Ammonium Nitrate Solution (ANSOL) is generated as a byproduct during explosives manufacturing process. Historically, ANSOL has been recycled for use in the mining industry, however, the unreliability of the recycling process over recent years has necessitated exploration of its treatment. The purpose of this study was to develop an electroactive membrane-based platform technology for the treatment of ANSOL. The process recovered ammonia via electroactive gas stripping membranes functioning as a cathode, while eliminating common ANSOL chemicals of concern, such as, RDX and Cr(VI). The transformation of waste streams from cost centers to profit centers has been generating significant interest from multiple industrial sectors, including farm waste, municipal water treatment, and chemical manufacturing. Therefore, a preliminary technoeconomic analysis (TEA) was also performed to study the economic viability of the treatment process.

The novel process employs three electroactive membranes to (i) facilitate oxidation of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), (ii) remove hexavalent chromium from solution, and (iii) to recover pure ammonium from the contaminated ammonium nitrate stream. For Cr(VI) removal, electrochemical reductions have been particularly effective. By integrating these methods with membrane-based processes (i.e., flow-though electrodes), which have been shown to minimize mass-transport limitations in electrochemical reactions, the system is designed to simultaneously reduce any Cr(VI) on the membrane/cathode while oxidizing RDX on the membrane/anode, resulting in a stream free of RDX and Cr(VI) that can be safely disposed of. Previous studies have successfully demonstrated the electrochemical reduction of chromium using electrically conductive ultrafiltration (UF) membranes at sufficiently high applied potentials. 

In addition to Cr(VI) reduction, electrically conducting membranes could also help in recovering ammonia from the contaminated high concentration ammonium nitrate solution. In an aqueous solution, ammonium ions exist in a pH-dependent equilibrium with dissolved ammonia (at a pKa of 9.24).  At a pH above the pKa, the majority of ammonia exists in its non-ionic form, which has a significantly higher volatility. This ammonia can be recovered through stripping across a hydrophobic membrane that allows for gas transport. Using electrically conductive membranes for this purpose enables the splitting of water and the generation of hydroxide ions at the membrane/water interface, increasing the local pH, and driving the formation of NH3 that can then be driven across the membrane. This approach allows for the capture of ammonium directly from the waste stream (ideally in pure form), eliminating the need for alternative treatment methods, and providing an ammonia stream, free of RDX, chromium, and other chemicals of concern, that could be applied for agricultural uses or be used as an energy source.

The team developed a platform technology utilizing electroactive membranes to effectively remove prevalent ANSOL contaminants, specifically Cr (VI) and RDX. This technology is also capable of valorizing ANSOL, converting it into valuable commodities like ammonium nitrate. By circulating the feed solution through the Ti₄O₇ (anode) pores, the team observed significant enhancements in both mass transfer and RDX oxidation rates. The electrode's porous structure grants it an expansive electrochemical surface area, leading to increasing oxidation rates as recirculation rates increase. Chromium removal was found to be pH dependent as the species was predominantly present as Cr(III); by adjusting the feed's pH to alkaline conditions above 9, Cr(III) is effectively removed through the precipitation of Cr(OH)₃. Ammonia recovery was also made possible with the use of a secondary hydrophobic membrane. The incorporation of a cation exchange membrane improved ammonia recovery rates allowing the pH adjacent to the CF-PTFE membrane to surpass the pKa of ammonium ions, thus elevating recovery. 

The technoeconomic analysis, spanning both experimental and industrial scales, revealed optimal performance at the highest current density combined with the lowest recirculation rate. Additionally, the TEA also revealed that material costs dominate the expenditure, comprising the bulk of overall expenses. The main issue with this technology is the relatively slow reaction rates which cause slow degradation rates when compared to alternative methods. Additionally, the long residences times coupled with the high energy demand yield high degradation costs. It is possible that better electrocatalysts will improve performance, however, this could increase costs further.