Solid Matrices as a Metal Source: Comparison
Please note this is a comparison between Version 2 by Conner Chen and Version 1 by Carlos Ocampo-López.

Metal recovery through recycling refers to reprocessing waste into new metal products to reduce greenhouse gas emission levels, conserve natural resources, and manage energy consumption. In Europe, metal recycling in household solid waste is integral to sustainable waste management. Metal supply depends on specific geological, physical, and industrial conditions, so one supply metric does not fit all metals. With waste production, a growing pool of metals is created, a pool of renewable resources that can replace limited metals from enrichment deposits in the geosphere.

  • biological extraction
  • metals
  • bioprocess
  • agricultural wastes

1. Solid Matrices as a Metal Source

Metal supply depends on specific geological, physical, and industrial conditions, so one supply metric does not fit all metals [10][1]. With waste production, a growing pool of metals is created, a pool of renewable resources that can replace limited metals from enrichment deposits in the geosphere [5][2].
Waste is defined as an object or substance that is discarded after its initial use. The broadest category of waste is solid waste, which can be divided into different types, agricultural waste, municipal solid waste, and industrial waste [31][3].
More than 12 billion tons of solid waste are produced annually worldwide [31][3], including approximately 1.55 billion tons of agricultural waste [32][4] and about 2 billion tons of municipal solid waste [31][3]. The generation of industrial waste is almost 18 times greater than that of municipal solid waste [33][5].
The wastes mentioned above contain metals in various concentrations [3,6,33,34,35,36[5][6][7][8][9][10][11][12][13][14],37,38,39,40], which have the potential as matrices to be subjected to the process of extraction of metals in the form of dissolution using the bioleaching process.
Figure 1 2 shows the different solid matrices with metal contents (SMMC) with the potential to be recovered by bioleaching.
Figure 21.
SMMC with the potential to be recovered by bioleaching. Source: Authors.

2. Agricultural Residues

Crop residues are relevant raw materials for metal extraction. Residues such as peanut shells, cow horn, corn cob ash, sugar cane bagasse, and rice husk are rich in silicon, calcium, magnesium [3[6][15],41], and other trace elements such as Fe, Al, P, Mg, Zn, and Mn [42][16].
Meanwhile, based on the FAO’s State of Food and Agriculture (2019) report, approximately 14 percent of global food (estimated to be worth USD 400 billion annually) still becomes lost post-harvest and before it reaches the market. Furthermore, as per the UNEP’s Food Waste Index Report, an additional 17 percent of our food is wasted in retail and by consumers, primarily in households. According to FAO’s calculations, the combined amount of lost and wasted food could provide sustenance for 1.26 billion undernourished individuals each year [43][17].
At a regional level, Sub-Saharan Africa has the highest losses at 21.4 percent. Least Developed Countries (LDCs) and Small Island Developing States (SIDS) also experience significant losses, with 18.9 percent and 17.3 percent, respectively. Structural deficiencies in these regions result in substantial food losses between the farm and retail levels. East and Southeast Asia also report significant food losses at 15.1 percent, mainly due to significant losses in the fruit and vegetable value chains. The lowest losses occur in Latin America and the Caribbean (12.3 percent) and Europe and North America (9.9 percent). All regions, except for Central and Southern Asia, record an increase in estimated losses in 2020 compared to 2016, with the highest increases observed in SIDS (an additional 1 percent), Oceania (an additional 1.2 percent), and North Africa and Western Asia (an additional 1.7 percent) [43][17].
It is estimated that the global percentage of food lost after harvest in farming, transportation, storage, wholesale, and processing was 13% in 2016 and 13.3% in 2020. These percentages correspond to a food loss index of 98.7 in 2016 and 101.2 in 2020 [44][18].
Even some typical wastes, such as ashes from sugarcane bagasse, rice husk, corn cob, bamboo leaf, corn stalk, and palm kernel husk, are regularly employed as metal matrices to reinforce materials [41][15].
On the other hand, the chemical composition of coconut shell contains mainly calcium oxide, followed by silica, along with traces of Al, K, Fe, and P, in the form of oxides or chlorides [3][6]. Corn leaves and cacao plants contain Cd [39][13], cattle manure constitutes an essential source of metallic elements such as Zn and Cu [35][9], and pig manure contains Cu, Zn, and Mn [36][10].
Studies by Ramírez-Carmona et al. resulted in a map of agricultural biomass residues in Antioquia, Colombia, showing the metal concentrations of 92 species and the Tm/ha produced per crop. For example, metals are found in papaya residues, 1993 mg/kg of P and 6800 mg/kg Mg; branch onion residues, 1060 mg/kg of P and 10,807 mg/kg of Fe; paprika residues, 680 mg/kg of P; and citron residues, 104,000 mg/kg of Ca, among others [42][16].

3. Municipal Solid Waste

Over 2 billion tons of solid urban waste is generated annually, with approximately 33% being improperly managed. This mismanagement poses significant health and environmental risks, including water, soil, and air contamination. Improper disposal methods such as open burning of hazardous waste not only harm waste workers and neighboring communities but also increase the vulnerability of children to adverse health outcomes [45,46][19][20].
Thirty-seven percent of municipal solid waste is disposed of in landfills, 33% in open spaces; 19% is recycled, and 11% is incinerated. These wastes contain metals due to industrial and anthropic activities [47][21], involving metals such as Zn, Pb, Cu, Cr, Hg [37[11][22][23][24],48,49,50], Cd [37[11][22][23],48,49], Ni [49][23], Co and As [37][11]. In addition, household wastes such as eggshells are rich in Ca [33][5]. In the study by Kuusiola et al., the compositions of the metal fractions of 54% tinned steel, 15% stainless steel, 24% aluminum, and 8% other metals are shown [6][7]; likewise, in landfills, the concentrations of leached metals are 108.5 g/Mg of Pb, 90 g/Mg of Cu, 560 g/Mg of Zn, 101.5 g/Mg of Cr and 2.24 g/Mg of Cd [48][22].
In municipal solid waste incineration processes, metals such as Zn, Cu, Cr, Pb, Ni, Cd, and Hg are found [49,50][23][24]. Zhang et al. reported that after 15 days of bioleaching, 80.7–82.1% of Cd, 72.5–74.1% of Zn, 42.8–43.9% of Cu, and 24.1–25.2% of Cr and 12.4–13.0% of Pb were removed [49][23].
Similarly, studies in Poland showed that metal concentrations in the soil near the municipal solid waste landfill were similar to geochemical reference levels in forest and agricultural soils [48][22].

4. Industrial Wastes

Industrial waste is produced by industrial activity, including any material rendered useless during manufacturing, such as from factories, industries, mills, and mining operations. Industrial waste includes soil and gravel, masonry and concrete, and electronic waste, among others [8][25].
Every year, American industrial facilities generate and dispose of around 7.6 billion tons of industrial solid waste. Additionally, approximately 54 million tons of e-waste, including electronic devices like TVs, computers, and phones, are produced annually, with a projected increase to 75 million tons by 2030. However, in 2019, only 17% of e-waste was documented as being adequately collected and recycled. Improper management of e-waste and its components poses significant risks, leading to adverse health and developmental effects, particularly in young children [46,51][20][26].
Metallurgical slag is among the most abundant by-products generated by such industries [3,9][6][27]. The production of this waste is used to recover iron [52][28]. On the other hand, titanium is concentrated in the blast furnace titanium slag as vanadium titanomagnetite, which contains SiO2, Al2O3, CaO, and MgO [53][29], and the solid residues from silica sand purification contain metals such as Ba and Cd [3][6].
Raw materials for vanadium products include the vanadium–titanium magnetite, vanadium slag, coal, petroleum coke, fly ash, and spent catalysts [54][30].
The WEEE, commonly called e-waste, contains up to 60 different elements in various concentrations, comprising base metals, critical metals, and platinum group metals mixed in a complex matrix of metallic and non-metallic materials [11,55][31][32]. For example, Au, Ag and Pd, Cu, and Ni are recovered from used cell phone printed circuit boards, with 540–880 mg/kg Ag, 168 mg/kg Au and 110 mg/kg Pd [33,34][5][8].
Industrial wastes such as fly ash, gypsum, and red mud are alumina, silica, and zeolite sources. Likewise, sewage sludge ashes contain metals such as Cu, Cr, Cd, Ni, Pb, Zn, and Co [39,40,56][13][14][33].
Finally, depleted mines, although they increase the percentage of waste generated [9][27], contain a diversity of metals in low concentrations [57][34] that can be extracted by biological rather than chemical methods, as mentioned above.

References

  1. Moreau, V.; Dos Reis, P.; Vuille, F. Enough Metals? Resource Constraints to Supply a Fully Renewable Energy System. Resources 2019, 8, 29.
  2. Steinbach, V.; Wellmer, F.-W. Consumption and Use of Non-Renewable Mineral and Energy Raw Materials from an Economic Geology Point of View. Sustainability 2010, 2, 1408–1430.
  3. Kuthiala, T.; Thakur, K.; Sharma, D.; Singh, G.; Khatri, M.; Arya, S.K. The Eco-Friendly Approach of Cocktail Enzyme in Agricultural Waste Treatment: A Comprehensive Review. Int. J. Biol. Macromol. 2022, 209, 1956–1974.
  4. Ramirez-Carmona, M.; Muñoz-Blandón, O. Agroindustrial Waste Cellulose Using Fermented Broth of White Rot Fungi. Rev. Mex. Ing. Quim. 2016, 15, 23–31.
  5. Medici, F. Recovery of Waste Materials: Technological Research and Industrial Scale-Up. Materials 2022, 15, 685.
  6. Yadav, V.K.; Yadav, K.K.; Tirth, V.; Gnanamoorthy, G.; Gupta, N.; Algahtani, A.; Islam, S.; Choudhary, N.; Modi, S.; Jeon, B. Extraction of Value-Added Minerals from Various Agricultural, Industrial and Domestic Wastes. Materials 2021, 14, 6333.
  7. Kuusiola, T.; Wierink, M.; Heiskanen, K. Comparison of Collection Schemes of Municipal Solid Waste Metallic Fraction: The Impacts on Global Warming Potential for the Case of the Helsinki Metropolitan Area, Finland. Sustainability 2012, 4, 2586–2610.
  8. Ippolito, N.M.; Medici, F.; Pietrelli, L.; Piga, L. Effect of Acid Leaching Pre-Treatment on Gold Extraction from Printed Circuit Boards of Spent Mobile Phones. Materials 2021, 14, 362.
  9. Sydow, M.; Chrzanowski, Ł.; Leclerc, A.; Laurent, A.; Owsianiak, M. Terrestrial Ecotoxic Impacts Stemming from Emissions of Cd, Cu, Ni, Pb and Zn from Manure: A Spatially Differentiated Assessment in Europe. Sustainability 2018, 10, 4094.
  10. Provolo, G.; Manuli, G.; Finzi, A.; Lucchini, G.; Riva, E.; Sacchi, G. Effect of Pig and Cattle Slurry Application on Heavy Metal Composition of Maize Grown on Different Soils. Sustainability 2018, 10, 2684.
  11. Mavakala, B.K.; Sivalingam, P.; Laffite, A.; Mulaji, C.K.; Giuliani, G.; Mpiana, P.T.; Poté, J. Evaluation of Heavy Metal Content and Potential Ecological Risks in Soil Samples from Wild Solid Waste Dumpsites in Developing Country under Tropical Conditions. Environ. Chall. 2022, 7, 100461.
  12. Susianti, B.; Warmadewanthi, I.D.A.A.; Tangahu, B.V. Characterization and Experimental Evaluation of Cow Dung Biochar + Dolomite for Heavy Metal Immobilization in Solid Waste from Silica Sand Purification. Bioresour. Technol. Rep. 2022, 18, 101102.
  13. Vácha, R. Heavy Metal Pollution and Its Effects on Agriculture. Agronomy 2021, 11, 1719.
  14. Malinowska, E.; Jankowski, K. The Effect of Different Doses of Sewage Sludge and Liming on Total Cobalt Content and Its Speciation in Soil. Agronomy 2020, 10, 1550.
  15. Joseph, O.O.; Babaremu, K.O. Agricultural Waste as a Reinforcement Particulate for Aluminum Metal Matrix Composite (AMMCs): A Review. Fibers 2019, 7, 33.
  16. Centro de Estudios y de Investigación en Biotecnología (CIBIOT); Centro de Investigación e Innovación Energía (CIIEN); Universidad Pontificia Bolivariana. Biomasa de Residuos Agrícolas en el Departamento de Antioquia, 2nd ed.; Universidad Pontificia Bolivariana, Ed.; Universidad Pontificia Bolivariana: Medellín, CO, USA, 2015; ISBN 9789586962292.
  17. FAO. Tackling Food Loss and Waste: A Triple Win Opportunity. Available online: https://www.fao.org/newsroom/detail/FAO-UNEP-agriculture-environment-food-loss-waste-day-2022/en (accessed on 29 May 2023).
  18. FAO. Sustainable Development Goals. Available online: https://www.fao.org/sustainable-development-goals/indicators/1231/en/ (accessed on 29 May 2023).
  19. United Nations Mensaje del Secretario General Para. 2023. Available online: https://www.un.org/es/observances/zero-waste-day/messages (accessed on 30 May 2023).
  20. World Health Organization. Compendium of WHO and Other UN Guidance on Health and Environment, 2022 Update; World Health Organization: Geneva, Switzerland, 2022.
  21. Sharma, P.; Dutta, D.; Udayan, A.; Nadda, A.K.; Lam, S.S.; Kumar, S. Role of Microbes in Bioaccumulation of Heavy Metals in Municipal Solid Waste: Impacts on Plant and Human Being. Environ. Pollut. 2022, 305, 119248.
  22. Gworek, B.; Dmuchowski, W.; Koda, E.; Marecka, M.; Baczewska, A.; Brągoszewska, P.; Sieczka, A.; Osiński, P. Impact of the Municipal Solid Waste Łubna Landfill on Environmental Pollution by Heavy Metals. Water 2016, 8, 470.
  23. Zhang, R.; Wei, X.; Hao, Q.; Si, R. Bioleaching of Heavy Metals from Municipal Solid Waste Incineration Fly Ash: Availability of Recoverable Sulfur Prills and Form Transformation of Heavy Metals. Metals 2020, 10, 815.
  24. Lin, S.; Jiang, X.; Zhao, Y.; Yan, J. Disposal Technology and New Progress for Dioxins and Heavy Metals in Fly Ash from Municipal Solid Waste Incineration: A Critical Review. Environ. Pollut. 2022, 311, 119878.
  25. Psomopoulos, C.S.; Kungolos, A.; Di Nardo, A. Advances in Industrial Waste Reduction. Appl. Sci. 2023, 13, 1403.
  26. U.S. Environmental Protection Agency. EPA’s Guide for Industrial Waste Management; U.S. Environmental Protection Agency: Philadelphia, PA, USA, 2023.
  27. De Colle, M.; Puthucode, R.; Karasev, A.; Jönsson, P.G. A Study of Treatment of Industrial Acidic Wastewaters with Stainless Steel Slags Using Pilot Trials. Materials 2021, 14, 4806.
  28. Lan, M.; He, Z.; Hu, X. Optimization of Iron Recovery from BOF Slag by Oxidation and Magnetic Separation. Metals 2022, 12, 742.
  29. Zheng, F.; Guo, Y.; Chen, F.; Wang, S.; Zhang, J.; Yang, L.; Qiu, G. Fluoride Leaching of Titanium from Ti-Bearing Electric Furnace Slag in - Solution. Metals 2021, 11, 1176.
  30. Liu, S.; Xue, W.; Wang, L. Extraction of the Rare Element Vanadium from Vanadium-Containing Materials by Chlorination Method: A Critical Review. Metals 2021, 11, 1301.
  31. Işıldar, A.; van de Vossenberg, J.; Rene, E.R.; van Hullebusch, E.D.; Lens, P.N.L. Biorecovery of Metals from Electronic Waste. In Sustainable Heavy Metal Remediation; Case Studies; Spring: Berlin/Heidelberg, Germany, 2017; Volume 2, pp. 241–278.
  32. Işıldar, A.; van Hullebusch, E.D.; Lenz, M.; Du Laing, G.; Marra, A.; Cesaro, A.; Panda, S.; Akcil, A.; Kucuker, M.A.; Kuchta, K. Biotechnological Strategies for the Recovery of Valuable and Critical Raw Materials from Waste Electrical and Electronic Equipment (WEEE)–A Review. J. Hazard. Mater. 2019, 362, 467–481.
  33. Latosińska, J.; Czapik, P. The Ecological Risk Assessment and the Chemical Speciation of Heavy Metals in Ash after the Incineration of Municipal Sewage Sludge. Sustainability 2020, 12, 6517.
  34. Čech, V.; Gregorová, B.; Krokusová, J.; Košová, V.; Hronček, P.; Molokáč, M.; Hlaváčová, J. Environmentally Degraded Mining Areas of Eastern Slovakia As a Potential Object of Geotourism. Sustainability 2020, 12, 6029.
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