## The sorption of Co II on the solid particles

The sorption of Co(II) on the solid particles is also affected by the surface properties of solid particles. At low pH values, the protonation reaction (≡ SOH + H+ → ≡ SOH2+, where ≡ S represents the surface of solid particles, and –OH represents the functional groups) may provide the positive charged surface, and the deprotonation reaction (≡ SOH → ≡ SO− + H+) may occur at high pH values. At low pH values, the positive charged Co(II) species on the positive charged surface of solid particles may be attributed to Sulfo-NHS-SS-Biotin exchange, which is also supported by the ionic strength dependent-sorption. The sorption of Co(II) is mainly via ion exchange with hydrogen and sodium ions that saturate the exchange sites of solid particles. The exchange can be depicted by the following reactions:1)to exchange with hydronium ions:equation(5)≡SOH+Co2+⇔≡SOCo++H+equation(6)2≡SOH+Co2+⇔(≡SO)2Co+2H+2≡SOH+Co2+⇔≡SO2Co+2H+equation(7)≡SOH2++Co2+⇔≡SOHCo++H+equation(8)2≡SOH+UO22+⇔≡SOH2Co+2H+2)to exchange with Na+ ions:equation(9)≡SONa+Co2+⇔≡SOCo++Na+equation(10)2≡SONa+Co2+⇔(≡SO)2Co+2Na+.2≡SONa+Co2+⇔≡SO2Co+2Na+.