JOURNAL OF COLLOID AND INTERFACE SCIENCE 181, 561–570 (1996) ARTICLE NO. 0413 The Zeta Potential of Iron and Chromium Hydrous Oxides during Adsorption and Coprecipitation of Aqueous Heavy Metals RUSSELL J. CRAWFORD,* ,1 IAN H. HARDING,* AND DAVID E. MAINWARING² *Centre for Applied Colloid and BioColloid Science, School of Chemical Sciences, Swinburne University of Technology, P.O. Box 218, Hawthorn 3122, Australia; and ² Department of Applied Chemistry, Royal Melbourne Institute of Technology, P.O. Box 2476V, Melbourne 3001, Australia Received October 23, 1995; accepted February 26, 1996 adsorption and coprecipitation. Changes to the sign of the The adsorption and coprecipitation characteristics of Cr(III), zeta potential (charge reversal), for example, are directly Zn(II), and Ni(II) with amorphous hydrous iron(III) oxide related to sudden changes in adsorption density (3, 4). (HFO) and those of Zn(II) and Ni(II) with amorphous hydrous James and Parks ( 5 ) have reviewed electrical double layer chromium(III) oxide (HCO) have been measured by the authors models for their suitability in estimating surface charges and (R. J. Crawford, I. H. Harding, and D. E. Mainwaring, Langmuir zeta potentials under a wide range of solution conditions, 9, 3050, 1993). In the current study, the zeta potentials of these including those in which an adsorbing heavy metal species is colloidal substrates have been measured as a function of pH during present. It is clear from their discussion that the simultaneous and under the same conditions as the adsorption and coprecipita- measurement of parameters such as surface charge, zeta po- tion processes. The Gou ¨y–Chapman–Stern–Grahame model of the electrical double layer has been used, with allowance for spe- tential, and degree of heavy metal adsorption is desirable. cific site interactions, to accurately model the pH-dependent zeta The surface charge of oxides has been measured under con- potential for the individual hydrous metal oxides involved. A sur- ditions identical to those in which metal ion adsorption has face site mixing model has been proposed which models the zeta taken place ( 6, 7 ) ; however, zeta potential measurements are potential of the oxide substrates in the presence of an increasing frequently performed under widely different solids content amount of a dissimilar second oxide surface, which results from conditions (8–11). The method normally adopted in these the process of metal ion adsorption or coprecipitation. This mixing instances is to centrifuge the suspension and then redisperse model was found to satisfactorily account for all trends observed a small amount of the solid into the supernatant (12) prior in the zeta potentials measured during the adsorption and copre- to zeta potential measurements. cipitation of Cr(III), Zn(II), and Ni(II) with HFO and those of Simultaneous measurement of metal ion adsorption and Zn(II) and Ni(II) with HCO.  1996 Academic Press, Inc. zeta potential is difficult because adsorption experiments re- Key Words: adsorption; coprecipitation; hydrous oxides; zeta potential; modeling. quire a high solids content, whereas zeta potential measure- ments require a low solids content. Mass transport instru- ments measure zeta potentials at high solids content; how- INTRODUCTION ever, they are slow, require large amounts of material for one measurement, and involve nonstatic conditions during The removal of heavy metal ions from both natural water the measurement. The development of recent instruments supplies and waste water streams is becoming increasingly such as electroacoustic devices suggests that high solids con- important as awareness of the environmental impact of such tent can be used. Hunter and James (13) have shown that pollutants is fully realized. Adsorbing colloid flotation ( 1, it is possible to measure zeta potential at high solids content, 2) techniques are being developed as one method for water and have done so to investigate charge reversal, but did not purification which utilizes the processes of precipitation, ad- perform simultaneous adsorption trials. The current study sorption, and / or coprecipitation to remove heavy metals exploits a Doppler effect light scattering apparatus to mea- from waste water streams. The efficiency of such water puri- sure zeta potential. This technique is normally assumed to fication techniques is strongly dependent on the efficiency require low solids content; however, in this study, a concen- of the adsorption and/or coprecipitation processes, which in tration regime has been found in which both adsorption/ turn are strongly influenced by the zeta potential of the col- coprecipitation and zeta potential can be measured directly loidal substrate. Conversely, the zeta potential of the colloi- and under the same conditions. dal substrate is itself strongly influenced by heavy metal Zeta potentials, in the absence of metal ion adsorption, have been effectively modeled using the Gou ¨y–Chapman– 1 To whom correspondence should be addressed. Stern–Grahame (GCSG) model of the electrical double 561 0021-9797/96 $18.00 Copyright  1996 by Academic Press, Inc. All rights of reproduction in any form reserved.