1. Introduction
Most batteries, alkaline and Zn-C, which are used as power sources of energy for electronic devices, are not disposable. In recent years, in Ecuador, there has been an increase in battery consumption. A study in Ecuador in 2020 found that the national population of 16.8 million inhabitants consumes 17 million batteries, representing a per capita consumption of 1.06 kghab
−1. Only 1.53 million of these batteries were rechargeable (9.06%) [
1]. It is estimated that 78.15% of the population dispose of batteries in common garbage, 6.08% burn or bury them, 5.95% keep them in their house, and only 8.21% deposit them in the proper collection center. This disposal practice leads to the accumulation of batteries in city landfills and results in pollution problems, particularly in water sources and soil, due to the leachates containing heavy metals [
2,
3,
4].
The Ecuadorian Institute of Standardization (INEN) establishes that used batteries in Ecuador must be handed over to professionals authorized by the Ministry of Environment or AAAR (Responsible Environmental Enforcement Authority), according to the INEN 2534 standard. The process to be followed from collection to recycling is also outlined by the INEN [
5]. However, unlike in Europe, where directives prohibit the disposal of batteries through incineration or dumping, there is no law in Ecuador prohibiting these methods [
6]. In the city of Cuenca, although EMAC is the public company in charge of garbage collection, it lacks the initiative to collect batteries separately from other types of waste. In this regard, ETAPA EP (the public company responsible for water treatment and distribution) has assumed the responsibility of preventing batteries from contaminating drinking water sources and the wastewater treatment plant; however, its action is limited [
7]. ETAPA EP collection points gather approximately 35–40 kg of batteries each month. After conducting a stabilization process, the batteries are confined [
7]. This strategy entails logistical problems such as a lack of space and potential hazards, including the possibility of metal leaching due to flooding.
An effective waste management program should aim to provide batteries with a final treatment that needs to be environmentally sustainable and cost-effective. A typical alkaline battery comprises a zinc anode and a high-density manganese oxide cathode (
In contrast, Zn-C batteries are predominantly made of zinc, followed by
. Then, from this composition, around 33% of zinc and 29% of manganese oxide can be recovered and used as raw material for industry [
8]. Some authors have even succeeded in obtaining zinc oxide from Zn-C batteries. For instance, one study used sulfuric acid (
) for reductive leaching and selective precipitation with NaOH at pH 10 [
9]. In contrast, another used an ionic liquid for battery leachate treatment [
4]. Additionally, a third study employed solvent extraction, electrodeposition, and precipitation methods [
10].
Currently, zinc oxide production mainly relies on chemical processes that utilize zinc as the raw material. Unfortunately, zinc is often mined alongside other metals, particularly lead, resulting in significant pollution with toxic metals in soil, water, and sediments from Pb-Zn mines [
11]. Mining activities also lead to a substantial release of heavy metals into the environment, with zinc alone contributing up to
in close mining regions [
12]. Such releases have been shown to cause freshwater and marine human toxicity, ecotoxicity, metal depletion, eutrophication, and soil damages [
13]. Zinc is an essential element in the human body. Nevertheless, its excessive intake causes stomach cramps, nausea, and vomiting, and long-term exposure can affect cholesterol balance, immune system function, and fertility [
12]. Additionally, heavy metals that incorporate into the soil can decrease pH, nutrients, and microbial diversity, rendering them unsuitable for agriculture since they leach into water bodies [
12,
14]. China, the biggest supplier of zinc in the world, has several regions next to mines with contaminated soil and water, leading to health issues for residents [
12]. Poland has also contaminated allotment gardens in closed Zn and Pb mines, with the concentration of metals in soil and crops exceeding European Quality Standards, resulting in the integration of these contaminants into the food chain [
14]. In Ecuador, mining areas have high concentrations of Zn in nearby rivers, with levels ranging from 513–2670
. This is due to the discharge of waste generated during gold and silver extraction processes into the rivers, which carry heavy metals such as zinc [
15]. Furthermore, ecosystem remediation from Zn-Pb mines can cost up to
$ [
16].
Another common waste product is cooking oil. It is often taken to be inoffensive; nevertheless, its inadequate disposal can cause significant environmental harm. Typically, it is poured down the drain, leading to contamination of water bodies and wastewater treatment plants, negatively affecting the ecosystem and water treatment efficiency, respectively [
17]. The oil forms a layer on the water that obstructs sunlight and limits oxygen absorption, leading to additional ecosystem damage. Additionally, removing oil from wastewater can be expensive [
18]. In Ecuador, ~54 million liters of discarded oil are generated, and ~70% of this waste comprises vegetable oil [
1]. ETAPA EP is responsible for collecting and managing this waste to avoid water contamination. Still, this action is limited to collecting and storing the oil and, in some cases, transferring it if an individual or institution requires it. Storing the oil is not a definitive solution because, in addition to the space problem, there is, as in the case of batteries, the danger of infiltration where the oil can leak [
19]. However, there are ways to revalue this waste, such as producing biodiesel [
20]. This is produced by transesterification, where used cooking oil triglycerides chemically react with alcohol to form fatty acid methyl esters [
21], see
Figure 1.
As shown in
Table 1 (Inputs 1 to 4), biodiesel production has been extensively studied using homogeneous catalysts. This type of reaction is characterized by being fast, with a yield of over 90%. Nevertheless, it has disadvantages such as loss of catalyst and the need for neutralization.
Table 1 also displays several investigations on biodiesel production utilizing different heterogeneous catalysts, along with 1% or less of the catalyst. The results indicate that the reaction is highly efficient, with the advantage of recovering the catalyst. Nevertheless, the reaction is a little slower in some cases due to diffusional mass problems. One of the catalysts used for the transesterification reaction is zinc oxide, indicated in
Table 1 (Inputs 6 and 7). The use of discarded batteries’ zinc oxide and used cooking oil for producing biodiesel can effectively reduce the amount of hazardous waste and its management costs [
22]. Biodiesel is a great candidate to replace fossil fuels as a clean energy source due to its advantages. The main one is that the emission of CO
2, CO, unburned hydrocarbons, and particles is lower compared to fossil fuels. Likewise, the emission of SO
2 during the biodiesel combustion process is lower, due to low sulfur content in the biodiesel raw materials. These emission gases are the main cause of atmospheric pollution. Some other advantages include the fact that biodiesel can be produced from recycled oils and fats, can be used directly in diesel engines, and reduce the dependence on fossil fuels [
23,
24].
This research explores alternative processes that can properly revalorize waste municipal organic oil and batteries, which currently need to be managed appropriately. The proposed approach involves preparing the zinc oxide from discharged Zn-C batteries by a novel hydrometallurgical method and preparing biodiesel from cooking oil by catalyzed transesterification. Moreover, a material flow analysis (MFA) is developed to analyze the inputs, outputs, and storage of materials during 2022. This work evaluates a real sample of batteries and discarded oil deposited in municipal warehouses contributing to a circular economy, considering the real conditions in which these are delivered. For instance, discarded oil used in this study is a mixture of various sources from the entire city, such as fast-food stalls, local houses, restaurants, etc. In fast food stalls, a highly saturated oil, due to its reuse, is handed over for recycling.
4. Conclusions
Obtaining biodiesel from recycled materials is a crucial step towards efficiently managing urban waste produced in Cuenca. If successful, this process could bring monetary benefits to ETAPA EP and promote the concept of a circular economy. This study transformed two wastes which are considered environmental liabilities into value-added products. Zinc oxide was obtained from zinc–carbon batteries, yielding 56% and a purity of 98%. A 5% zinc oxide catalyst was supported on a carbon rod, also recycled from the stack. The recycled oil was conditioned and characterized; it was determined that the fatty acid in major percentage was linoleic acid (18:2), 11.29%. The water amount was 0.15%, showing that the drying of the sample was effective. The density was 0.965 and the viscosity was determined to be 50.912 . Both parameters were within ASTM standards for recycled oils used to produce biodiesel. Better results for obtaining biodiesel with the pretreated vegetable oil and ethanol in a 6:1 ratio were obtained using the supported catalyst. The determination of the amount of water, viscosity, and density was the same as in the case of oil, obtaining the following values 0.005%, 0.892 gcm−3, and 4.1887 mm2s−1, respectively. These parameters are within the ranges determined by biodiesel standards INEN 2482, ASTM B 100, and EN 590. The catalyst obtained favored the generation of biodiesel from recycled vegetable oil and ethanol. This was evidenced in the reaction yield since when using it, since a yield of 70.91% was obtained compared to the non-catalyzed blank where the yield was 0.5%. When comparing the yields between the catalyzed reactions, the yield and viscosity were not significantly different; the differences were found in terms of density. The supported catalyst allowed easier recovery of the catalyst.
The production of biodiesel involves several processes such as the production of zinc oxide, carbon treatment, and oil treatment. The energy consumption of each of the involved processes has been evaluated and quantified, resulting in an energy consumption of 32.9 kWh for obtaining the catalyst and 4.25 kWh for the oil treatment, thus obtaining the biodiesel sample through the reaction, which gives a total of 37.15 kWh.