Iron Oxide Modified Sunflower Meal Hydrochar for Removal of Lead Ions from Aqueous Solutions: Sorption Kinetics

A method for producing a hybrid sorption material based on renewable agricultural plant raw materials synthesized by hydrothermal carbonization (HTC), the structure of which is modified with iron and/or iron oxide particles, is proposed. A series of samples was obtained and the effect of high-temperature carbonization and subsequent alkaline activation on the sorption capacity of materials was assessed. The physicochemical properties were determined using IR spectroscopy, X-ray diffractometry, and thermogravimetry. The X-ray diffraction patterns of samples showed the presence of an α-cellulose crystal lattice and an Fe3O4 or γ-Fe2O3 crystallographic plane. As a result of carbonization, the destruction of the α-cellulose phase and partial reduction of iron oxides occurs. Alkaline activation leads to complete reduction of the iron phase. The sorption capacity of materials was assessed using the example of removing heavy metal ions, lead, from model aqueous solutions. Kinetic studies of batch adsorption were carried out to determine the contact time and the adsorption mechanism. The sorption capacity of the initial HTC material for Pb2+ was ~132 mg/g at a contact time of 30 min; for carbonized and activated hydrochar, it was 66.8 and 99.2 mg/g, respectively. The adsorption kinetics of Pb2+ ions was described using pseudo-first-order and pseudo-second-order, Elovich, and intraparticle diffusion models. It was shown that sorption occurs mainly owing to the chemical interaction of metal ions with the functional groups of sorbents with a partial contribution of mixed-diffusion adsorption. Efficiency of using hydrochar based on plant raw materials modified with magnetic iron particles in the processes of liquid-phase removal of inorganic pollutants was proven.