## 1 Introduction  
Substantial quantities of radioactive contaminants are generated each year from nuclear weapon tests, nuclear facilities, and reprocessing of spent fuels due to military and industrial activities. A significant amount of soil [1] and water [2] is contaminated by these contaminants in some districts, which poses great risks to the environment and human health. It is known that radioactive \({}^{137}\)Cs and \({}^{90}\)Sr, with half-lives of 30.1 and 28.9 years, respectively, are major contaminants. \({}^{137}\)Cs is an analog of potassium that is easily incorporated in living organisms [3], and \({}^{90}\)Sr can be introduced into the food chain, which seriously threatens human health [4]. They also have an adverse impact on ecology [5, 6]. Thus, removal of radioactive contaminants from contaminated water has become a significant environmental matter in recent years. In addition, the removed \({}^{137}\)Cs and \({}^{90}\)Sr isotopes have potential application as gamma radiation sources for medical and industrial applications [7, 8]. Accordingly, further research for feasible separation processes to recover Cs(I) and Sr(II) is required. Solvent extraction has been proved to be one of the most effective partitioning and recovery techniques from the viewpoints of speed, simplicity and easiness of scaling up. This method has been used for the removal of Cs(I) and Sr(II) for long time. Thus, the selection of the extractants plays an important role in the extraction process, and can greatly affect the separation efficiency. In the past few decades, some conventional and newly developed extractants have been used to study solvent extraction of Cs(I) and Sr(II). Crown ethers, such as 4,5\"-bis(_tert_-octylbenzo)dibenzo-24-crown-8 [9], dibenzo-21-crown-7 (DB21C7) [10],and 4\({}^{\prime}\),4\({}^{\prime}\),(5\({}^{\prime}\))-di-(_tert_-butylcyclohexano)-18-crown-6 (DtBuCH18C6) [11]; calixarene-crown ethers, such as 1,3-alternate calix[4]arene-crown-6 conformers [12], calix[4]dibenzocrowns-6 [13], calix[4]arene-bis(naphthylrown-6) [14], 25,27-bis(isopropyloxy) calix[4]-26,28-crown-6 (iPr-C[4]C-6) [15], calix[4]arene-bis-(_tert_-octylbenzo-crown-6) (BOBCalixC6) [16; 17], and acryl-calix[4]-crown-6 [18]; and amide agents, such as _N,N,N,N_-tetraoctyl-3-oxapentanediamide (TODGA) [19], are potent extractants for separating Cs(I) or Sr(II) from acid solutions. Bis(2-propyloxy)calix[4]crown-6 (BPC6) [20], calix[4]arene-bis(crown-6-ethers) [21; 22], and 4-_tert_-butyl-2-(\(\alpha\)-methylbenzyl) phenol (_t_-BAMBP) [23] have shown excellent extraction ability for Cs(I) from aqueous solutions. In addition, di-2-ethyl hexyl phosphoric acid (D2 EHPA) [24] has been proved to be feasible for the separation of Sr(II) from alkaline solutions. The abovementioned studies showed that crown ethers are generally selective for Sr(II), and calixarene-crown ethers are mainly selective for Cs(I). Extractants that are selective for both Cs(I) and Sr(II) in an aqueous solution have not been reported yet. Using a single extractant with high selectivity for both Cs(I) and Sr(II) can simplify the treatment of wastewater. Therefore, an agent with the ability to extract them efficiently is highly desired. Calixarenes have been regarded as molecular platforms or building blocks for the construction of metal ion carriers, which is an attractive motif in coordination chemistry [25; 26]. The extraction process of calixarenes for industrial applications is extremely important [27]. Classical CH\({}_{2}\)-bridged calixarenes were studied in cations extraction and transport [28; 29], and calixarene amide ionophores for strontium extraction were also researched [30]. In our present work, low-cost _p-tert_-butylsulfonylclax[4]arene (SC4A), possessing the ability to extract Cs(I) and Sr(II) efficiently, was investigated.