Automatic pH control system enhances the dechlorination of 2,4,4′-trichlorobiphenyl and extracted PCBs from contaminated soil by nanoscale Fe⁰ and Pd/Fe⁰

Abstract

Purpose

Dechlorination of polychlorinated biphenyls (PCBs) by nanoscale zerovalent iron (NZVI) is often strongly hindered by increased pH because large amounts of H⁺ ions were consumed during the surface reaction. The main objective of this work was to evaluate the effect of pH control in acid on the dechlorination processes of PCBs and to compare the dechlorination efficiency between 2,4,4′-trichlorobiphenyl (2,4,4′-CB) and the extracted PCBs from the field PCBs-contaminated soil in this system.

Methods

The reaction solution pH was controlled to be weakly acid (4.90–5.10) with an automatic pH control system, in which the dechlorination of 2,4,4′-CB and extracted PCBs from a PCBs-contaminated soil by NZVI and palladized nanoscale zerovalent iron (NZVI/Pd) was investigated.

Results

To control the reaction solution pH to be acid actually increased the dechlorination rate of PCBs by NZVI and NZVI/Pd. The observed normalized pseudo-first-order dechlorination rate constants (k _obs) of 2,4,4′-CB increased from 0.0029 min⁻¹ (no pH control) to 0.0078 min⁻¹ (pH control) by NZVI and from 0.0087 min⁻¹ (no pH control) to 0.0108 min⁻¹ (pH control) by NZVI/Pd. In the case of NZVI/Pd, the chlorines in the para position were much more easily dechlorinated than ortho position, and biphenyl was the dominating product. As the solution pH was controlled at 4.90–5.10, the dechlorination rate constants of PCB congeners extracted from soil (k _obs) were 0.0027–0.0033 min⁻¹ and 0.0080–0.0098 min⁻¹ by NZVI and NZVI/Pd, respectively.

Conclusions

To keep the reaction solution to be weakly acid markedly increased the dechlorination rate of PCBs, which may offer a novel technology in the remediation of PCBs-contaminated soil.