Structure of Al-humic flocs and their removal at slightly acidic and neutral pH

Abstract

Using microscopic and optical monitoring techniques, the process of coagulation of humic substances was studied paying attention to the structure of flocs and their settleability and filterability with information from image analysis, and HPLC, zeta potential, TOC and UV absorbance measurements. The results show that Al-humic flocs formed at pH 5.0 appear more compact with higher fractal dimension while those formed at pH 7.0 are looser with lower fractal dimension. During growth, Al-humic flocs also undergo a decrease in their fractal dimension. The attainable removal of TOC is about 40-50% at both pH values, but the optimum Al dose at pH 7.0 is about 5 times of that at pH 5.0. Charge neutralization is believed to be the main mechanism of coagulation at pH 5.0 and sweep coagulation plays an important role at pH 7.0 as well as at pH 5.0 under an overdosing condition.

Measuring Floc Structural Characteristics

“…A value of close to zero indicates a shape approaching a straight line, whilst a value of 1 indicates the shape is a perfect sphere or circle. The shape factors may be of use to (Wang and Gregory, 2002). Two fundamental difficulties arise when using such an approach.…”

“…In most instances, carefully dropping a small sample of the suspension onto a microscope slide or into a measuring cell on a slide is suitable for particle size analysis under a microscope (Spicer and Pratsinis, 1996;Wang and Gregory, 2002).…”

“…Therefore this cannot be a true representation of the actual size distribution. Wang and Gregory (2002) use a similar method to withdraw flocs for analysis using a sampling tube. The contents of the tube were carefully emptied into a microscope cell previously filled with water.…”

“…Image analysis is often combined with microscopy by mounting a CCD camera onto the microscope (Li and Ganczarczyk, 1986;Cousin and Ganczarczyk, 1998;da Motta et al, 2001;Kobayashi, 2004). The advent of photography and image analysis has allowed much quicker measurements of an almost inexhaustible number of different floc size measurements to be made from floc samples when compared to traditional microscope methods (Wang and Gregory, 2002). However, it still relies upon an unbiased approach from the microscopist.…”

Aggregation of nano-sized alum–humic primary particles

“…Actually, it is usually not easy to distinguish between the mechanisms of precipitation and adsorption. The results of some study [6] showed that it was mainly determined by pH and coagulant dosage. Kim et al [7] found that flocs formed under sweep coagulation were larger and more compact than those formed under charge neutralization.…”