Kinetic, equilibrium, and thermodynamic studies of biosorption of mercury from aqueous solutions by canola stalks waste

Mercury (II) is widely applied in electricity generation and industrial production such as chlor-alkali, plastics, metallurgy, and electronics. Due to its volatility, persistence, and bioaccumulation, mercury (II) has been considered as one of the most toxic metals which can affect the health of human beings. Long-term exposure to large amounts of mercury (II) would harm the human brain, heart, kidneys, lungs, and even the immune system. Therefore, the removal of excess mercury (II) from water resources has attracted much attention. According to the Environmental Protection Agency (EPA), the permissible limits for mercury (II) in drinking water and wastewater are 0.001 and 0.005 mg L−1, respectively. According to the literature, there are several conventional technologies for mercury (II) removal from wastewater consisting of coagulation and flocculation, oxidation or ozonation, membrane separation, and adsorption. Most of these methods suffer from drawbacks like high capital and operational costs and there are problems in the disposal of the residual metal sludge. Recently, biosorption has become the focus of attention to remove mercury (II) ions because it has high efficiency, low cost, the possibility of metal recovery, and regeneration of the adsorbent. The high cost of common adsorbents such as activated carbon has inspired a search for suitable low-cost adsorbents. Canola is the world’s third-largest oilseed crop, after soybean and palm, and is a good candidate for adsorbent in the adsorption-desorption process. Canola stalk is considered as easily-available lignocellulosic waste all over the world because the production and use of vegetable oil are increasing and canola is extensively used for this purpose; therefore, canola stalk can be easily found due to its characteristics and it has been used in several studies to remove pollutants. The canola stalk was collected from cropland in a suburb of Fars, Iran, which was later followed by washing with distilled water to remove dust impurities, and then dried completely in natural sunlight. The dried materials were crushed into fine particles with the help of an electric mill. In this research, canola stalks waste was used to remove mercury (II) from aqueous solutions in a batch system under different conditions (pH, temperature, contact time, initial concentration of mercury (II), and adsorbent dosage). In this context, the adsorbent was characterized by several techniques including Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), BET surface area, and zeta potential. Furthermore, the adsorption of mercury (II) has been studied in terms of pseudo-first-order and pseudo-second-order kinetic models, and the Langmuir, Freundlich, Langmuir–Freundlich and Redlich–Peterson adsorption isotherm equations are applied to the experimental data to obtain information about the interaction between the mercury (II) and the canola stalks waste. The results showed that the optimal values of pH, adsorbent dosage, contact time, initial mercury (II) concentration and temperature were 7, 2 g L-1, 120 min, 2 mg L-1, and 60oC, respectively. Among the used isothermal models (Langmuir, Freundlich, Langmuir-Freundlich, and Ridlich-Peterson), the Freundlich model has a better fit on the experimental data, which indicates the multilayer adsorption of mercury (II) on the adsorbent surface. The maximum adsorption capacity of the adsorbent was found to be 58.4 mg g-1 according to the Langmuir-Freundlich isotherm model. The results showed that the mercury ion adsorption process followed the pseudo-second-order model, which indicates the chemical adsorption of mercury by the adsorbent. The thermodynamic parameters (ΔG°, ΔH°, and ΔS°) demonstrated that the adsorption of mercury (II) on canola stalks waste was feasible, endothermic, and spontaneous between 25 and 60 °C. The negative values of ΔG° confirm the feasibility of the process and also the spontaneous nature of adsorption with a high preference for mercury (II) by canola stalks waste. The positive value of ΔS˚ revealed the increased randomness at the adsorbent-mercury (II) solution interface during the adsorption process with increasing temperature. The adsorption process can be classified as physical adsorption and chemisorption by the magnitude of the enthalpy change. It is accepted that if the magnitude of enthalpy change is lesser than 84 kJ/mol, then the adsorption is physical. However, chemisorption takes place in the range of 84–420 kJ/mol. The value of enthalpy (38.06 kJ/mol) suggests that physisorption is much more favorable for the adsorption of mercury (II) by canola stalks waste. The desorption efficiency of mercury (II) using 0.2 M HCl was 86%. This study suggests that canola stalks waste has the potential to become an effective and economical adsorbent for the removal of mercury (II) ions.