Study On Development And Application Of Novel Adsorbents For Decontamination Of Radioactive Strontium And Cobalt From Aqueous Solutions

As nuclear industry develops rapidly these days,the risk of water polluted by radioactive nuclides is increasing.Among various radioactive nuclides,radioactive strontium(⁹⁰Sr)is one of the fission products and radioactive cobalt(⁶⁰Co)is a major nuclide of activated corrosion products.Both of the nuclides pose a great threat to public health because of their long half-life,strong mobility in solutions,and high biological toxicities.Hence,it is necessary and urgent to study the removal of ⁹⁰Sr and⁶⁰Co from aqueous solutions.Adsorption is a simple,safe,and efficient method,and it is a high demand to develop efficient adsorbents and investigate their applications in actual water matrixes.In this study,novel metal sulfide adsorbents were synthesized and a new adsorption process was developed for the application of the powdered adsorbents in Sr²⁺and Co²⁺removal.The aim of this study was to provide new adsorbent materials,knowledges,and approaches for radioactive wastewater treatment.The main contexts and results are as follows.(1)Three metal sulfide adsorbents,i.e.,K/Zn/Sn/S metal sulfide with mixed morphology(KZTS),K/Zn/Sn/S metal sulfide nanosheet(KZTS-NS),and Na/Zn/Sn/S metal sulfide(NaZTS),were synthesized using hydrothermal methods.The morphology and structure of KZTS-NS could be controlled by adjusting the reaction time and the volume of H₂O added during the synthesis process.The structures of these three adsorbents were similar;they were consisted of anionic frameworks and cationic ions located in the frameworks.The soft-base S^2-ligands in the frameworks endowed the adsorbents with high affinity toward soft-acid Sr²⁺and Co²⁺,and the cationic ions(i.e.,K⁺and Na⁺)were ion exchangeable.(2)The results of batch experiments showed that KZTS,KZTS-NS,and NaZTS exhibited remarkable adsorption performance for Sr²⁺,i.e.,rapid kinetics(equilibrium time of 1–10 min),wide active p H ranges(3–11),strong selectivity for Sr²⁺against other competing ions,and negligible Sr²⁺desorption.The isotherms could be well fitted using Langmuir model,and the maximum adsorption capacities of KZTS,KZTS-NS,and NaZTS at 25?were 19.3,55.7,and 32.3 mg/g,respectively.In addition,the adsorption mechanisms were all ion exchange between Sr²⁺and K⁺/Na⁺.Among KZTS,KZTS-NS,and NaZTS,KZTS-NS exhibited the best adsorptive performance without secondary pollution,and thus was considered as an effective and environmentally friendly material.(3)KZTS-NS showed excellent Co²⁺adsorption performance.The kinetics was fast with an equilibrium time of 40 min.The distribution coefficients of KZTS-NS for Co²⁺were much higher than other previously reported adsorbents in the p H range of2–8.Freundlich model was a better fit to the isotherms than Langmuir one,which was different from that of Sr²⁺adsorption,and the maximum experimental adsorption amount was 82.2 mg/g.KZTS-NS could simultaneously adsorb Co²⁺,Mn²⁺,and Ni²⁺,whereas showed much higher selectivity for Co²⁺.The adsorption mechanisms included not only ion exchange between Co²⁺and K⁺,but also surface adsorption by forming Zn_0.76Co_0.24S composite.(4)The practical application of powdered adsorbents was limited because powdered adsorbents were hardly separated from aqueous solutions,and the adsorption performance was significantly interfered by the competing ions in actual water matrixes.To overcome these limitations,a novel two-stage countercurrent adsorption-flocculation-microfiltration process was developed,in which KZTS-NS was the adsorbent and cationic polyacrylamide was the flocculant.The process could effectively removal Sr²⁺and Co²⁺from tap water,which was used for simulating polluted surface water,achieving decontamination factors of 1405 and 1176 for Sr²⁺and Co²⁺,respectively.The reasons of the excellent decontamination performance of the process included that(i)the selectivity of the adsorbent for Sr²⁺and Co²⁺was strong;(ii)the adsorption capacity of the adsorbent could be fully used by the arrangement of the process;and(iii)the interference of competing ions were significantly reduced in the process.