Bioresource Technology xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Effect of extracellular polymeric substances on corrosion of cast iron in the reclaimed wastewater Juntao Jin, Guangxue Wu, Zhenhua Zhang, Yuntao Guan ⇑ State Environmental Protection Key Laboratory of Microorganism Application and Risk Control (MARC), Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, Guangdong, China
h i g h l i g h t s EPS contained different components but with similar functional groups. EPS exhibited inhibition effects on corrosion of cast iron. EPS formed a protective film and retarded the cathodic reduction of oxygen. Excessive EPS promoted anodic dissolution of cast iron through EPS-Fe binding.
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Extracellular polymeric substances Corrosion Biofilm Reclaimed wastewater Cast iron
a b s t r a c t Microorganisms were cultured in the R2A medium with inoculum from biofilm in a reclaimed wastewater distribution system and then extracellular polymeric substances (EPS) were extracted from the culture. Characterization of EPS and their effects on the corrosion of cast iron were examined. EPS extracted from different culturing stages contained different proportions of protein and polysaccharide but with similar functional groups. All types of EPS could inhibit cast iron corrosion and the EPS from the stationary stage had the highest inhibition efficiency. The inhibition efficiency was increased with addition of a small amount of EPS while decreased with excessive amount of EPS. EPS formed a protective film on the metal surface, which retarded the cathodic reduction of oxygen. Excessive amount of EPS promoted anodic dissolution through EPS–Fe binding. The C@O and CA(O, N) in EPS could be the anodic electrochemical sites with possible products of CA(C, H). Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Reclamation of wastewater is an effective way to alleviate water resource scarcity. However, abundant organic matters and inorganic ions in reclaimed wastewater can cause serious corrosion of distribution pipes during transportation, especially when cast iron pipes are used. Pipe clogging and deterioration of reclaimed wastewater quality will occur concurrent with the corrosion of pipes in the distribution system (Larson and Sollo, 1967). In reclaimed wastewater distribution system, biofilm growth will accelerate the pipe corrosion due to microbiologically influenced corrosion (MIC). A biofilm composes of microbial cells, extracellular polymeric substances (EPS) secreted by the cells and metabolic products. Strongly adhesive EPS, mainly composed of polysaccharides, lipids and proteins, enable microorganisms to
⇑ Corresponding author. Tel./fax: +86 755 26036702.
bind to carrier surfaces tightly. In general, EPS contain functional groups that can bind metals easily (Geesey et al., 1986). The weak electrostatic interaction between EPS and metal ions has been shown to accelerate metal corrosion (Washizu et al., 2004). However, some studies showed that EPS had anti-corrosive properties. EPS of Desulfovibrio vulgaris inhibited corrosion of mild steel due to the prevention of biofilm formation and the exclusion of oxygen from the biofilm (Stadler et al., 2008). Finkenstadt et al. (2011) reported that purified Leuconostoc mesenteroides EPS inhibited the corrosion of low-carbon steel as determined by the electrochemical method. Dong et al. (2011) found that adsorbed EPS layers from thermophilic sulfate reducing bacteria could anti-corrosion of carbon steel by hindering the reduction of oxygen but could stimulate the anodic dissolution by chelation with Fe2+ ions. Moreover, the effect of biofilm on corrosion depended on culturing stages since biofilm could promote the corrosion in the initial growth stage but inhibit the corrosion in the later growth stage (Teng et al., 2008). EPS are main components of biofilm,
E-mail address: [email protected] (Y. Guan). http://dx.doi.org/10.1016/j.biortech.2014.01.117 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Jin, J., et al. Effect of extracellular polymeric substances on corrosion of cast iron in the reclaimed wastewater. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.01.117
2
J. Jin et al. / Bioresource Technology xxx (2014) xxx–xxx
whether their function on corrosion relates with the culturing stage is still unclear. The objectives of this study were to (i) characterize EPS extracted from microorganisms cultured at different stages, (ii) evaluate effect of EPS on corrosion of cast iron with the electrochemical method, and (iii) analyze the mechanism of EPS effect on corrosion.
spectroscopy (EIS). Polarization measurements were performed at a 0.01 V/s scan step with the scan range of +2 V to 2 V versus the open circuit potential (OCP). The EIS measurements were conducted around the OCP by applying a 5 mV sinusoidal perturbation ranging from 100 kHz to 10 mHz with 10 data-points per decade. ZSimDemo 3.30d software was employed to fit the EIS data using the equivalent circuit model as shown in Fig. S1. All experiments were run in duplicates.
2. Methods 2.4. Surface analysis 2.1. Extraction and characterization of EPS Biofilm was taken from cast iron pipes in Xili reclaimed wastewater treatment plant (Shenzhen, China) with a sterile brush and suspended in 10 mL 0.85% NaCl. The suspension was used as the inoculum and cultured in the R2A liquid medium at 30 ± 2 °C (180 rpm). EPS were extracted from the R2A liquid medium on Days 1, 2, 3 and 4. The liquid medium was firstly heated in a 60 °C water bath for 40 min and then centrifuged at 12,000g at 4 °C for 20 min. Dissolved EPS were dialysed for 24 h against deionized water using the 10 kDa molecular weight cutoff dialysis membrane (Sangon Biotech, China) for purification and then lyophilized at 60 °C. Adequate EPS were extracted from the medium at each culturing stage and used for the designed experiments. The protein content of the purified EPS was determined by the Bradford method (Bradford, 1976). The polysaccharide content of EPS was quantified using the phenol–sulfuric acid method (Dubois et al., 1956). The purified EPS were analyzed by Fourier transformation infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and scanning electron microscope (SEM). 2.2. Corrosion of cast iron in EPS-containing solution Batch experiments were carried out to examine the effect of EPS on corrosion under two conditions (i) the EPS produced at different culturing stages, and (ii) different EPS dosages. For the effect of EPS harvested at different culturing stages, the reclaimed wastewater was added with different types of EPS extracted from different culturing stages with the concentration of 200 mg/L. For the effect of EPS dosage, the reclaimed wastewater was added with different amount of EPS extracted from the stationary stage (0 mg/L, 100 mg/L, 200 mg/L, 300 mg/L and 400 mg/L, respectively). Experiments were carried out in 500 mL Duran bottles containing reclaimed wastewater and purified EPS. The cast iron coupons with dimensions of 12 cm ⁄ 1.5 cm ⁄ 0.2 cm were used. The coupons were polished using SiC grit paper up to grade 1200 and rinsed with deionised water, and then polished by aluminum powders (1.0 mm, 0.3 mm and 0.05 mm in sequentially) and rinsed with deionised water. The freshly polished coupons were immersed into the solution for 5d. In all batch experiments, the coupons were sampled at intervals for electrochemical measurements. Especially, coupon samples at the condition of 200 mg/L EPS were used for SEM and XRF analysis. In addition, the corrosion products containing absorbed EPS were gently removed from the surface of coupons, dried in a vacuum freeze drier, and stored overnight in an evacuated desiccator for XPS analysis. 2.3. Electrochemical measurements Electrochemical measurements were conducted in a conventional three-electrode corrosion cell with a saturated calomel reference electrode (SCE) and a platinum sheet as the counter electrode (CE). The cast iron coupon was used as the working electrode (WE). A CHI 660D electrochemical workstation (Chenhua, China) was used to measure polarization and electrochemical impedance
A mixture of purified EPS and KBr powder was pressed into a pellet and analyzed by the FTIR spectroscopy (Shimadzu, IRPrestige-21, Japan). Dried samples were compressed into a pellet and attached onto a carbon tape for the XPS analysis (Quantum, CA, USA). An XPS wide range (0–1350 eV at an emission angle of 45°) for binding energy spectra and XPS high resolution spectra of C 1s and O 1s scans were analyzed to investigate the differences in the chemical composition of EPS before and after interacting with the cast iron. SEM (Hitachi, S-4800, Japan) was used to observe the purified EPS and the EPS formed on the coupons surface after exposure. The coupons were dried in the vacuum freeze drier overnight and coated with platinum before observation by SEM. 3. Results and discussion 3.1. EPS production and characterization The growth curve of biofilm strain in the R2A medium was tested to distinguish the different culturing stages. In addition, polysaccharide and protein in the EPS extracted at different culturing stages (on days 1, 2, 3 and 4, corresponding to log phase, stationary phase, initial death phase and later death phase) were examined. The polysaccharide of the 2d-EPS (EPS extracted on day 2) was highest. However, the protein content increased from 0.03 g/L in 1d-EPS (EPS extracted on day 1) to 0.22 g/L in 2d-EPS, and decreased to around 0.1 g/L in 3d-EPS (EPS extracted on day 3) and 4d-EPS (EPS extracted on day 4). In the later culturing stage, the protein was constant while the polysaccharide was reduced. It might be ascribed to the fact that polysaccharide was preferred to be utilized than protein during the later stage of bacterial growth. As analyzed by FTIR spectroscopy, the polymers produced in all experiments had similar functional groups. The peaks at 3400 cm1, 2928 cm1 and 2355 cm1 were assigned to the hydroxyl group, CAH stretching and AN@C@OA group, respectively (Iyer et al., 2005; Mishra and Jha, 2009). Peaks around 1642 cm1 and 1416 cm1 corresponded to the ring stretching and the ACOOA group (Freitas et al., 2009). The stretching of CAOAC and CAO at 1000–1200 cm1 corresponded to the presence of carbohydrates (Mishra and Jha, 2009), and a peak at 1070 cm1 confirmed the presence of uronic acid (Bramhachari and Dubey, 2006). These results confirmed the presence of aliphatic methyl groups, primary amines, halide groups, uronic acid and saccharides. A wide XPS spectrum of EPS showed that the EPS contained mainly of C, O and N, while less of P and S. Subtle differences in XPS were observed between the 2d-EPS and others. For 2d-EPS, the binding energy around 80 eV was highest, which might be attributed to several phosphate groups. 3.2. Corrosion of cast iron in the EPS-containing solution The potentiodynamic polarization curves for the cast iron after 3 h exposure in solutions without and with the addition of EPS are shown in Supplementary Figs. S2 and S3. Values of the polarization parameters Ecorr, icorr, anodic Tafel slope (ba), cathodic Tafel slope
Please cite this article in press as: Jin, J., et al. Effect of extracellular polymeric substances on corrosion of cast iron in the reclaimed wastewater. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.01.117
3
J. Jin et al. / Bioresource Technology xxx (2014) xxx–xxx Table 1 Electrochemical parameters for cast iron in reclaimed water without and with various kinds of EPS from the Talfe plots. Polarization
EIS
Ecorr (V)
icorr (lA/cm )
bc (V/dec)
ba (V/dec)
g (%)
Rct (kXcm2)
Cdl 10–6 F
100 mg/L 200 mg/L 300 mg/L 400 mg/L
0.8 0.81 0.82 0.78 0.75
46.91 46.4 33.85 40.27 43.96
8.76 7.668 7.489 7.559 7.642
5.11 5.502 5.937 6.383 6.394
1.09 27.84 14.15 6.29
118.4 122.2 236.85 129.3 121.9
2.04 1.52 1.13 1.54 1.67
2d-EPS
100 mg/L 200 mg/L 300 mg/L 400 mg/L
0.83 0.83 0.85 0.84
23.23 15.76 22.79 26.2
7.965 7.528 8.097 8.192
5.575 5.611 5.871 5.947
50.48 66.40 51.42 44.15
133.9 363.2 317.2 195.8
2.007 0.37 0.58 0.694
3d-EPS
100 mg/L 200 mg/L 300 mg/L 400 mg/L
0.86 0.84 0.79 0.84
24.74 21.4 24.81 27.55
8.862 8.108 8.197 7.892
5.163 5.821 5.737 5.86
47.26 54.38 47.11 41.27
294.2 323.3 258.4 249.5
1.11 1.01 1.14 1.72
4d-EPS
100 mg/L 200 mg/L 300 mg/L 400 mg/L
0.85 0.86 0.80 0.75
36.27 30.82 32.31 42.31
8.312 8.61 5.944 8.834
5.39 5.572 5.667 5.761
22.68 34.30 31.128 9.811
141.7 169.83 156.2 133.4
1.35 0.579 0.82 1.86
Without EPS 1d-EPS
2
(bc), impedance parameters Rct and Cdl are presented in Table 1. The inhibition efficiency (g, %) was calculated by: g ð%Þ ¼
i0corr icorr , i0corr
0
where icorr and icorr are the corrosion current densities without and with the addition of EPS, respectively. 3.2.1. Effect of types of EPS on the corrosion As indicated in Table 1, the addition of EPS extracted at different culturing stages reduced icorr significantly, and the absolute value of bc was higher than ba, indicating that the cathodic reaction (oxygen reduction) controlled the rate of the corrosion process. bc decreased with the addition of EPS, showing that the cathodic process was suppressed. It could be attributed to the formation of an adsorbed EPS film on the cast iron electrode, retarding the mass transfer of oxygen. However, no obvious trend (
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
Effect of extracellular polymeric substances on corrosion of cast iron in the reclaimed wastewater Juntao Jin, Guangxue Wu, Zhenhua Zhang, Yuntao Guan ⇑ State Environmental Protection Key Laboratory of Microorganism Application and Risk Control (MARC), Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, Guangdong, China
h i g h l i g h t s EPS contained different components but with similar functional groups. EPS exhibited inhibition effects on corrosion of cast iron. EPS formed a protective film and retarded the cathodic reduction of oxygen. Excessive EPS promoted anodic dissolution of cast iron through EPS-Fe binding.
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Extracellular polymeric substances Corrosion Biofilm Reclaimed wastewater Cast iron
a b s t r a c t Microorganisms were cultured in the R2A medium with inoculum from biofilm in a reclaimed wastewater distribution system and then extracellular polymeric substances (EPS) were extracted from the culture. Characterization of EPS and their effects on the corrosion of cast iron were examined. EPS extracted from different culturing stages contained different proportions of protein and polysaccharide but with similar functional groups. All types of EPS could inhibit cast iron corrosion and the EPS from the stationary stage had the highest inhibition efficiency. The inhibition efficiency was increased with addition of a small amount of EPS while decreased with excessive amount of EPS. EPS formed a protective film on the metal surface, which retarded the cathodic reduction of oxygen. Excessive amount of EPS promoted anodic dissolution through EPS–Fe binding. The C@O and CA(O, N) in EPS could be the anodic electrochemical sites with possible products of CA(C, H). Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Reclamation of wastewater is an effective way to alleviate water resource scarcity. However, abundant organic matters and inorganic ions in reclaimed wastewater can cause serious corrosion of distribution pipes during transportation, especially when cast iron pipes are used. Pipe clogging and deterioration of reclaimed wastewater quality will occur concurrent with the corrosion of pipes in the distribution system (Larson and Sollo, 1967). In reclaimed wastewater distribution system, biofilm growth will accelerate the pipe corrosion due to microbiologically influenced corrosion (MIC). A biofilm composes of microbial cells, extracellular polymeric substances (EPS) secreted by the cells and metabolic products. Strongly adhesive EPS, mainly composed of polysaccharides, lipids and proteins, enable microorganisms to
⇑ Corresponding author. Tel./fax: +86 755 26036702.
bind to carrier surfaces tightly. In general, EPS contain functional groups that can bind metals easily (Geesey et al., 1986). The weak electrostatic interaction between EPS and metal ions has been shown to accelerate metal corrosion (Washizu et al., 2004). However, some studies showed that EPS had anti-corrosive properties. EPS of Desulfovibrio vulgaris inhibited corrosion of mild steel due to the prevention of biofilm formation and the exclusion of oxygen from the biofilm (Stadler et al., 2008). Finkenstadt et al. (2011) reported that purified Leuconostoc mesenteroides EPS inhibited the corrosion of low-carbon steel as determined by the electrochemical method. Dong et al. (2011) found that adsorbed EPS layers from thermophilic sulfate reducing bacteria could anti-corrosion of carbon steel by hindering the reduction of oxygen but could stimulate the anodic dissolution by chelation with Fe2+ ions. Moreover, the effect of biofilm on corrosion depended on culturing stages since biofilm could promote the corrosion in the initial growth stage but inhibit the corrosion in the later growth stage (Teng et al., 2008). EPS are main components of biofilm,
E-mail address: [email protected] (Y. Guan). http://dx.doi.org/10.1016/j.biortech.2014.01.117 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Jin, J., et al. Effect of extracellular polymeric substances on corrosion of cast iron in the reclaimed wastewater. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.01.117
2
J. Jin et al. / Bioresource Technology xxx (2014) xxx–xxx
whether their function on corrosion relates with the culturing stage is still unclear. The objectives of this study were to (i) characterize EPS extracted from microorganisms cultured at different stages, (ii) evaluate effect of EPS on corrosion of cast iron with the electrochemical method, and (iii) analyze the mechanism of EPS effect on corrosion.
spectroscopy (EIS). Polarization measurements were performed at a 0.01 V/s scan step with the scan range of +2 V to 2 V versus the open circuit potential (OCP). The EIS measurements were conducted around the OCP by applying a 5 mV sinusoidal perturbation ranging from 100 kHz to 10 mHz with 10 data-points per decade. ZSimDemo 3.30d software was employed to fit the EIS data using the equivalent circuit model as shown in Fig. S1. All experiments were run in duplicates.
2. Methods 2.4. Surface analysis 2.1. Extraction and characterization of EPS Biofilm was taken from cast iron pipes in Xili reclaimed wastewater treatment plant (Shenzhen, China) with a sterile brush and suspended in 10 mL 0.85% NaCl. The suspension was used as the inoculum and cultured in the R2A liquid medium at 30 ± 2 °C (180 rpm). EPS were extracted from the R2A liquid medium on Days 1, 2, 3 and 4. The liquid medium was firstly heated in a 60 °C water bath for 40 min and then centrifuged at 12,000g at 4 °C for 20 min. Dissolved EPS were dialysed for 24 h against deionized water using the 10 kDa molecular weight cutoff dialysis membrane (Sangon Biotech, China) for purification and then lyophilized at 60 °C. Adequate EPS were extracted from the medium at each culturing stage and used for the designed experiments. The protein content of the purified EPS was determined by the Bradford method (Bradford, 1976). The polysaccharide content of EPS was quantified using the phenol–sulfuric acid method (Dubois et al., 1956). The purified EPS were analyzed by Fourier transformation infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and scanning electron microscope (SEM). 2.2. Corrosion of cast iron in EPS-containing solution Batch experiments were carried out to examine the effect of EPS on corrosion under two conditions (i) the EPS produced at different culturing stages, and (ii) different EPS dosages. For the effect of EPS harvested at different culturing stages, the reclaimed wastewater was added with different types of EPS extracted from different culturing stages with the concentration of 200 mg/L. For the effect of EPS dosage, the reclaimed wastewater was added with different amount of EPS extracted from the stationary stage (0 mg/L, 100 mg/L, 200 mg/L, 300 mg/L and 400 mg/L, respectively). Experiments were carried out in 500 mL Duran bottles containing reclaimed wastewater and purified EPS. The cast iron coupons with dimensions of 12 cm ⁄ 1.5 cm ⁄ 0.2 cm were used. The coupons were polished using SiC grit paper up to grade 1200 and rinsed with deionised water, and then polished by aluminum powders (1.0 mm, 0.3 mm and 0.05 mm in sequentially) and rinsed with deionised water. The freshly polished coupons were immersed into the solution for 5d. In all batch experiments, the coupons were sampled at intervals for electrochemical measurements. Especially, coupon samples at the condition of 200 mg/L EPS were used for SEM and XRF analysis. In addition, the corrosion products containing absorbed EPS were gently removed from the surface of coupons, dried in a vacuum freeze drier, and stored overnight in an evacuated desiccator for XPS analysis. 2.3. Electrochemical measurements Electrochemical measurements were conducted in a conventional three-electrode corrosion cell with a saturated calomel reference electrode (SCE) and a platinum sheet as the counter electrode (CE). The cast iron coupon was used as the working electrode (WE). A CHI 660D electrochemical workstation (Chenhua, China) was used to measure polarization and electrochemical impedance
A mixture of purified EPS and KBr powder was pressed into a pellet and analyzed by the FTIR spectroscopy (Shimadzu, IRPrestige-21, Japan). Dried samples were compressed into a pellet and attached onto a carbon tape for the XPS analysis (Quantum, CA, USA). An XPS wide range (0–1350 eV at an emission angle of 45°) for binding energy spectra and XPS high resolution spectra of C 1s and O 1s scans were analyzed to investigate the differences in the chemical composition of EPS before and after interacting with the cast iron. SEM (Hitachi, S-4800, Japan) was used to observe the purified EPS and the EPS formed on the coupons surface after exposure. The coupons were dried in the vacuum freeze drier overnight and coated with platinum before observation by SEM. 3. Results and discussion 3.1. EPS production and characterization The growth curve of biofilm strain in the R2A medium was tested to distinguish the different culturing stages. In addition, polysaccharide and protein in the EPS extracted at different culturing stages (on days 1, 2, 3 and 4, corresponding to log phase, stationary phase, initial death phase and later death phase) were examined. The polysaccharide of the 2d-EPS (EPS extracted on day 2) was highest. However, the protein content increased from 0.03 g/L in 1d-EPS (EPS extracted on day 1) to 0.22 g/L in 2d-EPS, and decreased to around 0.1 g/L in 3d-EPS (EPS extracted on day 3) and 4d-EPS (EPS extracted on day 4). In the later culturing stage, the protein was constant while the polysaccharide was reduced. It might be ascribed to the fact that polysaccharide was preferred to be utilized than protein during the later stage of bacterial growth. As analyzed by FTIR spectroscopy, the polymers produced in all experiments had similar functional groups. The peaks at 3400 cm1, 2928 cm1 and 2355 cm1 were assigned to the hydroxyl group, CAH stretching and AN@C@OA group, respectively (Iyer et al., 2005; Mishra and Jha, 2009). Peaks around 1642 cm1 and 1416 cm1 corresponded to the ring stretching and the ACOOA group (Freitas et al., 2009). The stretching of CAOAC and CAO at 1000–1200 cm1 corresponded to the presence of carbohydrates (Mishra and Jha, 2009), and a peak at 1070 cm1 confirmed the presence of uronic acid (Bramhachari and Dubey, 2006). These results confirmed the presence of aliphatic methyl groups, primary amines, halide groups, uronic acid and saccharides. A wide XPS spectrum of EPS showed that the EPS contained mainly of C, O and N, while less of P and S. Subtle differences in XPS were observed between the 2d-EPS and others. For 2d-EPS, the binding energy around 80 eV was highest, which might be attributed to several phosphate groups. 3.2. Corrosion of cast iron in the EPS-containing solution The potentiodynamic polarization curves for the cast iron after 3 h exposure in solutions without and with the addition of EPS are shown in Supplementary Figs. S2 and S3. Values of the polarization parameters Ecorr, icorr, anodic Tafel slope (ba), cathodic Tafel slope
Please cite this article in press as: Jin, J., et al. Effect of extracellular polymeric substances on corrosion of cast iron in the reclaimed wastewater. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.01.117
3
J. Jin et al. / Bioresource Technology xxx (2014) xxx–xxx Table 1 Electrochemical parameters for cast iron in reclaimed water without and with various kinds of EPS from the Talfe plots. Polarization
EIS
Ecorr (V)
icorr (lA/cm )
bc (V/dec)
ba (V/dec)
g (%)
Rct (kXcm2)
Cdl 10–6 F
100 mg/L 200 mg/L 300 mg/L 400 mg/L
0.8 0.81 0.82 0.78 0.75
46.91 46.4 33.85 40.27 43.96
8.76 7.668 7.489 7.559 7.642
5.11 5.502 5.937 6.383 6.394
1.09 27.84 14.15 6.29
118.4 122.2 236.85 129.3 121.9
2.04 1.52 1.13 1.54 1.67
2d-EPS
100 mg/L 200 mg/L 300 mg/L 400 mg/L
0.83 0.83 0.85 0.84
23.23 15.76 22.79 26.2
7.965 7.528 8.097 8.192
5.575 5.611 5.871 5.947
50.48 66.40 51.42 44.15
133.9 363.2 317.2 195.8
2.007 0.37 0.58 0.694
3d-EPS
100 mg/L 200 mg/L 300 mg/L 400 mg/L
0.86 0.84 0.79 0.84
24.74 21.4 24.81 27.55
8.862 8.108 8.197 7.892
5.163 5.821 5.737 5.86
47.26 54.38 47.11 41.27
294.2 323.3 258.4 249.5
1.11 1.01 1.14 1.72
4d-EPS
100 mg/L 200 mg/L 300 mg/L 400 mg/L
0.85 0.86 0.80 0.75
36.27 30.82 32.31 42.31
8.312 8.61 5.944 8.834
5.39 5.572 5.667 5.761
22.68 34.30 31.128 9.811
141.7 169.83 156.2 133.4
1.35 0.579 0.82 1.86
Without EPS 1d-EPS
2
(bc), impedance parameters Rct and Cdl are presented in Table 1. The inhibition efficiency (g, %) was calculated by: g ð%Þ ¼
i0corr icorr , i0corr
0
where icorr and icorr are the corrosion current densities without and with the addition of EPS, respectively. 3.2.1. Effect of types of EPS on the corrosion As indicated in Table 1, the addition of EPS extracted at different culturing stages reduced icorr significantly, and the absolute value of bc was higher than ba, indicating that the cathodic reaction (oxygen reduction) controlled the rate of the corrosion process. bc decreased with the addition of EPS, showing that the cathodic process was suppressed. It could be attributed to the formation of an adsorbed EPS film on the cast iron electrode, retarding the mass transfer of oxygen. However, no obvious trend (
