ABE 527
Rabi H. Mohtar
Chemicals in the Environment
Factors affecting non-point source loads
1.
Source level
2.
Partitioning in the soil
3.
Degradation: microbial, photodecay
4.
Leaching – Subsurface Transport
5.
Runoff and sediment movement – surface transport
6.
Volatilization
7.
Plant uptake/Bioconcentration (animal uptake)
8.
Chemical oxidation
9.
Photo-transformation
10.
Hydrolysis
Phase Relationships
Mass
Volume
MA  0
Air
VA
M W   sVw
Water
VW
MW  sVs
Solid
VS
M tot  M w  M s
Bulk Density = b 
Water Content
Gravimetric
Volumetric
Vtot  VA  Vw  Vs
Ms
Vtot
w  MW M s
  Vw Vtot
Chemical transport by surface water
1.
2.
Particulate Phase
 Adsorption to sediment
 Chemical Solids (crystals or pellets)
Dissolved phase

Loading of chemical, A  YA g
runoff event

YA  YS S A  W CA
where
YS = Sediment Yield (Mg (metric tons)/event)
S A = Adsorbed Chemical Conc. g/Mg = μg/g
 = Volume Runoff m3
CA = Dissolved concentration IN RUNOFF g/m3
Adsorption Isotherms
Adsorption Models  Isotherms
1. Freundlich: Empirical Relationship
S A  KC Ab
where S A = adsorbed concentration μg/g
CA = solute concentration μg/cm3 = g/m3

3
K = adsorption coefficient cm
g

1
b
b = empirical coefficient
2. Langmuir: Based on monolayer of chemical at saturation
abC A
SA 
1  bC A
a = empirical value of max possible adsorption (saturation) for the soil (mg/g)
3
b = saturation coefficient (empirical) m
g
Example Phosphorus Adsorption
abC p
SA 
1  bC p
 
a  function (clay, oc)
a  3.5  10.7 % clay  49.5% oc
(for acid soils)
b  function (clay, oc, pH)
b 0.061  169,832 10 pH   0.027  % clay   0.76  % o.c.
Fertilizer Application Rate 90 kg/ha P
Mixed by Tillage to 30 cm
Initial P = 3.3 μg/cm3
90 kg
109  g
108 ha 2
ha
kg
cm
Padded 
 30  g 3
cm
30cm


 

mass P
soil volume
Ptot  S p b  C p  33.3  g
for our soil:
1)
SP 
2)
SP 
cm3
clay = 20%
silt = 55%
sand = 25%
oc = 1%
a = 260 mg/g
b = 1.53 cm3/mg
abC p
pH = 6
ρb = 1.5 g/cm3
= 0.4 cm3/cm3
1  bC p
P
tot
 C p 
b
Substitute and solve
abC p b   P  C p 1  bC p 
 b  C p2   abb    bP  C p  P  0
 0.612  CP2   546.15  C p  33.3  0
546.15  298, 280.9  81.52
1.224
CP  0.0618  g 3
cm
S P  22.2  g
g
Ptot  33.3  g 3 soil
cm

C P  0.025 g 3 soil
cm
SP b  33.3  g 3 soil
cm
CP 
For practical purposes:
Ptotal  S p b
A)
B)
Cp is very small!
Phosphorus Yield
Yp  AW  SDR  ER  SP
A = soil loss (tons/ha)
W = watershed area (ha)
SDR = sediment delivery ratio (unitless)
ER = sediment enrichment ratio (unitless)
SP = adsorbed P concentration (g/ton)=(μg/g)
YP = P yield (g)
For prior example, if:
A = 10 tons/ha
W = 50 ha
SDR = 0.2
ER = 1.3
SP = 22.2 g/ton
YP  10 50 0.51.3 22.2   2900 g  2.9kg
Suppose watershed empties into a 3 ha lake of depth H=5m. What is the loading to the
lake?
2,886 g
ha
P

 0.10 g 2
2
m
3ha
10, 000m
Pesticides
Amount of pesticides reaching waterways are function of:
1.
Rate of application
2.
Persistence [conservation or nonconservation]
3.
Mobility
3 classes of pesticides:
1.
Insecticides
a. Organochlorine – very conservative DDT10 yrs
b. Organophosphorus
c. Carbamate
2.
Herbicides
a. Generally non-conservative
b. Some may persist up to a year
3.
Fungicides
a. Typically in orchards and on vegetable farms
Pesticide Adsorption
Pesticide Mobility
Sorption of pesticides
Most are non-ionicadsorption is to organic matter less to clay
Some are organic cations – diquet and paraquet – adsorb strongly to clays
S=KC Special case Freundlich
S = adsorbed concentration μg/g
C = solute concentration μg/cm3
K = adsorption coefficient cm3/g
K = fnc(o.c., pesticide type)
K  K 1  %o.c./100
Mobility class
K1 =
fnc (pesticide type)
1
>1000
2
400
3
100
4
20
5
<15
K 1  K oc
Pesticides Example:
3
Lindane: class 1 pesticide  K oc  2000 cm
g
Application rate 11.4 kg/ha b  1.2 g 3
cm
o.c. soil = 3%
=0.3
3
cm
K  2000(0.6)  60
g
incorporated depth = 7.6 cm
Total Pesticide, T
kg ha 109  g 1
T  11.4
 15.2  g 3
8
2
cm
ha 10 cm kg 7.5cm
SL  KCL
1)
T  S L b  CL
2)


T   K    C
C  15.2
 60 1.2   0.3  0.21  g cm
3
 cm3 
g
g
S  60 
 0.21 cm3  12.6
g
 g 
T  15.14  0.06
adsorped
dissolved
Microbial Degradation
Pesticide Relationships
EPA/600/3-87/015
Processes, Coefficients, and Models for Simulating Toxic Organics and Heavy Metals in
Surface Waters
Biological Transformation
Michaelis-Menton kinetics
dT
  k bT
dt
where kb 
ˆ X
Y  Km  T 
̂ = max specific growth rate of culture (1/doubling time)
X = biomass concentration (cells/cm3)
Y = yield coefficient (cells produced/toxicant removed)
Km = ½ saturation constant (value of T where kb  1 ˆ )
2
 1  # cells 
 

t
vol 
kb   
 1
t
 # cells   M p 

 

 M p   vol 
 
Table A.1. (EPA Handbook)
T  To e  kbt
lnT
T
e-kt
k
t
Facors affecting biodegradation rates:
1.
Temperature
k  k20 T  20
2.
3.
4.
t
  1.04  1.13
Nutrients – necessary for growth
Acclimation – shock load toxicant may kill portion of organisms
Population Density – (a lag occurs if # of organisms too few)
Oxidation
2nd order
dT
  KOxT
dt
Ox =conc. of oxidant
Under natural waters Ox is constant source
1st order
dT
  K 1T
dt
Photolysis
Sun
Energy increase
molecule
Back to original
molecule
Excited electron state
Loses energy
+
fluorescence or
phosphorescence
dT
  kaT
dt
ka is rate constant for adsorption of light by the toxicant
number moles toxicant released

number einsteins adsorbed
(unit of light – molar basis of photons)
 = efficiency of transformation
ka=intensity  adsorption
Converted to
new molecule
Indirect Photolysis
Sun
energy
non-target
molecule
toxicant
molecule
release
energy
new molecule
dT
  k 2  T   k pT
dt
2nd order psuedo 1st order
k2 is indirect photolysis rate constant
x is conc. of non-target intermediary
Volatilization
Movement from liquid to gaseous phase.
fnc  molecular wt.
Two-Film Theory
Cg

≡
Gas film
Csg
Csl
Liquid film

≡
Cl
Mass moves from areas of high to low conc.
H=p/c Henry’s Law
H=Henry’s law constant
p=partial pressure of chemical in gas film
c=solubility of chemical in the fluid film
Liquid/gas film resistance
dimensionless (mg/l)/(mg/l)
units (atm-m3/M)
H  0.1
liquid film controls
3

3 atm  m 
2.2

10


mole 

H  0.1
gas film controls
Rate Transfer Eq. – Liquid to Gas
dc K L

 cs  c 
dt
d
cs = saturated chemical conc.
KL = rate transfer coefficient
d = avg. fluid depth
KL is computed as geometric meanresistance in series
1
1
1


K L K li K gi
liquid
film
rate
coef.
gas
film
rate
coef.
K li & K gi can be computed from diffusivities of the chemical in liquid and air,
respectively.
Liquid Film Transfer} Kli from comparing to oxygen transfer as standard
½
D
  Rivers & Lakes
K li  K a  l

 DO 2 
Ka = oxygen re-aeration coef.
cm2
@ 20 C
DO2 = diffusivity O2 in water = 2.4 105
s
Dl = diffusivity chemical in water
2
2
Dl   22 105  M w 3 cm
s



molecular
weight
Gas Film Transfer
2
0.001  Dg  3
K gi 

 W  Lakes
h  Vg 
h=water depth (m)
vg = kinematic viscosity air
w = wind speed (m/s)
 0.12  0.17 cm s 
2
Dg = diff. chemical in air
2
2
Dg  1.9M w 3 cm
s

