Immobilization of 60Co and 90Sr ions using red mud from aluminum industry

Abstract

The removal of 60Co and 90Sr from the aqueous phase was tested using red mud - the fine grained residue from bauxite ore processing. This industrial waste represents a mixture of numerous minerals, mainly oxides and hydroxides of Fe, Al, Si, and Ti. Experiments were conducted as a function of contact time, pH, and pollutant concentrations. Kinetic data were well fitted with a pseudo-second order equation. The calculated rate constants and initial sorption rates indicated faster sorption of Sr2+ ions. Removal of both cations rapidly increased with the initial pH increase from 2.5 to 3.5. With the further increase of pH, Co2+ sorption was nearly constant (98%-100%), whereas Sr2+ removal remained at the same level to initial pH ~8 and gradually increased to 100% at pH 12. Equilibrium sorption data followed the Langmuir model, with the maximum sorption capacities of 0.52 mmol/g for Co2+ and 0.31 mmol/g for Sr2+. Sorbed cations exhibited high stability in distilled water. Desorption of Co2+ was also negligible in the presence of the competing Ca2+ cation, while 42%-25% of Sr2+ ions were desorbed depending on the previously sorbed amount. The results indicate that red mud is of potential significance as Co2+ and Sr2+ immobilization agent due to its high efficiency, abundance, and low-cost. [Projekat Ministarstva nauke Republike Srbije, br. 43009]

Effect of experimental variables onto Co2+and Sr2+sorption behavior in red mud-water suspensions

“…[22] The calculated constants K L were also higher for raw sample, signifying higher surface affinity towards Co 2C and Sr 2C ions. The difference was particularly pronounced for Sr 2C ions.…”

“…Blank experiments with raw red mud sample from the same deposit showed wider buffering pH range (»3.5 to »10), and considerably higher pH at the plateau (9.5) due to the residual NaOH from the Bayer process. [22] The "liming" effect of red mud was attributed to several constituents: free caustic, sodalite and calcite. [36] Free caustic was washed out from RBRM sample; thus, the major alkaline anions are identified as OH ¡ , CO 3 2¡ /HCO 3 ¡ and Al(OH) 4 ¡ /Al (OH) 3 (aq), which originated from sodalite and calcite dissolution according to the reactions: [32] …”

“…Also, sorption of radionuclides from aqueous media using red mud was explored in several studies. [18][19][20][21][22][23] Red mud is a waste generated during alumina production from bauxite ore by the Bayer process. It is characterized by high alkalinity (average pH from 10 to 13) [24] and heterogeneous composition (hematite (Fe 2 O 3 ), goethite (a-FeOOH), boehmite (g-AlOOH), gibbsite (Al(OH) 3 ), anatase and rutile (TiO 2 ), quartz (SiO 2 ), sodalite (Na 4 Al 3 Si 3 O 12 Cl) or cancrinite-type sodium aluminum silicate (CAN), gypsum (CaSO 4 £ 2H 2 O), calcite (CaCO 3 ), whewellite (CaC 2 O 4 £ H 2 O), etc.)…”

“…Previously, the removal of Co 2C and Sr 2C from aqueous phase using raw red mud from "Bira c" factory deposit (Bosnia and Herzegovina) was preliminarily tested, with promising results. [22] 60 Co and 90 Sr are significant radionuclides present in the LRW. [27,28] Although they have a different origin in nuclear reactors, they contribute considerably to the overall radiation level.…”

“…Due to elevated alkalinity, significant amounts of Co 2C were precipitated in the form of insoluble hydroxide. [22] In such systems, contribution of red mud mineral matrix and the mechanisms of cation sorption by various solids remain largely unknown. High alkalinity and high sodicity, coming from free NaOH, [25] may also pose an impediment to some potential red mud applications, such as soil remediation processes or construction of barriers around radioactive waste disposal sites.…”