Exam 1 (ch 10, 11, 12, 13) Flashcards Preview

Environmental Chemistry > Exam 1 (ch 10, 11, 12, 13) > Flashcards

Flashcards in Exam 1 (ch 10, 11, 12, 13) Deck (62):
1

Uses for water

  • drinking
  • cleaning
  • agriculture
  • waste water
  • industry cooling
  • recreation
  • wildlife

2

Speciation

  • chemical species - structurally specific form of a chemical
  • multiple forms that a chemical can take
  • large molecules may be heavily effected by functional group speciation
  • evaluate small changes between structures
    • ligand binding
    • protonated/deprotonated
    • oxidation state (redox reactions)

3

Effect of Speciation

  • Some ox. states are toxic and others are not.
  • Free metal (2+) is typically most toxic
  • different environmental interactions
  • change in charge

4

Free Ion Activity Model

  • free metal is actually M(H20)6 2+
  • octahedral
  • aquo species

5

Sources of Metal in Aq. System

  • natural weathering of minerals and soils
  • background metal concs are not zero
  • enhanced by human activities (mining, construction)
  • rapid exposure of minerals to oxygen and water

6

Anthropogenic sources

  • originating from human activity
  • point sources - "end of the pipe", mining, smelting, manufacturing
  • nonpoint sources - diffuse, landscape level contributions
  • Zn from tires and Cu from brakes are nonpoint sources

7

Receiving water

any body of water that gets input of material from human activities

8

Metal Tox in Aquatic Systems

  • related to impacts on gills (like human kidney)
  • responsible for respiration and ion regulation (osmoregulation)
  • Cu tox in aqua is 10-100 ug/L drinking water tox is 3mg/L
  • biotic ligand is the target of metal
  • metal bind to ion transport protein, has higher affinity than major ions (Ca, K, Mg, Na)
  • Eq. process so LeChatelier's principle applies
    • competing constituents for metal can effect eq.

9

Biotic Ligand Model

  • predicts site specific water quality
  • effects:
    • pH
    • species with lone pairs
    • DOM (functional groups)
  • total amount of metal in water is NOT a good indicator of tox.

10

Complex equilibrium

  • common central species "parent material"
  • parent material has relatively low conc compared to ligand conc.
  • ligands are the "controlling variables"
    • conc is environmentally controlled. varies by location

11

Complex equilibrium steps

  • write all stepwise, one ligand exchange at a time
  • write overall. one reaction with all ligands to make product (beta equil constants)
  • write mass balance equation
    • algebraic rearrangement. betas, controlling variables and parent material
  • alpha expressions. ax = x/CT
  • [x] = alphax (CT)

12

Environmental Redox

  • natural systems - environment controls ox and red
    • env. controls one half of the redox reaction
  • Aerobic vs anaerobic
  • O2 is dominant oxidizing agent

O2 + 4H + 4e = 2H2O

  • other element for oxidation half reaction
  • aerobic environments likely for oxidation to occur
    • more oxidized speciation of element is more likely

13

Redox in Anaerobic

  • O2 is absent
  • wetlands (swamps), deep sediment, intestinal tract
  • saturated with water, high microbe activity, no sunlight, light organic matter
  • microbes consume oxygen during respiration
    • {CH2O} + O2 = CO2 + H20
  • oxygen diffusion in air is faster than in water
  • influenced by relative rates - O2 can diffuse in water, but may be consumed faster
  • sand/soil without microbes can be aerobic for 10s of meters

14

Anaerobic Microbial Activity

  • when O2 consumption > O2 diffusion
  • anaerobic respiration is less efficient than aerobic metabolism
  • less activity leads to more organic matter accumulation
  • {CH2O} = CO2 + 2H
    • Carbon from 0 to 4+ ox state
    • other element in environment will be reduced

15

Microenvironments

  • inorganic will have less microbes and more O2
  • organic matter will be home to microbes and less O2
  • can change from aerobic to anaerobic within mm of soil
  • organic matter zone could cause anaerobic zone
  • soil is heterogeneous

16

Soil solution

-centrifuge water out of soil

17

Soil/Water levels

-soil surface

-water table (unsaturated)

-groundwater (saturated pores)

18

Measuring Redox

  • redox potential measure:
    • electrochemical cell in lab
    • environmental water sample
      • surface water
      • soil solution
      • groundwater
  • Electrode
    • inert material
    • reference electrode, calomel electrode (Hg/Hg2Cl2)
    • correct measurement to standard hydrogen electrode
    • Estd = Ecal +0.242V

19

pE

  • conceptual representaion of the tendency for a system to donate or accept electrons
  • not real measurement, but conceptual representation
  • pH = -log ae
  • ae is the activity of electrons
  • pE ranges from -12 to 25. Lower values indicate high ae and reducing conditions
  • high pE is lack of electrons
  • based on stability of water. Cannot get so high or low that water is ox or red
    • dependent on pH

20

Water red and ox

oxidation - 6H2O = 4H3O+ + O2 + 4e

 

reduction - 2H2O + 2e = H2 + 2OH-

21

Pourbaix Diagram

  • speciation according to redox potential and pH
  • whole diagram is equil conditions
  • equal conc on species lines
  • vertical lines are acid base reactions
  • horizontal lines are redox reactions
  • further away from line, more dominant species in the center. Other species are NOT absent

22

Acid Volatile Sulfides

-model to predict metal toxicity in sediments

-based on affinity of transition metals for sulfide (Kf)

-formation constant is the inverse of the solubility constant

- MS is not biologically available for uptake so no toxicity effect. Free ion most toxic

23

AVS solubility products

H2S = H+ + HS-

HS- = H+ + S2-

Ksp based on following reaction:

MmSn + 2H+ = mM+ + nH2S

-dependent on pH

-more acidic, more soluble metal sulfides, more free metal ions

24

AVS species

  • most Sulfide (S2-) is bound to Fe2+. FeS
  • FeS serves as reservoir
    • Stronger competition will replace Fe.
    • K of replacement reaction is product of reactions

25

Metal Speciation in Sediments

  • can change based on environment
  • Sulfite has high affinity for thiol groups on cistine
  • Metal ion interaction with functional groups ins exchangable
  • adsorbed - exchangable, carbonates, Fe and Mn oxides
  • Extract w/0.5M HCl - absorbed fractions and M sulfides
  • other fractions include organic matter and residual
  • residual - in the particle, not coming out
  • adsorbed - on the surface

26

AVS extraction

  • FeS and MeS - molar total gives AVS
  • Simultaneously Extracted Metal (SEM) - Me2+ when HCl dissociates MeS and FeS
    • Add HCl and condense H2S volatile gas

27

Normalized Toxicity

  • cant impact tox based on total metal
  • normalize using mol SEM/AVS
  • SEM is potentially bioavailable
  • AVS - sulfide is available to complex metal ions
  • When SEM/AVS < 1 then little free metal
  • metal titration used to determine how much metal is dissolved when an amount of cadmium is added

28

Assimilative Capacity

-amount of a contaminant that can enter the environment w/o causing significant changes

-increased AVS increases capacity

29

AVS evaluation

  • evaluate ratio of SEM and AVS
  • SEM/AVS <1 then metal
  • SEM/AVS >1 then metal>sulfide, free metal present
  • limiting reagant problem

30

Dissolved Gas Importance

  • habitat quality (O2)
  • greenhouse gas emissions (CO2, CH4, N2O)
  • contaminant distribution
    • gasoline between water table and groundwater some equil and dissolves into watertable and groundwater

31

Simple and Reactive gases

  • simple - no reaction between gas and water as it dissolves
  • reactive - gas reacts with water
  • CO2 + H2O = H2CO3

32

Henry's Law

  • simple gases
  • valid for low conc such as environmental
  • equil of dissoved and gas phase
  • [G] = KHPG
  • PG : partial pressure of gas
  • watch the units
  • Temperature effects solubility. KH is only valid at one temp.
    • solubility decreases as temp increases

33

PO2

  • PO2 = PdryXO2
  • Pdry = Patm - PH2O
  • Pressure of water is highly variable
  • Atmospheric O2 is typically 20.9%, 2.04e4 Pa
  • [O2] = KH (2.04e4 Pa) = 2.7e-4 M
  • normally expressed as 8.5 mg/L
  • can range in waters from 5 - 14 mg/L
  • Assuming at equil

34

Gas in Water Equil Assumptions

  • sink - consumes the species
    • aerobic metabolism decreases O2
      • CH2O + O2 = H2O + CO2
  • source - produces the species
    • photosynthesis
    • 6CO2 + 6H2O = C6H12O6 + 6O2
  • Compare relative rates of sink and source

35

Time of day effect on O2

  • day time plants net produce O2
  • night time plants net consume O2
  • Highest O2 levels in afternoon/evening
  • Lowest O2 levels in early morning
  • At equil with the atmosphere twice per day

36

Physical Processes effect on Gas/Water Equil

  • effect mixing/diffusion
  • boundary layer - between the bulk air and fluid water
  • faster the flow, the thinner the boundary layer
  • aeration - increase surface area and increase flow (physical mixing)
  • rainwater and atmosphere are a good equil assumption
    • limited bio process (no sink or source)
    • small volume compared to surface area

37

Special Circumstance Gas/Water Equil

  • when gas phase is NOT the bulk atmosphere
    • closed container
    • soil pore spaces
  • In these cases equil assumption can be made and we can use Henry's law to assess  the equil

38

CO2 in Water

  • resultant species: CO2(g), CO2(aq), H2CO3, HCO3-, CO3 2-
  • Can increase overall aq concentrations
  • Environment can control aq. speciation
    • pH as controlling variable
  • CO2 can affect environmental pH

39

Atmosphere controls speciation

  • poorly buffered water then CO2 can control pH and conc of HCO3 and CO3
  • PCO2 is changing so pH can vary
    • can also change in microclimates like soil pores
  • pH of rain water that is at equil with the air is pH = 5.66

40

Acid Rain

  • ph 2 -4.5
  • due to H2SO4 from SO2 due to coal
    • S2- + O2 (heat)= SO2 from coal combustion
  • HNO3 from NOX
  • N2 + O2 (heat) = 2NO hot temp w/air as fuel

41

Time of day impact on CO2

  • pH can range from 5-10
  • noon - high photosynthesis which consumes CO2 so highest pH
  • midnight - respiration produces
  • At pH 10 then -OH is major factor

42

Sand

SiO2

43

CO2 and CaCO3

 

  • CaCO3 is important for seashells
  • 1014 is driver for more soluble species than otherwise predicted
  • CO2 + CaCO3 + H2O = Ca2+ + 2HCO3 -
  • Ksum = KHKa1KspKb(1/Kw) = 1.5 e 10-6
  • Ksum = [Ca2+][HCO3-]2 / PCO2
  • Increase CO2 then increase Ca2+ (dissolved shells)
  • HCO3- is a base species, higher pH than water at equil.

44

CaCO3 Solubility

  • Ksum = [Ca2+][HCO3-]2 / PCO2
  • Ksum = 4S3 / PCO2
  • 390 ppm CO2  parts per million by volume
  • 390e10-6 atm CO2 per 1 atm total gas
  • stable ph because more HCO3- so well buffered
  • Limestone increases pH and is more stable pH

45

Alkalinity

  • measures the capacity of a water body to neutralize acid
  • alkalinity = proton acceptors - proton donors
    • alkalinity = [OH] + [HCO3] +2[CO3] - H3O
  • Acid neutralizing capacity is similar
    • allows for other species of proton donors and acceptors
    • natural organic matter (NOM), silicates, phosphates, Al3+
  • Alkalinity of natural water ranges from 50 - 2000 uM

46

Measure Alkalinity

  • carbonate alkalinity
    • H + CO3 + OH = HCO3 + H2O
    • phenolphthalein as indicator pH 8.3
    • moles of H required to reach endpoint
  • Total alkalinity
    • H + HCO3 + CO3 + OH = H2CO3 + H2O
    • bromocresol green indicator pH 4.5
    • moles of H required to reach endpoint
  • Alkalinity is a capacity factor while pH is an intensity factor

47

Alkalinity Uses

  • quantify conc of carbonate ligands available for metal speciation
  • predict susceptibility of water body to acidification
    • <200uM high sensitivity
    • 200-400 uM moderate sensitivity
    • >400 uM low sensitivity
  • Concrete in urban areas can be source of artificial limestone. Increase alkalinity

48

natural organic matter (NOM)

  • product of bio activity (not synthetic)
  • has acid/base properties
  • can complex metals
  • DOM or POM (particulate organic matter)
  • Discreet small molecules (sugars, small acids, amino acids) or macromolecules (high MW, bio polymers)

49

Macromolecules

  • high MW, derived from biological polymers
  • cellulose: polysaccharides
  • lignin: aromatic polymer, diverse competition of alcohols
    • coumaryl alcohol
    • coniferyl alcohol
    • sinapyl alcohol

50

DOM vs POM

operational definition

use 0.45um filter

goes through it is dissolved. stuck in the filter than precipitate

Not always true: nanoparticle 100nm will go through filter

51

Humic Substances

  • operationally defined based on empirical properties
  • result from microbial and or abiotic degradation of biopolymers like lignin
  • represents fragments that are resistant to degradation
  • oxidation and hydrolysis of biopolymers and polymerization of small organic fragments

52

Plant material degradation

plant material

humin - insoluble at all pHs

humic acid - insoluble

fulvic acid - suluble at all pHs

small molecules

53

humic structures

  • humin - very large, few functional groups
  • humic acid - more oxidized groups, smaller overall, -COOH, -OH.    35% O by mass
  • fulvic acid - even smaller group, greater proportion of functional groups. 45% O by mass
  • As humics age, propertied change and higher % O. More oxidized. Greater water solubility. More fulvic acid

54

Characterizing humic substances

  • determine average properties
  • Spectroscopy - IR and NMR to find relative abundance of functional groups
  • use titrations to find average pka
    • COOH 2.5-5
    • phenols 9-10
  • Quantify Kf with metals
    • DOM contains -COOH, -OH, -N: , -SH
    • Kf changes with DOM location

55

Metal Biogeochemistry

  • pathways and cycles through which metals interact with soild, sediments, and biota
  • metal toxicity - major elements (Ca, Na, K, Mg) in high conc (essential nutrients)
    • trace elements - low conc, micronutrients at low levels, tox at higher levels

56

Classification of Metals and Ligands

  • Type A, hard metals: low polarizability, prefer O- and N- containing ligands
    • top left
  • Type B, soft metals: polarizable, prefer S-containing and heavy halogen ligands. Subject to alkylation (bind to C)
    • Cd, Zn, Hg

57

Aquo Complexes

M(H2O)62+

Some complexes are ionizable

M(H2O)62+ = M(H2O)5(OH)+ + H+

Metal can withdraw electron density from oxygen and reduce the strength of the OH bond

-more significant on cations with larger charges

Fe2+ pKa = 10.1   Fe3+ pKa = 2.19

  • metal salt solutions can be significantly acidic

58

Metal Complexes with Humics

  • functional groups on humic material can complex metals
    • can be multidentate ligands - more than one functional group on the same molecule binds to a single metal ion
  • factors
    • metal ion
    • pH - functional groups ionized or not (are H ion competing with the metal ion for the binding site)
    • ionic strength - activity
    • competing ions - other cation in solution?
    • location, source, age of DOM

59

Conditional Stability Constants

  • Kf'
  • only valid for a set of conditions (pH, ionic strength, competing ions, DOM source)

60

Binding Capacity of DOM

  • analogous to concentration of ligand
  • DOM is heterogeneous so cant determine molar conc
  • can determine moles of binding sites per L
  • found via titration
  • capacity rivers
  • due to age effects

61

62

DOC vs DOM

  • DOC - represents concentration based on mass of C (mg C/L)
  • measured instrumentally by combustion analysis
    • CO2 measured by IR detector
  • approximately 60% of DOM is C
  • DOM = 1.67*DOM
  • DOM measured by high temp oxidation >450C. Lost mass is presumed to be DOM (volatile solids)
    • must ensure that sample is totally dry
    • more time consuming than combustion analysis of C