Abstract
Okra extract is known to have potential application as a bio-flocculant for wastewater treatment. However, no research to date has given insight into the components responsible for the flocculating ability of okra extract or its flocculating mechanism. The work presented here addresses this knowledge gap showing that pectin, especially pectin homogalacturonan (HGA) regions, appear to be the polysaccharides responsible for the flocculating ability of okra extract. The way pectin works in flocculation may be best explained by a polymer bridging mechanism. Specifically, a linear relationship between okra bio-flocculating ability and pectin homogalacturonan region to rhamnogalacturonan-I region weight ratio (HGA/RG-I) was found (y = 2.0x+47.6, R2 = 0.93, when GalA content > 300 mg/g extract), which was also validated using commercial citrus peel pectin.
Highlights
Flocculation is one of the most widely used solid-liquid separation processes for the removal of insoluble suspended solids and colloids and/or soluble organic matter in water treatment (Renault et al, 2009)
Acid flocculants had the highest optimum yields at 38.0 %, followed by alkali flocculants at 32.3 % and water flocculants at 28.5 %. This is thought to be because compared to neutral pH solvent, acidic and alkaline conditions help to open the cell wall structure, which leads to better solubility of cell wall components into the extracting solvents and higher yields (Adetunji et al, 2017; Yeoh et al, 2008)
Previous studies have shown promising yields of okra extracts (Anastasakis et al, 2009; Lee et al, 2015) but they only used water as the solvent. This finding is opposite to some works that shown the use of strong acids provides high extraction yield and time-saving advantages when other biomasses materials were studied, for example citrus pomace (Lim et al, 2012), this present study suggests that the structure of okra pectin may be more fragile or less connected to the cell wall compared to those other biomass materials and the use of strong solvents is not of necessity
Summary
Flocculation is one of the most widely used solid-liquid separation processes for the removal of insoluble suspended solids and colloids and/or soluble organic matter in water treatment (Renault et al, 2009). This industrial process, with the help of flocculant, combines those fine divided or dispersed particles together into aggregates, facilitates their removal in subsequent sedimentation, floatation and filtration stages and causes clarification of the system (Renault et al, 2009; Sharma et al, 2006). There are many plant-based bio-flocculants including extracts from okra (Abelmoschus Esculentus) (Lee et al, 2015), mallow (Malva sylvestris) (Anastasakis et al, 2009), and modified konjak polysaccharides (Xie et al, 2007) etc
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