Silver Recovery from Laundry Washwater: The Role of Detergent

Nov 21, 2017 - The use of silver nanoparticles as an antimicrobial agent on textiles is rising. Ag leaching during laundry and its subsequent discharg...
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Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Silver Recovery from Laundry Washwater: The Role of Detergent Chemistry Tabish Nawaz† and Sukalyan Sengupta*,† †

Civil and Environmental Engineering Department, University of Massachusetts Dartmouth, 285 Old Westport Road, Dartmouth, Massachusetts 02747, United States S Supporting Information *

ABSTRACT: The use of silver nanoparticles as an antimicrobial agent on textiles is rising. Ag leaching during laundry and its subsequent discharge in the environment pose ecotoxicological risks. Removing Ag from laundry washwater is therefore an environmental necessity, but its recovery also leads to environmental sustainability. Low Ag concentration, competition from other cations (such as Ca2+, Mg2+, and Na+), and complexity of the detergent matrix make the recovery process challenging. The present study utilizes a thiol-group functionalized ion-exchange resin in a fixed-bed column to remove Ag from laundry washwater and recover it as Ag2S nanoparticles or high-purity powder. The role of each detergent component in affecting Ag speciation and the resin performance has been analyzed. Builders and bleaching agents are reported to negatively impact the resin performance. pH and cocationic species (Ca2+) concentration are reported to be critical parameters for the successful recovery scheme. The work demonstrates a closedloop sustainable scheme by recycling and reusing the resin and the regenerant solution over 5 cycles. KEYWORDS: Source-separated wastewater treatment, Resource recovery, Detergent chemistry, Ion-exchange resin, Sustainability, Silver recovery

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a previous work,19 we have demonstrated the application of a thiol-group-functionalized ion-exchange resin, Ambersep GT74, for selective recovery of Ag (at concentration 99%) grade powder from the laundry wash solution.

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EXPERIMENTAL METHODS

Interaction of Free Ag+ with Detergent Components Using ISE. Each detergent component was studied separately for interaction with free Ag+ ions using the ISE electrode. The initial ISE reading was recorded corresponding to the initial free Ag+ ion concentration of 5 mg/L. The detergent component was then added to the solution (1 L volume) simultaneously to achieve its respective concentration (Table S1). The experiment was run for a typical washing time of 15 min, and the ISE readings were noted at t = 0, 5, 10, and 15 min. The corresponding samples (10 mL) at these time intervals were withdrawn from the solution, immediately filtered (0.2 μm), and analyzed using AAS. The comparison of the two measurements enabled one to determine if the free Ag+ complexed, precipitated or remained undisturbed in the presence of the respective detergent components studied. Resin Uptake Performance in the Presence of Detergent Components. 1 L of 5 mg/L Ag+ solution was prepared in DI water in a tightly sealed glass jar. The detergent component being studied was then added to the solution at its corresponding concentration (Table S1). The solution was stirred for 15 min, after which it was filtered using a 0.2 μm filter. The filter paper was analyzed using EDS coupled with SEM for Ag-containing precipitate. A 10 mL aliquot was withdrawn for AAS analysis from the filtrate, and ∼0.1 g of the resin was introduced into the jar. In the part of the investigations that involved zeolite and perborate/TAED, Ag was lost to the solid (zeolite) phase and Ag0 particles, respectively (discussed in Results and Discussion); in these cases, a 0.2 μm syringe filter was used to collect the samples for AAS analysis. The tightly sealed jar was put in a rotary mixer for 24 h at a temperature of 20 ± 2 °C (cold wash). The chosen time of 24 h for attainment of equilibrium is based on our previous study with the same resin.19 At the end of 24 h, the resin was filtered out, and a 10 mL aliquot was taken from the filtrate for AAS analysis. In the study with zeolite, the resin and zeolite were separated using 90 μm pore size sieves (RETSCH Test Sieves, Fisher Scientific). The filtered resin was rinsed in DI water and was regenerated in 100 mL of acidified thiourea solution (0.5 M thiourea, 0.1 M nitric acid) for 2 h in the rotary mixer. The choice of 0.5 M thiourea as regenerant was determined by comparing its regeneration performance with a variety of regenerants (2 M HCl (8% regeneration); 1 M HNO3 (11% regeneration); and lower concentrations of thiourea (60−100% regeneration)). The rationale of the regenerant specifications (thiourea concentration and pH) is provided in detail in our earlier work.19 The Ag-loaded zeolite was regenerated using 100 mL of HNO3 solution (pH ∼3) in a rotary mixer with 2 h of stirring. The uptake/removal by the resin is calculated as

MATERIALS AND METHODS

Materials. The commercial resin Ambersep GT74 was obtained from Supelco Analytical (Bellefonte, PA), and prepared for use in this study using the methodology described in our previous work.19 Twelve different detergent components were studied (Table S1). Technicalgrade chemicals sodium dodecylbenzenesulfonate (SDBS), zeolite, diethylene triamine pentaacetate (DTPA), sodium metasilicate pentahydrate, sodium carboxy methyl cellulose (CMC) (Mw ≈ 90 000), sodium perborate tetrahydrate, and 4,4′-diamino-2,2′stilbenedisulfonic acid were purchased from Sigma-Aldrich; sodium carbonate, sodium chloride, sodium nitrate, calcium nitrate tetrahydrate, sodium hydroxide (10 N), and ethanol (denatured) were obtained from Fisher Scientific. Silver nitrate, nitric acid (67−70% strength), TAED, and silver sulfide standard were purchased from Alfa Aesar; thiourea and sodium dodecyl sulfate (SDS) were purchased from Acros Organics. Magnesium sulfate was obtained from Allied Chemical; silver nitrate standard (as 1000 ppm Ag+) was purchased from PerkinElmer. Silver ionic strength adjustor (ISA) was purchased from Thermo Scientific Orion (Cat. No. 940011). Berol 266 was kindly provided by Akzo Nobel USA. Synthetic Laundry Wash Water Preparation. 2 g/L of detergent solution was prepared (roughly twice the typical concentration)21 by dissolving the components at their respective concentrations (Table S1) in deionized (DI) water. It is acknowledged that tap water having chloride in it can reduce the free Ag+ concentration. However, Mitrano et al.,22 observed that the transformation of Ag+ in wash solution is independent of tap or DI water and is primarily guided by detergent chemistry. The study22 also reports that Ag+ remained dissolved in the solutions prepared from liquid laundry detergents, irrespective of chloride in the system. Since the aim of this study is to explore the role of liquid laundry detergent chemistry, DI water was chosen over tap water. On the basis of our previous study,19 60 mg/L Ca2+ as calcium nitrate tetrahydrate, 8 mg/ L Mg2+ as magnesium sulfate, and 5 mg/L Ag+ as silver nitrate were added to the detergent solution. The pH was adjusted using nitric acid or sodium hydroxide. Analytical Procedure. Total dissolved Ag in the solution was analyzed using a PerkinElmer Atomic Absorption Spectrometer (AAS) (AAnalyst 300) with 5 calibration points (blank, 1, 2, 5, and 10 mg/L Ag; linear calibration curve). The free Ag+ ion concentration was

uptake/removal = (C@t = 15min − C@t = 24h) × V where C@t=15min = Ag+ concentration after 15 min with the detergent component, C@t=24h = Ag+ concentration after 24 h with the resin and detergent component, and V = volume of the solution (980 mL). On the basis of the mass leached from the resin during regeneration, percent recovery is defined as B

DOI: 10.1021/acssuschemeng.7b02933 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 1. Interaction of Ag+ ions with the respective detergent components observed within a typical washing time (15 min) at temperature ∼20 °C; Initial [Ag+] ∼5 mg/L; ISE and AAS readings correspond to free Ag+ and total dissolved Ag concentrations, respectively.

% recovery =

Ag mass leached during regeneration × 100% initial Ag mass present in the solution

for analysis and water lost in the Ag2S precipitate over subsequent cycles by adding the required amount of the fresh regenerant.

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RESULTS AND DISCUSSION Ag + Ion Interaction with Individual Detergent Components. Figure 1 shows the concentration profiles for free Ag+ ions and total dissolved Ag in the presence of the detergent components at their respective concentrations and solution pH. In the presence of surfactants (SDBS, SDS, and Berol), free Ag+ ISE concentration shows negligible change, suggesting insignificant interaction of Ag+ ion with SDBS, SDS and Berol; and Ag dominantly exists as free Ag+ (Figure 1a−c). In the presence of zeolite (Figure 1d), the free Ag+ ion and total dissolved Ag concentrations dropped to almost zero, implying disappearance of Ag from the solution. This is due to ion-exchange of Ag+ ions with Na+ ions in the insoluble solid zeolite phase. However, reducing the pH to ∼3, restored the free Ag+ signal to its initial value, indicating the ability of zeolite to release Ag+ in the presence of high concentration of H+. In the case of metasilicate ions (Figure 1e), the free Ag+ ion concentration fell to 80% of its initial value; total dissolved Ag, however, remained constant. The free Ag+ concentration loss is due to the interaction of Ag+ with metasilicate anions, OH− ions (pH ∼10.9), or both. Since the total dissolved Ag remained constant, precipitation is ruled out. However, at a pH 10.9, Ag2O sols may form,23 but no color change was observed in the solution. Also, the EDS analysis of the filter paper (over which the solution was filtered) did not show any presence of an Ag precipitate. In a test study conducted at a lower pH (∼9.5) but with the same metasilicate concentration (∼0.267 g/L), free Ag+ concentration showed roughly the same loss (reduced to ∼77% of its initial value) (Figure S1a), whereas for the solution at pH ∼9.5 (no metasilicate), the free Ag+

Warm wash (35 °C) and hot wash (55 °C) studies were carried out in Incushaker 10L H1010, Benchmark Scientific with the same protocol as described above for the cold wash. Fixed-Bed Column Run: Uptake and Regeneration. Fixed-bed column runs (uptake and regeneration) were carried out according to the procedure described in our previous work.19 First, 1 g of the dry resin was used to prepare the fixed-bed. The influent detergent solution, after pH adjustment to ∼3.5−4, was passed through the column in downflow mode. The flow rate provided an empty bed contact time (EBCT) of ∼1 min. Precipitation of Silver Sulfide. NaOH solution (1 N) was added dropwise to the slowly stirred spent regenerant solution (∼50 mg/L Ag+) until the colorless solution turned light brown. A 10 mL aliquot was withdrawn for TEM and EDS analysis of the precipitate. To the remaining solution, 1 N NaOH was continuously added dropwise until the pH rose to 11−12 and a black precipitate appeared. The precipitate was filtered using 0.2 μm filter and collected on the filter paper. AAS analysis on the filtrate was done to ensure complete Ag precipitation. The precipitate collected on the filter paper was analyzed using SEM and EDS. Reuse of Resin and the Spent Regenerant. The reusability of the resin was studied in the batch mode. In the first cycle, 0.1 g of the resin was added to 1 L of the detergent solution (at pH ∼3.5−4) in a tightly sealed glass jar. The jar was then put in the rotary mixer for 24 h. Aliquots of 10 mL were withdrawn initially and finally for AAS analysis. The uptake was determined as defined above. The resin was then regenerated using fresh acidified thiourea solution. The resin after regeneration was reused for subsequent cycles according to the method described in our previous work.19 The regenerant solution after precipitation, subsequent filtration, and pH adjustment to ∼1.5 was reused for the regeneration of the resin in its subsequent cycles. We compensated for the volume loss of the regenerant (∼3−4% each cycle) due to sample volume withdrawn C

DOI: 10.1021/acssuschemeng.7b02933 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering concentration was reduced to only ∼97% of its initial value (Figure S1b). This shows that the reduction of free Ag+ concentration (to ∼77% of its initial value) is mainly due to the complexation of Ag+ with metasilicate anions. However, at pH 10.9, some loss due to high OH− concentration is not completely ruled out; in a separate test study, without metasilicate anion, the solution (at pH ∼10.9) exhibited reduction in free Ag+ concentration to ∼87% of its initial value (Figure S1c). It is inferred that the reduction in free Ag+ ions is due to its interaction with both metasilicate and OH− anions. In the pH range of 3.5−4, no reduction in free Ag+ concentration was observed (Figure S1d). Metasilicate anion exists predominantly as metasilicic acid at this pH range (pKa1 = 9.9, pKa2 = 11.8),24 which due to protonation is unable to coordinate with Ag+. The effect of 0.1 g/L sodium carbonate (pH ∼10.3) on free Ag+ concentration is negligible (∼6% loss of free Ag+ initial concentration) (Figure 1f). As total dissolved Ag remained constant, precipitation is ruled out. The minor loss can be due to the hydrolysis of Ag+ leading to the formation of its hydroxy complexes (pKa: ∼12),25 as the theoretical analysis reveals that roughly 2−3% of Agtotal exists as Ag(OH) and Ag(OH)2− at pH ∼10.3.25 Also, the solution without sodium carbonate at pH ∼10 resulted in a similar loss of free Ag+ concentration (∼6%) (Figure S2). In the presence of DTPA (Figure 1g), free Ag+ ions completely disappeared from the solution; however, total dissolved Ag remained constant. This shows that precipitation did not occur, and the loss in Ag+ signal was wholly due to chelation by DTPA via the carboxylate and amine groups.26 Since Ag−oxoanionic interaction is ionic in nature,27 introducing species with higher charge density than Ag+ was hypothesized to free up Ag+ ions. Therefore, in separate test studies with the same DTPA and Ag+ concentrations, adding a drop of 67−70% strength HNO3 (pH reduced to ∼5 from ∼10.3) improved free Ag+ concentration from ∼0 to ∼90% instantly. With further addition of a drop of the acid, which decreased the pH to ∼3.5, free Ag+ concentration was restored to 100% of its initial value (Figure S3a). The available literature26 also reports complete existence of Ag cations in free Ag+ form in the presence of DTPA at pH < 4. The hypothesis was also tested and confirmed with Ca2+ ions (60 mg/L as Ca) present along with Ag+ ions in the solution when DTPA was introduced (Figure S3b). These studies confirmed the hypothesis that DTPA chelated Ag+ through the Ag−O interaction of its carboxylate group and that Ag+ availability can be improved by reducing the solution pH to acidic range (3−4) or by introducing species like Ca2+ into the solution. In the case of ethanol, CMC, and stilbene (Figure 1h−j, respectively) negligible losses of ∼2, 5, and 6%, respectively, in free Ag+ signal were recorded, with no change in total dissolved Ag concentration. The minor loss may occur due to the Ag−O interaction and Ag−N/Ag−O interaction in cellulose and ethanol and stilbene, respectively. However, these signal losses are considered insignificant to disturb the recovery scheme. Figure 1k,l depicts the effect of perborate/TAED on free Ag+ ions at two different concentrations (Figure 1k, perborate: 0.52 g/L, TAED: 0.078 g/L (corresponds to typical maximum dose: ∼20% of total detergent dose);20 Figure 1l, perborate: 0.14 g/L, TAED: 0.021 g/L (corresponds to typical liquid laundry dose: 0−10% of total detergent dose)).28 In both the scenarios, the free Ag+ and the total dissolved Ag concentrations reduced to zero, implying Ag precipitation. The solution turned turbid

with a brownish-greyish tint, and gas bubbles were prominently seen attached to the beaker wall. The presence of bubbles, possibly of O2(g), indicated possible autodecomposition of H2O2 in the presence of trace heavy metal ions,27 Ag+ in this case. However, this did not explain the precipitate formed. The EDS analysis on the precipitate collected from the solution showed signal only for Ag, thus indicating reduction of Ag+ to Ag0 (Figure S4c). This indicates a redox reaction in the system. The species present in the system prior to precipitation are Ag+, NO3−, B(OH)4−, H2O2, HO2−, OH−, TAED, triacetlyethylenediamine (TriAED), diacetylethylenediamine (DAED), and peracetic acid (PAA).29,30 An earlier work4 shows that at pH ∼10 PAA does not react with Ag+. NO3− and B(OH)4− are ruled out as N and B are at their highest oxidation states. TAED was separately tested for the reduction of Ag+ in this work; none was observed. The role of TriAED and DAED are also ruled out as they are even more stable than TAED, with DAED being practically inert with resonance stabilization.30 McKillop and Sanderson29 report generation of H2O2/HO2− from perborate in aqueous solutions, both of which can act as the reducing agent. On the basis of the pKa value of H2O2 (11.6),29 at pH ∼10 the relative percent compositions of H2O2 and HO2− are ∼98 and ∼2%, respectively. Since in acidic pH range only H2O2 exists, we conducted the speciation study at pH ∼3. We noticed neither precipitation nor bubble formation, and both ISE and AAS readings remained steady at their initial values throughout the experiments (Figure S5a). This proved that H2O2 was not the reducing agent in the precipitation. Therefore, the active reducing agent in the system is HO2− leading to precipitation of Ag0 according to the following reactions:31 HO2− + OH−→ H 2O + O2 + 2e− Eox = −0.076 V

Ag + + e− → Ag 0

Ered = 0.799 V

(1) (2)

2Ag + + HO2− + OH−→ 2Ag 0 + H 2O + O2

Eoverall = 0.723 V

(3)

Since the reaction potential of eq 1 is lower than that of eq 2, the perhydroxyl ions can act as a reducing agent to reduce the free Ag+ ions. Moreover, the presence of bubbles in the beaker testifies to the evolution of O2(g), which is possible when O in HO2− (oxidation state: −1) is oxidized to O2(g) (oxidation state: 0). Geranio et al.4, working with a similar system, observed precipitation and loss of free Ag+ ISE signal. However, the mechanism leading to the precipitation and the precipitate identity was not elaborated upon. In our study, we report that the perborate/TAED bleach system in alkaline conditions (pH ∼9−10) reduces Ag+ to Ag0. Ag+ speciation analysis identifies mainly builders (zeolite, sodium metasilicate pentahydrate, and DTPA) and bleaching agents (sodium perborate tetrahydrate and TAED) in a typical laundry formulation as the key detergent components that may offer competition to the resin for Ag uptake. The role of pH and co-cations, like Ca2+, are considered critical for designing an effective recovery scheme, as species like H+ and Ca2+ screen Ag+ from the detergent components, and make it available for removal by the resin. Ag - Uptake Performance of the Resin: Influence of Individual Detergent Components. Figure 2 shows Ag D

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Figure 2. Ag uptake and recovery performance of the resin in the presence of the respective detergent components; initial [Ag+] ∼ 5 mg/L, resin mass ∼0.1 g, solution volume: 1 L, regenerant: 0.5 M thiourea and 0.1 M nitric acid; first bar (blue) from left represents initial [Ag+], second bar (red) is [Ag+] at t = 15 min after adding the detergent component, third bar (black) is [Ag+] at t = 24 h after adding the resin, fourth bar (green) is Ag mass removed from the solution after 24 h, and fifth bar (cyan) is Ag mass recovered after regeneration.

suggests Ag−thiol complexation as the primary Ag uptake mechanism. For CMC, the recovery is comparably low (∼80%). Since no precipitation was seen and almost all Ag disappeared from the solution at the end of 24 h, Ag uptake by the resin was confirmed, but Ag was not completely desorbed from the resin during regeneration. CMC is a soil anti-redeposition agent, which creates a protective layer around the solid surface sterically inhibiting redeposition of already removed particulate soil.20 Therefore, adsorption and subsequent spreading of CMC on the resin is possible, potentially resulting in steric hindrance, which may have prevented thiourea to access some of the Ag (∼15−20%) adsorbed in the resin during the regeneration. However, this behavior of CMC does not pose any problem for the Ag recovery from the detergent solution, since, in the presence of surfactants the adsorption of CMC is significantly reduced.20 This explains effective regeneration (∼100%) in the case where we have studied the overall detergent formulation (refer to Section S7, Supporting Information). In the case of zeolite, almost all Ag ended up in the solid zeolite phase (Figure 2d) at the end of 15 min. After regeneration, the resin and the zeolite phases leached ∼1.05 ± 0.09 mg (∼20% recovery) and ∼4.04 ± 0.12 mg of Ag, respectively. The size exclusion prevented the resin to access and capture Ag+ ions from the solid zeolite phase as resin harmonic mean size19 is 0.45−0.7 mm, whereas zeolite pore diameter is typically 98% of Ag was recovered from the resin phase (Figure S5b). The improvement in the resin performance is attributed to the prevention of Ag+ reduction by HO2−, as at this pH (∼3) only H2O2 exists in the solution, and Ag exists as free Ag+ ions. The studies under warm and hot wash thermal conditions yielded similar results as those under cold wash condition; this is reported in Table S2. The results discussed above indicate that for the resin to effectively remove Ag (>90% removal), the predominant form of Ag should be free Ag+. In cases where free Ag+ is severely compromised (zeolite, DTPA, and perborate/TAED), the resin removal performance has been low. However, introduction of Ca2+, or reducing the pH into acidic range (∼3−5) improves the resin performance. Therefore, to recover Ag from an overall F

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Figure 4. TEM image and EDS spectra of Ag2S nanoparticles obtained after alkaline hydrolysis of spent regenerant (∼50 mg/L of Ag) with recycled NaOH wash solution (∼1 N).

is still below its maximum uptake capacity (300 mg of Ag+ per g of dry resin).19 In our earlier work,19 the packed-bed column treated ∼20 000 BVs of the influent solution containing ∼5 mg/L Ag+ with the similar cationic system and composition but without detergent components. Since in the present study the availability of free Ag+ ions to the resin bed was ensured through pH adjustment to ∼3.5−4, the break through in ∼9000 BVs as compared to that in 20 000 BVs as noted earlier19 suggests fouling of the bed by the organic matter in the detergent formulation. This aspect of the recovery scheme is being explored and would form a separate study. The detergent chemistry impacts the saturation capacity of the resin bed via fouling of ion-exchange sites; however, it has limited bearing upon the selectivity of the resin. This is significant for the recovery scheme, as in real samples Ag is expected to be in trace concentrations, and selectivity would be a critical design parameter in such cases. The fixed-bed was regenerated (>90% regeneration) by passing 152 BVs of the regenerant solution (Figure 3b). The regeneration mechanism of the resin using acidified thiourea solution is described in our previous work.19 The peaks at BV# 87 and 115 appeared after the column regeneration was stopped overnight, and continued the next day. This is due to the intraparticle diffusion being the rate limiting step for the regeneration ion-exchange process. The resin and the regenerant were recycled and reused over 5 cycles without any significant loss in their performances. The detailed discussion on this aspect is available in Section S7. Precipitation of Silver Sulfide. The particles in size range of ∼3−7 nm were prominently seen for the precipitate withdrawn during the onset of precipitation (when the solution turned light brown at a pH ∼7−8 and with gentle stirring) (Figure S6a). EDS spectra of the particles shows prominent Ag and S peaks (Figure 4). UV−vis spectra of the solution (absorbance peak at ∼310 nm) is indicative of Ag2S nanoparticles37 (Figure S6b). For the black precipitate obtained after complete precipitation (pH ∼11−12), EDS analysis reveals prominent Ag and S peaks (Figure S6c). On comparing the EDS analysis of the precipitate obtained in this study with the assay of 99.5% pure Ag2S standard (Figure S6d), it is inferred that the precipitate is high-purity Ag2S. AAS analysis

detergent formulation using an ion-exchange system, the solution pH is deemed a critical parameter. Fixed-Bed Column Run: Exhaustion and Regeneration. Figure 3a shows that Na+, Mg2+, and Ca2+ brokethrough within ∼10, ∼20, and ∼40 BVs respectively, whereas Ag+ needed ∼9000 BVs to break-through. This shows preferential selectivity for Ag+ by the resin against Na+, Mg2+, and Ca2+. The selectivity sequence (Ag+ ≫ Ca2+ > Mg2+ > Na+) observed is similar to reported literature.19 The fixed-bed column treated 14 000 BVs of ∼5 mg/L Ag+ detergent solution, removing ∼84% of the influent Ag. The role of influent solution pH is critical to the performance of the fixed bed. In the present study, when the influent detergent solution was passed through the column at its typical laundry pH (∼10), Ag+ broke through within 100 BVs. The ISE and AAS analyses on the influent detergent solution (at pH ∼10) revealed free Ag+ and total dissolved Ag concentrations as 1.39 ± 0.03 and 4.99 ± 0.024 mg/L, respectively. The difference in the two analyses suggests that most of the Ag was in complexed state in the influent solution, and only 28 ± 0.86% was in free Ag+ form. The deficient performance of the column was attributed to the low availability of free Ag+ ions in the solution. Since we noticed increase of free Ag+ concentration in the acidic pH range with components like DTPA, metasilicate and perborate/TAED, a free Ag+ speciation study using ISE was conducted with ∼5 mg/L Ag+ detergent solution at pH values of ∼3.6, 5.7, and 7.5. The study noted that at pH 3.6, 5.7, and 7.5, free Ag+ concentrations (at t = 15 min) were 4.8 ± 0.23, 3.84 ± 0.14, and 2.27 ± 0.32 mg/L respectively. Therefore, for the column run a pH of ∼3.5−4 was selected for the influent solution. This improved the column performance (Figure 3a), confirming the low availability of free Ag+ ions as the cause for deficient column performance at the influent pH of ∼10. The effluent pH was in the range of ∼1.8−2. Under real circumstances, laundry wash water makes ∼15−20% of total domestic wastewater (pH ∼6.5−8), which in turn is ∼30% of municipal wastewater influent.36 Therefore, on account of dilution, we expect only a minor impact of acidic pH on subsequent wastewater operations. The column performance (176 mg of Ag+ per g of dry resin uptake), though improved at pH ∼3.5−4 compared to pH ∼10, G

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(7) Mitrano, D. M.; Rimmele, E.; Wichser, A.; Erni, R.; Height, M.; Nowack, B. Presence of nanoparticles in wash water from conventional silver and nano-silver textiles. ACS Nano 2014, 8 (7), 7208−7219. (8) Benn, T. M.; Westerhoff, P. Nanoparticle silver released into water from commercially available sock fabrics. Environ. Sci. Technol. 2008, 42 (11), 4133−4139. (9) Quadros, M. E.; Pierson, R., IV; Tulve, N. S.; Willis, R.; Rogers, K.; Thomas, T. A.; Marr, L. C. Release of silver from nanotechnologybased consumer products for children. Environ. Sci. Technol. 2013, 47 (15), 8894−8901. (10) Lombi, E.; Donner, E.; Scheckel, K. G.; Sekine, R.; Lorenz, C.; Von Goetz, N.; Nowack, B. Silver speciation and release in commercial antimicrobial textiles as influenced by washing. Chemosphere 2014, 111, 352−358. (11) Notter, D. A.; Mitrano, D. M.; Nowack, B. Are nanosized or dissolved metals more toxic in the environment? A meta-analysis. Environ. Toxicol. Chem. 2014, 33, 2733−2739. (12) Yeo, M. K.; Yoon, J. W. Comparison of the Effects of Nanosilver Antibacterial Coatings and Silver Ions on Zebrafish Embryogenesis. Mol. Cell. Toxicol. 2009, 5, 23−31. (13) Van Der Zande, M.; Vandebriel, R. J.; Van Doren, E.; Kramer, E.; Herrera Rivera, Z.; Serrano-Rojero, C. S.; Gremmer, E. R.; Mast, J.; Peters, R. J. B.; Hollman, P. C. H.; et al. Distribution, elimination, and toxicity of silver nanoparticles and silver ions in rats after 28-day oral exposure. ACS Nano 2012, 6, 7427−7442. (14) Gagné, F.; André, C.; Skirrow, R.; Gélinas, M.; Auclair, J.; van Aggelen, G.; Turcotte, P.; Gagnon, C. Toxicity of silver nanoparticles to rainbow trout: A toxicogenomic approach. Chemosphere 2012, 89, 615−622. (15) Ivask, A.; Elbadawy, A.; Kaweeteerawat, C.; Boren, D.; Fischer, H.; Ji, Z.; Chang, C. H.; Liu, R.; Tolaymat, T.; Telesca, D.; et al. Toxicity mechanisms in Escherichia coli vary for silver nanoparticles and differ from ionic silver. ACS Nano 2014, 8, 374−386. (16) Tan, J. M.; Qiu, G.; Ting, Y. P. Osmotic membrane bioreactor for municipal wastewater treatment and the effects of silver nanoparticles on system performance. J. Cleaner Prod. 2015, 88, 146−151. (17) Ahamed, M.; AlSalhi, M. S.; Siddiqui, M. K. J. Silver nanoparticle applications and human health. Clin. Chim. Acta 2010, 411 (23), 1841−1848. (18) Westerhoff, P.; Lee, S.; Yang, Y.; Gordon, G. W.; Hristovski, K.; Halden, R. U.; Herckes, P. Characterization, recovery opportunities, and valuation of metals in municipal sludges from US wastewater treatment plants nationwide. Environ. Sci. Technol. 2015, 49 (16), 9479−9488. (19) Nawaz, T.; Sengupta, S. Silver recovery from greywater: Role of competing cations and regeneration. Sep. Purif. Technol. 2017, 176, 145−158. (20) Smulders, E.; Von Rybinski, W.; Sung, E.; Rähse, W.; Steber, J.; Wiebel, F.; Nordskog, A. Laundry Detergents; Wiley: Weinheim, 2007. (21) Cameron, B. A. Family and Consumer Sciences. Fam. Consumer. Sci. Res. J. 2007, 36 (2), 151−162. (22) Mitrano, D. M.; Arroyo Rojas Dasilva, Y.; Nowack, B. Effect of variations of washing solution chemistry on nanomaterial physicochemical changes in the laundry cycle. Environ. Sci. Technol. 2015, 49 (16), 9665−9673. (23) Murray, B. J.; Li, Q.; Newberg, J. T.; Menke, E. J.; Hemminger, J. C.; Penner, R. M. Shape-and size-selective electrochemical synthesis of dispersed silver (I) oxide colloids. Nano Lett. 2005, 5 (11), 2319− 2324. (24) Lide, D. R., Frederikse, H. P. R., Eds. CRC Handbook of Chemistry and Physics, 76th ed.; CRC: Boca Raton, FL, 1995. (25) Baes, C. F.; Mesmer, R. E. Hydrolysis of Cations; Krieger: Malabar, FL, 1976. (26) De Stefano, C.; Lando, G.; Pettignano, A.; Sammartano, S. Evaluation of the sequestering ability of different complexones towards Ag+ ion. J. Mol. Liq. 2014, 199, 432−439. (27) Cotton, A. F.; Wilkinson, G.; Gaus, P. L. Basic Inorganic Chemistry, 3rd ed.; Wiley: New York, 1995.

on the solution pre-and post-precipitation revealed complete Ag precipitation. The mechanism of precipitation can be found in our previous work.19 In this work, we have developed a conceptual framework for Ag recovery from the detergent matrix, and successfully demonstrated its application aspect using a thiol-group functionalized ion-exchange resin, Ambersep GT74. In keeping with the idea of sustainability and green chemistry, we have reused the resin and the regenerant solution over multiple cycles in a closed-loop process. This methodology has resulted in reduction of the chemicals used, recycling and prevention of their disposal, particularly of thiourea, as it is toxic.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02933. Details on detergent composition, resin removal and recovery performance under warm and hot thermal conditions, additional information on Ag+ speciation study, notes on Ag0 precipitation in the presence of perborate/TAED; Ag2S precipitate analysis, and reusability results and discussion of the resin and regenerant (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sukalyan Sengupta: 0000-0002-1398-5107 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Dr. Chen-Lu Yang for his help in SEM and EDS analysis, Dr. Alexander Ribbe for TEM and EDS studies, Sidhartha Maiti for UV−vis analysis, and Jeff Beaudry for helping immensely at various stages of the study.

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REFERENCES

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DOI: 10.1021/acssuschemeng.7b02933 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering (28) Sachdev, A.; Krishnan, S.; Shulman, J. Heavy-duty liquid detergents. In Liquid Detergents, 2nd ed.; Lai, K. Y., Ed.; Surfactant Science Series Vol. 129; CRC Press: Boca Raton, FL, 2006; pp 239− 318. (29) McKillop, A.; Sanderson, W. R. Sodium perborate and sodium percarbonate: cheap, safe and versatile oxidising agents for organic synthesis. Tetrahedron 1995, 51 (22), 6145−6166. (30) Davies, D. M.; Deary, M. E. Kinetics of the hydrolysis and perhydrolysis of tetraacetylethylenediamine, a peroxide bleach activator. J. Chem. Soc., Perkin Trans. 2 1991, No. 10, 1549−1552. (31) Honda, M.; Kodera, T.; Kita, H. On the electrochemical behavior of H2O2 at Ag in alkaline solution. Electrochim. Acta 1983, 28 (5), 727−733. (32) Pearson, R. G. Hard and soft acids and bases. J. Am. Chem. Soc. 1963, 85 (22), 3533−3539. (33) Liu, J.; Sonshine, D. A.; Shervani, S.; Hurt, R. H. Controlled release of biologically active silver from nanosilver surfaces. ACS Nano 2010, 4 (11), 6903−6913. (34) Li, L.; Leopold, K.; Schuster, M. Effective and selective extraction of noble metal nanoparticles from environmental water through a noncovalent reversible reaction on an ionic exchange resin. Chem. Commun. 2012, 48 (73), 9165−9167. (35) Xu, B.; Gonella, G.; DeLacy, B. G.; Dai, H. L. Adsorption of anionic thiols on silver nanoparticles. J. Phys. Chem. C 2015, 119 (10), 5454−5461. (36) Metcalf & Eddy, AECOM. Wastewater Engineering: Treatment and Resource Recovery, 5th ed.; Tchobanoglous, G., Stensel, H. D., Tsuchihashi, R., Burton, F., Abu-Orf, M., Bowden, G., Pfrang, W., Eds.; McGraw Hill Education: New York, 2014. (37) Qian, X. F.; Yin, J.; Feng, S.; Liu, S. H.; Zhu, Z. K. Preparation and characterization of polyvinylpyrrolidone films containing silver sulfide nanoparticles. J. Mater. Chem. 2001, 11 (10), 2504−2506.

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DOI: 10.1021/acssuschemeng.7b02933 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX