生物炭Mg/Al-LDHs复合材料对磷的吸附特性及机理

为有效控制水体富营养化和实现农业废弃物资源化利用,以生物炭作为类水滑石(Layered Double Hydroxide,LDHs)的载体,采用共沉淀法制备出生物炭镁铝水滑石复合材料(Mg/Al-LDHs@BC),并研究其对水中磷酸盐的吸附特性。采用X射线衍射、扫描电子显微镜、Zeta电位仪、傅立叶红外光谱仪和X射线光电子能谱对其进行表征。结果表明,花瓣状Mg/Al-LDHs成功负载到生物炭表面上。Mg/Al-LDHs@BC对磷酸盐的吸附动力学过程更符合准二级动力学模型,等温吸附过程更适合Langmuir模型来描述,拟合得到的最大吸附量达71.37 mg/g,高于Mg/Al-LDHs,同时较生物炭提升约9倍。Mg/Al-LDHs@BC的零点电荷为5.39,在酸性条件下对磷酸盐的吸附性能优于碱性条件。Cl^-和NO₃^-对Mg/Al-LDHs@BC吸附磷酸盐干扰较小,吸附量仅分别下降3.66和5.93 mg/g;CO₃^2-和SO₄^2-对其干扰较大,吸附量分别下降了19.64和15.93 mg/g。Mg/Al-LDHs@BC对磷酸盐的吸附机理主要涉及阴离子交换、静电吸引和配体交换。研究结果可为农业废弃物资源化利用及水体富营养化防治提供理论依据。

Phosphorus adsorption characteristics and mechanism of biochar loaded Mg/Al-LDHs composites

Abstract: In order to effectively control the eutrophication of water bodies, and further realize the resource utilization of agricultural wastes. Taking the biochar as the carrier of hydrotalcite-like (LDHs), the biochar loaded magnesium-aluminum hydrotalcite composites (Mg/Al-LDHs@BC) was prepared using a co-precipitation approach. An attempt was also made to explore the adsorption characteristics of Mg/Al-LDHs@BC to phosphate in water. The crystal structure, morphology, and the zero charge point of Mg/Al-LDHs@BC were characterized by XRD, SEM, and Zeta potentiometer, respectively. FT-IR and XPS were selected to analyze the surface functional groups and the surface element properties of Mg/Al-LDHs@BC before and after the adsorption of phosphate. An investigation was also made to clarify the adsorption mechanism of phosphate on Mg/Al-LDHs@BC. The intraparticle diffusion model, quasi-first-order kinetic model, and quasi-second-order kinetic model were used to fit the kinetic experimental data. The Freundlich model, the Langmuir model, and Temkin model were selected to analyze the adsorption isotherm process. The results showed that the petal-like Mg/Al-LDHs was successfully loaded on the surface of biochar. The XRD results showed that the layered structure of LDHs on Mg/Al-LDHs@BC composite material was not destroyed after the adsorption of phosphate. The FT-IR results showed that the characteristic peak of phosphate at 1 060 cm-1 appeared after the adsorption of phosphate, indicating that the phosphate has been successfully adsorbed on Mg/Al-LDHs@BC. Furthermore, the phosphate adsorption kinetics of BC followed the quasi-first-order kinetic model, whereas, the phosphate adsorption kinetics of Mg/Al-LDHs and Mg/Al-LDHs@BC were accorded with the quasi-second-order kinetic model. The isotherm adsorption processes of BC, Mg/Al-LDHs and Mg/Al-LDHs@BC were all suitable to be described by the Langmuir model. The maximum adsorption capacity obtained from the Langmuir equation reached 71.37 mg/g, which was nearly 10 times higher than that of biochar, and was also higher than Mg/Al-LDHs as well. The zero point charge (pHpzc) of Mg/Al-LDHs@BC was 5.39, indicating the positive charge on the surface of material was beneficial to the adsorption of phosphate, as the pH of phosphate solution was lower than the value of pHpzc. When the pH of phosphate solution was higher than the value of pHpzc, the negative charge on the surface of material inhibited the adsorption of phosphate. The adsorption capacity of Mg/Al-LDHs@BC for phosphate decreased gradually with the increase of pH. It infers that the phosphate adsorption performance of Mg/Al-LDHs@BC under acidic conditions was better than that under alkaline conditions. Cl- and NO3- had little influence on the adsorption of phosphate on Mg/Al-LDHs@BC, where the adsorption amount only decreased by 3.66 mg/g and 5.93 mg/g, respectively, while, CO32- and SO42- showed distinct interference on this adsorption process, where the adsorption capacity significantly decreased by 19.64 mg/g and 15.93 mg/g, respectively. The XPS results showed that after the phosphate adsorption of Mg/Al-LDHs@BC, the percentage (the percentage of the area of a single peak to that of all peaks) of CO32- peak in O1s spectra decreased from 40.85% to 39.04%, indicating that CO32- located in the LDH layers exchanged with phosphate. In addition, the percentages of Mg-OH peak in Mg 1s spectra and Al-OH peak in Al 2p spectra both declined, indicating that the phosphate coordinated with the surface metal atoms by the form of inner-sphere complexes. Combining results of FTIR, XPS, Zeta potential, and pH experiment, it infers that the adsorption mechanism of phosphate on Mg/Al-LDHs@BC involved in the anion exchange, electrostatic attraction, and ligand exchange. The findings can provide a promising theoretical contribution to the utilization of agricultural wastes, and the scientific management of water eutrophication