Abstract: Heavy acid(HA) and HA-bound heavy in the sludge under composting were extracted using a sequential extraction method with H₂O and Na₄P₂O₇+NaOH, separately,for temporal variation of HA and HA-bound heavy metals. Results show that heavy metals in the sludge under composting were mainly in HA-bound form and a little in water soluble form. Besides, they were quite low in availability and mobility. At the end of composting, water-soluble (FA-bound) forms of heavy metals were low in content, while HA-Pb, HA-Ni and HA-Cu was apparently quite high in proportion to the total heavy metals and increased by 28.00%, 17.34%, and 1.40 percentage points, respectively, over that at the beginning of composting. Obviously, composting promotes transformation of water soluble form into HA-bound form of heavy metals, and HA capacity of binding heavy metals varies, displaying an order of Cu > Pb > Ni > Zn.

Key words: sludge compost, humic acid, heavy metal, effectiveness