Effects Of Bloom-forming Cyanobacterial Extracellular Polymeric Substances On The Adsorption Of Cd(Ⅱ) And Cu(Ⅱ) Onto Kaolinite And Its Mechanisms

Cyanobacterial blooms result in high level of cyanobacterial extracellular polymeric substances（EPS）in water bodies.The effects of bloom-forming cyanobacterial EPS on the distribution of cadmium and copper in the interface between sediment and water are unknown.Clay mineral is the main component of sediment.The effects of EPS,originated from a typical bloom-forming cyanobacterium Microcystis aeruginosa,on the adsorption and desorption characteristics of Cd（Ⅱ）and Cu（Ⅱ）by kaolinite were investigated in this study.The main contents were presented as follows:（1）The composite of kaolinite+EPS showed higher adsorption capacity toward Cd（Ⅱ）and Cu（Ⅱ）than pure kaolinite.The amount of adsorbed Cd（Ⅱ）increased 36.85% when 0.6 g/L of EPS was existed in the composite.The amount of adsorbed Cu（Ⅱ）was 4.5 times of that of pure kaolinite with the addition level of 0.3 g/L EPS.Hydroxyl groups were involved in the adsorption processes according to the FTIR spectrum.The results of XRD determination showed that the diffraction pattern of kaolinite was not modified by cyanobacterial EPS or Cd（Ⅱ）or Cu（Ⅱ）,indicating the adsorption process is mainly based on surface adsorption.The adsorption data of kaolinite fit Langmuir model well with the maximum sorption capacities of 18.42 mg/g for Cd（Ⅱ）（R²=0.976）and 6.536 mg/g for Cu（Ⅱ）（R²=0.998）,respectively,which suggesting the adsorption of Cd（Ⅱ）and Cu（Ⅱ）onto kaolinite is monolayer soption.Whereas,only Freundlich model well describes the adsorption data of Cd（Ⅱ）or Cu（Ⅱ）by the composite of kaolinite + EPS which belongs to multilayer adsorption.The values of n calculated from adsorption data were 1.31 and 1.828 for Cd（Ⅱ）and Cu（Ⅱ）,and the R square were 0.973 and 0.968,respectively.The adsorption of Cd（Ⅱ）and Cu（Ⅱ）by kaolinite,kaolinite + EPS composite is in accordance with the pseduo-second order kinetic equation.The adsorption of Cd（Ⅱ）reached equilibrium within the first 5 min.The adsorption of Cu（Ⅱ）reached equilibrium within 30 min.The adsorption Cd（Ⅱ）and Cu（Ⅱ）by kaolinite were exothermic process,whereas an oppiste result was observed when EPS existed in the composite.In the p H range of 5 to 9,Cd（Ⅱ）and Cu（Ⅱ）adsorption to kaolinite and the composite of kaolinite + EPS increased with the increase of pH.（2）In order to further elucidate the effects of bloom-forming cyanobacterial EPS on the adsorption behavior and adsorptional mechanism of cadmium and copper in the interface between sediment and water,the desoption characteristics of absorbed Cd（Ⅱ）and Cu（Ⅱ）from kaolinite and the composite of kaolinite + EPS were investigated.The results showed that the desorption rate of EDTA was the highest,followed by NH₄NO₃,indicating that surface complexation and ion exchange were the main adsorption mechanism.The desorption kinetics of Cd（Ⅱ）and Cu（Ⅱ）in deionized water,NH₄NO₃ and EDTA reached equilibrium at about 30 min.The desorption rates of Cd（Ⅱ）and Cu（Ⅱ）decreased with the increase of p H.The order of desorption rate of Cd（Ⅱ）and Cu（Ⅱ）for different cations was Ca²⁺ > Mg²⁺ > NH⁴⁺ > K⁺ > Na⁺.In the desorption experiments of different anions,the desorption rates of Cd（Ⅱ）and Cu（Ⅱ）were in the order of SO₄^2-> Cl^-> HCO₃^2-> CO₃^2-.