Grantee Research Project Results
Final Report: Fate and Transport of Heavy Metals in the Subsurface: Effects of Polymer-Surfactant Aggregates
EPA Grant Number: R826188Title: Fate and Transport of Heavy Metals in the Subsurface: Effects of Polymer-Surfactant Aggregates
Investigators: Dentel, Steven K. , Cha, Daniel K. , Huang, C. P.
Institution: University of Delaware
EPA Project Officer: Aja, Hayley
Project Period: September 1, 1997 through August 31, 2000
Project Amount: $295,582
RFA: Exploratory Research - Environmental Chemistry (1997) RFA Text | Recipients Lists
Research Category: Water , Land and Waste Management , Air , Safer Chemicals
Objective:
Land application of sewage sludge (biosolids) has been increasing in preference because of its cost-effectiveness and the fertilizing and soil conditioning ability of sludge. However, the disposed material contains pathogens, metals, toxic organic chemicals, and nutrients that require proper management for the sake of the environment. Without proper management, the pollutants may adversely influence the soil environment, migrate into groundwater, transport to surface waters, and finally, become dangerous to the environment. Both short- and long-term control of pollutants should set standards for sludge disposal. This necessitates the understanding of the fate and transport of pollutants in the soil system and the influence of sludge conditions on prevalent mechanisms. Although it has been assumed that biosolids offer a medium that enhances the soil's ability to immobilize metals, the interactions of biosolids with soils are complex, and the mechanisms that may be responsible for the ability of biosolids to immobilize metals are poorly understood. We hypothesized that polymer-surfactant aggregates (PSAs) are unique ingredients in biosolids that enhance immobilization of heavy metals. PSAs are formed by the combination of the surfactants present in biosolids, due to the high concentrations of anionic soaps and detergents in wastewater, with cationic polymers, which are used to chemically condition the biosolids prior to dewatering. Therefore, the primary objective of this research project was to examine the metal binding effects of polymer-surfactant combinations added to soils, particularly in comparison to soils with neither, or with one, of the two additives.
Summary/Accomplishments (Outputs/Outcomes):
To characterize such PSAs relevant to environmental systems, sodium dodecyl sulfate (SDS), an anionic surfactant, and Percol 757, a cationic polymer, were utilized as a model system. Their interactions and the resulting PSA structures were indirectly characterized via surface tension, viscosity, charge on aggregates, water-insoluble dye solubilization capacity, free surfactant concentration, and solution phase polymer concentration.
The strong interactions of the surfactant and polymer can be illustrated on a macroscopic basis, by adding 5,000 mg/L of a clear SDS solution to a clear solution of 1,000 mg/L Percol 757 in water (see Figure 1). A large-scale structure forms spontaneously, with cumulous sheets being visibly discrete from the aqueous phase. The laminae are readily disrupted by gentle mixing to form much smaller, settled agglomerates.
Figure 1. Quiescent Combination of SDS and Percol 757.
The PSA structures found in more heterogeneous or environmental settings are smaller, existing on a nanoscale and requiring indirect methods to characterize their presence and effects. However, they comprise a separate phase of assembled molecules, and our results show that they can collect environmental contaminants in a manner that changes the behavior of these constituents in biosolids, soils, or groundwater.
When the polymer was present in an aqueous system, even a small amount of SDS led to the formation of PSAs. For most experiments, we used 400 mg/L of Percol 757 polymer, as an amount typifying the dose added to wastewater biosolids prior to dewatering. PSAs were formed at the lowest SDS addition used, 5 mg/L, with the critical aggregation concentration (CAC) evidently well below this. However, the polymer's presence increased the critical micelle concentration (CMC) from 1,200 mg/L (SDS alone) to 2,000 mg/L.
Analyses show that this increase is due to the consumption of SDS by Percol, roughly at an equal charge (and mass) basis, in forming PSAs. However, in the intermediate concentration ranges, the charge ratio is not equal; therefore, the PSA varies in net charge. This causes important variations in PSA behavior, which were mapped out using a variety of techniques.
Surface tension was one such technique. These measurements (see Figure 2) showed that the polymer, which is not surface-active on its own, led to very significant decreases in surface tension in combination with relatively low amounts of SDS. At higher SDS concentrations, near the charge equivalence point (~400 mg/L SDS), the PSA phase separated, but beyond this, the precipitate was resolubilized due to the regain of charge. The charge transitions were confirmed by streaming current measurements. Viscosity and turbidity measurements (see Figure 3) indicated the charge neutralized range and the ultimate CMC for SDS to correspond with region amounts of the Percol polymer. The Percol polymer gave similar trends, which could be normalized to collapse onto one master curve.
Figure 2. Surface Tension of SDS With and Without Percol 757.
Figure 3. Streaming Current and Turbidity Varying SDS Concentration in Presence of 400 mg/L Percol 757.
When the PSA structure is phase-separated, this also is indicative of hydrophobic domains existing within the PSA structure. Using 1H nuclear magnetic resonance, we discovered a correlation between these domains and a side peak on the proton spectrum, providing a nanoscale confirmation of the important changes in PSA structure and properties at various concentration ratios.
The existence of both hydrophobic and highly charged domains within the PSA structures has environmental implications. We have previously demonstrated the partitioning of a hydrophobic organic pollutant, trichlorobenzene, into the SDS-Percol PSA, and in this report we correlate this phenomenon with specific concentration regimes and ratios. Orange OT, an organic dye, absorbs light at 490 nm only when in a nonpolar environment. The regions of absorbance were those in which hydrophobic domains exist (see Figure 4). Again, this pattern was identified at a variety of polymer and surfactant concentrations (see Figure 5).
Figure 4. Demonstration of Hydrophobic Domains at 400 mg/L and 1,200 mg/L SDS by Orange OT Absorbance in Presence of Varying SDS and With or Without 400 mg/L Percol 757.
Figure 5. Hypothesized Polymer-Surfactant Aggregates in the Solution Phase. Smallest entities are SDS monomers, which can form "bead and necklace" aggregates, or a variety of other ordered structures in the proximity of the oppositely charged polymer, or its own independent micelles. Each of these structures exists in a specific concentration region and leads to unique effects on contaminant uptake and mobility.
Divalent copper also was affected by the PSAs, although more research is needed to identify the mechanisms. SDS alone decreased the activity of the Cu2+ ion, but the presence of the Percol polymer accentuated this effect in the charge equivalence region (see Figure 6).
Figure 6. Reductions in Free Cupric Ion With Varying SDS and With or Without 400 mg/L Percol 757.
In an environmental setting, PSAs may assume certain chemical constituents, and they also may mobilize or immobilize them through its own behavior. Working with four different soil types and a montmorillonite clay, we found that media having an affinity for the polymer itself also had a greater affinity for the PSA. In these environments, sorbed PSAs should decrease the mobile contaminant concentration in solution. Soluble phase PSAs also were found in some conditions, which would increase contaminant mobility.
Significant concentrations of biosurfactants are produced by activated sludge microorganisms and soil microorganisms. A commercial biosurfactant, JBR 515, exhibited interactions with the Percol polymer, although not to the extent observed with Percol 757 and SDS. JBR 515 influenced the solubility of Orange OT dye and decreased the available Cu2+ in solution, similar to SDS. The presence of Percol 757 had a positive influence on the copper binding on JBR 515 below the biosurfactant's CMC. Thus, the role of both anthropogenic and biogenic surfactants is magnified in the presence of the cationic flocculant polymer. In land-applied biosolids, the effects of anionic surfactants and cationic polymers in combination may have significant effects on the uptake, or the mobilization and immobilization of contaminants in these media.
Journal Articles:
No journal articles submitted with this report: View all 6 publications for this projectSupplemental Keywords:
heavy metals, polymers, surfactants, soil, sludge, biosolids, groundwater, environmental chemistry, air, ecosystem protection/environmental exposure and risk, waste, chemistry, ecology, engineering, chemistry, physics, environmental chemistry, fate and transport, hydrology, anionic soaps, biosolid surfactants, cationic polymers, clay, fate and transport, heavy metals, metal binding, polymer surfactant aggregates, sludge, soil, subsurface., Scientific Discipline, Air, Waste, Ecosystem Protection/Environmental Exposure & Risk, Ecology, Hydrology, Environmental Chemistry, Chemistry, Fate & Transport, Engineering, Chemistry, & Physics, fate and transport, cationic polymers, soil , wastewater treatment, subsurface, biosolid surfactants, metal binding, polymer surfactant aggregates, anionic soaps, sludge, heavy metalsProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.