Remediation of carcinogenic chlorinated ethylenes (CEs) such as tetrachloroethylene (PCE), trichloroethylene (TCE), dichloroethylene (DCE), and vinyl chloride (VC) remains challenging due to their widespread and refractory nature. Reductive dechlorination is one of the most promising approaches to achieve full detoxification with a low risk of producing secondary pollution such as perchlorate from oxidative treatments. Nanoscale zero-valent iron (nZVI) and sulfidated ZVI (SZVI) are widely used and studied reductants for CEs remediation, but the application on large scale faces the limitation of high production cost and the potential influences on the biotic environment.
The naturally occurring iron mineral GR green rust (GR), which is a layered Fe(II)−Fe(III) hydroxide, is an ideal alternative for reducing CEs, however, the direct electron transfer from GR to CEs is hindered by the poor interaction and high reaction activation energy barrier. Previous research in the field of reductive dechlorination by GR has shown that applying carbonaceous catalysts (biochar) to facilitate the electron transfer is critical for guaranteeing the dechlorination performance, while there is little guidance for selecting or designing cost-effective carbonaceous catalysts with high reactivity.
Based on this knowledge gap, in this thesis, we first tested whether the catalytic reactivity of the biochar can be tailored by adjusting the composition of its precursor—biomass. By comparing effects derived from different amendments, doping biochar with nitrogen is identified as a very efficient approach for enhancing its reactivity for catalysis of TCE dechlorination by the GR.
In the next step, the feasibility of using this nitrogen-doping approach for improving the reactivity of different carbonaceous substrates (biosubstrates, commercial carbonaceous materials including carbon black and graphene nanoplatelets), using different nitrogen sources (urea, melamine) is tested. The doping conditions such as pyrolysis temperature, temperature ramping time, peak temperature holding time are adjusted to identify the most reactive carbonaceous catalyst for catalyzing the chemical reduction of TCE by GR. By correlating the physicochemical properties changed with the resulting reactivity, it is proposed that besides inducing N-functional groups, the N-doping process also creates intrinsic carbon defects as active sites for catalysis.
Finally, the most reactive nitrogen-doped carbonaceous catalysts were selected for preparing electrodes for electrochemical dechlorination of CEs, and hence to study the cathodic reduction instead of using GR as the reductant. We find that the nitrogen-doped carbonaceous catalysts optimized for chemical reduction of CEs also show high catalytic reactivity in the electrochemical reduction. The electrochemical dechlorination with nitrogen-doped carbonaceous electrode achieved a high efficient, low-cost, and chemical-free technique for CEs remediation in groundwater.