咖啡渣活性炭的制备、表征及其对Cr(Ⅵ)的吸附机制研究

以碘吸附值为评价指标，活化时间、活化温度和浸渍比为影响因素，采用响应面法试验设计对磷酸活化法制备咖啡渣活性炭的工艺条件进行优化，并通过静态吸附试验研究了不同吸附时间、溶液pH值和吸附温度条件下，活性炭对水溶液中Cr（Ⅵ）吸附性能的影响，最后利用Langmuir、Freundlich吸附等温方程、准一级动力学方程、准二级动力学方程和颗粒内部扩散方程进行拟合。试验结果表明，制备咖啡渣活性炭的最佳工艺条件为活化时间1 h、活化温度498℃、浸渍比1.72；在此条件下活性炭得率为30.4%，碘吸附值为（799±16）mg/g，比表面积为1 006 m²/g，孔容为0.779 cm³/g、微孔孔容为0.051 cm³/g、平均孔径为3.088 nm。较低pH值和较高温度能够促进活性炭对Cr（Ⅵ）的吸附；Langmuir等温方程能够更好地描述活性炭对Cr（Ⅵ）的吸附效果；活性炭对Cr（Ⅵ）的吸附分3个阶段：快速吸附阶段、慢速吸附阶段和吸附平衡阶段，10 min内可完成吸附总量的79%，360 min内达到吸附平衡，该吸附过程符合准二级吸附动力学方程。分析表明咖啡渣活性炭对Cr（Ⅵ）的吸附主要为单分子层的化学吸附。

Preparation and Characterization of Activated Carbon from Coffee Grounds and Its Adsorption Mechanism for Cr(Ⅵ)

The preparation conditions of activated carbon from coffee grounds, including activation temperature, activation time and impregnation ratio, were studied by using the response surface methodology with iodine adsorption value as evaluation index. And the effects of adsorption time, initial pH value and temperature on the Cr(Ⅵ) removal by the obtained activated carbon were investigated. In addition, the kinetic data were fitted by pseudo-first-order equation, pseudo-second-order equation and infra-particle diffusion model, and the equilibrium data was analyzed by using Langmuir and Freundlich isotherm models. The experimental results showed that the activated carbon had the best performance under the optimum conditions of activation time 1 h, activation temperature 498℃ and impregnation ratio 1.72. Under these conditions, the yield of activated carbon was 30.4%, and the iodine adsorption value got up to (799±16) mg/g, the relative surface area could reach 1 006 m²/g, the total pore volume was 0.799 cm³/g, the t-Plot micropore volume was 0.051 cm³/g and the average pore size was 3.088 nm. The adsorption data fitted Langmuir isotherm model. Higher temperature and lower pH could promote the adsorption of Cr(Ⅵ) on activated carbon. The adsorption of Cr(Ⅵ) on activated carbon was divided into three stages:fast adsorption stage, slow adsorption stage and adsorption equilibrium stage, the total adsorption of 79% was completed in 10 min, the adsorption process could reach equilibrium in 360 min, and the pseudo-second-order equation generated the best agreement with the experimental data for the adsorption systems. The results showed that the adsorption of Cr(Ⅵ) on the obtained activated carbon was chemical adsorption with monomolecular layer.