New insights into the debromination mechanism of non-metallic fractions of waste printed circuit boards via alkaline-enhanced subcritical water route

Abstract

The debromination of brominated epoxy resins (BERs) from waste printed circuit boards (WPCBs) is of great challenge due to its high structural stability. An enhanced subcritical water (SCW) process with alkaline additive is proposed for efficient debromination of BERs, and the mechanism of alkaline additive on bromine removal has been investigated. More than 95.11% of bromine is removed under the optimal conditions. Both enhanced fiberglass destruction and BERs degradation contribute to the efficient debromination process. Structure of Ca−Al, Si−O, and Al−O bonds in fiberglass is destroyed by alkaline corrosion, releasing BERs into the solution and facilitating the debromination process. Meanwhile, phenol, 2-bromophenol and catechol are the main products of BERs degradation. Nucleophilic substitution reaction by the bimolecular process dominantly contribute to the debromination. The energy barrier of the bimolecular process is only 15.9 kcal/mol, which is much lower than that of the direct dissociation energy of C−Br bond (168.3 kcal/mol). Ion association of KOH is weaker than that of NaOH in the alkaline-enhanced SCW process due to the difference of cation radius, proved by ab initio molecular dynamics simulation. An efficient approach for WPCBs debromination under mild conditions has been proposed.