Aluminium sulphate

Endpoint summary

Additional information

Abiotic degradation

The endpoints "Phototransformation of an element in water, soil or air" are not relevant for substances that are assessed using a read-across approach on an elemental basis, i.e., based on the exposure and effects of aluminium, expressed as elemental Al.

 

The term “Hydrolysis” refers to the “Decomposition or degradation of a chemical by reaction with water,” and this as a function of pH (i.e., abiotic degradation). The need for testing may be waived if “The substance is highly insoluble in water”, or if “The substance is readily biodegradable”. These column 2 waivers are designed for organic substances and are therefore not relevant in the case of metals. For aluminium salts, hydrolysis is a meaningful endpoint but hydrolysis does not result in hydrolytic half-lives that can be used to determine the ultimate disappearance of the substance in the environment. Rather under specific environmental conditions an equilibrium between bioavailable and non-bioavailable aluminium will be reached. The specific and relative concentrations of these states in water are influenced by pH, dissolved organic carbon and to some extent, hardness.

The speciation of Aluminium is pH dependent. At low pH values, dissolved aluminum is present mainly in the aquo form (Al3+). Aluminum hydrolyses increasingly with increased pH and is poorly soluble in the neutral pH range between 6.0 and 8.0 due to the formation of less soluble hydroxide complexes (e.g., Al(OH)2+, Al(OH)2+). Its solubility is at a minimum near pH6.5 at 20°C and then increases as the anion, Al(OH)4– becomes more abundant.

In the presence of complexing ligands and under acidic (pH < 6) and alkaline (pH > 8) conditions, aluminium solubility is enhanced.

 

At 20°C and pH < 5.7, aluminum is present primarily in the forms Al³⁺ and Al(OH)²⁺. In the pH range 5.7 to 6.7, aluminum hydroxide species dominate, including Al(OH)²⁺ and Al(OH)²⁺, and then Al(OH)3. At a pH of approximately 6.5, Al(OH)3 tends to predominate over the other species. In this range, aluminium solubility is low. At pH > 6.7, Al(OH)4–becomes the dominant species.

 

Due to its size fluoride readily substitutes for OH- and has a strong affinity for Al under acidic conditions although this is limited by the relative low abundance of fluoride in the environment. Aluminium-hydroxide complexes predominate over aluminium-fluoride complexes under alkaline conditions.

Sulphate also complexes with aluminium under alkaline conditions (>pH 6). Nevertheless aluminium-sulphate complexes were found to represent only 5% of monomeric aluminium in lake studies. While all aluminium species described are present simultaneously at any pH value the relative proportion of these species will depend upon pH in aquatic systems.

Aluminium-organic complexes were a major component of monomeric Al over a wide range of pH from 4.3 to 7.0 and were found to positively correlate with increasing DOC concentrations as pH decreased.

The hydrolytic products of mononuclear aluminum combine to form polynuclear species in solution in aquifers. Aluminum starts to polymerize when the pH of an acidic solution is above 4.5: 2Al(OH)(H2O)5²⁺Al2(OH)2(H2O)8⁴⁺+ 2H2O

Polymerization leads to larger structures, and ultimately to the formation of the Al13 polycation.

Biotic degradation

For inorganic substance like aluminium salts for which the chemical assessment is based on the elemental concentration (i. e., pooling all inorganic speciation forms together), biotic degradation is an irrelevant process, regardless of the environmental compartment that is under consideration: biotic processes may alter the speciation form of an element, but it will not eliminate the element from the aquatic compartment by degradation or transformation. This elemental-based assessment (pooling all speciation forms together) can be considered as a worst-case assumption for the chemical assessment.

Bioaccumulation

The available evidence shows the absence of aluminium biomagnification across trophic levels both in the aquatic and terrestrial food chains. The existing information suggests not only that aluminium does not biomagnify, but rather that it tends to exhibit biodilution at higher trophic levels in the food chain.

Adsorption/desorption

Due to its dynamic chemistry, the amount of aluminium associated with suspended particles is dependent on the chemical conditions. Factors that are known to affect aluminium speciation, such as pH and DOC, are also known to affect adsorption and desorption from particle surfaces. To illustrate this further, the amount of aluminium associated with suspended particles was estimated by chemical simulation that included aqueous aluminium speciation (inorganic and organic), aluminium solubility, and complexation by NOM. For these simulations a NOM concentration of 4 mg/L (2 mg/L as DOC) and a total suspended solids (TSS) concentration of 1 mg/L were chosen to represent a reasonable lower bound for the range of values of these substances that would be expected in the environment. Suspended particles were assumed to be composed primarily of silica (80%) with a small amount of clay (10%) and particulate organic matter (10%). Aluminium concentrations were set to the maximum allowable by solubility with amorphous gibbsite at a temperature of 20⁰C. Under these conditions, the amount of aluminium bound to particles as a result of surface complexation (i. e. adsorption) was pH dependent, but was typically less than 8% of the total aluminium at pH 6, and was further reduced to below 1% at pH values above 7 (Figure 4.2.1. -2A). This distribution was similar in both soft and hard waters. The corresponding Log Kd values for this distribution are shown in Figure 4.2.1. -2B, with values between 3 and 5.  Very similar results were obtained with higher DOC concentrations of 4 mg/L.