Preparation Of Pectin-based Microsphere And Its Adsorption Properties For Thorium And Cesium

Radioactive waste from radionuclides and nuclear reactors can have adverse effects on the environment and human health.After the accident at the Fukushima nuclear power plant in Japan in 2011,a large number of low-level waste liquid was released into the sea.Therefore,it is necessary to remove thorium ion and cesium ion from wastewater.Adsorption is widely used due to its advantages such as simple process,safety and high efficiency,and the choice of adsorbent is very important for the application of this method.Therefore,the selection of an adsorbent that is difficult to decompose in a wide p H range,has high selectivity,remains stable at high temperatures,and resists ionizing radiation becomes the key to the treatment of Th⁴⁺and Cs⁺in radioactive waste water.Two pectin-based sorbents were prepared by gel-sol method and embedding method.They were pectin-PDA microsphere and pectin/AMP-Zn microsphere adsorbents.The adsorption properties of the two adsorbents for Th???and Cs?I?mononuclear nuclides were discussed in detail.The adsorption mechanism of the two adsorbents and their resistance to irradiation,regeneration and post-treatment were investigated.The conclusions are as follows:?1?Pectin-Ca microspheres were prepared by sol-gel method with Ca²⁺as cross-linking agent,and then pectin-PDA microsphere adsorbent was modified by dopamine-modified pectin microspheres.The adsorption performance of the adsorbent for removing Th⁴⁺and the recycle regeneration and post-treatment of pectin-PDA adsorbent were discussed.The adsorption mechanism was analyzed by means of SEM,FTIR and XPS.The results showed that the pectin-PDA microspheres had the best effect at p H=3.5.The contact time was 1200 min at25?,p H=3.5,initial Th⁴⁺concentration of 20 mg/L,and adsorbent dosage of0.03 g.The coexisting ions Cs⁺,Sr²⁺,Mn²⁺and Mg²⁺had little effect on the adsorption of Th⁴⁺,indicating that the adsorbent had a good adsorption selectivity.Thermodynamic and kinetic studies showed that this adsorption process conforms to the Langmuir isotherm adsorption model and pseudo-second-order kinetic model.The maximum adsorption capacity of the adsorbent was 99.010 mg/g.Thermodynamic data showed that the adsorption of Th⁴⁺by pectin-PDA was a spontaneous endothermic process.After five cycles of pectin-PDA adsorbent,the desorption rate was still above 74%.After the inactivated adsorbent was incinerated,the burning loss rate was 92.51%.It could be observed by TG analysis that the thermal stability of pectin-PDA was improved compared to that before modification.The mechanism of adsorption of Th⁴⁺on pectin-PDA microspheres was the reaction of hydroxyl and amino groups on pectin-PDA with Th⁴⁺in solution.?2?The pectin/AMP-Zn microsphere was prepared by embedding ammonium phosphomolybdate?AMP?in pectin by electrostatic-injecting device using gel-sol method and Zn²⁺as cross-linking agent,which was used in the removal of Cs⁺in radioactive waste water.The adsorption properties of pectin/AMP-Zn removal Cs⁺and the cyclic regeneration and post-treatment of the adsorbent were discussed.The adsorption mechanism was analyzed by SEM,FTIR and XPS.The results showed that pectin/AMP-Zn can be used in the range of p H=3¹1,with good acid and alkali resistance.At 25?,p H=5,initial Cs⁺concentration of 120 mg/L,and adsorbent dosage of 0.05 g and contact time of330 min,the adsorption capacity of the adsorbent was 41.837 mg/g.The coexisting ions K⁺,Na⁺,Li⁺,Ca²⁺and Mg²⁺had little effect on the adsorption of Cs⁺,indicating that the adsorbent had better adsorption selectivity.The results of thermodynamics and kinetics showed that the adsorption process was in accordance with the Freundlich isothermal adsorption model and the pseudo-second-order kinetic model,and was a spontaneous exothermic process.After 5 cycles of pectin/AMP-Zn adsorbent,the desorption rate was 57%.After the inactivated adsorbent was incinerated,the burning loss rate was 86%.In addition,the adsorption mechanism of Cs⁺on pectin/AMP-Zn was ion exchange between Cs⁺and NH₄⁺on AMP.?3?The effects of column height,flow rate and concentration of Cs⁺on the dynamic adsorption of pectin/AMP-Zn microspheres were discussed.The results showed that the penetration and depletion points were delayed with the increase of the column height,and the breakthrough and depletion points were shortened with increasing concentration of Cs⁺and flow rate.