Effect of humic acid on sorption of neptunium on hematite colloids

Sorption of neptunium on hematite colloids has been studied at varying pH values (3-10) in absence and presence of humic acid using 239 Np as a tracer. The experiments were carried out under aerobic and anaerobic conditions. The results show negligible sorption of Np(V) on hematite colloids at lower pH < 5, beyond which it was found to increase sharply reaching ∼ 90% at pH 10. Presence of humic acid (2mg/L) was found to decrease the sorption of Np(V) on hematite at higher pH values while at lower pH the effect was found to be negligible. The stability constant of Np(V)-humate complex (log/3) was determined to be 4.2 ±0.2 by solvent extraction method. Sorption of humic acid on hematite as a function of pH was also measured by UV-visible spectrophotometry. Under nitrogen atmosphere and in presence of sodium dithionite, Np(V) was reduced to Np(IV) as evident by the enhanced sorption at all pH values, which was further enhanced in presence of humic acid at lower pH. However at higher pH values, humic acid does not affect the sorption of neptunium under reducing conditions, which could be due to competing reduction process involving ferric ions obtained from dissolved hematite.