<p>The mode of occurrence of arsenic and flue gas components significantly influences their migration behavior. However, research on microscopic forms of arsenic and the chlorination mechanism of arsenic remains limited. This study systematically elucidates arsenic’s speciation characteristics and chlorination reaction pathways through combined experimental and density functional theory (DFT) analyses. The sequential chemical extraction results show that arsenic has a high affinity for pyritic sulfur. In addition, DFT calculation results indicate that arsenic in pyrite exhibits two primary speciation modes, and the structure of substitutional arsenic is more stable compared to superficial arsenic. First, physisorption interactions between HCl and distinct FeS₂ (100) surface terminations containing arsenic are identified as the dominant pathway. For chlorination mechanism of arsenic, arsenic chloride desorption is the most difficult to occur, and arsenic chloride formation for the superficial arsenic structure is easier compared to the substitutional arsenic structure. Thermodynamic and kinetic analyses indicate that higher reaction temperatures can enhance the formation of arsenic chloride, and the more arsenic chloride will be generated on surface with superficial arsenic. The combined experimental and computational results systematically elucidate arsenic’s speciation characteristics during pyrite decomposition and delineate its chlorination reaction pathways, thereby establishing a theoretical framework for arsenic transformation dynamics and capture efficiency during combustion processes.</p> Graphical Abstract <p></p>