USING ORGANOCLAYS TO ENHANCE CARBON FILTRATION
Organoclays have found increased acceptance over the past few years as pre-polishers for activated carbon in both groundwater and wastewater cleanup. The reason is that activated carbon tends to become quickly blinded by large organic molecules of low solubility, particularly oils.
However, it is also well established that activated carbon tends to be more efficient at low concentrations of organic contaminants than at higher ones, i.e. at less than 1 ppm. With organoclays it is exactly the opposite, they are better at removing organics at higher concentrations, above
3 ppm. Therefore it makes sense in the case of water that is contaminated with various organics, to set up two or more vessels in series, the first one filled with organoclay, the remainder with activated carbon. The economics make sense, even though the organoclay is not regenerated, because
of the reduction in down time every time a carbon vessel has to be changed out. Organoclays have a removal capacity of 50% or more, by weight, for oils, or seven times more than activated carbon. In the case of other organic contaminants, as the solubility increases, the efficiency
decreases, except in the case of methylene chloride, which it removes at far higher efficiency then carbon.
Organoclays consist of bentonite that has been modified with quaternary amine cations. The positively charged nitrogen cation exchanges for sodium and calcium on the clay surface.
The clay now becomes organophilic, which means it will no longer swell in water, but instead in hydrocarbon fluids such as kerosene, diesel fuel, and so on. At the same time, when the organoclay is placed into water, the quaternary amine chains extend away from the clay surface into the water.
The amine chain is now neutral because the cation is attached to the clay surface, rendering the entire system into a non-ionic surfactant with a solid base. This chemical set up allows it to function as a non-ionic surfactant and remove oil and other organic compounds of low polarity by partition.
As oil droplets and other organic compounds float past an organoclay particle whose chains extend into the water, these chains will reach out like the tentacles of a squid, partitioning into the oil droplet and fixating it (the droplet) onto itself. Since this activity takes place outside
of the clay, no blinding occurs.
During the ion exchange of the quaternary amine with the sodium and calcium, only a part of the total number of ions is exchanged. The remainder are still present, and give the clay the ability to also remove inorganic metal cations such as lead, zinc, copper
and others. While the removal capacity is hard to predict, it nevertheless is a nice side benefit for the end user to have a product that removes organics and inorganics simultaneously.
Usually not all the quaternary amine chains are attached to the clay platelets. Some of them tie to the
attached chains inversely by a tail-tail interaction. Since now there is a positive charge extending into the water, negatively charged metal anions such as hexavalent chrome, selenite and arsenate are removed. Many humic acids are also removed by such organoclays.
If the removal of oil
is the main purpose of using the organoclay, and if it is placed into a carbon vessel, the clay is granulated and then blended with anthracite to keep the pore spaces open.
This article will present column studies that have been conducted with several organic compounds, such as benzene,
toluene, xylene, and naphtalene. Test data from different petroleum oils will also be shown, followed by some actual case studies.
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