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Formation and dissolution kinetics of Al(OH) 3 (s) in synthetic freshwater solutions
Author(s) -
Lydersen E.,
Salbu B.,
Polèo A. B. S.,
Muniz I. P.
Publication year - 1991
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/90wr02409
Subject(s) - dissolution , precipitation , aqueous solution , amorphous solid , kinetics , phase (matter) , chemistry , nuclear chemistry , analytical chemistry (journal) , mineralogy , crystallography , chromatography , organic chemistry , physics , meteorology , quantum mechanics
The precipitation of Al in aqueous solutions can be described as a two‐step process. When acidic inorganic Al solutions ( p H 4.5) were titrated with NaOH to p H levels between 5.5 and 6.0, an amorphous Al(OH) 3 (s) phase was formed instantaneously. During the first 5 min, the apparent half time for the reduction in dissolved Al species ( t 1/2 ) was 0.162 ± 0.07 hours ( n = 4). The decrease of dissolved Al species continued during the following 24 hours, but at a far slower rate ( t 1/2 = 55.6 ± 25 hours ( n = 4)). The highest precipitation rates were found in the solution of highest p H, and at approximately identical p H, the highest rate was found in the solution of highest temperature. The dissolution of amorphous Al(OH) 3 (s) can also be described as a two‐step process. When the amorphous Al solutions ( p H between 5.5 and 6.0) were acidified to p H about 4.5 (HCl), the initial dissolution was relatively fast, but not as instantaneous as in the first step of precipitation. The apparent t 1/2 after 5 min was estimated to be 0.65 ± 0.12 hours ( n = 4). During the first 2.67 hours (160 min) the dissolution step could be described by the equation C ( t ) = C 0 e −0.45 t , i.e., t 1/2 = 1.53 ± 0.60 hours ( n = 20) according to which the solid Al phase should dissolve totally (97%) during 7.65 hours. However, after 24 hours of storage 40 to 50% of the original Al(OH) 3 (s) remained in the solid phase. This is explained by the presence of a far lower dissolution rate in a second step, i.e., the remaining Al(OH) 3 (s) was less soluble. The average dissolution rate for the rest of the 24‐hour period (21.33 hours) could be described by the exponential function C ( t ) = C 0 e −0.0095t , i.e., t 1/2 = 73.2 ± 23.4 hours ( n = 4). This implies that a total dissolution of Al (97%) will occur after 15.3 ± 4.9 days. The implications of these findings are discussed with reference to the current geochemical models pretending to simulate and predict aluminum chemistry in soils and surface runoff.