Application of Linear and Non-linear Kinetic and Isotherm Models for Evaluation of Lead Removal Efficiency from Aqueous Solutions Using Biochars

Heavy metals pose a serious threat to both human and environmental health. As a result, practical and low-cost strategies to remove them are urgently needed. Biochar is such a sustainable adsorbent playing an influential role in reducing pollution caused by heavy metals. This study aimed to investigate the effects of two feedstock types (corn (Zea mays L.) residues and Conocarpus erectus L. wood) and pyrolysis temperatures (400°C and 700°C) on the removal of lead from aqueous solution and compare linear and nonlinear forms of kinetic and isotherm data of adsorption. For this purpose, characteristics affecting lead removal such as pH of the solution, biochar concentration, contact time, and pollutant concentration were evaluated. In brief, after finding the best pH of the solution, the effect of adsorbent concentration was investigated. Then, the optimum pH and adsorbent concentration were used to assess contact time. Finally, pH, adsorbent concentration, and contact time in the optimum condition were selected to assess the impact of pollutant concentration on the removal of lead by biochars. According to the results, the optimum pH affecting lead adsorption by biochars was 5. The most suitable biochar concentration regarding lead removal was 0.8 gL-1. Lead adsorption by biochars reached equilibrium after 120 minutes. Moreover, sorption capacity increased by elevating the heavy metal concentration. Corn residues biochar produced at 700°C and wood-based biochar fabricated at 400°C had the most and the least lead sorption capacity, respectively. Pseudo-second-order, intra-particle diffusion, and pseudo-first-order models in two linear and non-linear forms revealed the best to the worst description for the sorption process as kinetic models, respectively. According to the model efficiency criteria, Freundlich, Temkin, and Langmuir in non-linear form, and Freundlich, Langmuir, and Temkin in linear form showed the most to the least suitable fit. These results highlighted the importance of using models in their original type being non-linear form. According to the outcome of this study, biochars produced from corn residues had more ability to remove lead than wood-based biochars. Moreover, pyrolysis temperature of 700°C was more effective in the adsorbents' ability to eliminate lead than 400°C. Therefore, the application of low-cost adsorbents with a high capability to remove lead, such as biochars, could positively reduce pollutants from aqueous systems.