1. Introduction
Dishwashing sponges are a ubiquitous element in practically every kitchen around the world, where they are considered indispensable for efficiently cleaning dishes, pans, and cutlery to achieve hygiene [
1]. However, what often goes unnoticed is the considerable environmental impact these sponges have on our global ecology. To this end, the widespread use of dishwashing sponges represents a significant environmental concern, as sponges are often manufactured from synthetic materials, such as polyurethane and polyethylene, which originate from petroleum. The problems associated with these materials are caused by the inefficiency of recycling, where dishwashing sponges are not easily recyclable due to the complex combination of materials, which end up in landfills or being incinerated, further increasing environmental problems, especially in regions of countries considered in development. According to the World Wide Fund for Nature (WWF) [
2], Brazil is the fourth largest producer of plastic waste in the world, behind the United States, China, and India, with only 1.2% of the total recycled in 2016 second [
3,
4].
In response, it is necessary to look for ways to deal with the environmental impact of dishwashing sponges, aiming to transition to sustainable alternatives, which will require the active participation of consumers, manufacturers, and governments around the world [
5]. Such factors must be allocated to global management, which must be representative of the renewable environment, focusing on ways to inhibit the release of microplastics and recycling inefficiency, where the challenge of dealing with the environmental impact of dishwashing sponges is far from being resolved. Therefore, the transition to sustainable alternatives requires the active participation of consumers, manufacturers, and governments around the world. Furthermore, it is essential that scientific research continues to explore new materials and technologies that can replace the harmful materials traditionally used in dishwashing sponges [
6,
7].
Given the applied context, polymers come from the formation of macromolecules made up of repetitions of atoms (or groups of atoms) joined by covalent bonds. However, the main raw material for their production are oil, coal, and natural gas, as well as forms of natural origin, such as starch, silk, and cellulose [
8,
9].
The most used materials are PE, Polypropylene (PP), Polystyrene (PS), Poly Vinyl Chloride (PVC), PU, and Poly Ethylene Terephthalate (PET), which in 2017 corresponded to 92% of global consumption. Thus, the management of solid waste as well as the recycling of materials and products are needed to address the presence of polymers and their synthesis, becoming a challenge for society, companies, and public management in general.
In relation to PU, the majority of global production, there are aims to apply flexible foams, mainly in mattresses and automotive manufacturing [
10].
PU is recognized as a prominent material on the world market, among synthetic polymers, especially for applications involving flexible foams used for cleaning, upholstery, mattresses, and the automotive industry, among other global forms [
11].
The presence of microplastics is massive in all oceans on different continents, resulting from degradation by controlled methods or through adverse weather conditions. To this end, the decomposition of the polymer occurs according to its chemical composition, with the presence of reactive chemical groups that favor chemical reactions as well as the breaking of its bonds [
12,
13].
Thus, sponges are commonly used to clean household items; however, their deterioration or fragmentation can generate microparticles and cause the release of added chemical substances. Although there is evidence of the danger of microplastics, there is little knowledge about the effects caused by these microparticles and the chemical additives leached [
14].
However, pus can undergo degradation, with the ease of this process being influenced by structural characteristics such as the chemical groups present in the molecular chains, the level of crystallinity, and molecular orientation, among others [
15]. According to research conducted by [
16], the chemicals present in 55 polymers were identified and classified according to the environmental risk they present, where flexible PU was categorized as one of the most dangerous due to the toxicity, mutagenicity, and carcinogenicity of its monomers, catalysts, and degradation products, such as hydrogen cyanide [
17,
18,
19].
Due to their physical, thermal, electrical, and chemical properties, polymeric materials have been increasingly considered to replace products traditionally manufactured with metallic and ceramic materials. With the increase in polymer production, there has been a significant effort to conduct studies that seek to balance economic viability, environmental benefits, and positive social impacts [
20,
21,
22].
Adsorption comes from a physical–chemical phenomenon of surface adhesion, observed when a fluid, which can be liquid or gaseous, is transferred to the surface of a solid. It has been widely studied with the aim of developing low-cost alternative adsorbents, where the adsorbents are generally solid and with the presence of porous particles in their structure [
23,
24].
To this end, sustainability is one of the alternative priorities when obtaining low-cost adsorbents, especially those considered discardable such as polyurethanes, which have been attributed with a positive environmental impact and chemical and thermal resistance. These have shown promising results as they have good permeability, low density, and high porosity [
25,
26,
27].
Thus, the adsorption process can be classified into two types according to its intensity: physical and chemical. Where, for the physical adsorption process, the bond between the surface of the adsorbent and the adsorbate is considered relatively weak, this can be attributed to Van der Walls forces. However, the chemical adsorption process involves sharing or exchanging electrons, characterized by a possible new bond, stronger in relation to the physical adsorption process [
28].
Therefore, the application of PUs is intended as an adsorbent agent from various pollutants present in water, including heavy metals and volatile organic compounds (VOCs), as well as dyes. In this context, the functionalization of PUs through their functional groups increases the efficiency in removing certain pollutants present in drinking water. Thus, PUs are promising materials for chemical adsorption systems due to their high porosity, chemical versatility, good mechanical stability, and low cost [
29].
Within an attribution envisioning the quantity of PUs for kinetic treatments, in Brazil, several factors influence the useful life of a sponge made up of PUs due to its conditions of use and the ways in which consumers interpret its use. Thus, the city of Maringá/PR located in Brazil, treated as a city for ecological purposes, was used as the management context for the quantification of spongy material in kilograms (kg). The city of Maringá is made up of 415 thousand inhabitants and generates an average of 300 thousand kg of waste daily. Based on this principle, the replacement of dishwashing sponges is estimated to be changed every 15 to 20 days on average [
30].
Data attributed by the Brazilian Institute of Geography (IBGE), 2023, stipulate that Maringá is made up of 188,117 residences distributed in the master and urban plan. Thus, in the context described for the number of residences, when multiplied by the exchange of sponges on average every 15 days within a month, an estimated number of 376,234 units/month was obtained [
31].
However, to achieve this goal, it was necessary to include teams distributed across a region within the city, divided by regions in order to meet the selective collection of the material in question. The city hall, within its collection management, assigned as a team the application of 25 direct employees, and the use of five trucks to carry out collection management throughout the territorial extension of the master plan, representing collection from Monday to Friday during a period of 10 h of daily work. The present study aimed to use the used dishwashing sponges that are discarded incorrectly in the environment for the sustainable production of a new adsorbent material and analysis of its production costs.
2. Materials and Methods
2.1. Research Development
The experiments were developed in the Laboratory of Management, Control and Environmental Preservation at the State University of Maringá. This laboratory was created to promote research and practice in the fields of environmental management, control, and preservation, and to explore the impacts of human activities on the environment. Through the laboratory, the university actively engages in initiatives to protect and preserve the environment.
2.2. Material Preparation
The methodology for preparing the sponge for use in scientific purposes involves several steps. First, the sponge was collected carefully so as to not cause any damage. Then, it was chopped into small pieces in order to facilitate further steps. Next, deionized water at 100 °C was used to remove any possible contaminants present in the sponge. This was performed for 30 min, stirring occasionally for complete removal of contaminants. Finally, the sponge was placed in an oven and dried at 105 °C for 24 h. This step allowed any residual water and contaminants to evaporate and the sponge was ready to be used for scientific purposes [
32].
2.3. Material Characterization
The material was characterized in terms of its textural, structural, morphological, and chemical composition. Scanning electron microscopy (SEM) (Quanta 250 FEI) was used equipped with an EDX-type chemical analysis system with software support (Oxford Instruments Nanotechnology Tools Ltd., London, UK). The zeta potential of the surface was determined using a particle analyzer DelsaTMNanoC (Beckman Coulter) and Fourier transform infrared spectroscopy (FTIR) Vertex 70 v (Bruker) [
33].
2.4. Effect of Mass and pH
To investigate the effect of the mass and pH of a sponge on the release of paracetamol, the following method was used. Between 0.01 and 0.05 g of sponge was used by varying the mass in five increments, and an initial paracetamol concentration of 60 mg L−1 was added and stirred at 120 rpm at room temperature for 24 h. The pH values were varied to 4, 7, and 10 to investigate the effect of pH on the release of paracetamol. The mass that gave the best result was used in the study of the effect of pH on the release of paracetamol. The release of paracetamol was measured for all the different pH levels and the results were analyzed. The results were compared to identify the effect of the mass and pH of sponge on the release of paracetamol. We also looked at any potential interactions between the mass and pH values. The results will inform us on how to best optimize the release of paracetamol from a sponge and what mass and pH values we should use for further studies on the effect of a sponge on the release of paracetamol.
2.5. Kinetic and Equilibrium Study
The kinetic and equilibrium studies of the effect of dosage (0.01 g of sponge) and pH (7) on the adsorption of paracetamol onto sponge material was investigated in a laboratory set-up. The experiments were conducted at room temperature (25 °C) in a batch reactor with 30 mL of paracetamol solution at a concentration of 60 mg L
−1, with a stirring speed of 120 rpm, while pH was maintained at 7. Samples were collected at specific intervals (namely from one to 1440 min) and filtered through cellulose acetate membranes, in Equation (1):
The adsorption capacity was determined by measuring the amount of paracetamol adsorbed in the sponge material at equilibrium. Data obtained from the experiments were fitted to the pseudo first order and pseudo second order kinetic equations. The pseudo first order equation describes the rate of adsorption as a linear function dependent on the initial concentration of the adsorbate (Ci), while the pseudo second order equation relates the rate of adsorption to a product of the initial concentration (Cf) and the capacity of the adsorbent (qe).
The pseudo first order equation is given by qt = qe [1 − e−k1t], where qt is the adsorbed amount of paracetamol at any time ‘t’, qe is the adsorbed paracetamol at equilibrium, and k is the rate constant. The constant can be determined by plotting a graph of qt against t (time) providing a linear link between them. The pseudo second order equation is given by where k is the rate constant and Co is the initial concentration of paracetamol. By plotting qt against t (time), a linear relationship is obtained and the pseudo second order equation can be applied. The linear plots generated from the kinetic equations are used to calculate the rate constants (k) which are then used to determine the adsorption capacities expressed in terms of mg g−1. The data obtained from the experiments can thus be used to calculate the adsorption capacity (qe) and the rate constants (k) for pseudo first and pseudo second order equations. This in turn enables one to modify the existing conditions in order to increase the adsorption capacity or reduce the rate constant, as required.
2.6. Adsorption Isotherms
The adsorption isotherm experiments were conducted at three different temperatures 298, 308, and 318 K C using 30 mL paracetamol solution with varying concentrations from 10 to 250 mg L−1 in contact with 0.01 g in adsorbent pH 7 under stirring at 120 rpm for 720 min. From the calculations of the adsorption capacity, the most classic models of adsorption isotherms were evaluated, i.e., Langmuir and Freundlich.
The Langmuir isotherm is based on the assumption that there is a defined number of active sites with no competition between them, assuming that adsorption occurs in a location independent of the adjacent active sites occupied, and is presented in Equation (2).
where b
L is the Langmuir isotherm (L mg
−1) constant. The Freundlich model is applied to heterogeneous surface models, and therefore, does not provide assumptions about the adsorption capacity of a monolayer, and assumes that there is an interaction between the adsorbed molecules, and is presented in Equation (3):
where k
F is the Freundlich isotherm constant (mg L
−1) (L g
−1)
1/n. After applying the adsorption isotherm models, equilibrium data were used to calculate the thermodynamic parameters: enthalpy (ΔH), entropy (ΔS), K
c equilibrium constant, and Gibbs free energy (ΔG).
2.7. Cost Analysis Involving the Use of Dish Sponge
Cost can be interpreted as different concepts depending on the format in which the analyst interprets this function. Here, the study included the purchasing of raw materials, which must be aligned with production costs, especially due to their final composition, which will be the final product. Thus, the cost analysis was studied for both adsorbents in order to report all expenses involved in the production of the materials, that is, the financial outflows related to production and final costs attributed to the application of the adsorptive agent.