MATERIALS AND METHODS The leonardite used as adsorbent in this research was supplied by Sociedad Espa˜nola de Acidos H´umicos, SA (SEPHU, Zaragoza, Spain). Samples of adsorbent were sieved to a grain size of 0.09–0.2mm. The surface area, determined by the Brunauer–Emmett–Teller (BET) method, was found to be 19.9m2g−1. Some of the chemical characteristicsofthismaterialhavebeendeterminedpreviously.12 The elemental composition and humic acids content of leonardite are shown in Table 1. Heavy metal stock solutions were prepared with concentrations ofup to 1000mgdm−3 of metalnitrate saltsinMilli-Qwater.ThepHadjustmentsweremade with nitric acid or sodium hydroxide solutions. All chemicalswereofanalyticalreagentgrade (Scharlau, Barcelona, Spain). The amount of Ni(II) and Cu(II) in the solution before and after adsorption was determined by atomic adsorption spectrophotometry (AAS), using a Varian AA-1275 spectrophotometer. All experiments were performed twice. The repeatability of the results showed a variation coefficient of less than 5%.