Effects of Soil and Water Properties on Anionic Polyacrylamide Sorption
J. H. Lu, L. Wu,* and J. Letey
ABSTRACT
ture regarding the sorption reaction between soil and
PAM. In addition, the molecular weight of PAM used
in irrigation is much higher but the concentration is
much lower compared with those earlier studies. Information on PAM sorption at concentrations as low as
10 mg L⫺1 is sparse, possibly because of the difficulty
in determining PAM concentrations in soil solutions
(Lu and Wu, 2001).
Nadler and Letey (1989) and Malik and Letey (1991)
determined sorption isotherms of several types of polyanion by an Arlington sandy loam (coarse-loamy,
mixed, thermic Haplic Durixeralfs) through the use of
tritium labeled polymers. Their results suggested that
polymer sorption by soil was mostly limited to external
(outer) surface area and was considerably influenced
by water quality (Aly and Letey, 1988). Nadler et al.
(1992) also found that there was little desorption after
the polymers were adsorbed onto soil.
Soil and water properties such as texture, clay mineralogy, OM, and concentration of dissolved salts will
affect PAM sorption. Knowledge of the extent these
factors will affect PAM sorption is still lacking. The
objective of this study is to investigate the effects of
these soil and irrigation water properties on the sorption
of anionic PAM by soils.
Knowing the sorptive behavior of anionic polyacrylamide (PAM)
by soils is useful in predicting the appropriate application rate, depth
of effective treatment, and its mobility in soils. Sorption isotherms of
PAM by soil materials, six natural soils, and their subsamples with
partial organic matter (OM) removed by H2O2 oxidization under
different dissolved salt concentrations were examined. The PAM sorption isotherms can be well described by the Langmuir equation. Soil
texture, OM content, and dissolved salts (a combined contribution of
soil salinity and irrigation water quality) influenced the extent of PAM
sorption. Soils with high clay or silt content and low OM content
had a high sorptive affinity for anionic PAM. The amount of PAM
saturation sorption increased significantly as the total dissolved salts
(TDS) increased. Divalent cations Ca2ⴙ and Mg2ⴙ were about 28 times
more effective in enhancing PAM sorption than monovalent cations
Naⴙ and Kⴙ, mainly because of their stronger charge screening ability.
The effectiveness of cation enhancement on PAM sorption varied
with soil texture and was greater in fine soils than in sandy soils.
Organic matter had a negative effect on PAM sorption. Soil samples
after the removal of partial OM adsorbed more PAM than natural
soils. The negative effect of OM on PAM sorption was attributed to
the reduction of accessible sorption sites by cementing inorganic soil
components to form aggregates and to the enhancement of electrostatic repulsion between PAM and soil surface by its negatively
charged functional groups.
W
ater-soluble PAMs have been used as soil
amendments for various agricultural purposes,
such as minimization of surface water run-off, soil erosion and crusting, stabilization of soil structure, and
enhancing infiltration (Barvenik, 1994; Sojka and Lentz,
1996). High molecular weight (12–15 Mgm mol⫺1 ) and
moderately anionic charged (a substitution of NH2 by
OH at 苲10–20%) PAMs are the most effective types in
soil application (Barvenik, 1994; Anonymous, 1995).
Addition of PAM to soil will affect soil dispersion,
flocculation, and aggregation (Ben-Hur et al., 1992).
Knowledge of the sorptive behavior of PAM is useful
in predicting appropriate dose of application, depth of
effective treatment, its mobility in soil, and changes
in soil physical conditions. With good knowledge of
polymer–soil interactions, the optimal amount of polymer application can be potentially prescribed.
Polyacrylamide is one of the most widely used polymers. Its sorption by sand, silica, alumina, clay minerals,
latex, cellulose, and other materials has been the topic
of previous publications. Greenland (1972) and Theng
(1982) presented comprehensive reviews on the sorption of polymers in clay suspensions. Pefferkorn (1999)
discussed interfacial processes involving PAM sorption.
Nevertheless, little information was found in the litera-
MATERIALS AND METHODS
Anionic PAM (Celanese Corporation, Louisville, KY1 ) is
a high molecular weight polymer (10–15 million g mol⫺1 ). It
has 21% substitution of NH2 by OH so the polymer has a
moderate negative charge. The granular powder from the
manufacturer was purified three times by methanol precipitation prior to use in this study. The standard PAM stock solutions of various concentrations were prepared in deionized
(DI) water and in NaCl and CaCl2 solutions with a concentration range from 0.001 to 0.01 M. They were allowed to age
for 1 wk before use but if they were older than a month, new
solutions were prepared. The purpose of including NaCl and
CaCl2 in PAM standard solutions was to evaluate the effect
of dissolved salts on PAM sorption by soil. Besides PAM, all
reagents (Fisher or Aldrich Chemicals) used were analytical
or higher grades.
Six soils from the western United States of America, a
Linne clay loam (fine-loamy, mixed, superactive, thermic
Calcic Pachic Haploxerolls), an Imperial silty clay (fine, smectitic, calcareous, hyperthermic Vertic Torrifluvents), an Imperial silt loam (mixed, calcareous, hyperthermic Typic Torrifluvents), a Palouse silt loam (fine-silty, mixed, superactive, mesic
Pachic Haploxerolls), an Arlington loamy sand, and a Hanford
sand (coarse-loamy, mixed, superactive, nonacid, thermic
Typic Xerorthents), were selected to obtain a wide range of
1
Mention of a trademark proprietary product does not constitute
endorsement by University of California.
Dep. Environmental Sciences, Univ. of California, Riverside, CA 92521.
Received 15 Feb. 2001. *Corresponding author (laowu@mail.ucr.edu).
Abbreviations: CEC, cation-exchange capacity; DI, deionized; PAM,
polyacrylamide; OM, organic matter; TDS, total dissoved salts.
Published in Soil Sci. Soc. Am. J. 66:578–584 (2002).
578
579
LU ET AL.: SOIL AND WATER PROPERTIES AFFECTING ANIONIC POLYACRLAMIDE SORPTION
texture and OM contents (Table 1). Soil samples were collected from the top 10-cm layer of the profiles, air-dried, and
ground to pass through a 1-mm sieve. Organic matter content
was determined by 450⬚C combustion method (Davies, 1974),
and particle-size distribution by the hydrometer method (Gee
and Bauder, 1986).
To evaluate the effect of clay minerals on PAM sorption,
fine silica sand (No. 90, diam. ⬍0.15 mm, Paragon Building
Products, Inc., Norco, CA) and two clay minerals, a wellordered kaolinite (specific surface area of 10.05 m2 g⫺1 and
cation-exchange capacity [CEC] of 2.0 cmol kg⫺1 ) and a Cabased montmorillonite (specific surface area of 97.42 m2 g⫺1
and CEC of 120 cmol kg⫺1 ) were also used in this study. The
sand was thoroughly washed with DI water to remove any
fine particles and dissolved salts before use. The clay minerals
were purchased from the Source Clay Minerals Repository
(Columbia, MO). More information on the two clay minerals
can be found in Van Olphena and Fripiat (1979).
Sorption isotherms were determined by a standard batch
equilibrium method. Clay materials or soil samples were added
to PAM solution with concentrations ranging from 2 to 40 mg
L⫺1 in a series of 25-mL glass bottles. The solution/soil or clay
ratios ranged from 10 to 100 for soils, and 50 to 200 for clays
under different cation conditions to get an appropriate final
PAM concentration in supernatant. Preparatory experiments
showed that sorption of PAM increased slightly with increasing solution/soil or clay ratio, but ⬍10% within the above
ranges.
Preliminary experiments showed that more than half of
sorption occurred in the first 30 min and ⬎85% in the first
5 h. The final equilibrium took about 15 to 22 h, depending on
soil characteristics and dissolved salt concentrations in PAM
solution. In this research, capped bottles were shaken for 36 h
at 25 ⫾ 2⬚C on a reciprocal shaker. Following that, 10 mL
supernatant was centrifuged for 5 min. at 5000 ⫻ g to remove
possible suspended particulates before analysis. The amount
of PAM adsorbed was calculated from the difference between
the initial concentration of the PAM and the concentration
remaining in solution at the end of the sorption run. All experiments were duplicated.
Dry clay particles tend to coagulate when added to PAM
solution, which can cause incomplete mixing between the adsorbate and adsorbent. Thus, a clay suspension (1 g L⫺1 )
instead of dry clay was added to PAM solution in this research.
To achieve uniform distribution, the suspension was subjected
to vigorous stirring before it was taken out and added to the
PAM solution.
To evaluate the role of the OM in PAM sorption, isotherms
were also carried out on the same soils after removing a portion
of their OM. A range of OM contents was obtained by consecutive additions of aqueous 10% H2O2 (Palmer and Troeh,
1977). After one and two oxidation treatments, the OM contents of the Palouse silt loam was reduced from 54.5 to 33.4
and 13.2 g kg⫺1, and the Linne clay loam was reduced from
38.8 to 32.4 and 20.1 g kg⫺1, respectively. The Imperial silty
clay was reduced from 24.6 to 13.5 g kg⫺1, and the Arlington
loamy sand from 19.3 to 8.4 g kg⫺1 after one treatment. After
oxidization, the samples were heated to 95⬚C in a water bath
for 10 min to remove excess H2O2. They were then air-dried
and ground to pass through a 1-mm sieve.
The method of Lu and Wu (2001) was used to determine
PAM concentration in soil supernatants. This method was
based on determination of amide groups by the N-bromination
method and deduction of interferential moiety of dissolved
OM by spectrophotometry. It has a detection limit of 0.2 mg
L⫺1 and a linear range from 0.2 to 60 mg L⫺1 without dilution
of original samples. The concentrations of cations in soil super-
Table 1. Textural and chemical properties of the six test soils.
Soils
Clay Sand Silt OM†
Linne clay loam
Imperial silty clay
Imperial clay loam
Palouse silt loam
Arlington loamy sand
Handford sand
330
425
283
181
87
36
g
302
108
399
322
875
956
pH‡
kg⫺1
368
467
318
497
38
8
EC§
CEC¶
m⫺1
38.8
24.6
18.8
54.5
19.3
5.75
7.14
7.48
7.73
5.88
7.06
7.54
S
cmol kg⫺1
0.168
38.2
0.664
30.5
0.291
22.1
0.0742
25.1
0.868
13.9
0.0591
9.8
† Organic matter.
‡ pH was measured in saturated pastes of soils.
§ Electric conductivity (EC) are measured in extracts of soil pastes.
¶ CEC is cation-exchange capacity.
natants were determined by Inductively Coupled Plasma
(ICP) in an Optima 3000 DV Spectrophotometer (Perkin Elmer Corporation, Norwalk, CT). The analysis wavelengths for
Na⫹, K⫹, Ca2⫹, and Mg2⫹ are 589.592, 766.490, 317.933, and
285.213 nm, respectively.
RESULTS AND DISCUSSION
The PAM sorption isotherms for fine sand (silica),
kaolinite, and montmorillonite are shown in Fig. 1, and
for soils are shown in Fig. 2. These sorption isotherms
displayed an L-type shape (Giles et al., 1974) and tended
to reach a plateau within the range of PAM concentrations used in this study, which agrees with the results
of earlier research. It was found that the isotherms of
hydrolyzed PAM sorption by cationic polystyrene latex
(Meadows et al., 1989); cellulose (Lindstrom and Soremark, 1976), kaolinite (Nabzar et al., 1988) and montmorillonite (Ben-Hur et al., 1992) were all L-type.
Sorption isotherms were described by the linear form
of the Langmuir equation
1
C
C
⫽
⫹
A Asb As
[1]
where A is the amount of PAM adsorbed per unit weight
of soils (mg g⫺1 ), b is the constant related to the binding
energy, As is the amount of PAM saturation sorption
(mg g⫺1 ), and C is the equilibrium concentration in
solution (mg L⫺1 ). The values of As and b can be obtained from the slope and intercept of the regression
line based on experimental data. In this study, most of
the correlation coefficients were ⬎0.97 (n ⫽ 6). The
Langmuir equation describes the PAM sorption by soils
very well, as shown in Fig. 1 and 2.
The fitted amounts of PAM saturation sorption (As )
are presented in Table 2. Compared with most pesticides
and herbicides, the amount of PAM sorption was very
high. The high affinity of PAM to clay minerals and
soils are because of the mechanisms of multi-segment
sorption of its long chain (Theng, 1982).
Effect of Soil Texture
on Polyacrylamide Sorption
Unlike the small organic molecule or nonionic polymers, anionic polymers do not enter the interlayer space
of expanding layer silicates. The extended coils of PAM
because of charge repulsion on their long chains prevent
them from doing so. Thus their sorption is limited to
the accessible outer surface (Theng, 1982). Accordingly,
580
SOIL SCI. SOC. AM. J., VOL. 66, MARCH–APRIL 2002
agrees with the order of their specific outer-surface areas. In the presence of salts at a certain concentration
range, clay minerals can adsorb PAM several times
greater than soils do, and ten or several hundred times
greater than the fine sand does (Table 2).
As the chemical compositions of inorganic components of soils are mainly silicate or alumino-silicate materials, it is safe to assume that per unit surface area of
sand, silt, and clay fractions have similar active sites for
PAM sorption. Soils with fine texture, such as Linne
clay loam and Imperial silty clay, tend to adsorb more
PAM than coarse-textured soils such as Arlington loamy
sand and Hanford sand when the solution contains the
same type and concentration of cations (Table 2).
The effect of thickness and shape of clay tactoid on
PAM sorption was evaluated by adding various amounts
of salts to the suspension. When PAM was prepared in
DI water or when the cation was Na⫹ (PAM was prepared in NaCl solution), the amount of PAM sorption
by montmorillonite was only slightly higher than by
kaolinite. However, when Ca2⫹ was present, the amount
of PAM sorption by montmorillonite was two to three
folds of that by kaolinite, even though the salt concentration was the same (Table 2). In the presence of Ca2⫹,
the number of plates per tactoid of montmorillonite
tends to decline (Green et al., 1978), which provides a
relatively large total accessible surface area. However,
this does not happen with kaolinite. A PAM study conducted by Ben-Hur et al., (1992) using montmorillonite
and illite, showed similar results.
Effect of Dissolved Salts
on Polyacrylamide Sorption
Fig. 1. PAM sorption isotherms by sand and clay minerals under
various salt concentrations. PAM solutions were prepared in: (i)
䊉 DI (deionized) water; (ii) 䉭 0.002 M NaCl; (iii) 䉫 0.010 M NaCl;
(iv) 䉱 0.001 M CaCl2; and (v) 䉬 0.005 M CaCl2. Each data point
is the mean of two replicates and error bar is its average deviation.
Solid line is fitted Langmuir sorption isotherm.
factors that affect the accessible outer-surface area, such
as size, thickness, and shape of clay tactoids in suspension, will influence the amount of PAM sorption.
As a result, the amount of PAM sorption by fine
particles was greater than by large particles (Malik and
Letey, 1991). Figure 1 clearly shows that in solutions
of equal salt concentration, PAM sorption was in the
order of montmorillonite ⬎ kaolinite ⬎ fine sand, which
The isotherms in Fig. 1 and 2 indicate that the sorption
of anionic PAM by the same minerals and soils increased
significantly as the concentrations of dissolved salts increased. For all six test soils, the amount of PAM saturation sorption in 0.01 M NaCl was 0.5 to 1.5 and in 0.005
M CaCl2 was four to nine times greater than that in DI
water. Soils with high dissolved salt content, such as the
Arlington loamy sand, adsorbed more PAM than the
Palouse silt loam, even though the texture of the Arlington loamy sand was coarser than the Palouse silt loam
when PAM solution was prepared in DI water. Clay
minerals such as montmorillonite and kaolinite have
high accessible surface areas and can adsorb more PAM
than soils do, but the magnitude of PAM sorption was
similar by both the minerals and soils in DI water (Fig.
1 and 2), because of the lower salt concentration in clay
than in soil suspension. In contrast, minerals adsorbed
five to ten times more PAM than soils in 0.005 M CaCl2.
The results in Table 2 showed that clay minerals adsorbed relatively low amounts of PAM in DI water
relative to their particle sizes (Table 2), which was attributed to lower salt concentrations in the clay suspensions
than in the soil suspensions.
The nature of interactions between anionic polymer
and soil material surfaces is still not well understood.
Hydrogen bonding (Kohl and Taylor, 1961; Nabzar et
al., 1984) and ligand exchange (Theng, 1982) are two
LU ET AL.: SOIL AND WATER PROPERTIES AFFECTING ANIONIC POLYACRLAMIDE SORPTION
581
Fig. 2. PAM sorption isotherms by the six test soils under various salt concentrations. PAM solutions were prepared in: (i) 䊉 DI (deionized)
water; (ii) 䉭 0.002 M NaCl; (iii) 䉫 0.010 M NaCl; (iv) 䉱 0.001 M CaCl2; and (v) 䉬 0.005 M CaCl2. Each data point is the mean of two
replicates and error bar is its average deviation. Solid line is fitted Langmuir sorption isotherm.
bonding mechanisms often suggested by earlier researchers. H-bonds usually occur between the amide
group of polymer and the free hydroxyl group of the
adsorbent surface, which are not H-bonded with other
neighboring hydroxyls (Griot and Kitchener, 1965; Pefferkorn et al., 1990). While in ligand-exchange bonding
mode, the carboxylic group of the PAM enters the inner
coordination layer of the edge Al to form a coordination
complex. However, as both anionic PAM and surface
of soil materials are negatively charged in a normal soil
pH range of 5 to 9, electrostatic repulsion prevents PAM
sorption through H bonding, ligand exchange, or other
unknown mechanisms. As a result, the sorption process
is entirely governed by the competition between polymer
attractive interaction with soil surfaces and repulsive electrostatic forces, as observed in the case of hydrolyzed
PAM sorption by siliceous materials (Lecourtier et al.,
1990). The PAM sorption therefore increases rapidly
582
SOIL SCI. SOC. AM. J., VOL. 66, MARCH–APRIL 2002
Table 2. The amount of polyacrylamide (PAM) saturation sorption (As, mg g⫺1 ) by clay minerals and soils.
NaCl
CaCl2
Soil materials or soils
DI†‡
0.002 M
0.010 M
0.001 M
0.005 M
Fine sand
Kaolinite
Montmorillonite
Linne clay loam
Imperial clay loam
Imperial silty clay
Palouse silt loam
Arlington loamy sand
Handford sand
0.0127
0.1604
0.1944
0.2759
0.2104
0.2424
0.0815
0.1697
0.0787
0.0156
0.4081
0.4532
0.3357
0.2517
0.2913
0.1123
0.1721
0.1100
0.0189
0.7619
0.8013
0.3936
0.2857
0.3498
0.1431
0.1909
0.1202
0.0507
1.3341
2.5849
0.7082
0.4803
0.5715
0.2819
0.2595
0.1817
0.0982
3.0421
7.2323
1.9088
1.1962
1.5954
0.6848
0.6199
0.4493
† Deionized water.
‡ The signs in this row only indicate the salt concentration of PAM solutions when they were prepared. The exact salt type and concentration
in sorption solution is different from numbers listed in the table, as clays
and soils may release or adsorb some cations from aqueous solution
during the sorption process.
when electrostatic repulsion is reduced by increased
cation masking. Moreover, this reduction of electrostatic repulsion also favors the attraction between adsorbed and nonadsorbed PAM molecules, thereby
allowing more molecules to approach the interface and
to be adsorbed.
Another possible mechanism for enhanced PAM
sorption by cation masking may be attributed to the
change of configuration in PAM molecular structure.
Light-scattering measurement showed that the average
gyration radius of the hydrolyzed PAM molecule in
aqueous solution tends to decrease in the presence of
salts (Muller et al., 1979).
Major cations in soil solution include Na⫹, K⫹, Ca2⫹,
and Mg2⫹ (Wolt, 1994). To show the effect of different
valent cations on PAM sorption, the amounts of PAM
saturation sorption by six soils were plotted against the
concentration of monovalent cations (Na⫹ and K⫹ ) and
divalent cations (Ca2⫹ and Mg2⫹ ), respectively (Fig. 3).
The concentrations of cations were measured in the
supernatant after each sorption experiment with a 20
mg L⫺1 initial concentration of PAM. Figure 3 shows
that divalent cations (Ca2⫹ and Mg2⫹ ) are much more
effective than monovalent cations (Na⫹ and K⫹ ) in enhancing PAM sorption by soils at the same concentration, as indicated by the data points for all six soils being
steeper in case of Ca2⫹ and Mg2⫹ than those of Na⫹ and
K⫹. It was estimated from the average of slopes in Fig.
3a and 3b that the divalent cations are about 28 times
more efficient in enhancing PAM sorption than the monovalent cations in the range of concentrations used in
this study. This is in the same magnitude with their
relative flocculation power (Na⫹ ⫽ 1, K⫹ ⫽ 1.8, Mg2⫹ ⫽
27, and Ca2⫹ ⫽ 45; Rengasamy and Sumner, 1998). Since
the concept of relative flocculation power is a comprehensive index of charge screening ability for cations,
this phenomenon signified that the mechanism of PAM
sorption enhancement by salts is mainly because of their
charge screening ability. Another possibility for divalent
cations being more effective than monovalent is that
divalent cations, such as Ca2⫹, may act as a binding ion
between the carboxylate groups of PAM chain and the
anionic surface sites of soil particles (O’Gorman and
Kitchener, 1974; Theng, 1982).
Fig. 3. Influence of salt concentrations on the amount of PAM saturation sorption.
The enhancement of PAM sorption by cations varies
with soil texture (Fig. 3). When total concentration of
Ca2⫹ and Mg2⫹ in solution increased from 0.5 mM to
苲5.0 mM, the increasing amounts of PAM saturation
sorption by the six test soils were 0.37, 0.45, 0.60, 0.99,
1.54, and 1.63 mg g⫺1 for the Hanford sand, Arlington
loamy sand, Palouse silt loam, Imperial silt loam, Imperial
silty clay, and Linne clay loam, respectively. Roughly,
this follows the increasing order of clay and silt contents
of the six soils. This result suggests that the function of
charge screening of the same cations is more effective
in fine soils than in sandy soils, possibly because of the
higher negative charge density on the surface of fine
soils than the sandy soils. The same trend was also found
for the monovalent cations (Na⫹ and K⫹ ), except that
LU ET AL.: SOIL AND WATER PROPERTIES AFFECTING ANIONIC POLYACRLAMIDE SORPTION
583
the magnitude of increase in PAM sorption was much
less compared with divalent cations.
Electrostatic repulsion is strong enough to prevent
any PAM sorption in some cases. Lecourtier et al. (1990)
found that there exists a critical salt concentration level
for highly charged particles such as silicon carbide. No
sorption was observed below that level. For soils, previous studies also indicated that attractions between negatively charged soil and PAM requires a small quantity
of divalent cations in water to shrink the electrical double layer and bridge the soil and PAM negative charge
sites to enable flocculation (Wallace and Wallace, 1996).
Our results suggest that when anionic PAM is applied
to dispersive soils, which are usually highly negatively
charged, a certain amount of Ca2⫹ in irrigation water is
necessary to ensure the effectiveness of PAM application, since sorption is the prerequisite of PAM function
in stabilizing soil aggregates.
Effect of Organic Matter on PAM Sorption
The amounts of PAM saturation sorption by four
natural soils and their subsamples with OM lowered by
H2O2 oxidization are plotted versus their OM contents
in Fig. 4. It is obvious that the sorptive affinity of the
soils to PAM increased after some of the OM was removed. For example, the amount of PAM saturation
sorption of the Linne clay loam increased from 0.28 to
0.32 and 0.38 mg g⫺1 after its OM contents were lowered
from the original 38.8 to 32.4 and 20.1 g kg⫺1, respectively. This phenomenon was in agreement with earlier
observations by Nadler and Letey (1989). Since the oxidation process did not change texture and salinity, the
increase in PAM sorption was mainly because of the
decrease of OM content. Hence, it was concluded that
OM in soil had a negative effect on PAM sorption. This
helps to explain why soils with high OM content such
as the Palouse silt loam, have a relatively low PAM
sorption in DI water (Table 2), even though the texture
may be fine.
The extent of negative effect by OM on PAM sorption
is also related to soil texture. Figure 4 showed that in
fine soils such as the Linne clay loam and Imperial
silty clay, a small reduction of OM content led to a
considerable increase of PAM sorption; while in coarse
soils such as the Palouse silt loam and Arlington loamy
sand, the increase in PAM sorption was relatively small.
This phenomenon was attributed to the fact that OM
is more effective in cementing clay than sand together.
Earlier studies suggested that high molecule PAM
had limited ability to diffuse into soil aggregates (Elhardy and Abd El-hardy, 1989) and its sorption was
largely restricted to the external surface (Malik and
Letey, 1991). Soil OM plays an important role in forming
soil aggregates. Removal of soil OM breaks down soil
aggregates and exposes new accessible sites for PAM
sorption, which definitely increases the sorption amount
of PAM. In addition, in the normal soil pH range of 5
to 9, the majority of functional groups in OM carry a
negative charge. Decrease in OM content reduced the
electrostatic repulsion between soil particles and anionic
PAM molecules and thus increased PAM sorption.
Fig. 4. Effect of organic matter on the amount of PAM saturation
sorption. The same symbol in the graph represents the same soil,
but different OM contents.
It is true that some functional groups such as –OH,
–COOH, –Phenyl OH, and –NH2 might form H-bonds
with –COOH and –NH2 groups in PAM molecule. Thus
high OM may favor PAM sorption. However, the increase of PAM sorption after OM oxidization showed
that this kind of H-bonding could not negate the abovementioned two mechanisms, possibly because the direct
sorption of PAM by OM is only a small proportion of
the total PAM sorption. This is quite different from
the behavior of small molecular compounds such as
pesticides and herbicides. In the latter, OM may be
decisive in determining the amount of sorption, since
most pesticides or herbicides have hundreds or thousands times of affinity with OM than with inorganic components.
CONCLUSIONS
The sorption behavior of anionic PAM by soils was
quite different from small molecular organic compounds
such as pesticides. It has a high affinity on soil components owing to the multisegment sorption mechanism.
In concentrations ranged from 0 to 40 mg L⫺1, the PAM
sorption isotherms can be well described by the Langmuir equation. However, its large molecular size and
the stretched-out long chain nullify its access to the inner
sorption sites of soil particles and aggregates, limiting
sorption to the outer-surface area. As a result, the
amount of PAM saturation sorption is highly related to
soil texture and degree of aggregation.
In the normal soil pH ranged 5 to 9, sorption of the
anionic PAM must overcome electrostatic repulsion as
the majority of soil surfaces are negatively charged. The
presence of cations in aqueous solution, either from the
salinity of soils or from irrigation water, significantly
increases PAM sorption. The efficiency of the cation
enhancement depends mainly on their charge screening
584
SOIL SCI. SOC. AM. J., VOL. 66, MARCH–APRIL 2002
ability. For the six soils used in this study, divalent cations, such as Ca2⫹ and Mg2⫹, are about 28 times more
effective than monovalent cations such as Na⫹ and K⫹
under the same concentration.
High OM contents reduce the PAM sorption, possibly
because of the reduction of accessible sorptive sites since
OM can cement inorganic soil components into soil
aggregates and increase the electrostatic repulsion between particle surfaces and PAM molecules.
This research explored the effect of soil and water
properties on PAM sorption using a batch equilibrium
experiment. The isotherms of PAM sorption under the
field conditions might be different, which will be the
topic of our future research.
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