Experimental study and molecular dynamics simulation:Thermal behaviors, volatile products and chlorine migration characteristics during pyrolysis and combustion of 2,4-dichlorophenoxyacetic acid waste salt

An organochlorine pesticide, 2,4-Dichlorophenoxyacetic acid (24D), produces pesticide by-product waste salts that pose a potential risk of secondary contamination, creating significant challenges in disposal management. While thermal treatment has emerged as a viable method for managing 24D waste salt, reducing the thermal temperature and controlling the release of chlorine-containing organic compounds from volatile products remains a significant obstacle. In this work, the effects of pyrolysis and combustion on the 24D thermal treatment were explored by TGA-FTIR-GCMS coupling techniques and molecular dynamics calculations (ReaxFF). Combustion achieved a total organic carbon (TOC) removal rate of 93.17 % at 873K, which was higher than that achieved by pyrolysis. In terms of volatile products, the release of small molecule gases such as CO₂, CO, CH₄, and HCl is significantly enhanced during combustion, the HCl emission in the combustion process is 2.7 times that in the pyrolysis process. However, as the temperature of the combustion process increased, 24D synthesis reaction to form substances such as heterocyclic-Cl, the amount of which increased to 7.01 % at 873K. ReaxFF-MD simulations investigated the thermal treatment of 24D, revealing a complex pyrolysis and combustion mechanism, including multiple reaction pathways for dechlorination, dehydrogenation, dehydroxylation, and generation of chlorinated products HCl and CH₃Cl. The combustion process can increase the free radicals and provide more degradation pathways in the reaction system, which promotes the migratory conversion of organic-Cl to inorganic-Cl in 24D. Therefore, combustion treatment with low decomposition temperature and easy degradation of chlorine-containing intermediate products is a feasible method to purify 24D WS.