**1.2 Crowns and bridges**

Crown is technically a "Cap" placed on a tooth to protect it from fracture or sensitivity. On the other hand a bridge is a fixed replacement of missing teeth, with support from adjacent teeth. Both of these function similarly on biological and biomechanical terms, hence discussed together. These are of either *Porcelain fused to metal* (PFMs) or *full ceramics*. In case of PFMs, a metal core is placed in the tooth surface and ceramic is built on it. This is done by initially preparing the metallic portion by conventional casting techniques. Then the ceramic powder is incrementally painted on it and sintered. This is followed by glazing. In case of full ceramics, the wax pattern is prepared for the crown, it is invested and mould space prepared by lost wax technique. The ceramic is fused and typically pressed into the mould space.

Fig. 1. Ceramic Crowns and their anatomy.

Fig. 2. Clinical picture of crown. Note pink ceramic used of gum recession.

The classification based on microstructure will be dealt with in detail and an idea of classification by processing technique is added in this review. Other classifications are of

Basically ceramics are used as indirect restorative materials such as crowns and bridges, Inlays/Onlays and dental implants. Recently ceramic braces are used in orthodontics.

Crown is technically a "Cap" placed on a tooth to protect it from fracture or sensitivity. On the other hand a bridge is a fixed replacement of missing teeth, with support from adjacent teeth. Both of these function similarly on biological and biomechanical terms, hence discussed together. These are of either *Porcelain fused to metal* (PFMs) or *full ceramics*. In case of PFMs, a metal core is placed in the tooth surface and ceramic is built on it. This is done by initially preparing the metallic portion by conventional casting techniques. Then the ceramic powder is incrementally painted on it and sintered. This is followed by glazing. In case of full ceramics, the wax pattern is prepared for the crown, it is invested and mould space prepared by lost wax

technique. The ceramic is fused and typically pressed into the mould space.

Fig. 2. Clinical picture of crown. Note pink ceramic used of gum recession.

academic interest.

**1.2 Crowns and bridges**

Fig. 1. Ceramic Crowns and their anatomy.

Fig. 3. All Ceramic Crown.

Similarly, ceramic teeth are manufactured in various shades, shapes and sizes to be used in complete dentures. Also, in case of gum recession, in fixed prosthesis, pink coloured ceramics are placed in the lost gum region to make it look natural.


Recent Applications of Bioremediation and Its Impact

*Amara Dar and Arooj Naseer*

### **Abstract**

Socioeconomic concerns have increased the technology dependence to facilitate the increasing population on the earth. Number of anthropogenic sources are responsible for contaminating the natural environment. Effluents from industries contain many toxicants that cause lethal effects on human and animal life on earth. Many techniques are used so far for the abatement of such pollutants from the environment. As "nature heals itself" so dealing with such problems with bioremediation utilizing the invisible workers (microorganisms), plants and enzymes can help minimize and get rid of such pollutants. It is a greener way to conserve the environment and get rid of such awful substances. Bioremediation can help to get rid of contaminants either by in situ or ex situ approach. By using both ways, either ex situ or in situ, the decontamination of the environment can be successfully done. Using various plant materials and microorganisms by tailoring the surrounding environment to make it suitable for rectifying the contaminant issue is the main goal of bioremediation.

**Keywords:** bioremediation, microorganisms, enzymes, plants, sustainable development

### **1. Introduction**

Bioremediation is a process of converting harmful substances to environmentally safe substances by the action of the invisible workforce. This invisible workforce is the number of microorganisms working in sequence to degrade environmentally toxic substances. Bioremediation works with detoxification and eradication of chemically diverse and physically hazardous materials that cause a threat to the natural existence of the environmental setup.

Ecologically discussing bioremediation refers to the interaction between three factors; Contaminant, invisible workforce, and environment, as shown in **Figure 1**. Interaction of these factors in turn ensures the mobility of contaminant in the environment, the presence of suitable conditions to degrade the contaminant, and eradication or degradation of the contaminant by converting it into an environmentally friendly substance. Mobility or bioavailability of any contaminant is about the ease with which the contaminant is available to microorganisms. The microorganisms need a suitable set of conditions (like; as availability of electron acceptors, pH, and availability of nutrients) to function well and convert the environmentally harmful substances to environmentally benign substances. The biodegradability of the contaminant depends

#### **Figure 1.**

*Ecological interpretation of factors governing bioremediation [1].*

upon the presence of suitable microorganisms to eradicate that contaminant under the required conditions [2].

Eradication of contaminants depends primarily on the nature of the contaminant, which may include pesticides, herbicides, heavy metals, hydrocarbons, sewage, plastics, etc. nature of the contaminant, degree of contamination, environmental factors, contaminated sites, cheap policies for conserving environment are important selection study criteria that are considered while choosing any bioremediation technique [3, 4]. Although it is important to properly plan the selection criteria but other factors that involve the aerobic and anaerobic nature of the area under study, pH, and moisture content are equally important to be considered. Bioremediation strategies make it possible to increase the efficacy of the contaminant removal process. These strategies may be ex situ or in situ. Mostly, the bioremediation techniques work for the removal of hydrocarbon contaminating species from soil or water [5–8]. Various other cost-effective techniques can be efficiently applied to the contamination sites for the removal of hydrocarbons [9].


**3. Discussion**

*Goats (Capra) - From Ancient to Modern*

DTHs mortalities [11, 21].

relationships [4].

In contrast to cattle, many of Cestodes, Trematodes and Nematodes readily cause disease in goat as well as in sheep (**Figure 2**). Recently, some data also highlights differences in caprine and ovine species/strains, especially for nematodes [4, 19]. In goats, it is understood that they tend to accumulate parasites, which is assessed from constant monitoring of increasing number of eggs, keeping in view about seasonal differences in excretion [20]. Sheep acts in reverse [11]. In developed nations, the main magnitudes of these infections is reflected as spartan losses of production. Whereas in underdeveloped/developing countries it translates in more aggravate

After goat and sheep domestication, both independently settled down to different feeding habits. The sheep are grazers and prefer to take grass and broadleafed plant. Goats, on the other hand, are classified as browsers or intermediate browsers. They can ingest substantial amounts of woody plants, vines and brush according to their liking [3]. These feeding habits could upshot to sources of DTHs infestation and with distinct strategies with major consequences to host-parasite

In the caprine evolutionary processes, adaptation to this high miscellany of plants, direct for three consequences to regulate parasitic populations. They include

**160**

**Figure 2.**

**Figure 1.**

*Developmental stages of immune cells during Helminth infection.*

*Growth of sheep and goat population in the last 20 years.*

In goats, full immune response expression, seems to be delayed by 6 months i.e. 12 months versus 6 months in goat to sheep [22]. Immune differences in expression between the two hosts are also been documented [4, 23]. It is also assumed that goats tends to accumulate parasites more than sheep. Because of goats weak recognition, and expulsion systems, larval reduction and expulsion of larval or/and adult worms are rarely observed [11, 24]. DTH infections under ordinary circumstances could be reduced as a result in changes to; (i) helminthes resistance by developing an immune response (ii) infective contact especially by avoidance feeding pattern of goats; and (iii) self-medication as results of alleviating worm challenges [4].

In this modern era, helminth's genomics and proteomics understanding tend to provide dependable evidences on presence of large number of immunomodulatory products. These are abridged in number of articles. We can group them in immunological phases;


3.2.6. Synthesis of [Cu(dNQ)2(DMF)2] (**5**)

HdNQ (23.5 mg, 0.1 mmol) was dissolved in DMF (10 mL) and warmed to 60 ◦C. While continuously stirring, 10 mL of DMF solution of CuCl2 (8.5 mg of CuCl2·2H2O, 0.05 mmol) (warmed to 60 ◦C) was added. After 30 min of stirring, the beaker was laid

down at room temperature. After two days, green prisms of **5** had formed, and were filtered off, and dried in the air.

[Cu(dNQ)2DMF2] (**5**)—Calc. for C18H8N6O10Cu (531.84 g·mol−1 ): C, 40.65; H, 1.52; N, 15.80%. Found: not measured, IR (ATR, cm−1 ): *ν*(C–H)ar 3058 (vw), *ν*(C–H)al 2968 (vw), 2928 (vw), 2860 (vw), *ν*(C=O) 1651 (s), *ν*(C=C)ar 1601 (m), 1591 (m), 1525 (m), 1497 (m), *ν*(N–O)as 1569 (m), *ν*(C=N) 1454 (m), *ν*(C–C)ar 1399 (m), 1374 (m), 1260 (m), *ν*(N–O)sym 1323 (m), *β*(C–H) 1182 (m), 1151 (m), *ν*(C–O) 1122 (m), *δ*(CH3) 1098 (m), *δ*(NO2) 922 (m), 705 (m), *γ*(C–H) 831 (m), 801 (m), Ring breathing 756 (m), 733 (s), *β*(CCC) 660 (m), *β*(CNC) 591 (w), *β*(C–O) 527 (m), *γ*(CCC) 470 (w), 450 (w).

### 3.2.7. Synthesis of [Cu(ClNQ)2] (**6**)

HClNQ (22.5 mg, 0.1 mmol) was dissolved in 10 mL of DMF and warmed to 60 ◦C. While continuously stirring, 10 mL of DMF solution of CuCl2 (8.5 mg of CuCl2·2H2O, 0.05 mmol) (warmed to 60 ◦C), was added. After 30 min of stirring, the beaker was laid down and the precipitate of **6**, that formed during stirring, was filtered off. The mother liquor was laid to crystallise at room temperature. After five days, green needles of **6** had formed, and were filtered off, and dried in the air. The identity of the powder and crystals was verified by IR spectroscopy.

[Cu(ClNQ)2] (**6**)—Calc. for C18H8Cl2N4O6Cu (510.73 g·mol−1 ): C, 42.50; H, 1.78; N, 10.97%. Found: C, 42.33; H, 1.58; N, 10.97%. IR (ATR, cm−1 ): *ν*(C–H)ar 3078 (vw), *ν*(C=C)ar 1599 (m), 1482 (s) *ν*(N–O)as 1561 (m), *ν*(C=N) 1434 (s), *ν*(C–C) 1384 (m), 1374 (m), 1249 (m), 1220 (s), *ν*(N–O)sym 1323 (s), *ν*(C–O) 1123 (m), *β*(C–H) 1148 (m), 1052 (m), *ν*(C5–Cl) 984 (m), *δ*(NO2) 934 (m), 679 (m), *γ*(C–H) 831 (m), 818 (s), Ring breathing 754 (s), *β*(CCC) 726 (m), 679 (m), 655 (s), *β*(CNC) 613 (w), *β*(C–O) 535 (m), *β*(C5–Cl) 510 (w), *γ*(CCC) 476 (m).

### *3.3. Physical Measurements*

Infrared spectra of prepared complexes were recorded on a Nicolet 6700 FT-IR spectrometer from Thermo Scientific with a diamond crystal Smart OrbitTM, in the range 4000–400 cm−1 . Elemental analyses of C, H, and N were with a CHNS Elemental Analyzer varioMICRO from Elementar Analysensysteme GmbH. Absorption spectra were measured with a SPECORD 250 spectrophotometer (Analytik Jena, Jena, Germany), from 300 to 600 nm, in Nujol, and DMSO and DMSO/water (1:1) solutions at 24, 48 and 72 h intervals. IR and UV-Vis spectra were described in Origin 2022b [56]. The morphological characteristics of the samples were studied using a JEOL JSM 6510 scanning electron microscope (W cathode, 20 nm resolution at 1 kV). Semi-quantitative chemical analysis of the samples was determined using the attached Oxford Instruments EDS analyser INCA X act. Measurements were performed on native samples without any conductive coating.

### *3.4. X-ray Structure Analysis*

The crystal structures of **3** and **5** were determined using an Oxford Diffraction Xcalibur2 diffractometer equipped with a Sapphire2 CCD detector, while the structures of **1a**, **1b** and **6** were determined using a Rigaku XtaLAB Synergy S diffractometer with Hybrid Pixel Array detector (HyPix-6000HE). CrysAlis Pro software was used for data collection and cell refinement, data reduction and absorption correction [57]. Structures of prepared complexes were solved by SHELXT [58] and refined by SHELXL [59], implemented in the WinGX program [60]. For all non-H atoms, anisotropic displacement parameters were refined. Hydrogen atoms of XQ and DMF molecules were placed in calculated positions and refined riding on carbon atoms. Presence of hydrogen bonds was analysed by using SHELXL, while PLATON [61], running under WinGX, was used to analyse π–π interaction. Diamond was used for molecular graphics [62]. The summary of crystal data and structure refinements for **1a**, **1b**, **3**, **5** and **6** is presented in Table 7.

*Inorganics* **2022**, *10*, 223

**Table 7.** Crystal data and structural refinement of **1a**, **1b**, **3**, **5** and **6**.

### *3.5. Cell Cultures*

The human cancer cell lines were purchased from ATCC (American Type Culture Collection; Manassas, VA, USA). HCT116 (human colorectal carcinoma), HeLa (human cervical adenocarcinoma) and Jurkat (human leukemic T cell lymphoma) were cultured in RPMI 1640 medium (Biosera, Kansas City, MO, USA) while A549 (human alveolar adenocarcinoma), MCF-7 (human Caucasian breast adenocarcinoma), Caco-2 (human colorectal adenocarcinoma) and MDA-MB-231 (human breast cancer cell line) were maintained in a growth medium consisting of high glucose Dulbecco's Modified Eagle Medium (DMEM) + sodium pyruvate (Biosera). The human kidney fibroblasts (Cos-7) were cultured in DMEM medium (Biosera). All media were supplemented with a 10% fetal bovine serum (FBS; Invitrogen, Carlsbad, CA, USA), Antibiotic/Antimycotic Solution (Sigma, St. Louis, MO, USA) and maintained in an atmosphere containing 5% CO2 in humidified air at 37 ◦C. Prior to each experiment, cell viability was greater than 95%.

### Screening of Antiproliferative/Cytotoxic Activity

The antiproliferative/cytotoxic effect of copper complexes (concentrations of 10, 50 and 100 µM) was determined by resazurin assay in HCT116, Caco-2, A549, MCF-7, MDA-MB-231, HeLa, Jurkat and Cos-7 cells. Tested cells (1 × 104/well) were seeded in 96-well plates. After 24 h, final copper complex concentrations, prepared from DMSO stock solution, were added, and incubation proceeded for the next 72 h. Ten microliters of resazurin solution (Merck, Darmstadt, Germany), at a final concentration of 40 µM, was added to each well at the end-point (72 h). After a minimum of 1 h incubation, the fluorescence of the metabolic product resorufin was measured by the automated CytationTM 3 cell imaging multi-mode reader (Biotek, Winooski, VT, USA) at 560 nm excitation/590 nm emission filter. The results were expressed as a fold of the control, where control fluorescence was taken as 100%. All experiments were performed in triplicate. The IC50 values were calculated from these data.

### *3.6. Antibacterial Activity*

### 3.6.1. Microorganisms Used

The tested bacteria (S. aureus CCM 4223 and E. coli CCM 3988) were obtained from the Czech collection of microorganisms (CCM, Brno, Czech Republic).

### 3.6.2. Agar Well-Diffusion Method

The antibacterial properties of the four complexes, **1b**, **3**, **4** and **6**, and their ligands, HClBrQ, HBrQ, HdNQ and HClNQ, were evaluated by the agar well diffusion method using a slightly modified process compared to [63]. Firstly, each compound was dissolved in a small amount of 100% DMSO and then dissolved to 33.6 µM solution. As a positive control, gentamicin sulfate (Biosera, Nuaille, France), with concentration 50 µg/mL, was used.

Bacteria were cultured overnight, aerobically, at 37 ◦C in LB medium (Sigma-Aldrich, Saint-Louis, MO, USA), with agitation. The inoculum from these overnight cultures was prepared by adjusting the density of the culture to equal that of the 0.5 McFarland standard (1–2 × 108 CFU/mL), by adding a sterile saline solution. These bacterial suspensions were diluted 1:300 in liquid plate count agar (HIMEDIA, Mumbai, India) resulting in a final concentration of bacteria approximately 5 × 105 CFU/mL, and 20 mL of this inoculated agar was poured into a Petri dish (diameter 90 mm). Once the agar was solidified, five mm diameter wells were punched in the agar and filled with 50 µL of samples. Gentamicin sulfate, with a concentration of 50 µg/mL, was used as a positive control. The plates were incubated for 18–20 h at 37 ◦C. Afterwards, the plates were photographed, and the inhibition zones were measured by ImageJ 1.53e software (U. S. National Institutes of Health, Bethesda, MD, USA). The values used for the calculation were mean values calculated from 3 replicate tests.

The antibacterial activity was calculated by applying the formula reported in [63]:

%RIZD = [(IZD sample − IZD negative control)/IZD gentamicin] × 100

where RIZD is the relative inhibition zone diameter (%) and IZD is the inhibition zone diameter (mm).

As a negative control, the inhibition zones of 5% DMSO equal to 0 were taken. The inhibition zone diameter (IZD) was obtained by measuring the diameter of the transparent zone.

### 3.6.3. Determination of the Minimum Inhibitory Concentration (MIC) by the Microdilution Method

The minimum inhibitory concentration was determined by the microdilution method with a slight modification of the procedure described in [64].

Stock solutions of the test samples prepared in 5% DMSO (Sigma-Aldrich, USA) were two-fold diluted (1:1 to 1:8) in wells of a 96-well plate (Greiner Bio-One, Germany): the wells of the microtiter plate were filled with 50 µL of Mueller–Hinton Broth (MHB, HIMEDIA, Mumbai, India), and 50 µL of stock solution of the test substance was added to the first well (1:1 dilution). After mixing, 50 µL of this solution was transferred to the next well (1:2 dilution), etc. Each sample was tested in triplicate.

Bacteria were cultured overnight, aerobically, at 37 ◦C in MHB, with agitation. The inoculum from the overnight cultures was prepared by adjusting the density of bacterial suspension to 0.5 McFarland standard (1–2 × 108 CFU/mL) by adding sterile saline and then diluting 1:150 in MHB. Subsequently, 50 µL of this inoculum was added to each well with 50 µL of diluted test samples (final concentration of bacteria in the well ca. 5 × 105 CFU/mL). Wells filled only with MHB medium and bacterial suspension were used as a positive control, and wells filled with sterile MHB alone were used as a negative control. The plates were covered with a lid and incubated for 18–20 h at 37 ◦C. The evaluation was made by measuring the absorbance at 600 nm on a microplate reader (Synergy HT, Biotek, Santa Clara, CA, USA).

### *3.7. Radical Scavenging Experiments*

Radical scavenging activity of complexes and free ligands was estimated by DPPH (2,2-diphenyl-1-picrylhydrazyl) and ABTS (diammonium 2,20 -azino-bis(3-ethylbenzothiazoline-6-sulfonate) radicals, according to slightly modified methods described in literature [65,66]. Compounds were dissolved in DMSO and the resulting solutions were diluted with MeOH in 1:3 ratio. The prepared solutions were mixed with methanolic solutions of the respective radicals and incubated in the dark at room temperature (7 min ABTS and 30 min DPPH). The absorbance was recorded at 734 nm (ABTS) and 517 nm (DPPH), respectively. L-ascorbic acid was used as a standard, and experiments were performed in triplicate. The IC50 parameters were calculated from a linear plot of the inhibition percentage against the concentration of compounds.

### **4. Conclusions**

In this work, we present six new copper(II) complexes: [Cu(ClBrQ)2] (**1a**, **1b**), [Cu(ClBrQ)2]·1/2 diox (**2**) (diox = 1,4-dioxane), [Cu(BrQ)2] (**3**), [Cu(dNQ)2] (**4**), [Cu(dNQ)2(DMF)2] (**5**) and [Cu(ClNQ)2] (**6**), where HClBrQ is 5-chloro-7-bromo-8-hydroxyquinoline, HBrQ is 7-bromo-8-hydroxyquinoline, HClNQ is 5-chloro-7-nitro-8-hydroxyquinoline and HdNQ is 5,7-dinitro-8-hydroxyquinoline. Prepared complexes were characterised by IR spectroscopy (**1**–**6**), and elemental (**1b**–**4** and **6**) and X-ray structural (**1a**, **1b**, **3**, **5** and **6**) analysis. The sability of **1b**–**4** and **6** in DMSO and DMSO/water solutions was verified by UV-Vis spectroscopy.

Complexes **1a**, **1b**, **3**, **5** and **6** were molecular compounds. Crystal structures of **1a**, **1b**, **3** and **6** formed by square planar [Cu(XQ)2] complexes, in which copper atoms were coordinated by two pairs of oxygen and nitrogen atoms from two deprotonated XQ ligands. On the other hand, a tetragonal bipyramidal coordination of the copper atom in **5** was observed. The equatorial plane was occupied by the same atoms as in the mentioned [Cu(XQ)2] complexes, and oxygen atoms from DMF molecules occupied axial positions. Interestingly, despite similar structures, intramolecular interactions were observed only in

**5** and **6**, where hydrogen bonds existed. They created layers parallel with (01-1) (**5**) and (100) (**6**) plane.

After cytotoxic evaluation, only complex **3** showed promising potential as an anticancer agent, but, due to the low selectivity towards non-cancerous cells, modification of the complex was required.

Among the prepared compounds only **3** was active against both tested radicals (ABTS and DPPH). Antioxidant activity could be involved in the cytotoxic effects of **3** against the tested cell line; however, the free ligand showed even better antiradical properties. Apparently, the coordination of HBrQ ligand to Cu(II) decreased its antioxidant activity.

Only the HClNQ ligand showed antibacterial potential with concentration 33.6 µM.

**Supplementary Materials:** The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/inorganics10120223/s1, Figure S1: SEM micrograph (a) of **1a** along with EDS elemental analysis (b); Figure S2: Molecular structure of **1a**; Table S1: Hydrogen bonds [Å and ◦ ] for **5** and **6**.

**Author Contributions:** Conceptualization, I.P. and M.K. (Martina Kepeˇnová); investigation, M.K. (Martina Kepeˇnová), M.K. (Martin Kello), R.S., M.G., R.F., L'.T., M.L., J.Š. and I.P.; resources, M.K. (Martin Kello), M.G., M.L. and I.P.; writing—original draft preparation, M.K. (Martina Kepeˇnová), M.K. (Martin Kello), R.S., M.G. and I.P.; Writing—Review & Editing, M.K. (Martina Kepeˇnová), R.S. and I.P.; visualization, M.K. (Martina Kepe ˇnová) and J.Š.; supervision, M.K. (Martin Kello), M.G. and I.P.; project administration, M.K. (Martin Kello), M.G. and I.P.; funding acquisition, M.K. (Martin Kello), M.L. and I.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by VEGA 1/0148/19, VEGA 1/0653/19, APVV-18-0016 and VVGS-PF-2022-2134. Moreover, this publication is the result of the project implementation, "Open scientific community for modern interdisciplinary research in medicine (OPENMED)", ITMS2014+: 313011V455, supported by the Operational Programme Integrated Infrastructure, funded by the ERDF. We also thank the Research Infrastructure NanoEnviCz project, supported by the Ministry of Education, Youth and Sports of the Czech Republic, Project No. LM2018124 for instrumentation.

**Data Availability Statement:** CCDC 2216977-2216981 contain the supplementary crystallographic data for **1a**, **1b**, **3**, **5**, and **6**. These data can be obtained free of charge via http://www.ccdc.cam. ac.uk/conts/retrieving.html, (accessed on 5 November 2022) or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: (+44) 1223-336-033; or e-mail<EMAIL_ADDRESS>
**Acknowledgments:** Special thanks go to Petra Ecorchard. (Centre of Instrumental Techniques, Institute of Inorganic Chemistry of the CAS) for the preparing of SEM samples.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

### **References**

### *Article*


**5. Indonesia**

There is a severe reduction in fossil fuel supplies in Indonesia. The current oil and gas re‐ serves are reported to be approximately 747 million cubic meters (94.7 billion barrels) of oil and *2557* million cubic meters (*90,300* billion cubic feet) of natural gas representing a 13% reduction in supplies which is significant because the demand for fossil fuels has already ex‐ ceeded the supply capacity of Indonesian oil industry [23]. Among the several alternative renewable energies available for the country to be harnessed, bioenergy has generated con‐ siderable interests and shows theoretically an adequate potential to overcome the energy shortage and create a balance between the energy demand and supplies for Indonesia. Bio‐ energy is renewable and reduces *CO2* emissions when substituted for fossil fuels [23].

Indonesia is an agrarian country and has approximately 90 million hectares of forest indicat‐ ing that it should concentrate on the development of biomass-based energy programs. At the same time it was ranked among the top ten countries on the globe encountering a net loss of forest area during the era 2000-2005 [23] indicating that bioenergy development development of agro-industrial plantations [20]. The Government of Cambodia has been providing special concession scheme to investors to invest in biodiesel production that is mainly focused on Jatropha as feedstock crops. A number of initiatives are still under either

Bioenergy, energy efficiency, waste management, deforestation and forest degradation are the potential areas for CDM projects in Cambodia. There are four approved project on bio‐ gas, one on waste/heat gas utilisation and one on biomass and completion of these projects

that the country lost 29% of its primary evergreen forests to severe degradation between 2000 and 2005 [21]. A case study is under validation to manage these degraded evergreen primary forests in Cambodia for sustained flow of timber and other ecosystem services that could lead to financial incentives through a carbon payment scheme under global climate change mitigation through reduction emissions from deforestation and forest degradation

The aggregate technical potential for electricity generation from biomass consisting of forest products, agricultural crops and residues, municipal waste and sewerage has been estimat‐ ed using computer simulation techniques at18, 852 *GWh per year*. The findings do not ex‐ plicitly indicate the provision of efficiency in this analysis. Small scale projects based on simplified technologies are most appropriate as CDM projects for Cambodia. However sev‐ eral CDM projects are in implementation or registration/validation phases but low aware‐ ness among policy makers and the private sector, weak institutional capacity, lack of human and technical resources, inappropriate policies and strategies are the major limitations to avail opportunities for carbon trading through CDM [3]. Four out of five active CDM projects on rice husk cogeneration, rubber plantation, improved cookstove and biogas in

There is a severe reduction in fossil fuel supplies in Indonesia. The current oil and gas re‐ serves are reported to be approximately 747 million cubic meters (94.7 billion barrels) of oil and *2557* million cubic meters (*90,300* billion cubic feet) of natural gas representing a 13% reduction in supplies which is significant because the demand for fossil fuels has already ex‐ ceeded the supply capacity of Indonesian oil industry [23]. Among the several alternative renewable energies available for the country to be harnessed, bioenergy has generated con‐ siderable interests and shows theoretically an adequate potential to overcome the energy shortage and create a balance between the energy demand and supplies for Indonesia. Bio‐

energy is renewable and reduces *CO2* emissions when substituted for fossil fuels [23].

Indonesia is an agrarian country and has approximately 90 million hectares of forest indicat‐ ing that it should concentrate on the development of biomass-based energy programs. At the same time it was ranked among the top ten countries on the globe encountering a net loss of forest area during the era 2000-2005 [23] indicating that bioenergy development

Cambodia would reduce emission by *4.2 MT CO2e* over a period on 7-30 years.

[4]. Literature reports

would be able to reduce an annual emission of CO2 of 204, 308 *t year* −1

planning or implementation stages.

24 Sustainable Energy - Recent Studies

(REDD-plus) scheme [22].

**5. Indonesia**

projects should be designed in such a way that do not aggravate the loss of forests for sus‐ tainability and viability for long term applications. There was approximately *13 million Mg* oven-dry-weight of forest biomass in 2005 [24] another study reports that aboveground for‐ est biomass ranged from ∼5000 *to* 11, 000 *million Mg*[25]. It is reported that the quantity of wet biomass that can be harvested from 'production forest' and 'other land with tree cover' could be approximated in three ranges which are: *5083 million Mg* (a lower bound), *5410 mil‐ lion Mg* (a moderate bound) and *10,726 million Mg* (an upper bound). The wet biomass was converted to dry weight equivalent using data from the Global Forest Resources Assessment 2005: Indonesian Country Report [26], State of the World's Forest 2005 [24]; and global For‐ est Resourses Assessment 2005: progress towards Sustainable Forest Management [27]. Sun‐ tana et al. [23] reported that if Indonesia converts forest biomass into bio-methanol for electricity generation and as a gasoline substitute then annually 10,063,731 households could be provided with electricity continuously using a *1kW* fuel cell. The results reported are ob‐ tained using widely accepted calculation methods due to Vogt et al. [23A] which uses the quantity of biomethonal produced from the annually collected forest biomass and the amount of electricity and transportation fuel that could be substituted by the biomethanol produced from the annually forest materials. With the use of only 5% of forest biomass and converting it to bio-methanol as a gasoline substitute would be equivalent to the total quan‐ tity of gasoline consumed in Indonesia during the year 2005. The use of bio-methanol as a substitute for fossil fuel to power vehicles could avoid the emissions of 8.3-34.9% of the total carbon emitted in Indonesia. Timber extraction data from the 1980s reveal that *7.5 million m3 per year* log wastes are generated during harvesting operation that corresponds to about *3.75 million Mg* biomass and is equivalent to collecting biomass materials from 124, 000 *ha year* −1 of forest land. 29.5 million litres of bio-methanol can be produce with an efficiency of 25% and could avoid *21.7* Gigagrams *(Gg)* of carbon if it is substitute for natural gas-methanol in fuel cells or *1.97 Gg* of carbon when it is used to supplement gasoline.

Indonesia is the third-largest producer of rice in the world and produced 65,150,764 metric ton in 2010 compared with 64,398,890 metric ton in 2009. Rice bran containing 13.5% oil has a potential for extraction of biodiesel. Gunawan et al. [28] studied rice bran for a potential source of biodiesel production in Indonesia and claimed that 96,000 ton of biodiesel can be obtained from rice bran per year.

Oil palms is another energy crop which were grown on 3.6 Mhectares of land in 2005 and Indonesia is strengthening its production with the increasing worldwide demand for biodie‐ sel derived from oil palms. These trees start bearing fruits approximately 30 months after planting in field and continue to be fertile for a period of 20-30 years ensuring a consistent supply of oil. The estimate for the additional land demands for palm oil plantation in 2020 range from *1 to 28 Mha* in Indonesia that can be met to a large extent by degraded land as well as agricultural management such as implementation of best management practices and earlier replanting with higher yielding plants. Palm oil production has played a major role in land use change in Indonesia [29] and it produces 44% of the world's palm oil as per re‐ cords for the year 2009. It is predicted that palm oil would be the leading internationally traded edible oil by the year 2012 [30] and the palm oil industry in Indonesia looks forward for high pressure modern power plants to cope with future demand. It is estimated that the residue of palm oil consisting of empty fruit bunch, fiber, shell, wet shell, palm kernel, fronds and trunks has a potential for annual power generation of *5000 GWh* [31]. The transi‐ tion of energy scenario from fossil fuels to biomass has been underway using existing tech‐ nologies. In order to make it practically effective requires substantial investments in infrastructure, conversion technologies and in research and development (R&D) for palm oil biomass.

Out of six regional grids in Indonesia where the electricity from the project activities can be grid-connected, primary emission reductions potentials exist in Java, Bali and Southern Su‐

Potential and Use of Bioenergy in The Association of Southeast Asian Nations (ASEAN) Countries – A Review

http://dx.doi.org/10.5772/51917

27

Utilization of palm oil mill effluent (POME) to generate electricity by minimising the emana‐ tion of methane gas could reduce GHG emission of 47, 222 *tCO*2 per year. The feasibility study of this project that was funded by NEDO Japan is completed and is expected to be considered for CDM scheme and be financed by developed countries. The expected finance

A study financed by the World Bank revealed that the country has a potential to mitigate GHG emissions of over 3 billion tons of carbon dioxide equivalent (CO2e). There are large scale possibilities for emission reductions in the energy sectors. Biomass offers a large poten‐

Wood and charcoal were the most dominated traditional energy resources for the period 1996 to 2002 that account for about 75% of the total national energy consumption. Wood is mainly used for cooking and space heating and in rural areas still accounts for up to 90% of the energy consumption. An increase of 4.8% in the total energy use with reference to period 1996-2002 is noted. The Government is keen to develop bioenergy for which more than 2 million hectares of ideal land has been initially identified for biofuels feedstock plantations which is a major step to produce enough biofuels by 2020. Protected area management sys‐ tem is enforced in Lao PDR and a recent study on improvement and implementation of pro‐ tected area management with positive interaction between people and the natural environment was conducted using a simple simulation model, the "Area Production Model" aiming at evaluating different options for land use and primary production. The findings of this research reveal that the integrated land-use planning approach was found to be well adapted to the needs of the protected area management system [38]. The Ministry of Plan‐ ning and Investment signed a Memorandum of Understanding in June 2008 with private companies to construct two biodiesel factories with a production capacity of 50,000 tones each by 2010 [39]. A production of the Biodiesel (B100) was reported on July 7, 2011 at a rate of 40,000 Litres/month (*www.linkedin.com/groups/Green-Energy-in-Cambodia-Lao-3991528)*. A rural Renewable Energy Initiative in the great Mekong Subregion reports that Lao produces 223,300 tones of sugarcane and 55,500 tones of cassava in the year 2007 indicating that Gov‐

ernment policy towards the development of bioenergy is progressing.

,equivalent to 1922 *million l year* −1

A study on "Application of biofuel supply chains for rural development and Lao energy se‐ curity measurements" was conducted in March 2008 which claims that bioethanol could substitute for 20% of gasoline use in 2030 with the production of commercially viable Jatro‐ pha biofuel in four different phases starting from 2008 to 2030 over a total land of 1.1 million hectares [40]. Bush [41] discussed that bioenergy holds enormous potential of

of diesel fuel in Laos, with an abun‐

matera grids [36].

tial for CDM projects [3].

**6. Lao, P. D. R.**

18907 *MWh year* −1

for this project could be from Japan [37].

The other feedstocks for biofuel in addition to palm oil, forest biomass and rice bran are crops waste (rubber truck, coconut, sugarcane), waste of food crop products (cassava, sjatro‐ pha, sorghum,, soybeans, peanuts, maize, paddy) account for 12.77×106 tonnes per year and 87.45×106 tonnes per year, respectively [32]. The crops waste are residues left in field after grain harvest. The Government of Indonesia is in the process of preparing additional land for growing high-yield feedstocks to meet the country's biofuel production goals of 5.57 mil‐ lion kiloliters of biodiesel and 3.77 million kiloliters of bioethanol [33].

The U.S. Department of commerce claimed that biomass installed capacity for energy source in Indonesia is *445MW* which is only 1% of the total resource potential of 49,810MW. The country has targeted 810MW with a conversion efficiency of about 30% of biomass power by 2025 with an increase of 83% but still it is far less than the potential contribution [33].

Research conducted at BPPT-LSDE in Indonesia reported on a plan to construct 1500 litre per day capacity biodiesel using palm oil waste. The domestic manufacturing capacity of bi‐ omass gasifier is improved and capable of producing 15−100 *kWe*for rice mill and wood mill power supply as well as for rural electrification. The Indonesian Government has been fo‐ cusing its policy on bioenergy diversification and introduced a huge plantation of Jatropha curcas as an additional biodiesel source which is non-edible and has well known potential to be converted into biodiesel [34].

Jupesta [35] studied technological changes in the biofuel production system in Indonesia us‐ ing mathematical modelling consisting of two scenarios: the base scenario and the technolo‐ gy scenario. The base scenario assumes the conditions and data set in the Indonesian Government's Mix Energy policy that relies on an increase in biofuel production by increas‐ ing the land allocation for biofuel while the technology scenario concentrates technical change consisting of growth in yield and a cost reduction in addition to the growth in land allocation. The author reported that the highest contribution is likely to come from palm oil that accounts for 93% and 64% of the technology scenario and the base scenario, respective‐ ly. The excess production for export increases in both scenarios. But the technology scenario gives more competitive results.

The substantial amount of bagasse in sugar mills can provide fuel for electricity-generating projects in Indonesia that will most probably be considered for the Clean Energy Develop‐ ment Mechanism (CDM) scheme. A recent study concluded that this source has a potential of 260, 253 *MWh* that could generate a Greenhouse Gas (GHG) emission reduction of 240, 774(large scale) or 198, 177 *tCO*2(small scale) annually. The present low efficiency co‐ generation for those values lead to the earning of about US\$1.36 or 1.12 million respectively. Out of six regional grids in Indonesia where the electricity from the project activities can be grid-connected, primary emission reductions potentials exist in Java, Bali and Southern Su‐ matera grids [36].

Utilization of palm oil mill effluent (POME) to generate electricity by minimising the emana‐ tion of methane gas could reduce GHG emission of 47, 222 *tCO*2 per year. The feasibility study of this project that was funded by NEDO Japan is completed and is expected to be considered for CDM scheme and be financed by developed countries. The expected finance for this project could be from Japan [37].

A study financed by the World Bank revealed that the country has a potential to mitigate GHG emissions of over 3 billion tons of carbon dioxide equivalent (CO2e). There are large scale possibilities for emission reductions in the energy sectors. Biomass offers a large poten‐ tial for CDM projects [3].


**1. Introduction**

It is well-known that bismuth layer structure ferroelectrics (BLSFs) are one of the important ferroelectric oxides, which have a general formula (Bi2O2) 2+ (Am−1BmO3m+1) 2−, and their crystal structure composed of pseudo-perovskite blocks (Am−1BmO3m+1) 2− interleaved with bismuth oxide layers (Bi2O2) 2+ along the *c*-axis [1–3]. Generally, A represents a tetravalent, pentavalent, and hexavalent ion (such as k+ , Li1+, Zn2+, Ca2+, Sr2+, Cr3+ , or La3+) [4], or the mixture of them. About B, it represents a tetravalent, pentavalent, or hexavalent ion (such as Ti4+, Ta5+, Nd5+). m is the number of BO6 octahedra in the pseudoperovskite block (m = 1, 2, 3, 4, or 5) [5]. The CaBi4Ti4O15(CBT) shows the structure of A21am space group at room temperature, composing four perovskite-like TiO2 octahedron units stacked in between (Bi2O2) 2+ layers.

For Aurivillius oxides, CBT ceramics attracted much attention from years ago, with simple preparation, transferring speed, a high fatigue strength, and low leakage current density, which are widely used in large equipment [6]. With the advancement of the aerospace industries, the research of high temperature piezoelectric acceleration sensor is urgent and necessary. Due to the high cure temperature (*T*c = 790 ◦C) [7] and excellent

**Citation:** Wu, D.; Zhou, H.; Li, L.; Chen, Y. Gd/Mn Co-Doped CaBi4Ti4O15 Aurivillius-Phase Ceramics: Structures, Electrical Conduction and Dielectric Relaxation Behaviors. *Materials* **2022**, *15*, 5810. https://doi.org/10.3390/ma15175810

Academic Editor: Andres Sotelo

Received: 26 July 2022 Accepted: 16 August 2022 Published: 23 August 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

fatigue resistance [8,9], Bismuth layered piezoelectric ceramics are widely used in piezoelectric acceleration sensors. However, the low piezoelectric property limits the application of Pure CBT, because its own layer structures limit the material transportation when sintering progress and spontaneous polarization (along *a-b* plane) [10–12]. Moreover, a low spontaneous polarization (*P*s) and higher coercive field (*E*c) requires higher polarization voltage, and high electrical conductivity leads to high leakage current [13]. Therefore, it is of certain significance to study the high-temperature conductivity of CBT for operating in high-temperature environment. For Aurivillius piezoceramics, it is necessary to study electrical resistivity and conduction behavior at high temperature. Until now, many studies about CBT have been reported that concentrated on the structures and how to improve the *T*c or piezoelectricity [14–16]. For example, Gd3+ was found to reduce the leakage current and low loss [17]. Generally, the *p*-type conduction is mainly a conducting type for Aurivillius piezoceramics. As such, the dc conductivity can be reduced by donor doping [18]. There are few studies about the conduction behavior of CBT. For example, Xie et al. doped W+ into CaBi4Ti4O15 piezoceramics, the relaxation activation energy of the doped system was 1.45 eV, and its hopping conduction energy was 1.50 eV, while dc conduction energy was 1.39 eV [19], but the *d*33 of this system was only 17.8 pC/N. Many studies revealed that V5+, Nb5+, and W6+ can decrease the high-temperature conductivity and increase the piezoelectric properties of BLSF ceramics, since these donor-type substituted ions could release the distortion of the oxygen octahedral, as well as reduce the concentration of oxygen vacancies in the lattice [20–22]. This means that CBT ceramics may have two different conductive types at different temperatures. However, there are many studies on the conduction mechanism of bismuth layered oxide ceramics and various mechanisms are still not widely adopted. Therefore, it would be necessary to study the conductance mechanism of the CaBi4Ti4O15 ceramics, which is conducive to understanding of the microscopic motion energy of charge carriers [23].

In this work, a kind of Gd/Mn co-doped CaBi4Ti4O15 ceramics were prepared using the solid-state reaction method and the structures of samples were characterized by using XRD and SEM. The effects of Gd/Mn co-doping on the electrical conduction and dielectric relaxation behaviors of CaBi4Ti4O15 were studied in terms of the temperature dependent conductivity spectrum and electrical modulus analysis, with emphasis on the thermally activated motion of ionic defects, which predominates the dielectric behaviors at high temperature.