硝酸氧化改性褐煤提升吸附镉能力研究

Study on improving cadmium adsorption capacity of lignite modified by nitric acid oxidation lignite

采用固液吸附系统研究了褐煤和硝化褐煤对镉离子的等温吸附、动力学吸附等特征,用卡方检验、Avrami模型筛选和验证了2种材料适宜的吸附机制模型。结果表明,不同温度下硝化褐煤和褐煤最大单层吸附量分别为35.61~46.84、29.63~38.20mg·g^-1,硝化褐煤比褐煤增加吸附量在20.18%~22.62%。基本达到平衡吸附量时,分别用了150、330min。硝化褐煤在低温时符合Langmuir单层吸附和准二级动力学过程,褐煤适合Freundlich模型描述。Avrami模型可较好地描述2种材料的等温吸附和动力学吸附特征。不同温度的等温吸附特征和动力学结果表明,随温度升高,2种材料经历了单层吸附、多层吸附的物理-化学过程,吸附是自发的吸热过程。红外光谱分析表明,硝化褐煤拥有较多的功能团,与吸附镉有关的为含氧功能团区、未知吸附带、脂肪族羟基和芳香C—H结构。硝化褐煤中羰基碳相对含量较褐煤高,可能是结合Cd2+能力提高的原因。因此,褐煤改性后的硝化褐煤对镉离子的吸附量和吸附速率最大,硝化褐煤能提高对镉的吸附性能,热力学和动力学吸附机制在不同材料和不同温度情况下不同,模型适宜性需用误差函数卡方检验辅助确定。改变褐煤基吸附材料的关键是增加有效功能团。研究首次表明,吸附过程中动力学级数发生改变,脂肪族羟基区、未知吸收带2360.20cm^-1和2341.98cm^-1和含氧官能团是褐煤硝酸氧化后吸附镉能力提高的关键因素。

The isothermal and kinetic adsorption characteristics of lignite and lignite modified by nitric acid oxidation for Cadmium(II)ions were investigated though the method of solid-liquid adsorption.Nonlinear chi-square analysis and Avrami model were used to verify the optimum adsorption mechanism models of the two materials.The results showed that the maximum monolayer adsorption capacities of nitric acid oxidation lignite(NAOL)and raw lignite(RL)were 35.61~46.84,29.63~38.20 mg·g^-1.The adsorption amount of NAOL increased by 20.18%~22.62%compared with the RL.The adsorption equilibriums were achieved at about 150 minutes for NAOL and 330 minutes for RL.At low temperature,NAOL obeyed Langmuir isotherm model which describe monolayer adsorption and pseudo-second-order kinetic model,but RL were better described using Freundlich equation.Overall,all these lignite-based adsorbents could be best fitted with Avrami models at all temperature.The adsorption isotherm and kinetics of the adsorbents at different temperatures demonstrated that absorption process probably went first through monolayer sorption and successively multilayer absorption of physical-chemical process.Through thermodynamic parameters,it was verified that absorption processes were spontaneous,endothermal and favourable sorption.FT-IR analysis showed that NAOL had many functional groups,and the oxygen-containing functional group region,unknown adsorption zone,aliphatic hydroxyl group as well as aromatic C-H structure were related to the adsorption of cadmium.The relative content of carbonyl carbon in NAOL was higher than that in RL,which may be the reason for the improvement of Cd 2+binding capacity.Therefore,the adsorption capacity and adsorption rate of Cd 2+could reach to the maximum value treated by the NAOL.Though the NAOL improved the adsorption performance of Cd 2+,the model suitability also needed to be confirming by chi-square test of error function.That is because the thermodynamic and kinetic adsorption mechanism varied with different materials and temperatures.The key method of lignite-based adsorption materials was to increase the effective functional groups.The study indicated for the first time that the kinetic order had changed during the adsorption process.Aliphatic hydroxyl zone,unknown absorption zone 2 360.20 cm^-1 and 2 341.98 cm^-1,and the oxygen-containing functional group(mainly carbonyl carbon),were the key factors for the improvement of cadmium adsorption capacity of NAOL.