Water treatment has become a multi-billion dollar industry, with rapidly evolving technologies needed to meet ever more stringent regulations. This edition of The Drip to explains the science of adsorption in plain language with a few helpful analogies. The goal is to help readers visualize the process to help select technology and improve their daily operations.
There are broad categories of treatment, most of which involve removing some form of pollutant from a bulk stream of flowing water. Filtration, chemical precipitation, oxidation/reduction, ion exchange and adsorption are all common tools employed in modern wastewater treatment systems. As regulatory limits become lower (e.g. 14 ppb for copper in stormwater), adsorption becomes the tool of choice for the treatment system designer. This post will discuss adsorption of IONIC materials in aqueous systems.
Adsorption is the process of removing dissolved chemicals (species) from a bulk liquid phase like water usually onto a solid phase, or “media.” Ionic species are simply those that have some form of electrical charge. For the sake of full disclosure – no one really knows what electrical charge is made of – but we’ll pretend we do for convenience. We do know that “positive” charge is the opposite of “negative” charge, and that these two attract each other, like the poles of a magnet, in proportion to the amount of charge in each.
Charges never really exist independent of their opposite charge – but ionic species can be more comfortable (in a lower energy state) in some forms than in others. Adsorption media works by presenting an aqueous ionic species with a more comfortable (lower energy) place to be. It is helpful to think of the adsorption media providing a comfortable couch, or a parking place for ionic species. For example, some types of adsorption media have a negatively charged parking places for positively charged aqueous ionic species (e.g. metal cations).
Water treatment by adsorption provides these ionic parking spaces on a massive scale. Because ionic species can’t steer, they behave much like a driver with a blindfold looking for a parking place in a huge parking garage. When adsorption media is new and unused, the garage is empty and none of the parking places are occupied. It is easy for the blindfolded driver to find a parking place. As the media is used, the parking places start to fill up. Once the garage is about half full, some of the drivers begin to bounce all the way through the parking garage without finding a place. When the garage is 3/4 full, the drivers are frequently bouncing out of the garage. This is similar to the gradual failure of adsorption media long before the media is actually full, or “saturated.”
These factors must be considered when designing adsorption systems. The usual approach to resolving capacity issues is to design a parking garage that is much larger than actually needed.
Other design factors also fit this analogy well. For example, flow rates and residence times have a strong effect on the performance of adsorption media. If the blindfolded driver is given many hours to find a parking place, he’ll usually do so even if the parking garage is relatively full. This is analogous to a low flow rate, or long residence times in the adsorption media. If the residence time is short or the flow rate is high, this is analogous to giving almost no time for the blind driver to find a parking place. As a result, excessive flow rates or short residence times hurt adsorption performance.
Temperature is another factor that is well-suited to the analogy. The physical effects of increasing temperature on a molecular scale is a net increase in the velocity of a given atom or molecule within a confined area. For our parking analogy, that’s the same as increasing the speed inside of the parking garage, allowing our blindfolded driver to check more parking places at a higher rate while he’s still in the garage, hoping to finding an un-occupied space. In the same way, increasing temperature generally improves the performance of adsorption media.
The final parameter in this analogy is making sure that the parking spaces are the right size for the cars. This is the phenomena of “selectivity” whereby the adsorption media are designed to preferentially adsorb the targeted pollutants, and desorb (release) the benign ions into solution.
In the real world, all this means we need to understand how many drivers we have (concentration and flow), how many parking spaces we have in a given garage (saturation capacity), and design the garage for maximum efficiency. The Enpurion(TM) Metals Treatment process overcomes the design problem by placing adsorption columns one after another in series. This patent-pending feature is like always having an empty parking garage at the end of the parking structure. In this way, the adsorption systems can be designed to be flexible, modular, and can be re-arranged for any facility or any change in operating conditions. Since the first parking garage in the series is full before it is replaced, serial adsorption treatment systems are far more efficient and less costly to operate. Serial processes are a patented feature of the Enpurion(TM) metals treatment process, which can be accurately modeled for any industrial application.
The same analogy holds true for non-ionic species and non-polar media. For example, suspended oils are readily adsorbed using granular activated carbon (GAC). The behavior of non-polar species on a molecular level is quite different, but with the exception of temperature, all of the analogies remain true. Please feel free to email (email@example.com) with questions or for more information.