Colloids and Surfaces A: Physicochem. Eng. Aspects 443 (2014) 177–181 Contents lists available at ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects jo ur nal ho me page: www.elsevier.com/locate/colsurfa Investigation of the property of kaolin–alum flocs at acidic pH Wen-zheng Yu a , John Gregory b , Hui-juan Liu a , Jiu-hui Qu a,∗ a State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China b Department of Civil, Environmental and Geomatic Engineering, University College London (UCL), Gower Street, London WC1E 6BT, UK h i g h l i g h t s • There was full re-growth for monomeric Al or in situ polymeric Al 13 . • The break-up process displayed irre- versibility for unstable small poly- meric Al. • There was a significant increase of D f when the pH changed from 5.2 to 6.0. • The number of small flocs after break- age and re-growth decreased at acidic pH. • The strength of flocs is strongly cor- related with D f of flocs. g r a p h i c a l a b s t r a c t a r t i c l e i n f o Article history: Received 2 June 2013 Received in revised form 26 October 2013 Accepted 5 November 2013 Available online 14 November 2013 Keywords: Floc breakage Re-growth pH Fractal dimension Polymeric Al species a b s t r a c t The formation, breakage and re-growth of flocs were investigated using aluminium sulfate (alum) at acidic pH to explore the property of flocs and coagulation mechanism by different Al species formed in situ. The broken flocs can fully re-grow to the size before breakage, when monomeric Al or polymeric Al 13 forms dominate the coagulant species. However, for relatively unstable small polymeric Al (Al 3 –Al 8 ) and alum precipitate, the break-up process displayed a distinct irreversibility. There was a distinct increase of fractal dimension (D f ) when the pH changed from 5.2 to 6.0, as well as the strength of flocs (the size of broken flocs), which meant that the flocs structure changed significantly. Also the D f of broken flocs was higher than that of flocs before breakage, and the D f values of re-grown flocs were not greatly different from the values before breakage. The number of small flocs after breakage and re-growth was much lower than that before breakage at acidic pH (pH 5.2) when the average size of flocs was chosen at the same value before breakage and after re-growth. An important finding is that the strength of flocs is strongly correlated with D f of flocs. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. 1. Introduction Hydrolyzed coagulants are usually used in the purification of drinking water [1–3], and pH is of great importance for the coag- ulation process. The effect of pH on the binding between particles and/or precipitate and their surface charge determines the growth of flocs. Some researchers found that adjusting the pH downwards ∗ Corresponding author. Tel.: +86 10 62849128; fax: +86 10 62849160. E-mail addresses: wzyu@rcees.ac.cn (W.-z. Yu), j.gregory@ucl.ac.uk (J. Gregory), hjliu@rcees.ac.cn (H.-j. Liu), jhqu@rcees.ac.cn (J.-h. Qu). to acidic pH could improve the coagulation process [4,5]. But other researchers found that the coagulation efficiency was greater at neutral pH as the size flocs formed was much larger than the one at acidic pH [6]. The low growth ability of small flocs at the acidic pH is the main factor causing not too large flocs compared with sweep coagulation at neutral pH. Therefore, the cohesion/re- cohesion ability for the flocs growth should be explored. Improved knowledge of the factors affecting the re-growth abil- ity of broken flocs, and the effect of floc breakage and re-growth on particle separation processes would give a better understand- ing of the coagulation mechanism. Many researches [7,8] found that it was difficult for the broken flocs to fully return to their 0927-7757/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.11.005