Water Conditioning & Purification Magazine

The Importance of Activated Carbon Terminology: A Glossary of Terms

By H. George Nowicki, Chris Brunning, Barbara Sherman, Henry Nowicki

Introduction

This article provides some of the major definitions to help professionals working in the field of activated carbon. Using clear and precise words demonstrates your professional knowledge and enables you to make better decisions when purchasing activated carbon or using consultants and laboratory testing services. Vocabulary improvement and understanding of terms gives employees professional confidence, which will help them more effectively communicate with
others.

A myriad of uses

A layperson may think of activated carbon as ‘that stuff’ in water-filter pitchers, but people in the field know that point of use (POU) devices are only one of many applications for activated carbon. From municipal drinking water treatments to waste water treatment for vapor-phase odor removal and water filtration and purification of effluent waters, activated carbon has many uses in addition to de-chlorinating water in pitcher devices.

Activated carbon’s main uses are in pollution and odor control, color removal, catalyst support and preparation of drinking water. In fact, activated carbon is used in many of our everyday typical tasks: the water used to prepare the morning coffee has been carbon filtered at a municipal plant and then through your POU residential device; the sugar used to sweeten that coffee was decolorized by carbon; the cigarette some smoke with that coffee has activated carbon in the filter for toxic chemical removal; even some automobiles and petrol filling stations have carbon filters to recover and use the gasoline vapors. Batteries contain activated carbon. The odor-control in cat litter boxes is endowed by a special de-dusted activated carbon. The gold and platinum jewelry industries use activated carbon in the processes of recovering precious metals and it is also used in renal dialysis units, as well as in a device to improve invitro fertilization of human ova.

Taken for granted for a long time because it was a low cost commodity, the prices of activated carbon have increased dramatically over the last year because of the five-year US tariff that began in January of 2007, which will probably be re-issued for another five years when it expires in 2012. The increased costs for activated carbon due to the US import tariffs against Chinese steam-activated, bituminous coal-based activated carbon have created an economic need to better understand the associated terminology within the industry so as to make more educated decisions about this commodity.

Wikipedia, the Internet encyclopedia, defines activated carbon as: “a general term which covers carbon material mostly derived from charcoal… It is a material with an exceptionally high surface area. Just one gram of activated carbon has a surface area of approximately 500 m², typically determined by nitrogen gas adsorption and includes a large amount of microporosity. Sufficient activation for useful applications may come solely from the high surface area, though often further chemical treatment is used to enhance the adsorbing properties of the material.”

The vocabulary and other important terms listed here are official definitions as defined by the American Society for Testing and Materials (ASTM), the American Water Works Association (AWWA) and NSF International.

Activated carbon glossary

Abrasion resistance: the property of a particle to resist attrition or wearing away by friction.

Absorption: a process in which fluid molecules are taken up by a liquid or solid and distributed throughout the body of that liquid or solid.

Accelerated adsorption tests: adsorption tests in which the end point is hastened by testing at conditions more severe than those anticipated in services.

Accelerated service life: the elapsed time unit the end point is reached in an accelerated adsorption test.

Acid-extractable material: substances dissolved by an acid under specified conditions.

Activated carbon: a family of carbonaceous substances manufactured by processes that develop adsorptive properties.

Activation: a generic term used to describe the capacity to adsorb in general; also, the adsorptive capacity of an adsorbent as measured by a standard test.

Adsorbate: any substance that is or can be adsorbed.

Adsorbent: any solid having the ability to concentrate significant quantities of other substances on its surface.

Adsorption: a process in which fluid molecules are concentrated on a surface by chemical or physical forces, or both.

Adsorption wave: see mass transfer zone.

Adsorption zone: see mass transfer zone.

Ash: residue after the combustion of a substance under specified conditions.

As is basis: as received.

Breakpoint: the appearance in the effluent of a specified concentration of an adsorbate. Breakthrough: the first appearance in the effluent of an adsorbate of interest under specified conditions.

Capacity: the rated service cycle, expressed as a function of time or volume of water treated by the system between manufacturer-specified servicings (cleaning, regeneration or replacement) of the medium (media).

Channeling: the flow of fluid through passages of lower resistance which can occur in fixed beds or columns of particles due to nonuniform packing, irregular sizes and shapes of the particles, gas pockets, wall effects and other causes.

Chemical adsorption: see chemisorption.

Chemisorption (chemical adsorption): the binding of an adsorbate to the surface of a solid by forces whose energy levels approximate those of a chemical bond.

Coadsorption: the adsorption of two or more components on an adsorbent, each affecting the adsorbability of the other. Contact batch operation: an adsorption process in which an adsorbent is dispersed in a fluid to be treated and then separated when practical equilibrium is
attained.

Continuous moving bed: an adsorption process characterized by flow of a fluid through a continuously moving bed of granular adsorbent with continuous withdrawal of spent adsorbent and continuous addition of reprocessed or virgin adsorbent.

Countercurrent adsorption: an adsorption process in which the flow of fluid is in a direction opposite to the movement of the adsorbent.

Critical bed depth: the minimum depth of an adsorbent bed required to contain the mass transfer zone.

Crushing strength: the property of a particle to resist physical breakdown when contained and subjected to a slowly increasing, continuously applied force.

Degassing: removal of gases.

Density, absolute or true: the mass under specified conditions of a unit volume of a solid sorbent excluding its pore volume and inter-particle voids.

Density, apparent (density, bulk): the mass under specified conditions of a unit volume of a solid sorbent, including its pore volume and inter-particle voids.

Density, block: see density, particle.

Density, bulk: see density, apparent.

Density, particle (density, block): the mass under specified conditions of a unit volume of a solid sorbent including its pore volume but excluding inter-particle voids.

Desorption: the separation of an adsorbate as such from a sorbent.

Differential heat of adsorption: the heat evolved during the adsorption of an incremental quantity of adsorbate at a given level of adsorption.

Dosage: the quantity of substance applied per unit weight or volume of the fluid being treated.

Dry basis: exclusive of any moisture which may be present.

Dust: an imprecise term referring to particulates capable of temporary suspension in air or other gases; also, particles smaller than an arbitrarily selected size.

Dynamic adsorptive capacity: the quantity of a given component adsorbed per unit of adsorbent from a fluid or fluid mixture moving through a fixed bed at the break-point for that component.

Effective size: the particle size, in millimeters, which corresponds to 10 percent finer on the cumulative particle size distribution curve.

Electrical conductivity of a particulate substance: the current flowing through a unit cross section for an imposed unit potential gradient under specified conditions of packing.

Electrophoresis: migration of dispersed solid, liquid or gaseous material to one of two electrodes under the influence of an impressed direct-current voltage.

End point: the occurrence in the effluent of the maximum permissible concentration of an adsorbate of interest.

Equilibrium adsorptive capacity: the quantity of a given component adsorbed per unit of adsorbent from a fluid or fluid mixture at equilibrium temperature and concentration of pressure.

Expanded bed: a bed of granular particles through which a fluid flows upward at a rate sufficient to slightly elevate and separate the particles without changing their relative positions.

Extractant water: water that has been in contact with a system or component(s) for a specified duration.

Filterability: the rate at which particles can be separated from a slurry by means of a permeable medium under specified conditions.

Fines: particles smaller than the smallest nominal specification particle size.

Fixed bed: a bed of granular particles through which a fluid flows without causing substantial movement of the bed.

Fluidized bed: a bed of granular particles in which the fluid flows upward at a rate sufficient to suspend the particles completely and randomly in the fluid phase.

Freundlich adsorption isotherm: a logarithmic plot of quantity of component adsorbed per unit of adsorbent versus concentration of that component at equilibrium and at constant temperature, which approximates the straight line postulated by the Freundlich adsorption equation: X/M = kCn (where: X = quantity adsorbed; M = quantity of adsorbent; C = concentration; k and n = constants).

Granular activated carbon: activated carbon in particle sizes predominantly greater than 80 mesh.

Hardness: a generic term referring to the resistance of a particle to breakdown as measured by specific tests.

Heat of adsorption: the heat evolved during adsorpton.

Hydrolytic adsorption: the adsorption of a weakly ionized acid or base formed by the hydrolysis of some types of salts in aqueous solution.

Hysteresis loop: the divergence between the paths of the adsorption and desorption isotherms.

Ignition temperature (kindling point): the lowest temperature at which combustion will occur spontaneously under specified conditions.

Impact strength: the property of a particle to resist physical breakdown when subjected to a rapidly increasing applied force.

Influent challenge: the mixture of water and contaminants entering the system.

Integral heat of adsorption: the sum of the differential heats of adsorption from zero to a given level of adsorption.

Intermittent moving bed (pulse, slug): an adsorption process characterized by upward flow of a fluid through a fixed bed of granular adsorbent with periodic withdrawal of spent adsorbent from the bottom of the bed and additions of reprocessed or virgin adsorbent to the top of the bed.

Irreversible adsorption: adsorption in which the desorption isotherm is displaced toward higher equilibrium adsorption capacities from the adsorption isotherm.

Isobar: a plot of quantity adsorbed per unit of adsorbent against equilibrium temperature when concentration or pressure is held constant.

Isotere: a plot of equilibrium concentration or pressure against temperature when the quantity adsorbed per unit of adsorbent is held constant.

Isotherm: a plot of quantity adsorbed per unit of adsorbent against equilibrium concentration or pressure when temperature is held constant.

Langmuir isotherm: a plot of isothermal adsorption data, which to a reasonable degree, fit the Langmuir adsorption equation.

Macropore: pores with widths exceeding 50 nanometers (500 angstrom units).

Mass transfer zone (adsorption wave, adsorption zone): the region in which the concentration of the adsorbate of interest in the fluid decreases from influent concentration to the lowest detectable concentration.

Maximum contaminant level (MCL): the maximum permissible concentration of a contaminant in drinking water as established in the US EPA National Primary Drinking Water Regulations.

Mean particle diameter: the weighted average particle size, in millimeters, of a granular adsorbent computed by multiplying the percent retained in a size fraction by the respective mean sieve openings, summing these values and dividing by 100.

Media migration: the entrainment of a fraction of the media into the product water.

Mesopore: pores of widths between two and 50 nanometers (20 and 500 angstrom units).

Moisture content: the water content of a substance as measured under specified conditions.

Monomolecular layer: an adsorbed film, one molecule thick.

Multimolecular layer: an adsorbed film, more than one molecule thick.

Oven drying loss: the reduction in weight resulting when a substance is heated in an oven under specified conditions.

Physical adsorption (Van der Waals adsorption): the binding of an adsorbate to the surface of a solid by forces whose energy levels approximate those of condensation.

Pore diameter: the diameter of a pore in a model in which the pores in a sorbent are assumed to be cylindrical in shape and which is calculated from data obtained by a specified procedure.

Pores: the complex network of channels in the interior of a particle of a sorbent.

Pore volume: volume of the pores in a unit weight of a sorbent.

Pore volume distribution: the distribution of pore volume among pores of different size or diameter.

Powdered activated carbon: activated carbon in particle sizes predominantly smaller than 80 mesh.

Preferential adsorption: adsorption in which a certain component or certain components are adsorbed to a much greater extent than others.

Pressure drop: the difference between the inlet and outlet pressures of the system at the rated service flow.

Product water: water that has been treated by the system.

Reactivation (revivification): oxidation processes for restoring the adsorptive properties of a spent sorbent.

Raw water: untreated water or any influent water before it enters a specific water treatment component or system.

Regeneration: distillation or elution-type processes for restoring the adsorptive properties of a spent sorbent.

Relative efficiency: the rating of the adsorptive capacity of an adsorbent based on a comparison of its performance with that of a reference adsorbent in a defined test.

Retentivity: the ability of an adsorbent to resist desorption of an adsorbate.

Reversible adsorption: adsorption in which the desorption isotherm approximates the adsorption isotherm.

Revivification: see reactivation.

Service life (service time): the elapsed  time until the end point is reached in an adsorption process.

Sorption: a process in which fluid molecules are taken up by absorption and/or adsorption.

Split feed: a liquid-phase adsorption process in which a powdered adsorbent is added to the solution to be treated in two or more steps, with or without intermediate separation of the adsorbent.

Surface area (B.E.T.): the total surface area of a solid calculated by the B.E.T. (Brunauer, Emmitt, Teller) equation, from nitrogen adsorption or desorption data obtained under specified conditions.

Surface area distribution: the distribution of surface area according to some parameter such as pores of different size or diameter.

Surface oxides: oxygen containing compounds and complexes formed at the surface of an adsorbent.

Synthetic test solution: a solution of two or more components prepared under specified conditions for use in evaluation of adsorbents.

Threshold concentration: the minimum concentration at which a substance can be detected by the taste or odor test method employed.

Total dissolved solids (TDS): the remain ing solids from a filtrate evaporated to dryness and dried to a constant weight at 180°C (356°F) after passing through a 0.45-mm glass fiber filter.

Uniformity coefficient: the ratio of the particle diameter corresponding to 60 percent finer on the cumulative particle size distribution curve to the particle diameter corresponding to 10 percent finer on the same distribution curve.

Unit volume: total water-holding volume without the medium (media) or components in place.

Van der Waals adsorption: see physical  adsorption.

Water-extractable material: substances dissolved from other substances by water under specified conditions.

Wettability: the rate at which particles can be made wet under specified conditions.

Evolution of the activated carbon vocabulary

It is important that we all agree on the meanings of the words that we use to conduct day-to-day business. If you think there are any additional words that should be included in this list of  important activated carbon terminology or if you have any other input about the content of this list, please let the authors know. We are open to additions and any other suggestions, which will be considered and may be incorporated into an updated version that we will be making available on the PACS website www.pacslabs.com approximately two months from this article’s publication date.

Any additional definitions that we may have overlooked can be emailed to George Nowicki: GeorgePACS@aol.com. Please include references for any suggested vocabulary, so that any new additions for the list can be properly verified before we add them to the future update.

About PACS Inc.

Professional Analytical and Consulting Services Inc. (PACS Inc.) is an independent promoter of the activated carbon industry. The firm has been awarded eight government grants to conduct R&D on new carbon products. PACS provides testing services to over 500 clients and has participated in over 40 legal proceedings as science advisors. It has developed the Activated Carbon School as a part of the 60 short courses available to scientists. This October 1-15, 2008 PACS will provide the 22nd International Activated Carbon Conference and Courses program near the Pittsburgh International Airport.

About the authors

Corresponding author H. George Nowicki III, B.S., B.A., is a Technician in the PACS laboratory and helps direct the day-to-day testing services. He can be reached by email at GeorgePACS@aol.com or by phone at (724) 457-6576. Chris Brunning, B.A., designs and maintains the PACS website www.pacslabs.com and works in the PACS mobile laboratory, where he performs asbestoses inspections and management plans for how companies should handle asbestoses. He can be reached by email at ChrisPACS@aol.com. Barbara Sherman, M.S., is the Manager of Operations and directs the day-to-day PACS Short Courses and Conferences services. She can be reached by email at BarbPACS@aol.com. Henry Nowicki, Ph.D., directs the research and development, is a Senior Consultant and is the new business developer for PACS. He can be reached by email at HNpacs@aol.com.

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