Chapter 12 Organic Matter in Water

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Chapter 12Organic Matter in Water

• CH350/EV350
• Spring 2008

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Carbon in the environment

• Inorganic
• CO2, HCO3-, CO32-, graphite, diamond
• Most carbon on earth in inorganic form
• Organic
• Other molecules containing carbon of biological
  or anthropogenic origin
• Small concentrations in environment, but very
  important environmentally

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Organic Matter

• Organic Matter (OM)
• Decayed biomaterials
• Molecules secreted by organisms
• Anthropogenic organic compounds
• Sewage and pulp waste from paper mills

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Organic matter

• Aquatic NOM is often formed on land and leached
  into water bodies, but some NOM can be formed
  directly in the water column
• Two categories of OM
• Discrete small molecules, well defined chemical
  structures and properties
• Macromolecules, general chemical structures and
  properties, usually based on a definition rather
  than fundamental structural properties
• This chapter mainly deals with macromolecules

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Environmental issues related to OM

• Toxicity of specific organic compounds
• PAHs, PCBs, dioxins, pesticides, pharmaceuticals,
  etc
• Reaction with other aquatic species
• Inorganic tin reacts to form monomethyl tin
  (CH3Sn3) or dimethyl tin ((CH3)2Sn2)
• The methylated tin is more toxic
• Consumption of oxygen
• Bulk nonliving organic matter will be oxidized by
  oxygen (bacterial process)
• Oxygen concentrations can drop in areas of high
  organic matter leading to low pE environments

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Humic material

• Humic material (HM) is a form of environmental
  organic matter of biological origin
• Not well defined molecules
• Major component of soil and water OM
• HM makes up 50 of DOM in aquatic environments
• Divided into three categories
• Fulvic acids (FA) soluble at all pH values,
  smaller molecules
• Humic acids (HA) soluble at pH gt 2, medium
  sized molecules
• Humin (Hu) insoluble at all pH, larger molecules

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Formation of Humic materials

• CH2O O2 ? CO2 H2O
• Complete decomposition of organic matter
• Two theories both could be important
• Degradative pathwayPlant material ? Humin ?
  Humic Acid ? Fulvic Acid ? Small molecule
• Synthetic pathway
• Plant material ? Small molecule ? Fulvic Acid ?
  Humic Acid ? Humin
• Could explain similarities of humic materials
  from various sources

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Humic Materials

• Humic material is a stable intermediate of the
  break down of biomolecules
• CH2O O2 ? CO2 H2O
• Carbon-14 dating has been used to age humic
  materials
• Stream humate 20 years
• Soil humus 500-1000 years
• Could have a significant impact on global climate
  change if we lose or gain soil humus due to
  different land use practices
• Grasslands and forests typically have higher soil
  organic matter than cropland

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Composition of Humic Materials

• Elemental composition of humic materials
• Carbon 45-60, usually closer to 60
• OM 1.7(OC)
• OC is usually measured
• FAltHAltHu
• Soil humus gt humate from lakes and oceans
• Carbon content of sediment HM increases with
  depth
• Oxygen 25-45
• FAgtHAgtHu
• Humate from lakes and oceans gt Soil humus
• Hydrogen 4-7
• Nitrogen 2-5
• Humate from lakes and oceans gt Soil humus
• Inorganic elements (ash) 0.5-5

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Measuring Organic Carbon in Water Samples

• Acidify samples
• Puts all carbonate and bicarbonate species into
  carbonic acid form
• CO32- 2H ? H2CO3
• Purge sample with CO2 free air
• Removes carbonic acid and CO2 from water sample
• H2CO3(aq) ? CO2(aq) ? CO2(g)
• Oxidize organic carbon using a catalyst and heat
• CH2O O2 catalyst ? CO2 H2O
• Detect CO2 with infrared detector

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Structure of Humic Materials determined using IR
and 13C NMR

• Oxygen containing functional groups are
  responsible for many of the chemical and physical
  properties of organic matter
• Carboxyl (RCOOH) 2-6 mmol/g humic material
• Phenolic (RC6H4OH) 1-4 mmol/g
• Alcoholic (RCH2OH) 1-4 mmol/g
• Carbonyl (ketones and quinones) 2-6 mmol/g
• Methoxyl (ROCH3) 0.2-1 mmol/g
• Aliphatic and aromatic groups
• Amine groups

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Structure of a generic humic substance

• Functional groups
• Carboxyl (RCOOH)
• Phenolic (RC6H4OH)
• Alcoholic (RCH2OH)
• Carbonyl
• Ketones
• Quinones
• Methoxyl (ROCH3)
• Aliphatic
• Aromatic
• Amine groups

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Structure of a generic humic substance taken from
another source

• Functional groups include carboxyl (RCOOH),
  phenolic (RC6H4OH), alcoholic (RCH2OH),
  carbonyl (ketones and quinones), methoxyl
  (ROCH3), aliphatic, aromatic, and amine groups

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Aqueous HM as a proton acceptor

• Carboxylate groups (RCOOH)
• pKa 2.5-5
• Acidifies pure water to pH 5.5-6.5
• Unprotonated when dissolved in natural waters at
  low concentrations
• May be major source on anionic charge in
  dissolved phase
• DOM can be an important buffering agent in some
  natural waters

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Example 12.2 page 265

• Total positive charge 44.7 mmol/L
• Total negative charge 9.4 mmol/L
• Net positive charge of 35.3 mmol/L
• 8.0 mg/L(CCOO- mmol/g) 8CCOO- mmol/L 35.3
  mmol/L CCOO- 4.4 mmol/g
• Concentration of carboxylate groups in range of
  2-6 mmol/g
• pH 5.88 is 2 pH units above normal pKa so 100
  of carboxyl groups should be deprotonated
• Negative charge not accounted for with simple
  ions is accounted for when considering OM

• A sample of water from the Canadian Shield
  containing dissolved humic material at a total
  concentration of 8.0 mg/L has measurable
  concentrations of the following ions
• H pH 5.88
• NH4 3.6 mg/L (as N)
• Na 75.9 mg/L
• K 50.8 mg/L
• Mg2 124 mg/L
• Ca2 569 mg/L
• Cl- 138 mg/L
• NO3- 7.0 mg/L (as N)
• HCO3- 14.4 mg/L (as C)
• SO42- 59.4 mg/L (as S)

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Forms of Humic materials

• Free HM soluble or insoluble forms
• Complexed HM chemically bound to metals,
  inorganic species (phosphate), or organic
  molecules solution or particle form
• Surface-bonded HM chemically bonded to other
  solids such as clay minerals, iron oxides,
  aluminum oxides chemical properties of particle
  are determined by organic matter not the solid if
  whole surface is covered with HM

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Humic material as a complexing agent for metal
ions

• Complex stability constant (Kf) depends on
• Nature and number of binding sites on OM
• Properties of metal
• pH
• Presence of other competing ligands
• 2RCOO- Ca2 ? RCOO- Ca2 -OOCR

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Reactions between humic material and small
organic molecules

• Humic substances are capable of reacting with
  many specific organic molecules
• Weak attractions van der Waals
• Strong attractions
• Reaction between positively charged species and
  negatively charged humate sites
• Atrazine is positively charged when pH lt 8
  leading to attraction to negatively charged humic
  material

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Reactions between humic material and small
organic molecules

• Strong attractions
• Hydrogen bonding (oxygen and nitrogen functional
  groups) can occur between humic material and
  organic molecule
• Carbaryl can hydrogen bind to HM with hydrogen
  bonding

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Reactions between humic material and small
organic molecules

• Strong attractions
• Salt linkage of negatively charged humic material
  with negatively charged organic molecule
• 2,4-dichlorophenoxy acetic acid (2,4-D) is
  negatively charged when pHgt6
• Carboxylate groups on humic material is
  negatively charged when pHgt6

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Reactions between humic material and small
organic molecules

• Strong attractions
• Hydrophobic interactions occur between nonpolar
  molecules and nonpolar regions in humic material
• Structure of water (polar) has less disruption
  (lower in energy) when nonpolar molecules are
  near nonpolar regions of humic materials
• DDT is a nonpolar insecticide

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PAHs binding to OM effects of pH and ionic
strength

• Schlautman and Morgan EST 27961 1993
• Increasing pH and ionic strength decreases
  binding of perylene

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PAHs binding to OM effects of pH and ionic
strength

• Schlautman and Morgan EST 27961 1993
• Increasing ionic strength decreases binding of
  pyrene and anthracene (pH 7 10)

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Binding of organic pollutants to organic matter

• Binding of hydrophobic pollutants to OM can
  dramatically increase the solubility of
  pollutants in water
• If humic material becomes insoluble (when
  riverine sources encounter sea water) the OM will
  remove pollutant from water column as the OM
  precipitates out of the water
• Reactivity of pollutant is altered when bound to
  OM
• Some pollutant experience enhanced degradation
  others experience reduced degradation

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Forms of Humic materials

• Free HM soluble or insoluble forms
• Complexed HM chemically bound to metals,
  inorganic species (phosphate), or organic
  molecules solution or particle form
• Surface-bonded HM chemically bonded to other
  solids such as clay minerals, iron oxides,
  aluminum oxides chemical properties of particle
  are determined by organic matter not the solid if
  whole surface is covered with HM

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Surface bonded humic materials

• Interactions of humic materials with clay
• Clay surfaces are negatively charged
• Cant have electrostatic interaction
• Metals on clay surface interact with humic
  material
• Hydrogen bonding between clay surface and humic
  material