Adsorption and coadsorption mechanisms of Hg2+ and methyl orange by branched polyethyleneimine modified magnetic straw

•Branched polyethyleneimine modified magnetic straw was prepared.•Hg2+ adsorption arose through formation of chelate complex.•MO adsorption occurred primarily due to electrostatic interaction.•The presence of methyl orange significantly enhanced the adsorption capacity of Hg2+. Adsorption and coadsorption of Hg2+ and methyl orange (MO) were accomplished using branched polyethyleneimine modified magnetic straw. The adsorption equilibrium was achieved within 40 mins and the maximal adsorption amounts of Hg2+ and MO reached 320.4 and 510.2 mg/g, respectively. Thermodynamic parameters, such as ΔH0 and ΔS0 were found as −3.728 kJ/mol and 10.593 J/mol K for the adsorption of Hg2+, while ΔH0 and ΔS0 were found to be −2.119 kJ/mol and 16.123 J/mol K for the adsorption of MO. It was suggested that the adsorptions of Hg2+ and MO were exothermic processes with further increase in the degree of freedom. The respective adsorption mechanism of Hg2+ and MO involved complexation and electrostatic interactions. In the binary system of Hg2+ and MO, the adsorption capacity of MO was not affected by the presence of Hg2+, while the adsorption amount of Hg2+ significantly increased in the presence of MO through the mechanism of bridge enhancement. Specifically, when the initial concentration of Hg2+ was 0.005 mol/L, the adsorption amounts of Hg2+ were 510.2 and 1858.3 mg/g, respectively in the mono-component and binary systems.