Coal gangue geopolymers as sustainable and cost-effective adsorbents for efficient removal of Cu (II)

Coal gangue geopolymers (GP) were employed to eliminate Cu (II) as an economical absorber. This study explores the feasibility of employing coal gangue geopolymers (GP) to be an economical absorber with a function of eliminating Cu (II). The results showed that GP is a mesoporous structure, which pore size distribution is around 5.60 nm. The optimal adsorption capacity of Cu (II) was achieved when the ratio n(SiO2)/n(Na2O) was 1.2. Cu (II) adsorption capacity of GP may achieve a maximum of 72.3 mg/g. Silicon and aluminum hydroxide groups of GP facilitated strong interactions and effective trapping of Cu (II). Data analysis discovered that the adsorption procedure in examinations followed a pseudo-second-order kinetic model, with the Langmuir model providing the best appropriate to the adsorption isotherm data. These confirm that charge and chemisorption simultaneously appeared in the spontaneous Cu (II) adsorption on GP. The intra-particle diffusion model (IPD) and Boyd models supported by observation that positive relationships result in initial Cu (II) adsorption onto the outer layer of GP, followed by subsequent diffusion into internal pores. Internal and external diffusion processes jointly dominate the Cu (II) adsorption rate. Furthermore, the adsorption-related characteristics of a combination of Cu (II) and Cr (VI) demonstrated that GP effectively removes Cr (VI) without compromising its adsorption performance for Cu (II), without any discernible competition or synergy between the two. Notably, the GP could be reused for five times, and Cu (II) removal rate is 80.88%, indicating the possibility of its application as an effective, sustainable Cu (II) removal absorbent.