Superfund Research Program
Modified Clays for Environmental Remediation
Project Leader: Stephen A. Boyd
Grant Number: P42ES004911
Funding Period: 1995 - 2000
- Project Summary
Final Progress Reports
Over the past 10 years, project researchers have described the basic elements of an in situ trap and treat technology for contaminant plume management. The essential element of this technology is chemical modification of clays present in subsoils to enhance their ability to remove organic contaminants from water. This involves a simple one-step chemical modification reaction in which hydrophobic cationic surfactant molecules (e.g. quaternary ammonium cations) are attached to the surfaces of clays. This fundamentally changes the environment at the clay surface and in the clay interlayer regions from hydrophobic to organophilic. The organo-modified clays have substantially enhanced sorptive capabilities for organic contaminants and effectively remove them from water. Formation of organo-modified sorbent zones in the subsurface can be achieved by in situ injections of surfactant solutions. Properly placed, the sorbent zone can intercept and immobilize organic contaminant plumes within a confined region. The downgradient water emerging from such a zone is purified, and the immobilized contaminants can be subjected to remedial activities.
Success of the proposed in situ technology described above requires maintenance of the hydraulic conductivity to the sorbent zone to ensure flow through the zone and avoid fluid bypass. Hydraulic properties of sorbent zones were studied by measuring changes in the hydraulic conductivity of a sandy loam with different fine contents (6, 12, 18, and 24%) that were either untreated or batch treated with hexadecyltrimethylammonium (HDTMA) to 0.1, 0.7, 1.0, and 2.3 times the cation exchange capacities (CEC), and subjected to different effective stresses. Treatment levels achieved and the kinetics of HDTMA adsorption were evaluated using 14C-labeled HDTMA. Soil suspension turbidity was used to infer the degree of clay dispersion or flocculation; clays were dispersed in 0, 0.1, and 2.3 CEC treated soils and flocculated in 0.7 and 1.0 CEC treated soils. Consistent with their flocculated clay structures, the 0.7 and 1.0 CEC treated soils showed equal or higher conductivities compared to 0, 0.1, and 2.3 CEC treated soils. The 0.7, 1.0 and 2.3 CEC treated soil also showed higher compressibility. At levels up to 1.0 CEC, HDTMA adsorption occurred rapidly, but was kinetically limited at the high treatment level (2.3 CEC). Creation of sorbent zones by batch treatment with cationic surfactants was deemed hydraulically feasible as evidenced by equal or higher conductivity of HDTMA-treated soils to 0.7 and 1.0 CEC treatment levels, as compared to untreated soil, under all loads.
In a related study, the hydraulic properties of soils modified by injection of the cationic surfactant hexadecyltrimethylammonium (HDTMA) were investigated using soil columns and a fixed ring consolidometer. Oshtemo soil (87% sand, 10.5% clay, 2.5% silt), equilibrated with NaCl or CaCl2 solutions, was treated at two different effective stresses by recirculation to two different HDTMA soil concentrations, one above and one below the cation exchange capacity (CEC). No drastic changes in hydraulic conductivity occurred as a result of HDTMA treatment at any of the experimental conditions studied. Decreases in hydraulic conductivity were greater in soils treated to high HDTMA concentrations, and under higher effective stress. Conductivity of soils treated to 0.8 times the CEC were greater than untreated soils. Changes in hydraulic conductivity induced by the treatment were attributable to changes in effective pore size due to particle migration and pore blocking, and to partitioning of the pore space into smaller units. Treatment conditions which resulted in more clay dispersion, indicated by higher turbidity, caused a greater decrease in hydraulic conductivity, suggesting that turbidity may provide a good qualitative indicator of conductivity changes due to treatment by in situ injection. These results indicate that sorptive zones created in situ with HDTMA are hydraulically feasible.
An additional line of investigation is centered on the development of new fundamental materials, including sorbents and catalysts, for use in environmental remediation and pollution prevention. Researchers found that the displacement of half of the inorganic exchange cations in the synthetic smectite clay fluorohectorite by surfactant onium ions of the type [CnH2n+1ECmH2m+1]+, where E is N or P, n>12 and 3<m<5, results in the segregation of the two cation types into separate galleries. Moreover, the galleries stack in a regularly alternating pattern, forming a heterostructure with a basal spacing equal to the sum of the spacings characteristic of the parent end - member organic and inorganic clays. These mixed ion clay heterostructures are reminiscent of a staged graphite intercalation compound.
The number of interlayer cations occupying the galleries of smectite clays and other 2:1 layered silicates is determined by the permanent charge on the layers. Therefore, true staging behavior, with the appearance of vacant interlayers as seen in graphite, would seem implausible. Nevertheless, a peculiar irreversible stage two - like clay structure in fact has been observed for a synthetic fluoromica. This latter intercalate has a regular arrangement of empty and Li+ - occupied galleries. The vacant galleries are formed by the partial but irreversible migration of Li+ from the galleries into vacant octahedral positions within the silicate layers. In this thermally driven process, every other Li+ gallery is depleted of all its cations in order to fill all of the vacant octahedral sites in adjoining mica-like layers.
Layered silicate intercalates, with dissimilar yet regularly stacked gallery environments, are fundamentally interesting from a structural viewpoint. In addition, such amphiphilic heterostructures should have novel adsorption or ion exchange characteristics for the removal of pollutants from both aqueous and non-aqueous solutions. Project researchers have extended the chemistry of amphiphilic heterostructure formation to include compositions in which the fraction of onium ions on the exchange sites, fQ, is in the range = 0.35 - 0.50. Emphasis was placed on the possibility of forming heterostructured intercalates with inorganic to organic cation ratios of 1:2 and 2:1. The same gallery stacking sequence as in a 1:1 mixed ion heterostructure was observed, signifying that up to 30 % of the onium ions in the organic galleries can be replaced to form solid solutions. This alloy - like mixing of gallery organic and inorganic ions appears to be unique among metal oxide intercalation compounds. Phase segregation of the parent end member clays was observed at compositions outside the heterostructure composition range.
Having successfully combined the hydrophobic and hydrophilic properties of organic and inorganic clays into a single amphiphilic phase, the researchers are now investigating the new heterostructures for use in pollution control and environmental remediation.