Colloids and Surfaces A: Physicochem. Eng. Aspects 243 (2004) 1–10 Speciation stability of inorganic polymer flocculant–PACl Dongsheng Wang a,∗ , Wei Sun a , Yi Xu a , Hongxiao Tang a , John Gregory b a SKLEAC, Research Center for Eco-Environmental Science, Academia Sinica, P.O. Box 2871, Beijing 100085, China b Department of Civil and Environmental Engineering, University College London, Gower Street, London WC1E 6BT, UK Received 27 November 2003; accepted 27 April 2004 Abstract The speciation distribution, physico-chemical properties of coagulants, and principles for their further species transformation under various water and wastewater treatment conditions are of paramount importance in coagulation processes. In this paper, the effect of various factors on the speciation transformation of typical inorganic polymer flocculants (IPFs)–polyaluminum chloride (PACl) was investigated by ferron assay. Dilution and co-existing electrolyte exhibit little effect on the speciation distribution of PACl after dosing. Solution pH has significant effect on the species transformation, depending mainly on OH/Al ratio (denoted as B value). The lower the B value, the more significant the effect pH exhibits. A large amount of Al b , i.e. rapid reactive species by ferron assay, could form for alum and PACl 0 in the weak acidic pH range, which transforms quickly into Al c after aging. PACl, at high B values, maintains high speciation stability under the various conditions investigated. The ferron method provides a measure to explain the reactivity of different species with ferron. The classification of three kinds of aluminum hydrolysis species based on kinetic differences needs further modification. © 2004 Elsevier B.V. All rights reserved. Keywords: PACl; Speciation stability; Ferron method; Coagulation; Mechanism 1. Introduction Coagulants are one of the key factors in the coagulation process. Alum is the most frequently applied traditional coagulant in the world. Recently, based on conventional iron and aluminum salts, inorganic polymer flocculants (IPFs) have been developed rapidly and become applied widely, especially in China, Japan, Russia, and west Europe [1–5]. Many kinds of IPFs have been developed including aluminum-based, iron-based, inorganic–inorganic compos- ite flocculants, inorganic–organic composite flocculants, and multifunctional composite IPFs [1–8]. Among them, polyaluminum chloride (PACl) is one of the typical kinds and has become most widely applied. The property and general coagulation behavior of IPFs have been exten- sively investigated [2–5,8–11]. The differences between traditional coagulants and IPFs have been gradually ex- amined. However, previous studies paid most attention to the preparation, characterization, and application of IPFs. Little study has been focused directly on the features of ∗ Corresponding author. Fax: +86 10 62923543. E-mail address: wgds@mail.rcees.ac.cn (D. Wang). chemical speciation during coagulation. There exist few established conclusions as to why their efficiency is su- perior to the traditional coagulants. It is generally thought that the pre-produced products contain species of superior quality and possess structure fairly stable to further hydrol- ysis and solution chemistry, resulting in higher coagulation efficiency. The action and mechanism of IPFs are not fully understood. IPFs are actually clusters or aggregates of a combina- tion of intermediate products with various anions, during hydrolysis of metal salts under suitable conditions. Hence, the study of their aqueous chemistry, especially the species distribution and transformation principles, would build the theoretical basis for IPFs and their further improvement [2]. There are many excellent reviews with large amounts of literature on the aqueous chemistry of Al(III) and Fe(III) [2,12–15]. However, there exist a few firmly established principles due to the complex nature of hydrolysis. The hydrolysis of Al(III) follows multiple pathways. A series of hydrolysis species can be formed depending on various fac- tors such as aluminum concentration, sorts of bases adopted, methods of alkalization, speed for base addition, co-existing anions and particles. Therefore, the species formed are 0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.04.073