Biosorption
Encyclopedia
Biosorption is a physiochemical process that occurs naturally in certain biomass
which allows it to passively concentrate and bind contaminants onto its cellular structure . Though using biomass in environmental cleanup has been in practice for a while, scientists and engineers are hoping this phenomenon will provide an economical alternative for removing toxic heavy metals from industrial wastewater and aid in environmental remediation.
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....
which allows it to passively concentrate and bind contaminants onto its cellular structure . Though using biomass in environmental cleanup has been in practice for a while, scientists and engineers are hoping this phenomenon will provide an economical alternative for removing toxic heavy metals from industrial wastewater and aid in environmental remediation.
Environmental Uses
- Pollution interacts naturally with biological systems. It is currently uncontrolled, seeping into any biological entity within the range of exposure. The most problematic contaminants include heavy metals, pesticides and other organic compounds which can be toxic to wildlife and humans in small concentration. There are existing methods for remediation, but they are expensive or ineffective . However, an extensive body of research has found that a wide variety of commonly discarded waste including eggshells, bones, peat, fungi, seaweed, yeast and carrot peels can efficiently remove toxic heavy metal ions from contaminated water. Ions from metals like mercury can react in the environment to form harmful compounds like methylmercuryMethylmercuryMethylmercury is an organometallic cation with the formula . It is a bioaccumulative environmental toxicant.-Structure:...
, a compound known to be toxic in humans. In addition, adsorbing biomass, or biosorbents, can also remove other harmful metals like: arsenicArsenicArsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
, leadLeadLead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
, cadmiumCadmiumCadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
, cobaltCobaltCobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
, chromiumChromiumChromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
and uraniumUraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
. Biosorption may be used as an environmentally friendly filtering technique. There is no doubt that the world could benefit from more rigorous filtering of harmful pollutants created by industrial processes and all-around human activity.
- The idea of using biomass as a tool in environmental cleanup has been around since the early 1900’s when Arden and Lockett discovered certain types of living bacteria cultures were capable of recovering nitrogen and phosphorus from raw sewage when it was mixed in an aeration tank . This discovery became known as the activated sludge process which is structured around the concept of bioaccumulation and is still widely used in wastewater treatment plants today. It wasn’t until the late 1970’s when scientists noticed the sequestering characteristic in dead biomass which resulted in a shift in research from bioaccumulation to biosorption .
The Differences Between Biosorption and Bioaccumulation
- Though bioaccumulationBioaccumulationBioaccumulation refers to the accumulation of substances, such as pesticides, or other organic chemicals in an organism. Bioaccumulation occurs when an organism absorbs a toxic substance at a rate greater than that at which the substance is lost...
and biosorption are used synonymously, they are very different in how they sequester contaminants:
- Biosorption:
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- Biosorption is a metabolically passive process, meaning it does not require energy, and the amount of contaminants a sorbent can remove is dependent on kinetic equilibrium and the composition of the sorbents cellular surface . Contaminants are adsorbed onto the cellular structure. Bioaccumulation is an active metabolic process driven by energy from a living organism and requires respiration .
- Bioaccumulation:
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- Bioaccumulation occurs by absorbing contaminants which are transferred onto and within the cellular surface. Both bioaccumulation and biosorption occur naturally in all living organisms however, in a controlled experiment conducted on living and dead strains of bacillus sphaericusBacillus sphaericusBacillus sphaericus is an obligate aerobe bacterium used as a larvicide for mosquito control. It forms spherical endospores.Bacillus sphericum is a gram positive bacteria, with rod shaped cells that form chains....
it was found that the biosorption of chromium ions was 13 - 20% higher in dead cells than living cells .
- Bioaccumulation occurs by absorbing contaminants which are transferred onto and within the cellular surface. Both bioaccumulation and biosorption occur naturally in all living organisms however, in a controlled experiment conducted on living and dead strains of bacillus sphaericus
- In terms of environmental remediation, biosorption is preferable to bioaccumulation because it occurs at a faster rate and can produce higher concentrations . Since metals are bound onto the cellular surface, biosorption is a reversible process where as bioaccumulation is only partially reversible .
Factors Effecting Performance
- Since biosorption is determined by equilibrium, it is largely influenced by pHPHIn chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
, the concentration of biomass and the interaction between different metallic ions . In addition, no two sorbents are the same and thus individual specimens perform differently under different conditions.
- For example, in a study on the removal of pentachlorophenolPentachlorophenolPentachlorophenol is an organochlorine compound used as a pesticide and a disinfectant. First produced in the 1930s, it is marketed under many trade names...
(PCP) using different strains of fungal biomass, as the pH changed from low pH to high pH (acidic to basic) the amount of removal decreased by the majority of the strains, however one strain was unaffected by the change . In another study on the removal of copper, zinc and nickel ions using a composite sorbent as the pH increased from low to high the sorbent favored the removal of copper ions over the zinc and nickel ions . Because of the variability in sorbent this might be a drawback to biosorption, however, more research will be necessary.
Common Uses
- Even though the term biosorption may be relatively new, it has been put to use in many applications for a long time. One very widely known use of biosorption is seen in activated carbonActivated carbonActivated carbon, also called activated charcoal, activated coal or carbo activatus, is a form of carbon that has been processed to make it extremely porous and thus to have a very large surface area available for adsorption or chemical reactions.The word activated in the name is sometimes replaced...
filters. They can filter air and water by allowing contaminants to bind to their incredibly porous and high surface area structure. The structure of the activated carbon is generated as the result of charcoal being treated with oxygen . Another type of carbon, sequestered carbon, can be used as a filtration media. It is made by carbon sequestration, which uses the opposite technique as for creating activated carbon. It is made by heating biomass in the absence of oxygen. The two filters allow for biosorption of different types of contaminants due to their chemical compositions -- one with infused oxygen and the other without.
Biosorption in Industry
- Many industrial effluents contain toxic metals that must be removed. Removal can be accomplished with biosorption techniques. It is an alternative to using man-made ion-exchange resins, which cost ten times more than biosorbents. The cost is so much less, because the biosorbents used are often waste from farms or they are very easy to regenerate, as is the case with seaweed and other unharvested biomass.
- Industrious biosorption is often done by using sorption columns as seen in Figure 1. Effluent containing heavy metal ions is fed into a column from the top. The biosorbents adsorb the contaminants and let the ion-free effluent to exit the column at the bottom. The process can be reversed to collect a highly concentrated solution of metal contaminants. The biosorbents can then be re-used or discarded and replaced.
Citations
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