論文の公開元へStrength of Flocs Formed by the Complexation of Proteins and Humic Acid
概要
1.1 Colloidal particles in the environment
Colloidal substances are roughly from nanometer to micrometer in size and exist in dynamic systems (Lead and Wilkinson, 2007). In soil and water environments, most of the colloidal substances are clays and organic materials (Bünemann et al. 2018). These natural colloidal materials usually bear electrical charges on their surfaces. The amount of positive and/or negative charges depends on the environmental conditions such as pH of the medium, ionic strength, and concentration of the substances. The soil colloidal substances (e.g., humic substances, protein, and polysaccharides) have a property of binding to the different colloidal substances such as synthetic particles through several physical and chemical processes. These processes are referred to adsorption, coagulation, aggregation, and dispersion which could amplify the chemical activity, transportation action, and hydraulic properties of the substances (Giachin et al. 2017).
Electrostatic interaction, van der Waals forces, hydration phenomenon, and hydrodynamic interaction are the interactions involved in maintaining the stability of colloid. In meanwhile, an attractive force between colloidal substances can form aggregation. Considering the van der Waals force and diffuse double - layer interaction in determining the stability of colloids is referred to as classical DLVO (Derjaguin, Landau, Verwey, Overbeek) theory (Verwey 1947; Trefalt and Borkovec 2014a).
One of the characteristics of colloids in the natural environmental condition is charging behavior of the substances. This is an important aspect of controlling the aggregation and dispersion of colloidal suspensions. For example is the adsorption induced by charged ions, polyelectrolytes, clay and, surfactant (Kobayashi 2008; Beltrán-Heredia and Sánchez-Martín 2009). Reaching the point of zero charges, known as an isoelectric point (IEP), points out the start of the aggregation process. Charge reversal also known as overcharging usually occurs because of saturated adsorption of oppositely charged substances. There are additional attractive interactions, such as hydrophobic or van der Waals force, that occurs between the substances. Therefore, the interaction between colloidal substrate in the soil and water environment must be paid attention to consider especially when these interactions could affect the mobility of substances.
1.2 Transportation of colloidal matter in soil and water environment
Colloidal substances with pollutants in the environment have the potential to move with the water flow. In the worst-case scenario, the unsupervised situation can enhance the transport of pollutants, which leads to further contamination. The movement of such colloidal substances through soil pore entirely depends on the size of the substances (Hajra et al. 2002; Li et al. 2018; Oladoja and Pan 2015; Kobayashi 2005). For colloids to be mobile in soil pore, they must be stable, and the pore is larger than the colloid substances. Meanwhile, larger substances easily settle and deposit to the soil matrix. Other than the size of substances, the size of soil pore is also the factor in the mobilization of colloid (Enfield and Bengtssona 1988). The interaction between the substances also plays an important role in the aggregation of substances. The collision between primary substances and their interaction depends on the flow field (Kobayashi 2005; Hakim and Kobayashi 2019). The high shear flow would cause the breakage of the aggregated complex and permits the mobilization of substances in the soil environment. In a meanwhile, if the shear flow is low, the aggregated substances would not break and are retained in the pore.
1.3 Floc breakage and floc strength
The strength of flocs depends on the inter-substance binding forces in the aggregates to hold the flocs together (Parker 1972; Bache et al. 1997). The floc strength is also referring to the number of individual bonds and their strength within the flocs. The floc strength study was conducted through a converging flow field. The floc strength method can provide an insight into the strength of the binding forces between the flocs and controlling its parameter in the environment
1.4 Humic substances
Humic substances (HSs) are the significant fraction of natural organic matter (NOM) in soil and water environments comprised of degraded matters of bacterial matter, plant, and animal products. The HSs play an essential aspect in the nutrient sequence system and the transportation of pollutant or hydrophobic organic contaminants, metal ion, and radionuclides in the soil and water environment (Giachin et al. 2014, 2017).
The HSs are easily bound with the mineral surfaces (Ma et al. 2018), ions (Brigante et al. 2009; Hakim et al. 2019), organic molecules (Tan et al. 2008; Khodir et al. 2020; Santos et al. 2011), oxides (Pota et al. 2020), and polymer (Xu et al. 2011). HSs such as humic acid (HA) and fulvic acid (FA) are high molecular mass polymer. These HSs are considered a supramolecular organization composed of functional groups (carboxylic acid, phenolic acid), aliphatic carbon, polymethylenes (-CH2-), O, N, S, and P atoms, and others. The HSs composition has led to various interactions such as electrostatic interaction, hydrophobic interaction, van der Waals interaction, and hydrogen bonding (Brigante et al. 2009; Avena and Wilkinson, 2002). The composition of carboxyl, phenolic, and carbon content of LHA obtained from International Humic Substances Society (IHSS) is displayed in Table 1.1 below.
1.5 Lysozyme protein
To explore the relationship between HSs and protein, a detailed and well-organized framework is needed. It is essential to use a characterized and readily available such as Lysozyme (LSZ) protein. LSZ is a globular protein with an ellipsoidal shape with a molecular weight of 14.3 kDa and an isoelectric point near pH 11. Moreover, LSZ protein is positively charged substances with the size (radius) around 1.64 nm (Dabkowska et al. 2018). The LSZ is well known for the high structural stability that makes it useful as a model protein for many practical uses and widely studied. Unfortunately, the knowledge of the interaction between the natural particles is lacking. LSZ protein is a weak polyelectrolyte, so do the HSs.
1.6 Moringa oleifera seed protein
Moringa oleifera (MO) is a plant mainly found around the tropical country and currently commercializes in several industries such as food, pharmaceutical, and cosmetic. Due to its potential properties, MO seed (Figure 1.4) gains attention from researchers for its coagulating abilities. Unfortunately, a problem of using natural coagulant is the method of extraction and purification. There are several extraction and purification methods used previously. Due to protein solubility, the MO seed powder can use the water or salt solution to extract the protein. Some of the purification methods used are dialysis, free-drying, ion- exchange, delipidation, precipitation, and centrifugation (Yamaguchi et al. 2020). Details on the extraction and purification method and the molecular weight obtained are displayed in Table 1.2.
Therefore, in this study, we focused on the strength of flocs formed by protein and HS. The floc strength depends on the intermolecular binding within and between the aggregates. The interaction between substances influences the inter-and intra- molecular binding of floc. The physicochemical factors such as ionic strength, pH, and mass ratio can influence the interaction between substrates.
この論文で使われている画像
