Methylated mud snail protein as a bio-flocculant for high turbidity wastewater treatment

One of the most highlighted environmental footprints of rapid urbanization and industrialization is the production of a large volume of industrial effluents (Nasim & Bandyopadhyay 2012). Effluents from mining, smelting plants, coking factories and the construction industry all have the high turbidity characteristics (Chen et al. 2016; Zhao 2020). These high turbidity effluents with high levels of suspended solids, can increase the processing cost, cause insufficient light flux and also great harm to the aquatic ecosystem. To address this, flocculation, membrane technologies, electro-flocculation and constructed wetlands have been used for high turbidity water treatment (He et al. 2020; Ma et al. 2021; Saini et al. 2021). Flocculation is an industrially applicable and financially feasible method owing to its relatively simple operation, low cost and low energy consumption (Teh et al. 2016). To date, various flocculants have been used in municipal wastewater and heavily loaded industrial wastewater retreatment (Wang et al. 2011). However, traditional synthetic polymer flocculants caused severely secondary pollution to the natural environment during their application. For instance, polyaluminium chloride flocculant inhibits root elongation growth and disturbs the uptake of nutrient and water in plants (Sun et al. 2007), polyacrylamide flocculant is treated as suspicious carcinogen and presents high neurotoxicity (Liu et al. 2019). Thus, developing bio-flocculant from a natural polymers resource has received increased attention because of the advantages of eco-friendliness, non-toxicity, no secondary pollution and excellent biodegradability (Ferasat et al. 2020). In recent years, bio-flocculants as a facile and efficient way for high turbidity wastewater treatment has been applied to address the issue of global water scarcity. Various high abundant natural polymers have been used to produce bio-flocculants, including cellulose, lignin, chitin, cyclodextrin, chitosan and starch.

Protein, an abundant natural biomaterial worldwide, emerged as an promising candidate as a bio-flocculant because of its macromolecular and charged properties (Matthew et al. 2018). For instance, bio-flocculants based on cow's milk casein and soy protein flocculants have been applied to purify wastewater containing kaolin, kanto loam suspension and oil/water emulsion (Seki et al. 2004; Liu et al. 2012). However, China's soybean annum consumption excedes 110 million tons, nearly 80% of which depend on imports. Net imports of dairy increased with a growth rate in excess of 30%, leaving a huge gap in supply (Huang et al. 2014; Hairong et al. 2016). Thus, they are not the ideal source of protein-based bio-flocculants, which severely prevent their application on a large scale. Mud snail, is a mollusk, widely grown in the freshwater resources of the Yangtze River basin and most of the eastern provinces of China, its total annual yield in China is more than 10 million tons (Zhang et al. 2013). Mud snail is composed of a tough outer shell and a soft mud snail abdominal foot, 60% of which is mud snail protein (Msp). Thus, mud snail is an abundant low-cost underutilized animals protein resource (Xia et al. 2007). The Msp contains abundant leucine, lysine, glutamate, asparagine and arginine, indicating the existence of carboxyl functional groups. Prior published reports have demonstrated that these carboxyl of aspartic and glutamic acid residues in protein can react with methanol (Fraenkel-Conrat & Olcott 1945). After methylation reaction, the negative charge of proteins was eliminated, which is advantageous to destabilize the finely suspended solids during water purification. Owing to the high protein content, abundant resources, low-cost and low utilization characteristics, Msp shows considerable potential as an alternative source of protein-based bio-flocculant.

Here, the author reported a novel bio-flocculant composed of methylated mud snail protein (MeMsp) for high turbidity wastewater treatment. The Msp was extracted from mud snail abdominal foot using isoelectric precipitation (PSC-IP) and its flocculation performance was further enhanced by methylation. Structural characterization of FT-IR, zeta potential (Zp) analysis and elemental analysis were carried out and further confirmed the successful synthesis of the MeMsp. Their flocculation performance was evaluated by assessing the clarity of high turbidity water simulated by kaolin suspension, and MeMsp was applied to tunnel construction wastewater from the temporary sand sedimentation tank of the wastewater treatment station. In flocculation experiments, the effects of dosage, solution pH value, salt concentration and settling time on the removal rate of turbidity were investigated. The sedimentation characteristics and flocs particle size distribution were determined at the optimum dose by sedimentation balance test. In addition, the flocculation kinetics based on aggregation of particle and collision frequency models and the flocculation mechanism were investigated to comprehensively understand the flocculation of high turbidity water.

Mud snail was obtained live from a local supplier near the campus. Sodium hydroxide was provided by the TIAN LI Chemical Reagent Co., Ltd (Tianjin China). hydrochloric acid, ammonia and methanol were obtained from Tianjin Fuyu Fine Chemical Co., Ltd (Tianjin China). Kaolin was supplied by Tianjin FUCHEN Chemical Regent Factory (Tianjin China). Sodium chloride came from the Shanghai Chemical Industry Park (Shanghai China). The construction wastewater was obtained from the temporary sand sedimentation tank of the railway tunnel construction wastewater treatment station located in Shaanxi, China.

Msp was extracted from the mud snail abdominal foot part which had to be refrigerated within 48 h of treatment to avoid deterioration of extractable protein due to prolonged exposure. Before protein extraction, the outer shell and viscera were removed manually from the mud snail. The abdominal foot part was freeze dried 24 hours at −45 °C with freeze dryer (Ningbo Scientz Biotechnology Co. Ltd, Ningbo, China) and then crushed. This was followed by weighing the powder /0.01 M NaOH solution at a 1:40 ratio (2.5% w/v) and heated at a temperature of 40 °C, with magnetic stirring for 40 min. The ratio and temperature were selected according to previous reports (Xia et al. 2007). After centrifugation at 4,000 rpm for 10 min, an extra extraction was executed for 1 hour with half of the lye volume. The extract was pooled. The pH of the supernatant was adjusted to 4.3 using 0.1 M HCl and centrifuged as above. The precipitate was freeze dried.

Mud snail protein was dispersed in 100-fold amounts of methanol solution that included HCl (0.1 mol/L), next the mixed solution was stirred slowly by magnetic stirrer at room temperature for the required time (3–36 h) and neutralized with ammonia. Consequently, the MeMsp was obtained by centrifugation at 3,000 rpm for 10 min. Finally, the MeMsp was freeze dried as the samples of characterization tests (Liu et al. 2012).

In this section, FT-IR spectra were obtained using a FT-IR spectrometer (Nicolet) and KBr pellets, with the wave numbers range from 500 to 4,000 nm. The SEM images were recorded by a scanning electron microscope (SEM, Hitachi S-4800) to sense the surface morphology of free-sedimentation kaolin particles and flocs. The zeta potential was determined using a Zeta potential analyzer (Zetasizer Nano ZSE) to evaluate the surface electric potential of the Msp and MeMsp. The settling performance and flocs size distribution were obtained using a Particle Sedimentometer (JCJ04, POWEREACH, Shanghai, China). Elemental composition analysis of flocculant was carried out using an Elemental Analyzer (Vario EL cube Elementar Germany). pH values of solution were measured using a pH meter (PHSJ-3F).

It is generally accepted that the carboxyl ionization of aspartic and glutamic acid residues on these side chains of protein led to the majority of the negative charges on Msp (Xia et al. 2007). Modification of surface electrical properties can affect protein flocculant properties. Therefore, to improve the flocculation performance of Msp, methyl was introduced into MeMsp via a methylation reaction, as shown in Figure 1. After extraction and pulverization, Msp was modified with methanol using hydrochloric acid as catalyst, according the method reported by Fraenkel-Conrat and Olcott (Fraenkel-Conrat & Olcott 1945). The ionizable -COOH was substituted to nonionizable -COCH₃, so, the reaction eliminated the negative charge of Msp under a high pH environment.

FT-IR spectroscopy was used to detect the change in chemical bonds of Msp and MeMsp (6–36 h). The curves (a–e) depicted the FT-IR spectra of Msp and MeMsp at different reaction times. The strongest peak at 1,659 cm⁻¹ and the peak at 1,535 cm⁻¹ were due to the C = O stretching vibration and the N-H bending vibration of the amide groups, the small and broad peak around 3,300 cm⁻¹ is the secondary amides. These characteristic absorption peaks are all in good agreement with those absorption peaks for protein (Ying et al. 2005). In the FT-IR spectra of MeMsp (b–e), the peaks at 1,730 cm⁻¹ and 1,220 cm⁻¹ are ascribed to C = O and C-O-C asymmetric stretching vibrations of methyl ester groups (Wheelwright et al. 2012; Bhalkaran & Wilson 2016), which demonstrates that the carboxyl groups were successfully methylated. Another apparent evidence is the significant increase in the intensities of the CH₃ stretching vibration at the peaks of 2921 cm⁻¹ and 2,850 cm⁻¹ and the CH₃ bending vibration at 1,398 cm⁻¹(Wheelwright et al. 2012). All of these changes and transformation indicate that the methyl was successfully introduced to the MeMsp by methylation. The zeta potential of the Msp and MeMsp (6–36 h) at different pH was investigated and displayed in the Figure 2(b). It can be seen that zeta potential of Msp firstly positive and then negative as pH increased, the isoelectric point (pI) was observed at approximately pH 4.3 according to the curve, which was consistent with the Xia's report (Xia et al. 2007). After methylation, the pI of MeMsp6-36 samples was determined to be 7.6, 9.5, 9.8, and 9.9 respectively, all these values are greater than the Msp. It can be noted that methylation resulted in a higher positive potential, the increase in pI was due to the reduction of the negatively charged carboxylic groups on the surface of the Msp as the reaction proceeds. Furthermore, methylation with protein is reported to be a specific reaction that affects only the carboxylic acid groups, while other functional groups such as indole, amino, phenolic and thiol groups remain unchanged (Fraenkel-Conrat & Olcott 1945). So, the methylation process can be revealed by the changes in carbon content and C:N atom ratio (Bhalkaran & Wilson 2016). As shown in Table S1 in Supplementary Information, upon increasing the reaction time from 2 to 36 h, the carbon content increased from 49.97% to 52.87% and the C:N atom rate also increased due to the transformation from -COOH to -COCH₃. These trends indicate that Msp was successfully methylated, and the reaction occurred mainly during the first 24 hours. Therefore, 24 hours was chosen as the total reaction time.

Figure 3(a) shows the dosage on the flocculation performance of mud snail abdominal foot powder (powder), Msp and MeMsp-24. The results indicated that the powder has a slight clarifying effect, which may be because the attraction between particles and powder. Msp exhibited similar variation trend with MeMsp-24, the clarification efficiency first continuously increased and then decreased with increasing the flocculant dosage. The optimal dosage of Msp and MeMsp-24 was 31.5 mg/g, 27 mg/g, the corresponding clarification efficiencies were 80.82% and 97.46%, respectively. When flocculant levels were low the possibility of kaolin particles colliding with flocculants was so low that interaction was insufficient to induce flocculation (Yang et al. 2019). After increasing the dosage of flocculant, the adsorption bridging effect and the net trapping sweeping effect were enhanced. However, when dosage was excessive, for Msp, the electrostatic repulsive force dominated and intermolecular attraction became weaker, thereby becoming a detrimental condition for aggregation of particles (Liu et al. 2019). MeMsp-24 would cause a ‘cage effect’, the sites on surface of kaolin particles were highly occupied and generated steric resistance, which prevented the growth of flocs and thus decreased the clarification efficiency (Zhao et al. 2018). Therefore, from the aspects of optimal additive dosage and clarification efficiency, the effect of MeMsp-24 was superior to that of Msp.

The influence of pH on the flocculation performance is shown in Figure 3(b). MeMsp-24 was superior in reducing the turbidity of kaolin suspension, regardless of pH range. The clarification efficiency of MeMsp-24 was more than 97.13% at a broad pH range of 1–9, which was better than that of Msp (maintained 95.5% at pH 1–3). Further increasing the pH, their clarification efficiency gradually declined, and the powder did not exhibit flocculation activity in acidic and alkaline environments. The results depended upon the fact that protein is amphoteric, the ionization of carboxyl groups was blocked when pH was great than pI, which provided a positive ion environment. This was greatly facilitated to trigger the bridging effect between the interacting particles (Essandoh et al. 2020). Therefore, methylation is clearly beneficial to the pH range at which MeMsp-24 is efficient due to methyl blocking the carboxylic acid groups and giving a higher pI. As Figure 2(b) shows, MeMsp-24 can remain positive over a broad pH range of 1.0–9.8, this confirms that MeMsp-24 has a wide scope for flocculation. This is an advantage for MeMsp-24, which facilitates its adaptation to various flocculation environments. In addition, the optimal result can be achieved near neutral pH, this can reduce the possibility of corrosion of the equipment in acidic environments.

Highly saline wastewater, a most prevalent context in industrial fields, greatly affects the flocculation performance of flocculants. So, it is significant to investigate the effect of inorganic salt on the flocculation performance (Liu et al. 2012). Figure 3(c) shows that increase in NaCl concentration caused the powder to lose flocculation activity. In contrast, addition of NaCl to the kaolin suspension containing Msp and MeMsp-24 was found to improve the removal rate of kaolin. MeMsp-24 reached 98.35% at 0.5 mol/L, and Msp increased from 81.03% to 95.1% after increasing of NaCl concentration. This can be explained as electrostatic shielding caused by NaCl, on the one hand, the charge shielding possibly reduced the thickness of diffused electric double layers around the kaolin particles, which facilitated bridging of particles, allowing the particles be more prone to aggregate (Konduri & Fatehi 2017). On the other hand, the charge shielding allowed the structure of flocs to be less rigid and have a more relaxed configuration, which enhanced bridging flocculation. (Piazza et al. 2015) Therefore, both Msp and MeMsp-24 had excellent salt resistance and the potential to deal with high salt wastewater.

The settling dynamic of flocs in the coagulation–flocculation process is an important parameter in the design of settling tank, the effect of standing time on flocculation performance was investigated. (Liu et al. 2019) As Figure 3(d) shows, the major flocculation reaction took place within first 10 min, most of the impurities settled within 10 min. At 15 min, flocculation reaching equilibrium with clarification efficiency of Msp and MeMsp-24 reached 80.86% and 97.23% respectively, and the residue turbidity remained constant after 15 min. Therefore, MeMsp-24 can destroy the stability of high turbidity wastewater so that particles can settle rapidly in a short time to increase effectiveness and reduce the cost of the industrial wastewater treatment.

Settling performance and flocs size distribution are another important characteristics for evaluating the performance of flocculant (Yusoff et al. 2018). Therefore, settling curves and particle size distribution of kaolin and flocs were measured and displayed in Figure 4 and Table S2. Figure 4(a) shows that the cumulative weight of flocs formed by Msp and MeMsp-24 all presented initially rapid increases and then slowly increased to a plateau. According to the initial slope of the weight accumulation curves, the settling velocities of the flocs for Msp and MeMsp-24 were 1.2 mm/s and 1.33 mm/s, respectively. This result indicated that both Msp and MeMsp-24 significantly accelerated the rate of solid–liquid separation, but MeMsp-24 had the faster settling velocity. Furthermore, according to the easy accessibility model (EAM), MeMsp-24 has a larger radius of gyration due to its fast sedimentation speed, so that it is easier to agglomerate with solid particles to form flocs, and which is beneficial to enhance flocculation efficiency (Brostow et al. 2007). The particle size of kaolin was 842.7 nm (Table S2), the average size of flocs formed by Msp and MeMsp-24 was 10 μm and 14 μm, respectively (see Figure 4(c) and 4(d)), The larger floc size of MeMsp-24 is due to stronger electrostatic interactions between MeMsp-24 and kaolin, which facilitate the aggregation of kaolin particles. Figure 4(b) shows kaolin suspensions (4.0 g/L) before and after flocculation treatment with Msp and MeMsp-24. Clearly, MeMsp-24 triggered the precipitation of kaolin and produced a relatively transparent water sample, further demonstrating the excellent flocculation performance of MeMsp-24.

To demonstrate visually the flocs deposited, the surface morphologies of free-deposited kaolin (Figure 5(a) and 5(b)), flocs formed by Msp (Figure 5(c) and 5(d)) and flocs formed by MeMsp-24 (Figure 5(e) and 5(f)) were taken to depict the changes before and after the flocculation. Figure 5 shows that free-deposited kaolin exhibited a relatively decentralized state. After being flocculated by Msp and MeMsp-24, the flocs took on a tightly clustered reticulate structure, this greatly enhanced the rate of solid–liquid separation. In addition, the flocs were composed of densely aggregated and disorderly arranged fragments and particles of various size, showing irregular three-dimensional clusters. This structure is conducive to further enhancing the coagulation of particles and bridge effect because this can enhance the contact opportunities between flocs.

In order to better explain the flocculation process of Msp and MeMsp, the authors investigated the flocculation kinetics, which are significant for process control of solid–liquid system separation (Berlin & Kislenko 1995). According to the established flocculation kinetic model, flocculation is the balance of aggregation and breaking of flocs. Therefore, the current study concentrated on the kinetics based on particle aggregation and frequency of collisions of particles (Das et al. 2013).

According to Figure 6(a) and 6(b), the rate constants k₁ and k₂ and regression coefficient R² were determined from the Equation (3) and given in the Table S3. For Msp and MeMsp-24, with increase in flocculant dosage from 7 mg/g to 31.5 mg/g and 27 mg/g, respectively, the aggregation rate constant k₁ increased, whereas aggregate breakage rate constant k₂ decreased. This indicated that the higher dosages of flocculant generated harder flocs. However, with the further increase in flocculant dosage over the optimal dosage, flocculation performance declined as k₁ decreased and k₂ increased. This indicated that the decrease in the flocculation effect was caused by both the decrease in aggregation and the increase in fragmentation (Feng et al. 2020), which explained that the steric and electrostatic repulsion led to destabilization of flocs in the suspension. Therefore, excess of flocculant dosage affects the flocculation kinetics as well as flocculation performance (Das et al. 2013; Kaith et al. 2016).

According to Equation (5), the rate constant K is obtained from the slope of the fitted curves of the (N₀/N_t)^1/2 versus t of Msp and MeMsp-24 that is shown in Figure 6(c) and 6(d), and displayed in Table S3. The results indicated that the rate constant k reached the maximum at the optimal dosage, lower or higher dosages caused the decrease in k. This trend illustrated that most efficient collision between flocculant and kaolin particles took place at the optimal dosage. For Msp, the rate constants were always less than MeMsp-24, only 10.22 × 10⁻²⁰ at the optimal dosage, this because only adhesion and adsorption through van der Waals force led to a weak collision process. Increased flocculant dosage led to electrostatic repulsion that further reduced the probability of collision (Ma et al. 2017). For MeMsp-24, the low dosage flocculant caused weak interactions between MeMsp-24 and kaolin particles due to the low positive charge density in suspension, resulting in insufficient collision. When dosage was in excess, firstly, the bridging effect had difficulty taking place because sites on the surface of the kaolin particles were highly occupied without sufficient effective junction points for collision. Secondly, excess positive charge after neutralizing the negative charge of kaolin suspension generated the repulsive force between the flocs, both of these decreased the collision efficiency (Das et al. 2013; Kaith et al. 2016; Ma et al. 2017).

Currently, tunnel construction processing mainly adopts a borehole-blasting method and shield driving method, which will produce huge amounts of high turbidity wastewater due to spraying cooling dust-fall and water gushing coming from bad geological layers (Cho et al. 2016). As shown in Table S4, the railway tunnel construction wastewater through temporary sedimentation is a typical alkaline high turbidity suspension of nanoparticles, the main treatment process was flocculation + sedimentation + filtration. From the results presented in Figure 3(b), it is clear that MeMsp-24 allowed a turbidity reduction higher than 97% at pH values between 1 and 9. Therefore, MeMsp-24 was applied to tunnel construction wastewater, Figure 7(a) and 7(b) showed the effect of MeMsp-24 amount on the settling behavior of railway tunnel construction wastewater at natural pH (pH = 9.04). It is evident that the addition of MeMsp-24 increased the sedimentation velocity and the larger the amount of MeMsp-24 added, the higher was the clarification efficiency of the railway tunnel construction wastewater and the clarification efficiency could reach 92.12% at 150 mg/L. These results demonstrated the practical applications of MeMsp-24 in future sustainable high turbidity wastewater treatments.

As shown in Figure 7(c), the FT-IR spectra of flocs displayed characteristic peaks at 3,356 cm⁻¹ attributing to –OH stretching caused by inner surface –OH in-phase and interlayer water –OH. The peaks at 1,100 cm⁻¹ and 500 cm⁻¹ were due to Si-O and Al-OH, the area around 800 cm⁻¹ was due to Si-O-Si intertetrahedral bridging bonds in SiO₂ (Kumar & Lingfa 2020). In FT-IR spectrometry of flocs, no new peaks appeared in addition to the original peaks of kaolin, Msp and MeMsp-24, which indicated that the flocculation was not caused by a chemical reaction between kaolin and flocculants. Figure 7(d) shows that the zeta potential of the kaolin suspension was always negative; after flocculation by Msp and MeMsp-24, the zeta potential was close to zero at pH 3 and pH 3 and 7, respectively. Thsi apparently corresponds to higher clarification efficiency, indicating the charge neutralization is one of major mechanisms of flocculation processes in the environment with pH less than pI (Luo et al. 2020). When pH is more than the pI, Msp and MeMsp-24 showed the same charge for kaolin particles (see Figure 2(b)), so, adhesion and adsorption through relatively weak van der Waals forces played a critical role in contact and collision processes (Ma et al. 2017). Moreover, the expected zeta potential of the suspension after flocculation with only the presence of charge-neutralization mechanisms will be zero under the point of optimum dosage (Tripathy & De 2006). However, Figure 7(d) shows that the zeta potential tended to be negative at optimum dosage. This is because the macromolecular weight of flocculants favours bridging and electrostatic patch relative to the charge-neutralization mechanism and the zeta potential tends to become negative at optimum flocculation (Tripathy & De 2006).

According to the analysis above, a possible flocculation mechanism diagram with Msp and MeMsp was deduced and displayed in Figure 8. When the pH was less than the pI, the charge neutralization produced an interaction between particles and flocculant to overcome the potential barrier, and aggregation is allowed to occur. The kaolin particles and smaller flocs attach to the flocculant via strong bridging effects and electrostatic patch, generating larger ones with a compact floc structure. In the environment, where the pH is great than pI, adhesion and adsorption due to van der Waals forces plays a critical role in overcoming the electrostatic repulsion in contacting and collision processes. Finally, with decrease in floc strength, smaller and finer precipitates are generated through the bridging effect, and are accompanied by weak flocculation effectiveness.

In summary, the authors have developed a bio-flocculant MeMsp from the mud snail abdominal foot through extraction and methylation. Then, synthesis of MeMsp was confirmed by FT-IR spectra, elemental analysis and ZP analysis. The performance of Msp and MeMsp in flocculation of turbidity kaolin suspension, as well as the flocculation kinetics and mechanism were systematically studied. After flocculation trials, it was found that Msp worked well at a specific acid–base condition as pH (1–3), while MeMsp-24 showed excellent charge-neutralization capability due to elimination of the negative charge and showed a wide highly efficient range of pH (1.0–9.0) reducing turbidity levels. More specifically, the optimal dosage of MeMsp-24 can be reduced to 27 mg/g, while the turbidity reduced percentage could reached 97.4% at pH 7.0, which is much higher than the 80.8% of Msp; MeMsp-24 had faster settling velocity and larger average size. Moreover, in a realistic tunnel construction wastewater flocculation experiment, MeMsp-24 also performed in excellent flocculation (92.12% clarification efficiency). The flocculation kinetics and mechanism analysis revealed that the most effective particle collisions occurred at the optimal dosage, and charge neutralization and adhesion played irreplaceable roles in different environments, respectively. The renewable, non-toxic, and biodegradable MeMsp provides a new opportunity and a candidate for high turbidity wastewater treatment.

This work was supported by the Natural Science Basic Research Program of Shaanxi (Program No. 2021SF-497), and the Fundamental Research Funds for the Central Universities, CHD 300102291403.

All relevant data are included in the paper or its Supplementary Information.