Stability of green tea nanoscale zero-valent iron

Colloidal suspension of nano zero-valent iron (nZVI), obtained as a result of mixing green tea (GT) solution with iron solutions (FeCl₂ and FeSO₄ separately to form two suspensions: GT-nZVI_FeCl2 and GT-nZVI_FeSO4) was assessed in terms of stability. The particles of suspensions had a tendency rapidly aggregation to form larger aggregates, and then settled. The absolute value of zeta potential (ζ) of suspensions did not exceed 17 mV. To prevent the aggregation of nZVI particles, the pHs were changed by the addition of Na₂CO₃ to obtain higher values of ζ. For both suspensions, the zeta potential at a satisfactory level was obtained at pH values higher than 7.5. Then, the potential was lower than - 40 mV for GT-nZVI_FeCl2 and lower than - 25mV for GT-nZVI_FeSO4. This means that the first suspension had a better stability than second. The isoelectric point (IEP) of GT-nZVI_FeCl2 was in the range from 4.5 to 5, while in the case of GT-nZVI_FeSO4 all measured ζ were negative. To investigate the effect of increased ionic strength (IS) on the stability of suspensions at different pH values, NaCl and MgSO₄ were added to the irons solutions. The final IS in the GT-nZVI was 12 mM. In both of the suspensions the absolute values of ζ were lower than previous and were about 25 and 20 mV, for GT-nZVI_FeCl2 and GT-nZVI_FeSO4, respectively.