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

Abstract

The impact of the surface properties of Fe oxides formed both in the absence 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 pressure-jump relaxation spectrometry. The adsorption edge of Pb on these two 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 suspensions in the pH range of 3–5 indicate that Pb adsorption on these Fe oxides consisted of two elementary reactions. The results indicate that the mechanisms of Pb sorption in the pure goethite system derived by Hayes [Equilibrium, Spectroscopic, and Kinetic Studies of Ion Adsorption at the Oxide/Aqueous Interface. PhD thesis, Stanford Univ., Palo Alto, CA, 1987] were also acceptable for the Pb sorption in these two Fe oxide systems in the present 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. Cosmochim. Acta 62 (1998) 193]. However, the intrinsic forward rate and equilibrium 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 improvement of the crystallinity of Fe oxide (lepidocrocite) resulted from the presence of citrate at the concentration of 10 −5 M through catalysis and the subsequent decrease in the specific surface area. Compared to the Fe oxide formed in the absence of citrate ligands, the intrinsic backward rate constant 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 lower 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 less coordination of nitrate ions with Pb on the surface. Therefore, the presence of citric acid at the concentration of 10 −5 M during the formation of Fe oxides significantly influenced the intrinsic rate and equilibrium constants of Pb sorption at the Fe oxide/water interface through fundamental structural modifications and the subsequent alteration of their surface properties.