Adsorption Of Pb(Ⅱ) To Soil Active Components:Chemical Speciation Modeling

In soil environments humic substances(HS) and goethie are ubiquitous and very important reactive colloids. It can interact with heavy metal ions in soil strongly. It plays an important role in govrening the speciation distribution of heavy metals to affect its mobility, bioavailability and toxicity in nature. The adsorption of heavy metal ions to the HS, goethite and the goethite-HS complex can be described by the cheimical speciation model, such as NICA-Donnan model, CD-MUSIC model and LCD model, respectivety. They also can be use to reavel the adsorption mechanism and predict the adsorption behaviors and the speciation distribution of heavy metal ions in soil accurately. However, it always fail to give reasonable prediction for the Pb binding in the soil.In our studies, the HS and goethite were used for the soil active components. The binding characteristics of Pb to the HS, goethite and goethite-HS complex and the speciation distribution were investigated using a combination of acid-base titration, batch adsorption experiment, X-ray absorption fine structure(XAFS) spectroscopy and the chemical speciation modeling. Main results are below:1. The negative charge on the HS increased with the increasing p H and with the increaseding ionic strength because of the enhancement of the charge screening effect. The negative charge on the soil HAs were similar with that of PAHA. The negative charge on the JGFA was abo ut two times of that of HAs. The material-specific NICA-Donnan model parameters and proton affinity distribution for the proton binding indicated that there were significantly diference of the site density, heterogeneity, proton afinity of the binding site and particle size among the HS. The NICA-Donnan model with the generic parameters only can give reasonable prediction for the PAHA. For the soil HS, the prediction of the NICA-Donnan model with the generic parameters unsystematic deviated from the proton binding to the soil HAs and overestimated the proton binding to the JGFA. When the generic Qmax,H1 was replaced by the material-spcific Qmax,H1, the NICA-Donnan model with the generic parameters was improved significantly for the soil HAs.2. The amount of Pb binding to HS and theDH/DPb measured exchange ratio increased with the increasing p H and decreasing ionic strength. At low Pb(II) concentration, the amount of Pb(II) adsorbed by the HAs was higher than that of JGFA due to the higher Pb(II) affinity of HAs which strongly influenced by the structure of HS. The amount of Pb(II) adsorbed by the JGFA was much higher than that of HAs at high Pb(II) concentration where the site density may be the most important factor for the Pb(II) binding. The –COOPb and –OPb were the primary species for Pb binding to HS. The fraction of –COOPb increased with the decreasing p H, contrary to the –OPb. The NICA-Donnan model with the material-specific parameters can give accurate description for the Pb(II) binding to the HS. The NICA-Donnan model with the generic parameters can give reasonable prediction for the Pb(II) binding to the PAHA. However, it overestimated the Pb(II) adsorption to the soil HS. The model prediction with generic parameters could be strongly improved only when the generic nPb1 was replaced by the specific value of nPb1 observed of JGHA and JLHA. The combination of material-specific parameters and the parameters of EXAFS fitting implied that at low Pb(II) concentration the Pb(II) is bound by the carboxylic gro up in a monodentate structure, at high Pb(II) loading the Pb(II) tend to be predominantly complexed by one carboxylic and one phenolic group(salicylate-type) or two phenolic groups(catechol-type) in ortho position3. The charge of goethite increased with the increasing p H and ionic strength. The PZC of the goethite was 9.1. The Pb adosrption to the goethite increased with the increasing p H. The result of the model optimization indicated that the surface complexes of Pb adsorbed to the goethite were-(Fe OIIH)2Pb+1.22,-(Fe OIIH)2Pb OH+0.22,-(Fe OIIH)2Pb+0.43,-(Fe OIIH)2Pb OHFe3OI-0.57,-(Fe OIIH)2Pb Fe2OIH+0.99 and-(Fe OIIH)2Pb OHFe2OIH-0.01. The speciation distribution of Pb adsorbed showed that at low Pb concentration the-(Fe OIIH)2Pb OHFe2OIH-0.01 at 021 plane was the predominate species for Pb adsorbed; at high Pb concentration the-(Fe OH)2Pb OH+0.22 at 110 plane was the predominate species for Pb adsorbed.4. The measured relative charge of the goethite/HS system was lower than that of the sum of the pure goethite and HS at p H<PZC, and similar with it at p H≥PZC. The fraction of HS adsorbed by the goethite increased with the increasing p H and decreasing ionic strength. The JGFA is present in the Stern layer and adsorbed with strong conformational change. The JGHA was adsorbed without conformation change large part of the JGHA is located in the diffuse layer. The amount of JGHA adsorption to goethite is larger than that of JGFA and the density of JGHA adsorption is smaller than that of JGFA. The amout of net proton release is positive, suggesting that the effects of negative charge of humicson the oxide surface dominate.5. In the goethite/HS/Pb(II) ternary system, the fraction of Pb(II) adsorption to goethite increased and fraction of the HS adsorption decreased with the increasing p H. The presence of HS can promote the adsorption of Pb(II) to goethite and the high Pb(II) concentration increase the HS adsorption. At 5 mmol/L Pb(II) and acidic p H, the linear additivity model always underestimated the adsorption of Pb(II) to the goethite-HS complex in the ternary system. The LCD model which consider the interaction between goethite and HS and its effect on the Pb(II) adsorption can describe the adsorption of Pb(II) to the goethite-HS complex satisfactorily, except the high JGHA concentration at alkaline p H. The speciation distribution of the LCD model show that the Pb(II) are adsorbed to goethite by the G-Pb, G-HS-Pb and G-Pb-HS. At 5 mmol/L Pb the G-Pb-HS is the primary species for Pb(II) adsorption and at 1/0.5 mmol/L the G-Pb become more and more important.The electrostatic potential at the goethite/H2 O interface decreased with the increasing p H and HS concentration.