Water Well Journal
Issue link: https://read.dmtmag.com/i/692787
sare selectively transferred from the fluid phase to the surface of insoluble, rigid particles suspended in a vessel or packed in a column. The two most common methods of adsorption in water treatment use zeolites or activated carbon. Zeolites Zeolites are natural or synthetic crystalline aluminosili- cates, which have a repeating pore network and release water at high temperature. They are manufactured by a synthesis of sodium aluminosilicate or another silica source in an auto- clave followed by an ion exchange process with certain cations (such as sodium or calcium). The ion exchange process is followed by drying of the crystals, which can be pelletized with a binder to form macroporous pellets. Zeolites are used in the drying of process air, carbon diox- ide removal from natural gas, air separation, catalytic crack- ing, and catalytic synthesis and reforming. Non-polar (siliceous) zeolites are synthesized from alu- minum-free silica sources or by dealumination of aluminum- containing zeolites. The dealumination process is performed by treating the zeolite with high temperature steam, at a tem- perature typically higher than 930°F (500°C). This high tem- perature heat treatment breaks the aluminum-oxygen bonds and the aluminum atom is expelled from the zeolite frame- work. In addition to adsorption, zeolites were at one time widely used as ion exchange beds in domestic and commercial water purification, softening, and other applications. They are also still widely used as catalysts and sorbents. Their well-defined pore structure and adjustable acidity make them highly active in a large variety of reactions. These naturally-occurring and synthetic inorganic minerals are anions (ions with a negative electrical charge); therefore, zeolites can exchange only cations (ions with a positive electrical charge) such as sodium (Na) or calcium (Ca 2+ ). Since the ion exchange capacity of zeolites is not very high, a group of synthetic organic (carbon-based) ion ex- change resins were artificially developed during the mid-20th century in an effort to make water softeners more efficient and marketable. Activated Carbon Activated carbon is a highly porous, amorphous solid con- sisting of microcrystallites with a graphite lattice (structure), usually prepared in small pellets or as a powder. Although quite inexpensive to produce, one of its main drawbacks in use is that it reacts with oxygen at moderate temperatures. Activated carbon can be manufactured from carbonaceous material, including coal, peat, wood, or nutshells (coconut). The manufacturing process consists of two phases: carboniza- tion and activation. The carbonization process includes drying and then heating to separate the by-products, including tars and other hydrocarbons, from the raw material as well as to drive off any generated gases. The process is completed by heating the material to over 750°F (400°C) in an oxygen-free atmosphere that cannot support combustion. The carbonized particles are then "activated" by exposing them to an oxidizing agent, usually steam or carbon dioxide at high temperature. This agent burns off the pore-blocking structures created during the carbonization phase and they then develop a porous, three-dimensional graphite lattice structure. The size of the pores developed during activation is a function of the time they spend in this stage. Longer expo- sure times result in larger pore sizes. The most popular aque- ous phase carbons are bituminous because of their hardness, abrasion resistance, pore size distribution, and low cost. All the same, their effectiveness needs to be tested in each application to determine the optimal product. Besides water treatment, activated carbon is also used for the adsorption of organic substances and non-polar adsorbates and is also used for wastewater treatment. It is the most widely used adsorbent since most of its chemical and physical properties (pore size distribution and surface area) can be fine-tuned according to what is needed. Its usefulness also derives from its large micropore volume and the resulting high surface area of contact. Activated carbon is commonly used in water treatment for the removal of taste and odor offending compounds or organic materials. It can be found in two forms: granular activated carbon (GAC) or powdered activated carbon (PAC). GAC (Figure 1) is generally used as a granular media bed in a filter vessel where water comes into contact with the material as it flows through the bed. PAC is typically injected into a flow stream where the material attaches to the offending substance, causing the taste and/or odor substance to be removed. One particle of activated carbon has an extremely large surface area owing to its structure and diversity of pores, simi- lar to those found in a sponge. PAC, a finely ground charcoal, is used for this process. When PAC is added to the water, the ENGINEERING from page 40 Figure 1. Function of granular activated carbon. waterwelljournal.com 42 July 2016 WWJ