Grantee Research Project Results
1999 Progress 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 Period Covered by this Report: September 1, 1998 through August 31, 1999
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 living. Both short- and long-term control of pollutants should be taken into consideration to 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 ability of a soil 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 hypothesize that polymer-surfactant aggregates (PSAs) are unique ingredients in biosolids that enhance immobilization of heavy metals. PSAs are formed by combination of the surfactants present in biosolids?due to the high concentrations of anionic soaps and detergents in wastewater?with the cationic polymers used to chemically condition the biosolids prior to dewatering. Therefore, the primary objective of this research is 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.
Progress Summary:
The structures formed by sodium dodecyl sulfate (anionic surfactant), and Percol (cationic polyelectrolyte) have been studied at varying concentration ratios and 0.01 N ionic strength. The interactions between these two components were observed in the presence of only a small amount (significantly below the CMC) of sodium dodecyl sulfate (SDS). Consequently, we conclude that PSAs were formed at all of the examined concentrations. The characterization of aggregates was performed at 400 mg/L Percol with SDS varied over a wide concentration range. The critical aggregation concentration (CAC) was observed below 5 mg/L SDS. It could not be defined as an absolute value because of the experimental limitations at very low surfactant concentrations. The charge equivalence was calculated as 346 mg/L SDS using the charge density of 3.0 meq/g for Percol. Experimental observation of the charge equivalence at 400 mg/L SDS was in good agreement with the theoretical expectation. Because this experimental value is showing the total SDS concentration, its bound fraction can be assumed to be much closer to the calculated value. The critical micelle concentration in the SDS-Percol system (CMCPS) was observed at 2000 mg/L; this critical point for SDS solutions (CMCS) was obtained around 1200 mg/L. Phase separation phenomenon started just below the charge equivalence and continued up to the CMCPS. The resolubilization of the precipitate occurred because of the regain of charge of the system. The patterns that emerged from the data allowed identification of six distinct types of polymer-surfactant structures, each responsible for distinct rheological, electrokinetic, and solubilization behavior in a specific concentration regime.
The conformation of SDS molecules around Percol segments has been pictured for six regions of different charge and/or hydrophobicity behavior observed. The charge of the structures changed from positive to negative at the charge equivalence. Having positive charge, an increase in hydrophobic domains in Region-1, and then a decrease in Region-2 were observed. The SDS concentration separating these two regions is between 200 and 300 mg/L. The increase of hydrophobicity in the first region can be explained by the increasing binding of SDS to Percol in the micelle form. The source of the decrease in hydrophobic domains in Region-2 is the phase separation of these structures, and any other structures formed, as SDS concentration was increased. A low concentration of structures that remained in the solution may have only the SDS monomers bound to Percol. Region-3, as the first region after the charge equivalence, had no hydrophobic domain. The following three regions (Region-4, Region-5, and Region-6) showed a continuous increase in hydrophobic domains formed; in Region-4 and Region?5 this increase was due to the PSAs, in Region-6 it was due to the free SDS micelles. In Region-3, most of the structures were in the solid phase and it was hypothesized that binding as a second layer in the solution phase can happen only if the bound SDS monomers are far from each other and have contact to water. The data suggest that no resolubilization occurred in this region. This situation continued up to 1000 mg/L SDS, which is the end of Region-3. When resolubilization started and a significant amount of PSAs returned to the solution phase (Region-4), further binding created hydrophobic domains by forming a bilayer or with the increase of the aggregation number of micelles bound to Percol. There is no molecular level indication proving one of these hypotheses. The free SDS concentration increased with SDS added, while some portion was continuing to bind to Percol. When the Percol binding sites were saturated completely, the added SDS filled the SDS required in the solution to initiate free micelle formation (Region-5). The last region, Region-6, involves the increasing amount of free SDS micelles in the solution.
The characterization of the SDS-Percol system was repeated at four other
Percol concentrations, 100 mg/L, 200 mg/L, 600 mg/L, and 800 mg/L, and the same
behavior was observed in each case.
Future Activities:
The combination of a cationic flocculant and an anionic surfactant, as occurs in biosolids treatment, forms structures not evident when either of these components is present separately. Therefore, contaminant fate in wastewater sludges (biosolids), and in soils after land application of biosolids, may be altered due to the presence of polymer-surfactant aggregates. The polymer-surfactant structures demonstrate the ability to concentrate and immobilize a hydrophobic chemical compound. In the upcoming year, we will investigate whether the phenomena we have observed in systems comprised solely of the polymer and surfactant will be manifested when solid surfaces are present. A clay and several soils will be used for these experiments.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., 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.