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In a solution it is typical to observe how much of a substance can attach to another; leaching however, is the process of a solute becoming detached or extracted from the other substance by way of a solvent.

Leaching is a naturally occurring process which has been adapted to be used for a variety of applications with a variety of methods. Specific extraction methods are dependent on the soluble characteristics relative to the sorbent material such as concentration, distribution, nature, and size. Leaching can occur naturally seen from plant substances (inorganic and organic), solute leaching in soil, and in the decomposition of organic materials. Leaching, as previously mentioned, can also be applied affectedly to enhance water quality and contaminant removal, as well as for disposal of hazardous waste products such as fly ash, or rare earth elements (REEs). It is important to understand leaching characteristics in order to prevent or encourage the leaching process and in order to be prepared for it in the case where it is inevitable.

In an ideal leaching equilibrium stage, all the solute is dissolved by the solvent, leaving the carrier of the solute unchanged. The process of leaching however is not always ideal, and can be quite complex to understand and replicate, and often different methodologies will produce different results.

Leaching Processes
There are many types of leaching scenarios, therefore the extent of this topic is vast. In general however, the three substances can be looked at as a carrier, substance A, a solute, substance B, and a solvent, substance C. Substance A and B are somewhat homogenous in a system prior to the introduction of substance C. At the beginning of the leaching process, substance C will work at dissolving the surficial substance B at a fairly high rate. The rate of dissolution however will decrease substantially once it needs to penetrate through the pores of substance A in order to continue targeting substance B. This penetration can often lead to dissolution of substance A, or the product of more than one solute, both unsatisfactory if specific leaching is desirable. Things to be considered when observing the leaching process are the physiochemical and biological properties of the carrier and solute, and certain properties may be more important depending on the material, the solvent, and their availability. These specific properties can include, but are not limited to:


 * Particle size
 * Solvent
 * Temperature
 * Agitation
 * Surface area
 * Homogeneity of the carrier and solute
 * Microorganism activity
 * Mineralogy
 * Intermediate products
 * Crystal structure

The general process is typically broken up and summarized into three parts:


 * 1) Dissolution of surficial solute by solvent
 * 2) Diffusion of inner-solute through the pores of the carrier to reach the solvent
 * 3) Transfer of dissolved solute out of the system

Leaching Processes for Biological Substances
Biological substances can experience leaching themselves, as well as be used for leaching as part of the solvent substance to recover heavy metals. Many plants experience leaching of phenolics, carbohydrates, and amino acids, and can experience as much as 30% mass loss from leaching, just from sources of water such as rain, dew, mist, and fog. These sources of water would be considered the solvent in the leaching process and can also lead to the leaching of organic nutrients from plants such as free sugars, pectic substances, and sugar alcohols. This can in turn lead to more diversity in plant species that may experience a more direct access to water. This type of leaching can often lead to the removal of an undesirable component from the solid by water, this process is called washing. A major concern for leaching of plants, is if pesticides are leached and carried through stormwater runoff, ; this is not only necessary to plant health, but it is important to control because pesticides can be toxic to human and animal health.

Bioleaching is a term that describes the removal of metal cations from insoluble ores by biological oxidation and complexation processes. This process is done in most part to extract copper, cobalt, nickel, zinc, and uranium from insoluble sulfides or oxides. Bioleaching processes can also be used in the re-use of fly ash by recovering aluminum using sulfuric acid.

Leaching Processes for Fly Ash
Coal fly ash is a product that experiences heavy amounts of leaching when it is disposed of. Though the re-use of fly ash in other materials such as concrete and bricks is encouraged, still much of it in the United States is disposed of in holding ponds, lagoons, landfills, and slag heaps. These disposal sites all contain water where washing effects can cause leaching of many different major elements, depending on the type of fly ash and the location where it originated. The leaching of fly ash is only concerning if the fly ash has not been disposed of properly, such as in the case of the Kingston Fossil Plant in Roane County, Tennessee. The Tennessee Valley Authority Kingston Fossil Plant structural failure lead to massive destruction throughout the area and serious levels of contamination downstream to both Emory River and Clinch River.

Leaching Processes in Soil
Leaching in soil is highly dependent on the characteristics of the soil, which makes modeling efforts difficult. Most leaching comes from infiltration of water, a washing effect much like that described for the leaching process of biological substances. The leaching is typically described by solute transport models, such as Darcy's Law, mass flow expressions, and diffusion-dispersion understandings. Leaching is controlled largely by the hydraulic conductivity of the soil, which is dependent on particle size and relative density that the soil has been consolidated to via stress. Diffusion is controlled by other factors such as pore size and soil skeleton, tortuosity of flow path, and distribution of the solvent (water) and solutes.

Leaching Mechanisms
Due to the assortment of leaching processes there are many variations in the data to be collected through laboratory methods and modeling, making it hard to interpret the data itself. Not only is the specified leaching process important, but also the focus of the experimentation itself. For instance, the focus could be directed toward mechanisms causing leaching, mineralogy as a group or individually, or the solvent that causes leaching. Most tests are done by evaluating mass loss due to a reagent, heat, or simply washing with water. A summary of various leaching processes and their respective laboratory tests can be viewed in the following table: 3. References Wikipedia Article Sources -- Bibliography

1-Bärlocher, Felix (2005). Chapter 5: Leaching. In: Graça M.A., Bärlocher F., Gessner M.O. (eds). Methods to Study Litter Decomposition. Dordrecht, Netherlands: Springer. ISBN #978-1-4020-3348-3. Retrieved from: https://doi.org/10.1007/1-4020-3466-0_5

2-Richardson, J.F., Harker, J.H., Backhurst, J.R. (2002). Chapter 10: Leaching. In: Chemical Engineering (Fifth Edition) Volume 2: Particle Technology and Separation Processes. Amsterdam, Netherlands: Elsevier. ISBN #978-0-7506-4445-7. Retrieved from: https://doi.org/10.1016/B978-0-08-049064-9.50021-7

3-Dubus, I.G., Beulke, S., Brown, C.D. (2002). “Calibration of Pesticide Leaching Models: Critical Review and Guidance for Reporting”. Pest Management Science. Volume 58(8): 745-758. https://doi.org/10.1002/ps.526

4-Prosser, Alan P. (1994). “Review of Uncertainty in the Collection and Interpretation of Leaching Data”. Hydrometallurgy. Volume 41(2-3): 119-153. https://doi.org/10.1016/0304-386X(95)00071-N

5-Peelman, S., Sun, Z.H.I., Sietsma, J., Yang, Y. (2015). Chapter 21: Leaching of Rare Earth Elements: Review of Past and Present Technologies. In: Rare Earths Industry. Amsterdam, Netherlands: Elsevier. ISBN #978-0-12-802328-0. Retrieved from: https://doi.org/10.1016/B978-0-12-802328-0.00021-8

6-Perket, C. & Webster, W.C. (1981). “Literature Review of Batch Laboratory Leaching and Extraction Procedures”. Hazardous Solid Waste Testing: First Conference, edited by Conway, R. & Malloy, B. ASTM International: 7-27. https://doi.org/10.1520/STP28826S

7-Tukey Jr., H.B. (1970). “The Leaching of Substances from Plants”. Annual Review of Plant Physiology. Volume 21: 305-324. https://doi.org/10.1146/annurev.pp.21.060170.001513

8-Lyer, R. (2002). “The Surface Chemistry of Leaching Coal Fly Ash”. Journal of Hazardous Materials. Volume 93(3): 321-329. https://doi.org/10.1016/S0304-3894(02)00049-3

9-Rohwerder, T., Gehrke, T., Kinzler, K., Sand, W. (2003). “Bioleaching Review: Part A”. Applied Microbiology and Biotechnology. Volume 63(3): 239-248. https://doi.org/10.1007/s00253-003-1448-7

10-Addiscott, T.M., Wagenet, R.J. (1985). “Concepts of Solute Leaching in Soils: A Review of Modeling Approaches”. European Journal of Soil Science. Volume 36(3): 411-424. https://doi.org/10.1111/j.1365-2389.1985.tb00347.x

Article Evaluation -- Leaching Chemistry

1. Evaluating Content: Is everything in the article relevant to the article topic? Is there anything that distracted you? Is any information out of date? Is anything missing that could be added? What else could be improved? Is scientific information presented clearly, accurately, and without jargon? Does the article link to other Wikipedia articles for related topics?

''The article is not very fluid. In that I mean there does not seem to be good transitions between paragraphs and even some sentences seem to be randomly placed with little relation to the surrounding sentences. The introductory/overview section at points goes into detail that could definitely be summarized for that section, and then explained possibly in an "examples" section, or something similar. There is also quite a bit of repetition, for example the following passage:''

"...the solid is brought into contact with a liquid. The solid and liquid are in contact..."

''I think most of the jargon could be edited to become more clear and concise so that the topic can be understood accurately. Some of the linked Wikipedia articles are just words used throughout this article (i.e. "water", "batteries", etc.), which don't necessarily relate to the topic of the article. There is also a WikiLink for equilibrium, and it takes you to a nearly empty Wikipedia article, therefore it probably isn't very helpful. There are however, good WikiLinks also included in this article that are helpful, but it seems that in addition to the beneficial ones there are quite a few that are unnecessary or misleading. Further research may need to be done on the topic to get substantial information from it (such as accessing other articles). The equation provided looks very nice and has a link attached to it, however there is no thorough description of the equation and it provided no definitions to variables, making it difficult to use/understand. The article also lacked the use of images or figures, a crucial piece that seems to really set certain articles above others.''

2. Evaluating tone: Is the article neutral? Are there any claims that appear heavily biased toward a particular position? Are there viewpoints that are over-represented, or under-represented?

''The article is neutral in the sense that it is not bias. The contents in the article however, are not very even, and some need some more work. For example leaching processes for biological substances has the most written for it, and seems accurate. For each remaining section there is very little written, making it seem as though the person who primarily wrote this article new the most about biological leaching.''

3. Evaluating sources: Check a few citations. Do the links work? Does the source support the claims in the article? Is each fact referenced with an appropriate, reliable reference? Where does the information come from? Are these neutral sources? If biased, is that bias noted?

''The sources for this article all seem to be fairly good, coming from established journals and relating to the topic. The first reference however has a tag, even though it seems to be a reliable source. References two and three also don't have any links, because they are books, but in looking up the books they appear to be reliable sources.''

4. Evaluating talk page: Now take a look at how others are talking about this article on the talk page. What kinds of conversations, if any, are going on behind the scenes about how to represent this topic? How is the article rated? Is it a part of an WikiProjects?

''The talk page on this article is very slim, and no talk has occurred in over 10 years (April, 2009). It seems that there may have been some controversy on similar information being in more than one article (leaching and leachate), so talk of merging the articles occurred; it does not appear to have actually happened though. The article is rated as "Start-Class" on the project's quality scale and "Mid-importance" on the project's importance scale. The WikiProject it belongs to is chemistry, but is considered inactive.''

5. Adding to the talk page: Choose at least one of the four evaluation comments you wrote in your sandbox and leave at least one paragraph of evaluation on the article's Talk page. Be sure to sign your feedback with four tildes ~.