Lead Adsorption On Clay:CD-Music Modeling

Clay mineral is one of the most important sorbent for metal cation because of the high contents in many soils and sediments and large surface areas which contain both positive and negatively charged surface sites. Lead, one of the most toxic heavy metals, can lead to serious health disorders. Lead pollution has become a global environmental problem. In this study, kaolinite and montmorillonite, two representative clay minerals, are adsorbents. First, we study the surface charge characteristics of two kinds of mineral with the acid-base titration experiments. Then, we study the factors (different pH, ionic strength, lead concentration) affecting adsorption of lead on kaolinite and montmorillonite. And fit the experimental data with the CD-MUISC model, to obtain the morphology of adsorbed Pb on mineral surface. Main results are below:□Test kaolinite, montmorillonite, high purity, without other impurities, showed a sheet, and the latter sheet larger than the former. ATR-FTIR analysis showed that, kaolinite, montmorillonite contains abundant aluminum hydroxyl (Al2O-H), and silicone-based (Si-O-Si), both the structure and functional groups having a similar. Determination of specific surface area found that the specific surface area of kaolinite is23.4±0.6m2/g, the external surface area of montmorillonite is83.8±0.8m2/g. Total surface area of montmorillonite measured by glycol ether method (EGME)is420±20m2/g.□The acid-base titration results of kaolinite and montmorillonite at different pH and ionic strength, showed that the proton adsorption on mineral surface increased with the decrease of pH, decreased with the increase of ionic strength. In pH range of the experiment, the three titration curves did not intersect, which is different from the surface of the oxide mineral. This feature of clay minerals is the result of the permanent charge of base surface and the variable charge of the edge surface.□The adsorption isotherm experiment of lead adsorbed on clay mineral surface, at different pH, ionic strength, lead concentration, showed that pH significantly affect the adsorption of Pb on mineral surfaces. With the increase of pH, kaolinite, montmorillonite increased adsorption of Pb. With the decrease of ionic strength, kaolinite, montmorillonite increased adsorption of Pb. The effect of ionic strength decreased with the increase of pH.2:1montmorillonite which has large surface area and many permanent charges, adsorped a greater amount of Pb than1:1kaolinite.? By fitting the acid-base titration data of clay mineral surface with CD-MUSIC model, we can get the density of the mineral surface reaction sites, the proton binding constants, binding constants of salt ions. For kaolinite surface, dSOH=11.25site/nm2, dx-=-7.07site/nm2, proton binding constants, log K SOH20.5+=-2.31, log KX-H=3.67, salt ions binding constants, log K SOH-K05+=-7.29, log K SOH2-NO305-=-6.11, log K X-K=-6.74; For montmorillonite surface, dSOH=1.2site/nm2, dXK=1815.94site/nm2, proton binding constants, log K SOH20.5+=7.39, log K XH=4.86, salt ions binding constants, log K SOH-K05+=-0.41, log K SOH2-NO30.5-=7.27. By fitting lead adsorption data with the parameters obtained from acid-base titration data, we can get the morphology and the binding constant of lead adsorbed on mineral surface. In kaolinite surface, log K (SOH)2PbOH=-4.9, log K X2-Pb=-7. In montmorillonite surface, log K (SOH)2PbOH=0.49, log K X2Pb=1.76. At edge surfaces, they were hydroxylated bidentate mononuclear inner complex, charge distribution factor, f=0.6. At base surfaces they were the bidentate outer complexes.