The pH50 values (pH at 50% metal
adsorption) followed the order: upland soil > 8-Year paddy
soil > 85-Year paddy soil > 35-Year paddy soil > 15-Year
paddy soil. This order is consistent with that for the Cu(II)
adsorption isotherms shown in Fig. 1. With increasing of
pH, Cu2+ may hydrolyze to form Cu(OH)+, Cu(OH)2 or
even precipitates of copper hydroxides.