3.1 Biosorption of heavy metals:
3.1.1. Growth of MTB in presence of metals
Among the six metals used, it was observed that none of the metals were utilized within 24 h by any of the strains (Table 1). The strain MSR-1 showed meagre growth up to 48 h in presence of Cd (OD – 0.004). After 72 h, it exhibited notable growth in presence of Pb (OD – 0.005) and Cr (OD – 0.004). The strain effectively grew between 96 h (Cd – 0.037, Mn – 0.013, Pb – 0.024, Ni – 0.006, Zn – 0.009) and 120 h (Cd – 0.102, Mn – 0.045, Pb – 0.047, Ni – 0.034, Zn – 0.042) in presence of most of the metals. The strain RJS2 started growing only after 96 h in media supplemented with Cd (OD – 0.004), Pb (OD – 0.002), Ni (OD – 0.005), and Cr (OD – 0.002). After 120 h of incubation, only a slender growth was observed in presence of manganese (OD – 0.017) and zinc (OD – 0.009). However, poor growth of RJS5 except in presence of Cd (0.011) while RJS6 exhibited good growth in presence of Cd (OD – 0.008), Mn (OD – 0.009), Pb (OD – 0.007), and Cr (OD – 0.008) after 72 h of incubation. Good growth of RJS5 and RJS6 detected till 96 h in the existence of all the metals (Cd – 0.093, Mn – 0.013, Pb – 0.009, Ni – 0.01, Zn – 0.011, Cr – 0.009, Co –0.012), (Cd – 0.045, Mn – 0.017, Pb – 0.016, Ni – 0.014, Zn – 0.004, Cr – 0.021, Co – 0.007) respectively. RJS7 growth was observed in presence of Cd (OD – 0.093), Pb (OD – 0.009), Cr (OD – 0.009) after 96 h of incubation. Abundant growth in presence of all metals was noted in RJS5 (Cd – 0.156, Mn – 0.095, Pb – 0.089, Ni – 0.014, Zn – 0.013, Cr – 0.056, Co –0.017), RJS6 (Cd – 0.093, Mn – 0.051, Pb – 0.048, Ni – 0.041, Zn – 0.009, Cr – 0.099, Co –0.011), and RJS7 (Cd – 0.025, Mn – 0.008, Pb – 0.019, Ni – 0.008, Zn – 0.007, Cr – 0.027, Co –0.006) towards 120 h. Similar results were observed with Thiobacillus ferrooxidans which showed biosorption of Zn only after 288 h (Cho et al., 2002). Comparatively, MTB strains displayed an efficient biosorption after 72 h and therefore the MTB performs faster biosorption than other strains. Among the tested strains, both MSR-1 and the RJS6 strains exhibited notable growth in the media supplemented with all the metals. Further, they could efficiently utilize Cd within 48 h which was quite similar to Cd degradation by Pseudomonas aeruginosa JP-11 (Raj et al., 2016). Comparatively, RJS6 was more effective than MSR-1. RJS6 utilized four metals namely, Cd, Mn, Pb, and Cr within 72 h whereas MSR-1 could utilize three metals Cd, Pb, and Cr.
Table 1. Effect of time on biosorption of heavy metals by MTB
Metals
|
Strains
|
MSR-1
|
RJS2
|
RJS5
|
RJS6
|
RJS7
|
Time duration (hours)
|
24
|
48
|
72
|
96
|
120
|
24
|
48
|
72
|
96
|
120
|
24
|
48
|
72
|
96
|
120
|
24
|
48
|
72
|
96
|
120
|
24
|
48
|
72
|
96
|
120
|
control
|
-
|
+
|
+
|
+ +
|
+++
|
-
|
+
|
+
|
++
|
+++
|
+
|
+
|
++
|
+++
|
+++
|
+
|
++
|
+++
|
+++
|
+++
|
_
|
+
|
+
|
+
|
++
|
Cd
|
_
|
+
|
+
|
++
|
+++
|
_
|
_
|
_
|
+
|
++
|
_
|
+
|
+
|
++
|
+++
|
_
|
+
|
+
|
++
|
+++
|
_
|
_
|
_
|
+
|
++
|
Mn
|
_
|
_
|
_
|
+
|
++
|
_
|
_
|
_
|
_
|
+
|
_
|
_
|
_
|
+
|
++
|
_
|
_
|
+
|
+
|
++
|
_
|
_
|
_
|
_
|
+
|
Pb
|
_
|
_
|
+
|
++
|
++
|
_
|
_
|
_
|
+
|
++
|
_
|
_
|
_
|
+
|
++
|
_
|
_
|
+
|
+
|
++
|
_
|
_
|
_
|
+
|
++
|
Ni
|
_
|
_
|
_
|
+
|
++
|
_
|
_
|
_
|
+
|
++
|
_
|
_
|
_
|
+
|
++
|
_
|
_
|
_
|
+
|
++
|
_
|
_
|
_
|
_
|
+
|
Zn
|
_
|
_
|
_
|
+
|
++
|
_
|
_
|
_
|
_
|
+
|
_
|
_
|
_
|
+
|
+
|
_
|
_
|
_
|
+
|
+
|
_
|
_
|
_
|
_
|
+
|
Cr
|
_
|
_
|
+
|
+
|
+++
|
_
|
_
|
_
|
+
|
++
|
_
|
_
|
_
|
+
|
++
|
_
|
_
|
+
|
++
|
+++
|
_
|
_
|
_
|
+
|
++
|
Co
|
_
|
_
|
_
|
+
|
++
|
_
|
_
|
_
|
_
|
+
|
_
|
_
|
_
|
+
|
+
|
_
|
_
|
_
|
+
|
+
|
_
|
_
|
_
|
_
|
+
|
Signs used: +++ High turbidity, ++ medium turbidity, + Low turbidity, - no growth
3.1.2 Growth of MTB strains upon the various metal concentration
Different metal concentrations (1 ppm-100 ppm) were supplemented and the growth was monitored to determine the efficient biosorption. The strains MSR-1, RJS2, RJS7, and RJS5 showed abundant growth at 1 ppm and 10 ppm concentrations of all the metals, and the growth was declined upon the increasing concentration of metals (Table 2). However, RJS6 displayed good to abundant growth in a wide range of metal concentrations (1-100 ppm).
Table 2. Effect of metal concentration on biosorption of heavy metals by magnetotactic bacteria
Metal
|
|
Strain
|
MSR-1
|
RJS2
|
RJS5
|
RJS6
|
RJS7
|
Metal concentration(ppm)
|
1
|
10
|
100
|
1
|
10
|
100
|
1
|
10
|
100
|
1
|
10
|
100
|
1
|
10
|
100
|
Cd
|
+++
|
+
|
+
|
+++
|
-
|
-
|
+++
|
+
|
-
|
+++
|
+
|
+
|
+++
|
-
|
-
|
Mn
|
+++
|
_
|
_
|
+++
|
_
|
_
|
+++
|
-
|
_
|
+++
|
+
|
+
|
+++
|
_
|
_
|
Pb
|
+++
|
_
|
_
|
+++
|
_
|
_
|
+++
|
+
|
_
|
+++
|
_
|
_
|
+++
|
_
|
_
|
Ni
|
+++
|
_
|
_
|
+++
|
_
|
_
|
+++
|
-
|
_
|
+++
|
_
|
_
|
+++
|
_
|
_
|
Zn
|
+++
|
_
|
_
|
+++
|
_
|
_
|
+++
|
+
|
_
|
+++
|
_
|
_
|
+++
|
_
|
_
|
Cr
|
+++
|
_
|
_
|
+++
|
_
|
_
|
+++
|
-
|
_
|
+++
|
_
|
_
|
+++
|
_
|
_
|
Co
|
+++
|
_
|
_
|
+++
|
_
|
_
|
+++
|
-
|
_
|
+++
|
_
|
_
|
+++
|
_
|
_
|
Signs used: +++ High turbidity, ++ medium turbidity, + Low turbidity, - no growth
3.1.2.1. Biosorption of metal (1 ppm) by MTB strains
The media supplemented with different metals were inoculated with individual MTB strains and the samples after incubation were subjected to Atomic Absorbance Spectroscopy analysis to determine its biosorption potential. Among the chosen strains, the MSR-1 demonstrated an effective biosorption against chromium (100%), cadmium (55%), manganese (12%), cobalt (40%), and nickel (4%). The treatment of heavy metals with RJS2 strain revealed successful biosorption of cadmium (26.4%), manganese (28%), lead (96%), nickel (30%), chromium (51%), cobalt (48%) and zinc (9.2%). The strain RJS5 demonstrated a notable biosorption of all the metals [cadmium (95.8%), manganese (37%), lead (58%), nickel (57%), zinc (55%), chromium (27.5%) and cobalt (78%)]. The strain RJS6 displayed significant biosorption of six metals˗ cadmium (52%), manganese (17.2%), nickel (10%), zinc (34%), chromium (100%) and cobalt (59%). However, the strain RJS7 showed lesser biosorption against Cd (24%), Mn (22%), Ni (7.8%), Zn (40%), Cr (69%), and Co (28%) as shown in Table 3.
Among the tested strains, the RJS5 strain displayed eminent biosorption of all the metals. However, RJS2 strain highlights, promising biosorption of lead. Further, strains like MSR-1 and RJS6 have enormous biosorption potential for chromium. AAS analysis revealed, RJS5 as the potent strain for effective biosorption of heavy metals in a 1 ppm concentration.
Table 3. Percentage of metal recovery (1 ppm) concentration by the magnetotactic bacteria
Strains
|
Cadmium
|
Manganese
|
Lead
|
Nickel
|
Zinc
|
Chromium
|
Cobalt
|
MSR-1
|
55
|
12
|
0
|
3.9
|
0
|
100
|
40
|
RJS2
|
26.4
|
28
|
96.3
|
30
|
9.2
|
51
|
48
|
RJS5
|
95.8
|
37.1
|
57.9
|
57.3
|
54.8
|
27.5
|
78
|
RJS6
|
52
|
17.2
|
0
|
10
|
33.8
|
100
|
59
|
RJS7
|
24.4
|
22.2
|
0
|
7.8
|
40
|
68.9
|
28
|
3.1.2.2. Biosorption of metal (10 ppm) by MTB strains
The MTB strains were exposed to 10 ppm concentration of various metals and its biosorption was studied through AAS analysis. The ability of MSR-1 for significant biosorption of Cd (55.6%), Mn (7.5%), Pb (58%), Ni (64%), Zn (17.7%), Cr (13%) and Co (40%) was observed in Table 4. The strain RJS2 exhibited low biosorption against Cd (2%), Mn (6.2%), and Zn (0.58%) but high biosorption against Pb (64.5%), Ni (82%), Cr (39%) and Co (88%). The RJS5 strain demonstrated remarkable biosorption of Cd (78%), Mn (15%), Pb (80%), Ni (82%), Zn (50%), Cr (6.4%) and Co (62%). The RJS6 strain proved effective biosorption for metals like Cd (20%), Mn (44%), Pb (84%), Ni (4%), Zn (0.58%), Cr (16%) and Co (52%). Biosorption of metals [Cd (49%), Mn (25%), Pb (48%), Ni (17%), Zn (22.6%), Cr (4%) and Co (23.5%)] by RJS7 strain was comparatively lower than other MTB strains.
Comparative data indicates, RJS5 strain is a possible choice for metal biosorption studies at 10 ppm concentration. However, MSR-1 strain displayed better biosorption when exposed to 10 ppm concentration than lower concentrations for most of the metals. The MSR-1 strain has been widely studied in previous biosorption reports against copper (II) (Huiping et al., 2007; Wang et al., 2011). This study highlights the role of MSR-1 against seven lethal heavy metals namely Cd, Mn, Pb, Ni, Zn, Cr, and Co. The bacterial strain Pseudomonas gessardii strain LZ-E reduces 95% of Cr (VI) (Huang et al., 2016). Likewise, MSR-1 and RJS6 possess similar biosorption capacity against chromium. The hexavalent chromium contamination in drinking water is quite dangerous and even deadly to humans (Verma and Dwivedi, 2013). Therefore, exposing these strains to chromium contaminated areas could be beneficial. The conventional strain Acidithiobacillus thiooxidans displayed 67% chromium biosorption only after 144 h of incubation. (Ryu et al., 2003). Contrary, an enhanced chromium biosorption by 33% was attained within 72-96 h in MTBs. Some halophilic fungal species like A. restrictus and S. halophilus are also sensitive to lead exposure while producing minimum absorption of 44 and 57%, respectively (Bano et al., 2018). Lead is one of the toxic metals which reduce the efficiency of most microorganisms (Lyu et al., 2017; Jiang et al., 2019), however, MTB’s have proved quite beneficial to the scenario.
The MTBs demonstrated evident biosorption against Pb and Ni at 10 ppm rather than at 1 ppm. This reveals the high concentration metal tolerance capacity of MTBs.
Table 4. Percentage of metal recovery (10 ppm) concentration by the magnetotactic bacteria
Strains
|
Cadmium
|
Manganese
|
Lead
|
Nickel
|
Zinc
|
Chromium
|
Cobalt
|
MSR 1
|
55.66
|
7.5
|
58.06
|
64.43
|
17.76
|
13
|
40
|
RJS 2
|
2.2
|
6.25
|
64.51
|
82.34
|
0.58
|
38.7
|
88
|
RJS 5
|
78
|
15
|
80
|
82.34
|
50.3
|
6.4
|
61.8
|
RJS 6
|
20.2
|
43.75
|
83.87
|
4.06
|
0.58
|
16
|
52
|
RJS 7
|
49.49
|
25
|
48.38
|
16.95
|
22.64
|
4
|
23.5
|
3.1.2.3. Biosorption of Cd, Pb, and Zn (1˗50 ppm) by RJS5 strain
The AAS analysis revealed, RJS5 as the dominant strain for biosorption against 1 and 10 ppm concentration of metal. Therefore, the biosorption potential of the RJS5 strain was further exploited using various concentrations (1-50 ppm) of Cd, Pb, and Zn. Results for Cd exposer at various concentrations [1 ppm (96%), 2 ppm (40.6%), 5 ppm (83%), 10 ppm (76%), 20 ppm (70%), and 50 ppm (53.6%)] displayed gradual decrease in biosorption with higher Cd concentration (Table 5). Contrary, biosorption of Pb in RJS5 strain increased with elevated Pb concentration (1 ppm – 59%, 2 ppm – 28%, 5 ppm – 56%, 10 ppm – 82%, 20 ppm – 93%, and 50 ppm – 93%). RJS5 exhibited evident biosorption of Zn at various concentrations (1 ppm – 56%, 2 ppm – 67%, 5 ppm – 33%, 10 ppm – 48%, 20 ppm – 67%, and 50 ppm – 54%).
The biosorption capacity of the RJS5 strain against cadmium (1 ppm, 5ppm, and 10 ppm) was higher compared to conventional strains Desulfovibrio magneticus RS-1 (58%) reported earlier by Arakaki et al., 2002. C. bicolor showed 94% lead and 74% cadmium biosorption (Horsfall et al. 2006). Low biosorption of cadmium, could be since it can suppress the growth of microorganisms (Xu et al., 2017). This signifies the importance of MTB strains where strains RJS2 and RJS5 proved better biosorption capacity against cadmium and lead. Biosorption of RJS5 against zinc (86%) was higher than Cichy et al. 2016 (28.83%).
The known MTB strain, Alphaproteobacterium MTB-KTN90 possesses low biosorption against cobalt (19.2%) (Tajer-Mohammad-Ghazvini et al., 2016). Similarly, Acidithiobacillus ferrooxidans strain displayed biosorption against cobalt (65%) at 0.5 ppm concentration and nickel (59%) at 4.5 ppm only after 31 days (Mohapatra et al., 2008) Contrary, enhanced biosorption of cobalt at 10 ppm concentration (88%) and 1 ppm concentration (78%) was achieved within 120 h by strains RJS2 and RJS5 respectively. Moreover, both the strains RJS2 and RJS5 displayed evident biosorption against nickel at 10 ppm concentration (82%, 82%) within 120 h.
Table 5. The percentage recovery of lead, cadmium and zinc by RJS5
Concentration (mg/l)
|
Cadmium
|
Lead
|
Zinc
|
1ppm
|
95.8
|
59
|
56.3
|
2ppm
|
40.6
|
28
|
85.7
|
5ppm
|
82.7
|
56
|
33.4
|
10ppm
|
76
|
81.8
|
47.8
|
20ppm
|
70
|
93
|
67
|
50ppm
|
53.6
|
93.4
|
54
|
3.2 Biosorption of heavy metals in tannery effluents
The biosorption sufficiency of MTBs against heavy metals was further exploited and applied to remove metals from tannery effluents. The AAS analysis of the tannery effluent augmented with MSR-1 strain revealed biosorption against all metals [Cd (13%), Mn (18%), Pb (0.7%), Ni (0.75%), Zn (3%), Cr (2%) and Co (4%). The strain RJS2 showed better biosorption against metals- Cd (4%), Mn (82%), Pb (21%), Ni (3%), Zn (2%), Cr (6%), and Co (96%) from tannery effluent. The strain RJS5 displayed significant biosorption of metals like Cd (1.5%), Mn (5%), Pb (3.5%), Ni (29.5%), Zn (0.7%), Cr (42%) and Co (11%) from tannery effluent. The metal biosorption from tannery effluent was most evident in RJS6 strain (Cd – 36%, Mn – 4%, Pb – 13%, Ni – 88%, Zn – 81%, Cr – 3% and Co – 2%). The strain RJS7 also showed a good biosorption of six metals (Cd – 33%, Mn – 6%, Ni – 88%, Zn – 22%, Cr – 0.8%, and Co – 3.6%) from tannery effluent (Table 6).
Table 6. The percentage recovery of heavy metals from tannery effluents using magnetotactic bacteria
Strains
|
Cadmium
|
Manganese
|
Lead
|
Nickel
|
Zinc
|
Chromium
|
Cobalt
|
MSR1
|
13
|
18
|
0.7
|
0.75
|
3
|
2
|
4.2
|
RJS2
|
4.3
|
82
|
21
|
3
|
2
|
6
|
95.7
|
RJS5
|
1.5
|
5
|
3.5
|
29.5
|
0.75
|
42
|
11
|
RJS6
|
36
|
4
|
13
|
88
|
81
|
3
|
2
|
RJS7
|
53
|
6.4
|
0
|
89
|
22
|
0.8
|
3.6
|
Industries are the biggest source for emitting heavy metals into the environment through soil and water (Chen et al., 2018; Hu et al., 2018). To date, various microbial species like Klebsiella oxytoca, Allescheriella sp., Stachybotrys sp., Phlebia sp., Pleurotus pulmonarius, Botryosphaeria rhodina can convert toxic metals to less toxic form (Dixit et al., 2015; Pinedo-Rivilla et al., 2009; Gupta et al., 2018). Certain fungal species, Aspergillus parasitica, and Cephalosporium aphidicola can degrade soil contaminated with Pb (II) (Dixit et al., 2015). Some bacterial strains (B-21) can uptake various heavy metals like cadmium, iron, copper (El-Zahrani and El-Saied, 2011).
In our study, the analysis of the tannery effluent augmented with the various strains of MTBs support that these MTBs are potential bio sorbents of various heavy metals present in the effluents. However, the biosorption potential of each strain varies to a certain extent concerning the metal degrading property. The strain RJS2 displayed biosorption against Mn (82%) and Co (95.7%) from tannery effluent within 5 days while S. epidermidis reported 80% biosorption of Mn only after 20 days (Das et al., 2012). Moreover, the fast biosorption of Cr (42%) in the RJS5 strain within 2 days is important. Certain fungal species like Aspergillus Niger and Penicillium sp. presented higher biosorption of Cr (94%, 90%) from tannery effluent only after 20 days (Abioye et al., 2018). Though, the huge biomass generated during the process will further slowdown the refining process. The strain Pseudomonas aeruginosa exhibitbiosorption of Cr (86%), Ni (64%), and zinc (71%) from industrial effluents after 20 days (Pandian et al., 2014). The strains RJS6 and RJS7 showed significant biosorption of nickel i.e 88% and 89% respectively from tannery effluent within 5 days. Moreover, the strain RJS6 possesses better biosorption properties against zinc (81%) from tannery effluent. The MTB strains are proved as better bio sorbents than the conventional strains. Further, optimization of essential factors such as pH, temperature, pulp density can significantly increase the metal biosorption in the MTBs. Moreover, these MTBs could be applied in industrial sectors owing to the waste generation for the treatment of heavy metal contaminants.