Mingge Wu,Qianqian Li,Cheng Chen,Guijin Su,Maoyong Song,Bohua Sun,Jing Meng,Bin Shi
The high-efficiency photocatalytic debromination of hexabromocyclododecane (HBCD) as new POPs and discovery of involved mechanisms remain a challenge. Herein, challenging Pd-anchored hollow-rod-like-g-C 3 N 4 (Pd/HR-g-C 3 N 4 ) was constructed as visible-light-driven photocatalyst. Complete destruction of (±)-α, (±)-β and (±)-γ-HBCD with 92.6 % bromine-removal was achieved at 60 min in optimal system of anaerobic methanol-water (70:30) mixture with 0.5 % Pd/HR-g-C 3 N 4 , being superior to most studied HBCD catalytic systems. The higher photocatalytic reduction performance is contributed to enhanced photoexcited-carriers separation and more negative conduction band arising from distinctive 1D-cavity of HR-g-C 3 N 4 , and surface-atomically dispersed Pd. A never-reported degradation pathway in which hydrodebromination and addition ring-opening synergistically occur is proposed according to identified products of ten saturated chain alkanes, C 12 H 19 Br 5 and C 12 H 19 Br 7 . Ulteriorly, hydride-transfer resulted in rapid hydrodebromination of HBCD over Pd/HR-g-C 3 N 4 , where H chiefly derives from methanol instead of water although they synergistically promoted e − utilization. This provided a valuable sight into the cost-effective pollution-removal of HBCD.
Bromine removal is significant in the recycling of waste printed circuit boards (WPCBs). This study found that the critical factors limiting the debromination efficiency of conventional pyrolysis are the formation of coke impeding mass transfer and conversion of bromine into less volatile species, such as coking-Br and copper bromide. According to frontier molecular orbital analysis and thermodynamic equilibrium analysis, C–O bonds of resin are sites prone to electrophilic reactions and copper bromide in residue may undergo hydrolysis; therefore, introducing H2O during pyrolysis was a feasible method for thorough debromination. Through pyrolysis in a water vapor atmosphere, the diffusion limitation of debromination was overcome, and resin was converted into light components; thereby, rapid and deep removal of bromine was achieved. The result indicated that 99.7% of bromine was removed, and the residue could be used as a clean secondary resource. According to life-cycle assessment, pyrolysis of WPCBs in water vapor could be expected to reduce 77 Kt of CO2 emission and increase financial benefits by 60 million dollars, annually.
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