A Bipolar Membrane-Integrated Electrochlorination Process for Highly Efficient Ammonium Removal in Mature Landfill Leachate: The Importance of ClO^• Generation

Abstract

Electrochemical oxidation has been demonstrated to be a useful method for removing biorefractory organic pollutants in mature landfill leachate but suffers from low efficiency in eliminating ammonium because of its resistance to being oxidized by HO • or free chlorine (FC) at decreased pH. Here, we propose a new bipolar membrane-electrochlorination (BPM-EC) process to address this issue. We found that the BPM-EC system was significantly superior to both the undivided and divided reactors with monopolar membranes in terms of elevated rate of ammonium removal, attenuated generation of byproducts (e.g., nitrate and chloramines), increased Faradaic efficiency, and decreased energy consumption. Mechanistic studies revealed that the integration of BPM was helpful in creating alkaline environments in the vicinity of the anode, which facilitated production of surface-bound HO • and FC and eventually promoted in situ generation of ClO • , a crucial reactive species mainly responsible for accelerating ammonium oxidation and selective transformation to nitrogen. The efficacy of BPM-EC in treating landfill leachates with different ammonium concentrations was verified under batch and continuous-flow conditions. A kinetic model that incorporates the key parameters was developed, which can successfully predict the optimal number of BPM-EC reactors (e.g., 2 and 5 for leachates containing 589.4 ± 5.5 and 1258.1 ± 9.6 mg L −1 NH 4 + -N, respectively) necessary for complete removal of ammonium. These findings reveal that the BPM-EC process shows promise in treating ammonium-containing wastewater, with advantages that include effectiveness, adaptability, and flexibility.

Boosting Ammonium Oxidation in Wastewater by the BiOCl-Functionalized Anode

“…It is visible that the concentration of NH 4 + -N consistently decreased over time during electrolysis, regardless of the initial content (ranging from 20 to 500 mg L –1 ) (Figure S14a). The linear curves displayed similar slopes, indicating a zero-order reaction kinetics. ,, This finding further supports the conclusion that the predominant reaction facilitated by Cl • primarily occurs within the bulk solution rather than at the surface because of its relatively long lifespan (∼10 –6 s). , A minimal NH 4 + -N loss occurred when the electrolyte did not contain Cl – (Figure S14b), suggesting that direct anodic oxidation and HO • -initiated reactions were not favorable for the oxidation of NH 4 + -N. This observation aligns with existing literature on the subject. , The addition of Cl – into the system resulted in a significant enhancement of the NH 4 + -N removal rate, attributable to the increased levels of reactive chlorine species generated. However, the extent of improvement diminished as the Cl – concentration surpassed 50 mM, indicating that the reaction became limited by charge transfer rather than mass transfer. , Interestingly, at a Cl – concentration of 500 mM, the reaction was slightly inhibited, potentially due to the reaction between Cl • and excessive Cl – resulting in the production of Cl 2 •– , a species resistant to oxidizing NH 4 + -N, as elaborated in the previous section.…”

“…Our previous studies and relevant literature have shown that the oxidation of NH 4 + by free chlorine under acid conditions exhibits sluggish kinetics. ,, For instance, the rate constant has been reported to be less than 10 2 M –1 s –1 at pH < 4.0 . To further confirm this, a series of reference experiments were performed in the same pH range and by directly mixing NH 4 + with HClO at different concentrations.…”

“…In addition to the synthetic wastewater that contained NH 4 + -N concentrations ranging from 20 to 500 mg L –1 and Cl – concentrations ranging from 50 to 500 mM, the real pretreated mature landfill leachate was also subjected to the electrolysis tests. The detailed information regarding the pretreatment methods can be found in our previous studies . The leachate sample used in the experiments had the following characteristics: chemical oxygen demand (COD) of 270.1 ± 12.2 mg L –1 , NH 4 + -N concentration of 626.2 ± 8.1 mg L –1 , and Cl – concentration of 2510.1 ± 16.6 mg L –1 .…”

“…In recent years, the electrochlorination of ammonium (NH 4 + ) to N 2 for wastewater treatment has received growing interest from academia and industry, − because this process shows promise as an alternative to the prevailing biological approach for treating biorefractory wastewater with a low C/N ratio, high salinity, low pH, and high temperature. , The electrochlorination process relies on the in situ production of free chlorine, which is generated by electrolyzing Cl – -laden wastewater. The free chlorine plays a pivotal role in facilitating NH 4 + conversion into its elemental form, similar to the traditional breakpoint chlorination process. ,, Dimensionally stable anodes (DSAs) consisting of mixed metal oxides (MMOs), such as IrO 2 –RuO 2 and IrO 2 –Ta 2 O 5 , are ideal anode materials for chlorine evolution and are widely used for treating NH 4 + in various wastewaters. ,, For example, our research group has collaborated with industry partners to implement a large-scale electrochemical reactor equipped with multiple MMO anodes for removing NH 4 + in mature landfill leachate, which has been successful in eliminating the pollutant.…”

“…Based on our two-year operational experience, two major problems that cause inferior performance are identified: (i) insufficient amounts of electrogenerated free chlorine at decreased Cl – concentrations in the landfill leachate and (ii) severe decline in pH caused by protons released from NH 4 + oxidation. We conclude that the origin of these issues is the slow reaction kinetics between HClO and NH 4 + , especially at decreasing pH values. , To address the technical issue, we focus efforts on developing a chlorine radical-initiated NH 4 + oxidation process, leveraging the superior oxidizing strength of the chlorine radical (with an oxidation potential of 2.47 V vs NHE) and its remarkably higher rate constant with NH 4 + (as depicted in the potential reaction formula in Text S1). ,, …”