Alumina

Environment

International, Vol. 24, No. 8, pp. 899-910, 1998 Copyright 01998 Elsevier Science Ltd Printed in the USA. All rights resewed 0160-4120/98 $19.00+.00

PIISO160-4120(98)00073-7

SYNTHESIS AND SPECIATION OF POLYALUMINUM CHLORIDE FOR WATER TREATMENT
Yun-Hwei Shen
Department of Environmental Protection Technology, National Pingtung University of Science and Technology, Pingtung,Taiwan 91207, ROC

Brian A. Dempsey
Department of Civil and Environmental University Park, PA 16802, USA email: yhs@mail.npust.edu.tw Engineering, The Pennsylvania State University,

EI 9803-135 M (Received29 March 1998; accepted 8 August 1998)

In this study, synthesis and speciation of polyaluminum chloride (PAC) for application in water treatment was investigated using a calorimetricspeciation method. It was possible to produce stable preparations of PAC solutions in which a relatively stable cationic polymer predominated. The mode of preparation has a dramatic effect on the composition of PAC preparation. Some important parameters such as hydroxyl ligand number, mixing intensity, base injection rate and hia sbencc ~td method, and aging were identified in this study. @l$@sINTRODUCTION

Alum (aluminum sulfate) is one of the most widely used coagulant for water treatment in the United States and has been proven to be an effective coagulant for the removal of certain contaminants, turbidity and color. In recent years, the preformed polymeric aluminum salts have been used with some success. Polyaluminum chloride (PAC) may be produced by adding base to aluminum chlorideuntil an empirical formula ofAl(OH (with n from 1.O to 2.5) is achieved. A variety of species can be formed when stock solutions of PAC are added to a raw water. O’ Melia and Dempsey (1982), in a review of work done using a wide variety of PAC coagulants, proposed that some PAC formulations may contain aluminum precipitates. The positively charged precipitates of AI(OH) may improve flocculationkinetics in turbidity removal and
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adsorb humic substances. Based on the analysis of redissolved precipitates, solubility test, turbidity data and electrokinetic measurements, Van Benschoten and Edzwald (1990) concluded that alum and PAC precipitate to form different solid phases; the polymeric structure remains intact within the PAC precipitate and particles are more positively charged andproduce lower turbidity than for alum floe. Dempsey et al. (1985) have investigated the benefits of PAC relative to alum. The results indicate that PAC is especially effective when the concentration of fulvic acid or other species with high coagulant demand is low, or when the pH falls outside the range of 5.5 to 7:O. O’ Melia et al. (1989) concluded that PAC coagulants are effective at lowerdosages than other aluminum preparations for the coagulation of high turbidity

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Y.-H. Shen and B.A. Dempsey

waters, particularly at low temperature or acidic pH, also that PAC is an effective filter aid for low turbidity waters, providing for destabilization and subsequent filtration of particles at acidic and neutral pHs. Bottero et al. (1980) reported that partially neutralized aluminumchloride solutions were effective coagulants for clay suspensions. Some French investigators (Bottero et al. 1980; Leprince et al. 1984) advocate on-site preparation of these coagulants by partial neutralization or by heating, permitting a specific tailoring of the inorganic “polymeric” coagulant to the water to be treated. Several investigators have successfully prepared polyaluminum coagulantsin the laboratory (Van Benschoten and Edzwald 1990; Dempsey et al. 1985; Parthasarthy and Buffle 1985; Yao 1987). There is still confusion regarding the actual dominant aluminum species in PAC preparations and regarding the relationship between some important synthesis parameters and PAC speciation. In this study, effects of hydroxyl ligand number, mixing intensity, base injection rate and...