聚合氯化铁-腐殖酸(PFC-HA)冷冻干燥絮体的表面微观形貌及其孔表面分形特征

Surface micro-morphology and fractal nature of the pore surfaces of cryofixed polyferric chloride-humic acid (PFC-HA) flocs

采用低温急速冷冻-真空干燥技术制备了不同pH(9.0、7.0、5.0)原水在45.89mg·L-1(以Fe3+计,原水pH=9.0)或28.41mg·L-1(以Fe3+计,原水pH=7.0、5.0)投药量下混凝后形成的PFC-HA絮体的粉末样品,研究了它们的表面微观形貌和孔表面分形特征.SEM图像表明,PFC-HA絮体具有不规则的块、片状形貌,而且在原水pH=9.0或pH=7.0时形成絮体也存在簇状结构.3种样品对N2的吸附-脱附等温线上均存在滞后圈,但样品3的形状不同.它们的吸附量、BJH累积吸附孔体积与脱附平均孔径的大小顺序均为:样品3>样品2>样品1,即随原水pH的降低而升高,而且孔尺度分布曲线显示样品3的表面存在大孔.BET比表面积的变化趋势与它们不一致,且样品2的比样品3稍大,样品1的最小.基于分形FHH方程,采用吸附等温线和脱附等温线数据计算出样品1和样品2的孔表面分形维Ds数均在2.94左右,而样品3的均小于2.38.这些Ds值似乎不能完全表示出因孔表面粗糙性而导致的空间填充能力的大小,主要表达了3种样品的中孔甚至大孔的尺度分布引起的不规则性.本试验中采用热力学模型计算出的3个样品Ds值大都超过3,它们已经失去了维数的实际意义;但样品3因其吸附-脱附等温线上的滞后圈接近圆桶式的A类,如果缩小分形标度区间,可以得到与分形FHH方程计算出的Ds接近的值.

Polyferric chloride-humic acid （PFC-HA） noes, formed from flocculation of different HA water samples （initial pH = 9.0,7.0,5.0） at PFC dosages of 45.89mg·L^-1（as Fe^3＋, initial pH =9.0） or 28.41mg·L^-1（as Fe^3＋, initial pH = 7.0, 5.0）, were vacuum freeze dried and then characterized by SEM （ scanning electron microscopy） and microstrueture measurements. In the SEM images, there are irregular block and sheet structures, and some clusters were also found in 2 of the samples. The floe microstruetures were investigated by N2 absorption-desorption. The sample prepared at pH 5.0 had a hysteresis loop in the N2 absorption-desorption isotherm with a different shape from the other two samnles. The adsorption volume, BJH cumulative absorption volume of pores and BJH desorption average pore diameter of the sample at initial pH 5.0 were much higher than those of the other samples, and those prepared at initial pH 7.0 were somewhat higher than at initial pH 9.0. The pore-size distribution （PSD） of the initial pH 5.0 sample shows that some macropores exist on its surface. However, the BET specific surface area of these samples has a different trend from their surface geometrical parameters. The surface area of the initial pH 7.0 sample was a little larger than that of the initial pH 5.0 sample, and the initial pH 9.0 sample possessed the smallest surface area. Furthermore, the pore surface fractal dimensions Ds for the initial pH 9.0 and 7.0 samples, based on fractal FHH adsorption and desorption equations, were all close to 2.94, but the Ds values of the initial pH 5.0 sample were less than 2.38. It seems that these pore surface fractal dimensions of PFC-HA floc powders cannot totally represent the space-filling capability of the irregular pore surface, but mainly show the irregularity from the mesopore size distribution, and some macropore size distribution. In addition, compared with the fractal FHH equation, the much higher pore surface Ds values for these samples were calculated through thermodynamic model, and most of them exceed 3. Due to the hysteresis loop of the A model in the N2 absorption-desorption isotherm for the sample prepared at initial pH 5.0, the close values of its pore surface fractal dimension Ds could be determined by both the thermodynamic model and the fractal FHH adsorption equation if the fractal scale is reduced.