Overview of Organoclay Applications for trace heavy metal removal

Hugh McLaughlin, P.E. is an independent consultant. Below is a summary of problems solved by Organoclay. For more information on Organoclays go to http://biomininc.com

As a Chemical Engineering Consultant, I often find a significant portion of the engineering challenge is associated with cleaning aqueous effluents to acceptable levels for discharge or for recycling to reduce overall water consumption. Furthermore, having worked with activated carbon for over 30 years, I have come to appreciate that there are applications that activated carbon is sometimes the “miracle cure” or even the “only game in town”.

There are a number of applications where activated carbon alone is not efficient or is even ineffective at removing certain contaminants. Because activated carbon adsorbs the contaminants on the internal surface area of the activated carbon, it is really only able to remove organics that are soluble in water and diffuse into the carbon particles. When a separate organic phase is present, even as a relatively low level of emulsified hydrocarbon, the material has a tendency to coat the activated carbon particles on the outside — and block access to the internal surface area. It sort of reminds me of the old Sherwin-Williams slogan “Cover the Earth” — but the impact is to render the activated carbon a gooey ineffective mess, while simultaneously breaking through an unacceptable portion of the still emulsified phase.

Years ago, I was delighted to learn that Organoclays can be used ahead of the activated carbon adsorbers to effectively remove the emulsified organic phase. This pre-filter approach avoids the premature demise of the activated carbon beds, which then serve as polishing filters and operate for dramatically longer periods of time before breakthrough.

Another area where activated carbon is ineffective is in any ion exchange applications, since activated carbon has effectively no cation exchange capacity. Some activated carbon will oxidize over time to develop carboxylic acid functionalities, but the usable capacity is unaffordable. One might be tempted to try ion exchange resins in such instances, but the typical exchange capacities are unrealistic for system that are not regenerating the resins, considering the cost of the ion exchange media.

There is a growing body of experimental data and field experience that Organoclays also function to provide ion exchange capacity in many aqueous clean up applications. While the specific mechanism of ion exchange is unclear, the capacity and effectiveness for a wide range of trace ions, including fertilizers (phosphorus, nitrates) and heavy metals (chromium, arsenic, selenium — it is a long list), makes Organoclays deserving of consideration in a wide variety of ion removal applications.

The removal mechanism probably involves several possible ionic sites within the Organoclay, which are formed by modifying bentonite, with a starting ion exchange capacity of 70-90 meq/gram, with quaternary amines. The capability to remove emulsified non-aqueous phases is usually attributed to the quaternary amines, but it is likely that a significant portion of the original bentonite ion exchange capacity is still available for binding the ionized heavy metals and other ions in solution that are more highly charged than the typical background salinity of monovalent cations and anions.

In summary, just as Organoclays have become the “miracle cure of choice” for non-aqueous emulsions that vex activated carbon adsorption, there may be a growing class of instances where Organoclays effectively address a specific requirement for removing trace heavy metals and even low levels of common fertilizer anions. While the specific mechanism is complex and the capacity and effectiveness are hard to predict, the performance of Organoclays is easy to test. In those cases where Organoclays work, they often emerge as significantly less expensive than the short list of other options.