Lead ion Adsorption Performance and Mechanism of Amino Modified Porous Geopolymer

This paper was to prepare amino-modified porous geopolymer (NH₂-3HGeo) based on geopolymer via acid treatment and post-grafting, and investigate its adsorption performance and mechanism for lead in water. Acid treatment increased the specific surface area of the geopolymer and optimized its pore structure. The maximum adsorption capacity of NH₂-3HGeo for Pb²⁺ reached 521.6 mg·g^–1. The adsorption process followed the pseudo-second-order kinetics and the Langmuir model, indicating a monolayer chemical adsorption. Geopolymer could mainly adsorb Pb²⁺ through ion exchange, while NH₂-3HGeo primarily complexe with Pb²⁺ through functional groups. After five cycles, the Pb²⁺ removal rate of NH₂-3HGeo still reached 90.6%, demonstrating a good recyclability. The results indicated that the material could have a great potential in the field of heavy metal pollution control in water bodies.

In the preparation process of amino-modified porous geopolymer, 400 g of metakaolin and 100 g of white carbon black were uniformly mixed, and then put in a sodium hydroxide solution prepared via dissolving 200 g of NaOH in 400 mL of deionized water as an alkaline activator and 10 mL of hydrogen peroxide solution under stirring. The resulting slurry was cast into polypropylene cubic molds and finally cured in an oven at 80 ℃ for 24 h to obtain the geopolymer (Geo). Subsequently, Geo was crushed and sieved to a fine powder with the sizes of 20–40 mesh. 80 g of Geo was added into 400 mL of nitric acid solutions at different concentrations (i.e.,1, 2, 3, and 4 mol·L^–1). The mixtures were placed in a constant temperature oscillator and agitated in 200 r·min^–1 at 80 ℃ for 5 h. After the reaction, the mixtures were filtered through a 0.45 μm membrane, and the products were collected, washed until neutral, and dried in an oven at 105 ℃ for 12 h, resulting in acid-treated porous geopolymers (i.e., 1HGeo, 2HGeo, 3HGeo, and 4HGeo). Finally, 20 g of acid-treated porous geopolymer was placed in a round-bottom flask containing 100 mL of xylene and stirred for 30 min. Afterwars, 20 mL of aminopropyltriethoxysilane (APTES) dissolved in 100 mL of xylene was added to a flask and stirred with 200 r·min^–1 at 80 ℃ for 12 h. The solid product was collected, cleaned with xylene, and dried at 80 ℃for 12 h to obtain the amino-modified porous geopolymers (i.e., NH₂-1HGeo, NH₂-2HGeo, and NH₂-3HGeo).In addition, the control sample NH₂-Geo was also prepared by the same procedure.

This study prepares amino-modified porous geopolymers via acid treatment and post-grafting with porous geopolymers as a base material. The results of the BET specific surface area measurement indicate that acid treatment significantly increases the BET specific surface area of the geopolymer material. The specific surface area of sample 3HGeo (i.e., 343.87 m²·g^–1) and pore volume (i.e., 0.1679 cm³·g^–1) are 12.94 times and 3.29 times greater than the specific surface area (i.e., 26.57 m²·g^–1) and pore volume (i.e., 0.0511 cm³·g^–1) of sample Geo, respectively. The XRD patterns, FTIR spectra, and thermogravimetric analysis show that acid treatment increases the porosity and silanol content of the porous geopolymers, thereby increasing the grafting amount of amino groups. The amino groups are grafted onto the geopolymers. The results of adsorption test show that the adsorption amount is positively correlated to the BET specific surface area and amino content. The adsorption rate of NH₂-3HGeo for Pb²⁺ rapidly increases and stabilize as the adsorbent dosage increases. of adsorption time, the removal rates of NH₂-1HGeo, NH₂-2HGeo, and NH₂-3HGeo for Pb²⁺ are 48.1%, 70.1%, and 99.9% at 360 min, 300 min, and 240 min, respectively, with the increase of adsorption time. The adsorption amount of Pb²⁺ increases as the temperature increases. The adsorption capacities of NH₂-1HGeo, NH₂-2HGeo, and NH₂-3HGeo increase as the initial Pb²⁺ concentration increases from 100 mg·L^–1 to 700 mg·L^–1. The adsorption amount continuously increases as the solution pH vale increases. Different coexisting ions (i.e., K⁺, Na⁺, Ca²⁺, and Mg²⁺) affect the adsorption performance of NH₂-3HGeo for Pb²⁺, with divalent cations having a greater impact than monovalent cations. The adsorption kinetics follows the pseudo-second-order model, indicating the chemical adsorption. The results of adsorption isotherm indicate that the adsorption process involves monolayer adsorption. The adsorption thermodynamic results show that moderately increasing the temperature is beneficial to adsorption. The results of recycling experiment show that after five cycles of adsorption-desorption, the adsorption rate of NH₂-3HGeo remains 90.2%, having a good recyclability. The contribution of different adsorption mechanisms indicates that Geo mainly adsorbs Pb²⁺ through ion exchange, while NH₂-3HGeo primarily relies on a functional group chelation.

Amino-modified porous geopolymers with a high specific surface area and a multi-pore structure were prepared by acid treatment and amino modification with porous geopolymers as a base material. The results of nitrogen adsorption showed that acid treatment significantly increased the specific surface area of the geopolymer and effectively improved the pore structure. The thermogravimetric analysis indicated that acid treatment created more active sites on the surface of the geopolymer (such as silanol groups Si—OH), which could act as active centers for subsequent grafting reactions. The prepared NH₂-3HGeo sample had a high adsorption capacity for Pb²⁺ and a good recyclability, with a maximum adsorption capacity for Pb²⁺ of 521.6 mg·g⁻¹. After five adsorption-desorption cycles, the adsorption rate was still 90.2%, providing an effective approach for the application of geopolymer materials in the treatment of heavy metal-containing wastewater. The results of XPS analysis showed that the complexation between —NH₂ and Pb²⁺ essentially involved the protonation of —NH₂ to form —NH₃⁺. The adsorption mechanisms indicated that Geo mainly adsorbed Pb²⁺ through ion exchange, while NH₂-3HGeo primarily did that through functional group chelation. This work could lay a certain theoretical foundation for understanding the microstructure of geopolymers and modified geopolymers, as well as the mechanism of their adsorption of heavy metal ions.