Influence of Natural Organic Matter on Contaminant Removal by Permeable Reactive Barrier

Natural organic matter (NOM) is ubiquitous in surface soils and shallow aquifers where most anthropogenic contamination occurs. The influence of NOM, as a dissolved or colloidal phase, on the contaminant removal during contaminated site remediation has been of increasing concern in recent years. This chapter provides an overview of the characteristics and geochemical reactions of NOM in the environment and discusses the effects of NOM on the application of zero-valent iron permeable reactive barrier, which is a widely used technology for groundwater remediation. The adsorption of NOM may inhibit the contaminant removal due to competition for or steric blocking of reactive sites, decrease in mass transfer to/from the reactive surfaces, and alteration of surface electrostatic and reductive potentials. Moreover, complexation of NOM with iron released from corrosion results in the formation of soluble complexes, colloids (< 0.1 μm), and aggregates (> 0.45 μm) in solution. Substantial amounts of soluble and colloidal metal-NOM complexes in groundwater arouse aesthetic and safety concerns regarding groundwater use. Nevertheless, molecular weights of metal-NOM colloids increase with reaction time and metal-NOM aggregates are formed through inner-sphere complexation and hydrophobic interactions. Subsequent deposition of these aggregates provides an additional hydrophobic domain for partitioning that enhances the removal of organics to some extent. Nevertheless, continuous deposition of metal-NOM aggregates on the surfaces may result in a more noticeable blocking of reactive sites and flow paths, which should be monitored in the long term.