Solid Matrices as a Metal Source: History
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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 [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 [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 [3].
More than 12 billion tons of solid waste are produced annually worldwide [3], including approximately 1.55 billion tons of agricultural waste [4] and about 2 billion tons of municipal solid waste [3]. The generation of industrial waste is almost 18 times greater than that of municipal solid waste [5].
The wastes mentioned above contain metals in various concentrations [5][6][7][8][9][10][11][12][13][14], 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 shows the different solid matrices with metal contents (SMMC) with the potential to be recovered by bioleaching.
Figure 1. 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 [6][15], and other trace elements such as Fe, Al, P, Mg, Zn, and Mn [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 [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) [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 [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 [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 [6]. Corn leaves and cacao plants contain Cd [13], cattle manure constitutes an essential source of metallic elements such as Zn and Cu [9], and pig manure contains Cu, Zn, and Mn [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 [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 [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 [21], involving metals such as Zn, Pb, Cu, Cr, Hg [11][22][23][24], Cd [11][22][23], Ni [23], Co and As [11]. In addition, household wastes such as eggshells are rich in Ca [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 [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 [22].
In municipal solid waste incineration processes, metals such as Zn, Cu, Cr, Pb, Ni, Cd, and Hg are found [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 [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 [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 [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 [20][26].
Metallurgical slag is among the most abundant by-products generated by such industries [6][27]. The production of this waste is used to recover iron [28]. On the other hand, titanium is concentrated in the blast furnace titanium slag as vanadium titanomagnetite, which contains SiO2, Al2O3, CaO, and MgO [29], and the solid residues from silica sand purification contain metals such as Ba and Cd [6].
Raw materials for vanadium products include the vanadium–titanium magnetite, vanadium slag, coal, petroleum coke, fly ash, and spent catalysts [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 [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 [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 [13][14][33].
Finally, depleted mines, although they increase the percentage of waste generated [27], contain a diversity of metals in low concentrations [34] that can be extracted by biological rather than chemical methods, as mentioned above.

This entry is adapted from the peer-reviewed paper 10.3390/su151310222


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