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Aluminium is found in atmosphere and the atmospheric input fluxes and solubility are consistent with the seasonality of surface water concentration Data and this yield an average residence time in the upper 100m of 6.5yrs. Atmospheric deposition can either be in two ways – either as Dust or rainfall to the open ocean. Lithospheric cycle of aluminium can be in two forms primary and secondary mineral.Biospheric cycle of aluminium although is small but can be found in micro-organisms, plant, invertebrates, vertebrates and humans. All of the cycles overlap because there is a continuous flow of aluminium in the cycle. “Aluminium is characterised by its relatively short (2-6 yr) residence time in surface seawater” Exley (2003). This short residence time can largely be attributed to the element’s rapid hydrolysis rate and the extremely low solubility of the hydrolysis products Exley (2003).
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Aluminium is the most abundant metallic elements in the earth crust (8.1wt) and it is very reactive hence it is not found in nature. Aluminium is also presence in many silicate materials which include feldspars, Mica and amphiboles. The atmosphere plays an important role in material transported from land to sea, with dust deposition forming the major geochemical pathway for the delivery of a number of trace elements Bowie et al (2001). The distribution of aluminium in sea water can be explained by both fluvial inputs and Aeolian crust to the open ocean. The surface concentration of aluminium are predominately influenced by Aeolian dust deposition (Maring and Duce, (1987) However in regions were the dust is not significant, the concentration of aluminium are know to be below the 1Nm in surface water. The aluminium cycle is mostly dominated by it biotic cycles due to the relative abundance in humans. The depositions of aluminium in the atmosphere either wet or dry estimate the presence of aluminium in sea water Exley (2003).
Construction of the cycle
The retention of aluminium in the lithospheric cycle actively favours the concentration of dissolved aluminium that happens in the oceanic cycle fig1. The reason these were used for the modelling is that it actually measured the lithospheric and biogenic cycle of aluminium but does not detail the oceanic cycle of aluminium in a broad sense while on the other hand, Bowie et al (2007) explained the biogeochemical cycle of aluminium in the open waters and how sediments are dissolved from dust and rainfall provided by atmospheric deposition. The idea behind this was to determine the movement of aluminium between the atmosphere, lithosphere and the biosphere. The table was taken from Exley (2003) and Bowie et al (2007). The abundant of elements in the lithosphere and biosphere is expressed in ppm.
The lithospheric cycle
The retention of aluminium in the lithosphere is actively favoured as the concentration of dissolved aluminium is limited by the mineral phase of decreasing solubility and hence lower free energy Exley (2003). Primary aluminium rich minerals such as feldspar are formed as a result of cooling of magma in the Earth’s crust and are dissolved from parent rocks by weathering. The additional dissolution of carbonic acid weathering results in the formation of clay like materials such as kaolinite and other secondary mineral phases Exley (2003). High insoluble particulates absorbed by both the minerals and the organic surfaces are returned to the Earth’s crust through sedimentation and subduction into the magma. This is the continue cycle as illustrated on fig 1
The Biotic cycle
This cycle represent the life forms of micro-organism, plant, invertebrates and humans coming together to form a proportion of the total biotic abundance of aluminium in the cycle represented in fig1. All of these cycle overlap with each other to show that aluminium is present in all food chain Exley (2003). The biospheric abundance of aluminium is an estimate and does not reflect the total amount of aluminium in the cycle Exley (2003).
Aluminium in the atmosphere
The atmosphere plays an important role in material transports of aluminium from land to sea. With dust deposition forming the major geochemical path way for the delivery of aluminium to the remote open waters (fig 1) aluminium is chiefly associated with mineral aerosol. With the atomic weight of 8.1 and particle size class of >1um aluminium has been used as a valuable tracer of Aeolian inputs into the ocean. Between 1.5% and 10% of Al associated with mineral aerosol is predicted to be dissolved in open waters (Maring and Duce, 1987).
Aluminium in the lithosphere
This is formed from both primary and secondary minerals and colloidal phase which is taken back to the atmosphere through sedimentary processes Fig1. The earth curst has a thickness of 35 to 40 km at the continent. The retentions of aluminium is extremely with 99.999% of cycled aluminium that is left in the lithosphere (REF) Dissolution and precipitation plays an important role in cycling this element and the dissolution in the aqueous phase. The activity of AL+3 in soil and groundwater is controlled by the precipitation and dissolution reactions and this ca be estimated from the PH and the activities’ of F and S04-2. Ion exchange plays an important role in retention mechanism for aluminium in acid to neutral Ph regimes. The Aluminium occurs in many silicate rock minerals, such as feldspar, feldspathoids, micas and many amphiboles.
Aluminium in soil
In weathering process, delay weathering of primary minerals leads to the deposition of sedimentary clay minerals example is alumino -silicates kaolinte and mortmorillonites. When soil goes through weather, silicon is lost more rapidly than alunimuim. In sulfate rich environments, aluminium forms sulphate minerals such as alunite.
Much solid aluminium has been identified in soils and the most stable which include the hydroxide (gibbsite) and clay mineral. Aluminium level in soil solution is affected by acid precipitation due to the magnitude been higher than the level in a soil solution. The increase level of aluminium are caused by the lowered PH value of acid precipitation and also by the complexes formed by the inorganic and organic ligands. ” percolation of acid precipitation through the soil tends to dissolved the least stable soil minerals and raise the levels of aluminium significantly in the subsurface runoff, which ultimately finds it way into channel system of a watershed”( ref15).
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Figure 1 -Data taken from Exley (2003) (a); Bowie et al 2007 (b)
Aluminium in sediments
In general estuaries environment have been observed to trap aluminium elements which is usually present in greater concentrations in river water than in sea water. Sediments near the months of rivers contain amounts of aluminium (Ref 2). Clay is the most common sedimentary aluminium – bearing minerals, typically consist of alternating layers of silicon and aluminium.
The general absorption of aluminium by soil and soil clays which is affected by soil acidity determines the toxicity of the element in acid soils. Example is that the present of organic matter in soil lower the Ph of the soil solution below neutral , which makes aluminium more soluble.”Fulvic and humic acids act as strong agents for aluminium” (Ref 9)
Aluminium in Natural waters
The concentration of aluminium in natural waters depends primarily on pH. Inspite of the fact that aluminium is an abundant element, they rarely occur in natural waters in concentration greater than a few tenth of a milligram per litter as long as the Ph is nearly neutral.
The concentration of aluminium in sea water has been reported as 0.01mg/1, probably present as aluminium hydroxide (REF 7). The concentration of aluminium in river water can vary considerably with flow. It readily precipitates in natural water to form particulate or colloidal hydroxide; however under the turbulent conditions of high flow, it can be dissolved from suspended minerals (Ref 14).
Aluminium is readily soluble at pH <4. High concentration have been found in acid mine waters, acid geothermal waters and poorly buffered lakes, streams that receive large inputs of acid runoff. (Ref 13). In aqueous solutions, it does not occur as the free ion but is surrounded by sis molecules of water to form aluminium hydroxide.
The element’s extreme insolubility and biogeochemical reactivity results in very low dissolved AL concentration .Although this is due to the removal from the water column. Lithospheric abundance of aluminium is quiet an indication that it is close to the living organism at about 1mm. The biospheric abundance of an element is relative to its lithosphere which shows that geochemical origin with time and also gives more accurate reflections of the mechanisms which dominate the biogeochemical cycling of those elements (ref)
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