Performance of phosphoric acid activated montmorillonite as buffer materials for radioactive waste repository

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Abstract

In this study, the performance of phosphoric acid activated montmorillonite (PAmmt) was evaluated by cesium ions adsorption experiments. The PAmmt samples were obtained by activating with 1, 3 and 5 mol L−1 of phosphoric acid, respectively under reflux for 3, 12, and 24 h. Experimental results demonstrated that the treatment of raw K-10 montmorillonite with phosphoric acid increased the materials’ affinity for Cs uptake and no significant amount of suspension solids were produced. A relatively insignificant variation in the CEC value was observed. Furthermore, PAmmt also showed high adsorption selectivity toward Cs ions. The improved sorptive properties were mainly related to the increased surface area and the relatively higher surface charge density. Increased specific surface area was the resulted from partial decomposition of lamellar structure of mmt; while the higher surface charge density was caused by the protonation of octahedral Al–OH sites during the acid activation. Generally speaking, stronger acid concentration and longer activation times would produce relatively more decomposed PAmmt particles. However, as the activation exceeds 3 h, the precipitation of Si4+ would passivate PAmmt against further acid attacks. Based upon our results, acid activation by phosphoric acid could produce PAmmt samples with high sorption capacity and selectivity, and good structural integrity, which are beneficial to be used at radioactive waste repository.

Introduction

Activation by either chemical (acid/alkaline) or physical (thermal) treatment is widely applied to clay minerals such as montmorillonite (mmt) for preparation of functional materials such as sorbent toward environmental contaminants or radionuclides of concern. The thermal treatment, also known as the calcination, would evaporate lattice water molecules, which further induce the reorganization of the clay structure. For example, after calcinations under 450–600 °C, kaolinite will lose its lattice water and reorganize its Al–O network and is transformed into metakaolinite with highly disordered structure [1]. From the perspective of chemical activation, both acid and alkaline treatments are conducted to prepare high sorption capacity as well as affinity sorbents [2]. The advantage of using chemical activation lies in that it can be carried out under mild conditions, minimizing further separation and regeneration processes. During acidification, the H+ attack would induce the leaching of mineral impurities, the disaggregation of clay particles, and the dissolution of the external layers [3]. Also, the H+ attack is accompanied with the replacement of exchangeable cations that are originally adsorbed on the clay surface to compensate the negatively charged clay surface. As a result, this replacement produces certain number of acid centers and thus the catalytic property of acid activated clay particles is much different from that of the original clay minerals [4]. For example, the catalytic property of montmorillonite could be greatly altered by replacing the internal Al3+ or Fe2+ with H+ during acidification [5]. On the other hand, the surface areas of acid activated clay particles are greatly increased due to the decomposition of lamellar structure of montmorillonite [3]. During decomposition stage, octahedral aluminum ions are preferentially released from the clay structure, causing the formation of additional Al–OH and Si–OH vibration bonds without much affecting the original mineral structure [6]. With these additional hydroxyl groups the sorption capacity of these activated clay minerals is quietly promoted toward some environmentally hazardous contaminants such as Pb(II) [7], Ni(II), Cu(II) ions [8]. The removal of printing and dyeing waste in the wastewater treatment by using acid activated clay minerals is also reported in literature [9].

It is widely accepted that the surface charges, surface areas, and the structural integrity of clays would be significantly modified during acid treatment [5], but the extent of such alteration would be strongly correlated to the entity of acid solutions that was used. For example, it is reported that all the silicon and aluminum atoms are removed from the chemical structure of the clay lattice when using HF, HSO3Cl, or H2SO4 solutions whereas only a mild digestion, which increases the laminar distance of the montmorillonite, is observed by using HCl solutions [10]. The magnitude of disorders in lamellar structure by acid activation as a consequence increases the surface area and greatly influences the integrity of acid activated clay particles. The latter entity is the fundamental requirement for hazardous waste repository, especially for the radioactive waste repository. In this case, clay minerals (especially the montmorillonite) are usually applied as buffer material to tightly adsorb released radionuclides, reducing their threat to the biosphere. Due to strong affinity between these clay minerals and radionuclides, these adsorbed radionuclides are generally regarded as immobile. In most cases, the remobilization is not the result of desorption but rather mainly stems from the so-called colloid mediated transport. This is undertaken by the ablation of disruptive fraction on surface of clay particles into aquifer solutions, producing the observed suspension colloids. With suspension colloids, migration of these radionuclides, which adsorbed tightly on suspension colloids, becomes unpredictable, depending upon the hydrological environments of groundwater flow [11], [12]. That is, the migration of radionuclides would be far beyond the expected distance when considering only the adsorption between radionuclides and clay minerals.

Although many works have devoted to develop high affinity adsorbent for removal or remediation of hazardous wastes, few studies focused on the integrity of acid activated clay particles. As mentioned above, the integrity of these activated clay particles is important to reduce the probability of colloid mediated transport. Accordingly, phosphoric acid, which exhibits only mild acidity, is applied in this study to activate montmorillonite particles. Also, surface adsorbed phosphate anions, which behave as soft bases compared to sulfate and fluoride anions, are expected to have stronger affinity toward some soft cations such as the transuranium ones. The performance of these phosphoric acid activated montmorillonite (PAmmt) is examined by cesium sorption isotherms, while the integrity of PAmmt samples is evaluated by observing the amount of suspension solid produced during these sorption experiments.

Section snippets

Solid and chemicals

The montmorillonite clay (mmt) used here was purchased from Aldrich (cat. 281522) and was used without any further purification. The surface area of montmorillonite clays is 243 m2/g (N2-BET) and its cation exchangeable capacity (CEC) is 125 meq/100 g. The idealized formula of mmt is Na0.2Ca0.1Al2Si4O10(OH)2 (H2O)10. All chemical regents including phosphoric acid solutions are of analytical grade purchased from Aldrich. The specific surface area of raw (mmt) and phosphoric acid activated

Cs sorption studies

It must be emphasized that all the sorption experiments were conducted under the neutral (pH 7.2–7.5) conditions since the Cs sorption behavior is strongly affected by the environmental pH. Fig. 1 shows the amount of suspension solid produced in the sorption experiments with 5 M of phosphoric acid activated mmt samples for various Cs concentrations. The increased amount of suspension solid indicates that these PAmmt particles will be unsuitable for use in the radioactive waste repository because

Conclusion

In this study, montmorillonite K-10 (mmt) was subjected to acid treatment by using phosphoric acid to obtain phosphoric acid activated mmt (PAmmt) samples. The performance of PAmmt samples was evaluated by Cs isotherm experiments. Out results showed that very limited suspension solids were produced during Cs sorption experiments. Also, PAmmt samples possess a high sorption capacity together with high sorption selectivity. These promising properties demonstrate their prospective application to

Acknowledgement

The authors are grateful to National Science Council of the Republic of China (NSC 97-2221-E-007-006-MY3) for supporting this study.

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