(11a) Synthesis of Manganese Oxide Coatings for Adsorption of Trace Metals from Ground Water

Manganese oxide (MnOx) occurs naturally in soil and has high affinity for trace metals such as lead, chromium, cadmium and zinc. Such heavy metals, when present in groundwater, will cause health risks if they enter aquifer drinking water sources. Our aim is to produce and characterize manganese oxide coatings on aquifer soil materials. The long term goal is to form a Permeable Reactive Barrier (PRB) to adsorb trace metals. Syntheses of manganese oxide coatings were carried out in the laboratory using batch reactor experiments, using automated titrator experiments and continuous column reactor experiments on Ottawa sand and the aquifer soil. The batch studies performed on Ottawa sand used three oxidants: ozone, hydrogen peroxide and bleach. The batch studies were also performed on the aquifer soil using two oxidants: ozone and bleach. For batch synthesis, pH was investigated for a range of 6 to 9, and the initial amount of manganese was varied from 0.3 to 3.0 g MnNO3 per 100 g sand. The MnOx formed on the sand ranged from 63.6 mg/kg to 10372.8 mg/kg. The batch synthesis was repeated three times on the same sand sample to examine the sequential buildup of manganese oxide coatings. To improve precision and productivity, the batch synthesis was repeated using an automated titrator. The sand was stirred slowly, and then a specific quantity of MnNO3 solution, and NaOH was added to adjust pH to specified value. The oxidant was added by a second automated titrator at 0.25 mL/min over 40 min. The system monitored pH and added sodium hydroxide solution to maintain pH at the final desired value through the entire experiment. A computer was connected to the titrator to continuously monitor pH changes with the help of pH electrode and labX software which helped in continuous monitoring of pH changes during the experiment. Each sample was produced in triplicate at pH of 6, 7 and 8. Two oxidants hydrogen peroxide and bleach, and two types of soils, Ottawa sand and aquifer sand were used in the automated titrator synthesis. The manganese oxide coating increased as pH increased from 6-8. The greatest amount of coating, 5512 mg Mn/kg, was observed for a combination of Ottawa sand and oxidant bleach at pH 8. These coated soil samples were being assessed for adsorption capacity of trace metals like cadmium, chromium and zinc. In the column (continuous reactor) experiment, manganese and bleach solutions were cycled alternately through a column of sand. The pH and ORP (Oxidation Reduction Potential) were monitored during the coating process. The pH ranged from 4 to 9. The ORP electrode measured 600-800 mV when passing through bleach and manganese solutions, and 100-300 mV when passing through DI-water. The coating process was carried out for increasing numbers of cycles: 24, 48 and 72 and 96. Flow rate was kept constant at 4 mL/minute. After the coating process, a lead solution (50 mg/L) was passed at 4 mL/min through the coated sand to determine the lead adsorption capacity on manganese oxide coated sand. A lead selective electrode was used to plot breakthrough curves. The amount of lead adsorbed for 24, 48 and 72 cycles was 500 mg/kg, 560 mg/kg, 900 mg/kg and 2226 mg/kg respectively. An excavated aquifer soil mostly containing sand and silt was investigated for coating synthesis and lead adsorption. A the permeable reactive barrier is installed at 20-30 ft below the ground level, the installation can be carried out by first identifying the path of the contaminant plume and then digging a trench in its path. A concern of the permeable reactive barrier is the pore spaces in the barrier get clogged and reduce the permeability of the reactive barrier. An experimental result can be used in a model to accurately predict the functioning of the permeable reactive barrier and to estimate the service life.