Abstract

Perchlorate has recently received a great deal of attention due to high concentrations found in groundwaters and surface waters. The US Army and DoD facilities generate ammunition wastewater containing perchlorate (ClO ₄^-), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and 2,4-dinitroanisole (DNAN) (i.e. PAX-21 wastewater) from munitions manufacturing and demilitarization processes. Perchlorate is known to affect human health by interfering with the uptake of iodide into the thyroid glands. The US EPA recently set the perchlorate reference dose to 0.0007 mg/kg/day which corresponds to a drinking water equivalent level of 24.5 ppb. Aqueous perchlorate is both chemically stable in natural water and extremely soluble and mobile; as a result, many traditional wastewater treatment techniques that are commonly used for solvents and other organic pollutants are not effective for removal of perchlorate from contaminated water. 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. GAC processes are not only expensive but generate 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.

The overall objective of this research was to develop a novel treatment process for the removal of perchlorate and energetic compounds from mixed ammunition wastewater. We investigated two options for effective removal of perchlorate from mixed munitions wastewater that contains both perchlorate and energetic compounds: (1) increasing the solution temperature to overcome the kinetic barrier of perchlorate reduction by zero-valent iron; and (2) integrating Fe(0)-biological process for simultaneous removal of perchlorate and energetic compounds.

Perchlorate reduction by zero-valent iron at elevated temperatures was investigated with a continuous-flow system that consisted of an iron-packed pressure vessel and a heat exchanger. Results from the continuous-flow system showed that 99% of perchlorate was removed in 1 hour of contact time at 175°C. With decreasing pH to 2.5, more than 60% of perchlorate was removed with an iron contact time of 30 min at operation temperature of 95°C. Increasing the reactor temperature to 125°C resulted in 98% perchlorate removal at the same retention time of 30 min. This result demonstrated that Fe(0) technology combined with heating may be a viable option for the removal of perchlorate from Army ammunition wastewater.

Biodegradation experiments using glucose as the primary sources of electrons and carbon were conducted to evaluate microbial perchlorate reduction as a practical option for the treatment of perchlorate in PAX-21 wastewater. The results indicated 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 wastewater 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. Zero-valent iron pretreatment completely removed DNAN and RDX. After a 3-day acclimation period, perchlorate in iron-treated PAX-21 wastewater was rapidly decreased to an undetectable level in 2 days, which demonstrated that iron treatment not only removed energetic compounds but also eliminated the toxic constituents that inhibited the subsequent microbial process.

Finally, potential application of energetic compounds as the source of electrons for perchlorate reduction by PRB was investigated. It was hypothesized that the pretreatment of PAX-21 wastewater with Fe(0) would convert energetic compounds to products that are more amenable for biological oxidation and that these products can also serve as electron donors for PRB. Results of batch experiments showed that DNAN was completely reduced to 2,4-diaminoanisole (DAAN) and RDX was completely reduced to formaldehyde in the presence of cast iron granules within 2 h. Batch biodegradation experiments showed that formaldehyde can serve as an electron donor for perchlorate respiring bacteria. It was also demonstrated that complete reduction of perchlorate in iron-treated PAX-21 wastewater can be achieved without adding an exogenous electron donor. This study confirmed that iron pretreatment not only removed energetic compounds, but also transformed the energetic compounds to products that can serve as the source of electrons for perchlorate respiring bacteria. Based on the results, we proposed an integrated Fe(0)-biological process for simultaneous removal of perchlorate and energetic compounds, which consists of (1) a Fe(0) process for the reduction of electron-withdrawing nitro groups to biodegradable compounds and (2) an anaerobic biological treatment process containing perchlorate-respiring bacteria, which utilize Fe(0)-treated energetic compounds as electron donor and perchlorate as electron acceptor.