|
|
Aluminum Mobilization from the Forest Land
Xing Wang
Why Al is important
Aluminum is the most abundant metallic element in the earth's crust (accounting
for 8 percent in mass). It is an important constituent of many common
rocks. Since the mid 1960s there has been increasing concern about the
environmental behavior of Al because of its adverse effects on ecosystem
and human health.
High Al concentrations in soil solution can lead to toxicity to the plant root system and in turn, to reduced nutrient uptake by forest vegetation. Plants that grow poorly in very acid soils (pH<5) are generally affected by Al toxicity, which causes plant roots to become short, thick, and stubby. Most hardwoods (e.g., maples, oaks, and beeches) are more sensitive to acid soils than are pines and spruces. In natural water systems Al also causes acute toxicity to many aquatic organisms. For example, Aluminum appears to be toxic to fish at concentrations above the range 4-8 mmole/L (100 to 200 mg/L), which are often reported for acidic surface waters. In Europe and North America, Al mobilization in acidic waters is thought to be an important contributory factor to the decline of fisheries.
In addition, Aluminum has been implicated as a neurotoxic agent in a number of laboratory and epidemiological studies. Elevated Al concentrations have been related to impaired motor function and to a number of cognitive deficits in both humans and experimental animals. Some pathological conditions potentially associated with Al exposure in humans are dialysis encephalopathy, Parkinsonian dementia of Guam, Alzheimer’s disease and Osteomalacia.
In spite of its enormous quantity in soil Al seldom appears in aquatic systems, for most Al compounds are insoluble in the normal pH range occurring in natural waters. Only in certain conditions can it be released into water systems and exert the adverse effects described above. The term “Al mobilization” represents the processes by which Al is converted from insoluble forms into dissolved forms and carried into water systems.
Al mobilization: from soil to water
The importance of Al in nature is related to its capability to alter the
acid/base property of natural water system. In soil, Al occurs primarily
in chemical combination with oxygen or silicon, such as in aluminosilicate
minerals. Exchangeable Al3+ is usually bound closely with the cation exchange
complex because of its high strength of adsorption (much higher than soil
base cations: Ca2+, Mg2+, K+, Na+, etc).
With the increase of soil acidity most base cations in the soil solution are leached out in the order of their strength of adsorption to soil complex. At a pH as low as 5.0, H+ ions from the soil water, when adsorbed on the surfaces of soil mineral particles such as gibbsite, Al(OH)3, are exchanged for soluble Al3+ ions according to
Al(OH)3 + 3 H+ à Al3+ +3 H2O
Aluminum ion (Al3+) and its complex are able to combine with the hydroxyl ion (OH-) that is normally originated from the hydrolysis of water, leaving H+ behind in the soil solution. When released into the water, the Al3+ cations capture OH- ions to form hydroxyl – Al ions:
Al3+ + OH- ßà AlOH2+ + OH- ßà Al(OH)2+ + OH- ßà Al(OH)3
These reactions are reversible. Theoretically the three cationic species existing in equal concentrations at pH 5 if in equilibrium with gibbsite, but Al3+ is dominant at lower pH. In deeper, less acid soil horizons, the Al tends to hydrolyze to form colloidal Al(OH)3 and precipitate. The mobile Al cations may also capture F- and SO42- anions; in the latter case some of the Al may precipitate as insoluble basic Al sulphate ALOHSO4.
Besides these inorganic, labile, water-soluble species (Alim), Al also appears in organically complexed, non-labile form (Alom) which are much less toxic to fish and other aquatic biota. The complexation of Al by organic acids, such as humic and fulvic acids, is poorly understood but, in very acid soils (pH < 4.5) it appears that complexation is much reduced and the toxic inorganic ions, especially Al3+ and AlOH2+, predominate.
Concentrations of dissolved Al are generally low in natural waters due to the relatively low solubility of Al minerals (0.4 mmole/L). Dissolved mononuclear Al occurs usually in inorganic (labile) form, such as aquo Al and complexes with OH-, F-, SO42-, or in organic form (non-labile). Organic and F complexes have been observed as the predominant forms of Al in most stream waters. Inorganic forms of aqueous Al are significant when pH is lower than 5.5, whereas organic Al forms trend to prevail in surface water that are less acidic.
The Al in streamwater stems from Al mobilized in acidic soils by weak- or strong- acids. The concentration of inorganic forms of Al in stream increases with additions of H+ to soil solution whereas the concentration of organic forms of Al is strongly correlated with variations in organic carbon concentration of surface water. Mobilized Al can be transported to adjacent surface waters either in storm runoff or in subsurface flow.
As waters migrate over larger drainages areas, stream acidity is neutralized by the release of basic cations from thicker deposits of soil and glacial till, and/or retention of strong acid anions (e.g. SO42- and NO3-) from biological reduction process. Both mechanisms result in the immobilization of Al. Lakes are generally net sinks of Al.
Aluminum mobilization is highly correlated with the change of acid/base equilibrium of surface water. The sensitivity of surface waters to Al mobilization is often assessed by evaluating its acid neutralizing capacity (ANC). ANC is defined as: ANC = HCO3- + 2CO32- + OH- - H+. Chemical weathering of soil minerals is the primary source of ANC in most drainage waters. Generally a system with high value of ANC is capable of neutralizing more input of acid and resisting the sharp elevation of acidity and Al concentration.
Al mobilization: a process of soil development
Aluminum mobilization is a soil-forming process to some soil types. In
high elevation, northern temperate regions, the soils encountered are
genrally spodosols. The soils are low in calcium and magnesium and the
dominant exchangeable cation is Al. In these regions, the process of soil
development is thought to involve the mobilization of Al (and Fe) from
upper to lower mineral soil horizons by organic acids leached from foliage
as well as decomposition in the forest floor. Organic acids produced by
the decompositions of organic matter within the forest floor serve to
solubilize and mobilize Al from the E horizon. As alumino-organic solutes
are transported through the B-horizon a variety of processes cause the
retention of Al within the B-horizon. Further loss of Al can occur in
the deeper soil profile as increase in pH facilities the hydrolysis of
Al.
Accelerated Al mobilization
1. Acid rain
Leaching by strong acids can result in the transport of ecologically significant
concentrations of inorganic Al to surface waters. The input of acidic
atmospheric deposition facilitates the transport of Al from soil to surface
waters by leaching the Al-contained soil mineral. In many northeast regions
of the United States SO42- is the major acid input to many surface waters
and the dominant reason of surface water acidification. Large inputs of
SO42- can significantly reduce the ANC of surface waters. Strong acid
input is hypothesized to be neutralized by a two-step process. Initially,
elevated H+ was neutralized by rapid dissolution of soil Al. This was
followed by a more complete attenuation of H+ and Al due to the slower
release of basic cations. When the equivalence of acidic anions (such
as SO42- and NO3-) approaches or exceeds the equivalence of basic cations,
high concentrations of acidic cations (H+, Aln+) are observed in surface
waters. The leaching of calcium and the mobilization of Al could result
in Ca/Al ratios in the soil solution of less than 1.0. The ratio is often
considered a threshold for restricted plan growth and associated nutrient
uptake.
Aluminum is mobilized largely to acidic surface water from shallow, acidic soil in upland regions. Non-calcareous forest soils are often acid sensitive, as they contain low amounts of available basic cations, and are limited in the retention of strong acidic anions through either biotic or abiotic processes. High elevation watersheds with soils that are highly permeable, underlain by silicate bedrock, shallow, and acidic (low in exchangeable basic cations) are most sensitive to acidic deposition.
2. Forest harvesting
Different management measures on forestland may enhance the rate of the
Al mobilization. A study on the effect of whole-tree harvesting in Harbard
Brook Experimental Forest, New Hampshire revealed that a large increase
in stream NO3- and basic cation (Ca2+, Mg2+, Na+ and K+) concentrations
and a decrease in stream SO42- concentrations followed by a decrease in
pH and increase in Al concentrations. Concentrations of Alim below the
outlet of the treated watershed exceeded levels found to be toxic to fish,
but exhibited considerable temporal variation. These responses are explained
by increased soil nitrification coupled with decreased vegetative uptake
following the whole-tree harvest.
3. Episodic Al release
The amount of total Al and its species can vary greatly in response to
the stream episodic acidification, a sharp change of stream pH over short
time in certain hydrological events. Some streams and headwater lakes
in New York State exhibit episodically high values of both Altd and Alim
during the spring melt period. The release of Alim is coincided with elevated
concentration of strong acids (SO42-, NO3-) during the spring. Highly
acidic conditions during snowmelt appear to have facilitated the release
of alumino-organic solutes from the mineral soil. A large deposition of
Al occurs during the summer low flow season within the stream.
4. Mixing zone effect
The toxic effect of Al is dependent on its chemical species. At some certain
places, e.g. where two streams with distinct pH levels meet, the acidity
of the mixed water may fall into a range which favors the generation of
toxic species of Alim. This phenomenon has been observed in the restoration
of acidified lakes and streams. The addition of mineral lime (e.g. CaCO3)
is extensively used for the purpose, often permitting there-establishment
of fisheries destroyed by a combination of low pH and alkalinity, and
elevated concentrations of toxic species of Al. The liming of streams,
however, is complicated because the automatic dosers which are often used
for this purpose must be able to compensate for changes in flow rate,
as well as the input from acid tributaries which may be located downstream
from the doser. Another serious complication is the mixing of limed and
still-acid streams which might produce conditions extremely toxic to fish,
due to transformation processes influencing the aqueous species of Al.
Contact Information
Please feel free to contact us if you have any questions or comments.
Your feedback is greatly appreciated by Xing Wang
email: xwang08@mailbox.syr.edu
Dr. Ruth D. Yanai
email: rdyanai@mailbox.syr.edu
342 Illick Hall
State University of New York College of Environmental Science & Forestry
Syracuse, NY 13210
Useful links
Aluminum
toxicity
http://www.vitawise.com/Nutritional_Healing/aluminum%20toxicity.htm
Aluminum
toxicity in agriculture
http://www.microirrigationforum.com/new/archives/altox.html
EPA’s view on
Al mobilization
http://www.epa.gov/science1/epe95019.pdf
EPA’s acid rain
page
http://www.epa.gov/airmarkets/acidrain/
Al tolerance in
plants
http://www.unicamp.br/ib/wal/abstracts.html
Aluminum
compounds: review of toxicological literautre
http://ntp-server.niehs.nih.gov/htdocs/Chem_Background/ExSumPdf/Aluminum.pdf
References
Driscoll, Charles. T. 1985. Aluminum in acidic surface waters: chemistry,
transport, and effects. Environmental Health Perspectives. 63: 93-104
Lawrence, Gregory B. 1986. Aluminum chemistry of headwater streams at the Hubbard Brook experimental forest, before and after whole-tree harvesting (New Hampshire).
Baldigo, Barry P. and P.S. Murdoch. 1997. Effect of stream acidification and inorganic aluminum on mortality of brook trout (Salvelinus fontinalis) in the Catskill Mountains, New York. Can. J. Fish. Aquat. Sci. 54: 603-615
Rosseland, B.O. and others. 1992. The mixing zone between limed and acidic river waters: complex aluminum chemistry and extreme toxicity for salmonids. Environ. Pollut. 78: 3-8
|
|