in pH at the negatively charged electrode zone (anode zone) of the field. The
accumulation of divalent metal precipitates reduces the permeability of the soil
around the anode, thereby increasing the electric potential required to continue the
mobilization and recovery process to the point where further increase of potential is
not economically feasible. The inhibition of transport of dissolved charged species
due to precipitate accumulation at the anode is called the “near-anode focusing”
(Li et al. 2015). Strategies such as pulse charge (Reddy and Karri 2006), charge
reversal (Hunter and James 1992), and acid enhancing (Maturi and Reddy 2008)
have been applied in certain scenarios in the past to increase the recovery times by
decreasing or all together eliminating the near-anode focusing effect.
6.6 Biomineralization and Biocrystallization
The process of biomineralization and biocrystallization attempts to reduce the
metallic species to the zero-valent state and recover the metal in its pure form. The
metal can be recovered by physical means such as scrapping or used as an in situ
conductor collecting electrons (Yong et al. 2002; Foulkes et al. 2016; Lloyd et al.
2008). This biotechnological innovation has shown potential in the recovery of
precious metals (Mabbett et al. 2006) or in the assembly of biosynthetic electrodes
in microbial fuel cells (Yong et al. 2002). In the study by Yong et al. (2002), the
researchers managed for the first to demonstrate the biological reduction of palladium from palladium (II) to palladium (0) which resulted in the production of the
metallic form – Pd(0) – which was detected on cell surfaces using XRD and EDX
methods. The crystalline nature of the precipitate was confirmed by SEM-XRD
scans where the predominant element on the cell wall deposits was shown to be
palladium (0).
The process of biocrystallization will definitely be of economic importance in the
field of bio-purification and biorecovery of several beneficial products from the
energy conservation processes.
7 Environmental Application Strategies
As indicated above, the utilization of biological systems to remove toxic metals from
water, soil, and environmental systems has been investigated extensively since the
late 1980s. Metals can be removed through reduction to precipitable species and
oxidation to precipitable species or by biosorption taking advantage of the
ion-exchanging properties of cell surfaces of bacteria, fungi, and/or algae. The
following are examples of processes that have been studied extensively by the
research group at the University of Pretoria and our collaborators overseas:
40
E. M. Nkhalambayausi-Chirwa et al.
accumulation of divalent metal precipitates reduces the permeability of the soil
around the anode, thereby increasing the electric potential required to continue the
mobilization and recovery process to the point where further increase of potential is
not economically feasible. The inhibition of transport of dissolved charged species
due to precipitate accumulation at the anode is called the “near-anode focusing”
(Li et al. 2015). Strategies such as pulse charge (Reddy and Karri 2006), charge
reversal (Hunter and James 1992), and acid enhancing (Maturi and Reddy 2008)
have been applied in certain scenarios in the past to increase the recovery times by
decreasing or all together eliminating the near-anode focusing effect.
6.6 Biomineralization and Biocrystallization
The process of biomineralization and biocrystallization attempts to reduce the
metallic species to the zero-valent state and recover the metal in its pure form. The
metal can be recovered by physical means such as scrapping or used as an in situ
conductor collecting electrons (Yong et al. 2002; Foulkes et al. 2016; Lloyd et al.
2008). This biotechnological innovation has shown potential in the recovery of
precious metals (Mabbett et al. 2006) or in the assembly of biosynthetic electrodes
in microbial fuel cells (Yong et al. 2002). In the study by Yong et al. (2002), the
researchers managed for the first to demonstrate the biological reduction of palladium from palladium (II) to palladium (0) which resulted in the production of the
metallic form – Pd(0) – which was detected on cell surfaces using XRD and EDX
methods. The crystalline nature of the precipitate was confirmed by SEM-XRD
scans where the predominant element on the cell wall deposits was shown to be
palladium (0).
The process of biocrystallization will definitely be of economic importance in the
field of bio-purification and biorecovery of several beneficial products from the
energy conservation processes.
7 Environmental Application Strategies
As indicated above, the utilization of biological systems to remove toxic metals from
water, soil, and environmental systems has been investigated extensively since the
late 1980s. Metals can be removed through reduction to precipitable species and
oxidation to precipitable species or by biosorption taking advantage of the
ion-exchanging properties of cell surfaces of bacteria, fungi, and/or algae. The
following are examples of processes that have been studied extensively by the
research group at the University of Pretoria and our collaborators overseas:
40
E. M. Nkhalambayausi-Chirwa et al.