CHAPTER 2: Literature review
2.3 Metal recovery by biological sulfate reduction
105 ºC followed by incineration at 550 ºC for 1 h to remove all the volatile composites. The total ash obtained is then used for calculating the fixed solid balance.
In another study, Villa-Gomez et al. (2011) reported less than 50% recovery of Cu, Zn, Pb, Cd using an IFB reactor, which was less compared with the results reported by Gallegos-Garcia et al.
(2009). This difference could be attributed to the different SRB species used by Gallegos-Garcia et al. (2009) in their study. Another reason could be due to the method employed for metal recovery by Gallegos-Garcia et al. (2009) assuming that the total suspended solids (TSS) concentrations equal the metal sulfide composition (Kaksonen et al., 2003; Gallegos-Garcia et al., 2009). On the other contrary, Villa-Gomez et al. (2011) showed that in addition to the metal precipitates, salts present in the mineral medium as well contributed to the total suspended solids, which slightly increased the actual metal recovery value. Therefore, in order to exclude the salts present in the TSS for calculating the metal recovery value, Villa-Gomez et al. (2011) acidified the recovered solids before analysis of the metals present. Thereby resulting in accurate determination of metal recovery efficiency in the study.
Bijmans et al. (2008) studied selective recovery of nickel from nickel and iron containing solution by sulfate reduction at low pH (5.0) using a single-stage gas lift bioreactor. The results showed selective removal of nickel (>83 %) as nickel sulfide (NiS). At pH 5.5, the metal recovery obtained was more than 99.9%; however, iron removal and recovery as FeS could not be achieved at this pH value. Hence, controlling the bioreactor pH allows selective metal precipitation from wastewater.
Although by controlling the pH, selective recovery of different heavy metals can be achieved using a single-stage system, SRB are inhibited even at a slightly acidic pH (<5) due to increase in toxic
effect of sulfide and acetate at low pH. Copper can be precipitated as CuS even at an extremely low pH (≤1) whereas zinc does not precipitate as ZnS until pH 1.3 (Sahinkaya et al., 2009).
Although single-stage bioreactor system for selective heavy metal removal at low pH has several advantages such as low-operation cost, simple process design, homogeneous distribution of sulfide inside the bioreactor, etc. it is difficult to operate sulfidogenic bioreactor at low pH (<4). Hence, it is necessary to separate biological sulfide reduction step from metal precipitation to achieve both SRB activity (pH 7.0–8.0) and selective metal precipitation (pH 1.0 -7.0). Table 3 summarizes the results of metal removal as well as metal recovery efficiencies obtained using different single- stage bioreactor systems.
Table 2.5 Metal removal and metal recovery efficiency values obtained using different single- stage bioreactor systems
Reactor type Metals Metal removal (%) Metal recovery (%) References Inverse fluidized
bed reactor (IFBR)
Se 98.0 58.0 Sinharoy et al.,
2019 Inverse fluidized
bed reactor (IFBR)
Cu Zn Pb Cd
99.9 98.6 99.2 99.7
41.1 44.2 60.3 47.4
Villa-Gomez et al., 2011
Inverse fluidized bed reactor (IFBR)
Fe Zn Cd
99.7 99.3 99.4
~90.0
~90.0
~90.0
Gallegos-Garcia et al., 2009
Gas lift reactor (GLR)
Ni Not available (NA) >99.9 Bijmans et al., 2008
2.3.2 Metal recovery in two-stage or multistage system
In order to avoid the afore-mentioned drawbacks with single-stage system for metal recovery, Tabak et al. (2003) used a two-stage system for selective, sequential precipitation (SSP) of metals, such as copper, zinc, aluminum, iron and manganese, as hydroxides and sulfides from Berkeley Pit AMD and followed additional processing of the recovered metals into marketable precipitates and pigments. The system involved a separate unit for sulfate reduction and metal precipitation;
whereas the bioreactor was used to produce hydrogen sulfide in the first stage, the metal precipitation was carried out in a separate stage using the H2S produced in the first stage.
The metal recovery percentage obtained using the SSP process was: 100% Zn (as sulfide), 99.8%
Cu (as sulfide), 99.7% Al (as hydroxide), 97.1% Fe (as sulfide), 99.7% Cd (as sulfide), 99.1% Co (as sulfide), 97.1% Mn (as sulfide) and 47.8% Ni (as sulfide). The purity percentage of the different metal precipitates were: 92% copper sulfide, 81% ferric hydroxide, 97% zinc sulfide, 95%
aluminum hydroxide and 75% manganese sulfide. Following the SSP treatment of the wastewater, Tabak et al. (2003) reported sulfate and sulfide concentrations below permissible limits and only calcium and magnesium were present in the effluent.
In another study on multi-stage system for recovery of Cu and Zn from AMD, Sahinkaya et al.
(2009) utilized anaerobic baffled reactor (ABR) and the metals were precipitated separately based on their solubility product with gaseous sulfide produced in the first stage. The bioreactor was fed with ethanol (1340 mg/L) and sulfate (2000 mg/L) which yielded 65% sulfate reduction, 85%
COD removal and 320 mg/L sulfide production values. Whereas Cu was precipitated separately using sulfide produced from ABR at low pH (<2) within 60 min, Zn did not precipitate at this pH.
Following Cu removal, Zn recovery was achieved based on sulfide/Zn ratio with a Zn removal efficiency in the range 84-98%. Cu and Zn were precipitated as CuS and ZnS, respectively, with a
particle size in the range 10-50 µm. Table 2.6 presents the metal removal and recovery values with multi-stage bioreactor systems reported in the literature.
Table 2.6 Metal recovery using different multi-stage bioreactor systems Reactor type Metal Metal removal
(%)
Metal recovery (%)
References Hollow fiber
membrane bioreactor (MBR)
Al Cd Co Cu Fe Mn Ni Zn
Not available (NA)
99.8 99.7 99.1 99.8 97.1 87.4 47.8 100.0
Tabak et al., 2003
Anaerobic baffled reactor (ABR)
Cu Zn
84-98 100
NA Sahinkaya
et al., 2009
2.3.3 Single-stage vs multi-stage system
Table 2.7 compares the different parameters on metal sulfide precipitation using single-stage and multi-stage systems for the treatment of heavy metal containing wastewater by biological sulfate reduction.
Single-stage system is less labor intensive, quick, cost-effective and involves low operation as well as maintenance costs (Johnson et al., 2006; Villa-Gomez et al., 2011). Moreover, single-stage system has low area requirement, but the main advantage of multi-stage system is easy and selective metal recovery (Tabak et al., 2003).
Table 2.7 Comparison of advantages and disadvantages of single vs multi stage reactor systems
Parameter Single stage system Multistage system
Labor requirement Low High
Treatment area Less More
Metal recovery Difficult Easy
Cost Low High
Operation and maintenance expenditure
Low High
Inhibitory effect on SRB Low High