Transformation of Chloride to Organic Chlorine in Terrestrial Environments: Variability, Extent, and Implications

Chloride (Cl-in) is a key component in a number of biogeochemical risk assessment models and, as a rule, these models assume that Cl-in behaves conservatively in soil. However, this assumption is challenged by the growing number of studies on natural formation-mineralization of organically bound chlorine (Cl-org) in terrestrial environments. The authors synthesized and scrutinized previously reported transformation rates of Cl-in to Cl-org in terrestrial environments to assess whether the process is of sufficient size to impact risk assessment models significantly. Their conclusion is that there is increasing evidence that considerable amounts of Cl-in are transformed to Cl-org in or on plants and that the process consistently runs at relatively high rates. The findings imply that risk assessment models that include uptake-transport of Cl-in by vegetation are likely to be influenced by the transformation of Cl-in to Cl-org in or on vegetation. The review confirms previous observations that transformation rates in soils depend on a combination of a number of factors such as concentration or deposition of Cl-in, organic matter content, nutrient availability, redox, and season. The review reveals that there seems to be at least two pools of Cl-org in the terrestrial environment: one with slow transformation rates, low mobility, and with a comparably low chlorine-to-carbon ratio (leaves, litter, and bulk-soil), and a smaller pool with considerably higher transformation rates, higher mobility and higher chlorine-to-carbon ratios (organic matter in through fall and soil leachate). Formation of chlorinated organic matter has a strong biotic component, implying that models built on physicochemical parameters (e.g., sorption and Kd values) are insufficient to predict movement and fate of Cl-36. The study clearly demonstrates that it is necessary to develop reliable rate estimates and methods that enable a deeper understanding of how and why the Cl-in is transformed to Cl-org and how this influences transport, distribution, and fate of Cl-36 in different environments.