Attapulgite as an eco-friendly adsorbent in the treatment of real radioactive wastewater

Radioactive wastewater is one of the riskiest pollutants generated by energy stations, medical programs, and diverse extractive industries worldwide (Paranhos Gazineu et al. 2005). Effective treatments need to be cost-effective and safely reduce the volume of aqueous waste (International Atomic Energy Agency 2002; Cherif et al. 2017) containing long-lived beta/gamma activity stored in large tanks under nuclear sites. Cs-137 has a radioactive half-life of about 30 years and very high solubility in liquid systems, and incorporates into both the soil environment and aquatic organisms (Al-Alawy & Mzher 2019; Ahmed 2022). Liquids contaminated with Cs-137 are a potential environmental problem. High radioactivity aqueous wastes with long-lived radionuclides may be treated using different treatment technologies, including ion exchange/sorption, chemical precipitation, and/or evaporation, reverse osmosis, filtration, and solvent extraction (IAEA 1999). Many studies have found that adsorption is a good technique for removing radioactive materials from wastewater, with high activity and low operating cost (Alardhi et al. 2020; Kadhum et al. 2021; Ali et al. 2022a). The best media in the treatment of industrial wastewater were clay minerals whose features make them optimal adsorbents due to their low production cost, ready availability, non-toxic nature, high specific surfaces, excellent adsorption properties, and great potential for ion exchange (Al-Ani & Sarapää; 2008). Clay minerals adsorb cesium to balance the negative charge on the aluminosilicate structure caused by the counter-ions (e.g., Na, Ca, Mg, or K) as adsorption sites on the clay sheet surface, interlayers between sheets, and broken bonds at the edges of clay crystals (Wilson 2007; Yuan et al. 2013). Attapulgite is the rock name of palygorskite, a hydrated Mg–Al silicate material that has a 2:1 inverted structure, i.e., the apices of the silica tetrahedrons are regularly inverted along the a-axis. This results in parallel channels throughout the particles, which give these minerals a high internal specific surface containing exchangeable cations and water (Stewart & Mollins 1996). Large cation exchange capacities (CECs) and high total uptake of cesium occur when the interlayer sites are available for adsorption, as has been recorded in the cases of montmorillonite and palygorskite (Adebowale et al. 2006; Ohnuki & Kozai 2013; Okumura et al. 2013; Ali et al. 2022b). Several studies have inspected the mechanism of cesium adsorption by ion exchange with different potential sites on mineral surfaces and studied the effect of the structural characteristics of these clay minerals (Cornell 1993; Park et al. 2019; Zabulonov et al. 2021). Other research examined the parameters that influence adsorption, adsorption isotherms, thermodynamics, and kinetics for many clay minerals (Sheha & Metwally 2007; Hadadi et al. 2009; Akalin et al. 2018; Semenkova et al. 2018; Muslim et al. 2022).

In this work, natural attapulgite clay minerals from the Western Desert of Iraq were selected as potential low-cost, readily available, environmentally friendly adsorbents adopted for use in a batch adsorption system. Attapulgite was implemented to treat real radioactive wastewater containing Cs-137 that has been accumulated since 1991 underneath the Al-Tuwaitha Nuclear Research Center near Baghdad, Iraq. The influence of various variables on the adsorption process was investigated along with its isotherms and kinetics.

Clays have characteristics that depend on their geological formation and mining location. Deposits of attapulgite occur in Wadi Bashira in Iraq's Western Desert. The representative sample was crushed in a jaw crusher (Retsch BB 1, Germany) and then milled in a rotating cylinder ball mill to pass a 75-μm sieve opening. Wet chemical analysis to identify the attapulgite's chemical composition was done in the Central Laboratories Department, Iraqi Geological Survey. X-ray diffraction (XRD) mineralogical analyses were performed using the Ital structure model MPD 3000 (Spain, Al Razi Metallurgical Center, Tehran, Iran). Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were employed to investigate clays' morphologies with the MIRA3 TESCAN instrument (Australia, Al Razi Metallurgical Center, Tehran, Iran). Particle size distribution analysis was done using a Brookhaven Instruments (USA) 90Plus particle size analyzer (Nanotechnology Center, UOT, Iraq). The minerals' specific surface area (SSA) and CECs were obtained from technical reports of the Iraqi Geological Survey (Baghdad, Iraq). Fourier-transform infrared (FT-IR) spectroscopy analyses were run with a Bomem MB-Series FT-IR Spectrometer (France) and operated according to ASTM E 1252-98(21) to specify the functional groups.

The radioactive wastewater samples were taken from a reservoir underneath the destroyed Radiochemical Laboratories (RCL) at the Al-Tuwaitha site (Iraq). The gamma spectroscopy analysis was conducted using a closed-end, coaxial, p-type model (GEM65P4-95/ORTEC (USA, Al-Tuwaitha site, Iraq) high purity germanium detector (HPGe), yielding high-level waste (HLW) containing radioactive cesium (Cs-137) with a specific activity of 4.5 GBq/L (Ibrahim et al. 2018). As per the appropriate safety procedure, the sample was diluted with distilled water to a safe limit to be handled within the laboratory. The activity was reduced to about 6,372 Bq/L, which is considered the initial activity concentration.

The attapulgite sample contains predominantly montmorillonite (smectite) associated with palygorskite as the main minerals, in addition to impurities like silica, calcite, and gypsum, as shown in Table 1 (Al-Ajeel et al. 2008). The montmorillonite is considered a Ca-montmorillonite on the basis of the ratio of (Na₂O + K₂O) to (CaO + MgO), which is approaching 0.136 (Abdou et al. 2013).

In Figure 1, the XRD analyses illuminate a convergence for attapulgite, in which the major peaks are palygorskite at 20° and 8° diffraction angles (2θ), and the minor mineral is montmorillonite with 2θ at 20.5° and 7°. In fact, the sample is a montmorillonite-rich, palygorskite clay. The results show clearly that the clays have not been subject to any purification or modification processes.

According to the particle size analysis, attapulgite exhibited small particle size. The surface areas were measured using the Brunauer, Emmett and Teller (BET) method. Attapulgite's specific surface and cation exchange capacity are given in Table 2.

The extent of Cs adsorption on montmorillonite and attapulgite depends on the minerals' ion exchange site types, which is characterized by the function and availability of the interlayer site type (III) that offers high CECs and cesium uptake. In attapulgite, the active sites are only in the planar (basal) surface (type I) and the edges of the interlayers (type II), which both show low CECs compared to type III. In kaolinite, the ion exchange capability is due to broken bonds at the edges of the clay plates and hydroxyl groups on the basal lamellar. The results indicate that Cs is adsorbed not only at the ‘frayed edge’ sites but also at other sites where the adsorption is reversible, as reported by Erten et al. (1988), Comans et al. (1991), Comans & Hockley (1992), and Shahwan et al. (1999). One is instantaneous and reversible on a timescale of a few days or less. The other is irreversible, occurs at longer times, and is caused by Cs migration into the interlayers. Slow Cs migration into interlayers was also proposed by Evans et al. (1983). These were in accord with the extent of cesium adsorption (desorption) by attapulgite after 2 h in the results as clarified in Figure 6(b), because some of the cesium sites are reversible on attapulgite's basal planes. The results of this study are agreement with the mechanism of adsorption (Ali et al. 2023; Khader et al. 2023).

The q_e predicted for attapulgite by the pseudo-first-order model approached the experimental q_e, as shown in Table 3. The intraparticle diffusion adsorption kinetic model is based on the assumption that the rate-controlling step may involve valence forces through ion exchange, substitution, or complexation (Wei et al. 2019; Al-Rahmani et al. 2020).

Since the plot of q_t versus t^(0.5) in the intraparticle model, as shown in Figure 8(c), did not pass the origin, intraparticle diffusion did not wholly affect the adsorption process. Also, the diffusion model's correlation coefficient (R²) for attapulgite was lower than that of the pseudo-second-order (0.867), as shown in Table 3. The suitability of the pseudo-second-order model with the experimental result means that adsorption is controlled by ion exchange, in which electrostatic interactions play a significant part (Jiaojiao et al. 2009; Xiang et al. 2014).

The research focus is the exploration of novel and effective adsorbents for Cs removal. The widespread exploration of advanced functional materials for nuclide pollution control is driven by increasingly severe environmental problems. Researchers have extensively explored and designed adsorbents for Cs removal. A comparison between the results of this study and previous studies is illustrated in Table 4 and suggests that attapulgite is a strong, stable, and efficient sorbent for Cs-137 removal. Moreover, attapulgite is easily applied as an adsorbent with a natural, low-cost, eco-friendly, and simple batch sorption process compared with synthesized adsorbents, such as zeolites, composites, and bio-sorbents for Cs-137 radioactive decontamination. Excellent Cs-137 adsorption efficiencies were achieved by attapulgite (97%) for a 2-h equilibrium time without functionalization or treatment of its surface.

Attapulgite had a small particle size, a high specific surface, better cation exchangeability, and an effective functional site. Excellent adsorption efficiencies of Cs-137 were achieved by attapulgite (97%) for a 2-h equilibrium time. The high adsorption efficiencies achieved in this study resulted from using low Cs-137 radioactivity concentrations (∼6.372 KBq/L). The kinetics of Cs-137 adsorption on attapulgite were evaluated. The pseudo-second-order kinetic model produces a good fit with the experimental data. According to the results, the local raw attapulgite was suitable clay and should be selected to manage the removal of the Cs-137 from wastewater. The attapulgite adsorbents proved to be promising materials for removing Cs-137 because they are inexpensive, available, and effective.

We gratefully acknowledge the scientific support of the Department of Chemical Engineering, University of Technology-Iraq; Iraqi Atomic Energy Commission (IAEC)/Radiation and Nuclear Safety Directorate, Baghdad, Iraq, and the Iraqi Geological Survey/Ministry of Industry and Minerals, and the Department of Chemical and Petroleum Industries Engineering at Al-Mustaqbal University College in Babylon, Iraq.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.