Effect of reaction time on Pb removal from allophanic volcanic ash soil by acid-washing

To investigate the effects of reaction time on removal of heavy metals in acid-washing, we conducted acid-washing using hydrochloric acid on allophanic and smectitic soils contaminated with lead (Pb). The Pb removal percentage in the smectitic soil increased with reaction time. On the other hand, in the allophanic soil, it reached to a maximum at around 10 min, and then decreased monotonically. This suggests that effective removal of heavy metals from allophanic volcanic ash soils can be achieved by separating the washing solution from the soil as quickly as possible. The pH of the washing solutions of the two soils were similar just after the acid addition but it rose much more quickly in the allophanic soil than in the smectitic soil. Under comparable conditions, the pH of the washing solution and Al concentration were always far higher for the allophanic soil than for the smectitic soil, probably due to the higher solubility of allophane and related non-crystalline aluminum silicate in the former soil. These experimental results indicate that the Pb that was once released from allophane was re-adsorbed via surface complexation as a result of pH increase.