Risk Management of TNT/Heavy Metals in Active Firing Ranges by Mobility Control Using MKP/Bentonite

Abstract

however, the leachable fraction decreased to 10% of initial TNT when bentonite was also amended with MKP. When the amount of bentonite was higher than 10% of initial TNT, the increase in the amended amount of bentonite did not reduce the leachability of TNT, indicating that MKP and bentonite application at 5% and 10% of soil mass is the optimal condition for the minimizing the mobility of TNT. At last, the risk reduction for the nearby residents of Darakdae firing range through the mobility reduction of TNT and heavy metals was quantified. The nearby residents of Darakdae firing range may use the Hantan River for the extraction of potable (drinking) and showering/bathing water. For the risk evaluation, carcinogenic and noncarcinogenic risk assessments were conducted. Furthermore, because of the great migration of contaminants from the firing range in summer due to the heavy rainfall, a monthly risk evaluation was conducted. The most significant contributors to the carcinogenic risk and noncarcinogenic hazard index were hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), a type of explosive, and TNT, respectively. Even MKP/bentonite application could not reduce the mobility of RDX, the carcinogenic risk was acceptable (< 10-4) regardless of MKP/bentonite treatment and thus the carcinogenic risk to the nearby residents of the firing range was not found to be a significant concern. In contrast, in untreated condition, the noncarcinogenic hazard index was inacceptable (> 1) due to the migration of TNT in February, July and August. However, it was estimated that the noncarcinogenic hazard index could be reduced less than 1 by MKP/bentonite amendment. Thus, it can be concluded that MKP/bentonite application can reduce the risk to the local residents of the firing range by reducing the mobility of contaminants, especially TNT, released from the firing range.

Recently, firing range contamination has become an emerging environmental issue in Korea. The most critical aspect of firing range contamination is the migration of contaminants from firing ranges to nearby surface water and/or groundwater, which is used as a source of drinking water for local residents. The soils at the firing ranges usually contain divalent cationic heavy metals (e.g., Pb2+, Cu2+, Zn2+, and Cd2+) and explosives, released from unexploded ordnances (UXOs) and shells. Of the explosives, 2,4,6-trinitrotoluene (TNT) is a typical concern due to its high toxicity and carcinogenicity. Thus, the mobility control of TNT and the divalent cationic heavy metals is important for the environmental management of the active firing ranges. It is known that K+ and PO43- can reduce the mobility of TNT and heavy metals via the specific sorption enhancement of TNT to clay mineral surfaces and heavy metals precipitation, respectively. Therefore, theoretically, monopotassium phosphate (MKP, KH2PO4), which contains both K+ and PO43-, can reduce the mobility of both TNT and heavy metals simultaneously and this study identified whether or not MKP can reduce the mobility of both TNT and heavy metals in soils collected from an active firing range, Darakdae firing range, located in Yeoncheon-gun, Gyeonggi Province, Korea. Mobility control of heavy metals using phosphate has been widely investigated and the lab-scale and field-scale feasibility study of phosphate application has been widely conducted. On the other hand, no studies on the feasibility of potassium application for the mobility control of TNT in soil are available at the present time. The earlier studies on the sorption enhancement of TNT by K+ conducted using K+-saturated soil and have focused on the identification of the specific sorption mechanism only. For this, at first, it was identified that whether or not MKP application can enhance the specific sorption, and which specific sorption mechanism is dominant between n-π electron donor-acceptor (EDA) interaction and cation-polar interaction. In untreated soil, TNT was not specifically sorbed to soil but MKP application induced the specific sorption of TNT, which resulting in the TNT sorption enhancement. Also, it was identified that the mechanism of specific sorption was a cation-polar interaction. Then, the simultaneous mobility reduction of TNT and heavy metals in soil by MKP application was studied. The mobility reduction was identified through synthetic precipitate leaching procedure (SPLP), and the leachability of TNT and heavy metals were greatly reduced by MKP application. It seems that mobility reduction is due to the sorption enhancement of TNT and heavy metals precipitation. The MKP application increased the sorption coefficient of TNT from 6.6 mg1-n kg-1 Ln to 12.1 mg1-n kg-1 Ln, and Pb, one of the concerned heavy metals in the firing range soil, was precipitated from soil. Finally, the mobility reduction of TNT and heavy metals was further elucidated using cyclodextrin and sequential extraction test, respectively, and both tests results revealed that the extractability of TNT and heavy metals decreased greatly (i.e., mobility of TNT and heavy metals greatly decreased). This study also investigated the minimizing mobility of TNT by MKP application with bentonite by further enhancement of TNT sorption. The leachable fraction determined by SPLP was about 50% of initial TNT when only MKP was amended with 5% of soil mass (the maximum soluble amount of MKP in porewater when the soil moisture content is the maximum water holding capacity)