Removal of recalcitrant compounds from water using synthetic hydrotalcites

Resumen

Chlorinated hydrocarbons (CHCs) and poly- and perfluoroalkyl substances (PFAS) are recalcitrant compounds which are toxic to humans and ecosystems. The improper disposal of these compounds after their extensive use in industry has led to the contamination of groundwater and soils. Hydrotalcite (HT)-like compounds have emerged as promising sorbents for CHCs and PFAS due to their high anion exchange capacity and tunable properties. This thesis examines the sorption mechanisms and sorption capacity of CHCs and PFAS into HT-like compounds (organo-HT and inorganic-HT) under a range of conditions (e.g., varying pH and water chemistry and complex contaminant matrix). These compounds were first synthesized and characterized in a laboratory setting, but to bridge the gap between laboratory observations and field applications, sorption studies have been conducted with contaminated groundwater from a site in Spain and the long-term fate of HT-like compounds were studied under natural aquifer conditions in a site in Denmark. Organo-HT and inorganic-HT intercalated with different anions (i.e., sodium dodecyl sulfate, 1-dodecane sulfonate, nitrate and carbonate) were synthesized via the coprecipitation method. Results demonstrate that upon drying, organo-HT particles aggregate which decreases the number of sorption sites, reducing their sorption capacity for CHCs. After dispersion in an organic solvent inorganic-HT show no changes in the crystal structure but an increase in the specific surface area (SSA) and a decrease in the aggregate size. The sorption mechanism of halogenated compounds into HT depends on the nature of the HT (e.g., intercalated anion) and the physico-chemical properties of the sorbate. In the case of the sorption of CHCs into hydrophobic organo-HT, this process occurs via solute partitioning. Varying water chemistry, solution pH, the co-existence of multiple CHCs have little effect on sorption efficiency of organo-HT towards CHCs. Conversely, surface adsorption controls the sorption of PFAS molecules into carbonate intercalated HT, where a high SSA (> 100 m2/g) and small aggregate particle (<100 µm) are desirable attributes. In this case, the sorption process occurs via electrostatic attractions and hydrogen bonding. This sorption mechanism is valid for nitrate intercalated at low PFAS concentrations, while anion exchange and intercalation of PFAS may occurs as concentration of the adsorbate increases. The presence of non-ionic species (trichloroethylene) do not affect the sorption capacity of inorganic-HT towards PFAS, while an alkaline pH conditions and the presence of anionic species (dodecyl sulfate) reduce the sorption capacity of inorganic-HT towards PFAS. The stability of organo-HT and inorganic-HT during long-term exposure to aquifer conditions depend on the intercalated anion and groundwater dynamics, while the HT aggregate size only has a minor effect. The chemistry of groundwater influences the precipitation of insoluble species (CaCO3, and adsorbed sulfate) on the HT surface. Overall, organo-HT and inorganic-HT are potential sorbents for “ex situ” remediation treatment to decrease CHCs and PFAS concentrations in groundwater. Nevertheless, the instability of HT compounds especially in the case of organo-HT is a significant limiting factor for their future application as sorbents under dynamic flow conditions.