Not to be confused with adsorption (which is what a sponge does), absorption is the attraction of tiny particles or dissolved molecules to a solid surface and holding them there by weak intermolecular forces. It is similar in concept to magnetism and the attraction due to static electricity, but much weaker. In theory, every atom in the universe has some degree of affinity for every other atom in the universe, just like gravity. But, just as gravity requires enormous masses like planets and stars to show its effects, adsorption requires extremely tiny distances to show its effects. In adsorption, the particle in question is randomly bounced around the solution by collisions with water molecules and other molecules in the water. (This is called Brownian motion. It is estimated that an atom or molecule in water is involved in a million-billion-trillion or 1027 collisions with other atoms or molecules every second. This is part of the definition of temperature.) Eventually, by chance, it will be bounced so close to the surface of a wall or another larger particle that there are very few water molecules separating it from the surface. When that happens, those few collisions from the other sides and tends to become “plastered” to the surface by a continual barrage of collisions from the solution. This is the “physical” half of adsorption. The “chemical” half occurs if there is any chemical, affinity between the particle and the material of the surface. If there is, the particle will become attached (adsorbed) and stay there; if not, it will bounce off, right away or just diffuse away, later. The adsorptive forces (called van der Waals or London forces) are so weak that adsorbed substances can become desorbed rather easily-by adding certain, acids, by heating the system, or by merely removing the contaminant from the influent water. For example, activated carbon filters or ion exchange beds nearing exhaustion are subject to desorption if the water quality suddenly changes for the better. That shows that these treatment techniques are equilibrium (balance) phenomena in which sorption and desorption both occur and achieve an average condition, like a well-matched tug-of-war. Since adsorption requires a surface, commercial adsorbent materials have very large surface areas and are exemplified by activated carbon, activated alumina, and fine powders such as baking soda. But many substances are so very insoluble or otherwise so readily adsorbable that even small surface areas can make a big difference. For example, most heavy metal ions (lead. mercury, copper, cadmium, silver, chromium) adsorb so strongly to the walls of both glass and plastic sample bottles that more than half of the total contamination can be missed in an analysis if the sample bottles are not treated with nitric acid first, to cause desorption. Similarly, many chlorinated hydrocarbons biphenyls (PCBs) adsorb so readily to both metal and plastic plumbing and filter materials that even coarse prefilters remove them very well.
The adsorption and reduction of disinfectant chlorine by activated carbon is a special case. Activated carbon is a mild reducing agent and chlorine is a strong oxidizing agent, so after chlorine becomes adsorbed, it then actually reacts with the carbon. The chlorine is reduced to chloride ion (as in table salt and sea water), one atom of carbon is oxidized to carbon dioxide, and both are released to the solution (desorbed). Meanwhile, most of the spots on the activated carbon where all this took place become “auto-regenerated” back to their original, like new condition, ready to adsorb again. For free available chlorine (FAC), this takes only about fifteen minutes, which means that a small amount of carbon can achieve an acceptable steady-state condition if the flow rate is slow or intermitted. For “combined chlorine” (monochloramine), the reaction is much slower, and more carbon or more contact time is needed to achieve equivalent reductions. The chemical reactions between activated carbon’s “active sites” (C*) and these forms of chlorine are shown below. Note that any surface oxides’ on the carbon are recycled when reacted with monochloramine, while they are oxidized to CO2 and lost when reacted with free chlorine.
Molecules produce only a few collisions from that side, and the particle is overwhelmed by Free Chlorine
CI2 + H2O <=> HOCI + H+ + CI (forming “aqueous chlorine”)
C* + 2CI2 + 2H20 => C*O2 + 4H+ + 4CI– (overall reaction)
C* + HOCI => C*O + H+ + CI–
C*O + HOCI => C*O2 + H+ + CI–
Combined Chlorine: Monochloramine
C* + NH2CI + H2O => C*O + NH3 + H+ + CI–
C*O + 2NH2CI => C* + N2 + H2O + 2H+ + 2CI–
Finally, most dissolved/suspended particles and molecules in drinking water that are highly adsorbable to something usually do become adsorbed to a larger particle before reaching the point of use. Thus, adsorbable contaminants can often be removed by mechanical fine-filtration because the contaminant in question is already adsorbed to a larger particle. If you remove the particle, you remove the adsorbed contaminants along with it. This commonly applies to heavy metal ions, many pesticides, other chlorinated hydrocarbons, viruses, and asbestos fibers.
About Activated Carbon: Granular activated carbon (GAC) and powdered activated carbon (PAC) are the predominant adsorbents used in our industry. They can be made from nearly anything organic: coal, petroleum, wood, coconut shells, peach pits, ion exchange resin beads, fabrics, even waste plastics. The starting material is first charred-heated without air or oxygen, so it doesn’t burn up. Everything that can be vaporized or melted bubbles out as tar or pitch, leaving many holes and channels. Then the charred material is heated further, to above 1000ºC (hot enough to melt aluminum and lead), with the introduction of live steam or other activating chemicals. The superheated water vapor is extremely corrosive, etching more holes and extending channels to an amazing degree. Metallic impurities are preferentially attacked and washed out, resulting in a significant purification of the original material.
However, the heat of activation does more than extend holes and channels and increase the surface area of carbon: it also changes the fundamental crystal form from amorphous “carbon black” to the perfect crystalline array of graphite plates. The carbon atoms in graphite are arranged in sheets or plates of interlocking six-atom rings that look like slices through a honeycomb. Such a perfect arrangement causes the London forces to focus and concentrate at the surface, making activated carbon the best (strongest and most general) adsorbent known.
After activation, the carbon may be treated further to produce specific chemical qualities on the surface. For example, an acidic environment produces carbon with maximum capacity for heavy metals but minimal capacity for chlorinated organics, while an alkaline environment does the opposite. Most grades used in our industry are made for organic adsorption. When activation is complete, the carbon is a delicate, airy material that is so full of holes, it can barely hold together. It is crushed to a powder, and then proprietary binders are added to form granules of the desired size. The final product has a total internal and external surface area of more than 1000 square meters per gram, or half a football field inside a piece the size of a pea.
Activated carbon adsorption is useful because the material has strong chemical affinities for several important classes of contaminants that are common in water.
- Disinfectant chlorine: “Free available chlorine” (FAC) is readily adsorbed, then chemically reduced, and finally desorbed as chloride ion along with one molecule of carbon dioxide, with auto-regeneration of most of the carbon’s active sites and nearly infinite capacity. “Combined chlorine” (monochloramine) is less easily adsorbed, requiring more carbon or reduced flow rate for equivalent performance.
- Organic compounds containing chlorine and other halogens: Simple halogenated hydrocarbons are highly adsorbable to activated carbon. This includes a great many pesticides (DDT, Endrin, Lindane, Chlordane, etc.), industrial solvents (trichloroethylene, trichloroethane, tetrachloroethylene, carbon tetrachloride, etc.), and disinfection byproducts (THMs including chloroform, chloral hydrate, etc.).
- Organic compounds containing benzene rings: These include some of the most toxic chemicals, such as benzene, toluene, dioxins, polychlorinated biphenyls (PCBs), and phthalate esters (plasticizers for vinyls).
- Heavy metals: Lead, cadmium, and mercury adsorb readily, both as dissolved ions and colloidal oxide or carbonate particles, but the capacity is limited-similar to the capacity for THMs.
- Taste and Odor (T&O) compounds: The substances produced by microbes that are responsible for the common musty-earthy-mildewy T&O are extremely well adsorbed and with very great capacity.
There is great variation in the adsorbability of dissolved/suspended substances, and also great variability in the adsorptive capacity of different adsorbents. A bed of granular activated carbon (GAC) may be exhausted with respect to chloroform and other volatile organic compounds (VOCs) after only a few hundred bed volumes, yet continue to adsorb PCBs for many thousands more. Different grades and types of activated carbon have different capacities for the same contaminant as well as various contaminants, which means that one must be very careful and specific in making comparisons.
Freundlich carbon isotherm
Chemists have developed a standard procedure for comparison of adsorption (capacities, called an isotherm. A Freundlich carbon isotherm is determined by preparing several identical bottles of powdered activated carbon suspended in water. (In German, “eu” is pronounced “oi,” so Freundlich sounds like “Froindlich.”) Varying amounts of a contaminant are added to the bottles, and all are mixed until adsorption has reached equilibrium under those conditions of temperature and pressure. Then the carbon is filtered out and the solutions are analyzed to find how much contaminant remains unadsorbed in each one. The Freundlich equation is used to calculate the amount of contaminant that was adsorbed per milligram of carbon, and each data point is graphed on logarithmic graph paper with the carbon capacity in mg/g on the Y-axis and the final equilibrium concentration in mg/L on the X-axis. Finally, the “average” line representing all of the data points is drawn. That average line on the graph paper is called the isotherm for that contaminant and that carbon under those conditions. But that line covers a range of capacities; the one value used for comparison purposes has been designated by international agreement to be the capacity in mg/g of carbon on the Y-axis that corresponds to the value of 1.0 mg/L on the X-axis. If the isotherm does not cross the 1.0 mg/L point, the line is artificially extended (“extrapolated”) to that level for the purpose. The Freundlich Equation can be represented as:
Ce = concentration of contaminant in solution at equilibrium (X-axis value)
K = a constant
1/n = another constant
The Ce is determined by analysis; the qe is calculated using equation and the two Freundlich constants that were determined by the chemist who published the data.
By convention in our industry, chloroform, the main THM, has been selected as the least adsorbable contaminant that activated carbon can be claimed to adsorb effectively – any contaminant that has a Freundlich isotherm qe value less than that for chloroform cannot be said to be removed by carbon adsorption. We “draw the line” at chloroform; anything less adsorbable than chloroform is deemed not worth the trouble. Example: in one test series, the Freundlich capacity for chloroform is 2.6 mg/g GAC (the isotherm passes through 1.0mg/L above the X-axis at the point where the Y-axis reads 2.6 mg/g GAC), while the equivalent value for trichloroethylene is 30 mg/g GAC. That is a much higher value, meaning that the particular GAC will adsorb trichloroethylene much more easily than chloroform. However, in the same data set, the capacity value for methylene chloride is only 1.3 mg/g, and thus we say that methylene chloride cannot be adsorbed efficiently by that GAC. The carbon’s capacity for it is too small to make an economically viable product. See the example below.
It is important to remember that carbon capacity figures derived from Freundlich isotherms are to be used only for comparisons – e.g., carbon A is better than carbon B, or contaminant X is easier to remove than contaminant Y. they should not be used as concrete capacities to calculate how long a filter should last. The reason is that the isotherm data are produced at equilibrium, which may take several days of stirring in the lab to achieve. But a bed of GAC or a filter cartridge operates on a dynamic, flowing basis, and equilibrium conditions may not be achieved even after a weekend downtime.