The objective of this Statement of Need (SON) was to seek innovative research that leads to a better understanding of the physical-chemical properties of per- and polyfluoroalkyl substances (PFAS) in order to predict their behavior within various environmental systems, engineered treatment systems, and biota. Successful projects will address the need for determining critical physical-chemical properties of not only the neutral forms of PFAS, but also charged monomeric species. Specific research needs were as follows:

• Evaluate data on conventional physical-chemical properties for PFAS that have been reported in order to clarify inconsistencies in reporting, and identify and address critical data gaps. Such parameters include, but are not limited to, melting point, boiling point, vapor pressure, aqueous solubility, Henry’s Law constant, Kow, pKa, and critical micelle concentration. Properties should be assessed as a function of temperature, as possible. 
• Develop fundamental chemistry-based models that better estimate PFAS distributions between environmental phases and organism phases of individual PFAS in legacy and modern fluorotelomer-based aqueous film-forming foam (AFFF).
• Develop models that can predict properties from structure-activity relationships such as linear solvation energy relationships (LSERs) and poly-parameter linear free energy relationships (pp-LFERs). 
• Develop an improved understanding of the environmental phases/interfaces and corresponding phase distribution behavior that are controlling partitioning in the subsurface and impact fate and transport. Properties should be assessed as a function of temperature, as possible.
• Develop an improved understanding of the organisms and organism compartments that most strongly accumulate PFAS mass. Edible plants and higher trophic organisms are of interest, but proposers should provide the rationale for selected organisms of study.

One or more of the objectives listed above could be addressed in a single proposal. A strategic workshop on PFAS in the environment was held in 2022 in which research and demonstration needs were identified. Proposers were strongly encouraged to review the document Summary Report: Strategic Workshop on Management of PFAS in the Environment for additional information on these research needs. Proposers had to be cognizant of previous SERDP- and Environmental Security Technology Certification Program (ESTCP)-funded efforts that relate to this research area.

It was preferred that at a minimum, the 40 PFAS that can currently be measured by U.S. EPA Method 1633 was considered; deviation from this list was accompanied by the rationale for such deviation. Environmentally relevant concentrations of PFAS were of particular concern; proposed efforts were conducted at such concentrations or the rationale was provided if different concentrations were proposed. 

Research and development activities at bench-, mesocosm-, and field-scale were be considered, although work did not necessarily have to culminate in a field-scale effort. Physical-chemical properties preferably should have been applicable across multiple different environmental media (e.g., surface water, groundwater, soil, sediment, air), but could be proposed for a single specific medium. 

Developing an improved understanding of the physical-chemical properties of PFAS is critical to ultimately understanding their fate in the environment. Such information will ultimately lead to improved management of PFAS in the environment.

Any quantitative approach to analyzing the behavior of PFAS in treatment systems or in the environment requires knowledge of its distribution in the multiple phases present. For example, a successful treatment system should be designed such that the PFAS of interest is primarily associated with the treatment phase and not sequestered in other phases that are recalcitrant to treatment. This requires that PFAS speciation be considered (e.g., the relative quantities of free or complexed anionic or neutral species, cation-anion ion pairs, larger molecular aggregates, or pure phase if the solubility limit is exceeded). All of this information is required to build a reliable model of PFAS fate and transport within any media. 

If PFAS is primarily associated with stationary phases, its mobility is much less than if it is in the vapor or aqueous phases. Evaluation of the bioaccumulation and toxicity of PFAS also requires consideration of its phase distribution and speciation. PFAS bound to phases not exposed to the organism or distributed among multiple chemical species will accumulate differently in organisms depending on the bioavailability of each species and their toxic potential. This is not a new requirement for understanding transport, transformations, and toxicity of environmental chemicals of concern, but the challenges and unknowns are extreme for PFAS.

Improving the understanding of the organism compartment(s) that most strongly accumulate PFAS mass is critical to predicting its ultimate fate and transport. Partition coefficients for neutral and ionic PFAS species partitioning from water/environmental matrices to plants (i.e., the major biological components such as lipids, proteins, lignin) and animals (e.g., membrane and storage lipids and proteins of shellfish, fish, mammals, deer, and birds) is a particular data gap. Effort is needed for both equilibrium and food web-based exchanges. As an alternative approach, toxicokinetic models that account for PFAS-specific protein binding and trans compartment transport (e.g., organic anion transport proteins) also may need to be considered.

The cost and time to meet the requirements of this SON are at the discretion of the proposer. Proposers submitting a Standard or Limited Scope Proposal must provide the rationale for the proposed scale. The two options are as follows: 

Standard Proposals: These proposals describe a complete research effort. The proposer should incorporate the appropriate time, schedule, and cost requirements to accomplish the scope of work proposed. SERDP projects normally run from two to five years in length and vary considerably in cost consistent with the scope of the effort. It is expected that most proposals will fall into this category. 

Limited Scope Proposals: Proposers with innovative approaches to the SON that entail high technical risk or have minimal supporting data may submit a Limited Scope Proposal for funding up to $250,000 and approximately one year in duration. Such proposals may be eligible for follow-on funding if they result in a successful initial project. The objective of these proposals should be to acquire the data necessary to demonstrate proof-of-concept or reduction of risk that will lead to development of a future Standard Proposal. Proposers should submit Limited Scope Proposals in accordance with the SERDP Core Solicitation instructions and deadlines.