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Environmental fate & pathways

Hydrolysis

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Endpoint:
hydrolysis
Data waiving:
study scientifically not necessary / other information available
Justification for data waiving:
other:
Endpoint:
hydrolysis
Type of information:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Acceptable, well documented publication which meets basic scientific principles.
Qualifier:
no guideline followed
Principles of method if other than guideline:
Determination of pH effect on hydrolysis of aluminium chloride at low concentrations (1.5*10E-4 mol/L) was performed by using electrospray ionization mass spectrometry (ESI).
GLP compliance:
no
Radiolabelling:
no
Analytical monitoring:
yes
Details on sampling:
The sample was prepared freshly before each experiment. The test substance was diluted to a concentration of 1.5*10E-4 mol/L.
The pH values of fresh solutions were adjusted after dilution.
Buffers:
- The aged solutions were first diluted, then pH was adjusted after an aging time. Tetramethylammonium hydroxide pentahydrate, (CH3)4NOH.5H2O (TMA, Analytical Reagent, Beijing Chemical Reagents Company) or HCl solution was added into the solution to control pH.
- For the calibration of pH electrode, two buffers with pH 4.01 and 7.00 were prepared.

Results:

At the initial Al concentration of the coagulant (0.55 mol/L), hydrolysis of aluminum salts was very slow. When the AlCl3was diluted to a concentration of 1.5 *10E-4 mol/L, hydrolysis occurred immediately. At pH 4.0, mono- and dimeric aluminum species[Al(OH)2(H2O)2-3]+and[Al2O2(OH)(H2O)0-5]+were detected as main products. With increasing pH, hydrolysis and polymerization increased. At pH 4.8, mono- and dimeric aluminum species hydrolyzed and polymerized into small polymeric aluminum species (Al3-Al5). The dimers still dominated in the spectra, trimers[Al3O4(H2O)0-5]+was also detected as major species. At pH 5.0, aluminum species mainly aggregated and assemblied to median polymeric species (Al6-Al10species) and these to large polymeric species (Al11- Al21). At pH 5.8, metastable median and large polymers decomposed into small alluminum species and disaggregated into dimeric species. With pH 6.4, the majority of aluminum formed to Al(OH)3amorphous flocs.

Accordingly, hydrolysis mechanism from polymerization toward decomposition was proposed.

Endpoint:
hydrolysis
Type of information:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Acceptable, well documented publication which meets basic scientific principles
Principles of method if other than guideline:
Isopiestic measurements were used to determine the change of the water activity due to the variation of the aluminium content of alkaline aluminate solutions (method described in Szabo et al. 1975).
Radiolabelling:
no
Analytical monitoring:
yes
Estimation method (if used):
Gibbs-Duehm equation; initial

Results:

There was a general increase of water activity with increasing aluminium concentration.It can be attributed to two processes: 1) the coordination of hydroxide ion to aluminium; 2) dimerization connected with the dehydration of the monomeric species.

 On the basis of the GIBBS-DUHEM equation the water activity data gave an indication of the concentration ranges in which the sodium aluminate solution exists mainly as I) NaOH + monomeric Al(OH)4-and II) NaOH + dimeric Al2O(OH)62-ions.

The water activity at low aluminium concentrations showed a smaller, almost linear, increase. In case smaller mole ratios (greater aluminium concentrations) a greater increase of water activity occurred leading to a steeper straight line.The change in water activity can be described - depending on the total electrolyte concentration - by assuming the presence of Al(OH)4-and NaOH in the solution of smaller aluminium concentrations (1 -4 M/dm3) and NaOH and dimer Al2O(OH)62-complex aluminate ions in systems of higher aluminium concentrations (2 -7 M/dm3) under alkaline conditions.

Description of key information

Aluminium is the most abundant metal in the lithosphere, and is characterized by a complex biogeochemical cycle (Driscoll and Postek 1996; Exley, 2003). Aluminium can participate in hydrolysis reactions, thereby forming a number of monomeric and polymeric Al-hydroxides and this process is highly dependent on pH. Under REACH (ECHA 2008, Chapter R.7B – Endpoint Specific Guidance), the term ‘Hydrolysis’ refers to the “Decomposition or degradation of a chemical by reaction with water”, and this is a function of pH (i. e., abiotic degradation). Aluminium persists in the environment irrespective of whatever chemical species form as a result of hydrolysis, although it may form insoluble aluminium hydroxides that precipitate out of solution. Characterization of aluminium in environmental media is typically based on total aluminium concentrations inclusive of all specific chemical forms or species. Since hydrolysis changes the chemical form but does not decompose aluminium and since characterization of total aluminium considers all chemical forms, the concept of degradation of aluminium by hydrolysis is not relevant in the consideration of its environmental fate.

Key value for chemical safety assessment

Additional information

Hydrolysis of aluminium ions has two possible “directions” towards a neutral pH, i.e. base hydrolysis and acid hydrolysis. Both acid and base hydrolysis of aluminum results in precipitation of aluminium hydroxide.

 

Aluminum salts are used as coagulants and flocculants to cause fine materials that are suspended, soluble or both to agglomerate, for subsequent removal via sedimentation and filtration. As part of this agglomeration or coagulation process, most of the aluminum associated with the added aluminum salt hydrolyses to aluminum hydroxide, which precipitates and becomes part of the floc structure. As such, it makes up a part of the sludge generated by the treatment process. A small amount of the aluminum added may stay with the finished water in either colloidal particulate (Al(OH)3) or soluble form (e.g., AlOH2+, Al(OH)2+, Al(OH)3, Al(OH)4-), dictated by the conditions of the treatment process and in particular, the pH

Reaction mass of aluminium hydroxide and aluminium nitrate and aluminium sulphate decomposes to aluminium hydroxide when adds to water and can therefore not be considered as water soluble.

 

Zhao et al. (2009) investigated the effect of pH in the range of 4.0 to 6.4 on the aluminium chloride hydrolysis at low concentration level. As coagulant aluminium chloride was diluted with deionised water.At pH 4.0, mono- and dimeric aluminum species [Al(OH)2(H2O)2-3]+and [Al2O2(OH)(H2O)0-5]+were detected as main products. With increasing pH, hydrolysis and polymerization increased. At pH 5.0, aluminum species mainly aggregated and assembled to median polymeric species (Al6-Al10species) and these further to large polymeric species (Al11- Al21). At pH 5.8 metastable median and large polymers decomposed into small aluminum species and disaggregated into dimeric species. With pH 6.4 the majority of aluminum formed to Al(OH)3amorphous flocs. In alkaline solution, depending on the concentration of aluminate, the soluble aluminate ions, Al(OH)4 ‾ and Al2O(OH)62 -ions are considered to be the totally dominating species (Szabo et al. 1978).