The Properties and Composition of Seawater: An Elemental Overview

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The Properties and Composition of SeawaterAn
Elemental Overview

• Dave Cohrs
• Water Quality Manager
• National Aquarium in Baltimore, US

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The Properties and Composition of SeawaterAn
Elemental Overview

• Water and the Hydrological Cycle
• Elemental Trends The Composition of Seawater
• Sources of Constituents
• pH, Alkalinity, ORP
• Nutrients Natural vs. Artificial Environments
• Chlorination and Ozonation
• Disinfection byproducts

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Water
O
H
H
105o

• H2O
• Asymmetrical polar structure permanent dipole
• Highest heat capacity of all solids and liquids
  (except ammonia) 4.184kJ/mol
• Highest surface tension of all liquids
• Dissolves more substances, in greater quantities
  than any other liquid
• Seawater contains 3.5 dissolved substances
• Highly transparent

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The Hydrological Cycle
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The Hydrological Cycle

• (Water x 1015kg)
• Rivers and Streams (1)
• Soil Moisture and Seepage (70)
• Salt Lakes and Inland Seas (104)
• Freshwater Lakes (125)
• Groundwater (8400)
• Glaciers and Icecaps (29300)
• Total amount of Water on Land (38000)
• Total amount of Water in the Oceans (1322000)
• Total Water Supply (1360000)

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The Hydrological Cycle

• The Oceans contain 97 of the global water
  inventory
• Less than 1 is available for drinking
• Density of Freshwater 1.00x103 kg m-3
• Density of Seawater 1.03x103 kg m-3
• Salinity Average Concentration of Dissolved
  Substances
• Surface waters salinities range from 33 to 37 mg
  kg-1
• Average 35 mg kg-1 (3.5 by weight)
• Salinity is a function of density and temperature
  (oC)
• The density of seawater normally increases with
  depth
• Now measured as R conductivity of seawater
  sample
• conductivity of standard KCl
  solution
• Where KCl solution 32.4356 g kg-1

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The Composition of SeawaterIts Elementary my
dear Watson

• All of the naturally occurring elements are
  present in Seawater
• Water 96.5
• Everything Else 3.5

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The Composition of Seawater
Cl-

• Major constituents
• 11 Major Ions make up 99.9 of dissolved
  constituents by weight
• gt1mg L-1 Concentration
• 21.9 Anions (Negatively Charged)
• 12.6 Cations (Positively Charged)
• Overall Salinity 34.482 (g kg-1 solvent)
• Minor constituents
• Trace constituents

Na
Mg2
Ca2
K
Br-
Sr2
F-
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The Composition of Seawater
Major Constituents (mg kg-1)
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Water 97
Trace Constituents
Minor Constituents
Major Constituents
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The Composition of Seawater

• Minor constituents
• lt 1 mg L-1 but gt0.001 mg L-1 ( 1 ppb)
• Trace constituents
• lt0.001 mg L-1
• Other
• Nitrogen, Silicon, Oxygen not included
• Nitrogen is mostly present as a gas (N2)
• O, Si considered non-conservative (they vary
  considerably)
• Dissolved Organic Carbon
• Approximately 0.6mg L-1 in surface waters
  (Williams, Spotte)
• Carbohydrates, phenols from algae, phytoplankton
  blooms
• Higher in Aquarium Systems
• Yellow water

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The Composition of Seawater
Minor Constituents (mg kg-1)
13
Water 97
Minor Constituents
Major Constituents
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Trace Constituents (mg kg-1)
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Sources of Constituents

• Volcanic Activity/Atmospheric Interactions
• Gaseous Emission
• Chlorine (as Chloride) and other Halogens
• Sulfur
• Carbon Dioxide and Methane
• Activity on the Sea Floor
• Dissolution of minerals in rock of the oceanic
  crust from hydrothermal circulation
• Calcium, Magnesium other Alkali Earth Metals
  (Group II)
• Organically-Rich Marine Sediments (Copper,
  Uranium, Zinc)
• Weathering of Igneous and Metamorphic Rock by
  rainfall and other mass movement of water on land

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Sources of Constituents

• Major Constituents
• Most behave conservatively
• SiO2 and Ca2 are notable exceptions
• Bio-Unlimited Constituents (i.e. Sodium,
  Chloride)
• Bio-Intermediate Constituents
• Depleted in surface waters, but never exhausted
• Minor and Trace Constituents
• Unlike Major constituents, most Minors and
  Traces
• Behave non-conservatively
• Concentrations are affected by biological or
  chemical processes
• Depleted from or added to the water
• Toxic above certain concentrations
• (See AQUALITY discussion on heavy metals by
  Conklin)

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Sources of Constituents

• Gases
• Solubility of gases increases with decreasing
  temperature
• Nitrogen (N2)
• About 11 of the 11.5 mg L-1 total Nitrogen in
  seawater
• Oxygen (O2)
• Surface waters are consistently supersaturated
  due to liberation of oxygen by phytoplankton and
  wave activity driving gases into solution
• Carbon Dioxide (CO2)
• Present in seawater as carbonic acid,
  bicarbonate, and carbonate
• Only about 0.23 mg L-1 at 24oC as gas
• Increasing atmospheric content during the last 60
  years
• Other Gases
• Sulfur Dioxide (SO2) Air to Sea acid rain,
  vulcanism
• Carbon Monoxide (CO) Sea to Air microbial
  decomposition
• Nitrous Oxide (N2O) Sea to Air microbial
  decomposition
• Methane (CH4) Sea to Air anoxic
  conditions/microbial
• Methyl Iodide (CH3I) Sea to Air
  Phytoplankton/Anoxic conditions
• Dimethyl Sulfide ((CH3)2S) Sea to Air
  Phytoplankton/Anoxic conditions

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A Comparison of Seawaterto other waters(mg kg-1)
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pH of Seawater

• pH
• Concentration of Hydrogen (Hydronium Ion)
• The p stands for Power
• pH - log10H3O
• ExamplepH 8.2
• 8.2 - logH
• - 8.2 logH (flip the sign and take Antilog)
• H 10-8.2 mol L-1
• H 6.3 x 10-9 mol L-1
• pH ranges from 7.7 to 8.3 in surface waters
• pH Scale is logarithmic
• At pH 7, there is 10x more H3O(aq) than at pH 8
• Conversely, there is 10x more OH-(aq) at pH 8
  than at pH 7
• 10-14 HOH-
• 14 pH pOH
• 2H2O (l) H3O(aq) OH-(aq)

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Alkalinity of Seawater

• Formally, the net molar concentration of strong
  base cations in excess of the net molar
  concentration of strong acid ions (in terms of
  charge equivalents)
• A Strong Base Cations Strong Acid Anions
• A (Na K 2Mg2 2Ca2) (Cl-
  Br- 2SO42-)
• Seawater is electrically neutral
• This works out to about 2 mol m-3
• Therefore
• A HCO3- 2CO32-
• A 2 mol m-3 throughout the oceans (A 140
  mg kg-1)
• K H3OHCO3-
• CO32-
• H3O K HCO3-
• CO32-
• Therefore the ratio of the concentration of
  bicarbonate and carbonate ions must control the
  hydronium concentration and pH! As the ratio
  increases, so does the pH.

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pH Alkalinity
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ORP of Seawater

• Oxidation-Reduction Potential
• OILRIG (Oxidation is Loss of electrons,
  Reduction is Gain of electrons)
• The oxidation state of elements with more than
  one valence state greatly affects their
  solubility
• Oxidized form of Iron (III) Fe(OH)3 very low
  solubility, suspended colloid
• Reduced form Iron (II) Fe(OH)2 more soluble
• Therefore, for waters of High ORP
• Available Iron will be very low
• This is true for Cobalt, Manganese
• Precipitated as hydroxides or hydrated oxides
• Biological significance where ORP varies
  dramatically

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ORP of Seawater

• ORP of seawater
• Very complex due to all of the redox couples
  present
• Natural waters
• In natural waters, redox reactions are usually
  quite different from what would be predicted
  based on thermodynamics (Horne 1965, Stumm and
  Morgan 1981).
• Surface waters are oxygen rich Oxidizing
• Sediments, mangrove habitats, low oxygen,
  organics - Reducing
• Aquarium systems
• Generally considered to be a measure of the
  state of cleanliness
• Use a platinum/combination electrode
• Make sure the filling solution matches the ionic
  strength of the solution
• Check against reference standards (Zobells or
  pH buffers with Quinhydrone)
• 275 350mV is an acceptable safe range

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Nutrients in Seawater

• Nutrients
• Polyatomic compounds containing Nitrogen,
  Phosphorus, Silicon
• Nitrogen
• Ammonia (NH3NH4), Nitrite (NO2-), Nitrate
  (NO3-), N2
• Nitrogen gas 11 mg L-1
• NH3NH4 NO2- NO3- 0.5 mg L-1
• Phosphorus
• Phosphate (PO43- ) 0.06 mg L-1
• The Magic Ratio
• NitrogenPhosphorus 151 molar ratio
• Same in both Tissue and Seawater
• Nutrients are depleted in surface waters,
  increase with depth
• Biologically limiting

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Nutrients in Aquarium Systems

• - Ammonia (NH3NH4), Nitrite (NO2-)
• typically very low concentrations in stable
  systems where filtration is adequate, well
  maintained
• Toxic in relatively low concentrations
• Nitrate (NO3-)
• Typically accumulates in fish or marine mammal
  systems
• May be depleted in photosynthetically active
  systems (corals)
• Biological effects at high concentration (much
  debate!)
• Water exchanges
• Denitrification systems (see Hignette, Mort,
  Aiken)
• Algal turf scrubbing (Adey, et al)
• Electrochemical reduction
• Phosphorus
• Also can climb to undesirable levels,
  stimulating algal growth

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Chlorination and Ozonation

• Increasing ORP Add an oxidant
• Sanitization or sterilization
• Mammal, marine mammal, or avian pools
• Human/Animal interaction
• Control of coliform and other bacteria
• Improve clarity
• Improve color
• Consider Bromine and Chlorine in Seawater
• Bromine (as Bromide) 65 mg L-1
• Chlorine (as Chloride) 19000 mg L-1
• Both species react with Chlorine (typically
  NaClO) or Ozone (O3) to form weak acids
• Strong oxidizing power
• Increased ORP
• Side Effects

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Chlorination and Ozonation

• Inorganic Reaction Products of Chlorination
• NaOCl H2Olt-gt HOCl Na OH-
• HOCl lt-gt OCl- H
• hypochlorous acid - hypochlorite
• HOCl Br -lt-gt HOBr Cl-
• HOBr lt-gt OBr- H
• hypobromous acid - hypobromite
• HOCl OCl- Free Chlorine
• HOBr OBr- Active Bromine
• Bromine (as bromide impurity) is sometimes
  restricted when selecting salts for seawater
  formulation to prevent unwanted reactions

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Chlorination and Ozonation

• Inorganic Reaction Products of Ozonation
• O3 Cl- lt-gt O2 OCl-
• O3 Br - lt-gt O2 OBr -
• OCl- Br - lt-gt OBr - Cl-
• HOCl Br -lt-gt HOBr Cl-
• HOBr lt-gt OBr- H
• Monochloramines
• NH3 HOCl lt-gt NH2Cl H2O
• Mono and Dibromamines
• Monobromamines prevail at NH4-N gt0.8mg L-1
• Bromine oxidation predominates in seawater so
  bromamines are favored

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Disinfection Byproducts

• Nothing is Free. Unfortunately, Blue Water has a
  price
• Disinfection byproducts can and do form during
  Chlorination and Ozonation
• Dissolved organic substances
• Disinfection Byproducts
• THM trihalomethanes. Known mutagens and
  carcinogens!
• Humic and fulvic acids are precursors (cause
  yellow water)
• Chloroform, Bromoform, Bromochloromethane,
  Dibromochloromethane
• Packed column aeration for removal of volatile
  THM and NCl3
• Bromate, Chlorate potential carcinogens
• Regulated DBP in drinking water
• Both species are stable and hard to get rid of,
  once formed

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Disinfection Byproducts

• - Bromate and Chlorate
• O3 Br - lt-gt O2 OBr
• O3 OBr lt-gt 2O2 Br
• 2O3 OBr lt-gt 2O2 BrO3
• Sunlight induces conversion of up to 50 of OBr
  - to BrO3- in chlorinated seawater (Macalady et
  al. 1977)
• Chlorate production favored in chlorinated
  natural seawater receiving strong sunlight
  (personal observation)
• Bromate has been shown to be reduced to Br - by
  activated carbon (Marhaba, Medlar et al.)
• USEPA D/DBP MCL in finished drinking water
• Bromate to 10µg L-1
• Chlorite to 1000µg L-1

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Thank you,Enjoy the rest of the conference