SURFACE COMPLEXATION MODEL PARAMETERS FOR GOETHITE (ALPHA-FEOOH)

Potentiometric titration studies on a pristine goethite suspension (al pha-FeOOH) have suggested alternative surface complexation model (SCM) parameters to those currently in use. It is proposed that the intrins ic surface acidity constants pK(a1)int and pK(a2)int should generally be higher than usually reported and the inner-layer capacitance in the region of 0.4-0.8 F m-2 for ionic strengths of 10(-3)-10(-1) M. The s tudy demonstrates that some surface complexation model (SCM) parameter s may be incorrect due to an experimental artifact, though they give g ood simulations of experimental data. This may be due to the success o f least-squares fitting programs such as FITEQL at finding model param eters to describe experimental data sets. Previous studies have shown that goethite (alpha-FeOOH) suspensions purged with CO2-free N2(g) for up to two months have measured points of zero net protonic charge (PZ NPC) in the range 9.0-9.3, which is close to a theoretical value of 9. 4. However, most surface complexation models parameters, such as the t wo intrinsic surface acidity constants pK(a1)int and pK(a2)int and the inner layer capacitance kappa1, have been determined from data sets i n which the PZNPC was in the range 7.35-8.65. This study reports a set of surface complexation model parameters determined from potentiometr ic titration data obtained from goethite purged with CO2-free N2(g) fo r 2 months. Parameters were calculated using the computer program FITE QL within the context of three commonly used models, the diffuse doubl e layer model, the constant capacitance model, and the triple layer mo del. The values of pK(a1)int and pK(a2)int found in this study were la rger than previously reported values. In addition, the values of kappa 1 in the range 0.40. 8 F m-2 were necessary to explain the data for io nic strengths in the range 10(-3) to 10(-1) M, these values of kappa1 are lower than those currently in use. These revised parameters are co nsistent with the removal of a carbonate surface complex from the goet hite, and suggest that artifacts have been introduced into many previo usly calculated surface complexation parameters. This raises questions on the methodology to be used to obtain titration data for determinat ion of SCM parameters. (C) 1994 Academic Press, Inc.