Non-thermal plasma irradiated polyaluminum chloride for the heterogeneous adsorption enhancement of Cs⁺ and Sr²⁺ in a binary system

Highlights

• A DBD NTP irradiation was configured and incorporated into the synthesis of PAC.

• A new type of NTP-irradiated PAC material was synthesized to treat Cs and Sr contaminants.

• NTP irradiation changed the synthesis of alum pulp and steered the adsorption of Cs⁺ and Sr²⁺.

• Uptake of Cs⁺ and Sr²⁺ by NTP-PAC was achieved by both physisorption and chemisorption.

• This study explores the potential influence of NTP on the treatment of radioactive wastewater.

Abstract

The natural ecosystem will continually deteriorate for decades by the leakage of Cs and Sr isotopes. The exploration of the new materials or techniques for the efficient treatment of radioactive wastewater is critically important. In this study, a dielectric barrier discharge (DBD) configuration was constructed to operate the non-thermal plasma (NTP). The NTP was incorporated into the synthesis of polyaluminum chloride (PAC) in two different procedures to intensify the synthesis of PAC (NTP-PAC) and enhance the further removal of Cs and Sr from wastewater. The employment of NTP in two procedures both had significantly changed the physicochemical characteristics of PAC materials, which facilitated the further adsorption application of NTP-PAC on the treatment of Cs⁺ and Sr²⁺. Different molecular, morphological, and adsorption characteristics were confirmed to the NTP-PAC materials. The heterogeneous adsorption of the NTP-PAC can be appropriately fitted by both the pseudo-first-order kinetic model and the Elovich model. Both physisorption and chemisorption reaction mechanisms were ensured for the heterogeneous adsorption of the NTP-PAC material towards Cs⁺ and Sr²⁺, which guaranteed the excellent adsorption performance of NTP-PAC materials compared to PAC. The electron collisions caused by NTP with alum pulp created highly reactive growth precursors and intensified the nucleation and hydrolysis polymerization of PAC. The employment of NTP explicitly broadens the reaction pathways between PAC and cationic contaminants in the aqueous environment, which expands the application area of PAC materials in environmental sustainability.

Introduction

The inefficient and prolonged disposal of excess emissions of wastewater containing radioactive ions has become one of the main technical reasons hindering the sustainable development of the nuclear industry in the current time (Huang et al., 2021d, Isik et al., 2021). The malicious or accidental discharge of some radionuclides into the natural water by some irresponsible companies would pose a severe threat to the human living environment (Alby et al., 2018, Zhang et al., 2019). Cesium (Cs) and strontium (Sr) are two of the common fission elements of uranium. Among the radionuclides of Cs and Sr, ¹³⁷Cs and ⁹⁰Sr isotopes have a half-life of approximately 30 and 29 years, emitting strong gamma and beta rays, respectively (Alby et al., 2018, Olatunji et al., 2015). Once the ¹³⁷Cs and ⁹⁰Sr isotopes are circulated to the aqueous environments, the deterioration of natural ecosystems will continue for decades. The mobility of both Cs⁺ and Sr²⁺ is strong, making the incorporation of Cs⁺ and Sr²⁺ into the aquatic materials easier (Goyal et al., 2020, Liang et al., 2020, Uematsu et al., 2020). The exploration of new materials or techniques for the efficient treatment of radioactive wastewater is critically important.

Some techniques have been applied to cleanse the pollutants of Cs and Sr from wastewater, such as ion exchange, filtration, reverse osmoses, solvent extraction (Wang and Zhuang, 2020), and electrokinetic removal, etc. (Wang and Zhuang, 2019). The adsorption method is mostly employed among these choices due to its remarkable compatibility and operability (Wang et al., 2018, Xu and Wang, 2017). To obtain the desired adsorption, a huge variety of new materials that pursue high adsorption capacities and selectivity (Fuks and Herdzik-Koniecko, 2018), wide applicability in different scenarios (Alby et al., 2018, Abdel-Galil et al., 2019, Roh et al., 2015), and excellent recyclability (Delaval et al., 2020) have been synthesized and investigated, such as a porous polymer (Isik et al., 2021, Chung et al., 2021, Chen et al., 2019), mesoporous titania (Mironyuk et al., 2019), Na-palygorskite (Wei et al., 2019), Zn-doped GaAs nanowire (Z. Liu et al., 2019), crown ethers modified magnetic adsorbent (L. Liu et al., 2019), mercerized bacterial cellulose membrane modified with EDTA (Cheng et al., 2019), core-shell structured carbon nanofiber-Prussian blue composites (Park et al., 2020), lithium-modified montmorillonite caged in calcium alginate beads (Xia et al., 2018), etc. Generally, the development of the adsorption method mainly focuses on the breakthrough of new materials. To achieve excellent functions in a specific environment, the materials of adsorbents and the synthesis process are becoming more complicated. Especially in the last decade, the synthesis of a new material commonly involves a variety of elements, solvents, and precursors and needs multiple steps (Tang et al., 2021, Huang et al., 2020, Xing et al., 2019). A large number of wastes are produced during the process, which increases the risk of secondary pollution and goes against the concept of green chemistry.

Non-thermal plasmas (NTP) have been applied for clean and sustainable energy storage, the decontamination of hazardous wastes, and the synthesis of innovative materials, especially for those hard to be achieved from thermal equilibrium in recent years (Tatarova et al., 2014). The NTP can be achieved by different types of discharges such as glow discharge, corona discharge, microwave discharge, arc discharge, and radiofrequency discharge, etc. (Murugesan et al., 2020; Gupta and Ayan, 2019; Peng et al., 2018). The free plasma electrons can reach temperatures of 10,000– 100,000 K while the atmosphere can maintain an ambient temperature (George et al., 2021, Mouhammadoul et al., 2020, Shan et al., 2019). These hot electrons would collide with the molecules of gases and liquids, obtain an effective dissociation and ionization of precursors, and incur an electronic avalanche (Palma et al., 2020, Kortshagen et al., 2016). Some free radicals, heating energy, microwave irradiation, ultraviolet rays, and ultrasonication are concomitantly generated during the NTP. The radicals and ions heat and react on the surfaces of target materials. The heating energy, microwave irradiation, ultraviolet rays, and ultrasonication would further enhance the dispersion, synthesis, and catalytic process (Martini et al., 2019). Yu et al. (2021) prepared a highly dispersed transition metal oxide-supported activated carbon by NTP for the efficient removal of elemental mercury. Sun et al. (2020) utilized NTP to add S active sites to fly ash adsorbents for Hg removal from flue gas. H.H. Chen et al. (2020) enhanced the metal dispersion and the support structure of Ni/Silicalite-1 catalysts by NTTP for the activation of CO₂ hydrogenation. Some attributes set the NTP apart from the traditional gas-phase or hydrothermal synthesis, including the loading of free radicals, intense heating by surface reactions, striking mass transferring, reducing diffusion losses, avoiding agglomeration, driving reactions at low temperatures, and facilitating inclusions of dopants (Woodard et al., 2018, Liao et al., 2018, Sultana, 2018, Hati et al., 2018).

Polyaluminum chloride (PAC) is one of the most commonly employed flocculants in the treatment of wastewater (Alengebawy et al., 2021, Long et al., 2021, Cho et al., 2020). The alum floc would be generated at a rapid rate by hydrolysis once PAC is stirred into the aqueous environment (Cho et al., 2020, Teng et al., 2020, Toor and Kim, 2019). The contaminants would be removed by the alum floc to some extent through the roll sweeping and net catching mechanisms (Guo et al., 2017, Mahdavi et al., 2017, Murnane et al., 2016, Ali and Kim, 2016). However, to the removal of cationic contaminants, especially the cations, the PAC plays an inconspicuous role.

In this study, a dielectric barrier discharge (DBD) configuration was constructed to operate the NTP irradiation. To avoid the excessive pursuit of complicated new materials and develop the adsorption technique according to sustainability, the NTP was incorporated into the synthesis of PAC in two different procedures to synthesize new types of adsorbent materials (NTP-PAC). The synthesis of the NTP-PAC was conducted according to a central composite design (CCD), analyzed by significance analysis and response surfaces, and optimized by desirability function and a quadric predictor. The binary equilibrium adsorption of the NTP-PAC towards Cs⁺ and Sr²⁺ was set, carried out, and optimized in an orthogonal design. Four kinetics and four isotherms models were chosen and mathematically fitted to explore some potential mechanisms on the adsorption enhancement induced by the NTP. Some characterization strategies including a Fourier infrared spectrometer (FTIR), a thermogravimeter (TGA), an X-ray diffractometer (XRD), and a scanning electron microscope with energy-dispersive X-ray spectroscopy (SEM-EDS) were used to analyze and verify the influence of the NTP on the PAC and the reaction pathways referring to the heterogeneous adsorption. This study not only exemplifies the application of the NTP on the traditional materials and explores the potential influence but also supplies the mechanisms between the cations of Cs and Sr and the NTP-irradiated Al-based materials.