Enhancing the Surface Characteristics of Coal Gangue through Different Modification Methods and the Improvement of Saline-Alkali Soil

Abstract Coal gangue exhibits great application potential in saline-alkali soil amelioration. However, natural coal gangue has a limited improvement effect due to its small specific surface area and poor surface activity. In this study, different modification methods were employed to improve the surface properties of coal gangue. Changes in the surface properties of modified coal gangue and their improvement effects in saline-alkali soil were investigated by XRD, FTIR, SEM, static soil culture tests, and simulated leaching tests. Compared with original coal gangue, modified coal gangue shows significantly enhanced active components, in which the stable kaolinite is transformed into active Al2O3 and SiO2 according to XRD analysis. In addition, the micropore structure of modified coal gangue has changed, showing a rougher surface, larger pore diameter and specific surface area. Especially for calcination-modified coal gangue, the specific surface area is greatly increased from 6.9 m2/g to 34.8 m2/g. The application of coal gangue modified by calcination, acid treatment, and ball milling can effectively reduce the pH of saline-alkali soil from 9.89 to 7.99, 8.40, and 8.74, respectively. Meanwhile, their incorporation can reduce the exchangeable sodium content of saline-alkali soil by 36.2%, 33.6%, and 40.7%, respectively. The addition of modified coal gangue has a significant effect on reducing nutrient loss during the leaching of saline-alkali soil. When the amount of calcination-modified coal gangue added reaches 30%, the loss of soil total nitrogen and total phosphorus can be reduced by 94% and 90%, respectively. The results demonstrate that modified coal gangue, owing to its increased active components and surface sites, can not only reduce the pH and exchangeable sodium content of saline-alkali soil, but also effectively mitigate nutrient leaching losses. In comparison, calcination is the most effective modification method, and a 30% addition rate is sufficient to achieve a desirable amelioration effect on saline-alkali soil. This study provides methodological and theoretical foundations for enhancing the potential of coal gangue in saline-alkali soil improvement, which is of great significance for the resource utilization of coal gangue and the improvement of cultivated land quality.