Sorptionskinetik

Sorptionskinetik, or sorption kinetics, describes the rate at which a solute is transferred from a liquid phase to a solid sorbent (or between phases) and the mechanism by which sorption proceeds. Sorption encompasses adsorption, where molecules adhere to a surface, and absorption, where molecules are taken into the bulk. Kinetic behavior reflects the nature of the interactions, which can be physical (physisorption) or chemical (chemisorption), and is often influenced by diffusion processes both at the external surface and within the sorbent pores.

Several kinetic models are used to interpret sorption data. The pseudo-first-order model describes the rate of occupation of sorption sites proportional to the number of unoccupied sites with a single apparent rate constant. The pseudo-second-order model assumes the rate is controlled by chemisorption or by the square of the difference between equilibrium and current sorption, often fitting many data sets well. The intra-particle diffusion model (Weber-Morris) expresses sorption as qt = kid sqrt(t) + C, where a linear plot of qt versus sqrt(t) can indicate diffusion within pores as a rate-limiting step. The Elovich model is used in some chemisorption scenarios on heterogeneous surfaces. Interpretations of kinetic plots help identify the controlling step: linear qt vs sqrt(t) with zero intercept suggests pore diffusion control, while deviations may indicate boundary-layer effects or multi-step processes.

In practice, sorption kinetics inform the design of treatment systems and help compare sorbents in batch and continuous-flow setups. Key factors affecting rates include temperature, agitation, particle size, porosity, solution pH, ionic strength, and competing species. While kinetic models yield useful rate constants, they are empirical and may not capture all mechanisms in complex systems.