Detoxification of PAX-21 ammunitions wastewater by zero-valent iron for microbial reduction of perchlorate

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Abstract

US Army and the Department of Defense (DoD) facilities generate perchlorate (ClO4) from munitions manufacturing and demilitarization processes. Ammonium perchlorate is one of the main constituents in Army's new main charge melt-pour energetic, PAX-21. In addition to ammonium perchlorate, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and 2,4-dinitroanisole (DNAN) are the major constituents of PAX-21. In order to evaluate microbial perchlorate reduction as a practical option for the treatment of perchlorate in PAX-21 wastewater, we conducted biodegradation experiments using glucose as the primary sources of electrons and carbon. Batch experiments showed that negligible perchlorate was removed in microbial reactors containing PAX-21 wastewater while control bottles containing seed bacteria and glucose rapidly and completely removed perchlorate. These results suggested that the constituents in PAX-21 wastewater may be toxic to perchlorate reducing bacteria. A series of batch toxicity test was conducted to identify the toxic constituents in PAX-21 and DNAN was identified as the primary toxicant responsible for inhibiting the activity of perchlorate reducing bacteria. It was hypothesized that pretreatment of PAX-21 by zero-valent iron granules will transform toxic constituents in PAX-21 wastewater to non-toxic products. We observed complete reduction of DNAN to 2,4-diaminoanisole (DAAN) and RDX to formaldehyde in abiotic iron reduction study. After a 3-day acclimation period, perchlorate in iron-treated PAX-21 wastewater was rapidly decreased to an undetectable level in 2 days. This result demonstrated that iron treatment not only removed energetic compounds but also eliminated the toxic constituents that inhibited the subsequent microbial process.

Highlights

Ammonium perchlorate, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and 2,4-dinitroanisole (DNAN) are the major constituents of PAX-21. ► DNAN is identified as the primary toxicant responsible for inhibiting the activity of perchlorate reducing bacteria. ► Iron treatment not only removes energetic compounds but also eliminates the toxic constituents that inhibit the subsequent microbial process.

Introduction

Perchlorate has recently received a great deal of attentions due to high concentrations found in groundwater and surface waters. Perchlorate has been released to the environment primarily through the use of ammonium perchlorate as a propellant in missiles, rockets and explosives, as a pyrotechnic in fireworks, in magnesium batteries, paint and as an automobile air bag inflator [1], [2]. Aqueous perchlorate is known to be highly stable and non-reactive under ambient conditions. Perchlorate is not only chemically stable in natural waters, but also extremely soluble and mobile; as a result, only a limited number of technologies are capable of removing perchlorate from contaminated water [3], [4]. Recently, microbial reduction of perchlorate has been recognized as a promising technology [5], [6], [7], [8] for the treatment of perchlorate-contaminated water. Some facultative anaerobic bacteria can utilize perchlorate as an electron acceptor and reductively transform perchlorate to environmentally benign end product, chloride (Cl) (Fig. 1). Many researchers successfully showed that perchlorate can be effectively removed from the biological system by supplying sufficient electron donors. A variety of different electron donors, including ethanol, fatty acids (acetate, lactate, propionate, and citrate), vegetable oils, have been shown to support the growth of perchlorate respiring bacteria [6], [8]. Several perchlorate respiring bacteria have been shown to use inorganic electron donors including H2, sulfide and Fe(II) [9]. Perchlorate respiring bacteria (PRB) are ubiquitous in natural environment such as soils, sediments, surface water, and groundwater aquifers [10]. A variety of PRB have been isolated and many of which are members of the newly identified genera Dechloromonas, Dechlorospirillum, and Dichlorosoma [1], [6].

US Army and the Department of Defense (DoD) facilities generate perchlorate from munitions manufacturing and demilitarization processes. For example, ammonium perchlorate is one of the main constituents in Army's new main charge melt pour explosives, Picatinny Arsenal Explosive 21 (PAX-21). In addition to ammonium perchlorate, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and 2,4-dinitroanisole (DNAN) are the major constituents of PAX-21. Presently, most Army ammunition plants use granular activated carbon (GAC) adsorption and alkaline hydrolysis to separate and treat energetic compounds in wastewater from munitions manufacturing and demilitarization processes. The GAC process not only is expensive but generates explosive-laden spent carbon, which needs to be treated or disposed of properly to avoid secondary contamination problems. This additional treatment further increases the overall cost of wastewater treatment.

Literature review suggested that perchlorate in PAX-21 wastewater can be removed by perchlorate respiring bacteria when it is supplied by sufficient electron donor. However, there is little information whether energetic compounds in PAX-21 wastewater can influence the rates of perchlorate biodegradation. Many nitroaromatic compounds including 2,4-dinitrotoluene (DNT) and 2,4,6-trinitrotoluene (TNT) have been shown to be toxic or mutagenic to various organisms [11]. Davies and Provatas [12] showed that DNAN was mutagen in bacteria and it exhibited larger toxicity to bacteria than munitions Composition B (60% RDX, 40% TNT). Recently, it was shown that zero-valent iron (Fe(0)) can enhance biodegradation of recalcitrant nitroaromatic compounds by removing electron-withdrawing nitro groups [13], [14]. Perey et al. [13] showed that elemental iron pretreatment of azo dye-containing wastewater can reductively transform the electron-withdrawing constituents on the azoaromatic compounds and make them more amenable for aerobic biodegradation. Oh et al. [14] also showed that Fe(0) treatment transformed recalcitrant RDX to ring cleavage products, formaldehyde, that are more amenable to mineralization by aerobic bacteria. If constituents in PAX-21 wastewater have toxicity to perchlorate respiring bacteria, Fe(0) pretreatment of PAX-21 wastewater may also be considered for enhancing the biodegradability in the similar manner.

The objective of this research is to determine the toxicity of energetic compounds to perchlorate respiring bacteria and evaluate microbial perchlorate reduction as a practical option for the treatment of perchlorate in PAX-21 wastewater. Biodegradation experiments, using glucose as the primary source of electrons, were conducted to determine whether energetic compounds in PAX-21 wastewater can influence the rates of perchlorate biodegradation. We also evaluated iron pretreatment of PAX-21 wastewater to increase perchlorate removal rates, and finally we propose an integrated iron pretreatment and anaerobic biological treatment process for treatment of perchlorate containing PAX-21 wastewater.

Section snippets

Chemicals

Glucose, 2,4-dinitroanisole (C7H6N2O5, DNAN, 98%), and 2-methoxy-5-nitroanlinine (C7H8N2O3, 98%) were purchased from Sigma (St. Louis, MO). 4-Methoxy-1,3-phenylenediamine sulfate hydrate (2,4-diaminoanisole sulfate hydrate, 99.5%) was purchased from Chem Service (West Chester, PA) and 4-methoxy-3-nitroanlinine (C7H8N2O3, 97.8%) was purchased from ChemPacific (Baltimore, MD). Sodium perchlorate monohydrate (NaClO4·H2O, ∼100%) was purchased from Fisher Scientific (Pittsburgh, PA). PAX-21

Batch biodegradation experiments

Table 1 summarizes the characteristics of the PAX-21 wastewater from the Holsten Army ammunition plant. Fig. 3 illustrates the disappearance of perchlorate in batch reactors under different conditions. After an acclimation period, perchlorate was rapidly and completely removed to an undetectable level in cell-control bottles containing only glucose and seed microorganisms (Fig. 3a). In contrast, negligible amount of perchlorate (less than 2%) was removed in batch reactors containing PAX-21

References (16)

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