History of APTsorb
Peat: A good start to a great solution
The story of APTsorb starts 8,000 years ago in the low-lying depressions of the northern boreal forests. After the last glacial retreat, bodies of water naturally aged and filled with vegetation, helped along by cold climate and poor drainage. This unique combination of an anaerobic decomposition environment and cool temperatures resulted in partially decayed plant matter, called peat, with a different kind of chemistry.
Numerous classifications for peatland systems exist. The botanical classification focuses on vegetation, and in general, two types of peat dominate: Sphagnum and reed-sedge. Sphagnum peat consists primarily of partially decayed mosses of the Sphagnum genus. It generally looks very fibrous and is commonly sold at home improvement stores in tightly compressed bales or bags. Reed-sedge peat, as the name implies, consists of partially decayed reeds, sedges, grasses and cattails. When decayed long enough, reed-sedge peat is dark and looks like rich humus soil.
Reed-sedge peat harvest field
Peat as a sorption media
Reed-sedge peat is a complex material consisting mostly of lignin, hemicellulose, cellulose and humic substances. These constituents bear functional groups such as alcohols, aldehydes, ketones carboxylic acids, phenolic hydroxides, esters and ethers that are primed and ready for chemical bonding. Further, it’s the negative charge of these functional groups that make peat a star performer against toxins with a positive charge, including the cationic form of many heavy metals.
With such an extensive list of functional groups, it’s little wonder that the mechanisms of sorption for both APTsorb and natural peat remain under debate. There are five commonly recognized mechanisms for the reaction of peat with metals:
|As the name implies, ion-exchange is simply an exchange of one ion, often a hydrogen ion on a carboxyl group, for another ion, such as a metal cation. This mechanism is often employed by traditional ion-exchange resins.|
|Surface adsorption is a weak, non-chemical attraction between the peat surface and a metal ion. This type of bond is easily reversed.|
|Chemisorption, or chemical adsorption, is a chemical bond between the surface of the media and a metal cation. Unlike ion-exchange, there is no exchange of ions, but electrons may be shared or exchanged at the active site. Chemisorption results in a strong bond that is not easily broken.|
|Complexation is another type of chemical bond, but this time between two or more functional groups and a metal cation. The functional groups are often carboxyl and hydroxyl groups, and during the formation of the bond, the morphology of the media surface may change. A hydrogen ion may be released, depending on the functional group.|
|Lastly, adsorption-complexation is a hybrid type of mechanism, with a weak physical attraction forming between the media surface and a metal-anion molecule while a functional group, such as a hydroxyl group, forms a chemical bond with the same molecule to balance out the charge.|
The APTsorb Advantage
Natural peat has too many short-comings to be used as-is. Raw peat does not have favorable hydraulic characteristics; water simply doesn’t want to flow through it. Generally speaking, when raw peat is used as a water remediation media, the addition of large amounts of sand is required to maintain hydraulic conductivity in the bed. Also, raw peat is dusty, it’s difficult to wet, and its low density makes shipping a challenge.
APTsorb harnesses the attractive qualities of peat while overcoming the shortfalls. APTsorb is a hardened granular media with a hydraulic conductivity of about 1 cm/sec. It is a robust media that can contend with variations in influent water. It does not require the addition of sand, which means that 100 percent of the treatment bed is dedicated to metals sequestration. It wets readily and has minimal dust.
Need to know
APTsorb’s diverse mechanisms of sorption result in loading curves that cannot be simply described by, say, a Freundlich equation. Coefficients of kinetic loading are not uniform across the different functional groups. That presents some challenges to using APTsorb, but also makes it an attractive solution for many applications.
For example, ion-exchange is one of APTsorb’s mechanisms of adsorption. Those sites are very active. As a result, APTsorb is an excellent candidate for a final polishing step in a conventional treatment regime. As those very active ion-exchange sites start to fill, other secondary mechanisms, which are not as active, start to predominate. Those functional groups are not as efficient as the ion-exchange sites, but are very effective at removing the bulk of the influent metals. Because those secondary sites are more numerous, they are able to treat a larger volume of water.
|This characteristic makes APTsorb a prime candidate for a lead/lag system.||Lead/lag system set up|
|Initially, the lead tank removes the bulk of the metals and the lag tank polishes the water of the final traces.||Water flow at start of system|
|Once the lag tank is no longer performing up to specifications, it is moved into the lead position and the lead tank media, now presumably exhausted, is replaced and moved to the lag position.||Water flow after lead/lag switch|
Like all media, APTsorb exhibits affinities for some ions over others. The loading curves for the different functional group can also vary with the particular metal being loaded. However, despite the diverse nature of the APTsorb surface, it is possible to predict media life.