Dual Effects of Humic Acid in Trichloroethylene Removal from Groundwater by Zero-Valent Iron: Hydrophobic Partition and Surface Adsorption

Abstract

Natural organic matter (NOM) in groundwater is a factor of concern in long-term operation of Fe0 permeable reactive barrier (PRB). In this study, humic acid, a major component of NOM, showed dual effects in trichloroethylene (TCE) removal from simulated groundwater by Fe0 in batch and column experiments. In the initial stage of contacting with Fe0, humic acid promoted TCE removal due to its hydrophobic partitioning towards TCE and the subsequent fast adsorption onto Fe0 surfaces. In a long run, humic acid inhibited TCE removal because the buildup of adsorbed humic acid on Fe0 surfaces passivated Fe0 reactivity and limited TCE mass transfer. Ca2+ enhanced the co-aggregation of humic acid with Fe0 corrosion products and led to a faster depletion of TCE removal capacity by diminishing Fe0 matrix porosity. Revealed by FTIR analysis, part of TCE removed through hydrophobic partitioning was retained in humic acid accumulated on Fe0 surfaces rather than reductively degraded by Fe0. It raises a concern of using Fe0 PRB to treat organic contaminants in NOM-rich groundwater. Releasing back of NOM retained organic contaminants might take place once the accumulated NOM is desorbed or detached from Fe0 surfaces, resulting in a rebound of organic contaminants in the groundwater beyond the confine of PRB.