Dual Effects of Humic Acid in Trichloroethylene Removal from Groundwater by Zero-Valent Iron: Hydrophobic Partition and Surface Adsorption
2016
Rao, Pinhua | Liu, Tongzhou | Liang, Xihua | Ding, Guoyu
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.
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