Effects of Ionic Strength on MES Sorption and Ni Sorption Kinetics at the Goethite-Water Interface.
Brian Rosen, Ryan Tappero, Kristian Paul, and Donald Sparks. Univ of DE, 531 S. College, 152 Townsend Hall, Newark, DE 19717
Soil chemists use batch sorption experiments to measure metal and oxyanion retention by soil and soil components to predict their fate and transport in the environment. Solution pH is a master variable influencing the thermodynamics and kinetics of sorption reactions on mineral surfaces, and therefore must be carefully controlled in batch sorption experiments. Buffers offer an effective means to control pH. However, buffers may interact with dissolved species or mineral surfaces. Batch sorption experiments (pH 6.5, 1.0 g/L solid, 500 µM sorbate, 48 hrs) were conducted to investigate the interaction between a common sulfonic acid buffer MES (2-[N-Morphalino]-ethanesulfonic acid) and goethite (a-FeOOH). Preliminary data indicated MES sorbed to goethite (Γ= 12 µmol/m2) in low ionic strength conditions (I < 0.01 M), but sorption was not evident at higher ionic strengths (I > 0.05 M), which indirectly suggested MES formed an outer-sphere complex. Additional evidence from ATR-FTIR flow-cell experiments reinforced the macroscopic batch data. Splitting of a single IR active band (~1050 cm-1) into two IR active bands occurred after exposing a MES buffer solution (500 µM) to a thin film of a-FeOOH, which indicated an interaction between MES and the mineral surface. However, nearly complete disappearance of the bands occurred rapidly when the influent flow-cell solution was changed to 0.01 M NaCl, suggesting MES was only weakly bound to the a-FeOOH surface. The influence of MES buffer-mineral interactions on Ni sorption kinetics was investigated. Total Ni loading was similar in buffered and unbuffered systems at both low and high ionic strengths (Γ= 1.2 µmol/m2), which indirectly suggested Ni forms an inner-sphere complex on a-FeOOH. However, the short-term Ni sorption kinetics (< 20 min) were different for the buffered systems with high and low ionic strength, which suggested Ni sorption was influenced by interaction of MES with a-FeOOH. Results suggest “Better” buffers may not always be considered as inert components in experimental batch systems, and may unexpectedly affect sorption kinetics.