Kinetic and equilibrium studies on the adsorption of calcium lignosulfonate from aqueous solution by coal fly ash

Experiments were performed to determine the adsorption of calcium lignosulfonate (CLS) onto Shand and Boundary Dam (BD) coal fly ashes. The results showed that both Shand and BD fly ashes exhibited potential for CLS adsorption. The removal efficiency increased with increasing adsorbent dosage and the maximum was obtained in the dosage of 20 g/L. Adsorption processes were time-dependent and the maximum adsorption capacity was achieved in the contact time of 120 min. Also, the processes were temperature-dependent and the maximum adsorption capacity was obtained at 20 °C. Additionally, adsorption of CLS could well fit the Pseudo-second-order kinetic model and the isotherm experimental data were correlated well with the Freundlich and Langmuir equations. Thermodynamic studies suggested that the adsorption of CLS onto both kinds of fly ashes was exothermic, the thermodynamic parameters were as follows: ΔG (4.56, 5.06 kJ/mol), ΔH (−15.58, 12.12 kJ/mol), ΔS (−0.0688, −0.0588 kJ/(mol K)) for Shand and BD fly ashes, respectively. Adsorption efficiency decreased with increasing pH over the range pH 2–8 and there was a slight increase as pH further increased. The maximum was obtained in pH 2–3. Also, the adsorption behaviors of CLS onto fly ash could be influenced by the ionic strength and the maximum was obtained in NaCl concentration of 0.1 mol/L. The results will have significant implications for the treatment of complex pulp and paper effluent by adsorption on low-cost adsorbents.

► Lignosulfonate (LS) is the important pollutant in pulp and paper mill wastewater.
► The kinetic and equilibrium studies on LS adsorption by fly ash were conducted.
► LS could be adsorbed onto fly ash with good removal efficiency.
► The adsorption process could be influenced by the solution chemistry.
► Fly ash as low-cost adsorbents showed the potential for the removal of LS.