Influence of pH The effect of pH on the adsorption of Ni(II) and Cu(II) by leonardite was studied with aqueous metal solutions of 5mgdm−3 concentration. The initial pH was adjusted to pH 2, 3, 4, 5 and 6 with nitric acid (0.1moldm−3) or sodium hydroxide (0.1moldm−3) solutions. Metal solutions without initial pH adjustment were also used. Leonardite (200mg) was added to 200cm3 of nickel and copper solutions in stoppered conical flasks. A blank was preparedbyadding200mgofleonarditeto200cm3 of Milli-Qwatertoverifywhetherthesorbenttransferred Ni or Cu to the water. To check if chemical precipitation was involved together with sorption in this range of pH, leonardite-free blanks, adjusted to different pH values, were also investigated. The mixtures were agitated with a magnetic stirrer (SBS) for 2h at a constant temperature of 25◦C and then, filtered through a 0.45µm Millipore (Billerica, USA) mixed cellulose esters filter and the residual concentrations of Ni(II) and Cu(II) were determined.