Basic Research On Desorption Of [PdCl₄]^2- Loaded On Quaternary Ammonium Rice Straw Lignin And Reduction Of Pd（Ⅱ）

At present,palladium is widely used in the fields of Industrial catalysis,electronic products,nuclear energy and automobile manufacturing because of the excellent physical and chemical properties.With the wider field of application,the demand is expected to increase.The traditional processes to extract palladium from palladium sources include pyrometallurgy and hydrometallurgy and ion exchange.The pyrometallurgy has the advantages of wide adaptability of raw materials and simple equipment,but it has the disadvantages of high energy consumption,serious pollution and low recovery rate of palladium as well.The process of hydrometallurgy is of the higher recovery rate of palladium than that of pyrometallurgy,but the process is long and the pollution is serious.Moreover,the wastewater is difficult to be treated and it is difficult to separated palladium from the complex ion system.The process of ion exchange method has the advantages of high recovery rate of gold and short process,but ion exchange resin is expensive and pernicious when being burned because of the tar and benzene.Therefore,achieving the extraction metallurgy to be efficient,energy-saving and environmental friendly has been the most urgent problem for the present.The quaternary ammonium rice straw lignin has the obvious advantages of reducing cost,improving metal recovery and reducing pollution.But,there are some problems such as the quaternary ammonium modification of rice straw lignin,the adsorption performance of quaternary ammonium rice straw lignin on[PdCl₄]^2-and the process of desorption,the species and its distribution characteristics,the desorption behavior and the reduction of Pd（II）after desorption of the[PdCl₄]^2-loaded quaternary ammonium rice straw lignin.According those problems,the lignin was extracted by sulfuric acid method and modified by quaternary ammonium based on rice straw.After that,the quaternary ammonium rice straw lignin was used to adsorb[PdCl₄]^2-.The mechanism and behavior of desorption of[PdCl₄]^2-that loaded on quaternary ammonium rice straw lignin were studied.In addition,the reduction of Pd（II）in the desorption solution was studied as well and the following conclusions were obtained.（1）The lignin was extracted by sulfuric acid method and then quaternary ammonium rice straw lignin was prepared after phenolization,crosslinking,chlorination and amination based on rice straw.The samples were characterized by sanning electron microscope（SEM）,fourier transform infrared spectroscopy（FT-IR）,thermogravimetric analysis and differential scanning calorimetry（TG-DSC）.The results showed that the quaternary ammonium salt bending characteristic peak appeared at 1 469 cm^-1,indicated that the grafting reaction of 3-Chloro-2-hydroxypropyltrimethylammonium chloride onto lignin,the quaternary ammonium straw lignin was produced.The synthesis of quaternary ammonium straw lignin was porous,rough and irregular and it had good thermal stability below 200℃.（2）The effects of the value of pH,the initial concentration of[PdCl₄]^2-,the adsorption time on the adsorption properties of quaternary ammonium rice straw lignin were investigated.Furthermore,the selectivity for[PdCl₄]^2-was also discussed.The results showed that the adsorption process could be described by pseudo-second order kinetic equation and adsorption isotherm could be simulated by Freundlich equilibrium adsorption equation.Moreover,the saturated adsorption capacity was 1.120 mmol·g^-1,the optimum value of pH for adsorption was 2 and the time of adsorption equilibrium was 8 h.In addition,the quaternary ammonium rice straw lignin was of excellent selectivity for[PdCl₄]^2-when the concentration of hydrochloric acid was less than 1mol·L^-1.（3）The process of desorption and thermodynamic analysis showed that the[PdCl₃SC（NH₂）₂]^-was the mainly form of desorption product.The process of desorption was that[PdCl₄]^2-was exchanged by Cl^-through ion-exchange at first,and then the palladium-thiourea complexes were abtained through[PdCl₄]^2-reacts with thiourea.（4）The quaternary ammonium rice straw lignin loaded[PdCl₄]^2-,which could be effectively desorbed by a mixture of hydrochloric acid and thiourea.The optimal desorption rate was 96.26%when the concentration of hydrochloric acid was 1 mol·L^-1,the concentration of thiourea was 0.6 mol·L^-1,the time of desorption was 70 min and the liquid-solid ratio was 2.With the increasing of"adsorption-desorption"regeneration cycle,the adsorption capacity of quaternary ammonium rice straw lignin decreased gradually,and the optimal cycle times were 4.According to the analysis of FTIR,the quaternary ammonium rice straw lignin was regenerated after desorption.（5）The results showed that iron/zinc powder were able to reductive palladium,and zinc powder had stronger reducing power.To improve the reduction effect of iron powder,iron powder reduction assisted Fenton oxidization technic was used to deal with desorption solution.The results showed that the Fenton reagent could destroy the palladium thiourea complex well and liberalize the palladium,the reduction rate of iron powder was improved effectively,the optimal reduction rate was 99.30%when the value of pH was 2,the time of reduction was 1 h,the dosage of H₂O₂ was 1 mL per 10mL desorption solution and the amount of iron powder was 10 mg per 20 mL desorption solution.This study is of great significance to extract palladium economically,efficiently and friendly.