Reaction mass of aluminium hydroxide and aluminium nitrate and aluminium sulphate

Endpoint summary

Additional information

The complex environmental chemistry of aluminium is addressed in several reports and public available sources to date (ATSDR (2008), Environment Agency (2007), Environment Canada and Health Canada (2010), EURAS (2007), IPCS (1997), RIZA (2002), WHO (2010)). All of them describe that aluminum is a naturally abundant element, the third most common element of the earth's crust. It is naturally released to the environment from the weathering of rocks and volcanic activity. It also enters the environment from anthropogenic sources such as drinking water and wastewater treatment. Treatment with aluminum salts may not necessarily increase the total aluminum concentration in finished drinking water, as the aluminum associated with suspended solids is removed but there are no reliable estimates of the quantities of aluminum released to the environment by natural processes on a global scale, most of which comes from natural aluminosilicate minerals (Environment Canada and Health Canada, 2010).Aluminum present in surface waters due to man-made applications cannot be distinguished from natural aluminum released during weathering of aluminum-bearing minerals.

  

It is found as a variety of forms, depending on pH, alkalinity, temperature, dissolved organic carbon, dissolved inorganic carbon and anion concentration. Hydroxides are the most common form occurring in water but the chemistry of aluminum becomes more complicated due to the presence of organic and inorganic ligands, which compete for complexation with the hydroxide ion (OH-). Due to this complicated chemistry all assessment reports also describe that aluminum is hard to assess.

 

Aluminum can be analysed under different forms, but historically results were reported mostly as total aluminum because of the easy analysis. In many cases, results are also available for extractable or dissolved aluminum. Total aluminum represents all the aluminum present in a water sample, including the particulate fraction. Extractable aluminum includes both the “dissolved” fraction and weakly bound or sorbed aluminum on particles, and “dissolved” aluminum represents the fraction present in a sample filtered through a 0.45μm membrane. All the bioavailable aluminum is considered to be present in this fraction, but not all the dissolved aluminum is bioavailable. Colloidal aluminum (0.01 to 0.1μm) and organic aluminum (aluminum bound with soluble organic ligands) that are included in this fraction are generally thought to be less bioavailable than truly dissolved forms of the metal (Text taken from Environment Canada and Health Canada, 2010).).

Speciation and solubility of aluminum and therefore toxicity depends on pH. Inorganic monomeric forms of aluminum are of particular importance as these are the most toxicologically active species. AtpH <5.5, the free ion (Al3+) becomes the prevalent form, along with the inorganic monomeric complexes [AlF, Al(OH)x and Al(SO4)]. The increased availability at this pH is reflected in higher toxicity. At pH 6.0–7.5, solubility declines due to the presence of insoluble Al(OH)3. At higher pH (pH >8.0), the more soluble Al(OH)4 - species predominate, which again increases availability.

 

The presence of natural organic matter can also markedly alter aluminum speciation. Indeed, the presence of moderate amount of organic matter results in complexation of aluminum, being the dominant form when the pH is between 4 and 7. Above pH 7, anionic aluminum hydroxides predominate over the organically complexed aluminum.

In addition, there is a marked tendency for dissolved aluminum-hydroxy and hydroxysilicate species to precipitate, resulting in the removal of some dissolved forms from the water column. This effect is particularly marked at pH values in the range from 6 to 8, the pH range found in most surface waters. The consequence of this is that the dissolved fraction of Al in water is often an insignificant proportion of the total quantity of metal, i.e. the Al(OH)3 precipitate will be the dominant species.

 

The most relevant way of describing toxicity is to express toxicity in terms of concentrations of dissolved aluminum as this is considered the bioavailable part. Many studies describe aluminum toxicity, but only a part of them expresses toxicity as dissolved aluminum concentrations. There is at present no justifiable reason to differentiate between the different aluminum compounds, as no indication is given that aluminum toxicity in the environment would depend on the salt that was applied (as aqueous solution). Moreover, at the moment, there are insufficient data enabling an effect assessment for each aluminum compound. Finally, it is doubtful whether discrimination between aluminum compounds would have an ecological relevance, as the compound will change with time and under various environmental conditions. Hence, it can be concluded that the aluminum compounds can be pooled with regard to toxicity.

 

Eight aluminum compounds are assessed here:

Aluminum sulphate, CAS-RN: 10043-01-3

Aluminum chloride, basic, CAS-RN: 1327-41-9

Aluminium chloride, CAS-RN: 7784-13-6

Aluminium chloride (hexahydrate), CAS-RN: 7446-70-0

Alumnium sulphate-14-hydrate, CAS-RN: 16828-12-9

Aluminum chloride hydroxide sulfate, CAS-RN: 39290-78-3

Aluminium oxide, CAS-RN: 1344-28-1

Aluminium, CAS-RN: 7429-90-5

 

During their use in water treatment, aluminum salts react rapidly, producing dissolved and solid forms of aluminium and some are released to surface waters. The amount of anthropogenic aluminium released is small compared with natural aluminum releases (Environment Canada and Health Canada, 2008).

A large selection of products and methods has become established in practice for tackling the problem of turbidity and particle removal from water. In water circuit systems mainly aluminium sulphate and polyaluminiumchloride are used. Although polyaluminiumchloride possesses many advantages against aluminiumsulphate, in "green" closed water circuits an enrichment of chloride occurs which can lead to corrosion of metal and steel parts

Through the registered substance reaction mass of aluminium hydroxide and aluminium nitrate and aluminium sulphate, nitrate is added to the water circuit. The introduced nitrate can be degraded by many microorganisms to gaseous nitrogen, which is environment-friendly and degases from the water circuit only the aluminium is relevant for assessment.