Spent Coffee Bioelastomeric Composite Foams for the Removal of

Research Article pubs.acs.org/journal/ascecg

Spent Coffee Bioelastomeric Composite Foams for the Removal of Pb2+ and Hg2+ from Water Asmita A. Chavan,†,‡ Javier Pinto,† Ioannis Liakos,† Ilker S. Bayer,† Simone Lauciello,§ Athanassia Athanassiou,† and Despina Fragouli*,† †

Smart Materials, Nanophysics, Istituto Italiano di Tecnologia (IIT), via Morego 30, 16163 Genova, Italy Università degli Studi di Genova, via Balbi 5, 16126 Genova, Italy § Nanochemistry, Istituto Italiano di Tecnologia (IIT), via Morego 30, 16163 Genova, Italy ‡

S Supporting Information *

ABSTRACT: Herein we present an interesting approach for the reutilization of coffee waste in water remediation. This is achieved by the development of bioelastomeric foams composed of 60 wt % of spent coffee powder and 40 wt % of silicone elastomer using the sugar leaching technique. In this study, we present the necessary characteristics of the developed “green” foams for the successful removal of Pb2+ and Hg2+ ions from water, and we identify the involved mechanisms. The capability of the bioelastomeric foams to interact with Pb2+ and Hg2+ is not affected by the presence of other metal ions in water as tests in real wastewater demonstrate. The incorporation of the spent coffee powder in a solid porous support, without compromising its functionality, facilitates the handling and allows the accumulation of the pollutants into the foams enabling their safe disposal. The fabricated foams can be used for the continuous filtration and removal of metal ions from water, demonstrating their versatility, in contrast to the sole coffee powder utilized so far, opening the way for the reutilization and valorization of this particular waste. KEYWORDS: Agrowastes, PDMS, Porosity, Remediation, Heavy metal ions

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INTRODUCTION The agrofood industry generates enormous amounts of inedible residues, originating from processed edible vegetables and cereals.1 They are usually composed of lignin and cellulose as major constituents and may also include other polar functional groups such as alcohols, aldehydes, ketones, carboxylic, phenolic, and ether groups. Due to their unique chemical composition, abundance, renewable nature, and low cost, they represent a viable option for aquatic pollutants’ remediation.2−4 Indeed, advances in the use of economical and sustainable materials, such as agricultural wastes, for water treatment can reduce various disadvantages associated with high fabrication and operational costs, handling and disposal problems, high energy of consumption, expensive equipment requirements, etc. Among several agricultural wastes that have been used as biosorbents for water treatment, coffee residue is one of the most studied.5−8 Spent coffee, the waste produced after the beverage preparation, is readily available in large quantities (worldwide, ∼6 million tons yearly9) from domestic and restaurant consumption but also from coffee manufacturing industries. Converting the waste into a useful resource could decrease the burden of waste management. Due to its functional chemical composition (contains fatty acids, lignin, cellulose, hemicellulose, polyphenols, etc.10−12), spent coffee has been reutilized as fertilizer, animal feedstuff, biodiesel source, and metal ion adsorbent.13−15 In the latter case, researchers are focused on the appropriate functionalization and utilization of © XXXX American Chemical Society

the spent coffee in water remediation, mostly for the removal of heavy metal ions.5,16−19 Although these studies show the efficient use of the spent coffee powder for water remediation, collection of the powder after the remediation process is cumbersome, requiring post-processes such as centrifugation and filtration of the segregated particles.20,21 Apart from the utilization of the pristine spent coffee, additional studies have been made for further functionalization or modification of the powder. One example is its conversion into activated charcoal, on one hand, improving its performance as adsorbent for heavy metal ions, dyes, or gases, but on the other hand, adding further preparation steps and costs.22 Fixing the powder in a solid compact configuration without compromising its chemical affinity to the metal ions could be an interesting solution for the improvement of the usability in the specific application. Nonetheless, the utilization of spent coffee in the form of polymer composite for water remediation is still unexplored. Spent coffee, or coffee parchments and husks, have been solely utilized so far as natural fillers in different polymers for mechanical reinforcement.23,24 Herein we report a novel approach to utilize spent coffee powder as active filler for the formation of bioelastomeric composite foams. The fabricated porous bioelastomers are successfully utilized for the removal of heavy metal ions from Received: May 20, 2016 Revised: August 5, 2016

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DOI: 10.1021/acssuschemeng.6b01098 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

The measurements were carried out under chamber pressure ∼2 × 10−9 mbar. The parameters such as energy pass, energy step, and scan number were set to 50 eV, 0.5 eV, and 1, respectively, in the case of wide scans, and for the high resolution scans to 30 eV, 0.1 eV, and 5, respectively. A flood gun was used to neutralize the surface charge with energy 7 eV and the filament current 2.6 A. The spectra were referenced to C 1s at 284.8 eV, and were analyzed using CasaXPS software. Quantitative estimation of the concentration of the ions was performed using an ICP-OES spectrometer (inductive coupled plasma-optical emission spectroscopy) (iCAP 6300, Thermo). A 250 μL portion of the collected solutions during the adsorption experiments was mixed with 2.5 mL of aqua regia and left overnight for the complete digestion of the metal ions. Then, the solutions were further diluted with Milli-Q water up to 25 mL and filtered through 0.45 μm PTFE filters prior to elemental analysis. Adsorption Experiments. For the adsorption study, 20 mL portions of aqueous solutions with different concentrations of Pb2+ (ranging from 9 to 1091 ppm), Hg2+ (ranging from 16 to 1162 ppm), and Cu2+ (ranging from 13 to 75 ppm) were prepared. Then, 0.2 g of the bioelastomeric foam (containing ∼0.12 g or 60 wt % spent coffee powder) was immersed in the metal ion solution and kept under stirring at RT. Aliquots were collected from the solutions at specific time intervals for further analysis. The effect of pH on the adsorption process was determined by dipping the foams in aqueous Pb2+ and Hg2+ ion solutions with fixed concentration (100 ppm) and different pH values ranging from 2 to 8. The pH of the solutions was controlled with either 1 M NaOH or 1 M HCl. The performance of the bioelastomeric foams was tested also in the presence of other metal ions in 20 mL of wastewater, containing Pb2+, Hg2+, Cu2+, Al3+, Mg2+, and Co2+ with different concentrations. Prior to reaction, the pH of the wastewater was adjusted to 4.5 by adding an aliquot of 1 M NaOH aqueous solution. Then, 0.2 g of a bioelastomeric foam (containing ∼0.12 g or 60 wt % spent coffee powder) was immersed in the wastewater and kept under stirring at RT. Aliquots were collected from the solution at specific time intervals for further analysis. The time dependent removal efficiency and the equilibrium adsorption capacity of the foams were calculated by eqs 2 and 3.32

water. We prove that the prepared foams are able to remove Pb2+ and Hg2+ from water efficiently, also in the presence of other metal ions. Compared to the so far published studies on the sole spent coffee powder, the presented bioelastomeric foams have improved performance, but most importantly they can be easily handled and disposed. One of the proposed applications is their use as thin filters for the continuous-flow cleaning process of contaminated waters.

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MATERIALS AND METHODS

Materials. Acetoxy polysiloxane (acetoxy-PDMS, Elastosil E43) was purchased from Wacker Chemie AG. The polymer curing is performed at room temperature (RT) under the influence of atmospheric moisture.25 Poly(ethylene oxide)-co-polydimethylsiloxane (PEO-b-PDMS) was purchased from Polysciences Inc. Lead nitrate, mercury chloride, copper nitrate, toluene, and hexane were procured from Sigma-Aldrich, whereas mixed metal ion wastewater was collected from the Nanochemistry facilities of the institute. Granulated sugar (granule size: 400−500 μm) was purchased from the local market, and spent coffee powder was collected from the canteen of the research institute. All of the above chemicals were used without any further purification. Fabrication of the Bioelastomeric Foams. The collected spent coffee powder was dried in an oven at 110 °C overnight, and subsequently ground using an electric grain mill (WonderMill) resulting in fine powder with mean diameter of 163 ± 62 μm as measured by the FIJI/ImageJ software.26 The hydrophilic foam was fabricated using the sugar leaching technique.27,28 In brief, 4 g of elastomer (Acetoxy-PDMS) and 0.04 g of the surfactant additive PEOb-PDMS29 (wt. ratio 10.0:0.1) were mixed in 3 mL of hexane. Subsequently, 60 wt % of spent coffee powder with respect to the polymer was slowly added in the solution under constant stirring. Then, 8 g of granulated sugar was gently mixed with the obtained mixture. The mixture was poured in a Petri dish and placed under a fume hood overnight at RT to allow the solvent evaporation and the polymer curing. The fully cured composite was then placed in hot water under ultrasonication for ∼2 h in order to dissolve the sugar, and as a result, the bioelastomeric foam was formed. Characterization Techniques. The surface properties of the fabricated foams were characterized using an OCAH 200 video based optical contact angle measuring instrument (DataPhysics, Germany). Water drops of 5 μL were placed on the surface of the foams, and the change of the drop volume during time was recorded and analyzed. The characteristics of the spent coffee powder as well as the porous structure of the foams were studied by SEM microscopy using a JEOL JSM-6490LA (JEOL, Tokyo, Japan) operating at 15 kV acceleration voltage. Compositional contrast was achieved using backscattered electron imaging with a backscattered electron imaging (BEI) detector. The average pore size of the porous structure of the foams was defined with FIJI/ImageJ.26 The density (ρf) of the foams was calculated by measuring their weight and geometrical volume. Then, the corresponding porosity (or volume fraction of voids, Vf) of the foams was calculated from the relative density (the ratio between the density of the porous (ρf) and the solid (ρs = 1 g/cm3) materials) using eq 1.30

Vf = 1 −

ρf ρs

⎡ (C − Ct ) ⎤ removal efficiency (%) = ⎢ 0 ⎥ × 100 C0 ⎣ ⎦

(2)

⎛ mg ⎞ ⎡ (C0 − Ce) ⎤ adsorption capacity, qe⎜ ⎟=⎢ ⎥×V ⎦ W ⎝ g ⎠ ⎣

(3)

Here, the following abbreviations apply: C0 is the initial concentration of ions (in ppm), Ct is the concentration (in ppm) at a given time t (h), qe is the adsorption capacity (mg/g) at the equilibrium, Ce is the ion concentration (in ppm) at the equilibrium conditions (usually after 30 h), V is the volume of solution in liters, and W is the weight of the foam in grams. The experimental data were fitted with the Langmuir adsorption isotherm model. The Langmuir model assumes a surface with homogeneous binding sites, equivalent sorption energies, and no interaction between sorbed species, and is described by eq 4.33,34

× 100 (1)

qe =

The pore connectivity degree (% of pores accessible from the outer part of the sample following a path across other pores) was estimated by measuring the accessible volume of the foams using a liquid pycnometer, and equations as proposed in ASTM D-2662.31 Energy dispersive X-ray photoelectron spectroscopy (XPS) measurements were performed on the foams before and after the reaction with metal ions, using a Specs Lab2 electron spectrometer equipped with a Mg Kα X-ray source (set at 1253 eV) and a PHOIBOS Analyzer HAS 3500 (hemispherical energy analyzer). The set voltage (15 kV) and current (10 mA) were applied to the source.

KLq maxCe 1 + KLCe

(4)

Here, the following abbreviations apply: Ce (mg/L) is the equilibrium concentration of the metal ions, qe (mg/g) is the specific metal uptake, qmax (mg/g) is the maximum adsorption capacity, and KL (L/mg) is the affinity of the sorbate for the binding sites. Moreover, the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor RL that is given by eq 5.33,35 RL = B

1 1 + KLC0

(5) DOI: 10.1021/acssuschemeng.6b01098 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 1. (a) Photograph of the bioelastomeric foam. SEM images of (b) the bioelastomeric foam with the spent coffee powder indicated by the yellow circles and the inset, and (c) the pure elastomeric foam. Here, C0 (mg/L) is the initial concentration of adsorbate, and KL (L/ mg) is the Langmuir constant. Values of RL between 0 and 1 indicate favorable adsorption.33 Continuous-Flow Filtration Experiments. A 0.65 g portion of a circular bioelastomeric foam of diameter 25.6 mm and thickness ∼1.9 mm (which contains 0.5 g or 60 wt % spent coffee powder) was fixed inside a plastic syringe of diameter 25.6 mm, and sealed to the syringe walls using acetoxy-PDMS to ensure no leaks. Then, the syringe was coupled with a syringe pump of continuous-flow rate (see Supporting Information Figure S1). A 20 mL portion of Pb2+ ions solution (concentration ∼294 ppm) was pumped into the system at different flow rates, i.e., 2, 5, 10 mL/h, and a much faster flow rate of about 1200 mL/h.

When water drops are placed on the fabricated bioelastomeric foams, they initially show water contact angles of 45.0 ± 9.4°, and subsequently they penetrate into the bulk of the foams within 70 s, as shown in Supporting Information Figure S3. This is attributed to the PEO-b-PDMS surfactant, which contains both the PDMS part that anchors to the cured PDMS, and the PEO hydrophilic pendant chains that are exposed on the elastomer surface and are sufficient to make the final system hydrophilic.29 In fact, the surfactant free bioelastomeric foams are quite hydrophobic and waterproof (with stable water contact angle of 109.00 ± 0.01°). Therefore, the PEO-b-PDMS modification is necessary in order to allow the interaction of the foams with the polluted water and therefore with the present heavy metal ions. Prior to the systematic study of the performance of the foams for metal ion removal, the effect of the pH of the water containing metal ions on the adsorption process is investigated. The pH is an important parameter which can affect the metal adsorption capacity, since it can influence both the surface charge of the functional groups available on the adsorbates of the surface and the metal speciation.36 The pH of the solution in which the dipped foam displays zero net surface charge is called “point of zero charge” (PZC). When the pH is higher than the PZC of the foams, their surface becomes negatively charged and provides electrostatic interactions that are favorable for adsorbing cationic species.7,37 Alternatively, when the pH of the solution is lower than the PZC, the surface charge is neutralized or becomes even positive; thus, the cations’ adsorption efficiency should decrease. For the interaction of the bioelastomeric foams with the Pb2+ ions (100 ppm), it was found that the adsorption capacity, calculated by eq 3 after 30 h of interaction time, is significantly different

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RESULTS AND DISCUSSION Figure 1a presents the fabricated bioelastomeric foam, with its color indicating the presence of the coffee powder filler. According to the SEM microscopy studies (Figure 1b), the use of the granulated sugar particles of size ∼400−500 μm provides the final foams’ pore sizes of 445 ± 114 μm, while the porosity was calculated to be 39 ± 4% and the pore connectivity degree 71 ± 11%. With a closer look to the image of Figure 1b, it can be noticed that not only are the pores of the foams submillimetric, but also micron scale porosity is present. The latter one is attributed to the porosity of the coffee powder (see inset of Figure 1b and Supporting Information Figure S2) which is preserved in the final structure and can be easily identified between the bigger pores of the foam. In the absence of spent coffee, the pure elastomeric foams (Figure 1c) have significantly lower porosity, 15 ± 5%, and pore connectivity degree of 62 ± 2%, demonstrating that these parameters depend not only on the fabrication process but also on the presence of the spent coffee filler. C


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divalent heavy metal ions36 as also shown in the Supporting Information for Hg2+ ions (Figure S4). Therefore, for the systematic study of the metal ions’ removal by the bioelastomeric foams, the pH was adjusted to 4.6 ± 0.2 assuring in this way that the capacity measured in each case is the maximum possible one. The bioelastomeric foams (0.2 g of foam contains 0.12 g or 60 wt % of spent coffee powder) were dipped in water containing different concentrations of Pb2+ ions, and Figure 3a demonstrates the time dependent removal efficiency in each case as calculated by eq 2. For all cases, the concentration of Pb2+ ions in water decreases as the interaction time between the foam and the solution increases (see Supporting Information Figure S5), while the removal efficiency increases with the interaction time, until a time above which a plateau is reached. On the other hand, the final removal efficiency of ions increases with decreasing initial concentration. Indeed, a removal efficiency higher than 90% was reached after ca. 9 hours of interaction between the foam and the ions at the lowest concentrations studied, i.e., up to 37 ppm, while at the end of the process (after 30 h) the removal efficiency reached 99%. However, as the initial ion concentration increases, the removal efficiency is reduced, falling below 50% for initial concentrations above 200 ppm, indicating that the concentration of the ions present in the water is much higher compared to that of the ions that can be adsorbed in the foams. As shown in Figure 3b, the calculated adsorption capacity (q) per unit weight of the foam in the equilibrium (after 30 h of interaction) significantly increases with the increase of the ions present in the water until it reaches the maximum value (qmax) of 13.5 mg/g. It can be assumed that at low concentrations most of the metal ions available in the solution interact with the foams’ active surface sites, which are more numerous than the total metal ions. As the concentration of the ions in water increases, the capacity of the foam increases until all the possible active surface sites become occupied, and therefore, no more ions can interact with the foam. The uptake of Pb2+ ions by the bioelastomeric foams was investigated using the Langmuir isotherm model (see Materials and Methods section; eq 4) to fit the results of Figure 3b. As shown, the model fits the experimental data remarkably well, while the qmax found is 13.4 mg/g, very close to the experimental one, 13.5 mg/g. From the fitting parameters,

for different pH values (Figure 2). In particular, as the pH increases the adsorption capacity increases too, until it reaches a

Figure 2. Effect of pH on the capacity of the bioelastomeric foam to adsorb Pb2+ ions (20 mL of Pb2+ ions with initial concentration 100 ppm and interaction time 30 h).

maximum value at pH ca. 5. Above this pH the capacity decreases. As will be shown below, the coffee powder is the only active component for the metal ions’ adsorption. Thus, taking into account that its pHPZC is about 3.9,7,38 the increase in the removal capacity as the pH increases can be attributed to the decrease in the positive surface charge, which results in a lower electrostatic repulsion between the surface and the metal cations. Specifically, at low pH (below 2), a large number of hydroxonium (H3O+) ions39 surrounds the adsorbent functional groups making the surface positively charged, thus reducing the chances for the formation of heavy metal ion− functional group complexes. As the pH increases (from 2 to 5), the concentration of H3O+ decreases, and thus, more adsorbent sites of the foam become available for the metal ions to bind. The maximum adsorption capacity is reached at about pH 5 where the surface is negatively charged. Nonetheless, at pH values exceeding 5, the adsorption capacity decreases, possibly due to Pb(OH)2 precipitate formation and therefore to the decrease of the number of ions present in water.40 This behavior is quite general and representative also for the other

Figure 3. (a) Effect of the initial Pb2+ ions concentration (C0) on the time dependent removal efficiency (%), and (b) dependence of the equilibrium adsorption capacity (qe) of the foam on the equilibrium concentration (Ce) of the Pb2+ ions in water after 30 h of interaction. The data were fitted with the Langmuir isotherm model (solid line). The initial ions’ concentrations (C0) are also shown in the graph. D


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Figure 4. XPS spectra of bioelastomeric foam: wide scan of (a) before and (b) after adsorption of Pb2+ ions. Deconvolution of the O 1s peaks (c) before and (d) after adsorption, and of the C 1s peaks (e) before and (f) after adsorption of lead.

Information Figure S6), confirming that there is no effect of the polymer on the adsorption of metal ions. Due to the absence of interaction between the polymer matrix and the metal ions, the morphology of the foam after the reaction with the Pb2+ ions features no changes (see Supporting Information Figure S7). However, as indicated by the abovementioned results, the metal ions are interacting with the spent coffee present in the foams, and therefore, the overall surface chemistry of the nanocomposite should change after the adsorption process. This is confirmed by the EDS analysis (see Supporting Information Figure S8), where the presence of lead was observed on the foam after the reaction. In order to identify the sites of the foams which interact with the metal ions, a more detailed study on their surface chemistry (before

the parameters of the Langmuir model are calculated (Supporting Information Table S1) and used to define the dimensionless parameter RL (see Materials and Methods section, eq 5) in order to identify whether the ions’ adsorption is favorable or not. In the entire range of Pb2+ concentrations, the value of the RL lies in the range 0 < RL < 1 (Table S1), confirming that the Langmuir adsorption is favorable for all the studied cases.33 In order to verify that the presence of the spent coffee powder in the foams is responsible for the removal of the metal ions, pure elastomeric foams (i.e., without coffee powder) were introduced in a 239 ppm Pb2+ ion solution. The concentration of Pb2+ ions in water remains stable (see Supporting E


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Figure 5. Time dependent removal efficiency (%) of Pb2+ ions using bioelastomeric (a) membrane under continuous-flow conditions, and (b) foam under static conditions. Both the membrane and foam consist of 0.5 g or 60 wt % of spent coffee powder, and experiments were carried out in a 20 mL of aqueous solution of ∼294 ppm Pb2+ ions.

removal efficiency decreases (Supporting Information Figure S9a). The maximum adsorption capacity (qmax) measured was 17.1 mg/g, and it was reached after ca. 1000 ppm (Supporting Information Figure S9b). This value is higher than the qmax of the Pb2+ ions (13.5 mg/g reached after ca. 400 ppm), indicating higher affinity and possibly different interaction mechanisms with the spent coffee functional groups. The efficiency of the bioelastomeric foams was further tested in the presence of mixed metal ions in wastewater. In the presence of Pb2+ (22 ppm), Hg2+ (30 ppm), Cu2+ (10 ppm), Al3+ (14 ppm), Mg2+ (12 ppm), and Co2+ (10 ppm), the bioelastomeric foam presents significant removal efficiencies of 48.04%, 68.15%, and 19.65% for Pb2+, Hg2+, and Cu2+ ions, respectively, after 30 h of interaction, with corresponding adsorption capacities of 1.03, 2.05, and 0.19 mg/g (Supporting Information Figure S10). It should be noticed that these adsorption capacities are not far from the ones obtained when only the isolated ions are present (i.e., 2.62 mg/g for 26 ppm of Pb2+, 3.82 mg/g for 40 ppm of Hg2+, and 0.9 mg/g for 13 ppm of Cu2+ (Supporting Information Figures S9 and S11)). In addition, it was found that the foams are able to remove the Co2+ and Al3+ from wastewater, with lower efficiencies of about 3.55% and 10.83%, respectively. These results confirm the ability of the bioelastomeric foams to remove Pb2+ and Hg2+ ions without significant alteration of their functionality under the presence of other ions. Finally, the adsorption performance of the bioelastomeric foams was tested under continuous-flow experiments, in order to determine if they can be employed in more practical filtration processes. As a proof of concept, the removal of Pb2+ ions was tested when 20 mL of an aqueous solution of 294 ppm Pb2+ was passing through a bioelastomeric membrane at different rates. As shown in Figure 5a the removal efficiency increases when the flow rate decreases, starting from a value of 4.25% at the fastest (1200 mL/h) and reaching 66.72% at the slowest rate applied (2 mL/h), indicating that the interaction time between the ions and the foam is an important parameter for efficient adsorption. At high flow rates the ions do not have enough time to interact and be adsorbed on the active surface sites of the membrane. The increment of the interaction time (lower flow rates) allows the ions to bind on the active sites of the membrane, resulting in higher removal efficiency (Figure 5a). In a comparison with the performance reached in a static experiment where the same quantity of the spent coffee in

and after reaction) was performed by XPS. From the wide scan spectra shown in Figure 4a,b (before and after the Pb2+ adsorption, respectively), peaks arising from the PDMS and coffee, such as Si 2p, O 1s, and C 1s, are clearly detected in both cases. On the contrary, the Pb4f peak was only detectable after reaction (Figure 4b), demonstrating the Pb2+ adsorption on the bioelastomeric foam. Furthermore, after the Pb2+ adsorption, a noteworthy difference in the binding energy of the oxygen (O 1s) peak is observed (shift from 532.30 to 531.80 eV) accompanied by the decrease of the atomic percentage from 21.85% to 18.49%. Hence, lead adsorption is accompanied by a change in oxygen binding state, providing evidence that oxygen containing groups take part in this process.41 The deconvolution of the high resolution O 1s spectra of the foam (Figure 4c,d) shows that the oxygen of the CO group after reaction has lower binding energy than before (at 531.84 eV from 532.62 eV), whereas for the CO component no displacement of the binding energy was observed (534.32 eV before and 534.31 eV after reaction). Although the overall position of the C 1s peak does not change, the deconvolution of the high resolution C 1s spectrum before and after reaction also showed significant differences. In particular, before reaction the C 1s peak (Figure 4e) consists of three peaks, with binding energies of 284.81, 286.70, and 289.09 eV, assigned to CC, CO and OCO bonds, with the latter two corresponding to alcohol and carboxylate groups.36 After the Pb2+ adsorption, the binding energy of the carbon of the carboxylate group decreases from 289.09 to 288.33 eV (Figure 4f), indicating a change in its state, whereas the other binding energies remain unaltered. All these observations can be assigned to the interaction of the metal ions with the oxygen of the carboxylate groups of the spent coffee present in the bioelastomeric foams, as also observed by others.41,42 It should be noted that this experimental confirmation of the interaction of the Pb2+ ions with just one functional group of the spent coffee powder further supports the use of the Langmuir isotherm model, as it is based on monolayer sorption onto a surface containing a finite number of identical sorption sites (i.e., the carboxylate groups). To evaluate the possibility of a broader use of the developed bioelastomeric foams, the removal of Hg2+ from water was tested. Also, in this case, the bioelastomeric foam is able to remove the ions with efficiency about 99% at low metal ion concentrations (16 ppm). As the concentration increases, the F


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ACS Sustainable Chemistry & Engineering bioelastomeric foam is employed for the removal of the same quantity of Pb2+ ions from a 20 mL solution (Figure 5b), it was found that in both cases after 9−10 h of interaction about 67% of Pb2+ ions was removed. As can be seen, the performance between the non-continuous-flow and continuous-flow removal approaches are similar. Thus, it is highly likely that the noncontinuous adsorption test results obtained in this work can be extended to the continuous filtration process. In a comparison to other published works, the present bioelastomeric foam has good adsorption performance, as well as additional advantages such as being user-friendly and easy to recover after adsorption. Specifically, Utomo et al. used waste coffee powder for the removal of 50 ppm Pb2+ ions, with an adsorption capacity of 0.78 mg/g,5 much lower than the one obtained in the present work (4.66 mg/g for 50 ppm of metal ion solution). In another study,6 coffee grounds were used to remove Pb2+ ions (10 ppm or 2 mg of ions in 200 mL) by varying the adsorbent mass (2 to 10 g), and the overall removal efficiency was ∼80% for all the adsorbent masses employed. Here, a similar adsorption capacity was observed for a similar quantity of ions in water (82 ppm or 1.64 mg of ions in 20 mL) by using only 0.2 g of bioelastomeric foam which contains 0.12 g of spent coffee. The highest Pb2+ ion adsorption capacities reported in the literature using coffee were achieved with waste coffee residues,43 reaching adsorption capacity of 27.6 mg/g. This capacity is almost double compared to that of the present study, but it should be considered that herein the capacities are calculated per gram of foam and not per gram of coffee powder. With the consideration that the removal of ions is attributed solely to the spent coffee of the foam, the qmax calculated per gram of coffee powder used in our study becomes 22.5 mg/g and therefore is in the same range. Furthermore, in the so far reported works, post-treatment processes (e.g., filtration) are needed to remove the reacted coffee waste from the water. In the current study, composites of spent coffee powder with elastomer are used, requiring no additional separation techniques, and offering, therefore, significant advantages with respect to other approaches presented so far. In conclusion, the present study demonstrates the formation of spent coffee based bioelastomeric foams by a straightforward method, and their successful use for the removal of heavy metal ions from water. The coffee loading was performed during the fabrication of the foam, without specific reaction conditions. These materials showed high adsorption capacities of 13.5 and 17.1 mg/g for Pb2+ and Hg2+ ions, respectively. The adsorption mechanisms of the Pb2+ ions in the bioelastomeric foam were identified, revealing the interaction of the metal ions with the carboxylate groups of the coffee filler. In addition, we successfully demonstrate the removal of the above-mentioned metal ions from wastewaters with six metal ions present. Hence, the present foams have improved performance compared to earlier studies, with a remarkable adsorption capacity. Other additional advantages are that they can be easily handled and disposed, and employed as filters for the continuous-flow metal ions adsorption, enabling new methods for the reutilization and valorization of this particular waste.

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Experimental setup and additional characterization data, including SEM images, water drop volume evolution, effects of experimental conditions, Langmuir parameters, ICP analysis, and EDS analysis (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors kindly acknowledge Mr. F. Drago (Istituto Italiano di Tecnologia) for ICP-OES measurements, and Mr. S. Nitti (Istituto Italiano di Tecnologia) for providing the mixed metal ion wastewater.

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REFERENCES

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