Effect of limestone grain size on the mobility of green nanoiron suspension

Nano zero valent iron (nZVI) is a promising agent for in situ remediation of contaminated groundwater and soil. However, the limited mobility of iron nanoparticles in calcareous soils may inhibit the efficiency of this technology. Aim of this study is the evaluation of the mobility of nanoimn synthesized with green tea polyphenols in porous media containing limestone by conducting columns tests. The effect of grain size of limestone particles added in columns on nanoiron suspension transport was evaluated. It was shown that the mobility of iron nanoparticles was strongly affected by the residual acidity and the aquatic iron (Fe(III)) of the nZVI suspension in contact with limestone which caused the neutralization of acidity and the precipitation of iron oxyhydmxides on limestone surface. It was shown that the interference of Fe(III) precipitation on the nZVI suspension mobility becomes more important as the size of calcite grains decreases. Namely, the total Fe content in the solids at the end of column tests, was 4.4 mg/g in the column containing the coarser CaCO3 grains (d = 3.4 mm) and increased up to 44.8 mg/g in the column with the finer grains (d = 0.22 mm), due to higher available specific surface area. The corresponding amount of retained nanoiron, nFe (0), was 0.75 and 4.6 mg/g in the columns with the coarser and finer CaCO3 grains respectively.