Immobilization of cadmium and zinc in soil by Al-montmorillonite and gravel sludge

We investigated the potential of montmorillonite, Al‐montmorillonite and gravel sludge to immobilize polluting heavy metals in agricultural soil. Batch experiments showed that both Al‐montmorillonite and montmorillonite immobilized zinc and cadmium. Zinc was bound specifically on Al‐montmorillonite and became increasingly incorporated into the interlayer hydroxy‐Al polymer, whereas there was no specific sorption on montmorillonite. Cadmium was bound on montmorillonite and Al‐montmorillonite unspecifically by cation exchange, but there was no incorporation into the lattice. In pot experiments montmorillonite, Al‐montmorillonite, or gravel sludge were added to a soil contaminated with zinc and cadmium. Increasing doses of these agents decreased the concentrations of NaNO3‐extractable zinc and cadmium. Aluminium‐montmorillonite and gravel sludge were more efficient than montmorillonite in immobilizing both zinc and cadmium. Remobilization tests at pH between 4 and 5.5 showed that cadmium and zinc desorbed more easily from montmorillonite than from Al‐montmorillonite. Gravel sludge application increased the buffer capacity of the contaminated soil substantially. The binding agents decreased zinc concentrations in red clover (Trifolium pratense), and gravel sludge also reduced the cadmium concentrations