Carbon composites using lignosulfonate for efficient palladium recovery under strong acidic conditions

Journal Pre-proof Facile Fabrication of Cux Sy /Carbon Composites using Lignosulfonate for Efficient Palladium Recovery under Strong Acidic Conditions Hao Liu (Conceptualization) (Methodology) (Software) (Formal analysis) (Writing - original draft), Qing-da An (Supervision) (Project administration), Jeonghun Kim (Writing - review and editing) (Supervision), Lin Guo (Writing - review and editing), Yu-meng Zhao (Writing - review and editing), Zuo-yi Xiao (Supervision), Shang-ru Zhai (Supervision) (Writing - review and editing) (Writing - review and editing)

PII:

S0304-3894(20)30241-7

DOI:

https://doi.org/10.1016/j.jhazmat.2020.122253

Reference:

HAZMAT 122253

To appear in:

Journal of Hazardous Materials

Received Date:

12 December 2019

Revised Date:

31 January 2020

Accepted Date:

5 February 2020

Please cite this article as: Liu H, An Q-da, Kim J, Guo L, Zhao Y-meng, Xiao Z-yi, Zhai S-ru, Facile Fabrication of Cux Sy /Carbon Composites using Lignosulfonate for Efficient Palladium Recovery under Strong Acidic Conditions, Journal of Hazardous Materials (2020), doi: https://doi.org/10.1016/j.jhazmat.2020.122253

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Facile Fabrication of CuxSy/Carbon Composites using Lignosulfonate for Efficient Palladium Recovery under Strong Acidic Conditions Hao Liu1, Qing-da An1, Jeonghun Kim2, Lin Guo1, Yu-meng Zhao1, Zuo-yi Xiao1, Shang-ru Zhai1* 1. Faculty of Light Industry and Chemical Engineering, Dalian Polytechnic University, Dalian 116034, China.

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2. Department of Chemistry, Kookmin University, 77 Jeongneung-ro, Seongbuk-gu, Seoul, 02707 Republic of Korea *Corresponding author:

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Name: Shang-ru Zhai

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E-mail: [email protected]

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Graphical abstract:

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Preparing CuxSy/Carbon Composites with Lignosulfonate as dual carbon and sulfur

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precursors.

Achieving tailored morphologies from different preparation conditions.

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Highlights:

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Demonstrating excellent sorption capacity towards palladium under strongly acidic conditions.

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Adsorption mechanism over synthesized composites was discussed in details.

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Abstract The recovery of noble metals from aqueous systems is of great significance for constructing sustainable framework of modern industry yet remains challenging. Herein, CuxSy/Carbon composites with superior thermal stability and adsorption capacity were successfully synthesized via one-pot hydrothermal method using lignosulfonate as dual role of raw materials. The optimal synthesis conditions were investigated via tailoring the temperature and the mass ratio of reagents. The

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morphologies and physical properties of the composites were characterized by scanning

electron microscope (SEM), transmission electron microscopy (TEM), X-ray

diffraction (XRD), and thermogravimetric analysis (TGA). The surface chemistry was

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analyzed by Zeta potential analysis, Brunauer-Emmet-Teller (BET), and X-ray

photoelectron spectroscopy (XPS). The Langmuir model and the pseudo-second-order

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model well described the adsorption of Pd(II) and Pd(IV) delivered by fabricated

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composites. The adsorption capacity obtained from Langmuir isotherm model towards Pd(IV) was 114 mg/g and Pd(II) was 101 mg/g, respectively. More importantly, the

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adsorbed palladium species could be desorbed with hydrochloric acid and thiourea, which suggested good durability and recycling performance of the typical composite.

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This work might provide a new guidance for the utilization of lignin or its derivatives and enriched the research in the field of noble metal recovery.

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Keywords: biomass; lignosulfonate; recovery; palladium; high-efficiency

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1. Introduction Palladium, a well-known noble metal, has long been used in modern industrial fields such as aviation, spaceflight, navigation, weapon and catalyst technology (Cui et al., 2018, Lei et al., 2019, Ruiz-García et al., 2019). Nevertheless, the upcoming application of palladium would have been limited due to the limited reserves and the difficulty of extraction (Hageluken, 2006, Ortet and Paiva, 2015), which makes recovering palladium from wastewater an economic and ecological issue worthy of

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attention (Yue et al., 2017). Over the past few decades, various methods have been

developed to recover noble metals from aqueous solutions, such as precipitation

(Guibal et al., 2002), adsorption (Zhang et al., 2019), filtration (Syed, 2006),

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electrochemical (Cui and Zhang, 2008), ion-exchange (Gomes et al., 2001), etc. Nevertheless, these methods are costly and complex, and can even produce toxic sludge

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and causes secondary pollution. Amongst them, adsorption with high-efficiency

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sorbents, as the easy-to-handle process with low energy consumption and no secondary pollution, has been paid particular attention (Goksungur et al., 2005, Huang et al., 2010).

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Recently, the researches on exploring various adsorptive materials for precious metal recovery have been conducted out, such as activated carbon (AC) (Soleimani and

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Kaghazchi, 2008), modified metal-organic framework (m-MOF) (Lin et al., 2017), cellulose derivatives (Yang et al., 2014), thiourea modified fibers (Zhang et al., 2019),

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thiourea modified sawdust (Losev et al., 2018), a-WO 3/PAN nanofiber (Wei et al., 2018), chitin nanofibrous membrane (Wang et al., 2019) and functional textiles (Opwis et al., 2016). The most famous industrial adsorbents for noble metals pre-concentration are Lewatit TP-214 and Purolite S-940 (Aktas and Morcali, 2011, Won et al., 2011, Xue et al., 2016). They have exerted relatively high adsorption capacity (30-80 mg/g), but the limitation of poor kinetics has restricted their widespread applications in practical

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processes. Although these related adsorbents have filled the gap toward noble metal adsorption researches, their significant drawbacks are unfeasible and uneconomic (Losev et al., 2018). Accordingly, potentially alternative types of adsorbents with high selectivity and facile fabrication process are in great demand to treat the troublesome issue. Meanwhile, the rise of biomass materials may provide a new guidance to these problems. Nowadays, biomass resources have been extensively investigated as green fuel

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and raw materials for chemicals and functional materials (Hubicki and Wolowicz, 2009,

Ong et al., 2007, Patil and Yan, 2016, Zakzeski et al., 2010). Amongst the various biomass materials, cellulose and hemicellulose have been maturely used and studied

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(Liu et al., 2019). By contrast, there are still a large number of vacancies in the research

toward the valorization of lignin or its derivates due to the unique molecular structure

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(Meng et al., 2019). Although varied processing conditions can prepare or separate

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different kinds of lignin, most of them are difficult to be fully dissolved in water, which might be a major obstacle for their potential potentiality for fabricating high-

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performance adsorptive composites (Yang et al., 2017). Desirably, lignosulfonate (LS), as a typical highly condensed waste of sulfite

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pulping process (Hu et al., 2018), can be dissolved in solution (Yan et al., 2010), which makes it to be a favorable choice for applying in various fields such as be used as

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plasticizer (Baumberger et al., 1997, Kalliola et al., 2015), surfactant (Alwadani and Fatehi, 2018), dispersant (Qiu et al., 2015) and corrosion inhibitor (Ouyang et al., 2006). And more and more lignosulfonate-based supported catalysts have been reported (Lai et al., 2018, Lee, 2013, Sun et al., 2014, Zhang et al., 2013). In addition, LS can act as both carbon and sulfur source in synthetic materials, making it a favorable raw material for applications in the field of noble metals recovery. In general, according to the theory

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of soft and hard acid-base (HSAB), S donor atoms can interact strongly with soft acids like precious metals (Lin et al., 2018). Accordingly, various sulfur-containing have been reported to demonstrate substantial improvements in the recovery performance of precious metal, such as sulfur-modified chitosan (Arrascue et al., 2003), thioureamodified chelating resin (Moyers and Fritz, 1976), thiolene hydrogel (Fırlak et al., 2014), and nanostructured transition metal dichalcogenides (TMDCs) (Yao et al., 2019). Among them, TMDCs (e.g. MoS2, WS2 and CuS) have attractive growing attentions

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owing to their advanced application in noble metals recycling (Feng et al., 2018, Yao et al., 2019). It should be pointed out that although these works provide favorable ideas, the use of a large number of organic solvents makes these studies insufficient in terms

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of economics and environmental protection. And at present, there have been few reports on the capture of palladium ions (Pd(II) or Pd(IV)) from aqueous solutions, and the

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utilization of LS as raw materials to fabricate sulfur-containing bio-sorbents has not yet

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reported.

In a consequence, a new type composite, integrating copper sulfur carbonaceous

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matrix (CuxSy/Carbon), was prepared in a controllable manner which using lignosulfonate as a precursor. The adsorption capacity and selectivity for palladium ions

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over as-fabricated CuxSy/Carbon composite was studied, and the optimum reaction conditions for the synthesis of CuxSy/Carbon were investigated by regulating the raw

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material mass ratio and hydrothermal temperature accordingly. The adsorption process was studied by the intermittent adsorption method with various concentrations of metal ions, and the kinetics were studied with the time as the variable. The properties, morphologies, and applicability of the CuxSy/Carbon were analyzed by various characterization methods, and the possible adsorption mechanism was explained in details.

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2. Experimental details 2.1. Materials and reagents The K2PdCl4, K2PdCl6, Ethanol, HCl (37%), thiourea (CH4N2S) and sodium lignosulfonate were purchased from Aladdin Ltd. The C4H6CuO4·H2O was obtained from Shanghai Macklin biochemical technology Co., Ltd. Pd(II) and Pd(IV) solutions were made of K2PdCl4 and K2PdCl6 dissolved in deionized water. Also, all reagents were analytical grade and used directly.

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2.2. Preparation of CuxSy/Carbon composites

CuxSy/Carbon Composites were prepared via a one-step hydrothermal method. Dissolving 2g LS and then add 6 mmol C4H6CuO4·H2O in deionized water (150ml).

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Afterward, the above LS solution was adjusted to pH = 1 by HCl (37%). Followed by shaking in the magnetic stirrer at 70 rpm for 60 min and transferring the solution to an

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autoclave at 200 o C for 720 min, the final products were separated via centrifugation

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and cleaned three times, then dried overnight under vacuum condition (60 o C). The obtained composite was named as CuxSy/Carbon-A-B, where the A is the temperature

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of hydrothermal synthesis and the B is the dosage ratio of LS/C4H6CuO4. Experiments of A = 120, 140, 160, 180, 200 (o C) and B = 1, 2, 3, 4 were conducted out as a reference

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to explore the optimal conditions. 2.3. Material characterization

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The morphologies were imaged through emission scanning electron microscopy (SEM, SIGMA 500) and transmission electron microcopy (TEM, CENTRATM 100 100KV). The XRD-6100 X-ray diffractometer were analyzed X-ray diffraction (XRD) over an angular range of 10-70 o. X-ray photoelectron spectroscopy (XPS) analysis of the distribution of each element in the composite by Thermo Scientia ESCALAB250 spectrometer (Thermo VG, USA) equipped with an Al-Kα X-ray source (1486.6 eV).

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Thermogravimetric analysis (TGA) was investigated by TG209F1 Libra at the temperature ranging from 30 o C to 800 o C by 10 K/min. 2.4. Batch adsorption experiments In the batch adsorption experiments, the CuxSy/Carbon composites (20 mg) were added to the conical flask that contains certain concentration metal ions solution, then the above device was set in a magnetic stirrer at 70 rpm for 4 h. The remaining solution was separated by a filter after adsorption equilibrium and determined the metal ion

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concentration by atomic absorption spectrometry (AAS). Repeating each experiment

three times, take the average and mark the error bars. The sorption isotherms of palladium ions were studied with the concentration of metal ions as variable, and the

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sorption kinetics of palladium ions were studied with the time as the variable. All the adsorption experiments were conducted out at room temperature. The sorption

(CO  Ce )V m

(1)

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Q

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performance (Q, mg/g) was computed through the following equation (1):

In which CO and Ce refer to the initial and residual concentration of metal ions

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(mg/L), respectively. V represents the volume of the solution (L), and m means the weight of the adsorbent (g).

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2.5. Desorption and recycling test

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The CuxSy/Carbon-200-2 composites that reached adsorption equilibrium were collected for adsorption reuse experiment. The composites (20 mg) were desorbed by HCl (0.4 M) and thiourea (0.6 M) mixed solution (30 ml). After the eluted CuxSy/Carbon composite was complete dried, the next sorption-desorption experiment was conducted out.

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3. Results and discussion 3.1. Characterization of the CuxSy/Carbon composites Fig. S1 illustrates the synthesis process of the CuxSy/Carbon composites via a onepot method using LS, and the reaction mechanism is speculated and shown in Fig. 1. The LS and Cu2(CH3COO)4 aqueous was adjusted to pH = 1 by HCl before transferred to the autoclave, during which the sulfonic acid groups were protonated, promoting to the formation of CuxSy compounds. In addition, one part of LS was hydrolyzed into

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monomer and dimer, which is polymerized to form the amorphous carbon by

dehydration reaction between monomer and dimer (Zhao et al., 2017). Meanwhile, an

amount of S ion, which is from the protonated sulfonic acid groups, form the CuxSy

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with Cu ions under hydrothermal conditions. It also should be mentioned that the

composition of the final products would alter as the temperature of hydrothermal

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synthesis and LS/Cu2(CH3COO)4 mass ratio change, which will be further discussion

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as below.

3.1.1. Effect of LS dosage on CuxSy/Carbon composites

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To further explore the influence of composition on various composites, the phase composition and structure of CuxSy/Carbon-200-B (B = 1, 2, 3, and 4) were analyzed

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by XRD (Fig. 2a). For the CuxSy/carbon-200-1 and CuxSy/carbon-200-2, the sharp peaks found in the patterns show ordered crystallization, and the diffraction peaks

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correspond to Cu8S5 (Geerite) (Inorganics PDF#33-0491) and Cu2S (ICSD PDF#721071). With the addition of excess LS in the system, the characteristic peaks of Cu8S5 are weakened in the patterns of the CuxSy/Carbon-200-3 and CuxSy/Carbon-200-4 composites. However, the peaks at 37o, 46o, and 48o remain, which can be indexed to the (1 0 2), (1 1 0), and (1 0 3) crystal planes of Cu2S (ICSD PDF#84-0206). Another interesting finding is that the diffraction peaks from carbon (JCPDS PDF#25-284)

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consistently strengthen as LS mass increases, indicating that hydrothermal carbonization shows increased levels of carbon with an increase in lignin concentration. Based on the above discussion, the co-existence of Cu8S5 and Cu2S exist in the CuxSy/Carbon-200-1 and CuxSy/Carbon-200-2 patterns, which is due to a small amount of carbon reducing portions of Cu2+ to Cu+, and the combinations of S2- and Cu ions are known to be (Cu+)2(S2-) and (Cu+)6(Cu2+)2(S2-)5 (i.e. Cu2S and Cu8S5). Nevertheless, the amount of carbon constantly increases as the dosage ratio of LS/Cu2(CH3COO)4

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increases to 3 and 4. Then, most of the Cu2+ will be reduced to Cu+ and combined with S2- to form Cu2S.

Also, the N2 adsorption-desorption isotherms of CuxSy/Carbon-200-B (B = 1, 2, 3,

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and 4) composites are presented in Fig. 2 b. From the concave curves, all CuxSy/Carbon200-B composites show type III isotherms, indicating a strong interaction between

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CuxSy molecules. All CuxSy/Carbon-200-B composites have a small specific surface

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area (maximum SBET = 10.3 m2 g-1), which decreases as B increases. This phenomenon may be because small pores and surface of the CuxSy/Carbon-200-B being covered with

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amorphous carbon, which agrees with the pore size distribution results (Fig. S2) and above XRD analysis.

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3.1.2. Effect of hydrothermal synthesis temperature on CuxSy/Carbon composites To explore the influence of temperature on composites during hydrothermal

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synthesis, five reference experiments with A = 120, 140, 160, 180, and 200 (o C) were run, where all were performed under the same condition of B = 2. Notably, the composites exhibit a different brown color from other materials (Fig. S3) when A = 120 o

C and 140 o C. To better understand this phenomenon, the compositions of the above

composites were determined by XRD (Fig. S4). Both CuxSy/Carbon-120-2 and CuxSy/Carbon-140-2 show peaks at 28.5 o, 47.3 o, and 46.2 o that are attributed to Cu8S5

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(Inorganics 33-0491), indicating that only Cu8S5 exists in the composite when A = 120 o

C and 140 o C. To further research these changes, SEM images of CuxSy/Carbon-120-2 and

CuxSy/Carbon-140-2 were analyzed in Fig. 3a and 3b. Due to the agglomeration caused by the high surface energy of Cu8S5 nanoparticles (NPs), both exhibit a single uneven morphology. A similar conclusion was obtained based on BET (Fig. 2b) studies. A new morphology was observed when A ≥ 160 o C showing spherical, smooth, and large

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particles which was different from Cu8S5 as indicated by the red circles in Fig. 3 (c-d). As temperature increases, the number of regular spheres also increases. Combined with the XRD results (Fig. 2a), it can be inferred that the new morphology in the SEM images

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is ascribed to Cu2S. To verify this, the SEM image of CuxSy/Carbon-200-4 containing a large amount of Cu2S is shown in Fig. 3f, and its spherical morphology is consistent

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with that in CuxSy/Carbon-160-2, CuxSy/Carbon-180-2, and CuxSy/Carbon-200-2. In

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Fig. S5, the particle size distributions are presented, revealing Cu8S5 with a uniform particle size of 150 ± 40 nm and a relatively large sphere of Cu2S at 2 ± 0.5 μ m. The

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TEM image of CuxSy/Carbon-200-2 (Fig. 3g) also shows the amorphous carbon and nanoparticle morphology of CuxSy in the composites. As the EDS images show in Fig.

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2h-k, it was found that the Cu, S, O, and C elements were spread about the pattern, which further validates the generation of CuxSy/carbon samples. In summary, the sulfur

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in the sulfonic acid group tends to combine with Cu ions to form Cu8S5 when the temperature of hydrothermal synthesis is less than 160 o C. As the temperature increases, more Cu2S and carbon begin to appear in the composites. 3.2. Adsorption behaviors of CuxSy/Carbon 3.2.1. Optimized synthesis conditions for CuxSy/Carbon composites To explore the effects of different composite compositions on adsorption capacity,

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simple adsorption tests of CuxSy/Carbon-200-B (B = 1, 2, 3, 4) were conducted. The results in Table S1 show that CuxSy/Carbon-200-2 has the maximum adsorption capacity. Then, as B increases, the adsorption capacity of CuxSy/Carbon-200-B rapidly decreases until B reaches 4, which may be due to the excess LS creating excess carbon in the composites reducing the proportion of CuxSy in the composites. In addition, the presence of elemental sulfur will also reduce the proportion of CuxSy in the composite at B = 1. Hence, B = 2 is determined as the optimal condition.

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Material yield is also an important factor when optimizing hydrothermal

temperature. Herein, the yields of CuxSy/Carbon-B-2 (B = 120, 140, 160, 180, and 200 o

C) increase as B increases up to 200 (Table S2). Notably, although the CuxSy/Carbon-

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120-2 and CuxSy/Carbon-140-2 show relatively pure CuxSy, yields are low, thereby

limiting the useful application space (Table. S2). CuxSy/Carbon-160-2, CuxSy/Carbon-

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180-2, and CuxSy/Carbon-200-2 composites have similar adsorption capacities, as

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shown in Table S1. To sum up, when A = 200 o C was determined as the optimum temperature, the products has favorable yield and adsorption capacity.

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3.2.2. Effect of acidity on CuxSy/Carbon-200-2 composites sorption Different acidity can not only alter the surface potential of the material but also

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change the existing form of Pd anion. Therefore, determining an optimum acidity is important for exploring adsorption behavior and avoiding the hydrolysis of metal ions.

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However, since both K2PdCl6 and K2PdCl4 have intense hydrolysis ability, their solutions would produce brick red flocculent precipitation under weakly acidic conditions (PdCl62- begin to precipitate even at pH around 3.5) (Fig. S6). Therefore, the influence of acidity on the adsorption capacity of CuxSy/Carbon-200-2 was investigated in a pH range of 1 to 3. As depicted in Fig. 4, the adsorption capacity of Pd(II) decreases from 89.1 mg/g to 49.9 mg/g with the increase of pH range from 1-3, while the capture

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capacity of Pd(IV) increases from 72.6 mg/g to 102.5 mg/g with increasing pH = 1 - 1.5, and then decreases to 62.5 mg/g at pH = 3. It illustrates that CuxSy/Carbon-200-2 presents an excellent capture performance for Pd(IV) and Pd(II) at pH 1.5, attributing to the major complex of palladium are [PdCl4]2- and [PdCl6]2- at corresponding pH and the strong interaction between the electron-donating sulfur atoms contained in CuxSy/Carbon-200-2 and [PdCl4]2- or CuxSy/Carbon-200-2 and [PdCl6]2- (Yao et al., 2019).

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3.2.3. Adsorption isotherms

Adsorption isotherm method was utilized to investigate the maximum sorption

capacity of CuxSy/Carbon-200-2 and the curves of adsorption presented typical

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adsorption behavior. When the adsorbent reached adsorption equilibrium, the

calculated data of Ce and Qe were fitted to the Langmuir model and the Freundlich

Ce 1 1  Ce  Qe Qmax K l Qmax

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Langmuir equation：

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parameters. (equation 3 and equation 4)

(3)

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Freundlich equation：

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model to express solid-liquid sorption performance and predict their equilibrium

1 log Qe  log K f  log Ce n

(4)

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where Ce (mg/L) is the concentration of metal ions in the solution at the equilibrium of adsorption. Qmax is the maximum adsorption capacity. Kl (L/mg) is the Langmuir adsorption constant. For the Langmuir isotherm model, Qmax and Kl could be obtained from the slope and intercept of the linear plot of Ce/Qe vs Ce. Kf (L/mg) represents the Freundlich adsorption constant. n is the Freundlich exponent. Kf and n can be obtained by determined the slope and intercept of the linear plot of log Qe vs log

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Ce. When the 1/n is between 0 and 1, it is beneficial to the adsorption process; otherwise, it is not conducive to the whole adsorption. The fitting plots of adsorption isotherm of two precious metal ions on CuxSy/Carbon-200-2 are shown in Fig. 5, and the fitting results of the different models are computed and listed in Table 1. By comparing the maximum adsorption amount, it can be seen that CuxSy/Carbon-200-2 has a better ability to capture Pd(IV) than Pd(II) at the same concentration. Also, correlation coefficient of linear Freundlich model is

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lower than that of the linear Langmuir model, indicating the capture of Pd ions by CuxSy/Carbon-200-2 is more conform with Langmuir model, and the monolayer

adsorption and chemisorption are the main way in this process (Yao et al., 2019).

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Notably, the value of n is between 3.2 and 3.8, suggesting that the capture process is

conducted out easily. The Qmax of Pd(II) and Pd(IV) reach 101.5 mg/g and 114.5 mg/g,

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respectively. And the comparison of maximal sorption capacity in previous literature is

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further concluded in Table 2. Notably, the CuxSy/Carbon-200-2 has not only relatively high uptake capacity, but also shorter reaction time. At the same time, it is ease of

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preparation and easy source of raw materials. To sum up, the CuxSy/Carbon-200-2 has great potential in noble metals capture (Bratskaya et al., 2016, Lin et al., 2017, Zhou et

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al., 2018).

3.2.4. Adsorption kinetics

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Adsorption kinetic parameters could reflect the residence time of the palladium ions uptake at the solution and CuxSy/Carbon-200-2 interface that is of great significance for the equilibrium time and capture rate study. Add 20 mg of CuxSy/Carbon-200-2 to palladium ion solution (20 mL) at a concentration of 50 mg/L. To make the experimental data more accurate, eleven experimental devices were prepared and simultaneously performed. All the experiments were carried out at pH =

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1.5 and 298K unless otherwise stated. The experimental data is summarized in Table S3 and Fig. 6a. To examine the adsorption behavior, pseudo-first-order (PFO-model) and pseudo-second-order (PSO-model) were investigated in Fig. 6b and 6c. And all experimental data are compared with the fitted data, and the kinetic equations are shown as equation 5 and equation 6:

ln(Qe  Qt )  ln Q1  k1t

(5)

PSO-model equation:

t 1 t   2 Qt k 2Q2 Q2

(6)

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PFO-model equation:

In which Qe (mg/g) and Qt (mg/g) refer to the amount of Pd adsorbed on the

CuxSy/Carbon-200-2 at equilibrium and at any arbitrary time, respectively. Q1 (mg/g)

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and Q2 (mg/g) represent the adsorption capacity of the PFO model and the PSO model,

respectively. k1 (1/min) and k2 (g/mg·min) signified the rate constant of the PFO model

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and the PSO model.

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When the experiment was carried out for 240 minutes, the Qe of Pd ions reached 39.24 and 35.91 mg/g, respectively. And the Qe remained essentially unchanged until 360 minutes, indicating that the adsorption of Pd ions by CuxSy/Carbon-200-2 can reach

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equilibrium within 240 minutes. It is worth noting that within 5 minutes of the start of the experiment, the AP of Pd(IV) reached 54.7% and the AP of Pd(II) reached 50.8%.

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The initial fast adsorption might be ascribed to the extensive of empty sites on

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CuxSy/Carbon-200-2, and the subsequent adsorption rate gradually slows down as a large number of sites are occupied and gradually reaches the adsorption equilibrium. Generally, the adsorption rate of ordinary physical adsorption and electrostatic attraction is relatively slow, suggesting that physical adsorption and electrostatic attraction are not the main adsorption modes of the process (Zhang et al., 2019), which is consistent with the BET and Zeta potential analysis.

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The fitting results obtained from PFO model and PSO kinetic models are summarized in Table 3. The correlation of PSO-model (R2 = 0.9934 and 0.9626) is better than that of PFO-model (R2 = 0.7441 and 0.8657), implying Pd(II) and Pd(IV) capture onto the CuxSy/Carbon-200-2 composite is mainly controlled by chemisorption. 3.2.5. Thermodynamic studies The influence of ambient temperature on CuxSy/Carbon-200-2 adsorption of Pd(II) and Pd(IV) is studied in a temperatures range of 278K to 308K (Fig. 7a), illustrating

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that the temperature rise is beneficial to capture capacity. The thermodynamic parameters have a bearing on the adsorption, e.g. free energy change (ΔG°), enthalpy change (ΔH°) and entropy change (ΔS°), which were computed by the following

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equation. And the calculated values of ΔH°, ΔG° and ΔS° at different temperature are summarize in Table S4.

S o H o 1 ln Kd   R R T

(7)

(8) (9)

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G o  H o  TS o

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Qe Ce

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Kd 

Where Kd is the thermodynamic equilibrium constant. T (K) refers to absolute

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temperature. Qe (mg/L) stands for the amount of Pd captured on the CuxSy/Carbon-2002 at equilibrium. Ce (mg/L) represents the concentration of remaining metal ions in the

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solution at adsorption equilibrium. ΔHο and ΔSο could be computed through the slope and intercept of the linear plot of lnKd vs 1000/T (Fig. 7b). ΔG° is obtained by equation (9).

The ΔH° is a positive value, indicating the increase of temperature is promoting for the adsorption, which indicates that the adsorption process was entirely endothermically driven. The positive values of ΔS° mean that the degree of freedom

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increases at the CuxSy/Carbon-solution interface during the adsorption process. The ΔGo is a negative value (T ≥ 298K) and the absolute value increased as the temperature increased, indicating the sorption process is spontaneous and active. 3.2.6. Selective adsorption In practical applications of adsorbents, selectivity is also an important evaluation indicator. Add 20 mg of CuxSy/Carbon-200-2 to a mixed solution (20ml) containing Pd ions and other interfering metal ion, such as Co(II), Fe(III), Mg(II), Cu(II) and Ni(II),

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the experiments were conducted out at 298K and pH = 1.5. The separation coefficient

(βRE1/RE2) and distribution coefficient (D) were calculated according to the equation (9) and (10).

 RE1  RE 2

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Co  Ce V mCe DRE1 DRE 2

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D

(9)

(10)

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Where m (mg) means the weight of the CuxSy/Carbon-200-2. Co and Ce represent the initial and residual concentration of metal ions in aqueous solution (mg/L). D refers

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to the affinity between the composite and metal ions, and V (L) is the volume of the solution, and RE1 and RE2 represented different metal. βRE1/RE2 stands for the separation

ur

coefficient of different metal and the higher βRE1/RE2 means the greater selectivity in capture Pd ion. Interestingly, the maximum βPd/Ni and βPd/Cu could reached 249 and 124,

Jo

and the adsorption capacity summary suggests the parallel results (Fig. 8a), illustrating that under the condition of pH = 1.5, CuxSy/Carbon-200-2 has a significantly selective adsorption capacity for Pd(IV) than the other four metal ions, and the composite can barely capture Ni and Cu ions. It is because of the intensive affinity between sulfurcontains and precious metal ions that the composite could have outstanding selectivity in the capture of palladium.

18

3.2.7. Stability and regeneration studies In practical application, reusability and stability are important for the adsorbent. The CuxSy/Carbon-200-2 regenerated by HCl-thiourea were collected and cleaned by deionized water. After being completely dried, it was used for the next adsorption experiment. As shown in Fig. 8b, the recovery of CuxSy/Carbon-200-2 could remain above 87% after five consecutive cycles, which indicates that the reusability and stability of CuxSy/Carbon-200-2 are excellent.

ro of

The thermal stability of the CuxSy/Carbon-200-2 was performed in Fig. 8c. The rapid decline in weight between 50 o C and 100 o C is mainly due to the evaporation of

residual moisture, and between 280 o C to 400 o C, the weight is reduced by about 13%

-p

due to the evaporation of sulfur. The final phase of weightlessness began around 400 o

C, which was mainly due to the evaporation of the remaining carbon. Since the

re

adsorption process conducts out under room temperature, CuxSy/Carbon composite

lP

could maintain favorable stability at practical application. 3.2.8. Adsorption mechanism

na

To elucidate the adsorption mechanism, XPS analysis was performed for the CuxSy/Carbon-200-2 before and after the capture of Pd ions. After the capture of Pd,

ur

the characteristic peaks of Pd 3d3/2 and Pd 3d5/2 appeared obviously in the spectrum (Fig. 9b), which suggests that Pd ions are captured on the CuxSy/Carbon-200-2

Jo

composite. The high-resolution scanning of the S 2p region illustrates the presence of S2- (Fig. 9c) with a peak of 165.2 eV decreases after adsorption, and a new peak appears at 161.5 eV due to coordination interaction with Pd. Fig. S7a gives the XRD patterns of the noble metal composite adsorbent before and after sorption. Compared with CuxSy/Carbon-200-2, a slight signal of Pd NPs was observed in the adsorbed composite, suggesting the redox reaction between Pd ion and Sulfur-containing compound, which

19

was demonstrated in the previous literature studies (Yao et al., 2019). Besides, the SEM images of CuxSy/Carbon-200-2 after Pd ion capture can hardly show that the Pd NPs on the composite (Fig. S7c), which suggests that Pd NPs may cover the composite in a super-small state. Generally, the adsorption mechanism of metal ions on adsorbents includes physical-adsorption sand chemisorption. Materials with the high specific surface area have better physical adsorption capacity, but the specific surface area of CuxSy/Carbon-

ro of

200-2 is only 5.060 m2/g as shown in above N2 adsorption-desorption isotherm, and there is no obvious variation after adsorption (Fig. S7b), which clarifies the weak

physical sorption performance. Besides, the average Zeta potential of composite is -

-p

6.7137 mV at pH 1.5 (Fig. S8), hence the composites could not adsorption the negatively charged metal chloride anionic complex, such as [PdCl4]2- and [PdCl6]2-.

re

Since sulfur is electron-rich, noble metal ions can easily combine with donor sulfur by

lP

coordination. In strong acidic media, the metal chloride anionic complex attacks the sulfur sites in CuxSy and then displaces hydrogen ion owing to intense oxidizing

na

capacity of [PdCl4]2- and [PdCl6]2- species. And [PdCl6]2- with a higher electron cloud density have a greater interact with S atoms. To sum up, the sulfur-rich compounds have

ur

a unique combination with noble metals, namely redox reaction and coordination interaction. Then chemisorption may be the main adsorption mechanism of

Jo

CuxSy/Carbon-200-2 composite to Pd ions. 4. Conclusion

A novel type of CuxSy/Carbon composites that have favorable selectivity and

adsorption capacity were synthesized using lignin sulfonate as raw materials via a onepot hydrothermal method. Optimum synthesis conditions for the adsorbent were investigated via controlling the synthesis temperature and the mass ratio of the reagents.

20

The materials synthesized under different conditions were analyzed and characterized. Notably, the CuxSy/Carbon-200-2 showed high selectivity to Pd in mixed solutions containing varied metal ions. Combined with BET and Zeta potential analysis, the unique adsorption capacity of CuxSy/Carbon-200-2 for Pd(II) and Pd(IV) had nothing to do with physical adsorption and electrostatic attraction. The capture mechanism can be ascribed to the redox reaction or coordination interaction between CuxSy/Carbon200-2 and Pd(II) or Pd(IV) chloro-complex in strongly acidic aqueous solution. By

ro of

virtue of this mechanism, the adsorbent showed a high capture capacity for Pd ions, e.g.

114 mg/g of Pd(IV) and 101 mg/g of Pd(II). The adsorption process reached equilibrium within 240 min. And it is worth noting that within 5 minutes of the start of the

-p

adsorption experiment, the AP of Pd(IV) reached 54.7% and the AP of Pd(II) reached

50.8%. The capture process of Pd(II) and Pd(IV) on the CuxSy/Carbon-200-2 followed

re

the pseudo-second-order kinetic model. The material can be recovered and reused many

lP

times by cleaning the mixed solution of hydrochloric acid and thiourea without significant loss in adsorption capacity. This study might provide a new guidance for the

na

application of lignosulfonate and enrich the research of precious metal adsorption.

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CRediT author statement

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Hao Liu:Conceptualization,Methodology,Software,Formal analysis,Writing - Original Draft. Qing-da An:Supervision,Project administration. Jeonghun Kim:Writing - Review & Editing,Supervision. Lin Guo:Writing - Review & Editing. Yu-meng Zhao:Writing - Review & Editing. Zuo-yi Xiao:Supervision. Shang-ru Zhai:Supervision,Writing - Review & Editing,Writing - Review & Editing.

21

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments The study was gratefully supported by the National Key Research &

ro of

Development Program of China [2017YFB0308701], the National Natural Science Foundation of China [21676039], State Key Laboratory of Bio-Fibers and EcoTextiles [2017kfkt12] and Dalian Leading Talents Project [No. 2018−192].

-p

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26

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Jo

ur

na

lP

re

-p

54, 9977-9980.

27

re

-p

ro of

Figure captions:

Jo

ur

na

lP

Fig. 1. Possible formation mechanism of CuxSy/Carbon composite.

Fig. 1, Zhai et al.,

28

●

a)

CuxSy/Carbon-200-4



Intensity (a.u.)

●

 



CuxSy/Carbon-200-3

 



●

 

● carbon  Cu8S5

CuxSy/Carbon-200-1



●

 Cu2S

 Cu S /Carbon-200-2 x y  









Cu8S5 PDF#33-0491

10

20

30

Cu2S PDF#72-1071

40

50

60

70

2Thete (degree) 2 -1

CuxSy/Carbon-200-4

SBET=3.4 m g

CuxSy/Carbon-200-3

SBET=4.9 m g

CuxSy/Carbon-200-2

SBET=5.6 m g

ro of

Quantity Intensity (a.u.)

b)

2 -1

0.2

0.4

0.6

0.8

Relative Pressure (P/P0)

1.0

lP

re

0.0

2 -1

SBET=10.3 m g

-p

CuxSy/Carbon-200-1

2 -1

Fig. 2, Zhai et al.,

na

Fig. 2. the XRD images of CuxSy/Carbon-200-1, CuxSy/Carbon-200-2, CuxSy/Carbon200-3 and CuxSy/Carbon-200-4 (a); The Nitrogen adsorption and desorption isotherms

ur

images of CuxSy/Carbon-200-1, CuxSy/Carbon-200-2, CuxSy/Carbon-200-3 and

Jo

CuxSy/Carbon-200-4 (b).

na

lP

re

-p

ro of

29

Fig. 3. The SEM images of CuxSy/Carbon-120-2 (a), CuxSy/Carbon-140-2 (b), (c),

ur

CuxSy/Carbon-160-2

CuxSy/Carbon-180-2

(d),

CuxSy/Carbon-200-2

(e),

CuxSy/Carbon-200-4 (f); The TEM image of CuxSy/Carbon-200-2 (g); Elemental

Jo

mappings of CuxSy/Carbon-200-2 composite (h-k).

30

89

75

.1 88

2.5

.6 82

.2

100

.3 74

67

.9

.2

80 6

56

2.5

60

.4

40

4

9.9

20 0

1 .0

)

( Pd

2 .0

Ⅱ)

2 .5

( Pd

3 .0

ro of

Ⅳ

1 .5

pH

Adsorbed amount / mg g-1

10

-p

Fig. 4, Zhai et al.,

Jo

ur

na

lP

re

Fig. 4. Effect of pH on the adsorption.

31

b)

110

4

2+

90 80 70

Pd4+

60 50

Pd2+

40

2

Pd R =0.9728 4+ 2 Pd R =0.9952

100

Ce /Qe (mg/L)

Adsorbed amount (mg/g)

a)

3

2

1

0

30 0

50

100

150

200

250

300

0

50

100

Equilibrium concentration (mg/L)

2.1

log (Qe)

2.0

2+

2

Pd R =0.8922 4+ 2 Pd R =0.9618

1.9 1.8 1.7 1.6 1.5 0.0

0.6

250

300

ro of

2.2

200

1.2

1.8

2.4

3.0

na

lP

re

log (Ce)

-p

c)

150

Ce (mg/L)

Fig. 5, Zhai et al.,

Fig. 5. Adsorption isotherm of Pd(II) and Pd(IV) by CuxSy/Carbon-200-2 (a); Fitting

Jo

ur

results of the adsorption isotherm data in models of Langmuir (b), Freundlich (c).

32

a)

50

4+

Pd Pd2+

45

40

35

2

Pd R =0.8657 2+ 2 Pd R =0.7441

2

ln(Qe-Qt)

Qe (mg/g)

3

b)

4+

1 0 -1

30

-2 25

0

50

100

150

200

250

0

20

7

4+

2

Pd R =0.9934 2+ 2 Pd R =0.9626

6 5 4

t/Qt

60

80

100

120

ro of

c)

40

Time (min)

Time (min)

3 2

0 0

50

100

150

200

250

lP

re

Time (min)

-p

1

Fig. 6, Zhai et al.,

na

Fig. 6. Time-dependent kinetics of two ions capture with CuxSy/Carbon-200-2 (a), Linear plot of pseudo-first-order kinetic model (b) and pseudo-second-order kinetic

Jo

ur

model (c).

33

a)

40

3 y=

2 R2 =0

1 30

lnKd

Adsorbed amount ( mg/g)

b)

2+ Pd Pd4+

50

20

0 275

.99

R2 =0 .99 6 37 .12 9 88 -10 .5

0

.43

10

7-

9.1

Pd4+ Pd2+ 78

x

y=

-1

10

31

85

280

285

290

295

300

305

-2 3.2

310

3.3

3.4

x

3.5

3.6

1000/T

Tempereture

ro of

Fig. 7, Zhai et al.,

Fig. 7. Effect of temperature on the capture capacity of two noble metal ions (a). A plot

Jo

ur

na

lP

re

-p

of lnKd vs 1000/T (b).

34

a)

b) 100

30

Recovery %

25

Qe

20 15 10

50

5 0

4+

Pd Co

2+

2+

Cu

0

2+ Ni2+ Fe3+ Mg

1

2

3

cycle number

4

5

ro of

weight (%)

c) 100 80

60

40

100 200 300 400 500 600 700 800

-p

0

o

lP

re

Temperature ( C)

Fig. 8, Zhai et al.,

Fig. 8. Adsorption capacities of the CuxSy/Carbon-200-2 for Pd(IV), Co(II), Cu(II),

na

Ni(II), Fe(III), Mg(II) in the mixed solution (a). Recovery of Pd(IV) by CuxSy/Carbon-

Jo

ur

200-2 in five cycles (b). TGA curves of CuxSy/Carbon-200-2 (c).

35

a) C 1s

Intensity (a.u.)

Cu 2p

O 1s

S 2p

O 1s Cu 2p

Pd 3d C 1s S 2p

1000

800

600

400

200

0

Binding energy (eV)

c)

S 2p

348

344

340

336

332

328

-p

Intensity (a.u.)

Intensity (a.u.)

Pd 3d

ro of

b)

175

170

165

160

155

Binding energy (eV)

lP

re

Binding energy (eV)

Fig. 9, Zhai et al.,

Jo

ur

and S 2p (c).

na

Fig. 9. XPS survey of CuxSy/Carbon-200-2 (a); XPS high-resolution scans of Pd 3d (b)

36

Table 1. Adsorption isotherm parameters of two metal ions at 298 K and pH = 1.5. Langmuir model Metal ions

qmax

Freundlich model

KL

R2

n

K

R2

101.530

0.027

0.9728

3.282

17.2222

0.8922

Pd4+

114.508

0.046

0.9952

3.762

26.1042

0.9618

Jo

ur

na

lP

re

-p

ro of

Pd2+

37

Table 2. Comparative study on palladium adsorption capacity of CuxSy/Carbon-200-2 and related sorbents. Adsorbent

Capture capacity

pH

Time (min)

(mmol/g) 1.7 (Pd2+)

2M HCl

60

PEPEI [46]

4.6 (Pd2+)

0.1M HCl

1080

UiO-66-NH2 [47]

0.85 (Pd2+)

UiO-66 [47]

0.61 (Pd2+)

GO [48]

0.41 (Pd2+)

This work

0.52 (Pd2+)

This work

0.59 (Pd4+)

1.0

1440

1.0

1400

-p

re

lP na ur Jo

ro of

Sawdust-based biosorbent [17]

6.0

600

1.5

240

1.5

240

38

Table 3. Fitted kinetic parameters of Pd2+ and Pd4+ adsorption at 298K and pH = 1.5.

pseudo-first-order qe, exp

qe, cal

k1

(mg/g)

(mg/g)

(mg/g)

Pd2+

36.35

35.157

0.233

Pd4+

39.75

37.266

0.242

R2

k2

(mg/g)

(mg/g)

0.8657

36.926

0.0126

0.9934

0.7441

39.287

0.0117

0.9626

-p re lP na ur Jo

R2

qe, cal

ro of

Metal ion

pseudo-second-order