Activated carbon
Activated carbon, also called activated charcoal, is a form of carbon commonly used to filter contaminants from water and air, among many other uses. It is processed (activated) to have small, low-volume pores that greatly increase the surface area available for adsorption or chemical reactions. (Adsorption, not to be confused with absorption, is a process where atoms or molecules adhere to a surface). The pores can be thought of as a microscopic "sponge" structure. Activation is analogous to making popcorn from dried corn kernels: popcorn is light, fluffy, and its kernels have a high surface-area-to-volume ratio. Activated is sometimes replaced by active.
Because it is so porous on a microscopic scale, activated carbon has a surface area of over 3,000 square metres per gram (920,000 square feet per ounce), as determined by gas absorption and its porosity can run 10ML/day in terms of treated water per gram. Researchers at Cornell University synthesized an ultrahigh surface area activated carbon with a BET area of 4,800 m2/g (1,500,000 sq ft/oz). This BET area value is the highest reported in the literature for activated carbon. For charcoal, the equivalent figure before activation is about 2–5 square metres per gram (610–1,530 sq ft/oz). A useful activation level may be obtained solely from high surface area. Further chemical treatment often enhances adsorption properties.
Activated carbon is usually derived from waste products such as coconut husks in addition to other agricultural wastes like olive stones, rice husks and nutshells which are being upcycled into activated carbon, diversifying feedstock supply. Waste from paper mills has been studied as a possible source of activated carbon. These bulk sources are converted into charcoal before being activated. Using waste streams not only reduces landfill burden but also works to lower the overall carbon footprint as previously discarded waste is repurposed. When derived from coal, it is referred to as activated coal. Activated coke is derived from coke. In activated-coke production, the raw coke (most commonly petroleum coke) is ground or pelletized, then "activated" via physical (steam or CO2 at high temperature) or chemical (e.g., KOH or H3PO4) methods to introduce a porous network, yielding a high-surface-area adsorbent which is referred to as activated coal.