Discipline: Microbial Diversity and Bioremediation Important - PowerPoint PPT Presentation

Discipline: Microbial Diversity and Bioremediation Important Concepts of Chemical for Bioremediation Process Professor Luciana Maria Saran Technology Department School of Agricultural and Veterinarian Sciences São Paulo State University Jaboticabal, SP, Brazil lmsaran@fcav.unesp.br 1

1. Oxidation States • It is the number of eletrons (e - ) that an atom loses, gains or appears to use when joining with another atom in compounds. • It determines the ability of an atom to oxidize or to reduce other atoms or species. • OXIDATION: results in an increase in the oxidation state. • REDUCTION: results in a decrease in the oxidation state. 2

1. Oxidation States • Almost all of the transition metals have multiple potential oxidation states. WHY? Transition Metals (d-block) 3

1. Oxidation States • It is relatively easy to lose electron(s) for transition metals compared to the alkali metals (Li, Na, K) and alkaline earth metals (Ca, Mg, Ba). WHY? • Alkali metals: 1e - in their valence s -orbital. Their oxidation state is almost always +1 ( from losing it ). • Alkaline earth metals: 2e - in their valences-orbital, resulting with an oxidation state of +2 ( from losing both ). 4

1. Oxidation States • Transition metals: have 5 d-orbitals and d-orbital has a variety of oxidation states. • These d-orbitals have unpaired valence electrons. • These unpaired valence electrons are unstable and eager to bond with other chemical species. • Highest Oxidation State for a Transition Metal = Number of Unpaiered d-electrons + Two s-electrons 5

1. Oxidation States • Example: iron ( 26 Fe) Electronic Configuration: 1s 2 2s 2 2p 6 3s 2 3p 6 3d 6 4s 2 Electronic Argon (Ar) Configuration  [Ar] Therefore: 26 Fe: [Ar] 4s 3d Possible oxidation states for iron: +2, +3, +4, +5 and +6. 6

1. Oxidation States Somes rules: • Free elements (elements are not combined with other elements) have an oxidation state of zero. Example: the oxidation state of Fe (iron) is 0. • For monoatomic ions, the oxidation is equal to the charge of the ion. Example: the ion Fe 3+ (ferric ion) has oxidation state of +3. • The oxidation state of a neutral compound is zero. 7

1. Oxidation States • What is the oxidation state of Fe in FeCl 2 ? a) Chlorine (Cl) is in the halogen group of the periodic table, than it has a charge of -1 or simply Cl - . b) Since there are 2 Cl atoms the negtive charge is -2. c) There is no overall charge for FeCl 2 and therefore the oxidation state of Fe is +2. • Its name is: ferrous chloride or iron(II) chloride. 8

1. Oxidation States • What is the oxidation state of manganese (Mn) in - ? MnO 4 - is a poliatomic ion. a) MnO 4 b) This especies has an overall charge of -1. It is not neutral. c) Generally oxygen (O) has an oxidation state of -2 and there are four O atoms and these atoms contribute with a total charge of -8. d) Since the overall charge of the ion is -1, the Mn atom must have an oxidation state of +7. 9

1. Oxidation States Oxidation states of a metal affect: • the solubility of its compounds; • its mobility; • its reactivity; • its cellular permeability; • its toxicity. 10

2. “Heavy Metals” and Oxidation States • “ Heavy metals ” are naturally occurring elements that have a high atomic weight and a density at least 5 times greater than water ” . • “ Heavy metals ” include metalloids , such as arsenic (As), that are able to toxicity at low level of exposure. • They are also considered as trace elements because of their presence in trace concentrations (ppb range less than 10 ppm) in various environmental matrices. Source: Tchounwou P.B. et al. 2012 . Heavy Metal Toxicity and the Environment . In: Luch A. (eds) Molecular, Clinical and Environmental Toxicology. Experientia Supplementum, v. 101. Springer, Basel. DOI https://doi.org/10.1007/978-3-7643-8340-4_6. 11

2.1 “Heavy Metals”: Meaning According to the IUPAC (International Union of Pure and Applied Chemistry): • The term “ heavy metals ” is both meaningless and misleading . Why??? Source: Duffus, J.H. Pure Appl. Chem . 2002 , 74 (5), 798-807. 12

2. “Heavy Metals” and Oxidation States 2.1 Mercury (Hg) • This “heavy metal” naturally ocorrs in the environment in metallic form (Hg 0 ) and it is a liquid metal at a typical ambient temperatures and pressures. • In addition to its elemental state, Hg exists in two 2+ and Hg(II) or Hg 2+ . oxidation states Hg(I) or Hg 2 • Hg 0 (elemental state); Hg 2 2+ (mercurous ion); Hg 2+ (mercuric ion) . 13

2. “Heavy Metals” and Oxidation States 2.1 Mercury (Hg) • It is not essential for plant or animal life. • Mercury and its compounds are toxic to humans, aquatic organisms, terrestrial organisms and microorganisms. • The toxicity varies among the different types of mercury. • Generally, organic forms are much more toxic than the inorganic form. 14

2. “Heavy Metals” and Oxidation States 2.1 Mercury (Hg) • Ionic mercury, under reducing conditions in the environment, changes to the uncharged elemental mercury (Hg 0 ) wich is volatile and may be transported over long distances by ar. • Hg 0 has na average residence time in the atmosphere of about one year. • Elevated levels of mercury can be found in remote areas far from the sources. 15

2. “Heavy Metals” and Oxidation States 2.1 Mercury (Hg) • Oxidised mercury (Hg 2+ ) may be deposited relatively quickly by wet and dry deposition processes leading to a residence time of hours to months. • It may be chemically or biologically transformed to organic forms: methylmercury and dimethylmercury. • Methylmercury represents the most important toxic impact of mercury to humans. 16

2. “Heavy Metals” and Oxidation States 2.1 Mercury (Hg) methylmercury dimethylmercury (bioaccumulative) (volatile) 17

2. “Heavy Metals” and Oxidation States 2.1 Mercury (Hg) • It is toxic to microorganisms. • Inorganic mercury: has been reported to have toxic effects at concentrations of the metal in the culture medium of 5 µg L -1 (or 5 ppb). • Organomercury compounds: have toxic effects at concentrations at least 10 times lower than 5 ppb. 18

2. “Heavy Metals” and Oxidation States 2.2 Chromium (Cr) • Chromium has several oxidation states. • The trivalent, Cr(III) and hexavalent, Cr(VI) states are the most stable. • Cr(III): in small amounts is an essential component of a balanced human and animal diet. • Cr(VI): is a potent, extremely toxic carcinogen and may cause death to animals and humans. 19 Source: Zayed , A.M.; Terry, N. Plant and Soil . 2003 , 249 , 139-156.

2. “Heavy Metals” and Oxidation States 2.2 Chromium (Cr) Chemical Oxidation Remarks Species State Elemental Cr Cr(0) Does not occur naturally. Divalent Cr Cr(II) Unstable. Oxidized to Cr(III). Trivalent Cr Cr(III) Forms stable compounds. Tetravalent Cr Cr(IV) Does not occur naturally. Pentavalent Cr Cr(V) Does not occur naturally. 2 nd most stable state of Cr. Hexavalent Cr Cr(VI) Source: Zayed , A.M.; Terry, N. Plant and Soil . 2003 , 249 , 139-156. 20

2. “Heavy Metals” and Oxidation States 2.2 Chromium (Cr) • Cr(III) and Cr(VI) are drastically different in charge, physicochemical properties as well as chemical and biochemical reactivity. • Cr(III) forms strong complexes with various ligands. It has an affinity for O-, N- and S-containing ligands and forms many organic complexes. • The solubility of Cr(III) is limited by the formation of highly insoluble oxides, hydroxides and phosphates. 21

2. Heavy Metals and Oxidation States 2.2 Chromium (Cr) • In the presence of excessive oxygen, Cr(III) oxidizes into Cr(VI). • Cr(VI) is the principal species in surface waters and aerobic soils. • Cr(VI) forms stable anions: Cr 2 O 7 2- (dichromate) and 2- (chromate). CrO 4 22

2. “Heavy Metals” and Oxidation States 2.2 Chromium (Cr) CrCl 3 .6H 2 O K 2 CrO 4 K 2 Cr 2 O 7 [Cr(H 2 O) 6 ] 3+ (aq)  Cr(III) 2- (aq)  Cr(VI) 2- (aq)  Cr(VI) CrO 4 Cr 2 O 7 23

2. Heavy Metals and Oxidation States 2.2 Chromium (Cr) • Cr(VI) is in general more toxic to organisms in the environment that the Cr(III). • Almost all the Cr(VI) in the environment is a result of human activities. • Cr(VI) is relatively stable in air and pure water. • Cr(VI) is reduced to the Cr(III) when it comes into contact with organic matter in biota, soil and water. 24

2. “Heavy Metals” and Oxidation States 2.2 Chromium (Cr) • Cr(VI): is a strong oxidizing agent. It is not readily adsorbed to surfaces. • The high oxidizing potential, high solubility and ease of permeation of biological menbranes make Cr(VI) more toxic than Cr(III). 25

3. “Heavy Metals” Speciation • Speciation refers to the various physical and chemical forms in wich an element may exist in the system. • It can have an affect on bioavailability and accumulation of these species. • “Heavy metal” toxicity is dependant upon chemical speciation. Source: Ansari, T.M.; Marr, I.L.; Tariq, N. J. Applied Sci . 2004 , 4 (1), 1-20. 26