Chapter 13 Metals and semimetals in the hydrosphere

1
Chapter 13Metals and semi-metalsin the
hydrosphere

• CH350/EV350
• Spring 2008

2
Metals in the hydrosphere

• Ocean water
• Sodium, Magnesium, and other trace metals
• Fresh water
• Calcium and other trace metals
• Ground water in South Asia
• Contaminated with As as arsenite (AsO33-) and
  arsenate (AsO43-)
• Causes serious disease

3
Aqua complexes

• No other ligands forming complexes with the metal
  other than water (some of which can lose protons)

4
Aqua complexes

• pKa is the pH at which the aqua complex is
  present with half the molecules in fully
  protonated form and the other half having lost 1
  proton
• When pH lt pKa all water molecules are protonated
  (reaction is predominately on the left)
• When pH gt pKa most molecules have one water
  molecule deprotonated (reaction is predominately
  on the right)

5
Aqua complexes

• From pKa data
• In 1 ions (Na and K) waters of hydration
  exist in fully protonated form
• In 2 ions water of hydration is deprotonated
  more easily
• Somewhat dependent on large charge to radius
  ratio (Z2/r)
• Be2 pKa is lowest of main group 2 ions
• In 3 ions (Al3 and Fe3) one water of hydration
  is deprotonated at most natural water pH

6
Aqua complexes

• Metal aqua complexes with low pKa can lose more
  than one proton and become neutral and insoluble
  in water

7
Classification of Metals

• Traditionally the term heavy metal has been
  used to discuss metals of biological concern
• Not a good term because there are no definitions
  to define heavy
• Lead 207 g/mol, 11.3 g/mL
• Mercury 200 g/mol, 13.6 g/mL
• Aluminum 27 g/mol, 2.7 g/mL
• Arsenic (metalloid)

8
Classification of Metals Type A

• Type A hard sphere metals
• d0 noble gas electron configurations
• Na, K, Mg2, Ca2, Al3
• Electrostatic model explains stability of
  metal-ligand complexes
• Stability of metal-ligand complexes correlated
  with Z2/r
• Mg2 gt Ca2 gt Sr2 gt Ba2
• Preference for forming complexes with O or F
  containing ligands over sulfur, chlorine,
  bromine, or nitrogen
• Ions may form insoluble OH-, CO32-, or PO43-
  compounds
• CaCO3 and AlPO4 are important forms

9
Classification of Metals Type B

• Type B soft sphere metals
• nd10 and nd10(n1)s2 type electron
  configurations
• Zn2, Cd2, and Hg2, Ag, Pb2
• Covalent bonding important in complex formation
• Electrostatic model does not fully explain
  stability of metal-ligand complexes
• Stability of metal-ligand complexes correlated
  with electronegativity differences between metal
  and ligand
• Stability Zn2 gt Cd2 gt Hg2
• Form more stable complexes than Type A metals do
• Complex stability with halides follows trend
  opposite that of Type A metals
• I- gt Br- gt Cl- gt F-
• Complexes with ligands containing nitrogen are
  more stable than those with oxygen
• Complexes with sulfide are common
• Complexes with carbon are observed
• CH3Hg and (CH3)2Hg

10
Classification of Metals Borderline Metals

• Borderline metals have properties between Type A
  and Type B
• ndx (0ltxlt10) electron configurations
• Fe2, Cu, Mn2
• Type B character tends to increase down and to
  the right in the periodic table
• Stability of complexes tends to increase as you
  move right on the periodic table

11
Environmental classification of metals

• Takes into account both covalent and ionic
  character of metals
• Toxicity Type B gt borderline gt Type A

12
Environmental Classifications

• All metals form aqua complexes
• Small Z2/r fully protonated
• Larger Z2/r partially deprotonated
• Redox active metals are present as different
  species in oxidizing or reducing environments

13
Environmental Classifications

• High concentrations of Cl- in seawater favors
  formation of Cl complexes
• Complexes likely to form complexes with ligands
  other than water
• Type B gt borderline gt Type A

14
Metal complexation with organic matter

• Functional groups of organic matter can complex
  metals

15
Metal complexation with organic matter

• Stability of complex depends on
• Nature of the metal ion (ionic or covalent
  character)
• Trivalent (Al3) bind more strongly than
  monovalent (Na)
• Alkaline earth metals (Ca2 and Mg2) bind less
  strongly than other metals with more covalent
  character (Cu2 and Pb2)

16
Metal complexation with organic matter

• Stability of complex depends on
• Solution pH
• Deprotonation of metal waters of hydration
• Protonation of carboxyllic acid groups on organic
  matter inhibits forming a complex
• Humic materials can change conformational shape
  at different pH which can affect binding

17
Metal complexation with organic matter

• Stability of complex depends on
• Solution ionic strength
• Higher ionic strength decreases binding of metals
  to organic matter
• Competition for ligand sites by other cations in
  solution
• Complexation between metals and anions such as
  Cl-, SO42-, and HCO3-
• Availability of functional groups

18
Metal complexation with organic matter

• Stability of metal complex with organic matter
  varies
• Mg and Ca dont bind as strongly as transition
  metals

19
Concentrations of binding sites

• Complexation capacity
• Rivers 1-2 mmol/L
• Lakes 2-5 mmol/L
• Ponds 5-15 mmol/L
• Swamps gt15 mmol/L

20
Binding of metals in academic laboratory waste to
garden compost.

• Matthew P. Fasnacht,
• Jessica Boester, and Shannon Willis
• Presentation given at 41st Midwest Regional
  Meeting of the American Chemical Society
• October 27, 2006
• Quincy, IL

21
Qualitative Analysis Scheme
Taken from Lagowski and Sorum. 2005. Introduction
to Semimicro Qualitative Analysis eighth edition.
22
Qualitative Analysis Waste

• Aqueous solution containing suspended solids
• pH 1
• Pb2, Ag, Hg22, Bi3, Cu2, Cd2, AsO43 Sb3,
  Sn2, Fe3, Al3, Cr3, Zn2, Ni2, Co2, Ba2
• Ca2, Mg2, Na, K, NH4
• 450 per 55 gallon drum of waste not containing
  mercury
• 300 per 2.5 L bottle of waste containing mercury

23
Biosorption

• The goal of this work was to find a biomaterial
  to sorb toxic metals in laboratory waste
  solutions.
• Previous work of biosorption of metals from
  aqueous solution
• Seaweed
• Chicken feathers
• Compost
• Electroplating waste (Cr, Zn, Cd, Ni, Cu)
• Most work has focused on simple solutions
  containing 1-3 metals

24
Garden Compost

• Compost made on campus of Southeast Missouri
  State University using waste plant and soil
  materials from grounds maintenance.
• Compost contains high concentrations of humic
  materials decayed plant waste

Tisdale, Samuel L., Nelson, Werner L., Beaton,
James D., Havline, John L. Soil Fertility and
Fertilizers, 5th ed. Page 93. New York Macmillan
Publishing Co. 1993.
25
Goal of this work

• Determine if garden compost would bind toxic
  metals present in qualitative analysis waste at
  pH 1 and pH 4.5.
• Are the concentrations of toxic metals in
  solution lowered enough by the garden compost to
  dump the resulting solution down the drain?

26
Methods

• Glass wool was placed in bottom of drying tube.
• 2 g of compost was added to the tube
• Waste solution was added using a burette
• Some of the waste was adjusted to pH 4.5
• The solution filtered through the compost and
  into a test tube
• Fractions were taken every 3 mL to determine the
  extent of metal removal

27
Methods

• Fractions were analyzed for metals using a Perkin
  Elmer Optima 3000 DV ICP-OES
• Emission was measured at the recommended
  wavelengths for all metals
• Metals tested Ag, Al, As, Ba, Bi, Ca, Cd, Cr,
  Co, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, Sb, Sn, and Zn

28
Ni binding
29
Cd binding
30
Cu binding
31
Sn binding
32
Pb binding
33
Cr binding
34
Volume of waste solution safe to discharge when
using 2 g of compost
Environmental Protection Agency. (June 2001).
Small Entity Compliance Guide Centralized Waste
Treatment Effluent Limitations Guideline and
Pretreatment Standards (40 CFR 437). Retrieved
April 10, 2006 from World Wide Web
35
Summary

• Metals in aqueous Qualitative Analysis Waste do
  bind to garden compost
• Higher pH generally causes greater binding of
  metals to compost
• Using 2 g of compost
• pH 1.2 solution, 6 mL of solution were treated
  successfully
• pH 4.9 solution, 10 mL of solution were treated
  successfully
• 6-10 g of liquid waste was converted to 2 g of
  solid waste

36
Metal Species and Bioavailability

• Total concentration of metals in solution is not
  a good predictor of metal toxicity to organisms
• Metals complexed with different ligands show
  varying amounts of bioavailability
• Metal has to bind to cell wall (preferentially
  over other ligands in solution) and be
  transferred into cell

37
Ligands in environmental systems

• Cl-, Br-, I-, S2-, SO42-, HCO3-, CO32-, H2O,
  anions of organic acids, NOM
• Anthropogenic many ligands are used to chelate
  metals, but can cause environmental damage
• Ammonia (NH3) decay of nitrogen containing
  organic wastes
• Sulfide (S2-) and sulfate (SO42-) discharged
  from pulp and paper mills
• Phosphate (PO43-) some detergents contain
  phosphates and phosphate fertilizer runoff
• Cyanide (CN-) extraction of gold from ore
  minerals
• EDTA (ethylenediaminetetraacetic acid)
  industrial cleaning and some detergents
• NTA (nitrilotriacetic acid) a detergent builder
  (15 by mass) used instead of phosphates

38
Calcium Case Study

• 2 oxidation state only, Type A metal
• Associated with oxygen-donor ligands
• Water, phosphate, carbonate, and sulfate
• Limited binding to NOM at low pH, some binding at
  neutral pH

39
Copper Case Study

• Trace mineral, large amounts toxic
• Cu2 most common oxidation state in water, but
  Cu can be found in reducing conditions
• Significant binding to OM as well as other ligands

40
Mercury Case Study

• Toxic metal
• Can exist in 0, 1, 2 (aerobic conditions)
  oxidation state in hydrosphere
• Aqua complexes deprotonate easily
• Binds strongly to OM and other ligands
• Methylation occurs under anaerobic conditions
• CH3Hg and (CH3)2Hg (insoluble and volatile)

41
Mercury Case Study
42
Mercury Case Study Amazon Basin

• Gold rush in the Amazon Basin
• Dredging river sand
• Sieve to retain heavy particles
• Particles mixed with mercury which dissolves the
  gold
• Roasting boils off the mercury leaving the gold
  behind
• If done in open air large amounts of mercury are
  released
• 100 tons per year estimated releases
• Atmospheric mercury is oxidized to Hg2 by
  photochemical reactions involving ozone and water
  vapor
• Hg2 is rained out onto land or water surfaces
• Methylation can then occur in water bodies