Passive Treatment of Low-Ph, Ferric Iron-Dominated Acid Rock Drainage in a Vertical Flow Wetland I: Acidity Neutralization and Alkalinity Generation

Abstract

Abstract. Passive treatment of acid rock drainage (ARD) is typically limited by the chemistry of the ARD. Anaerobic, ferrous iron-dominated ARD can be treated directly with limestone in an anoxic limestone drain (ALD), but alkalinity generation is limited because the high pH (5 -6) reduces limestone solubility. Oxygenated, ferrous iron-dominated ARD cannot be treated directly with limestone due to the potential for armoring by iron oxyhydroxide precipitates. Vertical flow wetlands that rely on biological processes are typically employed. Reducing and alkalinity producing systems (RAPS) are one type of VFW that requires biological pretreatment of ARD to remove oxygen. The biological pretreatment typically adds alkalinity to the ARD, limiting the alkalinity generated in the RAPS via limestone dissolution by lower the solubility of limestone. The low pH (< 3) of oxygenated, ferric iron-dominated ARD is prohibitive to the biological processes typically required for effective passive treatment. In this study, highly oxidized (i.e., 99% Fe 3+ ), low-pH (2.4), ARD is treated with a VFW system amended with fine-grained (1.2 mm) limestonebuffered organic substrate (LBOS). Nearly 100% of the influent acidity (averaged >1300 mg·L -1 ) is neutralized in the LBOS with an average 600 mg·L -1 of additional alkalinity measured in the effluent; total alkalinity generated in the LBOS averages > 1800 mg·L -1. Limestone dissolution accounts for 80 -95% of the total alkalinity generated in the system. Limestone dissolution is very rapid and occurs in a thin (2 -5 cm) dissolution front that advances as the limestone is depleted. Armoring due to iron oxyhydroxide precipitates apparently does not limit limestone dissolution, as limestone consumption above the dissolution front is nearly complete with greater than 85% of the limestone removed; limestone below the front is apparently unaffected. Sizing recommendations are made based on the influent acidity load and the volume of limestone in the LBOS.Additional Key Words: limestone-buffered organic substrate, limestone dissolution, armoring, biological oxidation of organic matter, anaerobic wetland 1

Passive Treatment of Low-Ph, Ferric Iron-Dominated Acid Rock Drainage in a Vertical Flow Wetland Ii: Metal Removal

“…Watzlaf (1997) also found that limestone dissolution occurs along a sharp reaction front in the compost layer of a reducing and alkalinity producing system (RAPS) -type VFW column study receiving low pH, ferric iron-dominated ARD. This dissolution front advances through the compost layer as the substrate's buffering capacity is consumed along the front (Thomas and Romanek, 2002;Watzlaf, 1997). Both studies observed apparent geochemical and mineralogical similarities above and below the reaction front that noteworthy.…”

“…Details regarding sampling of the influent ARD and the VFW effluent are presented in the accompanying paper (Thomas and Romanek, 2002). Water samples were taken weekly from April 1999 to December 2000.…”

“…In the previous paper (Thomas and Romanek, 2002), we demonstrated that ferric irondominated ARD can be effectively treated using a vertical flow wetland (VFW) amended with a limestone-buffered organic substrate (LBOS). Limestone accounted for over 80% of the total alkalinity generated in the LBOS with dissolution occurring in a thin (2 -5 cm) zone.…”

“…Below the reaction front, the pH was circumneutral, the same as the final effluent (Thomas and Romanek, 2002;Watzlaf, 1997).…”

“…Thomas and Romanek (2002) provide details of a limestone dissolution front that develops in this LBOS over time. The dissolution front is demarcated by distinct mineralogical zonation, similar to Watzlaf's (1997) findings.…”