磁性生物炭的制备、表征及对磷的吸附特性

On the synthesis,characterization and phosphate removal of the biocharbased magnetic composites

处理成本高、分离难度大是当前湖库等相对封闭水体磷治理工程中的主要问题。为此,以廉价农林废弃物花生壳为主要前驱物与Fe_3O_4纳米颗粒复合制备一种低成本、可磁分离的磁性生物炭吸附剂,研究了热解温度(400℃、500℃、600℃)、溶液pH值、阴离子(Cl^-和SO_4^(2-))共存等因素对复合材料吸附性能的影响。结果表明,复合后的磁性生物炭最大吸附量相比复合前提升了3~5倍。动力学数据拟合结果表明,拟二级动力学方程能较好地拟合吸附过程。磁性生物炭的零电荷点(均<7.5)与前驱物的热解终温在试验范围内呈现明显的正相关,表明引入Fe_3O_4后可明显增加磁性生物炭表面电荷,进而有效提升吸附性能。此外,阴离子共存试验表明,磁性生物炭对磷具有较好的选择性。

The objective of this article is to discuss how to prepare a kind of inexpensive,magnetically separable adsorbent for phosphate,known as the biochar-based magnetic composites（short for BMCs）.The ready-made BMCs can be synthesized from the magnetic iron-oxide（Fe3O4）nanoparticles and the biochars（BCs）derived from the cheap and ordinary farming wastes,such as the peanut shells through chemical co-precipitation processing.As is well known,it is difficult to separate the adsorbent from the phosphorus-contaminated water,particularly from the enclosed or semi-enclosed waters,for instance,lakes and reservoirs,which makes it highly expensive for the treatment due to two main factors,that is,the limited application of the adsorption method and the physico-chemical properties of BMCs and the BCs response to the pyrolysis temperature,which involve complicated scanning of the electron microscope（SEM）,the X-ray diffraction（XRD）,the Fourier transform infrared spectroscopy（FTIR）and the point of zero charge（PZC）measurement.In addition,it is also necessary to conduct a batch of experiments to assess the effects of the pyrolysis temperature（400℃,500℃and 600℃）,the p H value of the solution and the coexistant anions（Cland SO42-）on the performance of the phosphate removal when the BMCs and BCs are to be used,respectively.The result of the physico-chemical characterization indicates that the morphological state of BMCs tends to be significantly different from that of BCs＇surface area whereas the point of zero charges tends to be positively related to the pyrolysis temperature to a certain extent.What is more,the effect of BMCs on the phosphate removal proves higher than that on BCs for a factor of 3-5 probably because the process involves metal oxide at a higher point of zero charge（for example,the point of zero charge of Fe3O4is about 9）,which can significantly promote the surface charge capacities of BMCs,and,therefore,can contribute significantly to the phosphate removal.The removal of phosphate can also be controlled mainly by the chemical actions,for which the adsorption kinetics of BMCs and BCs is usually described by pseudo-second kinetic equation better than with other methods.Besides,higher adsorptive capacity of the adsorbent can also be found in the product of BMC synthesized from the biochar produced at higher pyrolysis temperature at the rate of 22.32 mg/g as can be predicted by Langmuir model.And,finally,it is also necessary to point it out that BMCs is in a position to display perfect selectivity on phosphate in case that there exist Cl^-,SO4^2- and H2PO4- in the solution.