Preparation of foaming agent from photosynthetic bacteria liquid by direct thermal alkaline treatment

Construction and Building Materials 238 (2020) 117715

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Preparation of foaming agent from photosynthetic bacteria liquid by direct thermal alkaline treatment Jiaxuan Cai a, Guangming Zhang b,a,⇑, Zhouhua Xie c, Yichun Zhu d a

School of Environment and Natural Resource, Renmin University of China, Beijing 100872, China School of Energy & Environmental Engineering, Hebei Institute of Technology, Tianjin 300401, China c Chongqing Nankai Secondary School, Chongqing 400030, China d School of Architectural and Surveying & Mapping Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, China b

h i g h l i g h t s
Photosynthetic bacteria can be used as a new source of protein-based foaming agents.
A foaming agent was prepared from photosynthetic bacteria liquid directly by thermal alkaline treatment.
A central composite design was applied to optimize the thermal alkaline treatment.
Foaming agents with SDS or CTAB showed better foamability than commercial product.

a r t i c l e

i n f o

Article history: Received 7 May 2019 Received in revised form 11 November 2019 Accepted 24 November 2019

Keywords: Foaming agent Photosynthetic bacteria Thermal alkaline treatment Response surface methodology Surfactant

a b s t r a c t This study confirmed that photosynthetic bacteria (PSB) can be used as a protein source for foaming agents. A foaming agent was successfully prepared from PSB liquid directly via thermal alkaline treatment, thus eliminating protein extraction and purification. Further optimization of the important process parameters was conducted through a central composite design to achieve the highest foamability. Results indicated that the PSB foaming agent prepared under a reaction temperature of 100 °C, reaction time of 1 h, and pH of 12.5 presented the highest foamability (476 mL). Adding 1.2 g/L sodium dodecyl sulfate could produce a foaming agent with a higher foaming capacity than a commercial high-performance product. These findings provide a new source of bio-building materials and expand the application of PSB. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Foamed concrete, a type of lightweight concrete, exhibits some special characteristics, because of the random air bubbles within its porous structure. Low self-weight, minimum consumption of input material, good mechanical strength, high flowability and improved workability have led to a growing interest in using foamed concrete [1–3]. As an essential raw material, foaming agents play an important role in the production of foamed concrete. The properties of foamed concrete are critically dependent on the quality of the foaming agent, which includes synthetic surfactants, proteins, aluminum powder, glue resin, and other ingredients [4–7]. Synthetic surfactants and protein foaming agents are used most widely [8]. Both types of agent reduce the surface ten⇑ Corresponding author. E-mail addresses: [email protected] (J. Cai), [email protected] (G. Zhang), [email protected] (Y. Zhu). https://doi.org/10.1016/j.conbuildmat.2019.117715 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

sion of the solution, which facilitates the formation of stable air bubbles. Synthetic surfactants are traditional foaming agents, which have high foamability even at low concentrations [9]. As for protein-based agents, its foaming capacity is somewhat inferior to that of some synthetic surfactants, but the foam stability is much greater [6,8]. Therefore, protein foaming agents, which show a balance of properties, are considered as an ideal choice. Furthermore, protein foaming agents can be used to produce foam fireextinguishing agents that are environmentally safe [10,11]. Various animal and vegetable proteins have already been used as raw materials [2,12–14]. Recently, alternative sources of protein have been explored in terms of reusability and environmental sustainability. To date, the feasibility of proteins extracted from wastes such as excess sludge [15,16] and vinasse [17] has been demonstrated. The recovered proteins are more widely available, and the use of them confers some social benefits. Nevertheless, the preparation process of protein foaming agents remains complex. For instance, to make use of animal protein, horn meal,

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blood, bones, or other animal products are hydrolyzed by heating with acids or bases, followed by water wash as purification. After that, the extraction process, which includes pressure filtration, vacuum concentration, air drying and grind will be done to get the desired protein product [11,18]. The purification and extraction process time consuming and energy consuming, so the application can be promoted if there is a suitable source of protein with a simpler preparation process. Photosynthetic bacteria (PSB) are a group of bacteria that can use light energy and organic matter to grow and are widely distributed in nature [19]. PSB can be used as the protein sources because of their high protein content and high growth rate. Yang et al. [20] reported that PSB cells have protein content of 45%–65% and in some cases up to 80%. Moreover, like other single-cell proteins, the proteins in PSB are abundant, with stable characteristics and lower cost than traditional raw materials, making them a viable option for foamed concrete [21]. Additionally, PSB have great potential in treating wastewater effectively and converting organics in wastewater into biomass, which is rich in single-cell proteins [22]. Therefore, using PSB to produce protein foaming agents is environmentally and economically beneficial because the cells can come from wastewater treatment. Therefore, both pollution control and resource recovery can be realized. However, to the best of our knowledge, there is no information on producing protein-based foaming agents from PSB. Since PSB have high protein content and more digestible cell walls, it might be possible to prepare foaming agents directly from PSB liquid. If so, the production sequence can be simplified. Thermal alkaline treatment is one of the most effective and intensively studied methods for breaking up microbial cells to extract protein [23,24]. Therefore, this high efficiency method was chosen. To fully exploit these thermal alkaline effects, critical process parameters such as reaction temperature, reaction time, and pH must be investigated [25]. The objectives of this study were to test the feasibility of using PSB protein as a source of protein foaming, as well as making a protein foaming agent directly via thermal alkaline treatment without the tedious process of extraction or purification. The important parameters of thermal alkaline treatment—reaction temperature, reaction time, and pH—was then optimized by means of response surface methodology (RSM). To further improve the foamability of the prepared PSB protein foaming agent, additives were also investigated. 2. Materials and methods

tion was centrifuged at 9000 rpm for 30 min, and the crude protein was collected. The method of purification was modified from Garg [27]. The crude protein was sealed in a dialysis bag and then placed in a beaker containing Tris-HCl buffer for 12-hour dialysis. The dialysis was repeated twice so that the purified protein solution could be obtained and the foamability of the solution could be measured. The solutions produced by PSB liquid using thermal alkaline treatment method should also be evaluated. Thermal alkaline treatment was performed in 500 mL beakers. For the treatment, sodium hydroxide (1 mol/L) was added to the PSB liquid to adjust the pH of the solution. The beakers were then heated to target reaction temperatures with an electronic constant temperature water bath (HH-4, Jintan Honghua Instrument Factory, China) for a certain time. Then the properties of the solution could be tested. The effects of sodium dodecyl sulfate (SDS), hexadecyl trimethyl ammonium bromide (CTAB), gelatin, and acacia were tested, and the added concentrations were chosen on basis of the literature [28–31]. 2.3. RSM design RSM was used to obtain an optimum condition that exhibited the highest foamability [32]. The effects of reaction temperature (X1), reaction time (X2), and pH (X3) on foamability of the PSB protein foaming agent were studied. From a literature review, it was determined that the temperature should be higher than 60 °C, reaction time was hardly less than 1 h, and pH usually ranged from 10.0 to 12.5 [33–36]. Moreover, given the practical problems, such as operability and equipment depreciation, it would be better not to use excessive reaction temperature or pH. Preliminary studies of the effects of reaction temperature, reaction time, and pH were conducted to determine the appropriate experimental ranges of those variables. Table 1 shows the experimental ranges and chosen levels of independent variables used in this study. A total of 20 experimental runs with different values of the three variables were carried out by central composite design (CCD), and a center point was tested in sextuplicate. Analysis of experimental data was carried out in Design-Expert 11.1.0 software (Stat-Ease, Inc., USA). 2.4. Analysis methods Foamability was determined according to the method of Wang et al. [37] with a slight modification. First, 100 mL of solution was placed in 1000 mL beakers and heated to 40 °C in a water bath. The solution was mixed with an electric mixer (JJ-1A, Shanghai Lichenbangxi Scientific Instruments Co., Ltd., China) at 1200 rpm for 5 min and then was transferred into a graduated cylinder. The volumes of both the protein solution and the formed foams were recorded as foamability (mL). Foam stability (mL) in this study was evaluated by measuring the volume of solution and foams that remained 5 min after the foamability was estimated [16,38]. The pH was measured with a pH tester (PHS-3E, INESA Scientific Instrument Co., Ltd., China). The biomass was measured according to Lu et al [39]. The protein content was measured and calculated according to Yang et al. [39,40]. 2.5. Statistical analysis Measurements were conducted in triplicate to ensure the accuracy of the data. The standard deviations of all measurements were less than 5%.

3. Results and discussion

2.1. Materials

3.1. Feasibility study

The PSB (Rhodopseudomonas) used in this study was cultured at room temperature under 1500–2500 lx, by fluorescent lamp, in a PSB culture medium (Beijing Fisheries Biotechnology Co., Ltd., China). PSB liquid in the exponential growth phase was then used for the experiments. The pH of the PSB liquid was 8.9, the biomass was 1145 mg/L, and the protein content of PSB cells was 49.18%. The concentrated high performance concrete foaming agent used in this study as a reference for the properties was purchased from Pengyi Chemical Co., Ltd (China). All chemicals used were of analytical reagent grade.

3.1.1. Preliminary study with purified protein and PSB liquid A preliminary study was undertaken on the foaming properties of protein purified from PSB. The foamability of the protein solution (1%) was 465 mL, so it demonstrated that PSB has the potential to be used as a source of protein foaming agents (Fig. 2). Moreover, foamability of the solution treated by thermal alkaline method reached 421 mL under a reaction temperature of 80 °C, reaction time of 3 h, and pH of 11, comparable to the foamability of a purified protein solution. Thus, a foaming agent can be prepared directly from PSB liquid without a series of protein extraction and purification operations, making it easier and cheaper to produce.

2.2. Experimental methods To investigate whether PSB protein can be used as a source of protein foaming agent, the foamability of the purified PSB protein solutions was assessed. In this experiment, as shown in Fig. 1, the protein was extracted and purified after breaking up PSB. The protein extraction method was modified from Krisna et al. [26]. PSB liquid was centrifuged at 6000 rpm for 10 min, and the supernatant was discarded. The sediment was resuspended in Tris-HCl buffer (10 mmol/L) with a pH of 8.0 and centrifuged two more times to collect bacteria. Cell walls were broken by sonication method (10 °C, 400 W for 5 min). Then, 1% Triton-X100 and sodium chloride (0.1 mol/L) were added. After stirring in the dark for 2 h at room temperature, the solution was centrifuged at 9000 rpm for 30 min to collect supernatant. The supernatant was subsequently placed in a refrigerator at 4 °C for 1 min to precipitate the protein after adding 60% ammonium sulfate solution into it. Then the solu-

3.1.2. Comparison with literature and commercial high-performance product Xiang et al. [16] used 60Co c-ray/H2O2 treatment to improve the foaming properties of the sludge protein solution. The foam height of 200 mL solution could reach 23.2 cm, as measured by a RossMiles foam meter. Therefore, the method in this article achieved

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Fig. 1. Schematic diagram of the process route for producing PSB protein solution.

Table 1 Experimental range and levels of independent variables used in this study. Variables

Symbol

Reaction temperature (°C) Reaction time (h) pH

X1 X2 X3

Range and level 2

1

0

1

2

60 1 9

70 2 10

80 3 11

90 4 12

100 5 13

performance foaming agent were tested to serve as reference standards (foamability of 636 mL and foam stability of 632 mL). The foamability of the product in this study was similar to those in other studies. Thus, a conclusion could be drawn that PSB can be used to prepare foaming agents, but improvement was needed to achieve the best effect. Therefore, the thermal alkaline treatment was examined in following experiments. 3.2. Optimization of thermal alkaline treatment

Fig. 2. Foamability of the PSB protein solution.

a foamability of about 300 mL. Ni et al. [15] reported that the foamability of sludge protein solution with additives approached to 770 mL. In addition, the properties of a commercial high-

3.2.1. Model fitting and statistical analysis A desired level of foamability and foam stability is the first consideration for foaming agents [41]. It is a common characteristic of protein foaming agent to have an advantage in foam stability while being deficient in foamability [8], as verified in the feasibility study. The stability of the foam depends mainly on the speed of liquid precipitation and the strength of the liquid film. The active material in protein foaming agents has a high molecular weight, and the interaction force between molecules is strong, thus contributing to a high liquid film strength [13], so the foams are not likely to break easily. Therefore, in this experiment foamability was considered the primary criterion, and foam stability was considered secondary.

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Experimental results from the 20 runs of the CCD design are listed in Table 2. Under various treatment conditions, the foamability of the products ranged from 333 to 461 mL. The results were further analyzed by regression analysis in Design-Expert. They suggested an empirical relationship between foamability and the test variables. The predicted full quadratic model for foamability is illustrated in Eq. (1), where X1, X2 and X3 were the coded values for reaction temperature, reaction time, and pH respectively, and Y was the foamability of the product.

Y ¼ 424:50 þ 19:44X 1 þ 9:00X 2 þ 26:94X 3  7:87X 1 X 2  4:12X 1 X 3  6:00X 2 X 3  4:31X 21  1:31X 22  9:87X 23

ð1Þ

An analysis of variance (ANOVA) was conducted to examine the adequacy of the quadratic model, and the results are presented in Table 3. The low p-value (<0.0001) indicates the high significance of the model. The coefficient of determination (R2) was the proportion of variability in the data explained or accounted for by the model. The R2 value of 0.9760 indicated that only 2.40% of the total variations could not be explained by the model, suggesting a reasonable agreement with observed values and predicted values. Fvalue for the lack of fit (p > 0.05) and a very low value (1.80%) of

coefficient of the variation (CV) clearly confirmed the high reliability and validity of the model. In a word, the model was sufficient for prediction within the range of experimental variables. On the other hand, the p-value was an indicator for the significance of each model term. The smaller the p-value was, the more significant the corresponding variable was [42]. The variables with greatest influence were pH (X3), reaction temperature (X1), and the quadratic term of pH (X 23 ). 3.2.2. Response surfaces and contour plots analysis Response surfaces and contour plots (Fig. 3) were obtained by the model. They depicted the relationship between variables and responses vividly. As shown in Fig. 3(a), the interaction of reaction temperature and reaction time was significant. A longer reaction time would be beneficial for foamability. The most significant effect was observed between 60 °C and 70 °C, a mild environment, where the reaction time demonstrated a linear increase in the response. When the reaction temperature was no more than 90 °C, the longer the reaction time was, the higher the foamability was. This can be attributed to the increase in protein content and the moderate degradation of some protein molecules [16]. PSB are gramnegative bacteria, which have a high fat content in the cell wall.

Table 2 CCD experimental data corresponding foam capacity with different combinations of three independent variables. Run

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Experimental factors

Foamability (mL)

Observed foam stability (mL)

Reaction temperature(°C)

Reaction time (h)

pH

Observed

Predicted

60 [2] 70 [1] 70 [1] 70 [1] 70 [1] 80 [0] 80 [0] 80 [0] 80 [0] 80 [0] 80 [0] 80 [0] 80 [0] 80 [0] 80 [0] 90 [1] 90 [1] 90 [1] 90 [1] 100 [2]

3 2 4 2 4 3 1 3 3 3 3 3 3 5 3 2 4 2 4 3

11 [0] 10 [1] 10 [1] 12 [1] 12 [1] 9 [2] 11 [0] 11 [0] 11 [0] 11 [0] 11 [0] 11 [0] 11 [0] 11 [0] 13 [2] 10 [1] 10 [1] 12 [1] 12 [1] 11 [0]

373 338.5 372 401 433 333 402.5 428.5 424 420.5 418.5 436 424 440.5 441.5 392.5 417 461 439 446

368.38 335.64 381.38 409.76 431.50 331.14 401.26 424.50 424.50 424.50 424.50 424.50 424.50 437.26 438.90 398.50 412.76 456.14 446.40 446.14

[0] [1] [1] [1] [1] [0] [2] [0] [0] [0] [0] [0] [0] [2] [0] [1] [1] [1] [1] [0]

370.5 336.5 370.5 397 432 331 401 424.5 419.5 415 413.5 433.5 420.5 435 440.5 389 415.5 458 435.5 441.5

Table 3 ANOVA for the established regression model of PSB protein foamability. Source

Sum of squares

df

Mean square

F-value

p-value

Model X1 X2 X3 X1X2 X1X3 X2X3 X 21

22488.93 6045.06 1296.00 11610.06 496.13 136.13 288.00 467.60

9 1 1 1 1 1 1 1

2498.77 6045.06 1296.00 11610.06 496.13 136.13 288.00 467.60

45.28 109.54 23.48 210.37 8.99 2.47 5.22 8.47

< 0.0001 < 0.0001 0.0007 < 0.0001 0.0134 0.1474 0.0454 0.0155

X 22

43.31

1

43.31

0.78

0.3965

X 23 Residual Lack of fit Pure error Cor total

2451.82

1

2451.82

44.43

< 0.0001

551.88 354.50 197.38 23040.80

10 5 5 19

55.19 70.90 39.48

1.80

0.2680

R2 = 0.9760, CV = 1.80%.

J. Cai et al. / Construction and Building Materials 238 (2020) 117715

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Fig. 3. Response surface curves (left) and contour plots (right) of the effects of reaction temperature, reaction time, and pH on the foamability: (a) fixed pH at 11; (b) fixed reaction time at 3 h; (c) fixed reaction temperature at 80 °C.

Fat can be hydrolyzed in an alkaline solution, weakening cell walls. The thermal alkaline hydrolysis method accelerated the decomposition of organic matter by heating, and the addition of alkali reduced resistance of the cells to high temperatures [35]. A higher degree of cell disruption indicated that more protein would be released into the solution and more foams could be produced. What’s more, the proteins would be degraded when exposed to

high pH or high temperature. When protein degraded partially, more buried hydrophobic amino acids were exposed to the aqueous solvent as intramolecular or intermolecular bonds broke down. Such modifications generally increased the flexibility and surface hydrophobicity of degraded proteins, resulting in improved proportions [43–45]. However, it was clear that increasing reaction time led to lower foamability only in the case of the reaction tem-

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peratures greater than 90 °C. The foamability of the product produced at 100 °C for 5 h was about 5% less than that of 1 h. A similar tendency was reported in previous studies. Cui et al. [46] found that in order to extract more proteins from sludge, at 130 °C and pH = 13, the best reaction time was 3 h. But when the temperature rose to 140 °C, the optimal reaction time dropped to 2 h. Harsh treatment conditions not only decreased foamability, but a pungent odor was produced. This odor was attributed to the proteins being excessively degraded into ammonia nitrogen, which reacted with alkali to form ammonia gas (citation). Thus, the foamability decreased. The effects of reaction temperature and pH on foamability are presented in Fig. 3(b). As seen here, the foamability increased smoothly with increases in reaction temperature and pH unless the environment was too harsh. Furthermore, as pH increased, the concentration of ions in the solution increased, providing a certain ionic environment for the formation and stabilization of the foam. Because protein molecules were amphiphilic, they were adsorbed on the liquid film after foams are formed. Therefore, the surface of the liquid film was positively or negatively charged. The addition of inorganic ions could cause the surface of the liquid film to carry the same electric charge. When the liquid film was impacted, electrostatic repulsion prevented the liquid film from draining, thus avoiding rapid breakage of the foam after generation and prolonging stabilization time [29]. From the contour plot, the highest foamability (>450 mL) was achieved under the condition of 90 °C–100 °C, with pH around 12. Conditions above this optimum range decreased the protein foamability because the Maillard reaction might occur. The protein content decreased while less degradable melanoid was produced, so the foamability decreased [33,47]. This change affected the quality of products, decreasing their foaming capacity. It was consistently observed during the experiment that the color of the solution was significantly darker. In addition, the contour plot displays rounded ridgelines running diagonally on the diagram partly, indicating that reaction temperature and pH were slightly interdependent [25]. This result was also supported by the p-value (0.1474) shown in Table 3, which implied that interaction between reaction temperature and time might not affect the foamability considerably. Fig. 3(c) depicts the effects of reaction time and pH on foamability. When pH was low, the foamability increased almost linearly with the increase in reaction time. However, if pH was about 11, the effect of longer reaction time was less impressive. Only a marginal change in foamability was observed. Although bases saponified the membrane lipids, its ability was limited. At low pH, more time was needed to break up the cell wall. Yet at a pH of 13, increasing reaction time decreased foamability. High alkali might induce protein denaturation [36], making the protein particles less likely to be dissolved in water. For this reason, the agent became less favorable for foaming. To sum up, the increasing of reaction temperature, time and pH value had obvious promoting effect on cell disruption and protein degradation, thus improving the foamability of the product. However, if the process parameters exceeded certain values, excessive degradation, Maillard reaction as well as other effects might happen and the foamability would decrease. 3.2.3. Optimization and model verification The main purpose of the optimization is to ascertain the strengthening effects of experimental conditions to maximize the foaming properties of the model based on experimental data. According to Design Expert, the optimal conditions are a reaction temperature of 100 °C, reaction time 1 h, and pH of 12.557. Taking the practical situation into consideration, the adjusted optimal conditions were 100 °C, 1 h, and pH of 12.5, and the estimated foamability was 478.2 mL.

To further verify the reliability of the model, the treatment was performed by adopting the program of optical analytical model. The experiment was repeated three times, and a mean value of 476 mL was obtained. The results of the analysis confirmed that the response model was adequate for reflecting the expected optimization. 3.3. Selection of additives in PSB foaming agents The foamability and foam stability of the agent produced in the adjusted optimal conditions were 476 and 474 mL, respectively. The agent had great foam stability, but the foamability was inferior to that of some commercial foaming agents. Adding some surfactants or organic macromolecular additives is an effective and simple method to improve the properties of foaming agents. SDS, CTAB, acacia gum, and gelatin are common additives [26–29], so they were examined in this study. The results showed that SDS and CTAB were far superior to acacia gum and gelatin. Acacia gum and gelatin can increase the viscosity of the solution. Thus, the viscosity of the foam film will increase while the gas permeability will decreases [48]. Normally, increasing viscosity can increase foam stability greatly while increasing foamability slightly because the newly created foam

Fig. 4. Effects of SDS (a) and CTAB (b) on the foamability.

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J. Cai et al. / Construction and Building Materials 238 (2020) 117715 Table 4 Evaluation of SDS and CTAB as additives for PSB protein solution. Additive SDS CTAB

Price (USD/kg) 1.187 9.640

Dosage (kg/L) 3

1.2  10 2.1  103

does not break down as soon as it formed. However, if the solution is too thick, the viscous resistance is difficult to overcome, and the foamability decreases [29]. In this study, because the foaming agent already had high foam stability, the effect on the stability was neither obvious nor necessary. Therefore, surfactant is more suitable as an additive for protein foaming agents. 3.3.1. Effects of SDS and CTAB A series of experiments were conducted to study the effects of SDS and CTAB. As shown in Fig. 4, SDS and CTAB both had a significant influence on the foamability of the PSB protein foaming agent. The properties of the agent were improved continuously by the addition of SDS, and foamability reached 158% of the initial value (476 mL) when 1.2 g/L was added. As for CTAB, the maximal foamability of 715 mL was obtained when 2.1 g/L was added. Then the foamability decreased notably with further additions. Similar observations have been reported previously [49]. Within certain limits, increasing surfactants can increase the foamability of the agent dramatically, because the addition of surfactants reduces the surface tension of protein foaming agent, which increases foamability. However, the effects turned out to be slightly negative at higher concentrations. This was because that the surface active material was present in the foaming liquid in the form of micelles when the addition was excessive, and the surface tension of the system was no longer lowered. Moreover, surfactants had a dispersive effect and caused the molecular chain of macromolecular substances in the protein foaming agent to unfold. Those substances became more uniformly dispersed in the liquid film, thereby increasing the overall strength of the liquid film, and both foamability and foam stability could therefore be improved [30]. However, when the amount of surfactants exceeds a certain limit, excessive surface active molecules are enriched in the hydrophobic position of the protein, hindering the interaction of the reactive groups and having negative effects on the properties of the foam. As for the superiority of SDS over CTAB, the main reason is that the polarity head of CTAB is too large, which affects the tight alignment between molecules, and a dense surface film is less likely to form. It may also be that CTAB binds to proteins and reduces the number of groups on the surface; CTAB has greater steric hindrance and less diffusion capacity [49]. Therefore, the foaming capacity of CTAB is slightly lower than that of SDS. 3.3.2. Overall evaluation of SDS and CTAB additions A comprehensive analysis was needed in order to choose between SDS and CTAB. A cost comparison for SDS and CTAB was conducted (Table 4). The cost of SDS was 1.424  103 USD/L, which was less than the cost of CTAB, 2.024  102 USD/L. Therefore, SDS appeared to be more attractive in terms of cost. With regard to safety, both SDS and CTAB had low toxicity. Considering all these factors, SDS is clearly superior to CTAB. 4. Conclusions The potential of PSB as a source of protein foaming agents was verified. Foaming agents can be made from PSB liquid directly by thermal alkaline treatment. To optimize this thermal alkaline treatment, RSM and CCD were used to determine the effects of

Cost (USD/L) 3

1.424  10 2.024  102

Economy

Safety

Foamability

*** *

** **

*** **

reaction temperature (60–100 °C), reaction time (1–5 h), and pH (9–13) on the foamability of the foaming agents. The results showed that the optimum conditions were 100 °C, 1 h, and pH of 12.5, and the highest foamability (476 mL) could be achieved. In addition, SDS, CTAB, acacia gum, and gelatin additives were studied to further improve the properties of the foaming agent, and SDS was considered as the best choice. Adding 1.2 g/L SDS increased foamability from 476 mL to 754 mL, exceeding the foamability of commercial high-performance foaming agents. Therefore, there is great potential for using PSB to produce protein foaming agents in view of their low cost, abundance, and environmental safety. CRediT authorship contribution statement Jiaxuan Cai: Investigation, Methodology, Writing - original draft, Writing - review & editing. Guangming Zhang: Conceptualization, Supervision, Writing - original draft. Zhouhua Xie: Visualization. Yichun Zhu: Validation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment The authors are grateful for the financial support from National Natural Science Foundation of China (51868025). References [1] H.K. Kim, J.H. Jeon, H.K. Lee, Workability, and mechanical, acoustic and thermal properties of lightweight aggregate concrete with a high volume of entrained air, Constr. Build. Mater. 29 (2012) 193–200, https://doi.org/10.1016/ j.conbuildmat.2011.08.067. [2] A. Remadnia, R.M. Dheilly, B. Laidoudi, M. Quéneudec, Use of animal proteins as foaming agent in cementitious concrete composites manufactured with recycled PET aggregates, Constr. Build. Mater. 23 (2009) 3118–3123, https:// doi.org/10.1016/j.conbuildmat.2009.06.027. [3] B. Chen, Z. Wu, Ni. Liu, Experimental research on properties of high-strength foamed concrete, J. Mater. Civ. Eng. 24 (2012) 113–119, https://doi.org/ 10.1061/(ASCE)MT.1943-5533.0000353. [4] L. Zhang, D. Zhou, W. Yang, Y. Chen, B. Liang, J. Zhou, Preparation of ceramic foams suitable for aircraft arresting by the airport runway based on a protein foaming agent, J. Wuhan Univ. Technol. Sci. Ed. 29 (2014) 980–989, https://doi. org/10.1007/s11595-014-1031-3. [5] C. Sun, Y. Zhu, J. Guo, Y. Zhang, G. Sun, Effects of foaming agent type on the workability, drying shrinkage, frost resistance and pore distribution of foamed concrete, Constr. Build. Mater. 186 (2018) 833–839, https://doi.org/10.1016/ j.conbuildmat.2018.08.019. [6] D.K. Panesar, Cellular concrete properties and the effect of synthetic and protein foaming agents, Constr. Build. Mater. 44 (2013) 575–584, https://doi. org/10.1016/j.conbuildmat.2013.03.024. [7] M. Siva, K. Ramamurthy, R. Dhamodharan, Development of a green foaming agent and its performance evaluation, Cem. Concr. Compos. 80 (2017) 245– 257, https://doi.org/10.1016/j.cemconcomp.2017.03.012. [8] Y.H.M. Amran, N. Farzadnia, A.A.A. Ali, Properties and applications of foamed concrete; a review, Constr. Build. Mater. 101 (2015) 990–1005, https://doi.org/ 10.1016/j.conbuildmat.2015.10.112. [9] A.J. Hamad, Materials, production, properties and application of aerated lightweight concrete: review, Int. J. Mater. Sci. Eng. 2 (2014) 152–157, https://doi.org/10.12720/ijmse.2.2.152-157. [10] D. Kubo, Y. Fukuda, Protein from fire-extinguishing chemical and an aqueous foam solution, 6495056 B2, 2002.

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[11] J.K. Fink, Hydraulic Fracturing Chemicals and Fluids Technology, Gulf Professional Publishing, Waltham, 2013, pp. 147–150, https://doi.org/ 10.1016/B978-0-12-411491-3.00012-1. [12] L. Yin, X. Zhou, J. Yu, H. Wang, Z. Liu, Protein foaming method to prepare Si3N4 foams by using a mixture of egg white protein and whey protein isolate, Ceram. Int. 40 (2014) 11503–11509, https://doi.org/10.1016/j. ceramint.2014.03.043. [13] B.S. Murray, Stabilization of bubbles and foams, Curr. Opin. Colloid Interface Sci. 12 (2007) 232–241, https://doi.org/10.1016/j.cocis.2007.07.009. [14] L. Verdolotti, B. Liguori, Synergistic effect of vegetable protein and silicon addition on geopolymeric foams properties, J. Mater. Sci. 50 (2015) 2459– 2466, https://doi.org/10.1007/s10853-014-8801-3. [15] H. Ni, T. Chen, Y. Li, Research on foaming characteristics and application of foaming agent of high stability sludge protein, Chinese J. Environ. Eng. 3 (2009) 2254–2260. [16] Y. Xiang, Y. Xiang, L. Wang, Z. Zhang, Optimization of foaming properties of sludge protein solution by 60Co c-ray/H2O2 using response surface methodology, Radiat. Phys. Chem. 127 (2016) 249–255, https://doi.org/ 10.1016/j.radphyschem.2016.07.016. [17] P. Wang, H. Zhou, Preparation of protein foaming agent by Baijiu vinasse, China Brew. 36 (2017) 85–89, https://doi.org/10.11882/j.issn.02545071.2017.08.019. [18] Q. Zhou, P. Zhang, G. Zhang, M. Peng, Biomass and pigments production in photosynthetic bacteria wastewater treatment: effects of photoperiod, Bioresour. Technol. 190 (2015) 196–200, https://doi.org/10.1016/j. biortech.2015.04.092. [19] A. Yang, G. Zhang, F. Meng, P. Lu, X. Wang, M. Peng, Enhancing protein to extremely high content in photosynthetic bacteria during biogas slurry treatment, Bioresour. Technol. 245 (2017) 1277–1281, https://doi.org/ 10.1016/j.biortech.2017.08.109. [20] F. Meng, A. Yang, G. Zhang, H. Wang, Effects of dissolved oxygen concentration on photosynthetic bacteria wastewater treatment: pollutants removal, cell growth and pigments production, Bioresour. Technol. 241 (2017) 993–997, https://doi.org/10.1016/j.biortech.2017.05.183. [21] F. Meng, A. Yang, H. Wang, G. Zhang, X. Li, Y. Zhang, Z. Zou, One-step treatment and resource recovery of high-concentration non-toxic organic wastewater by photosynthetic bacteria, Bioresour. Technol. 251 (2018) 121–127, https://doi. org/10.1016/j.biortech.2017.12.002. [22] Y. Xiang, Y. Xiang, L. Wang, Kinetics of activated sludge protein extraction by thermal alkaline treatment, J. Environ. Chem. Eng. 5 (2017) 5352–5357, https://doi.org/10.1016/j.jece.2017.09.062. [23] L. Shen, X. Wang, Z. Wang, Y. Wu, J. Chen, Studies on tea protein extraction using alkaline and enzyme methods, Food Chem. 107 (2008) 929–938, https:// doi.org/10.1016/j.foodchem.2007.08.047. [24] I. Kim, J.I. Han, Optimization of alkaline pretreatment conditions for enhancing glucose yield of rice straw by response surface methodology, Biomass Bioenergy 46 (2012) 210–217, https://doi.org/10.1016/j. biombioe.2012.08.024. [25] K.C. Duong-Ly, S.B. Gabelli, Salting out of proteins using ammonium sulfate precipitation, Methods Enzymol. 541 (2014) 88–93, https://doi.org/10.1016/ B978-0-12-420119-4.00007-0. [26] D. Garg, D.M. Kaur, Extraction, purification and characterization of enzyme amylase from Bacillus amyloliquefaciens, Int. J. Adv. Eng. Sci. 3 (2013) 158– 159. [27] T. Chen, M. Huo, Y.B. Li, R. Guo, Y.D. Li, Research on the influence of different stabilizers on foaming characreristics of sludge protein, Chinese J. Environ. Eng. 4 (2010) 1181–1185. [28] X. Yue, W. Zhang, J. Xu, Y. Zhao, Modification of the protein foaming agent extracted from microorganism in sludge, Chem. Ind. Eng. Prog. 30 (2011) 1316–1319, https://doi.org/10.16085/j.issn.1000-6613.2011.06.038. [29] D.L. Manyala, G. Rajput, N. Pandya, D. Varade, Enhanced foamability and foam stability of polyoxyethylene cholesteryl ether in occurrence of ionic surfactants, Colloids Surf. A Physicochem. Eng. Asp. 551 (2018) 81–86, https://doi.org/10.1016/j.colsurfa.2018.04.069.

[30] S. Qian, J.J.J. Chen, Effect of SDS (sodium dodecyl sulfate) foam on polarized light characteristics, Chem. Eng. Res. Des. 104 (2015) 218–236, https://doi.org/ 10.1016/j.cherd.2015.08.008. [31] C.N. Carmita, Optimization of cutting parameters using Response Surface Method for minimizing energy consumption and maximizing cutting quality in turning of AISI 6061 T6 aluminum, J. Clean. Prod. 91 (2015) 109–117, https://doi.org/10.1016/j.jclepro.2014.12.017. [32] E. Neyens, J. Baeyens, C. Creemers, Alkaline thermal sludge hydrolysis, J. Hazard. Mater. 97 (2003) 295–314, https://doi.org/10.1016/S0304-3894(02) 00286-8. [33] Y. Tzeng, L.L. Diosady, L.J. Rubin, Production of canola protein materials by alkaline extraction, precipitation, and membrane processing, J. Food Sci. 55 (1990) 1147–1151, https://doi.org/10.1111/j.1365-2621.1990.tb01619.x. [34] A.G. Vlyssides, P.K. Karlis, Thermal-alkaline solubilization of waste activated sludge as a pre-treatment stage for anaerobic digestion, Bioresour. Technol. 91 (2004) 201–206, https://doi.org/10.1016/S0960-8524(03)00176-7. [35] H. Nolsøe, I. Undeland, The acid and alkaline solubilization process for the isolation of muscle proteins: state of the art, Food Bioprocess Technol. 2 (2009) 1–27, https://doi.org/10.1007/s11947-008-0088-4. [36] C. Wang, Z. Tian, L. Chen, F. Temelli, H. Liu, Y. Wang, Functionality of barley proteins extracted and fractionated by alkaline and alcohol methods, Cereal Chem. 87 (2010) 597–606, https://doi.org/10.1094/CCHEM-06-10-0097. [37] S. Damodaran, Protein stabilization of emulsions and foams, J. Food Sci. 70 (2006) 54–66, https://doi.org/10.1111/j.1365-2621.2005.tb07150.x. [38] H. Lu, G. Zhang, T. Wan, Y. Lu, Influences of light and oxygen conditions on photosynthetic bacteria macromolecule degradation: different metabolic pathways, Bioresour. Technol. 102 (2011) 9503–9508, https://doi.org/ 10.1016/j.biortech.2011.07.114. [39] O.H. Lowry, N.J. Rosebrough, A.L. Farr, R.J. Randall, The folin by oliver, J. Biol. Chem. 193 (1951) 265–275. [40] A. Yang, G. Zhang, G. Yang, H. Wang, F. Meng, H. Wang, M. Peng, Denitrification of aging biogas slurry from livestock farm by photosynthetic bacteria, Bioresour. Technol. 232 (2017) 408–411, https://doi.org/10.1016/j. biortech.2017.01.073. [41] C. Hill, J. Eastoe, Foams: from nature to industry, Adv. Colloid Interface Sci. 247 (2017) 496–513, https://doi.org/10.1016/j.cis.2017.05.013. [42] A. Witek-Krowiak, K. Chojnacka, D. Podstawczyk, A. Dawiec, K. Pokomeda, Application of response surface methodology and artificial neural network methods in modelling and optimization of biosorption process, Bioresour. Technol. 160 (2014) 150–160, https://doi.org/10.1016/j.biortech.2014.01.021. [43] J. Cui, A. Dong, W. Zhang, F. Miao, Experimental investigation of extraction protein by alkaline thermal sludge hydrolysis from sludge, Chinese J. Environ. Eng. 3 (2009) 1889–1892. [44] E.A. Foegeding, P.J. Luck, J.P. Davis, Factors determining the physical properties of protein foams, Food Hydrocoll. 20 (2006) 284–292, https://doi.org/10.1016/ j.foodhyd.2005.03.014. [45] I.V.D. Plancken, A.V. Loey, M.E. Hendrickx, Foaming properties of egg white proteins affected by heat or high pressure treatment, J. Food Eng. 78 (2007) 1410–1426, https://doi.org/10.1016/j.jfoodeng.2006.01.013. [46] J.M.R. Patino, J.M. Conde, H.M. Linares, J.J.P. Jimenez, C.C. Sanchez, V. Pizones, F. M. Rodriguez, Interfacial and foaming properties of enzyme-induced hydrolysis of sunflower protein isolate, Food Hydrocoll. 21 (2007) 782–793, https://doi.org/10.1016/j.foodhyd.2006.09.002. [47] J. Dwyer, D. Starrenburg, S. Tait, K. Barr, D.J. Batstone, P. Lant, Decreasing activated sludge thermal hydrolysis temperature reduces product colour, without decreasing degradability, Water Res. 42 (2008) 4699–4709, https:// doi.org/10.1016/j.watres.2008.08.019. [48] J. Wang, A.V. Nguyen, S. Farrokhpay, A critical review of the growth, drainage and collapse of foams, Adv. Colloid Interface Sci. 228 (2016) 55–70, https://doi. org/10.1016/j.cis.2015.11.009. [49] S. Jiang, L. Wang, K. Feng, Compound modified experiment of protein-concrete foaming agent, J. Funct. Mater. 46 (2015) 56–61.