Aluminium mobilization in soils and waters

Abstract

The toxicity of dissolved aluminium to many plant and animal species is one of the major deleterious effects of the acidification of the environment. AI mobilization from insoluble forms in minerals and rocks is governed mainly by the pH of the solution surrounding weathering minerals. To a first approximation, the solubility product of gibbsite gives an indication of AI concentrations expected in solution. Secondary silicate minerals such as kaolinite, imogolite and allophane, and sulphate minerals such as alunite, maintain lower AI concentrations in solution than does gibbsite, particularly at pH < 5 . The cation exchange system of acid soils provides a large reserve of ionic AI, which can be brought into solution when soluble salts percolate through soils. Ligands such as fluoride and organic anions, which form soluble AI complexes, combine with much of the AI in solution when they are present in suitable concentrations, and therefore maintain higher concentrations of AI than might be expected from mineral solubility equilibria, particularly at pH 5-7.Slow reactions involving solid phases, and uncertain data for possible AI species present, limit the rigorous application of equilibrium thermodynamics to these systems, but the factors mentioned provide a reasonable understanding of the major features of aluminium mobilization.

The distribution of solute processes on an acid hillslope and the delivery of solutes to a stream: II. Exchangeable Al3+

“…The exchangeable Al 3 in acid soils provides a large reserve of adsorbed ionic Al 3 that can be rapidly mobilized into solution, whilst the dissolution of Al oxides from any primary aluminosilicate minerals may also play a major role in the buffering of mineral soil solution acidity (Bache, 1986;McBridge, 1994).…”

“…If it is assumed that precipitation of Al oxides occurs first, the illuviated Al oxides from upslope may be easily dissolved by the strong acidity and convergent throughflow in this slope position. Because amorphous Al oxides are those most easily dissolved Al 3 reservoirs in soils (Bache, 1986), illuviated Al oxides may be acting as a pool for the continuous release of Al 3 to the stream. On the other hand, the direct removal of Al oxides to the stream is also possible.…”

“…At this pH range (pH`4Á5), aluminosilicate clays and Al oxides begin to dissolve, releasing Al-hydroxy cations and Al 3 , replacing base cations from soil colloids (Bache, 1986). It may be concluded that exchangeable Al 3 is mainly generated from Al oxides under the high soil acidity of the study slope.…”

“…The soil acidification and leaching processes of Al 3 are largely governed by cation exchange reactions in the soil (Bache, 1986;Robarge and Johnson, 1992). In a correlation matrix of cationic composition of each cation to ECEC (Table III) AS is lowest in topsoil and rapidly increases to 30 cm depth ( Figure 6); however, there is little difference in the level of AS below this depth.…”

“…The increase of dissolved aluminium (Al) in soil solution and surface water is one of the most important applied issues for hydrochemical study at the catchment scale (Ball and Trudgill, 1995). Al in soils is mobilized from upper soil horizons (A and E horizons) as either complexed forms with organic materials (Alchelates) or soluble inorganic Al±Si±Fe sols (proto-imogolite and allophane).…”