| Adsorption is an efficient and convenient method to remove the heavy metals in wastewater. Biochar is a high-efficiency adsorbent and it has been widely studied to remove heavy metals in wastewater in recent years. In this paper, rice husk, a kind of agricultural waste material, was charred, modified with acids, alkalis and oxidants, and screend out a biochar, which was one of the best adsorbents with the optimum effect removing Pb2+and Cd2+ions from wastewater. Then, the biochar was loaded with magnetic Fe3O4 in order to reuse with an external magnet, and a new adsorbent material was made (marked as BC-Fe).In this study, the BET surface area, pore structure analysis, SEM, FTIR, and XRD of BC-Fe were analyzed. Meanwhile, the effects of contact time, solution pH, adsorbent dose, and temperature on BC-Fe removing Pb2+ and Cd2+ions in simulated wastewater of single Pb2+ or Cd2+ systems and complex Pb2+ and Cd2+ systems were investigated. The adsorption kinetics and thermodynamics were also studied in order to find out the characteristics and adsorption mechanism of Pb2+ and Cd2+on BC-Fe. In addition, the adsorption/desorption experiment was conducted to test recycle ability of BC-Fe and removal capacity of Pb2+ and Cd2+ on BC-Fe in actual wastewater was also experimented. The main results were given as follows:(1) The results of modified biochar preparation indicated that with increase in the carbonization temperature, the removal rate of Pb2+ and Cd2+ on biochar from solutions was increased. While the initial concentrations of Pb2+ and Cd2+ as 50 mg/L and 20 mg/L, respectively, the adsorption ratio of Pb2+ and Cd2+ on 4-BC-HNO3 (carbonazition temperature 400℃, modified with HNO3) achieved to 97% for both ions. BC-Fe removal efficiency of Pb2+ was better than that of Cd2+ either in the single heavy metal adsorption system or the mixed heavy metal adsorption system. While the concentrations of Pb2+ and Cd2+ were equal in the mixed adsorption system, the unit mass adsorption amount of Pb2+ on BC-Fe was higher than that of Cd2+ either, which proved that Pb2+ had more predominance than Cd2+ in the competitive adsorption, and the adsorption selection order was Pb2+> Cd2+. In addition, BC-Fe could be separated from solutions easily by an external magnet and be reused.(2) Characterization analysis of biochar showed that magnetic increased the specific surface area of rice husk biochar more than 40 times, which provided more adsorption sites. BC-Fe surface was irregularity with few microspores duo to many Fe3O4 particles adhere to the surface, and BC-Fe could be very convenient to recycle from wastewater. BC-Fe had many active groups like unsaturated carbon bonds, carboxyl, and hydroxyl, which could adsorb metal ions through ion exchange, chelating and other ways. Rice husk biochar proved to be non-crystal structure containing SiO2, and magnetic added crystal of Fe3O4 on BC-Fe.(3) In the single heavy metal adsorption experiment, the adsorption equilibrium reached in 2.0 h for Pb2+ and 1.0 h for Cd2+, respectively. The adsorption effect was mainly affected by pH value of the solution, adsorbent dose, and contact time. In the condition of adsorbent dose 0.1 g, solution volume 50 mL, solution pH 5.0, temperature 25 ℃, contact time 2 h, the initial concentration of Pb2+ 100 mg/L, the removal ratio and adsorption capacity of Pb2+ on BC-Fe was 81.3% and 40.6 mg/g, respectively, and when the initial concentration of Cd2+ was 50 mg/L, the removal ratio and adsorption capacity of Cd2+ on BC-Fe was 72.9% and 18.2 mg/g, respectively.(4) BC-Fe kinetic analysis showed that the adsorption reaction of Pb2+or Cd2+ on BC-Fe in simulated wastewater was satisfied well with the dynamics equation of Lagergren quasi two-level and the correlation coefficient R2 were greater than 0.99. Moreover, the model equilibrium adsorption capacity qe(mod) was very close to the experimental value qe(exp). The adsorption process of Pb2+ or Cd2+ on BC-Fe fitted well with Langmuir adsorption isotherm model and the reaction was similar to monolayer adsorption. When the temperature was 45 ℃, the theory saturated adsorption of Pb2+ or Cd2+ was 53.2 or 18.8 mg/g, respectively. Thermodynamics data indicated that the adsorption of Pb2+ or Cd2+ on BC-Fe was a spontaneous and endothermic process, and the adsorption effect was better at higher temperature.(5) The regeneration experiments of BC-Fe showed that nitrate acid could desorb abundant of Pb2+ and Cd2+ which were adhered on BC-Fe surface, and reutilization of the regenerated BC-Fe still could remove most of heavy metal ions in solution, indicating that BC-Fe had a great potential to be recycled. The removal ratio of Pb2+ or Cd2+ on BC-Fe form wastewater maintained still around 60% even after being reused for five times. Besides, BC-Fe was a kind of ideal adsorbents for the deep treatment of industrial wastewater, and it could remove heavy metals from the preprocessed actual wastewater effectively and meet the national integrated wastewater discharge standard. |
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