Adsorption of lead (Pb2+) and uranium (UO22+) cations by brown algae; experimental and thermodynamic modeling

•Chemically treated brown algae, Cystoseira indica, was used as biosorbent.•Synthetic wastewaters containing UO22+ and Pb2+ ions were treated by this biosorbent.•Non-ideal adsorption equilibrium models were derived for single and binary adsorption.•Vacancy solution theory was considered to model the present adsorption system.•Non-ideality of solutions was considered by comprising ion activity coefficients.

Batch experiments were conducted to assess the effect of metal concentration and initial pH of solution on the biosorption of UO22+ and Pb2+ by chemical modified Cystoseira indica. The sorption capacity of Pb2+ was enhanced by increasing initial pH from 3.0 to 5.5 and maximum sorption of UO22+ biosorption was occurred around initial pH 4.0. The single metal isotherm data, at 25 °C, were fitted on Langmuir equation and vacancy solution theory model using Flory–Huggins activity coefficients (VST-Flory–Huggins). The equilibrium adsorption experiments of the binary solution systems were carried out and the results were modeled by four types of extended Langmuir and multi-component VST-Flory–Huggins isotherm equations. Notwithstanding presence of any adjustable parameter, it was concluded that multi component VST-Flory–Huggins predicted the binary solution isotherms more accurate than extended Langmuir isotherms.