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What is the History of Organoclays?

 

History and Information on Organoclays for Water Treatment

A wealth of information has been discovered since 1949, when Jordon first published “Organophilic Bentonites. I. Swelling Organic Liquids”, which promoted the idea of the potential use of this family of materials for treating contaminated groundwater and wastewater.

In 1985, Wolfe's paper “Interaction of Aliphatic Amines with Montmorillonite to Enhance Adsorption of Organic Pollutants,” noted in their introduction that "Although organoclays have been recognized as adsorbents of such organic compounds, few studies have been conducted to determine whether certain clays could serve as practical adsorbents in the treatment of water and wastewater." They investigated the removal of eleven organic pollutants, including: butanol, hexanol, octanol, benzene, toluene, nitrobenzene, phenol, chloroform, dimethylphthalate, acetaldehyde and acetone; which were treated with organoclays created from the primary amines propylammonium (PA), dodecylammonium (DA) and dodecyldiammonium (DDA). Eight concentrations of each contaminant were prepared. X-ray diffraction studies of the various organoclays saturated with water and pollutants demonstrated "significant intercalcation" of the pollutant molecules between the clay platelets.

Five years later, Jaynes and Boyd's 1990 paper, “Trimethylammonium-Smectite as an Effective Adsorbent of Water Soluble Aromatic Hydrocarbons,” investigated the ability of trimethylphenylammonium (TMPA) and tetramethylammonium clays (TMA) to adsorb soluble hydrocarbons including benzene, toluene, ethylbenzene, p-xylene, butylbenzene and naphthalene. They were also able to construct adsorption isotherms for each contaminant and concluded that TMPA-smectite was potentially useful as a liner ingredient for petroleum storage facilities such as tank farms and underground storage tanks.

Their work also supported the leading hypothesis that describes how organoclays adsorb hydrocarbons - when large QACs are exchanged on montmorillonite, the organic cations form pillars that spread the clay platelets apart. The large interlamellar space allows for a hydrocarbon partition to form between the clay platelets. These changes in interlamellar distance have been measured by several authors using x-ray diffraction. Jaynes and Boyd also discovered that high cation-exchange montmorillonite may be less effective as a substrate than lower cation-exchange clay because the denser packing of QAC pillars between the clay plates may restrict the movement of aromatic molecules into the interlamellar space.

The term "isotherm" is used for convenience because the adsorption behavior of many organoclays follows a Langmuir isotherm, which was developed to describe the adsorption behavior of activated carbon. It assumes that there is a fixed number of adsorbent sites available on the surface and that adsorption is reversible. Unfortunately, the concept of an isotherm is misleading in its implication that the adsorptive properties of organoclay change with temperature or that the adsorption is reversible. Data which follows a Langmuir isotherm does not necessarily infer that the previously stated assumptions are valid.

Another 1990 paper, “Adsorption of Benzene, Toluene, and Xylene by two Tetramethyl-ammonium-Smectites Having Different Charge Densities” (Lee), also studied the adsorption of vapors and aqueous solutions of benzene, toluene, and xylene by several organoclays. They concluded that "low charge Wyoming TMA-smectite was very effective in removing benzene from water and may be a useful material for purifying benzene contaminated water."

In 1993 Zhang addressed some of the practical issues that needed to be resolved before organoclays could be recommended or manufactured for commercial water treatment. First, they investigated the factors that control the adsorption of organic cations on clays. It was determined that the smaller organic cations sorb onto clays to a level exactly equal to the cation exchange capacity of the clays. They also discovered that large QACs (larger than 12 carbons) could possibly sorb at nonexchangeable sites because they were able to exchange these materials at levels higher than cation exchange capacity. The degree to which QACs were adsorbed at nonexchangeable sites was directly related to the length of the alkyl chains of the amines.

Second, they discovered that more than 95% of the available sodium ions could be exchanged with QACs. Potassium, on the other hand, was replaced at only 70-75% of the sites. This indicates that sodium montmorillonite is the preferred raw material.

Finally, they conducted long-term desorption studies to discover if the sorption of organic cations was truly "permanent". They concluded that within their 180 days of investigation, the exchange was essentially irreversible. Further, they concluded that desorption did not increase with longer equilibrium time and found that the degree of desorption also decreased in inverse proportion to the length of the alkyl chains of the amines.

The 1993 study was followed by Kukkadapu and Boyd's 1995 study which prepared tetramethylphosphonium (TMP)-clay and tetramethylammonium (TMA)-clay in order to study their ability to adsorb benzene, toluene and carbon tetrachloride vapor. Aqueous solutions of benzene, toluene, ethylbenzene, styrene, xylene and carbon tetrachloride were also studied. They also found that the adsorption of organic vapors by dry organoclay media was higher than in the aqueous solutions. The potential use of organoclay to adsorb organic vapors has apparently not been investigated further.

Finally, in 1996 Dentel created and evaluated organoclays made from trimethylammonium (TMA), hexadecyltrimethylammonium (HDTMA), and dioctadecyl-dimethylammonium (DDDMA). He noted that organoclay made from QACs containing small functional groups behave differently than those made from QACs containing larger functional groups (12-18 carbons). In the former case, adsorption is preferential or competitive and the magnitude of uptake is not dependent on the solubility of the organic solute. In the latter case, the magnitude of uptake is inversely proportional to the solute's water solubility and adsorption is not preferential or competitive. This latter behavior was observed in several of the case studies.

All of this exhaustive research and investigation can be summarized as follows:

  • Many primary and quaternary amines are used as clay surface modifiers.
  • All organoclays that have been studied will adsorb organic compounds to some degree.
  • Of those clays that have been studied, several have been found to remove organic contamination from wastewater in potentially commercial quantities.
  • The adsorption behavior of organoclay is fundamentally different from that of activated carbon. In certain circumstances, a hydrocarbon partition can be created between clay platelets. The generic term "sorption" is a more accurate description of the phenomenon.
  • Adsorption and desorption tests have strongly suggested that commercial products should be manufactured from large molecular weight QACs. While this formulation hinders the adsorption of soluble hydrocarbons such as BTEX, it is optimized for the removal of free and dispersed oil.
 
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