Changes in the extractability of heavy metals on the interaction of sewage sludge with soil

On first mixing digested sewage sludge with soil the proportions of the Cu and Zn in the sludge that are extractable to conventional extractants fluctuate considerably. Thus Cu extractable from sludge alone shows a marked initia fall to a minimum at 8–29 days and then rises to near its initial value; from a sludge—soil mixture there is a less marked decrease (over 8–16 days) and the extractable Cu returns to near its initial value over 90–100 days. The Zn extractable from sludge alone falls to a minimum at 8–16 days and then rises to near the initial value, but the Zn extractable from sludge-soil mixtures rises sharply over 2–4 days and then falls to near the initial value over 50–100 days.

There appears to be little further change in the extractable Cu, Ni or Zn during the next 600 days, although the metals in the sludge are still relatively more extractable than those ‘native’ to the soil. It is likely that similar processes operate under field conditions, but more slowly.