Abstract
Opaline mudstone (OM) composed of opal-CT (SiO2·nH2O) has high potential use as a cesium (Cs) adsorbent, due to its high specific surface area (SSA). The objective of this study was to investigate the Cs adsorption capacity of chemically activated OM and the adsorption mechanism based on its physico-chemical properties. We used acid- and base-activation methods for the surface modification of OM. Both acid- and base- activations highly increased the specific surface area (SSA) of OM, however, the base-activation decreased the zeta potential value more (− 16.67 mV), compared to the effects of acid-activation (− 6.60 mV) or non-activation method (− 6.66 mV). Base-activated OM showed higher Cs adsorption capacity (32.14 mg/g) than the others (acid: 12.22 mg/g, non: 15.47 mg/g). These results indicate that base-activation generates pH-dependent negative charge, which facilitates Cs adsorption via electrostatic attraction. In terms of the dynamic atomic behavior, Cs cation adsorbed on the OM mainly exist in the form of inner-sphere complexes (IS) containing minor amounts of water molecules. Consequently, the OM can be used as an effective Cs adsorbent via base-activation as an economical and simple modification method.
Highlights
The removal of radioactive Cs (137Cs) from the Cs-containing wastewater is an important environmental issue, following the nuclear power plant accident at Fukushima Daiichi in Japan. 137Cs has a half-life of about 30.2 years with high solubility and mobility rates in water, and due to its chemical properties being similar to those of potassium (K), it can accumulate in plants and animals, increasing the health risk to h umans[1,2,3]
The X-ray diffraction (XRD) results show that Opaline mudstone (OM) is mostly composed of opal-CT with a minor component of quartz and feldspar (Fig. 1a, and Fig. S1 of the SI)
Based on the Langmuir model, the acid-activation of OM slightly decreased the adsorption capacity from (15.47 to 12.22) mg/g, while the base-activation of OM largely increased the qm value from (15.47 to 32.14) mg/g. These results indicate that base-activation further enhanced the Cs adsorption capacity of OM more as compared to acid-activation, and it corresponded to the TEM–EDS results
Summary
The removal of radioactive Cs (137Cs) from the Cs-containing wastewater is an important environmental issue, following the nuclear power plant accident at Fukushima Daiichi in Japan. 137Cs has a half-life of about 30.2 years with high solubility and mobility rates in water, and due to its chemical properties being similar to those of potassium (K), it can accumulate in plants and animals, increasing the health risk to h umans[1,2,3]. Zeolites as aluminosilicates consist of three-dimensional frameworks, which give them the microporous structures, in addition to the negative charges induced by isomorphic substitution in the mineral s tructure[7,8,9,10] These properties of clay minerals and zeolites facilitate cationic adsorption, and various studies have investigated Cs adsorbents by focusing on the two mineral groups. The experiment results of this study showed that the FES of the acid-activated biotite was more effective for Cs adsorption than the pH-dependent negative charge of base-activated illite. Other studies[24,26] reported that the diatomite was used to remove the radioactive Cs. the Cs adsorption capacity of diatomite was lower than that of other geological materials such as clay minerals and z eolite[26], diatomite has the potential to be an effective Cs adsorbent due to the porosity and low-cost
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