Application of the Southwell Plot method to determine equilibrium time in phosphate adsorption

Among the different factors that influence the liquid-solid adsorption technique, equilibrium time is one of the most relevant and requires a large number of experiments over a long period of time for its determination. This work evaluates the Southwell Plot as a further tool that can contribute to determining the equilibrium time in adsorption processes. It can also optimize the operating conditions in a batch system for the removal of phosphate in adsorbents produced from domestic sewage sludge and clam shell residue. Sewage sludge and clam shell residues were ground, sieved and sintered at 700 °C for 1 h. The material was characterized by thermal analyses (TG/DTG), chemical analysis (EDX), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and adsorption studies. The kinetic studies were investigated by varying the initial concentration of the phosphate solution and mass of the adsorbent. The equilibrium time was determined by applying the Southwell Plot method to the kinetic data and the results showed some fluctuations as a function of the adsorbent mass. At 0.30 g of the adsorbent in 30 mL of the phosphate solution, regardless of the initial phosphate concentration, the equilibrium time determined by the Southwell Plot was 4 h. The maximum phosphate adsorption capacity in this condition, determined by the Langmuir equation, was 49.45 mg g −1.