Chemical Reactions

1
Chemical Reactions

• What are some major chemical reactions in water?
• Solution-precipitation, acid-base, redox,
  ion-exchange, complexation, etc.
• Also check out Textbook pages 12-13 for example
• Well talk about some of these in-depth in coming
  weeks

2
Dissolved Gases, Alkalinity-Acidity Hardness

• Why and how gases dissolve in water?
• Henrys law Ci KiPi
• Role dissolve gases (DO) play in water chemistry
• Oxygen Org CO2 water (BOD and/COD)
• Oxygen dissolved iron water rust acidity
• CO2 dissolves easily (K 3.3810-2) gt increase
  in H (pH5.7) help dissolve carbonate
  subsurface (soil) gt CaCO3H Ca HCO3

3
Acidity vs. Alkalinity

• Acid (donate H) vs. base (accept H)
• Difference between pH and acidity
• pH -log (H) gt (H) 10-pH
• Acidity amount of base needed to change pH to
  endpoint
• Why acidity important?
• Sources and types of acidity
• Mineral acidity vs. gas acidity
• how to determine and/or calculate acidity

4
Net Acidity and Net Alkalinity

• Sources of alkalinity and types of alkalinity
• How to determine alkalinity
• How to convert Alk. in CaCO3 mg/L to HCO3-
• HCO3 Alk61/1(210-10.3/10-pH)50
• Net acidity hot acidity gt Calculated acidity
  Alkalinity -Net alkalinity
• Net alkalinity alkalinity - acidity

5
Hardness in Water

• Caused by Ca and Mg containing minerals and
  other divalent cations (Fe, Mn, etc.)
• Causes problems scales, less suds, soap scum
• Ca 2HCO3 -gtCaCO3 H2O CO2
• Measured as CaCO3 (mg/L) or grains 17 ppm/grain
• gt5 grains per gallon (gt100 ppm) need treatment
  (softener)
• Softener are based on Ion Exchange (Ca w/ Na)
• People w/ high pressure should ne cautious about
  Na
• Hardness measured by titration using blue
  indicator EDTA
• Total hardness as CaCO3 Ca hardness Mg
  hardness
• Ca and Mg (mg/L) can be calculated from hardness
  data
• Ca Hardness 0.4 Ca ppm
• Mg Hard 0.84gtMgCO3.29 Mg ppm

6
Ion Exchange and Complexation Reactions

• Cations/Anion with similar charge/radii can be
  exchanged between solution and exch. surface
• Water softener exchanges Na for Ca and Mg
• Important process in soil water solution for
  plants
• Complexation are charged compounds
• Important for polyvalent ions (Cr, Fe, Al, Pb,
  As..)
• Provide a means for trace metal transport
• PHREEQC can calculate easily (manual calc. hard)

7
Soil Erosion We Cant Afford It

• While the farmer holds title to the land, it
  belongs to all of the people because civilization
  itself rests upon soil.
• Thomas Jefferson
• When the soil is gone, men must go and the
  process does not take long.
• Theodore Roosevelt
• Soil is a few inches between humanity and
  starvation. (source unknown)
• Like Geology or Biology, Soil Science is a
  discipline

8
Soil Irrigation Water Chemistry

• Definition and Importance
• Crop, removal of bio-chem. contaminant, nutrient
  recycling, building materials, waste disposal,
  eng. etc.
• Composition minerals clay, nutrients,
  organisms, organics (humus), water, and gases
  (air)
• Soil Formation Factors ClORPT
• Soil Classification Sand-Silt-Clay ratio
  (Ternary)
• Nutrients macro (NPKS, Ca, Mg), micro (Fe, Mn,
  Zn, Cu, Co), and trace (Pb, Cd, Hg, B, Ce)

9
Soil Chemistry (contd)

• Processes Solution-Precip
• Adsorption-Desorption (AlgtHgtCagtMggtKgtNa)
• Cation Exchange (humusgtvermiculitegtsmectititegtilli
  tegtchloritegtkaolinitegtFe, Al oxides),
• Complexation (for poly-valent cations),
• Chealation (FegtCu, ZngtMg,),
• Plant Uptake, Transportation/Mobility
• Factors pH and acidity (active vs. reserve),
  Eh, CEC and B.S., Humus, Clay (dep. on types)

10
Soil Chemistry (contd)

• Determination of lime requirements (for acidic
  soil),
• Gypsum (for sodic soil), nutrients (K, N, P),
  organic content (LOI) for CEC adsorption.
• Factors Effecting Soil Erosion (R)USLE
• A (tons/hec)RKLSCP (explain terms)
• EI (RKLS)/T Ttolerance in Mg/Hec (used in
  USDAs CCP)
• T 4-11 Million grams/hec Egt8 is a problem
• Sources of Soil Contamination and Soil
  Remediation
• LUST, pesticides, trace/heavy metals, cleaning
  agents
• Excavation disposal to landfill, soil venting,
  bio/phytoremediation, onsite treatment (venting,
  bioenhanced farming, thermal treatment)

11
Irrigation Water

• Salinity (EC in mmohs/cm) Plant sensitivity to
  salt, gt3 is high risk
• SAR Na/sq.rt.CaMg/2 in meq/L
• gt18 high risk
• RSC(CO3HCO3)-(CaMg) in meq/L
• gt2.5 high risk
• Toxic elements good in moderation, but bad in
  excess
• B ( helps cell division),
• Na (damages soil structure CEC),
• Co, Ni, As, Zn
• Cu (helps metabolism)
• Mn (imp. for photosysn.)
• Mo (helps w/ N assimilation)
• Zn (promotes hormone growth) for irrigation
  water.