Open Access

ARTICLE

Leaching Characteristics and Mobility Assessment of Heavy Metals from Coal-Based Solid Wastes under Varying Ph Conditions

Ying Zhou^1,2, Changyong Zhu³, Huijun Yang^1,2, Yunhuan Jing¹, Fei Xie¹, Xudong Song^2,*, Yonghui Bai², Guangsuo Yu^2,4,*
1 Institute of Coal Chemical Industry Technology, National Energy Group Ningxia Coal Industry Co., Ltd., Yinchuan, China
2 State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, College of Chemistry and Chemical Engineering, Ningxia University, Yinchuan, China
3 National Energy Group Ningxia Coal Industry Co., Ltd., Yinchuan, China
4 Institute of Clean Coal Technology, East China University of Science and Technology, Shanghai, China
* Corresponding Author: Xudong Song. Email: ; Guangsuo Yu. Email:

Energy Engineering https://doi.org/10.32604/ee.2026.080004

Received 01 February 2026; Accepted 21 April 2026; Published online 08 June 2026

Abstract

Coal-based solid wastes, including coal gangue (CG), fly ash (FA), and coal gasification slag (CS), are generated in large quantities during coal utilization and are inherently enriched with various heavy metals. These toxic elements are susceptible to leaching under natural conditions, posing significant threats to surrounding soil and water ecosystems. This study systematically investigated the leaching behaviors and mobilization characteristics of heavy metals from these wastes across a pH gradient of 3 to 9. Through integrated leaching experiments, microstructural analyses, and speciation characterization, the regulatory effects of pH on metal release were quantitatively evaluated. The results revealed that Hg, Pb, and As exhibited high mobility, with maximum leaching concentrations reaching 0.0214, 0.0249, and 0.0085 mg/L. In comparison, Cr and Cd showed moderate mobility, while Zn, Ni, and Mn remained relatively stable across the tested pH range. Leaching agent type also significantly influenced extraction efficiency: hydrochloric acid achieved the highest extraction of Pb and Cr, whereas sulfuric acid was more effective for Hg and As. The chloride ions provided by hydrochloric acid can form stable soluble chloro-complexes with various heavy metal ions. This complexation significantly reduces the activity of free metal ions in the leachate, thereby disturbing the dissolution equilibrium and promoting the continuous release of heavy metals from the solid phase into the liquid phase. Among the three solid wastes, fly ash demonstrated the strongest enrichment and release potential for Pb, Cr, and As. These quantitative findings highlight the critical role of pH in controlling heavy metal mobilization and identify fly ash as a priority material for environmental management. This work provides a theoretical foundation and data-driven support for the risk assessment, pollution control, and sustainable utilization of coal-based solid wastes.

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Keywords

Coal-based solid wastes; heavy metals; leaching characteristics; pH value; environmental risk

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