Pressure-jump relaxation studies on kinetics of lead sorption by iron oxides formed under the influence of citric acid

The impact of the surface properties of Fe oxides formed both in the absenc e and the presence of citric acid at the citrate concentration of 10(-5) M on their kinetics and mechanisms of Pb sorption was investigated by using p ressure-jump relaxation spectrometry. The adsorption edge of Pb on these tw o Fe oxides can be described by the modified triple layer model (TLM) using the FITEQL program. Two relaxation times observed in the Pb ion-Fe oxide s uspensions in the pH range of 3-5 indicate that Pb adsorption on these Fe o xides consisted of two elementary reactions. The results indicate that the mechanisms of Pb sorption in the pure goethite system derived by Hayes [Equ ilibrium, Spectroscopic, and Kinetic Studies of Ion Adsorption at the Oxide /Aqueous Interface. PhD thesis, Stanford Univ., Palo Alto, CA, 1987] were a lso acceptable for the Pb sorption in these two Fe oxide systems in the pre sent study. The proposed mechanisms are in accord with the formation of the Pb surface complexes on goethite surfaces established by X-ray absorption spectroscopy (XAS) in other studies [Langmuir 7 (1991) 367; Geochim. Cosmoc him. Acta 62 (1998) 193]. However, the intrinsic forward rate and equilibri um constants for the two parallel reactions of Pb sorption by the Fe oxide formed at the citrate/Fe(II) MR of 0.001 were significantly lower compared with the Fe oxide formed in the absence of citrate. This is due to the impr ovement of the crystallinity of Fe oxide (lepidocrocite) resulted from the presence of citrate at the concentration of 10-5 M through catalysis and th e subsequent decrease in the specific surface area. Compared to the Fe oxid e formed in the absence of citrate ligands, the intrinsic backward rate con stant for the second reaction of Pb sorption by the Fe oxide formed in the presence of citrate was significantly higher. This is attributed to the low er point of zero salt effect (PZSE) of Fe oxide formed at the citrate/Fe(II ) MR of 0.001 and more net negative charges at the same pH, which caused le ss coordination of nitrate ions with Ph on the surface. Therefore, the pres ence of citric acid at the concentration of 10(-5) M during the formation o f Fe oxides significantly influenced the intrinsic rate and equilibrium con stants of Pb sorption at the Fe oxide/water interface through fundamental s tructural modifications and the subsequent alteration of their surface prop erties. (C) 2001 Elsevier Science B.V. All rights reserved.