Biochar pyrolyzed from wheat straw was innovatively used for the adsorptive removal of cationic dye methylene blue through exposure to a magnetic field. The adsorption capability of the biochar pyrolyzed at 200 °C exceeded that of samples pyrolyzed at higher temperatures. The surface acidic functional groups of wheat straw biochar were deduced to be more sensitive to the effects of the external magnetic field. The enhancement of the magnetic field achieved by increases in the initial dye concentration, and a decrease in the biochar dosage and solution pH, were more significant compared with those caused by other conditions. Kinetic experiments indicated that chemisorption occurred during adsorption. The qmax values for dye adsorption without, and with, an external magnetic field were found to be 46.6 and 62.5mg/g, respectively. These demonstrated that wheat straw biochar could be used for the efficient adsorption of pollutants when assisted by an external magnetic field.
As dissolution of raw biomass is serious when used as an adsorbent, the cheap biochar pyrolyzed from biomass might be a good matrix. Raw cornstalk biochar was intentionally modified by cetyltrimethylammonium bromide (CTAB) to prepare the composite adsorbent designed for the removal of negatively charged pollutants. After modification, the removal efficiency for anionic dye Orange II (ORII) increased from 46.9% of the virgin cornstalk biochar to 99.7% of the CTAB-modified cornstalk biochar. The uptake of ORII proved to be favorable under acidic conditions but unfavorable under alkaline conditions. By nonlinear simulation, the Elovich model was the best to describe the adsorption kinetics. For linear simulation of the kinetic data, the pseudo-second-order kinetic model fitted the experimental points better than the pseudo-first-order model. Kinetic analysis indicated that the ORII adsorption process on the CTAB-modified cornstalk biochar might be chemical adsorption accompanied by ion exchange. At 298 K, the maximal adsorption capacity of the modified biochar is 26.9 mg/g by the Langmuir model. The adsorption of ORII increased with a rise in the reaction temperature. The enthalpy and entropy of the adsorption process are calculated to be 38.45 KJ mol−1and 185.0 J mol−1 K−1, respectively. The negative values ofΔG0at 288, 298, and 308 K were −14.92, −16.50, and −18.62 KJ mol−1, respectively. The above thermodynamic analysis demonstrates that the adsorption process was endothermic and spontaneous.
A natural mineral diatomite was modified with lanthanum species using an ion exchange process to improve its adsorption performance for tetracycline removal. The prepared lanthanum-modified diatomite was characterized by scanning electron microscopy, X-ray diffractometry and Fourier transform infrared spectroscopy. The results showed that lanthanum was successfully immobilized onto diatomite, with a content of lanthanum element of about 1.5% (atomic ratio). The prepared adsorbent was evaluated for the adsorptive removal of tetracycline, and the adsorption isotherm, kinetics and mechanism were investigated. The adsorbent exhibited higher adsorption capacity than other adsorbents reported in literature, reaching 1056.9 mmol/kg. Langmuir model better fitted the experimental data than did other models. The removal of tetracycline was favorable at near neutral pH conditions. The tetracycline adsorption well followed pseudo-second-order kinetics model, and most of tetracycline was adsorbed within the initial 15 min. The increase in ionic strength reduced the tetracycline adsorptive removal, indicating that tetracycline adsorption on La-modified diatomite may be attributed to the formation of out-sphere surface complexes.
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