Synthesis, characterization, and application of zero-valent iron nanoparticles for TNT, RDX, and HMX explosives decontamination in wastewater

Highlights

• An inter-technique study of explosives decontamination onto nZVI was conducted.

• Effective reduction of TNT, RDX, and HMX over nZVI nanocomposites.

• Explosives were successfully decomposed to CO₂, N₂O, and CH₄.

• 2 g/kg of nZVI facilitated a most complete degradation of TNT within 6 h.

• Efficiency of TNT degradation was higher than that of RDX and HMX.

Abstract

This study has evaluated the efficiency of zero-valent iron nanoparticles (nZVI) for the remediation of soil and groundwater containing 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), and 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX). It was conducted via different treatment methods (batch experiments, column tests, and a permeable reactive barrier (PRB)) system. The combinative studies could be used to develop a more effective remediation technique. The chemical reactions that occurred upon mixing nZVI and contaminants provided the batch experiments, leading to rapid reduction of the explosives. There was a decrease in the removal efficiency from 95% to less than 30% for the batch experiments, which was due to the lake of stirring facilities in the column test, PRB system, and the interaction of the soil and explosives with nZVI. Kinetics studies indicated a more significant and rapid degradation of TNT than that of RDX and HMX, which was consistent with the lower activation energy of TNT. The X-ray spectroscopy results highlighted that during the reduction process, nZVI was transformed into core-shell structures with Fe(0) core and Fe₃O₄ shell. High-performance liquid chromatography-mass spectrometry (HPLC/MS) tests depicted the decomposition of explosive contaminants into simple elements, such as carbon dioxide, nitrous oxide, and methane, over cleavage of the ring structure.

Introduction

At the beginning of the twentieth century, over sixty highly explosive complexes were fabricated for the army, protection of national boundaries, and mining operations. These high-energy explosives generated by military factories include 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and are the predominantly used explosives in the world [1], [2], [3], [4] (Scheme 1). These contaminants are referred to as nitro aromatic or nitramine compounds. The compounds have contaminated the soil, surface, and ground water. Their degradation residues, which have severe toxicity even at low concentrations are toxic and genotoxic to living organisms, can infiltrate into the soil and flow to the groundwater, thus leading to severe pollution of environmental/ecological and human species. These compounds have severe toxicity at low concentrations [5], [6], [7].

Owing to the adverse effects of explosives on the environment, the U.S. Environmental Protection Agency (USEPA) has declared seven nitro-substituted explosives, including TNT, RDX, and HMX, as the priority contaminants. In addition, TNT, HMX, and RDX contaminants can still be found in several land areas. These areas have been contaminated with the highly explosive contaminants for many years. Most of these land areas cannot be used and will remain unworkable until large-scale in-situ decontamination exercises are instituted [8,9]. In addition, RDX can have a severe effect on the central nervous system of a human being and may cause cancer [10], [11], [12], [13], [14]. Thus, removal of such contaminants from contaminated waste water is very important to prevent the severe effects they can bring to human being and other living organisms.

Over the years, several remediation methods for the explosive compounds have been reported. However, the methods are expensive and often require contaminants to be disposed of as a hazardous waste. Nevertheless, nano-zero-valent iron (nZVI) has received immense attention for the remediation of several contaminants. The cost-effective and environmentally-friendly nZVI has been used for heavy-metal soil and water remediation, and acceptable results have been shown in laboratory and pilot-scale studies [15], [16], [17], [18], [19]. The high surface area of nZVI increases its reactivity with contaminants. Similarly, nZVI oxidation enhances electron transfer to the pollutants, thus degrading them to harmless compounds [20], [21], [22], [23]. nZVI can be used for the decontamination of several heavy metals because of its ability to be transported by groundwater and injected as sub-colloidal metal nanoparticles into contaminated wastewater [24,25]. nZVI is predominantly utilized for the remediation and removing organic compounds from water. The degradation efficiency of nZVI is based on its adsorption and reducing ability. It is adeptly of reducing and modifying numerous organic compounds by transforming them from toxic to less or none-toxic compounds. Over the years, nZVI has been used for the remediation of dense non-aqueous phase liquids (DNAPLs), pharmaceutical contaminants, nitrates and nitrites, and many more [4,20,24,25,33]. Mesoporous materials, such as nZVI, with regularly controlled pore and channel structures are useful for adsorption and removal of pollutants from water. Numerous studies have shown the successful reduction of TNT, RDX, and HMX contaminants with nZVI, showing a fast reduction of RDX using nZVI and discovered nitroso intermediates, and N₂O as RDX reduction products [4,[26], [27], [28], [29], [30]] (Scheme 1).

Hence, this work was aimed at studying the kinetics of TNT, RDX, and HMX explosive degradation using synthesized nZVI. Thus, the degradation of contaminated soil and groundwater using the nZVI was evaluated based on different concentrations, time scales, and temperatures. The degradation efficiency was compared through batch experiments, a column test, and a lab-scale PRB system. Finally, the mechanism and detection of the final products after the reaction were determined. Most importantly, this study will provide an insight about the transformation of explosive contaminants after the treatment with nZVI, thus assisting in the optimization of remediation processes.