Design of a multi-stage stirred adsorber using mesoporous metal oxides for herbicide removal from wastewaters

• A stirred sorption system for the removal of herbicides was designed for the first time.
• A mathematical model describing single and multi-stage adsorbers was developed.
• The optimum conditions were found for single-, double- and triple-stage adsorbers.
• Double-stage systems led to a large reduction of the required sorbent amount.
• Triple-stage systems allowed little improvements, though requiring higher costs.

Stirred adsorbers offer a number of advantages in comparison to fixed bed systems, as they reduce mass transfer resistances, pressure drops and fouling of the sorbent particles. Single-, double- and triple-stage adsorbers were designed for the adsorption of two herbicides, 6-chloro-N,N′-diethyl-1,3,5-triazine-2,4-diamine (Simazine) and (4-chloro-2-methylphenoxy)acetic acid (MCPA), on two mesoporous metal oxides, aluminum oxide (Al2O3) and iron(III) oxide (Fe2O3).The equilibrium data were described by the Freundlich model (for Simazine) and Langmuir model (for MCPA). The sorption kinetics was analyzed adopting pseudo-first (for MCPA) and the pseudo-second (for Simazine) order models. Based on the kinetic and equilibrium equations selected, a mathematical model describing single and multi-stage sorption systems was developed to predict the minimum amount of adsorbent required to remove a given fraction of herbicide from a fixed volume of wastewater.The replacement of a single-stage system with a two-stage system results in a significant reduction (from 78 to 91%) of the overall required sorbent amount that balances the higher cost of the two-stage plant. On the contrary, the adoption of a three-stage system allows only a limited reduction (from 24 to 41%) of sorbent in comparison to a two-stage system having the same total volume.