Self-supported geopolymer-zeolite bead adsorbents with improved microstructural properties for enhanced adsorption

Alkali-activated materials (AAM) are being investigated around the globe as alternative cementitious binders, primarily due to their low CO2 footprint in comparison to ordinary Portland cement (OPC). As the name states, silica, alumina, and calcium containing raw materials (metakaolin, coal fly ash, blast furnace slag, etc.) are activated using a concentrated alkali activator (normally NaOH and Na2SiO3-based) to get equally strong and durable binders compared to OPC. Based on the Ca content in the mixture, AAMs can be divided into three classes: (1) Low-Ca AAMs, (2) High-Ca AAMs, and (3) Blended systems. Among these three classes, the low-Ca AAMs, often referred as geopolymers, possess the potential for a number of applications owing to their specific chemical composition. Geopolymers mainly consist of highly cross-linked amorphous aluminosilicate gel. The aluminosilicate chains in amorphous geopolymeric gel are somewhat similar to the chains in crystalline zeolites, hence zeolitic phases are often detected in geopolymers. This suggested that the synthesis of zeolites in geopolymeric gel can induce multifunctionality in these binders. Zeolite crystals can be supported by the dense geopolymeric gel so that these can be used as self-supported geopolymer-zeolite adsorbents. Since mainly the industrial by-products (e.g., coal fly ash, blast furnace slag) are used for making geopolymers, benefits of synthesizing such valuable products will be twofold i.e., no need of external support and circular utilization of by-products. This is the main attraction for developing geopolymeric adsorbents in comparison to the other adsorbents. In this regard, PI has worked on synthesis of zeolites in geopolymers by hydrothermal treatment [1,2]. Zeolitic phases, primarily zeolite Na-P1 along with some minor phases, were successfully synthesized and microstructural properties of the synthesized samples were reported. Additionally, Cs+ and Pb2+ adsorption capacities of about 15 and 38 mg/g, respectively, were achieved. These values might be relatively low compared with the powder adsorbents; however, their comparison with other bulk-type adsorbents (reported in the literature) revealed that their performance was much better. On the other hand, it was observed that the adsorption rate of solid bulk-type samples was much slower than their pulverized/powder counterpart. Unlike powder adsorbents, much less area of adsorbents comes in direct contact with adsorbate ions. Adsorbate ions have to defuse within the matrix of bulk-type adsorbents, which suggests that the specific surface area and porosity are the key factors for adsorption rate and capacity of these adsorbents. The enhancement of surface area and porosity can potentially result in improved adsorption rate and capacity, paving the way for the industrial utilization of these newly developing bulk-type adsorbents. Considering the findings of the previous works, the proposed project is aimed at tuning the microstructural properties (i.e., circumferential area, surface area and porosity) of the samples to achieve enhanced adsorption efficiency of geopolymer-zeolitic adsorbents. Spherical bead samples will be prepared under similar hydrothermal conditions, previously reported by the PI, targeting the easy integration of these adsorbents in the industry. For tuning the porosity of the samples, certain amounts of foaming agent (H2O2) will be added in the slurry mixture which will produce pores within the matrix. The formation of pores will potentially increase the pores connectivity, ultimately resulting in enhanced open porosity. Then the physicochemical properties of control (un-foamed and foamed) and hydrothermally treated (un-foamed and foamed) samples will be studied by XRD, BET/BJH, SEM, and FTIR, followed by adsorption experiments on all the synthesized samples to directly compare their adsorption efficiency with microstructural properties. In this regard, preliminary experiments were conducted by the PI to cast the bead samples with typical solution casting method (i.e., dropping mixture drops into high viscosity solution, usually PEG solution, which keep the particles suspended until the gain enough strength). However, it was found that the solution casting method for preparing beads of these mixtures might not be suitable because of the low viscosity of slurry mixture, resulting from higher water/binder ratio requirement for synthesis of zeolites in these binders. Hence, mold casting method is proposed in this project. A new mold has been already designed in which size and shape of the beads can be perfectly controlled. Both the methods (solution casting and mold casting) will be compared in terms of protocol simplicity, efficiency and scalability. Consequently, bead samples of different sizes will be prepared for further investigation.