# Utilization of Thermal-Activated Coal Gangue to Enhance the Properties of Sandy Soil Composites
## 3 Results and Discussion
### Nano-Pore Size Distribution of Modified Sandy Soil  
According to the results of the nitrogen adsorption-desorption curves of the mixed sandy soil under different treatments, the pore distribution curves of the mixed sandy soil samples under various treatment conditions can be obtained by using the BJH theory [24]. As can be seen in Figure 9, the pore size distribution of sandy soil was modified after mixing with coal gangue and calcined coal gangue. The pore volume around 4 nm was significantly increased after the incorporation of coal gangue and calcined coal gangue. CK + 600 \({}^{\circ}\)C exhibited the highest pore volume, around 4 nm, which provided more pores for water adsorption during the isothermal water adsorption test under relatively low P/P\({}_{0}\). In other words, the presence of these small pores contributed to the water capture ability at low relative humidity. This will be helpful for the water retention of sandy soil. It is clear to see that the calcination temperature shows obvious effects on the pore structure of the calcined coal gangue. The high-temperature (850 \({}^{\circ}\)C) calcination reduced the small pores in coal gangue particles, as well as the water retention ability of the corresponding mixed sandy soil. This further indicates that the application of coal gangue and calcined coal gangue changed the original pore distribution structure of the sandy soil and improved the adsorption capacity of the mixed sandy soil. TACG (600 \({}^{\circ}\)C) exhibited the best performance on the nano-pore volume enhancement, which resulted in the best performance of water adsorption under low relative humidity. This property will be helpful to the re-cultivation of sandy soil under a water-scarce environment. The Adsorption and the Desorption Properties of NH\({}_{4}\)\({}^{+}\), PO\({}_{4}\)\({}^{3-}\), and K\({}^{+}\) in Modified Sandy Soils  
The NH\({}_{4}\)\({}^{+}\), PO\({}_{4}\)\({}^{3-}\), and K\({}^{+}\) are the most often detected in soils, which can be dissolved in the pore solution of soils and absorbed by the roots of plants. Therefore, these elements  
Figure 9: Pore distribution curve of mixed sandy soil. could be the key factor to evaluate the quality of soils in agriculture. The adsorption and desorption of NH4\({}^{+}\), PO4\({}^{3-}\), and K\({}^{+}\) by modified sandy soils are shown in Figure 10 (Treatment 1 is CK + raw coal gangue, Treatment 2 is CK + 600 \({}^{\circ}\)C calcined coal gangue, and Treatment 3 is CK + 850 \({}^{\circ}\)C calcined coal gangue). The adsorption amount of NH4\({}^{+}\) by different modified sandy soils was enhanced compared to CK. It indicates that the application of coal gangue enhanced the adsorption capacity of modified sandy soil for NH4\({}^{+}\) and effectively enhanced the fixation capacity of modified soil for NH4\({}^{+}\). After the calcined coal gangue was applied, the adsorption amount of NH4\({}^{+}\) by 10%, 15%, and 20% of raw coal gangue-blended sandy soil was enhanced by 13, 11, and 12 mg/kg compared to CK. The desorption rate was enhanced by 30%, 1%, and 12%. It can be seen that the calcination temperature showed strong effects on the NH4\({}^{+}\) adsorption and desorption of modified soils. The increased calcination temperature of coal gangue from 650 \({}^{\circ}\)C to 800 \({}^{\circ}\)C reduced both adsorption and desorption of NH4\({}^{+}\). This could be induced by excessive temperature affecting the pore structure of the gangue material, which in turn causes the adsorption/desorption capacity to change as described above. Comparing the adsorption/desorption of PO4\({}^{3-}\) by the composite soil under different temperature treatments. After applying the original gangue composite soil, the adsorption  
Figure 10: (**a**) Changes in NH4\({}^{+}\) adsorption/desorption under different treatments. (**b**) Changes in PO4\({}^{3-}\) adsorption/desorption under different treatments. (**c**) Changes in K\({}^{+}\) adsorption/desorption under different treatments.