NITRATE REMOVAL USING ELECTROKINETICS/IRON WALL PROCESS
|C.F. Chew and T.C. Zhang, Department of Civil Engineering, University of Nebraska- Lincoln at Omaha Campus, Omaha, NE 68182-0178.||
The feasibility of remediating nitrate in ground water using a combination of zero-valent iron treatment wall and electrokinetic processes was investigated. Several bench scale units consisting of 2.5-inch diameter by 7-inch length acryclic glass tubes were designed to simulate the remediation process. A nitrate-contaminated soil specimen sandwiched by an inch of sand and graphite mixture on both ends was loaded into the cell.
A small, 20-watt DC cell and graphite electrodes inserted into the sand/graphite layers supplied current/voltage to the cell. Vents were provided so as to release any gases that may have formed during the remediation process. Water elevations at the influent and effluent were maintained at the same elevations to study the effects of electrokinetics and limit the hydraulic influences.
Three runs without adding iron treatment wall were conducted at constant voltages of 5, 10, and 15 volts for a period of 6 days. The results indicated that nitrate reductions ranged from 19 to 37% with nitrogen gas being the primary product. Next, another four runs were conducted with constant voltages of 5, 10, 25, and 20 volts. However, in these runs an iron powder wall was placed next to the anode region. The amount of nitrate reduced ranged from 54 to 87% for the 5 to 20 volt runs. As the transformation rate improved, more ammonia-nitrogen was produced while the nitrogen gas produced remained relatively constant.
To evaluate the effects of current on the process, the DC power cell was switched to the constant current mode and was used to supply 1 and 2 mA to the cells with an iron wall located adjacent to the anode. The amounts of nitrate removed were 88% (1 mA run) and 84% (2 mA run). Final temperature in the soil increased slightly in the 20-volt and 2-mA runs and may account for the low flow rates towards the end of the 6- day period.
At a constant current of 1 mA, the results for the no iron wall cell indicated that pH decreased with time along the soil, indicating the movement of an acid front. In the iron wall run, the results indicated that the pH near the anode decreased with time but a alkaline front appeared to be moving towards it. Electrokinetics are reported to be suitable for dissolving metal contaminants (low pH conditions) and moving them to the cathode where treatment can be provided. However, when an iron wall is included, the pH condition changes and may not be suitable for removal of certain contaminants. Therefore, a pilot or treatability study should be performed to determine the effectiveness of the electrokinetic/iron wall process.
Key words: electrokinetics, iron, nitrate, remediation, treatment wall
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