In this study, six strains of microbial agents were investigated as environmently friendly scale and corrosion inhibitors for industrial cooling water applications. The static jar tests along with characterization methods were applied to evaluate the scale inhibition performance. Results showed that under a concentration of 240 mg/L, the nitrobacteria, denitrobacteria and Lactobacillus agents reached high CaCO3 scale inhibition efficiencies of 83, 82, and 86% respectively. Characterization methods indicated the deposited crystals morphologies were modified and the crystals peak intensities were lowered. In addition, weight loss measurements, electrochemical measurements, surface characterization analyses were conducted to study the corrosion inhibition performances and mechanisms. It was found that at 40 °C, Bacillus cereus agent with 200 mg/L possessed the highest corrosion inhibition efficiency of 60.11% at 3 d, together with the second-lowest current density of 13.0 μA cm−2 at 12 d. The corrosion inhibition mechanisms were attributed to biofilm accumulation and biomineralization on Q235 CS surfaces to form protective film. The results suggested microbial agents have promising potential as environmently friendly scale and corrosion inhibitors for industrial cooling water applications.
The feasibility of microbial agents using as scale and corrosion inhibitors was confirmed.
The CaCO3 scale inhibition efficiency of Lactobacillus agent after 24 h activation reached 87.60%.
Bacillus cereus agent displayed superior corrosion inhibition performance with a first accelerated, then inhibited, finally accelerated corrosion process.
The scale and corrosion inhibition effect closely related to microbial metabolism.
Cooling water systems are used to remove extra heat from heat exchangers in industrial production, during the operation, the continuous evaporation deteriorates water quality and causes corrosion and scaling problems. Such problems have a great impact on both economic and technical perspectives, therefore, proper treatment is required to eliminate the corrosion and scaling problems in cooling water system (Gan et al. 2018).
In industry, adding chemical agents is a most common approach to control both scale deposition and material corrosion problems. Traditional scale and corrosion inhibitors such as phosphonate, chromate, and molybdate have been restricted due to secondary pollution. Environmentally friendly polymer inhibitors like polyacrylic acid, polyepoxysuccinic acid (PESA) (Huang et al. 2019) and polyaspartic acid (PASP) (Gao et al. 2015) need periodical addition, which further brings potential labor costs. In this perspective, there is a significant nee to develop new corrosion and scale inhibitors for cooling water systems with high inhibition performances, environmentally friendly, and less dosing frequency.
Microbial agents, as an exogenous bacterial population, have wide applications in fields such as microbial organic fertilizer, agricultural feed additives and fermentation production (Jin et al. 2005; Jia et al. 2021; Poveda et al. 2021). Another major application area of microbial agents is wastewater treatment, which uses microbial biological metabolic process to achieve the purpose of pollutant removal. During the biological process, functional microorganisms decompose organic matter to obtain energy and produced amino acids, lipids and carbohydrates as their primary metabolites (Wang et al. 2019). Such metabolites, namely microbial products, with superior characteristics of eco-friendly, strong complexing affinities, and wide range of sources, may have the potential of being corrosion and scale inhibitors in cooling water system. Several studies have shown the microbial product own validity in scale and corrosion control. The first study on utilizing biological method on scaling control was conducted by Kawaguchi (Kawaguchi & Decho 2002), which proved that soluble extracellular polymeric substances (s-EPS) from one cyanobacterium (Schizothrix sp.) was capable of inhibiting CaCO3 precipitation. A recent study also showed the s-EPS of Bacillus cereus exhibited an 87.60% CaCO3 inhibition efficiency (Li et al. 2019b). Except for directly extracted microbial EPS, bio-materials derived from microbial metabolism, like xanthan gum, alginate, and itaconic acid, also possessed certain scale inhibition capacities. The investigations suggested that negatively charged functional groups in their molecular might interact with positively charged calcium ions leading to crystals distortion and scale inhibition (Yang & Xu 2011; Karabelas et al. 2017; Cui & Zhang 2019). Furthermore, microorganism adhesion to metal surfaces usually accelerates corrosion rate. It has been reported that some microorganisms were capable of retarding metal corrosion, namely through microbiologically influenced corrosion inhibition (MICI). In the study of Suma et al. (2019), Pseudomonas putida attached on mild steel surfaces by secreting adhered EPS and accelerating biofilm formation. The Fe-EPS and stable vivianite constituted the biopassivated layer and offered mild steel a long-term corrosion. Qu et al. (2015) found that, as biofilm grew with immersion time, the corrosion behavior of cold rolled steel in the presence of Bacillus subtilis was first accelerated and then inhibited. Wu et al. (2016) affirmed that, compared with the abiotic blank group, the corrosion rates of Q235 carbon steel under Desulfovibrio sp. and Pseudoalteromonas sp. di-cultures were lower. The above investigations suggested that microorganisms and their metabolites have promising potential in scale and corrosion inhibition.
At this time, research studies into microbial products in scale and corrosion inhibition are scarce, previous studies have been mainly carried out in the seawater environment, therefore there is a lack of research in circulating cooling water environments. Therefore, the aim of this study was to develop an environmentally friendly scale and corrosion inhibitor for circulating cooling water applications. The adopted microbial agents with low cost and environmental friendly characteristics possessed great potential. In this study, the scale and corrosion inhibition performances of six microbial agents (nitrobacteria, denitrobacteria, Lactobacillus, Bacillus cereus, Lactobacillus reuteri, and Pseudomonas fluorescens) were investigated. Their scale and corrosion inhibition mechanisms were discussed using surface characterization techniques.
MATERIALS AND METHODS
Preparation of microbial agent
Microbial agents used as corrosion and scale inhibitor were wettable dry powder of nitrobacteria, denitrobacteria, Lactobacillus, Bacillus cereus, Lactobacillus reuteri, and Pseudomonas fluorescens, the bioactive cell number in per gram of dry powder as well as their producers are shown in Table S1.
Activation & inoculation
Before use, microbial agents were activated by mixing with brown sugar, egg white and deionized water in a conical flask, the mixture was subsequently placed in a 37 °C water bath for 24 h before inoculation. During activation, pH variation was recorded every 4 h to characterize microbial activity and further determine the optimum inoculation time. Next, the supernatants of the activating liquid were used as scale and corrosion inhibitors in this study. The photograph of six microbial agents after activation is presented in Figure 1(a).
Static test experiments
Corrosion weight loss measurements
The RCC-II tester was conducted as the platform for 12 days of electrochemical experiments. Electrochemical measurements were carried out in a CHI760E (produced by Shanghai Chenhua) electrochemical workstation. A three-electrode system including a working electrode, an auxiliary electrode, and a reference electrode were used, in which the Q235 CS coupons clamped with a polytetrafluoroethylene (PTFE) platinum plate electrode holder served as the working electrodes. The operating parameters were the same as weight loss measurements. Saturated calomel electrode and platinum electrode were used as a reference and counter electrodes, respectively. To ensure stability, electrochemical impedance spectroscopy (EIS) was obtained at the end of open circuit potential (OCP) measurement by applying an alternating voltage of 5 mV over frequencies ranging from 0.01 to 105 Hz. The polarization curves were measured at the scan rate of 1 mV/s and the potential applied was in the range of −500 and 500 mV with respect to OCP.
Characterization of scale deposits
Based on the results of static tests, the CaCO3 precipitation samples in the absence or presence of four antiscaling microbial agents were collected, washed, dried, and then characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) to explore the scale inhibition mechanisms.
Surface analysis for corrosion inhibition
According to the results of weight loss and electrochemical measurements, the Q235 coupons after 3 days of weight loss measurements with Bacillus cereus and Pseudomonas fluorescens agents were taken out, washed with distilled water and dried in air at room temperature. The corrosive morphologies were recorded using a scanning electron microscope. Simultaneously, to further analysis the component of corrosion product, the surface layer was carefully scraped with a sterile spatula from another parallel specimen and examined with X-ray photoelectron spectroscopy (XPS; Thermo VG, USA).
RESULTS AND DISCUSSION
pH variation of microbial agents' activated solution
The pH variation during the microbial agents activation process under 37 °C is shown in Figure 1(b). In general, pH initial values differed by microbial agents' species, the pH variation tendency was approximately identical accompanied by significantly decline throughout the whole 36 h activation period. During the first 4 h of activation, the pH of most microbial agents increased slightly while pH of Pseudomonas fluorescens and Lactobacillus reuteri agents showed dramatic decline from 6.05 to 5.71 and 6.96 to 4.21, respectively. This may due to powder carrier brought organic acid and other acidic materials gradually dissolved as the activation proceeded (Li et al. 2012). Later, microbial agents' pH continuous decrease during 4 to 24 h of activation, demonstrating that dormant bacterial cells were gradually activated, their metabolism produced a variety of organic acids which increased the acidity of the activation solutions. After a sharply decrease, pH of microbial agents reached a base plateau and kept at a steady state between 20 and 24 h of activation; this indicated that bacterial cells were in a stable growth and metabolism phase. In the last activation of 24 to 36 h, most microbial agents remained stable, yet, pH levels of nitrobacteria and denitrobacteria agents increased from 3.85 to 4.45 and 3.79 to 4.35, respectively. This can be explained by ammonia release corresponded to microbial activity decrement (Gigliotti et al. 2012). Hence, according to the pH variation, this study chose 24 h activated microbial agents as inoculum for further scale and corrosion inhibition experiments.
Scale inhibition performance against calcium carbonate
Comparison scale inhibition under different activation time
The activation time is an important factor as discussed above, thus, to further verified the correctness of determination on inoculating time, the CaCO3 scale inhibition performance of six microbial agents after 12 h activation was compared with that after 24 h activation. The results are shown in Figure 2(a). As shown in Figure 2(a), under a concentration of 240 mg/L, the scale inhibition performances of microbial agents activated 24 h improved in varied degrees, contrasted with that of activated 12 h. The Lactobacillus agent displayed the highest improvement in scale inhibition after an additional 12 h activation, from 20% to 86%, followed by Nitrobacteria and Denitrobacteria agents, from 36% to 83% and 58% to 82% respectively. Beyond that, the additional 12 h activation had a weakened impact on Bacillus cereus and Pseudomonas fluorescens agents, which presented a slight increase in scale inhibition efficiency. The scale inhibition efficiency of Lactobacillus reuteri agent remained unchanged when activated for 12 and 24 h. Considering the pH values of the Lactobacillus reuteri agent were almost constant during the corresponding activation period, it can be speculated that the microbial growth and metabolism remained in relatively stable states. Therefore, the additional 12 h activation time did not contribute to its scale inhibition performance.
The comparison of scale inhibition of microbial agents after different activation times (12 and 24 h) combined with pH changes the results in Figure 1(b). We speculated that microorganisms gradually recovered from dormancy after activation, they generated organic acids through metabolic activities that lowered the pH of the surrounding environment. pH values before and after 12 h activation still showed a decreasing trend, indicating that microorganisms are still in the state of adaptation to the environment. However, at around 24 h activation, the pH values were lower and remained relatively stable, suggesting there was a deeper augmentation of organic acids produced by microbial activities. These organic acids were the main forces in the scale inhibition process (Mao et al. 2018). Thus, the 24 h activation was selected as the optimum inoculating time for microbial agents.
Effect of dosage
After activation for 24 h, a bacterial suspension of microbial agents was applied to investigate the dosage effects on calcium carbonate scale inhibition efficiency, the results are plotted in Figure 2(b). As can be seen, in measured dose range, scale inhibition efficiency gradually increased, yet, a certain degree of variation still existed between different species of microbial agents. Among six tested microbial agents, the Lactobacillus reuteri agent showed almost no scale inhibition while Bacillus cereus and Pseudomonas fluorescens agents showed slightly increased scale inhibition efficiencies from about 30% to 50% with dosage increase from 80 to 240 mg/L. Moreover, nitrobacteria and denitrobacteria agents showed extraordinary performances against CaCO3 formation, their scale inhibition efficiency increased dramatically from 7% to 83% and 5% to 82% with respect to experimental dose range. The optimum antiscaling performance appeared for the Lactobacillus agent, which exhibited a marked antiscaling increment from 26% to 86% between the dosage of 120 and 240 mg/L.
The scale inhibition effect can be explained as multiple organic acids, proteins, and peptides secreted during microorganism metabolism complexed with free calcium ions to inhibit the formation of calcium carbonate (Li et al. 2019b). In addition, typical functional microorganisms such as Lactobacillus were capable of producing lactic acid to bind calcium ions (Lv et al. 2021), nitrification of Nitrobacteria effected ammonium nitrogen to oxidized N (nitrite and nitrate) could also cause a decline in pH, making free Ca2+ ions. All these could achieve the effect of CaCO3 scale inhibition (Zhao et al. 2020).
Characterization of CaCO3 scales
In order to reveal the underlying scale inhibition mechanisms, the surface morphology of precipitated CaCO3 scales in the presence of four antiscaling microbial agents was further investigated by direct observation using SEM (Figure 3). As shown in Figure 3, in contrast with blank CaCO3 crystals with regular rhombohedral form and glaze surfaces (Figure 3(e) and 3(e’)), CaCO3 crystal formed under microbial agents became irregular with rough surfaces, indicating that microbial agents had an impact on CaCO3 crystalline morphology (Figure 3(a)–3(d)). In addition, CaCO3 crystals morphologies differed by different strains of microbial agents, with the addition of nitrobacteria and denitrobacteria agents, CaCO3 crystals surfaces became cratered with fractured defects, the number of crystalline step edges increased and the morphology showed a tendency towards a spherulitic shape (Figure 3(a), 3(a’), 3(b) and 3(b’)). In addition, CaCO3 crystals morphologies changed into elongated dumbbell shapes with rough surfaces in the presence of Lactobacillus agent (Figure 3(c) and 3(c’)), while the CaCO3 crystals formed an overgrowth towards a sequential direction under Pseudomonas fluorescens agent (Figure 3(d) and 3(d’)).
The observed phenomena concluded that adding microbial agents significantly changed the CaCO3 crystal morphology, leading either a lattice distortion or altering the crystal growth orientation, this can be explained as EPS secreted during microbial metabolism contains abundant organic substances (Sheng & Yu 2006), which are capable of occupying the active growth site of calcite crystal through complexation between the functional groups (carboxyl groups (-COOH), hydroxyl groups (-OH), sulfonic groups (-SO3)) and free calcium ions (Zhuang et al. 2018). What is more, it was found that in biomineralization, some specific amino acid and proteins play an important role in controlling CaCO3 nucleation and crystallization, resulted in a preferential crystals growth or a spherulitic crystals morphology (Kong et al. 2018; Wada et al. 2018).
Figure 4 shows the XRD spectrum of CaCO3 crystals precipitated under blank conditions and in the presence of four antiscaling microbial agents. The diffraction peaks at 29.24° (104), 35.89 (110), 39.33 (113), 43.07 (202), 47.43 (018), and 48.41(116) corresponds to calcite crystal, indicated that scale deposition with and without microbial agents were mainly calcite form. Moreover, adding microbial agents (nitrobacteria and Lactobacillus) slightly reduced the intensities of characteristic peaks at 29.24°, indicated that scale inhibition was mostly taking place in the (104) plane (Figure 4(a) and 4(c)), such results were in line with previous studies (Elkholy et al. 2018). In addition, the spectrum also detected that CaCO3 deposited with the nitrobacteria agent showed diffraction peaks at 31.61° and 45.33°, corresponding to halite crystal surfaces of (200) and (220) respectively (Figure 4(a)). This can be explained by ion exchange between biomolecules of EPS and solution. The antiscaling mechanisms on CaCO3 can be explained by biomolecules of EPS such as proteins, humic acids, and polysaccharides releasing Na+, while offering the binding sites to free Ca2+ to facilitate the formation of EPS–Ca2+ complexes (Li & Yu 2014).
Corrosion inhibition of microbial agents
Corrosion weight loss measurements
Corrosion inhibition efficiency of microbial agents at the concentration of 200 mg/L was performed by weight loss measurements. Table S2 illustrated the corrosion rate (v) and inhibition efficiency (η) of Q235 CS after immersing in simulated cooling water for 72 h. The corrosion situation of the blank sample with no microbial agents added was rather serious, of which the CS corrosion rate was 0.7243 (mm/a). However, addition of microbial agents showed a certain corrosion inhibition. Among the six tested microbial agents, Bacillus cereus agent possessed the maximum corrosion inhibition efficiency of 60.11%, along with the lowest corrosion rate of 0.2889 mm/a, followed by Lactobacillus reuteri agent of 0.4074 mm/a corrosion rate and 43.75% inhibition efficiency. From the above results, it was concluded that microbial agents possessed a certain protective effect on Q235 carbon steel corrosion in simulated cooling water systems.
To further investigate the Q235 CS corrosion with the presence of microbial agents, EIS measurements were conducted. Figure 5 presents the EIS spectra of Q235 CS during 12 days of repeated tests with different microbial agents and without microbial agents, time-dependent Nyquist plots are shown in Figure 5(a)–5(g) and the corresponding Bode plots are displayed in Figure 5(a’)–5(g’). Generally, a relatively larger diameter of Nyquist plot in biotic medium represents a higher corrosion resistance. As shown in Figure 5, the diameter of the Nyquist plot obtained in abiotic medium displayed a continuous downward trend, validating an increasing corrosion impact on rotatory coupons. However, Nyquist plot in biotic medium followed another tendency during a whole 12 days rotatory tests, the relatively larger diameter first decreased in the initial 2 days, then gradually increased to reached a maximum value on day 7, indicating that adding microbial agents introduced a corrosion inhibition effect. This effect was attributed to attachment and colonization of microorganisms, the microbial metabolically secreted organic molecules adsorbed on the Q235 CS surface and decreased the corrosion rate. Afterwards up to day 12, the diameter became narrowed and even smaller than the initial value, which suggested that destruction of the protective layer occurred overtime which was possibly due to exfoliation of the biofilm-corrosion product complex or local defect (Suma et al. 2019).
A corresponding equivalent circuit (EC) was used to describe the corrosion process in Figure 5(h), corresponding to the electrochemical parameters of the EC that are tabulated in Table S3. As shown in Figure 5(h), the Rs and Rct means the solution resistance and charge transfer resistance, respectively. Cf represents the capacitances of the biofilm and a constant phase element (CPE), Qdl, is introduced to represent the electrical double layer (EDL), the mathematical constant relates to the surface roughness, n, represents deviation from ideal capacitive behavior (Qu et al. 2015).
As shown by quantitative fitting parameters in Table S3, Rs in medium with microbial agents was different from the blank sample and showed undulation over the whole test period, indicating that dosed microorganisms could affect the corrosion conductivity. In addition, variation in Rct reflects the corrosion status of Q235 CS (Liu et al. 2021). Rct of Q235 CS without microbial agents gradually decreased with immersion time, demonstrating a decline in electron transfer resistance and accelerated corrosion rate. Furthermore, Rct values under 200 mg/L concentration of activated microbial agents were a magnitude higher, meaning that microbial agents obviously protected Q235 CS from corrosion.
Despite different strains for the microbial agents, their Rct values exhibited the same tendency during the whole experimental period, it decreased within the first tested 2 days, then significantly increased from day 4 to reached a peak value in day 7, corresponding to a first accelerated, then inhibited corrosion process. This phenomenon can be explained as follow. After inoculation, the addition of electrolyte caused a slight increase in corrosion rate. Simultaneously, microbes gradually acclimatized to the environment, they adhered on to the metal surface and formed inhibitive biofilms to hinder the contact between metal surface and corrosive medium (Liu et al. 2021). Afterwards, Rct values were then rapidly reduced in the remaining test indicating that longer run operations led to the loss of the protective function of the complexed film, such results showed good consistency with the Nyquist plot (Mehta et al. 2021a). The overall results provided a promising method of utilizing microbial agents in Q235 CS corrosion protection.
Potentiodynamic polarization test
Figure 5(i) represents the Tafel polarization curves of Q235 CS coupons after 12 days of rotation tests with and without microbial agents. As can be seen, a 200 mg/L dosage of microbial agents significantly lowered the current density compared with the blank group, suggesting that microbial agents performed a corrosion inhibition effect on Q235 CS. The electrochemical parameters obtained from Tafel analysis, including corrosion potential, Ecorr, corrosion current density, Icorr, anodic Tafel slope, βa, cathodic Tafel slope, βc, are presented in Table S4.
As shown in Table S4, the variation in the corrosion potential of Q235 CS changed with different strains of microbial agents. Compared with the blank sample, Ecorr of nitrobacteria, denitrobacteria, Lactobacillus, and Bacillus cereus agents shifted toward positive while Ecorr of Lactobacillus reuteri and Pseudomonas fluorescens agents showed more negatively. Furthermore, inoculation of microbial agents resulted in a marked decrement in the corrosion current density. The Icorr value of the Q235 CS coupons after 12 days rotatory test with Pseudomonas fluorescens agent was 10.6 μA cm−2, which was the lowest value among the tested microbial agents; followed by Bacillus cereus agent, which possessed a second lowest Icorr of 13.0 μA cm−2. Other microbial agents including nitrobacteria, denitrobacteria and Lactobacillus agents also showed relatively lower Icorr values. Lactobacillus reuteri agent possessed the maximum Icorr value of 40.1 μA cm−2 among the tested microbial agents, but this value was still lower than the blank group. The analysis of potentiodynamic polarization curves was in line with EIS data above, demonstrating that microbial agents were able to protect CS from corrosion.
Surface morphology of Q235 CS
The pristine Q235 CS coupon along with corrosive morphologies after 3 days of corrosion weight loss measurements with and without microbial agents are presented in Figure 6. Contrasting with the smooth and shiny surface displayed by pristine CS that had not been immersed (Figure 6(d)), the tested samples suffered different degrees of erosion with corrosion product on their surfaces. The most serious cases appeared on blank sample, on its surface, black iron oxide corrosion areas surrounded by yellow-brown rust were observed (Figure 6(c)). However, the presence of 200 mg/L microbial agents brought a certain protection, the Bacillus cereus agent gave coupons a homogeneous surface morphology with only speckled corrosive pits existing, while coupons with Pseudomonas fluorescens agents possessed inhomogeneous rust coverage (Figure 6(a) and 6(b)). Thereby, it was obvious that Bacillus cereus and Pseudomonas fluorescens agents could enhance the corrosion inhibition effect on Q235 CS coupon.
To further analyse the corrosion inhibition effect of microbial agents, two areas were boxed and selected from coupons after 3 days of immersion (in Figure 6) to represent typical Q235 CS corrosion morphologies. Blue and red boxes represent mild and deep corrosive regions, respectively, the surface morphology was characterized using SEM and the results are displayed in Figure 6(e)–6(g): SEM images of red boxed area, (e’)–(g’): SEM images of blue boxed area in the above photos).
As shown in Figure 6, morphologies of corrosion products varied from different selected regions. In the blue boxed area of biotic medium, the corrosion product showed different dimensions of enlarged rectangular crystal deposition, the depositions were closer to needle-like shapes under Pseudomonas fluorescens agent and turned to a rhombic-diamond morphology under Bacillus cereus agent. By contrast, in abiotic medium, the deposition was irregular with stony-like morphology. For the red boxed area of the biotic medium, the corrosion products were multilayered and heterogeneous, fluffy cauliflower-like structures were observed on coupons exposed to biotic medium. In abiotic medium, a high number of cracks in the background layer overlapped with scattered flowery structures, indicating severe corrosion damage had occurred.
Corrosion products analyses
An XPS spectrum was collected to validate the composition of complex corrosion product on Q235 CS coupons after 3 days of corrosion weight loss immersion. The surface survey spectra were shown in Figure 7 and the corresponding atomic percent of each main peak were given in Table S5. It was found that under the presence of microbial agents, the proportions of C and N which represent the organic constituents slightly increased, while the proportion of Fe which was attributed to corrosion level was decreased, indicating that the presence of Bacillus cereus and Pseudomonas fluorescens agents reduced the Fe release and increased the proportion of organic matter.
The high resolution images of Fe 2p are detailed in Figure 7(d)–7(f) and O 1s spectra are detailed in Figure 7(d’)–(f’), respectively. As displayed in Figure 7(d)–7(f), Fe 2p spectra were deconvoluted into four peaks. In the presence of microbial agent, the peak located in 710.3 eV represents FeO, while in absence of microbial agent, the peak at 707.7 eV represents metallic iron (Fe0) species (Njoku et al. 2021). Peaks at 711.3 eV, 711.4 eV and 710.5 eV are associated with ferric oxide/hydroxide species such as Fe2O3, Fe3O4 and FeOOH, while high binding energy of peaks at 713.4 eV, 713.6 eV and 712.6 eV are attributed to FeSO4 (Bouanis et al. 2009; Wu et al. 2014), the peak at around 719 eV is ascribed to the satellite of Fe3+ and peaks that appeared at 724 and 725 eV are α-Fe2O3, Fe3O4 and FeOOH with respect to Fe 2p1/2 (Pandarinathan et al. 2014).
In the deconvoluted O 1s spectra (Figure 7(d’), 7(e’) and 7(f’)), the peaks located at 529.8 eV, 529.9 eV and 529.7 eV in three tested sample may be assigned to iron oxide (Mehta et al. 2021b) and the peaks at 531.3 eV, 531 eV and 531.3 eV are the binding energy of the O2— and OH— ions, corresponding to the iron oxide/hydroxide layer as detected in the Fe 2p3/2 spectrum (Boumhara et al. 2015). Peaks at high binding energies of 532.3 eV, 532.6 eV and 532.8 eV are associated with the contributions of organic oxygen in single bonds of C-O (Boumhara et al. 2015), which may be the result of microbial metabolism. In addition, the peak observed at 530.6 eV for the Bacillus cereus group may relate to the presence of CaO, and the peak located at 531.7 eV for Pseudomonas fluorescens group may relate to CaSO4. As for blank sample, both two components were found corresponded to peak at 530.5 and 532 eV respectively (Ghods et al. 2011; Lu et al. 2017; Vanthana Sree et al. 2020).
Scale and corrosion inhibition mechanism of microbial agent
Scaling and corrosion problems are a serious threat to power plant safety operations towards eliminating the environmental pressure caused by chemical inhibitors. In our study, microbial agents were used as scale and corrosion inhibitors and its application feasibility was confirmed. The schematic illustration is shown in Figure 8.
Microbial agents of nitrobacteria, denitrobacteria and Lactobacillus played an important role in calcium carbonate deposition inhibition. During the activation process, microorganisms gradually recovered, their metabolism could promote the accumulation of organic acids, the lysis of bacterial biomass also contributed to the generation of large amounts of dissolved organic matter. These organic matters contain abundant negatively charged functional groups such as O-H and N-H groups, amide group, carboxylic groups, and C-O-C. Therefore, they chelated with free Ca2+ to further reduce the precipitation-forming Ca2+, or adsorbed on growth sites of micro-nuclei and prohibited the nucleation process (Li et al. 2019b). Moreover, highly electronegatively charged molecules of EPS could occupy growth sites to interfere with regular scale crystal lattices, resulingt in a distorted crystal structure with modified crystal morphology rather than regular oriented calcite growth without scale inhibitor (Shen et al. 2012).
In addition, microbial agents of Bacillus cereus also inhibited the Q235 CS corrosion process in simulated cooling water systems. Initially, the activated microorganisms started to adapt to the environment and secreted organic acid, some adhered onto the CS surface, given an accelerated corrosion rate along with a reduced radius of the Nyquist plot in EIS spectra. Subsequently, the attached bacteria on the CS surface colonized and formed compact biofilms, the covered biofilms could produce a microenvironment at the metal/biofilm interface What is more, sessile bacteria under the biofilm consumed oxygen for respiration to impede the oxygen cathodic reduction and prevent its diffusion to the metal surface (Khan et al. 2020), therefore the diameter of the Nyquist plot became larger and the corrosion process was inhibited. Furthermore, the Bacillus cereus agent also induced mineralization as a variety of microbials has been reported to be able to induce calcium precipitation (Han et al. 2016). As indicated by XPS spectra, the metabolically secreted exopolysaccharides could absorb Ca2+ to serve as the nucleation site of calcium carbonate, such complex biomineralized films acted like a barrier to prevent CS contact with the corrosive medium. The final loss of CS corrosion protection corresponded to the death of functional microorganisms and the detachment of cells from the biofilm due to starvation (Rochex & Lebeault 2007).
This study proved the feasibility of using microbial agents as scale and corrosion inhibitors with environmentally friendly and cost efficient results. At 40 °C, activated microbial agents showed excellent scale inhibition performances, 240 mg/L of Lactobacillus agents exhibited the maximum CaCO3 scale inhibition efficiency of 86%, nitrobacteria and denitrobacteria agents also showed high scale inhibition efficiencies of 83 and 82%, respectively. The SEM and XRD results showed that, under the presence of microbial agents, the calcite crystals morphologies were significantly modified from regular rhombohedral to rougher and more disordered shapes.
In addition, microbial agents also exhibited obvious corrosion inhibition effects on Q235 CS, the most prominent corrosion inhibitor was Bacillus cereus agent, which showed 60.11% corrosion inhibition efficiency under 200 mg/L by 3 d weight loss measurements. In the following 12 d of electrochemical tests, the Bacillus cereus agent group showed a weak current density of 13.0 μA cm−2, which is one order of magnitude lower than the blank group. Surface analysis indicated that biofilm colonized by microbial aggregation, mixed with biomineralization barrier and corrosion product, may take the main responsibility for the corrosion inhibition mechanisms. The results showed that microbial agents have a promising potential as corrosion and scale inhibitors for industrial cooling water applications.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.