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Water Res. 2018 Oct 15;143:346-354. doi: 10.1016/j.watres.2018.06.063. Epub 2018 Jun 27.

Effect of crystallization of settled aluminum hydroxide precipitate on "dissolved Al".

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Key Laboratory of Drinking Water Science and Technology, Research Centre for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China; Department of Civil and Environmental Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK. Electronic address:
Centre for Water Resources Research (CWRR), School of Civil, Structural and Environmental Engineering, University College Dublin, Newstead Building, Belfield, Dublin 4, Ireland. Electronic address:
BGI-Shenzhen, Shenzhen 518083, China; Hammersmith Hospital, Imperial College London, Ducan Road, London W12 0NN, UK. Electronic address:
Department of Civil, Environmental and Geomatic Engineering, University College London, Gower Street, London WC1E 6BT, UK. Electronic address:


When aluminum salts are added to water at around neutral pH, a precipitate of Al hydroxide is formed very rapidly. Initially the precipitate is in the form of nano-scale primary particles, which then aggregate to form flocs. The nature of the flocs depends greatly on the solution composition, for instance on the presence of humic acid (HA), which not only increases the size of the primary nanoparticles, but also decreases the connection points between them. The nanoparticles become smaller with aging, both with and without HA, as a result of crystallization. The aggregated amorphous nanoparticles (settled flocs) undergo a room temperature structural modification best characterized as a disorder-to-order transition, following elimination of water. During this process, the apparent Al concentration in the supernatant of water increases with age. The "dissolved Al" concentration in the supernatant becomes higher with increasing pH and, to some extent, in the presence of HA. However, it can be shown that the "dissolved Al" in the supernatant exists in the form of crystalline nano-particles or larger clusters, which are detached from the settled flocs. TEM results confirmed that HA only adsorbed on the surface of nano-particles during the coagulation process, which shows precipitate nanoparticles formed firstly during sweep coagulation before the adsorption of HA or complexed Al3+-HA. However, the adsorbed outer layer of HA does not change the crystallization process for the inner part of nano-particles. This laboratory study may have implications for the release of Al from sediments into lake water, following addition of coagulants to lower phosphorus concentrations.


Ageing; Crystallization; Flocs; Nanoparticle; Residual Al

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