Development of biochar produced by pyrolysis through hydrothermal heteroatom doping and its application in heavy metal removal

In this thesis study, corn cobs, an agricultural residue, were investigated as a potential alternative adsorbent due to their high availability, carbon-rich composition, and suitability as a valorized byproduct. Corn cobs were pyrolyzed at 600 °C under an argon atmosphere to produce biochar. To enhance the surface functionality of the resulting biochars, hydrothermal carbonization was performed at different temperatures (100 °C, 140 °C, 180 °C, and 220 °C). The hydrothermally carbonized biochar samples were treated with H3PO4, H2SO4, HNO3, and NH3 solutions, which are commonly used in the literature for heteroatom doping/functionalization, thereby introducing phosphorus, sulfur, oxygen, and nitrogen groups onto their surfaces. The resulting heteroatom-modified biochars were then evaluated for their adsorption performance in the removal of Pb(II) and Ni(II) ions from wastewater. The produced biochar samples were characterized by FTIR, TGA-DTA, SEM, SEM-EDX, and BET analyses after the optimization of process variables. According to the characterization findings, the BET surface areas of the heteroatom-doped biochars ranged between 365–434 m2/g, with the highest surface area (434 m2/g) obtained for the NH3-3–BC sample.". The pore volumes of the samples ranged from 0.159 to 0.191 cm3/g, and the average pore diameters were between 1.73 and 1.77 nm, indicating a predominantly micro porous structure. SEM images showed that all samples exhibited porous, irregular, and fractured structures, and it was confirmed that heteroatom doping enhanced pore development. FTIR, SEM-EDX, and BET analyses revealed that heteroatom doping positively contributed to the heavy metal adsorption performance. To determine the optimum conditions, parameters such as adsorbent dosage, initial concentration, pH, contact time, and temperature were investigated. Biochar samples with high surface area efficiency were used for the removal of lead and nickel from aqueous solutions, and adsorption kinetics, thermodynamics, and isotherm studies were evaluated. In conclusion, this study demonstrates that biochars derived from corn cobs, when functionalized with heteroatoms, exhibit high adsorption capacities for heavy metals and can serve as low-cost, promising adsorbents for environmental applications.