Understanding Ammonium Transport in Bioelectrochemical Systems towards its Recovery.

Ying Liu,Zhen He,Shuai Luo,Rui Qiao,Mohan Qin

doi:10.1038/srep22547

Abstract

We report an integrated experimental and simulation study of ammonia recovery using microbial electrolysis cells (MECs). The transport of various species during the batch-mode operation of an MEC was examined experimentally and the results were used to validate the mathematical model for such an operation. It was found that, while the generated electrical current through the system tends to acidify (or basify) the anolyte (or catholyte), their effects are buffered by a cascade of chemical groups such as the NH3/NH4+ group, leading to relatively stable pH values in both anolyte and catholyte. The transport of NH4+ ions accounts for ~90% of the total current, thus quantitatively confirming that the NH4+ ions serve as effective proton shuttles during MEC operations. Analysis further indicated that, because of the Donnan equilibrium at cation exchange membrane-anolyte/catholyte interfaces, the Na+ ion in the anolyte actually facilitates the transport of NH4+ ions during the early stage of a batch cycle and they compete with the NH4+ ions weakly at later time. These insights, along with a new and simple method for predicting the strength of ammonia diffusion from the catholyte toward the anolyte, will help effective design and operation of bioeletrochemical system-based ammonia recovery systems.

Highlights

In BES-based ammonia recovery systems, organic compounds in wastewater are oxidized by the exoelectrogens growing on an anode, producing a current through its external circuit[7]
In a seminal simulation study in which key transport phenomena and chemical reaction within the BES are modeled comprehensively, it was found that the diffusion of NH3 from the catholyte to the anolyte can greatly reduce the efficiency of recovery[20]
We first validate the mathematical model for ammonia recovery using microbial electrolysis cells (MECs) by comparing its predictions against experimental data obtained under the same operation conditions

Summary

Introduction

In BES-based ammonia recovery systems, organic compounds in wastewater are oxidized by the exoelectrogens growing on an anode, producing a current through its external circuit[7]. Since many cations such as Na+ ions exist in the anolyte, for a given current through the BES, they may compete with NH4+ ions for transport across the CEM11,16,21 How strong such competition is and how it affects the ammonia recovery is not well understood. Another open issue is how the pH in the anolyte and catholyte is regulated. In the only comprehensive model for BES-based ammonia recovery, the transport and chemical reactions of major species in BES was studied at a steady state[20] The predictions of this model agree qualitatively with experimental observations and they led to critical insight into the effects of current density and membrane properties on the ammonia recovery. We have studied the ammonia recovery in a representative BES – microbial electrolysis cells, by integrating experiments with simulations

Methods

Results

Conclusion