoscale Kirkendall effect. The effective synergistic effect of hollow CoO nanoparticles and N‐doped mesoporous graphene is helpful to obtain excellent electrocatalytic activity with four‐electron selectivity, long‐term stability and good anti‐methanol.

Cobalt/Salox‐Catalyzed Enantioselective Dehydrogenative C−H Alkoxylation and Amination

AbstractThe past decade has witnessed a rapid progress in asymmetric C−H activation. However, the enantioselective C−H alkoxylation and amination with alcohols and free amines remains elusive. Herein, we disclose the first enantioselective dehydrogenative C−H alkoxylation and amination enabled by a simple cobalt/salicyloxazoline (Salox) catalysis. The use of cheap and readily available cobalt(II) salts as catalysts and Saloxs as chiral ligands provides an efficient method to access P‐stereogenic compounds in excellent enantioselectivities (up to >99 % ee). The practicality of this protocol is demonstrated by gram‐scale preparation and further derivatizations of the resulting P‐stereogenic phosphinamides, which offering a flexible asymmetric alternative to access P‐stereogenic mono‐ and diphosphine chiral ligands. Preliminary mechanistic studies on the enantioselective C−H alkoxylation reaction suggest that a cobalt(III/IV/II) catalytic cycle might be involved.

Cobalt/Salox-Catalyzed Enantioselective Dehydrogenative C-H Alkoxylation and Amination.

The past decade has witnessed a rapid progress in asymmetric C-H activation. However, the enantioselective C-H alkoxylation and amination with alcohols and free amines remains elusive. Herein, we disclose the first enantioselective dehydrogenative C-H alkoxylation and amination enabled by a simple cobalt/salicyloxazoline (Salox) catalysis. The use of cheap and readily available cobalt(II) salts as catalysts and Saloxs as chiral ligands provides an efficient method to access P-stereogenic compounds in excellent enantioselectivities (up to >99 % ee). The practicality of this protocol is demonstrated by gram-scale preparation and further derivatizations of the resulting P-stereogenic phosphinamides, which offering a flexible asymmetric alternative to access P-stereogenic mono- and diphosphine chiral ligands. Preliminary mechanistic studies on the enantioselective C-H alkoxylation reaction suggest that a cobalt(III/IV/II) catalytic cycle might be involved.

Electrospun Hollow Carbon Nanofibers Decorated with CuCo2O4 Nanowires for Oxygen Evolution Reaction

In recent years, spinel-type structural cobalt salts (NiCo2O4, CuCo2O4, etc.) have been widely used electrocatalysis because of their superior properties such as large crustal reserves, low cost, environmental friendliness, high electrochemical activity, abundant oxidation valence, and stable chemical properties. In this paper, hollow carbon nanofibers loaded CuCo2O4 nanowires (CuCo2O4@CNFs) were prepared by electrospinning technique and solvothermal method. The CuCo2O4@CNFs exhibit enhances electrocatalytic activity for oxygen evolution reaction (OER), requiring an overpotential of 273 mV in a 1.0 M KOH solution to achieve a current density of 10 mA cm−2. In addition, the overpotential remained almost constant after 3000 cycles of voltammetry measurements. The enhanced electrocatalytic activity may be attributed to the unique one-dimensional hollow nanostructure of CNFs and high dispersion of CuCo2O4 nanowires, which enhanced the charge transfer and improved the diffusion of the electrolyte ions at the surface.

BIOTRANSFORMATION OF WASTEWATER PRODUCTION OF BAKERY YEAST WITH BIOGAS GENERATION

Baking yeast enterprises are a source of environmental pollution by concentrated wastewater. They are usually diluted with water and discharged into sewerage. It is rational to use methane fermentation, which will ensure the removal of pollutants and make the process cost-effective through the use of biogas and digestion. Therefore, it is relevant to study the question of the influence of fermentation parameters on the cleaning efficiency. The aim of the work is to study the methane fermentation of yeast production effluents in a continuous mode. Objectives: analysis of solving the problem of wastewater treatment of yeast plants, study of the influence of process parameters (dilution rate, addition of cobalt salts) on the efficiency of treatment; gas generation; vitamin production. Initial COD of effluents 4500 mg O2/dm3, pH 6. Cultivation regime – continuous, dilution rate 4,1·10–3, 6,2·10–3, 8,2·10–3, 12,4·10–3 hours–1. Cleaning efficiency 78,9 %. High values ​​of dilution rate cause overload of activated sludge, which leads to reduced cleaning efficiency. The presence of cobalt also has a depressant effect. A significant amount of biogas (up to 5,2 dm3/dm3) is produced with a high content of methane (up to 85 %), which is an alternative fuel. As the dilution rate increases, the biogas and methane content decrease. With increasing dilution rate from 4,1·10–3 to 12,4·10–3 hours–1, biogas decreased from 1,11 to 0,94 dm3/g CODloading, and from 1,43 to 1,39 dm3/g CODfermentation. Similarly to the effect on the depth of purification, the inhibitory effect of cobalt on methanogeneration is observed. Digestion is a valuable fertilizer with a significant content of cobalamin vitamins (up to 95 μсg/g). The addition of cobalt salts stimulates the synthesis of vitamins, providing an increase of 26,7 to 51,6 %, improving the ratio between active and inactive forms.

Effects of Heavy Metals on Bacterial Growth, Biochemical Properties and Antimicrobial Susceptibility

Heavy metals are metallic elements that occur naturally and have a high atomic weight and a density which is five times greater than the density of water. Several studies have demonstrated that metal contamination in natural environments could have an important role in the development and spread of antimicrobial resistance. This is of particular concern, considering that heavy metals usually occur at higher levels than pharmaceutically produced antimicrobials. Also, the bio-accumulative and non-biodegradable nature of heavy metals may result in long- term antimicrobial resistance selection pressure. Therefore, this study assessed the effects of Copper, Cobalt, Cadmium, Zinc and Lead salts on growth, biochemical properties and antimicrobial susceptibility of Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus to commonly used antimicrobials. The selected bacteria were exposed to varying concentrations of the heavy metal salts ranging from 1 ppm to 1000 ppm after which growth was measured using a spectrophotometer. The effect on biochemical reactions and antimicrobial susceptibility were also tested by the use of conventional biochemical tests and Kirby-Bauer disk diffusion methods respectively. Different heavy metals affected the growth of each individual microorganism differently. Despite that, the growth of E. coli, P. aeruginosa and S. aureus was observed to be inversely proportional to the concentrations of the heavy metals used. Following exposure to heavy metals, E. coli and S. aureus showed no changes in their biochemical properties but P. aeruginosa gave a positive urease result contrary to the control. E. coli developed resistance to Levofloxacin, Meropenem and Tetracycline while S. aureus to Azithromycin and Gentamicin and P. aeruginosa developed resistance only to Meropenem. These results confirm the adverse effects of heavy metals on bacterial growth and reveals that heavy metals can cause changes in some biochemical properties of bacteria that produce any of these properties under stressful growing conditions. This research also supports the findings that heavy metals enhance the development of antimicrobial resistance in otherwise antimicrobial sensitive strains of bacteria. This study therefore, gives insight into the likely state of the proliferation of heavy metal induced antimicrobial resistance in heavy metal contaminated areas.