A Comparison of Batch‐ and Flow‐Generated Anion Adsorption Isotherms

Solute transport models often use batch‐generated adsorption isotherms to partition solute between the aqueous and adsorbed phases, but the relationship at equilibrium between aqueous and adsorbed solute concentrations may be different in closed (batch) and open (flow) systems. Adsorption isotherms were generated for orthophosphate/goethite, silicate/goethite, orthophosphate/soil clay and orthophosphate/arsenated‐goethite systems using batch and flow (miscible displacement) techniques. The contact times were 168 and 96 h in the batch and flow experiments, respectively. The shapes of the isotherms generated by the two methods were very similar in all cases, although the flow‐generated isotherms were displaced slightly upward relative to the batch isotherms. Simple Langmuir plots of both batch and flow data were nonlinear for all systems, while in all systems containing phosphate, flow‐generated distribution coefficients were greater than the corresponding batch‐generated values, particularly at low surface coverage. In addition, adsorption maxima and values of the two‐surface Langmuir parameter b 1 were consistently greater in the flow systems. The results indicate that provided transport processes do not keep the flow system far from chemical equilibrium, batch‐generated anion adsorption isotherms are likely to underestimate the extent of adsorption in the corresponding flow system, particularly at relatively low aqueous phase sorbate concentrations. This appears to be due, in part, to the removal of competitive antecedent species in the flow system effluent. It is suggested that flow systems are thermodynamically equivalent to batch systems having very wide solid/solution ratios.