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Published online 12 March 2007
Published in Soil Sci Soc Am J 71:537-544 (2007)
DOI: 10.2136/sssaj2006.0163
© 2007 Soil Science Society of America
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SOIL & WATER MANAGEMENT & CONSERVATION

Soil Factors Influencing Suspended Sediment Flocculation by Polyacrylamide

Richard A. McLaughlina,* and Nathanael Bartholomewb

a Dep. of Soil Science, North Carolina State Univ., Box 7619, Raleigh, NC 27695-7619
b Dep. of Soil, Water, and Climate, Univ. of Minnesota, St. Paul, MN 55108

* Corresponding author (rich_mclaughlin{at}ncsu.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Turbidity in stormwater runoff may be treated with polyacrylamide (PAM) to flocculate suspended sediment, but the relationships between PAM properties and those of the suspended sediment have not been widely studied. Our objective was to determine how soil physical and chemical properties affected flocculation by PAMs with a variety of characteristics. Subsoil materials were collected from 13 active construction sites around North Carolina. These were tested for flocculation using PAMs with charge densities of 0 to 50% and molecular weights of 14 to 28 Mg mol–1, at concentrations of 0.025 to 10 mg L–1. Soil was added to solutions of single and mixed PAMs with various molecular weights and charge densities and the turbidity was measured 30 s after mixing. Five kaolinitic subsoils had linear responses to PAM regardless of molecular weight or charge density, with an optimal dose of 1 to 2 mg L–1 to obtain >96% reduction in turbidity. Increased turbidity, indicating stabilization, occurred for two additional soils with anionic PAM concentrations >0.5 mg L–1, but a neutral or a mixed anionic PAM product reduced turbidity at those concentrations. The remaining six suboils had widely differing patterns of response, including little or no turbidity reduction with any single anionic PAM. Increasing smectite and vermiculite content (>20%) in suboils from the Coastal Plain reduced the effectiveness of single, anionic PAMs for flocculation. The mixed anionic PAM, however, reduced turbidity in most subsoils and did not have a stabilization reaction at higher concentrations (1–5 mg L–1). Texture, mineralogy, and extractable Fe were highly correlated with reductions in turbidity with PAM, but most of the differences in flocculation occurred in subsoils with 20% or more smectite or vermiculite.

Abbreviations: CBD, citrate–bicarbonate–dithionite • PAM, polyacrylamide


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Polyacrylamide has been used for many years in water treatment and industrial processes to settle suspended solids (Peng and Di, 1994; Lentz and Bjorneberg, 2003). It is also used to reduce erosion and sediment transport in agricultural fields and construction sites (Sojka and Lentz, 1997; Flanagan et al., 2003). There is increasing interest in using PAM to settle suspended sediment in runoff from construction sites. Because PAM is a class of chemicals that has many different properties, the selection of a specific PAM for a given soil or sediment is often accomplished through a batch screening process. The critical PAM characteristics are net charge, charge density, and molecular weight. Cationic PAMs are usually avoided in environmental applications due to their potential aquatic toxicity (Barvenik, 1994), although suspended solids in water typically found under natural or disturbed conditions are likely to greatly reduce toxicity by binding to the PAM (Biesinger et al., 1976; Beisinger and Stokes, 1986; Goodrich et al., 1991).

The effectiveness of PAM as a flocculant is affected by the characteristics of the material being treated. Laird (1997) demonstrated that the efficacy of anionic PAM for clay flocculation varies with mineralogy (kaolinite > illite >> quartz). In addition, Ben-Hur et al. (1992) found adsorption of anionic PAM on illite to be 200 to 400 times greater than on smectite clays. Anionic PAM has generally been found to be much more effective at acidic pH levels than neutral or higher levels (Laird, 1997; Deng et al., 2006), although Peng and Di (1994) reported an optimal range of pH 5 to 6 for kaolin. Goldberg and Glaubig (1987) and Arora and Coleman (1979) demonstrated that mixtures of kaolinite and smectite behaved more like montmorillonite in coagulation experiments. Polyacrylamide flocculation of soils containing mixtures of smectite and kaolinite is generally less than in soils containing kaolinite alone. Polyacrylamide adsorption to quartz is poor due to lack of aluminol groups (>Al–OH), which are present on lateral edges of kaolinite and illite (Laird, 1997). Aluminol groups are positively charged under acidic conditions, so the edges of kaolinite and illite would be available for adsorption of anionic PAM. A bridge between negatively charged polymer and clay surfaces can also be formed with cations, with divalent cations more effective than monovalent cations (Nadler and Letey, 1989; Letey, 1994; Laird, 1997; Lu et al., 2002). The significance of divalent cations in solution or on exchange sites may differ according to soil mineralogy (Peng and Di, 1994; Laird, 1997).

Most previous studies of PAM adsorption and flocculation have used relatively high (≥10 mg L–1) concentrations of PAM and pure, cation-saturated minerals. Shrestha et al. (2006) found that much lower concentrations of PAM were needed for flocculation of a natural soil, similar to our findings. Our objective was to determine if there were relationships between PAM and soil properties that would be indicative of the flocculation success of each PAM–subsoil combination. The work described here involved only anionic and nonionic PAMs.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil Samples
Thirteen subsoil samples were provided by the North Carolina Department of Transportation (NCDOT) from active construction sites within 13 of their 14 geographic divisions (Fig. 1 ). This provided a fairly comprehensive range of materials representing North Carolina subsoils with a wide variety of mineralogies. These tend to have increased smectite and decreased Fe and Al oxides in the lower Coastal Plain compared with the Piedmont and Mountain physiographic regions (Coleman et al., 1949). The samples were taken by NCDOT staff from representative surface materials that were exposed during the grading process, although the original depths or locations are not known. It is common for the grading process to result in a surface layer of a mixture of subsoil materials from the nearby area or brought in as fill from other areas. Many of these NCDOT divisions contain multiple soil systems so the source area does not imply a specific mineralogy. Samples were allowed to air dry, and were then ground with a mortar and pestle, and passed through a 2-mm sieve.


Figure 1
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  		Fig. 1. Map of North Carolina Department of Transportation (NCDOT) divisions of the state from which the samples were obtained. No sample was obtained from Division 10.

 
Particle size analysis was completed on duplicate samples of each subsoil by the hydrometer method (Gee and Bauder, 1986) after sample organic matter was removed by the addition of H2O2 (Day, 1965). Chemical dispersion was accomplished by the addition of sodium hexametaphosphate. Samples that had already been chemically and physically dispersed in the particle size analysis were used for mineralogical analysis (Kunze and Dixon, 1986). The clay fraction was separated by centrifugation into <1- and 1- to 2-µm fractions for mineralogy determination by x-ray diffraction analysis (Whittig and Allardice, 1986) for 10 of the 13 subsoils. X-ray diffraction analysis was completed using a Diano Series 2000 diffractometer (Diano, Woburn, MA) with unresolved (Ni-filtered) Cu K-{alpha} radiation (wavelength = 0.1542 nm). X-ray diffraction patterns were interpreted by integrating the area under the curve of each clay mineral (smectite, vermiculite, mica, and kaolinite) in the Mg-glycerol-saturated samples. The area of each clay mineral curve was divided by the total area of all clay mineral curves to give the fraction of each clay mineral present in the soil.

The pH of each subsoil was determined in duplicate using a pH electrode with distilled water and a 1:1 soil/water ratio (Thomas, 1996). Organic C was determined by combustion using a CHN Elemental Analyzer (Model 2400 Series II, PerkinElmer Corp., Norwalk, CT). Exchangeable cations (Ca, Mg, and Na) were extracted by the Mehlich-3 procedure (1:10 w/w soil/0.2 M CH3COOH + 0.25 M NH4NO3 + 0.015 M NH4F + 0.13 M HNO3 + 0.001 M EDTA) and measured using an inductively coupled plasma (ICP)-emission spectrometer (Mehlich, 1984). The sodium adsorption ratio (SAR) was calculated from these extracts using the formula

Formula
Extractable Fe was determined by ammonium oxalate and citrate–bicarbonate–dithionite (CBD) extraction and determination by ICP-emission spectrometry (Jackson et al., 1986; Phillips and Lovley, 1987).

Polyacrylamide Materials
Eleven PAM products were included that had a wide range in molecular weight and charge density (Table 1). The molecular weight of the polymers ranged from 14 to 28 Mg mol–1 and the charge density ranged from neutral to 50% anionic molar charge. The single PAM products with known properties were: Superfloc 1606, A150, A150 HMW, A100, and N300 (Cytec Industries, West Patterson, NJ), Chemtall 923 VHM (Chemtall, Riceboro, GA), and Soilfix Polybead (Ciba Specialty Chemicals, Suffolk, VA). In addition, we included four products being sold in solid blocks, which are placed where runoff will intercept them, theoretically dissolving the PAM and initiating flocculation. These were APS Floc Logs 702aa, 702b, 702c, and 730b (Applied Polymer Systems, Woodstock, GA). No information is available on the content of these other than that they are primarily anionic PAM. Initially, a single flocculation test of all PAMs (11) with all subsoils (13) was performed to select the best four PAMs for each subsoil, and only these data are presented.


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  		Table 1. Polyacrylamide (PAM) properties used in this study.

 
Stock solutions of PAM (0.1% w/w) were made using the products as received, added to distilled water and stirred for at least 24 h at room temperature. The flocculation tests were conducted by placing 5 g of subsoil into a 100-mL specimen cup and adding 100 mL of distilled water. This concentration is in the range of that of our runoff plots on construction sites (Hayes et al., 2005). The PAM was then added by pipette to bring the PAM concentration in the cup to a range from 0 to 2 mg PAM L–1 for most tests. Some subsoil–PAM combinations had higher concentrations, up to 10 mg PAM L–1, to determine the direction of the turbidity curve.

Each subsoil suspension with PAM was shaken for 10 s and placed on the lab bench to settle under gravity. Twenty seconds after shaking, a nephelometer (Analite 152, McVan Instruments, Mulgrave, Australia) probe was inserted into the solution and a nephelometric turbidity unit reading was taken 10 s later at a depth between 10 and 38 mm. Preliminary experiments indicated longer shaking times did not increase flocculation significantly and longer settling times tended to reduce the ability to detect differences. The nephelometer was calibrated against a formazin turbidity standard (Hach Co., Loveland, CO).

The criteria for the selection of the most effective PAM were a combination of the greatest turbidity reduction, the lowest concentration of polymer needed to decrease turbidity, and variety of charge densities and molecular weights. Cytec Industries Superfloc A100 was included for all subsoils for comparison and Superfloc A110 was added because its charge density (18%) was intermediate among most of the PAMs tested. Another product, APS 705 powder, was tested for six of the subsoils in comparison to the other PAMs of known composition. The APS 705 contains a mixture of two anionic polymers of different molecular weights and charge densities, along with small amounts of conditioners (Steve Iwinski, Applied Polymer Systems, personal communication, 2004). It is referred to as "mixed charge density, mixed Mg mol–1" in the graphs. Although the components of this mixed PAM product are proprietary, it was included in our testing because of its widespread use in our region.

For statistical analyses, subsoils were initially grouped according to subsoil mineralogy. Subsoils that had <20% smectite or vermiculite in either the fine or coarse clay fraction with dominant kaolinite clay mineralogy were grouped together (Subsoils 5, 9, 11, 12, 13, 14). Subsoils 12 and 14 were presumed to have low smectite content due to their source locations, although mineralogical analyses were not performed. Subsoils 1, 2, 3, 4, 6, and 7 all had >20% smectite or vermiculite in either the fine or coarse clay fraction. Sample 8 was not analyzed for mineralogy, but because it could have been in either group, it was not included in statistical tests for either one. The turbidity reduction relative to the untreated control in each replicate was calculated to determine treatment differences. It was transformed for statistical analyses using the following equation:

Formula

For each subsoil, the turbidity reduction means at each PAM concentration were compared using the PROC GLM procedure and Tukey's multiple comparison test (SAS Institute, 1999). Only selected comparisons are presented to avoid excessive clutter on the graphs. To determine relationships between subsoil properties and turbidity reduction, Pearson correlation coefficients were determined for subsoil properties (texture, exchangeable cations, pH, organic matter, and Fe oxides) for all subsoils together and for both mineralogical groups. The correlation coefficients were determined using the SAS A regression PROC NLIN (SAS Institute, 1999). Graphs of turbidity reduction were created using Microsoft Excel 2003 using the smoothed line option.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
The results of the physical and chemical analyses of the subsoils are presented in Table 2. Dominant clay mineralogies were smectite, vermiculite, kaolinite, and mica (Table 3). The subsoil properties are in the range of soil properties reported for North Carolina (Daniels et al., 1999). A general trend of greater smectite and vermiculite fractions in the Coastal Plain (Subsoils 1–4 and 6) than the Piedmont and Mountain soils was evident, as well as increased sand content.


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  		Table 2. Soil source location and characteristics.

 

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  		Table 3. Soil source location and mineralogy in two size classes.

 
Turbidity Reduction
In the screening process, none of the solid block PAMs reduced turbidity as well as the single PAMs and therefore were not included in the replicated tests. Because solid blocks are made of binders and other ingredients, as well as multiple PAMs, it is probable that the same weight of material in our tests actually contained much less PAM and this is why there was less flocculation.

There were essentially three patterns of turbidity response to increasing doses of PAM: steady decline to 1 to 2 mg L–1, steady decline with a stabilization (increased turbidity) response initiating at or below 1 mg L–1, and low or erratic changes. Five of the subsoils, all from the Piedmont and Mountain regions, tended to respond equally among the PAMs tested regardless of charge density (Fig. 2 ). These exhibited a linear–log relationship between turbidity and PAM concentration. There were a few statistically significant differences among the PAMs at some concentrations (not shown), but generally the characteristics of the PAM did not substantially change the response. The optimal dose appeared to be between 1 and 2 mg L–1 and the resulting decrease in turbidity ranged from 96 to almost 100%. Subsoil 14 exhibited a stabilization effect, but only above the 2 mg L–1 concentration, which appeared to be the optimal dose for these subsoils, as discussed above. These subsoils were predominantly kaolinitic with little smectite, and all had relatively high CBD-extractable Fe.


Figure 2
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  		Fig. 2. Turbidity reduction as a function of polyacrylamide (PAM) concentration for polymers with different properties: results for Subsoil 14, which was representative of soils with linear responses (Subsoils 5, 9, 12, 13, and 14). Figure shown in linear–log scale. Letters associated with selected data points indicate significant differences (P = 0.05); c.d. is charge density.

 
The pattern of steady turbidity reduction with a stabilization response above 1 mg L–1 was evident for two subsoils, Subsoil 7 from the Piedmont and Subsoil 11 from the Mountain region. Each had an optimal turbidity reduction using single anionic PAMs at around 0.5 mg L–1, with increasing turbidity at higher concentrations (Fig. 3 ). These two soils are different from each other in a number of characteristics, such a higher clay and vermiculite content in Subsoil 7, but their response curves were surprisingly similar. The mixed PAM product had a linear response curve for Subsoil 11, with the lowest concentrations being less effective than the single anionic PAMs but no stabilization at the highest concentrations. While more of the mixed product was required for maximum turbidity reduction, the overall reduction was greater than the single PAMs. Similarly, a nonionic PAM was included in testing Subsoil 7 with response very similar to the mixed PAM response for Subsoil 11 (data not shown).


Figure 3
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  		Fig. 3. Turbidity reduction as a function of polyacrylamide (PAM) concentration for polymers with different properties: results for Subsoil 11, which was representative of soils with stabilizing responses (Subsoils 7 and 11). Figure shown in linear–log scale. Letters associated with selected data points indicate significant differences (P = 0.05); c.d. is charge density.

 
The remaining subsoils (2, 3, 6, and 8) exhibited patterns of low, erratic, or no response to increasing concentrations of PAM (Fig. 4 ). Compared with the soils described above, Subsoil 3 had a much lower response to increasing PAM concentrations and a maximum reduction of 85% (Fig. 4b). Although the single, anionic PAMs had a flat response above 1 mg L–1, the neutral and mixed PAM products continued a slight decline in turbidity at the 5 mg L–1 concentration. At the highest dose (5 mg L–1), the neutral and mixed PAMs, along with the lowest charge density anionic PAM, were the most effective. Samples 2, 6, and 8 had a relatively flat response to PAM dose until 0.1 mg L–1, after which the lower charge density PAMs tended to flocculate more than the higher charge density PAMs (Fig. 4a–4d). Some stabilization was evident for the single PAMs for Subsoil 2, but this may be an artifact of having tested higher concentrations of PAM for that subsoil. The mixed PAM was more effective than the single PAMs for Subsoil 2, but only once concentrations reached 1 mg L–1 or higher, and no rebound effect was present. The higher charge density PAMs were also effective for Subsoils 6 and 8 only at those highest concentrations.


Figure 4
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  		Fig. 4. Turbidity reduction for subsoils with low or erratic responses as a function of input concentration for polyacrylamide (PAM) with different properties: (a) Subsoil 2, (b) Subsoil 3, (c) Subsoil 6, and (d) Subsoil 8. Each data point is the average of three replications. Figure shown in linear–log scale; c.d. is charge density.

 
An increase in turbidity of high-concentration PAM solutions may have been induced steric stabilization or repulsion (Gregory, 1989; Lentz et al., 1996). This occurs when excess polymer is adsorbed to particles and segments of polymer chains extend into the aqueous phase, creating a thick layer around particles in suspension. This polymer layer makes the distance between individual particles large enough that van der Waals attraction may be too weak to cause adhesion and flocculation (Gregory, 1989).

Subsoils 1 and 4 were unresponsive to increasing concentrations of single PAMs, having nearly flat dose response curves (Fig. 5 ). The only PAM product that proved effective was the mixed PAM, which significantly reduced turbidity at concentrations up to the maximum dose. The highest concentration of any PAM tested was the 10 mg L–1 used for Subsoil 4, and at that concentration the mixed PAM response curve began to show evidence of a rebound effect.


Figure 5
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  		Fig. 5. Turbidity reduction as a function of polyacrylamide (PAM) concentration for polymers with different properties: results from Subsoil 4, which was representative of soils with poor responses (Subsoils 1 and 4). Letters associated with selected points indicate significant differences (P = 0.05). Figure shown in linear–log scale; c.d. is charge density.

 
Soil–Polyacrylamide Relationships
Subsoils that demonstrated the greatest turbidity reduction with PAM have several common properties. They all were >14% clay and 22% silt. Silt may be involved in the flocculation of clays through bridging by polymer chains to flocculate and settle more readily than clay-to-clay flocculation. Subsoil chemistry, however, may also be a factor in PAM responses. Piedmont and Mountain subsoils have much higher amounts of Fe oxides than subsoils from the Coastal Plain (Coleman et al., 1949). Our most responsive subsoils were all from Piedmont and Mountain sources (5, 7, 9, 11, 12, 13, and 14) and had the highest CBD-extractable Fe (88–761 mmol kg–1) of all subsoils tested. The dominant clay mineralogy of these subsoils was kaolinite in both size fractions (68 and 90% or greater, respectively) and mica. The two other subsoils (6 and 8) from the Piedmont had CBD-extractable Fe in the low end of the range (68–135 mmol kg–1) and more sand and less silt than the more responsive subsoils.

Anionic PAM flocculated the kaolinite-dominated soils more readily than subsoils with significant smectite or vermiculite, similar to the results of Laird (1997) for kaolinite and illite. Deng et al. (2006) found no flocculation of Na-saturated kaolinite using anionic PAM, but they used 10 mg L–1 or higher, which is in the stabilization range for PAM in our tests. Also, because our subsoils had multivalent cations (Fe, Ca, Mg, and Al) present on the clay surfaces, the bridging between them and the PAM was probably much greater than on Na-clays. Shrestha et al. (2006) also found that concentrations of PAM >1 mg L–1 inhibited flocculation of a 2 g L–1 natural soil suspension, but increasing the soil to 10 and 20 g L–1 eliminated the inhibition at 5 and 10 mg PAM L–1. At pH levels similar to ours, Peng and Di (1994) had maximum flocculation of kaolin in the 5 to 8 mg PAM L–1 range, with a rapid decline above 10 mg L–1.

The correlation between the turbidity reduction and kaolinite in the clay fraction (<1 µm) was high (r2 = 0.80). Since smectites and vermiculites have up to 10 times the specific surface area of kaolinites and micas (Sparks, 2003), the PAM dose response might be expected to be proportionately increased. This may partially explain why higher kaolinite fractions correlated with greater flocculation. The presence of exchangeable Ca2+ and Mg2+ (24–3600 mg kg–1) indicates that these were not limiting flocculation. All of the subsoils also had relatively low organic matter (0.9–7.2 g kg–1), which may have also improved flocculation (Helalia and Letey, 1988; Nadler and Letey, 1989; Kretzschmar et al., 1993).

There were no apparent similarities between subsoils that have a diminished flocculation at high PAM concentrations (11, 2, and 7), except perhaps low CBD-extractable Fe. In each of these subsoils, turbidity was reduced to a minimum level with PAM concentration at or near 1 mg L–1, above which turbidity increased. Montmorillonite had a similar stabilization trend as anionic PAM concentrations increased, but this disappeared as the ionic strength of the solution was increased, suggesting that available cations can be a limiting factor (Aly and Letey, 1988). We used distilled water in our tests and so any available cations were from the subsoils themselves. The addition of gypsum actually inhibited flocculation in Piedmont and Mountain subsoils and marginally improved it in the less responsive subsoils (Bartholomew, 2003).

There were no unique properties associated with subsoils that exhibited a stabilization trend at high PAM concentrations. We did not test all subsoil–PAM combinations for a stabilization point, but it is likely that it occurs above a certain PAM concentration for all soils; however, the concentration that causes diminished flocculation effect varies considerably among materials. This optimal PAM application falls within the stabilization trend region for Subsoils 2, 7, 11, and 14.

Subsoils 1 and 4 had little or no flocculation with any of the single polymers. Both subsoils had high amounts of 2:1 clays (smectite and vermiculite), which may have caused poor flocculation (Laird, 1997). Both subsoils also had very low amounts of CBD-extractable Fe (8 and 18 mmol kg–1) compared with the others. There was not a common reason for poor flocculation related to exchangeable cations or pH, because the subsoils have contrasting values for both. The multiple PAM product, APS 705, worked significantly (Tukey's test, P = 0.05) better on both of these subsoils than all other PAM products. This suggests that mixtures of polymers, and possibly other materials, may lead to better flocculation of recalcitrant soils than single polymers.

The Pearson correlation for the texture and chemical properties against the greatest amount of turbidity reduction for any PAM was the highest for sand or silt content (Table 4). Chemical properties CBD-extractable Fe, Ca, and pH also had significant correlations. The same analysis against only the results of 1 mg L–1 of the A100 PAM, which we included in all tests, had similar results but with lower correlations (data not shown).


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  		Table 4. Pearson correlation coefficients for soil properties and turbidity reduction by polyacrylamide.

 
For the six subsoils with known or expected dominance by kaolinite, only Ca had a significant correlation with turbidity, while the six with significant smectite or vermiculite content had high correlations with sand, silt, and CBD-extractable Fe. The response curve for turbidity reduction vs. CBD-extractable Fe is quite steep among the smectite or vermiculite subsoils but relatively flat among the kaolinite subsoils (Fig. 6 ). There appears to be a critical minimum extractable Fe beyond which higher amounts have little effect on flocculation by anionic PAM.


Figure 6
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  		Fig. 6. Relationships between citrate–bicarbonate–dithionite (CBD)-extractable Fe and turbidity, sand, and kaolinite in soil samples.

 
There were high correlations among the subsoil properties that were associated with turbidity reductions by PAM. Extractable Fe (CBD Fe) content was correlated with sand and kaolinite (Pearson r = –0.82 and 0.63, respectively), which is not surprising since the sandier subsoils with greater smectite and vermiculite content were from the Coastal Plain. Greater turbidity reductions from PAM treatment correlated with increasing CBD-extractable Fe and kaolinite and decreasing sand content (Fig. 6). It is possible that Fe oxides reduced the dispersion of the suspended particles (Goldberg and Glaubig, 1987), allowing PAM to form larger flocs. These correlations were relatively strong among all subsoils, but most of the kaolinitic subsoils occur on the right of the graph where the response is flat. The inflection points suggest there are critical concentrations or physical distributions below which PAM may be less effective. When the subsoils are divided by dominant mineralogy, the relationships were clearly different. The number of samples (13) is relatively small for the establishment of indicators of turbidity response, but for these samples sand content was as good a predictor of response to PAM as any other variable. If this relationship held with further testing, this would be a relatively simple method of determining where PAM may not be as effective in controlling turbidity in runoff from disturbed areas. While we do not think that the sand content had direct impacts on PAM flocculation, it did appear to be a good surrogate for determining PAM effectiveness for a given subsoil sample.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
In most cases, anionic PAM substantially reduced turbidity in samples of subsoils from across North Carolina by >90%. Subsoils from the Piedmont and Mountain regions, with little smectite or vermiculite, responded relatively linearly to increasing concentration up to 1 mg L–1, above which there was often a negative response. No clear advantage of a particular charge density or molecular weight was found. Subsoils from the Coastal Plain contained significant amounts of smectite and vermiculite and tended to be less responsive to PAM treatments. Two subsoils had little or no reduction in turbidity with the single anionic PAMs tested, but a commercial product containing a mixture of PAMs and other undisclosed ingredients did reduce turbidity. These subsoils had the highest smectite or vermiculite content of all soil materials tested. The relationships between PAM effectiveness and subsoil properties were found to be strong for particle size distribution, with increasing sand content having a negative effect on turbidity reduction. Extractable Fe and Ca correlated with turbidity reduction for all subsoils, as well as pH to a lesser extent. The Coastal Plain subsoils had different statistical relationships between turbidity reduction and chemical properties than the Piedmont and Mountain subsoils, and a critical minimum for some properties (extractable Fe and kaolinite content) appeared necessary for a single anionic PAM to be effective. Turbidity reduction by PAM was highly correlated with sand content, which was strongly correlated with other soil properties such a kaolinite and extractable Fe, suggesting that it may be a suitable indicator for our region.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication April 21, 2006.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome