Synthesis of fly ash based composites and investigation of strontium sorption behavior

In the present study, the composite adsorbents based on fly ash were synthesized and the effectiveness in the removal of strontium from liquid radioactive waste was investigated. Three adsorbents, un-doped modified fly ash (FA), 10 w% zirconium-doped (FA-Zr) and 10 w% titanium-doped (FA-Ti), were synthesized by hydrothermal treatment. NaOH solution used as activation agent, synthesis were carried out at 120 ºC with stirring speed 500 rpm for 24 hours. Fly ash and synthesized adsorbents were characterized by XRF, XRD, SEM-EDX, DTA/TGA and BET surface area analysis. The results show that the adsorbents are containing glassy phases and crystalline phases. Also, surface area is increasing with zirconium additives. Surface areas of synthesized adsorbents were found as; BETFA: 61.17 m2/g, BETFA-Zr: 87.34 m2/g and BETFA-Ti: 62.35 m2/g. The sorption behavior of adsorbents for Sr2+ ions were examined in dynamic conditions with column technique. The effects of pH, initial Sr2+ concentration, flow rate and bed height parameters were investigated via experimental design approach with central composite design (CCD). Surface contour plots were drawn by obtained results. The ANOVA (variance analysis) of the model at 95 % confidence level was examined. The predicted and experimental values are well suited. The values of F, P>F, Lack of fit F, R2, adjusted R2 and predicted R2 show that the obtained models for all adsorbents can be used to navigate the model spaces. The breakthrough curves were obtained with experiments which were carried out at optimum conditions for strontium adsorptions of adsorbents and breakthrough times and capacities, saturation times and capacities and column efficiencies were calculated. The breakthrough time is extended and efficiency of column is higher for FA-Zr. Column performances were modeled via Thomas, Yoon-Nelson and Bohart-Adams approaches. According to results, the sorption behavior of FA and FA-Zr columns can be explained with Thomas model while FA-Ti columns can be explained with Bohart-Adams model. The selectivities of adsorbents were determined in Sr2+, Ca2+ and Ba2+ multi-component system. The selectivity order of all adsorbents is as follows; Ba2+>Sr2+>Ca2+ which indicate that ion exchange is the dominant in sorption mechanisms. Keywords: Fly ash, hydrothermal synthesis, strontium, adsorption, experimental design, column, dynamic capacity