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Environmental fate & pathways

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Aluminium ammonium sulfate is very soluble in water (120 g/l at 20°C corresponding to a maximum value of 7.15 mg/l of aluminium total) and completely dissociated to various species on contact with water: its two component salts, aluminium sulfate and ammonium sulfate, and further completely to SO4 2-; NH4+ and Al3+ ions.

Aluminium

As aluminium is the third most common element in the earth’s crust, the literature is voluminous and the reviews, together with the number of reference papers on which they are based, that have been used in the compilation of this submission are the most recent and are listed below.

- ATSDR (2008). Agency for Toxic Substances and Disease Registry. Toxicological profile for aluminium. September 2008.

- Environment Canada and Health Canada (2010). Priority Substances List Assessment Report for Aluminium chloride, Aluminium nitrate and Aluminium sulphate, January 2010.

- Kaplan DI (2005). Recommended Distribution Coefficients, Kd Values, for Special Analysis Risk Calculations Related to Waste Disposal and Tank Closure on the Savannah River Site (U). Westinghouse Savannah River Company, Savannah River Site Aiken, SC 29808; Prepared for the U.S. Department of Energy Under Contract Number DE-AC09-96SR18500

- WHO (1997). Environmental Health Criteria 194. United Nations Environment Programme, International Labour Organisation, World Health Organization, International Programme On Chemical Safety.

Natural sources of aluminum release to aquatic systems include weathering of rocks, glacial deposits and soils and their derivative minerals, and atmospheric deposition of dust particles. The typical range of aluminium in soils is from 1% to 30% (10,000 to 300,000 mg Al kg-1), with naturally occurring concentrations varying over several orders of magnitude.The most obvious increases in aluminum concentrations have consistently been associated with environmental acidification.

 

Aquatic compartment

The fate and behaviour of aluminum in the aquatic environment are very complex: as an element, aluminum cannot be degraded in the environment, but may undergo various precipitation or ligand exchange influencing partitions between solid and liquid phases. Aluminum speciation and solubility are affected by a wide variety of environmental parameters, including pH, solution temperature, dissolved organic carbon (DOC) content, and the presence and concentrations of numerous ligands. Aluminum in compounds has only one oxidation state (+3), and would not undergo oxidation-reduction reactions under environmental conditions. Aluminum can be complexed by various ligands present in the environment (e.g., fulvic and humic acids). The solubility of aluminum in the environment will depend on the ligands present and the pH.

Metals in solution may be present as dissolved complexes, in association with particles, as colloids or as solids in the process of precipitating. Colloidal particles are important in the transport of metals in stream ecosystems, as well as the accumulation of metals in sediment and biofilm, and the transfer to biota. The reactivity of aluminum, as well as geochemical behaviour, bioavailability and toxicity, are dependent upon its speciation.

Interactions with pH and DOC are of primary importance to the fate and behaviour of aluminum. DOC will complex with aluminum in water, forming aluminum-organic complexes and reducing concentrations of monomeric forms.

Aluminum is relatively insoluble in the neutral pH range (6.0–8.0) and availability to aquatic biota should be low. The solubility is at a minimum near pH 6.5 at 20°C and Al(OH)3 predominates over all the other species. In the presence of complexing ligands and under acidic (pH < 6) and alkaline (pH > 8) conditions, aluminum solubility is enhanced. Al(OH)4 – becomes the dominant species at higher pH and at lower pH, aluminum is present primarily in the forms Al3+ and Al(OH)2+ and Al(OH)2.

Temperature has been shown to influence the solubility, hydrolysis and molecular weight distribution of aqueous aluminum species as well as the pH of solutions.

There are two general types of ligands that can form strong complexes with aluminum in solution: inorganic ligands including anions such as sulphate (SO42-), fluoride (F-), phosphate (PO43-), bicarbonate (HCO3-) and hydroxide (OH-) and organic ligands including oxalic, humic and fulvic acids.

Mononuclear aluminum hydrolytic products combine to form polynuclear species in solution when the pH of an acidic solution increases to over 4.5.

Aluminium form metal hydroxides that are rapidely removed from the water column at various pH values. With time, these hydroxides either polymerise to form larger insoluble stable complexes or they are trapped and buried in sediments (Guidance on the Application of Regulation EC No 1272/2008, ECHA 2009).

For Al fate the aquatic compartment is the most important with an environmental Al background concentration of median 17.7 µg / L (average natural background concentration in European streams) and mean 75.5 µg / L. There are in Europe large regional differences in Al background concentration.

Sediment and soil compartments

Aluminum is present in many primary minerals and is found in the soil complexed with other anions, such as fluoride, sulfate, and phosphate. The weathering of these primary minerals over time results in the deposition of sedimentary clay minerals, such as the aluminosilicates kaolinite and montmorillonite. The weathering of soil results in the more rapid release of silicon, and aluminum precipitates as hydrated aluminum oxides such as gibbsite and boehmite, which are constituents of bauxites and laterites. Aluminum is found in the soil complexed with other anions, such as fluoride, sulfate, and phosphate. The background concentrations vary widely and can be over 100 g/kg (ATSDR, 2008).Terrestrial organisms are exposed to added aluminum when alum sludge from water treatment facilities, primarily MWWTPs, is applied to agricultural soils.

In general, the solubility and mobility of aluminum in soil is greatest when the soil is rich in organic matter capable of forming aluminum-organic complexes and when the pH is low, such as in areas prone to acid rain or in acidic mine tailings. Under relevant environmental conditions, at pH values between 5.5 and 8.0, Al3+ ion will adsorb immediately to organic particles and precipitates, forms inorganic alumino-silicate minerals (clay and silt) or reacts to insoluble Al(OH)3. Kaplan (2005) published several Kd values based on measured groundwater and sediment concentrations. For sandy soils / sediment the value of 3700 L/kg was established as representative. According to Environment Canada and Health Canada (2010), because the bioavailability of aluminum in sediment is low due to the high pH and the presence of ligands and DOM.

Air

Aluminum-containing particulate matter in the atmosphere is mainly derived from soil and industrial processes where crustal materials (e.g., minerals) are processed. Aluminum is found as silicates, oxides, and hydroxides in these particles. Aluminum compounds cannot be oxidized and atmospheric transformations would not be expected to occur during transport. If aluminum metal particulates were released to air during metal processing, they would be rapidly oxidized.

Bioaccumulation and magnification

The potential for accumulation of aluminum has been studied in several aquatic species including algae, fish, aquatic invertebrates, amphibians and snails and several terrestrial species including plants and birds. The results including BCFs values show that aluminium have a low potential of bioaccumulation and biomagnification. For synthetic organic compounds, the use of a BCF and BAF threshold value provides valuable information for the evaluation of hazard and risk. Bioaccumulation is more complex for naturally occurring inorganic substances such as metals, however, as processes such as adaptation and acclimation can modulate both accumulation and potential toxic impact. All biota will naturally accumulate metals to some degree without deleterious effect and as some metals are essential elements, bioaccumulation does not necessarily indicate the potential for adverse effects. While metal bioaccumulation is homeostatically regulated for metals essential to biological function, non-essential metals may also be regulated to some degree as these homeostatic mechanisms are not metal-specific. Thus, interpretation of the toxicological significance of bioaccumulation data for metals such as aluminum is complex.

Conclusion of the exposure assessment of aluminium

Aluminum occurs ubiquitously in natural waters as a result of the weathering of aluminum-containing rocks and minerals. Of the known geochemical responses to environmental acidification, the best documented is the mobilization of aluminum from terrestrial to aquatic environments. This mobilization of aluminum is often episodic in nature and is associated with pH depressions (acidification) occurring during the spring snowmelt or associated with erosion from specific storm events. The complexity of Aluminum speciation in water and from there the phases (solid, adsorbed or dissolved) makes it impossible to predict an environmental relevant concentration of the most toxic species to aquatic organisms (Al3+).

Considering the natural occurring background concentration and comparing this with anthropogenic point sources (which have the highest concentration of Al-salts), that are mostly STPs and water purification plants that have a legal obligation to release water with 6<pH<9 it can be concluded that the emission of dissolved Al3+ will be very low.

 

Ammonium sulfate

The SIDS Initial Assessment Report (OECD SIDS, 2004) reports the exposure assessment of ammonium sulfate.

Ammonium salts and sulfates are abundant in the environment. Approximately 90 % of ammonium sulfate is used in products applied directly to land, for example as fertilisers or as a component in herbicides.

 Based on the physico-chemical properties of ammonium sulfate, water is expected to be the main target compartment.Depending on pH, ammonia (NH3) exists in equilibrium with the ammonium ion (NH4+), according to the following relationship:

NH4++ H2O → NH3 + H3O+

In general, as pH and temperature increase, the fraction of the total ammonia which is un-ionized increases. For example, at 5 °C and pH 6.5, 0.0395 % of the total ammonia is present as NH3. Increasing the pH from 6.5 to 8.5 will increase the un-ionized ammonium by a factor of approximately 100. Increasing the temperature will also increase the percentage of unionized ammonium. For example, in seawater at 25 °C and pH of approximately 8.1, approximately 7 % of the total ammonia is present as NH3.

The fraction of un-ionized ammonia in aqueous solution at different pH values and temperatures can be calculated from data in Emerson et al. (1975).To calculate the amount of un-ionized ammonia present, the Total Ammonia Nitrogen (TAN) must be multiplied by the appropriate factor selected from this table using the pH and temperature from your water sample.

Due to the salt-character of the substance the calculation of a fugacity model and Henrys Law Constant is not appropriate. Although ammonium sulfate can be created in the atmosphere from ammonia and sulfur dioxide, this process is limited by atmospheric sulfur dioxide, not by ammonia, which has many natural sources. Particulate ammonium sulfate is removed from air by wet and dry deposition. There is no evidence for photodegradation of ammonium sulfate. In unsterilized soil, ammonium sulfate is mineralized fairly rapidly, and subsequently nitrified. Nitrification and denitrification processes also occur naturally in streams and rivers, as well as in many secondary sewage treatment processes.

Based on the high water solubility and the ionic nature, ammonium sulfate is not expected to adsorb or bioaccumulate to a significant extent. However, mobility in soil may be reduced through ion-ion interactions.

References:

- Emerson, K., R.C. Russo, R.E. Lund, and R.V, Thurston. 1975. Aqueous ammonia equilibrium calculations: effect of pH and temperature. Journal of the Fisheries Research Board of Canada. 32:2379-2383.

- OECD (2004). SIDS Initial Assessment Report: Ammonium sulfate (7783-20-2). UNEP publications.