**Citation:** Santucci, V.; Fiore, S. Recovery of Waste Polyurethane from E-Waste. Part II. Investigation of the Adsorption Potential for Wastewater Treatment. *Materials* **2021**, *14*, 7587. https://doi.org/10.3390/ma14247587

Academic Editor: Juan F. Rodriguez

Received: 6 November 2021 Accepted: 6 December 2021 Published: 10 December 2021

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#### **1. Introduction**

According to the latest report published by the association of plastic manufacturers Plastic Europe [1], the demand for polyurethane in Europe was equal to 4 Mt in 2019, representing 7.9% of the total plastic demand. The main contributors to polyurethane requirement are the manufacturing of pillows and mattresses (31%), and the construction and building (24.5%), electrical and electronic (21.3%), and automotive (11%) sectors [1]. Of the 4 Mt/y polyurethane requested in Europe in 2019, approximately two thirds are in the form of foams (1.68 Mt flexible foam, 1 Mt rigid foam) [2]. In China, polyurethane output in 2011 reached 7.5 Mt, and polyurethane foam (PUF) accounted for 60% [3]. PUF wastes are product scraps, as the production of rigid polyurethane foam usually creates 15% of waste [3] and post-consumer waste materials. Of the total 29.1 Mt of plastic generated in Europe in 2019, approximately 1.5 Mt are made by PUF, of which one third is recycled, while the rest is incinerated or sent to landfill.

The scientific and technical literature offers several potential perspectives for material recovery from waste PUF, mostly as an oil absorbent [4–7], additive for construction materials [8–11], and adsorbent of pollutants from wastewater [12,13]. Nowadays, the market competition in the field of wastewater treatment technologies is increasing due to the need of achieving effective removal performances with limited costs. The most common adsorbent at the state-of-the-art level is activated carbon (AC), as dust or granular material, suitable for a variety of applications for drinking water, swimming pools, urban and industrial wastewater, etc. Alternatives to AC are oxides and zeolites, polymeric adsorbents (intended for application in industrial wastewater treatments, but their high costs of production and regeneration have prevented a broader application), and, developed more

recently, low-cost adsorbents derived from wastes [14]. The literature is rich of studies that investigated the adsorption potential of industrial and agricultural wastes, particularly for the removal of dyes or metals from wastewater (Table 1) [15–24].

**Table 1.** Overview of studies describing the properties and performances of commercial and novel adsorbents towards different contaminants (SSA: specific surface area; CLi: initial concentration in the liquid phase; qeq: amount adsorbed on the solid phase; teq: contact time).


Table 1 provides an overview of the literature data describing the properties and performances of adsorbents deriving from different "parent" materials, categorized per type of contaminant. The most promising experimental applications of low-cost adsorbents were industrial wastewater containing dyes, metals, and halogenated compounds. The dose of adsorbent was in the range of 0.1–20.0 g/L, though it was higher for the removal of phenols. The specific surface area (SSA) directly affects adsorption, and high values are usually

desirable to provide many adsorption sites. AC exhibited SSA values between 500 and 1500 m2/g [15,21]; however, adsorbents with relatively low values (<200 m2/g) could also achieve good adsorption capacities towards metals such as lead, cadmium, nickel, and cobalt [17]. The application of PUF as an adsorbent material for the removal of several pollutants from wastewaters is a recently investigated perspective [12,24]. PUF-based adsorbents achieved adsorption capacities between 20 and 30 mg/g for copper, cadmium, and chromium [13], making them less performant than commercial products, but still with a good adsorption capacity, higher than other waste-derived materials such as fly ash and hemp. Commercial AC is usually made from non-renewable resources or biomass transported over long distances, resulting in high environmental impacts due to feedstock and transportation, and in relevant energy demand [25]. The estimated impact on climate change of granular AC is 1.44 Kg CO2/kg adsorbent [20,26]. To sum up, a good adsorbent should: be made from a porous raw material with high SSA; have good affinity for the target contaminants; and have limited costs for raw material procurement-also including transportation, and for its preparation. To limit the environmental impacts, adsorbents with minimal energy consuming pre-treatments are preferable, and the feasibility of their regeneration after adsorption must be considered as well.

The interest of the scientific and industrial worlds is shifting towards waste-derived non-conventional adsorbents, derived from biological, agricultural, or industrial processes, which are available almost free of cost [27,28]. The porous structure of PUF is a desirable feature for an adsorbent because it provides numerous potential sites of adsorption; also, open-cell PUF can be successfully used in columns for the treatment of large volumes of wastewaters [14]. The potential as adsorbent of virgin PUF in combination with different reagents has been previously tested [29,30] but, to our knowledge, there are not many studies specifically exploring the application of "plain" (e.g., without modification of its chemistry) waste PUF as an adsorbent. The main goal of this study is to investigate the adsorption potential of waste PUF in the field of wastewater treatment technologies for the removal of inorganic and organic pollutants. Waste PUF is employed "as such" separated from end-of-life (EoL) refrigerators, after the application of minimal and simple physic treatments to eliminate the impurities (i.e., sieving and washing with water). The perspective explored by this study is coherent with the Circular Economy strategy of the actual European policy and regulations. This solution, if proven effective, can lead to a double potential benefit when costs and environmental burdens are reduced in comparison to the use of conventional adsorbents.

#### **2. Materials and Methods**

#### *2.1. Waste PUF Origin and Characteristics*

The tested material was waste PUF in a loose granular form derived from the shredding of EoL refrigerators (category 1 WEEE) at a TBD treatment plant managed by AMIAT in the metropolitan area of Turin, Italy. The waste PUF was sampled across 5 weeks (one sample per week) to account for any composition variability. The samples (1 kg each) were collected according to standard methods UNI 10802:2013 and UNI 14899:2006 at the end of the working day. The samples were assumed to be representative, considering that 3300 t/y EoL refrigerators entering the plant roughly correspond to over 300 items shredded per day [4]. The collected samples were quartered to obtain representative secondary samples for the characterization and adsorption tests. A complete characterization of the waste PUF is reported in a previous study [4], describing the investigation of the oil absorption potential of the same material (whole material and selected particle-size fractions). Compared to our previous study [4], this research explored the adsorption potential for wastewater treatment of the fraction of waste PUF with dimensions between 0.71 and 5 mm. The main features of the considered waste PUF are reported in Table 2. Commercial powdered Activated Carbon (AC) FILTERCARB RO, provided by Carbonitalia srl (Livorno, Italy) was chosen as reference material for the adsorption tests (Table 2).


**Table 2.** Main features of the considered waste PUF and of the reference commercial AC.

#### *2.2. Pre-Treatment*

The waste PUF sampled in the WEEE shredding plant contained impurities such as plastic, paper, and metal. Before the adsorption tests, the waste PUF (fraction having dimensions between 0.71 and 5 mm) underwent a washing pre-treatment (0.125 L water/g PUF) aimed at removing the impurities as higher density (sink) fraction after 15 min of shaking at 150 rpm in an ARGOLAB SKI 4 orbital shaker. After a 3 min rest, the floating particles of PUF were collected and wet sieved at 0.71 mm with 0.03 L water/g PUF. The washed samples were drained for 10 days in ambient conditions (21 ◦C, relative humidity 63%) and stored in a dry container.

### *2.3. Target Pollutants*

Three pollutants were considered in the adsorption tests: methylene blue, an organic compound present in paints used in textile and plastic industries; phenol, an organic pollutant derived from the polymer, chemical, and food industries; and mercury, a carcinogenic metal well known for its bioaccumulation potential in water reservoirs affected by industrial or mining activities [31]. The target pollutants solutions were prepared from the dilution in deionized water of: 1000 mg/L mercury solution Chem-Lab (Zedelgem, Belgium) 99.5+% phenol pellets Chem-Lab (Zedelgem, Belgium); 99.5% methylene blue (C16H18ClN3S · 3H2O) CarloErba Reagents (Cornaredo, MI, Italy).

The analyses of phenol and methylene blue were performed directly on the aqueous phases through an ONDA UV-30 SCAN UV-VIS spectrophotometer (at 269 and 668 nm, respectively). Mercury was analyzed through an NEX DE VS Rigaku XRF spectrometer.
