Leaching is the process of a
solute
In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. If the attractive forces between the solvent ...
becoming detached or extracted from its carrier substance by way of a
solvent
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
.
Leaching is a naturally occurring process which scientists have adapted for a variety of applications with a variety of methods. Specific extraction methods depend on the soluble characteristics relative to the
sorbent
A sorbent is a material used to absorb or adsorb liquids or gases. Examples include:
*A material similar to molecular sieve material, which acts by adsorption (attracting molecules to its surface). It has a large internal surface area and good t ...
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 can also be applied affectedly to enhance
water quality
Water quality refers to the chemical, physical, and biological characteristics of water based on the standards of its usage. It is most frequently used by reference to a set of standards against which compliance, generally achieved through tr ...
and contaminant removal,
as well as for disposal of
hazardous waste
Hazardous waste is waste that has substantial or potential threats to public health or the environment. Hazardous waste is a type of dangerous goods. They usually have one or more of the following hazardous traits: ignitability, reactivity, co ...
products such as
fly ash
Fly ash, flue ash, coal ash, or pulverised fuel ash (in the UK) plurale tantum: coal combustion residuals (CCRs)is a coal combustion product that is composed of the particulates (fine particles of burned fuel) that are driven out of coal-fired ...
,
or
rare earth elements
The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silve ...
(REEs).
Understanding leaching characteristics is important in preventing or encouraging the leaching process and preparing 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 described 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 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 desired. The physiochemical and biological properties of the carrier and solute should be considered when observing the leaching process, 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
Particle size is a notion introduced for comparing dimensions of solid particles ('' flecks''), liquid particles ('' droplets''), or gaseous particles ('' bubbles''). The notion of particle size applies to particles in colloids, in ecology, in ...
*
Solvent
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
*
Temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
* Agitation
*
Surface area
The surface area of a solid object is a measure of the total area that the surface of the object occupies. The mathematical definition of surface area in the presence of curved surfaces is considerably more involved than the definition of arc ...
*
Homogeneity
Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...
of the carrier and solute
*
Microorganism
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
activity
*
Mineralogy
Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical) properties of minerals and mineralized artifacts. Specific studies within mineralogy include the proces ...
* Intermediate products
*
Crystal structure
In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystal, crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric pat ...
The general process is typically broken up and summarized into three parts:
# Dissolution of surficial solute by solvent
#
Diffusion
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
of inner-solute through the pores of the carrier to reach the solvent
# Transfer of dissolved solute out of the system
Leaching processes for biological substances
Biological
Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary in ...
substances can experience leaching themselves,
as well as be used for leaching as part of the solvent substance to recover
heavy metals
upright=1.2, Crystals of osmium, a heavy metal nearly twice as dense as lead">lead.html" ;"title="osmium, a heavy metal nearly twice as dense as lead">osmium, a heavy metal nearly twice as dense as lead
Heavy metals are generally defined as ...
.
Many plants experience leaching of phenolics,
carbohydrate
In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or ma ...
s, and
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
s, and can experience as much as 30% mass loss from leaching,
just from sources of water such as
rain
Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...
,
dew
Dew is water in the form of droplets that appears on thin, exposed objects in the morning or evening due to condensation.
As the exposed surface cools by radiating its heat, atmospheric moisture condenses at a rate greater than that at wh ...
,
mist
Mist is a phenomenon caused by small droplets of water suspended in the cold air, usually by condensation. Physically, it is an example of a dispersion. It is most commonly seen where water vapor in warm, moist air meets sudden cooling, such a ...
, and
fog
Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth's surface. Reprint from Fog can be considered a type of low-lying cloud usually resembling stratus, and is heavily influ ...
.
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
Added sugars or free sugars are sugar carbohydrates (caloric sweeteners) added to food and beverages at some point before their consumption. These include added carbohydrates ( monosaccharides and disaccharides), and more broadly, sugars natu ...
,
pectic
Pectin ( grc, πηκτικός ': "congealed" and "curdled") is a heteropolysaccharide, a structural acid contained in the primary lamella, in the middle lamella, and in the cell walls of terrestrial plants. The principal, chemical component o ...
substances, and
sugar alcohol
Sugar alcohols (also called polyhydric alcohols, polyalcohols, alditols or glycitols) are organic compounds, typically derived from sugars, containing one hydroxyl group (–OH) attached to each carbon atom. They are white, water-soluble solids ...
s.
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
pesticide
Pesticides are substances that are meant to control pests. This includes herbicide, insecticide, nematicide, molluscicide, piscicide, avicide, rodenticide, bactericide, insect repellent, animal repellent, microbicide, fungicide, and lampri ...
s 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
Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...
to human and animal health.
Bioleaching
Bioleaching is the extraction of metals from their ores through the use of living organisms. This is much cleaner than the traditional heap leaching using cyanide. Bioleaching is one of several applications within biohydrometallurgy and several ...
is a term that describes the removal of metal
cations
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
from insoluble
ore
Ore is natural rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Retrieved 7 Apr ...
s by biological
oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
and
complexation
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
processes.
This process is done in most part to extract
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
,
cobalt
Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, pr ...
,
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...
,
zinc
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
, and
uranium
Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...
from insoluble
sulfide
Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds lar ...
s or
oxide
An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...
s.
Bioleaching processes can also be used in the re-use of
fly ash
Fly ash, flue ash, coal ash, or pulverised fuel ash (in the UK) plurale tantum: coal combustion residuals (CCRs)is a coal combustion product that is composed of the particulates (fine particles of burned fuel) that are driven out of coal-fired ...
by recovering
aluminum
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has ...
using
sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...
.
Leaching processes for fly ash
Coal fly ash is a product that experiences heavy amounts of leaching during disposal.
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,
lagoon
A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') a ...
s,
landfill
A landfill site, also known as a tip, dump, rubbish dump, garbage dump, or dumping ground, is a site for the disposal of waste materials. Landfill is the oldest and most common form of waste disposal, although the systematic burial of the waste ...
s, 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
Kingston Fossil Plant, commonly known as Kingston Steam Plant, is a 1.4-gigawatt (1,398 megawatt, MW) coal-fired power plant located in Roane County, Tennessee, Roane County, just outside Kingston, Tennessee on the shore of Watts Bar Lake. It is o ...
in
Roane County, Tennessee.
The
Tennessee Valley Authority
The Tennessee Valley Authority (TVA) is a federally owned electric utility corporation in the United States. TVA's service area covers all of Tennessee, portions of Alabama, Mississippi, and Kentucky, and small areas of Georgia, North Carolina ...
Kingston Fossil Plant structural failure lead to massive destruction throughout the area and serious levels of
contamination
Contamination is the presence of a constituent, impurity, or some other undesirable element that spoils, corrupts, infects, makes unfit, or makes inferior a material, physical body, natural environment, workplace, etc.
Types of contamination
W ...
downstream to both
Emory River
The Emory River is a river draining a portion of Tennessee's Cumberland Plateau. It flows for just over from its source near Frozen Head State Park to its mouth along the Clinch River at Kingston, Tennessee.
Hydrography
The Emory River rises on ...
and
Clinch River
The Clinch River is a river that flows southwest for more than through the Great Appalachian Valley in the U.S. states of Virginia and Tennessee, gathering various tributaries, including the Powell River, before joining the Tennessee River in Ki ...
.
Leaching processes in soil
Leaching in
soil
Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
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
Darcy's law is an equation that describes the flow of a fluid through a porous medium. The law was formulated by Henry Darcy based on results of experiments on the flow of water through beds of sand, forming the basis of hydrogeology, a branch of e ...
,
mass flow
Mass flow, also known as mass transfer and bulk flow, is the movement of fluids down a pressure or temperature gradient,Moyes & Schulte (2008). Principles of Animal Physiology. Pearson Benjamin Cummings. San Francisco, California particularly in ...
expressions, and
diffusion
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
-dispersion understandings.
Leaching is controlled largely by the
hydraulic conductivity
Hydraulic conductivity, symbolically represented as (unit: m/s), is a property of porous materials, soils and rocks, that describes the ease with which a fluid (usually water) can move through the pore space, or fractures network. It depends on th ...
of the soil, which is dependent on
particle size
Particle size is a notion introduced for comparing dimensions of solid particles ('' flecks''), liquid particles ('' droplets''), or gaseous particles ('' bubbles''). The notion of particle size applies to particles in colloids, in ecology, in ...
and
relative density
Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest ...
that the soil has been consolidated to via stress.
Diffusion is controlled by other factors such as pore size and soil skeleton,
tortuosity
Tortuosity is widely used as a critical parameter to predict transport properties of porous media, such as rocks and soils. But unlike other standard microstructural properties, the concept of tortuosity is vague with multiple definitions and vari ...
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
Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical) properties of minerals and mineralized artifacts. Specific studies within mineralogy include the proces ...
as a group or individually, or the solvent that causes leaching.
Most tests are done by evaluating mass loss due to a
reagent
In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...
, heat, or simply washing with water.
A summary of various leaching processes and their respective laboratory tests can be viewed in the following table:
Environmentally friendly leaching
Some recent work has been done to see if organic acids can be used to leach
lithium
Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense solid el ...
and
cobalt
Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, pr ...
from spent
batteries with some success. Experiments performed with varying temperatures and concentrations of
malic acid
Malic acid is an organic compound with the molecular formula . It is a dicarboxylic acid that is made by all living organisms, contributes to the sour taste of fruits, and is used as a food additive. Malic acid has two stereoisomeric forms ...
show that the optimal conditions are 2.0 m/L of organic acid at a temperature of 90 °C. The reaction had an overall efficiency exceeding 90% with no harmful byproducts.
:4 LiCoO
2(solid) + 12 C
4H
6O
5(liquid) → 4 LiC
4H
5O
5(liquid) + 4 Co(C
4H
6O
5)
2(liquid) + 6 H
2O(liquid) + O
2(gas)
The same analysis with
citric acid
Citric acid is an organic compound with the chemical formula HOC(CO2H)(CH2CO2H)2. It is a colorless weak organic acid. It occurs naturally in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in t ...
showed similar results with an optimal temperature and concentration of 90 °C and 1.5 molar solution of citric acid.
See also
*
Extraction
*
Leachate
A leachate is any liquid that, in the course of passing through matter, extracts soluble or suspended solids, or any other component of the material through which it has passed.
Leachate is a widely used term in the environmental sciences wher ...
*
Parboiling
Parboiling (or leaching) is the partial or semi boiling of food as the first step in cooking. The word is from the Old French 'parboillir' (to boil thoroughly) but by mistaken association with 'part' it has acquired its current meaning.
The wo ...
*
Surfactant leaching
*
Sorption
Sorption is a physical and chemical process by which one substance becomes attached to another. Specific cases of sorption are treated in the following articles:
; Absorption: "the incorporation of a substance in one state into another of a dif ...
*
Weathering
Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement), ...
References
{{Authority control
Industrial processes
Solid-solid separation