Effect of Nanoscale Zero-Valent Iron Treatment on the Characteristics of a DNAPL Source Zone in Sandy Soils(Abstract)

摘要

Subsurface placement of nanoscale zero-valent iron (NZVI) is an emerging technique for in-situ remediation of chlorinated hydrocarbons such as trichloroethylene (TCE) and perchloroethylene (PCE). While dechlorination by NZVI is fairly well understood on the batch scale, the interactions between NZVI and chlorinated hydrocarbons in soils under realistic flow conditions are both more complex and have been less investigated. Espe- cially in dense nonaqueous phase liquid (DNAPL) source zones, where dissolution and dechlorination occur as coupled processes, the net effects of NZVI placement on reduc- ing DNAPL source mass and mass emission from source zones are poorly understood. To address this issue, a well-controlled laboratory experiment was conducted in a small two- dimensional (2D) sand tank. The 2D test setting allows for the creation of realistic flow fields around the sources, compared to 1D column tests. An immobile DNAPL TCE source was created within a coarse sand lens surrounded by finer sand and groundwater under anaerobic conditions was allowed to flow through the tank. Bare reactive nano-iron particles (RNIP) were injected directly into the DNAPL source zone and the changes in the source mass and dissolved TCE concentrations away from the source were deter- mined with GC/FID. All dechlorination byproducts were also measured. It was observed that effluent TCE concentrations increased directly after RNIP injection indicating that NZVI treatment enhances TCE dissolution. Acetylene, ethene, ethane and non- chlorinated C4 hydrocarbons were also observed showing that a portion of TCE was fully dechlorinated. However, intermediate byproducts, i.e. cis-, trans- and 1-1- dichloroethylene as well as vinyl chloride were also detected down-gradient from the DNAPL source. These observations suggest that NZVI treatment of a DNAPL source zone by injecting NZVI into the source is likely to produce faster dissolution and thereby reducing the longevity of the source. However, directly after NZVI injection, effluent concentrations of chlorinated hydrocarbons may initially increase and chlorinated by-products may be formed and transported downstream of the source, requiring further treatment of the dissolved plume. NZVI treatment of DNAPL source zones should con- sider these potential effects on contaminant mass emissions when developing an injection and monitoring scheme.