Flocculation ability of alginate-protamine complex flocculant × SDGs
A certain amount of waste (garbage) is inevitably generated in the production of processed marine products. Appropriate treatment and reuse of this waste is important for the effective use of resources. I would like to contribute to the SDG 12 (RESPONSIBLE COMSUMPTUION AND PODUCTION) by developing methods to retrieve the waste and utilize it as energy such as biodiesel.
The United Nations has designated the decade starting in 2021 as the "Decade of Ocean Science," with the aim of contributing to the SDGs. Ocean science, as defined by the UN, includes the field of fisheries.
The flocculation of suspensions by chemical flocculants is used to improve solid-liquid separation in mineral processing operations, wastewater treatment, and so on. The currently accepted chemical flocculants are synthesized high-molecular-weight polymers and alum. Unfortunately, both of these flocculants are environmentally undesirable, since chemically synthesized polymer flocculants remain in natural environments for long periods without degradation to less harmful forms.
In recent two decades, many studies concerned environmentally friendly biodegradable flocculants have been reported; naturally existing polymers such as chitin or chitosan, methylated proteins, extracellular polymers produced by bacteria, chemically modified natural polymer or polysaccharide (starch-based flocculant, cellulose-based flocculant, alginate-based flocculant, and so on).
On the other hand, let me tell you something else, fillet of salmon, cod, and so on have been used for food all over the world. In the other hands, however, fish milt of salmon, cod, sandfish and so on, have been used for food from old times in Japan only, and most salmon milt has been discarded because of an inherent fishy smell, a fast decline of freshness, and difficulty for food processing due to lacking heat coagulation. In the world, a huge amount of fish milt has been discarded. Salmon milts include a lot of protamine as a nucleoprotein, which has a basic group, such as amino or guanidine functional group, and has a positive charge in the neutral pH region.
A surface charge of solid particles in the aqueous phase is known as a negative charge mostly. These suggest that protamine can adsorb onto suspended particles surface and can become a flocculant for suspended solid particles.
However, judging from the molecular weight of protamine (Mw. ca. 4600), the molecular size of protamine should be relatively small than other proteins used as flocculants. Novel flocculants are needed to have a certain size for making effective flocs in the flocculation process.
Bridging flocculation could occur under the situation which flocculant size is larger than the closest possible distance between surfaces of adjacent solid particles. The distance would exist because of electrostatic repulsive force between surfaces which were same charge. It has been pointed out that one of the conditions for having an excellent flocculant is that it is important to have a size exceeding this distance.
Figure 1 Schematic diagram of electric double layer existing at surface of suspended particle in aqueous environment and the proposed protamine-alginate complex flocculant.(Reference: Kashiki and Suzuki, Ind. Eng. Chem. Fundam. 25, 444 (1986))
As described above, polysaccharides have been employed as bio-flocculants. Major polysaccharides used as bio-flocculant were mainly alginate, chitosan, cellulose, starch, pullulan, xanthan and pectin and so on. Among these, one of the polysaccharides, alginate is a major structural polysaccharide from brown seaweeds, Brown algae are very familiar to us because they are widely distribute along the coast of Japan. In addition there are many unlitilied and unedible species of brown algae.
Alginic acid, a linear polysaccharide of beta-D-mannuronic acid and alpha-L-guluronic acid, is a main cell-boundary constituent of brown algae. Alginic acid contains about 4x103 mol/g of carboxylic groups. By these ways, cationic properties were added to alginate polymer to bind with solid surfaces charged negatively in aqueous environment.
We proposed the utilization of a complex of two substances, alginate polymer and protamine as an environmentally preferable flocculant in the aqueous phase, and examine the expression of flocculation ability and the capabilities of the alginate-protamine complex to flocculation of suspended solid particles. By this method, relative larger size and charged positively polymer as a flocculant could be obtained without some complicated chemical modifications.
Kaolin and quartz were chosen model solid particles. We investigated especially the expression of flocculation ability of the complex, the influence of the mass ratio of alginate and protamine and pH value of the suspension on the flocculation in the present study.
Protamine sulfate from salmon, Sodium alginate, Quartz sand and kaolin were purchased from Wako Pure Chemical Industry (Japan). was purchased from Kanto Chemical (Japan). Hydrochloric acid and sodium hydroxide were used analytical grade reagents. These reagents were used without further purification.
Preparation of suspensions
The quartz sand was washed with distilled water and was dried at 60℃. Then, it was pulverized for 48 h using ceramic ball mill. The quartz powder was washed with distilled water repeatedly and was dried at 60℃ for 48 hr. The size distributions were measured with a laser-scattering size distribution analyzer (LA-300 HORIBA, Ltd., Japan). The mean particle diameters for kaolin and quartz were 6.5 and 47.3 μm, respectively. In most experiments, concentrations of both powder suspensions were 3 g/L.
Quartz or kaolin (3 g/L, 100 mL) suspension was poured into a 100 mL glass graduated cylinder. The protamine and alginate solutions prepared at desired concentrations were mixed each 2 mL, respectively. Then, the mixture solution (4 mL) was stirred hardly and 2 mL of the mixture was immediately added to the 100 mL of suspension with a macro pipette. The suspension added flocculant was agitated with a magnetic stirrer at 500 rpm for 5 minutes. After this, the suspension was left to stand without agitation for 1 minute, then, a 2 mL sample was taken from the supernatant layer at a position of 2 cm under the liquid surface of the suspension. The absorbance of the sample was measured at 700 nm spectrophotometrically.
The flocculation efficiency was evaluated with the relative absorbance, A/A0, where A and A0 represent the absorbance of the suspensions in the presence and the absence of flocculant, respectively.
Typical results are shown in figures 2 and 3.
Figure 2 Influence of mass ratio of sodium alginate (Alg) and protamine (Pro) on the relative absorbance, A/A0, for (a) kaolin and (b) quartz particles. Each symbol corresponds to protamine content of the mixture.
Influence of the mass ratio of aliginate and protamine on the flocculation efficiency is shown in Figure 2 for kaolin (Fig. 2a) and quartz (Fig. 2b), respectively. The experiments were conducted at pH 6.9-7.0. For both kaolin and quartz, the degree of clarification was higher in the range of mass ratio ca. 0.3-0.8. Especially, in cases of 4.0x10-4 and 6.0x10-4 g for protamine content in the mixture for quartz and kaolin, the most effective clarification was observed in the experimental range of this study.
Figure 3 Influence of pH value of the kaolin suspension and mass ratio of sodium alginate (Alg) and protamine (Pro) in the mixture on the relative absorbance, A/A0, of kaolin suspension. The content of protamine is 1.5x10-4 g.
Influence of the pH value on the flocculation efficiency is shown in Figure 3. kaolin particle was employed as a model solid particle and the pH value of the 3 g/L suspension was varied by addition of HCl or NaOH solutions. The content of protamine in the mixture solution was fixed at 1.5x10-4 g and that of alginate was varied. At pH 3, 5 and 7, A/A0 value mostly became under ca. 0.1, thus, good clarification by flocculation was observed. On the other hand, at pH 9 and 11, flocculation obviously did not occur in the system. At pH 8, flocculation ability became getting worse with an increase in the mass ratio of alginate and protamine in the mixture solution. The reason why the complex flocculant has the flocculation efficiency in pH 7 was described in the section 3.3. On the other hand, in the alkaline pH region (pH 8-11), especially, a hydrogen ion dissociated from the guanidine and the amino groups, which resulting the both functional groups became uncharged state. This should be considered that the complex as the flocculant could dissociate into protamine and alginate and the flocculation performance disappeared.
A novel bioflocculant, alginate-protamine complex, was developed and was proposed. Alginate and protamine have no flocculating ability solely, however, the mixture solution of alginate and protamine or alginate-protamine complex showed a good flocculating ability. The flocculation performance was evaluated by clarification experiment with kaolin and pulverized quartz sand in terms of relative absorbance.
The flocculation performance was examined by varying experimental conditions, especially, alginate/protamine mass mixing ratio and pH. The flocculation performance was significantly affected by the mass mixing ratio and pH. The most effective mass mixing ratio was found at 0.4-0.8 in the pH range of 3-7. The flocculation performance was also affected significantly by pH. Good flocculation performance was provided at pH 3-7, however, the performance was getting worse at pH 8-11.