Teaching Adsorption Chemistry by Constructing Surface Complexation Models (SCM) in PHREEQC

Abstract

Adsorption modeling is important to understand and predict both how water contamination occurs, and how it might be prevented or remediated. Surface complexation models (SCM) help us understand adsorption under changing environmental conditions. This computational class introduces undergraduate students to adsorption and surface complexation and provides a foundation in geochemical modeling using freely available PHREEQC software from USGS. Practical capabilities are developed by (1) performing aqueous speciation reactions, (2) determining the surface charge of suspended metal oxide/mineral powders using potentiometric titration data, (3) calculating pH adsorption edges and adsorption isotherms, (4) applying PHREEQC to answer possible water treatment scenarios, and (5) using Visual Basic coding with loops to generate large data sets. This class is contextualized around arsenic contaminated groundwater and its treatment using iron oxide minerals, with hundreds of millions at risk worldwide (e.g., India, Bangladesh, Mexico). Participants report increases in their understanding of adsorption chemistry and confidence in the application of geochemical models to perform predictive SCM calculations.