IntroductionAdsorption is one of the most effective heavy metal ion treatment techniques available. Common heavy metal ion adsorbents include activated carbon, chitosan, cellulose and natural zeolite. Geopolymers, as a new type of adsorbent material, show great potential and value for application in the field of zinc-containing heavy metal wastewater treatment. Blast furnace slag is a type of bulk solid waste in the metallurgical industry, and the current high value utilisation rate is low. The use of blast furnace slag to prepare geopolymer adsorbent materials to treat heavy metal wastewater is one of the effective ways to realise the comprehensive utilisation of blast furnace slag in a high value way, which can achieve the purpose of ‘waste for waste’. At present, the related research mainly focuses on the adsorption mechanism and adsorption effect of geopolymers on heavy metal ions, while less attention has been paid to the desorption of heavy metal ions after adsorption and the recycling of geopolymers, and only a few scholars have carried out preliminary explorations by using inorganic acid, EDTA-2Na, and NaCl solution as desorbing agents. In this study, the geopolymer adsorbent material was prepared from blast furnace slag to investigate its adsorption performance and mechanism of action on zinc Zn²⁺, and nitric acid was used as a resolving agent to investigate the desorption effect of Zn²⁺ and the recycling performance of geopolymer adsorbent, which is of great guiding significance for the high value-added utilisation of metallurgical solid wastes and the treatment of zinc-containing heavy metal ion wastewater.MethodsDetermination of major oxides and their contents in blast furnace slag using XRF. Particle size distribution of finely ground blast furnace slag was analysed using a laser particle size meter. Analysis of the crystalline phase structure of blast furnace slag and geopolymers using XRD. The specific surface area of the geopolymer and its pore size were characterised using BET. The concentration of Zn²⁺ in solution was detected by ICP. SEM-EDS was used to analyse the crystalline structure, micro-morphology and compositional distribution of the geopolymer before and after adsorption. Kinetic analysis of geopolymers using quasi-primary and quasi-secondary kinetic models. Adsorption isotherms were simulated for geopolymers using the Langmuir adsorption isotherm model and the Freundlich adsorption isotherm model.Results and discussionThe surface and interior of geopolymers based on blast furnace slag (BFSGP) contains a large number of pore structures，which are favourable for the migration of Zn²⁺ in the solution, and at the same time increase the adsorption sites, thus effectively improving the adsorption capacity and adsorption efficiency of BFSGP. The N₂ adsorption and desorption isothermal curves and pore size distribution curves of BFSGP indicate that BFSGP is a typical mesoporous structure. The adsorption of Zn²⁺ by BFSGP increased significantly with the increase in the initial ion concentration of the solution. The adsorption process of BFSGP followed the quasi-secondary kinetic model and the Langmuir model, thus indicating that the adsorption process of BFSGP was dominated by chemisorption, and its adsorption process was strongly influenced by the chemical reaction between BFSGP and Zn²⁺, and at the same time conformed to the characteristics of the monomolecular layer adsorption.In the desorption experiments, within the first 20 min, nitric acid rapidly enters into the pores of the adsorbent and acts to break the chemical bond between Zn²⁺ and BFSGP, and the desorption rate is fast. The desorption rate decreases as the readily desorbed Zn²⁺ decreases. Despite the loss of some active adsorption sites due to chemical bonding and Al³⁺ leaching during the cyclic adsorption-desorption process, BFSGP still maintains good adsorption performance for Zn²⁺, which provides a new idea for the recycling of BFSGP as well as for the recovery of geopolymer-loaded heavy metals by the pickling method.ConclusionsThe adsorption of Zn²⁺ by BFSGP increased with the initial concentration of the solution. When the initial ion concentration was 200 mg/L, the adsorption amount and adsorption rate of Zn²⁺ by BFSGP were 196.14 mg/g and 98.07%, respectively. The adsorption of Zn²⁺ by BFSGP is in accordance with the quasi-secondary kinetic model and Langmuir model and is a chemical monolayer adsorption. BFSGP showed better adsorption performance in cyclic adsorption-desorption experiments, the first desorption rate of nitric acid was 45.52%, and after two cyclic adsorption-desorption cycles, the adsorption rate of BFSGP on Zn²⁺ was still up to 56.83%.