SURFACE-CHARGE AND SURFACE CHEMICAL CHARACTERISTICS OF MAGNETITES SUBSTITUTED WITH NICKEL, COBALT AND CHROMIUM

Magnetite (MAG) and magnetites substituted with nickel, cobalt and chr omium (NF, COF and CRF, respectively) have been prepared over the whol e range of composition, MexFe3-xO4 (x=0-3, Me=Ni, Co or Cr) using the coprecipitation technique and have been characterized for their chemic al composition, crystallinity and surface area. The types of sorption sites present in them were characterized by analysing the water sorpti on isotherms ultilizing the D'Arcy and Watt equation. Intrinsic dissoc iation constants, pK(ai) and point of zero charge, PZC, were evaluate d by analysing the pH-titration curves in acid and alkaline media. A s imilarity in the trend was observed for the crystallinity, surface are a, sorption sites and surface charge characteristics as a function of the composition of the substituted MAGs (i.e. x). The overall behaviou r of the substituted MAGs could be grouped into two: mixed oxides havi ng composition with x=0-1 and those in the composition region x=1-3. T hose in the composition region x=0-1 were crystalline with a lower sur face area, less hydrophilic in nature and have surface charge characte ristics lower than that of MAG. Those mixed oxides in the composition region x=1-3 nearly had the opposite characteristics, with increasing value of x. The hydrophilicity of the substituted MAGs decreased in th e sequence CRF>NF>>COF, and the acidic nature decreased in the sequenc e NF>CRF>COF. (C) 1998 Elsevier Science B.V. All rights reserved.