Trace Metal Cycling Flashcards
What are trace elements?
Trace elements are those naturally present at very low concentrations but despite this, they can be important in, for example, many biochemical processes
Give examples of trace elements important for human and animal health
Zinc, Iodine and Selenium
Give examples of trace elements important for biochemical processes in plants
Copper
What happens when trace elements are present in high concentrations?
Toxic
Why are trace elements important within an industrialised society?
Rare earth elements are important components of computers and mobile phones e.g yttrium, lanthanum
Give examples of how industrial cycles are built on trace elements
Nickel - alloys Cu- fungicides Cd- rechargeable batteries As - insecticides, electronics ( Arsenic) Hg ( Mercury) dental uses
How do Human activities influence the global cycle of trace elements? Give an example of the human activity and give an example for one of the 2 ways we influence their cycle
Human activities such as mining trace elements disturb their natural cycling
1) Altering the rate at which metals are transferred between reservoirs
2) Change the chemical form of the metal. For example, many trace element ores form sulphide or oxide minerals e.g galena, cassiterite ( tin oxide). Smelting the ore minerals converts the metals to elemental forms
How are trace elements distributed? What classification helps explain this? Describe this classification
According to their chemical properties. The Goldschmidt classification is a geochemical classification which groups the chemical elements within the Earth according to their preferred host phases: lithophile (O or Si loving ), siderophile (iron-loving), chalcophile (Sulfur loving), and atmophile ( Atmosphere loving) or volatile (the element, or a compound in which it occurs, is liquid or gaseous at ambient surface conditions).
Describe the abundance of trace elements in the earths crust. What does there abundance depend on?
Trace element abundance is low in the Earth’s crust and their availability depends upon the relative abundance of that element and the stability of the elements in which they occur ( stability of the minerals of which it is a major component)
Cu Hg Sn scarce, but easily isolated from mineral deposits
CuS2 HgS2 SnO2
What is mobilisation in this context?
Making an element available for transport
How are trace metals mobilised naturally?
By weathering processes which may be chemical, physical or biological.
Chemical weathering : ZnS + 2O2 = Zn2+ + SO42-. Rainwater reacts with rocks
Acid rain.
Physical weathering: frost fracturing ( Water gets into cracks and joints in bedrock. When the water freezes it expands and the cracks are opened a little wider. Over time pieces of rock can split off a rock face and big boulders are broken into smaller rocks and grave)
Biological weathering: plants can grow into small spaces and gaps in rock. When these roots grow, they exert pressure on the rock around them, causing the gaps to widen or even crack
when plants break up rocks with their growing roots or plant acids help dissolve
Some are also released in volcanic gases because they are so volatile, e.g Hg
Anthropogenic also : mining increases the mobility of trace elements as it increases exposure of the minerals to the environment. Mineral processing such as smelting will also change trace element mobility as the form of the element is changed
What are the main processes controlling the behaviour Of trace elements ( I.e metals)?
Complexation Redox changes Volatilisation/ anthropogenic activities Precipitation Adsorption
Describe how complexation controls the behaviour of trace metals?
If the element forms an ion
It may be complexed and mobilized in waters. An example is the divalent cooper ion ( Cu2+) which is solvated by six water molecules. The slightly negatively charged oxygen atoms of the water molecule will orientate themselves towards the positive charge on the copper ion. Other ions e.g chloride can replace some of the water molecules to form a complex.
How do redox changes control the behaviour of some trace elements?
Changes in oxidation state will also control the behaviour of some trace elements e.g iron, arsenic and chromium. Iron as Fe2+ is soluble and can be transported in solution, iron as the more oxidised Fe3+ is not soluble.
Background: The redox state of groundwater—whether the groundwater is oxic (oxidized) or anoxic (reduced)—has profound implications for groundwater quality. Knowing the redox conditions of groundwater can help determine whether it contains elevated levels of many contaminants, including arsenic, nitrate, and even some manmade contaminants. The redox conditions of groundwater strongly affect the mobility and persistence of many contaminants in groundwater. Redox conditions determine whether some chemical constituents, like arsenic and manganese, are released from the aquifer rocks and sediments into the groundwater
Groundwater is the water found underground in the cracks and spaces in soil, sand and rock. It is stored in and moves slowly through geologic formations of soil, sand and rocks called aquifers.
Arsenic can be present in water as As3+ ( arsenite) As5+ ( arsenate) or organic- As forms. Arsenite and organic- As species are more mobile than arsenate in ground waters that are reducing and therefore tend to be present in higher concentrations.
Chromium has 2 main oxidation states; Cr3+, which is an important trace element in human diets and Cr6+ ( hexavalent chromium) which is carcinogenic. The problems of elevated hexavalent chromium concentrations in drinking water were highlighted by Erin Brockovitch who sued the Pacific Gas and Electrivity Company for contaminating water supplies in Hinckley, California.
How are trace elements affected by volatilisation/ anthropogenic activities?
A few trace elements are volatile and their distribution can be affected by anthropogenic processes including smelting and release from power station chimneys as a result of burning of fossil fuels. They can also be distributed by natural processes including volcanic gases and geothermal activity
Most volatile trace metals: Hg, As, Cd,Pb, Zn
High temperatures ( smelters, power plants)
