|Effects of nano-Fe3O4 loaded acid-modified biochar on Pb2+ and Cd2+ adsorption in water
|英文关键词: nano-Fe3O4, HNO3, modified biochar, adsorption, Pb, Cd
| 为探讨纳米Fe3O4负载联合硝酸改性椰壳炭对Pb2+、Cd2+单一及复合溶液的吸附特性，通过静态吸附实验，针对吸附剂的表面特性、投加量、溶液初始pH、吸附时间、重金属初始浓度等影响因素进行了探讨，应用等温吸附模型及吸附动力学模型对吸附特性进行了研究。结果表明，纳米Fe3O4负载酸改性炭比表面积较未改性椰壳炭增加了221.03 m2·g-1，表面含氧官能团如O-H、C=O、C-O-C增加，芳香性增强，等电点提高至5.68。从经济效率角度考虑5 g·L-1为合理吸附剂用量，pH为5.0时，吸附效果最好，吸附在4 h达到平衡。准二级动力学模型对吸附的拟合度更高，吸附主要是化学吸附，吸附由快速外扩散和颗粒内扩散共同作用，Pb2+、Cd2+的吸附分别更符合Langmuir和Freundlich等温吸附模型。纳米Fe3O4负载酸改性椰壳炭对Pb2+、Cd2+的最大吸附量（Qm）分别达42.54 mg·g-1和25.79 mg·g-1，为未改性椰壳炭的1.87倍和2.23倍，复合溶液中Pb2+、Cd2+的Qm分别为单一溶液的65.16%和54.21%，这揭示了离子共存条件下的吸附竞争现象。研究表明，纳米Fe3O4负载联合硝酸改性提高了椰壳炭对Pb2+、Cd2+的吸附能力，且Pb2+的吸附性能及吸附竞争性优于Cd2+。
| To better understand the adsorption characteristics of Pb2+ and Cd2+ in their single and binary solutions by nano-Fe3O4 loaded acid-modified biochar, influencing factors such as adsorbent surface properties, additive dosage, initial solution pH, adsorption time, and initial metal concentration were studied via static adsorption. Moreover, models of isothermal adsorption and dynamic adsorption were applied to analyze the relevant adsorption kinetics and mechanisms. The obtained results indicated that the specific surface area of nano-Fe3O4 loaded acid modified biochar was higher than that of unmodified coconut shell biochar by 221.03 m2·g-1. In addition, the amount of surface functional groups such as O-H, C=O, C-O-C as well as the aromaticity were higher, while the isoelectric point of the biochar increased to 5.68. The applied biochar dosage of 5 g·L-1 was considered to be optimal in consideration of cost-effectiveness. The adsorption efficiency was preferable when ionic solution pH was 5.0 and adsorption equilibrium was achieved at around 4 h. The adsorption process was more aligned with the pseudo-second-order kinetic model, which suggested that chemisorption consisted of rapid out-diffusion and intra-particle diffusion were mainly accounted for the adsorption of Pb2+ and Cd2+. Furthermore, Pb2+ and Cd2+ adsorption were found to be well fitted to the Langmuir and Freundlich isothermal adsorption models, respectively. The maximum adsorption capacities(Qm) of Pb2+ and Cd2+ in single solution by nano-Fe3O4-loaded acid modified biochar were 42.54 mg·g-1 and 25.79 mg·g-1, respectively, which were 1.87 and 2.23 times higher than those by unmodified biochar. The maximum adsorption capacities of Pb2+ and Cd2+ in composite solution were 65.16% and 54.21% of those in single solution, revealing that the phenomenon of adsorption competitiveness took place in the presence of both ions. The results indicated that a nano-Fe3O4 loaded modification combined with acid modification enhanced the Pb2+ and Cd2+ adsorption capacities of coconut biochar. Moreover, the adsorption capacity and competitiveness for Pb2+ were higher than for Cd2+.