Efficient Separation and High Selectivity for Cobalt and Nickel from

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Efficient separation and high selectivity for cobalt and nickel from manganese-solution by a chitosan derivative: competitive behavior and interaction mechanisms Bing Liao, Na Guo, Shi-jun Su, Sang-lan Ding, and Wei-yi Sun Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04919 • Publication Date (Web): 05 Mar 2017 Downloaded from http://pubs.acs.org on March 5, 2017

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Industrial & Engineering Chemistry Research

Efficient separation and high selectivity for cobalt and nickel from manganese-solution by a chitosan derivative: competitive behavior and interaction mechanisms Bing Liao, Na Guo, Shi-jun Su, Sang-lan Ding, Wei-yi Sun∗ College of Architecture and Environment, Sichuan University, Chengdu, 610065, China

Abstract The competitive performance and selectivity for the separation of Co2+ and Ni2+ from Mn2+solution by a triethylene-tetramine-modified cross-linked chitosan derivative were explored through adsorption isotherm and kinetic models of single, binary and ternary component in static adsorptive experiment and displacement experiments. In the single system, the adsorption capacities of Co2+, Ni2+ and Mn2+ onto the adsorbent followed the order of Ni2+ > Co2+ > Mn2+. As to binary and ternary system, strong competitive and selective adsorption behaviors of Co2+ and/or Ni2+ over Mn2+ were observed. Separation factors suggested the extremely higher selectivity for Co2+ and Ni2+ against Mn2+. Displacement experiments showed that the initially adsorbed Mn2+ on the adsorbent could be displaced by subsequently adsorbed Co2+ and/or Ni2+ from the solution, but not the opposite. The results proved that the chitosan derivative was a potential and promising adsorbent for the selective removal of Co2+ and Ni2+ from manganiferous wastewater. Key words: chitosan; derivative; selective adsorption; mechanism

∗

Corresponding author. Tel./fax: +86 28 8546 0916. E-mail address: [email protected] (W.Y. Sun).

ACS Paragon Plus Environment


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1. Introduction Cobalt, nickel and manganese, as national strategic resources, are widely used in various industries because of their superior properties. However, an increasing demand of these metals leads to huge consumption, shortage of resources as well as more and more discharge of toxic metal effluent, thereby causing serious problems of environment pollution and human health1-3. Wastewaters containing Co2+, Ni2+ and Mn2+ are mainly produced from mining, metallurgy (hydrometallurgical process of iron, manganese, copper, cobalt and nickel ore), and battery production, especially the process of the precursor material preparation for anode material of lithium ion battery (such as lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, or lithium cobalt nickel manganese oxide anode material)4-6. At the same time, these wastewaters are precious secondary resources, which are valuable for recovery and reuse of metals in the view of sustainable development and environment. In addition, sometimes the removal of the undesired toxic metals is needed to recover the main metal for producing high-purity metal products in some industries. Classical methods such as chemical precipitation, ion exchange or solvent extraction are inefficient or uneconomical to separate specific toxic metals5, 7-10. In comparison with these methods, adsorption is proved to be an effective and promising process with advantages of low cost, extensive resources and non-secondary-pollution11. Thus, it is of great need to develop efficient and highly selective adsorbent for separation of toxic metals wastewaters to recover valuable metals. Recently, competitive and selective adsorption for toxic metals removal by various adsorbents have drawn much attention. For example, Li et al.12 systematically investigated the adsorption property of Cu(II), Pb(II) and Cd(II) onto an iminodiacetic acid chelating resin in binary solution and the results indicated that the direct displacement impact was the main interaction

mechanism

between

the

metal

ions.

Liu

et

al.13

discovered

that

the

diethylene-triamine-functionalized polymertic adsorbent displayed an excellent selectivity in the adsorption of copper ions over lead ions through an adjacent attachment and repulsion mechanism. The study by Tan et al.14 showed that direct displacement was the interaction mechanism between the favorable component and other metal ions when they investigated the competivtive adsorption properties of Cu2+, Cd2+ and Ni2+ from an aqueous solution on a novel graphene oxide menbrane. Tao et al.15 used purolite S984 resin to prepare 5N (the purity of the metal solution reaches


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99.999%) high-pruity nickel from 3N nickel-solutions and found that direct competition for the active sites between copper and nickel. From the studied results, it is found that the competitive and selective adsorption performance of different metal ions by various adsorbents are involved with different interactive mechanisms. Although lots of work have been carried out to explore the adsorption performance of different adsorbents for the removing Co2+, Ni2+ and Mn2+ from the aqueous solution11,

16-22

,

studies on the competitive adsorption performance of Co2+, Ni2+ and Mn2+ from aqueous solution and the separation of Co2+ and/or Ni2+ from manganiferous solution for metal recovery by adsorption are rarely reported in literatures. On the other hand, the determination of an interactive effect between toxic metals is various because of the combined impacts of the different metal ions and various adsorbents used22-25. Therefore, great effort is devoted to develop an efficient adsorbent for separating Co2+ and Ni2+ from Mn2+-solution in this work. In this study, triethylene-tetramine-modified cross-linked chitosan (CCTS) was produced according to our previous work26 and used as the adsorbent to separate Co2+ and Ni2+ from Mn2+-solution. The main objective was to determine the competitive and selective adsorption behavior of Co2+ and Ni2+ against Mn2+ by the adsorbent through adsorption kinetic and isotherm studies in single, binary and ternary aqueous solution and displacement experiments for each two metal ion. Furthermore, Fourier Transform Infrared Spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses were also employed to explore the selective and displacement mechanisms. The results indicated that the triethylene-tetramine-modified cross-linked chitosan was proved to be an efficient adsorbent for separation of Co2+ and/or Ni2+ ions from the manganiferous wastewater.

2. Experimental 2.1 Materials and solution The chitosan derivative used in the experiment was produced according to the method reported in our previous work26. The details of synthesis process were as follows. Firstly, raw chitosan was dissolved in a 3 % (in volume) acetic acid solution under magnetic stirring until completely dissolved to obtain a transparent gel solution, then the mixture was dropwise added into 1 mol/L NaOH solution to obtain chitosan beads. Secondly, the chitosan beads were crosslinked with epichlorohydrin using the mixing solvent of isopropanol alcohol and water. After



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stirring for 10 h, the beads were washed with ethanol and distilled water to remove the unreacted epichlorodydrin. Finally, the epoxy chitosan was grafted with triethylene-tetramine in N,N-dimethylformamide solution for 4 h and washed with ethanol and distilled water to remove the unreacted triethylene-tetramine. The surface area and pore volumes of the chitosan derivative were 1.2 m2/g and 0.0012 cm3/g, respectively. Also, the element contents of atoms C, H and N for the chitosan derivative were 41.03, 7.89 and 12.48 %, respectively. All reagents used in this study were of analytical grade and purchased from Chengdu Kelong Chemical Co., Ltd. Metal stock solutions (1000 mg/L) were prepared by adding MSO4 (M was Co, Ni or Mn) to deionized water. The test solutions were prepared via subsequent dilution of the stock solutions. 2.2 Adsorption experiments 2.2.1 Adsorption in single system For all the single experiments, the initial concentration of metal ion was about 200 mg/L and 0.05 g adsorbent was added into 25 mL metal ion solution and shaken at 120 rpm and 30 °C for 24 h. The effect of pH value of aqueous solution on the adsorption performance was explored in the range of 1-7. The pH was adjusted to the desired value with either diluted NaOH or H2SO4. Samples were taken at certain time intervals and the initial and final metal ions concentration was determined to examine the adsorption capacity of the adsorbent for each metal ion. The removal efficiency of the metal ion and adsorption capacity of the adsorbent are separately calculated by the following equations: =

=

× 100%

(1) (2)

whereη is the removal efficiency of metal ion, %; Q is the adsorption capacity per unit weight of adsorbent, mg/g; C0 and C are the initial and equilibrium metal concentrations, respectively, mg/L; V is the volume of aqueous solution, L; and m is the mass of adsorbent used, g. The adsorption isotherm was studied at the initial concentration of each metal ions varying in the range of 5-200 mg/L. The linear forms of the pseudo first-order, pseudo second-order and intra-particle diffusion kinetic models used for representing the kinetics data in this study are expressed as follows27:


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− = −

=

+

.

! !

= " √$ + %

(3) (4) (5)

where Qt and Qe are the uptake capacities of adsorbent at time t and equilibrium, respectively, mg/g; K1, K2 and K3 are the equilibrium rate constant of the pseudo first-order and second-order kinetics, and rate constant of the intra-particle diffusion respectively, 1/h, g/mg. h and mg/g. h0.5; t is the contact time of adsorption process, h. Linear forms of Langmuir and Freundlich used for fitting the experimental data are represented by the following equations27:

=

&'(

+

) &'(

= "* + + %

(6) (7)

where Qe and Qmax are adsorption capacity at equilibrium and the maximal adsorption capacity for monolayer adsorption, respectively, mg/g; KL and KF are Langmuir and Freundlich constant, respectively, L/mg and mg/g; Ce is equilibrium concentration of metal ions, mg/L. 2.2.2 Adsorption in binary system In the binary adsorption experiments, the initial concentration of each metal ion was about 200 mg/L, and each two metal ions were mixed at the volume ratio of 1:1. The pH value of the mixed solution was kept at about 5 for each experiment. Each adsorbent of 0.05 g was placed into 7 conical flasks of 100 mL containing 25 mL metal ions aqueous solution, and shaken at 120 rpm and 30 ℃ for 24 h. Adsorption kinetics study was carried out and the samples were withdrawn at different time intervals (10, 30, 60, 120, 360, 720, 1440 min) to determinate of the metal ions concentration. Adsorption isotherms experiments were proceeded at the initial concentration of about 5, 10, 20, 50, 100, 150, 200 mg/L for each metal ion at the mixed solution. In order to investigate the behavior of competitive adsorption between each two metals ions, separation coefficient is introduced to determine the selectivity. The separation factor in this study is defined as12: ",

/

=

. .!


(8)


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"/ =

∙

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(9)

where Ks and Kd are separation coefficient and distribution coefficient, 1 and L/g, respectively. 2.2.3 Adsorption in ternary system The uptake of transition metal ions from solution containing three types of metal ions was conducted at the volume ratio of 1:1:1 at the initial concentration of about 66.7 mg/L for each metal ions. Each 0.05 g of adsorbent was placed into 7 conical flask of 100 mL, and then 25 mL metal ions mixed solution was added and the flasks were shaken at 120 rpm and 30 ℃ for 24 h to reach the equilibrium. Samples during the adsorption process were taken out for determination of the concentration for each metal ions at time intervals of 10, 30, 60, 120, 360, 720, 1440 min, respectively. 2.2.4 Displacement experiments To further understand the relationship between each two of the metal ions selectively adsorbed on the adsorbent, experiments were carried out to determine whether one metal ion adsorbed first on CCTS would be displaced by another metal ion subsequently adsorbed from the solution. CCTS of 1.6 g was added into 800 mL Mn2+ solution with an initial concentration of about 200 mg/L and shaken at 30 ℃ for 24 h. Then the loaded adsorbent was separated from the solution and divided equally into two. One was placed into 400 mL distilled water first for 24 h of vibration, separated from solution and then added into 400 mL Co2+ solution with an initial concentration of about 200 mg/L. Another one was directly added into 400 mL Co2+ solution with an initial concentration of about 200 mg/L and shaken at 30 ℃ for 24 h. Samples were taken out at preset time interval for monitoring the changes of the metal ions concentration as a function of reaction time. The experiments for determining possible displacement reaction between Mn-Ni, Co-Mn, Co-Ni, Ni-Co and Ni-Mn were conducted with the same condition as mentioned above for Mn-Co. 2.2.5 Adsorption behavior in pyrolusite leaching solution In order to evaluate the possibility of the chitosan derivative application in the real solution, a leaching solution of pyrolusite (whose main component was MnO2) with SO2 as the reductant was chosen as the target solution used in this study. Before adsorption experiment, the solution was


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conducted through oxidation precipitation to remove iron and aluminium and sulfuration to remove most of the transition metals. The concentrations of the main component of the solution after pretreatment were as follows: Mn2+, 78.47 g/L; Co2+, 18.07 mg/L; Ni2+, 12.80 mg/L. The effects of pH value (range of 2-6) and reaction time on the selective adsorption performance of Co2+ and Ni2+ from high Mn2+-solution were conducted. Each 0.1 g of CCTS was added into 5 conical flasks of 100 mL containing 25mL solution, and then the pH value was adjusted to desired value. The flasks were shaken at 120 rpm and 30 ℃ for 24 h to reach equilibrium. A number of 100 mL conical flasks were each filled with 0.1 g adsorbent and 25 mL leaching solution under vibration at 30 ℃ and 120 rpm for 24 h. Samples at desired time intervals were taken out for determination of the metal ion concentration. 2.3 Analysis The metal concentration was determined by an ICP-MS (NEXION 300X, PE) equipped with an auto sampler (SC2 DX, ESI). The Fourier-Transform Infrared Spectroscopy (FTIR) was used to determine the change of the chemical bonds of the adsorbent before and after metal ion adsorption, which was recorded on a spectrometer (FTIR 6700, Nicolet, USA) over the wavenumber range 400–4000 cm-1 at a solution of 4 cm−1. X-ray photoelectron spectroscopy (XPS) was applied to characterize the surface chemical composition of the adsorbent before and after metal ion adsorption, using a XSAM-800 spectrometer (KRATOS, UK) with Al (1486.6 eV) under ultra-high vacuum (UHV) at 12 kV and 15 mA.

3. Results and discussion 3.1 Single component adsorption The effects of pH value on the adsorption process of each metal ion by CCTS were illustrated in Fig. 1. From the results in Fig. 1(a), all the adsorption amounts at adsorption equilibrium for the three metal ions increased quickly with the increase of pH value in the range of 1-4. Furthermore, the adsorption amount for Ni2+ and Mn2+ continued to rise with the increase of pH value, showing that higher pH value will benefit the adsorption process. On the one hand, higher pH would decrease the protonation of active sites of CCTS and the metal ions chelating with the active functional groups on the CCTS surface were the main reaction during the adsorption process. On the other hand, occurrence of metal ions hydrolytic precipitation also contributed to the increase of



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the adsorption capacity. However, when the pH increased up to about 6.0, the adsorption capacity of Co2+ dropped slightly. The reason was that small amount of chitosan derivative chains may transform into -O- with increasing the solution pH and hence reduce the uptake capacity of the adsorbents28. Furthermore, the recovery of cobalt, nickel and manganese were 72.35%, 85.71%, and 44.85% with the initial concentration of 200 mg/L, respectively. From the results, it was found that the adsorption capacities of the three metal ions always followed the order of Ni2+ > Co2+ > Mn2+ in all the solution pH range of this experiment, indicating the difference of binding capacity between the metal ions and adsorbent. Fig. 1(b) showed the adsorption kinetics of the three metal ions in single component solution. It was found that the amount of Co2+, Ni2+, and Mn2+ uptake by CCTS increased with reaction time and adsorption equilibrium was almost reached within 5 h for Co2+ and Mn2+, but for Ni2+, it took a longer time to obtain equilibrium. A linear fitting of the pseudo first-order, pseudo second-order and intra-particle diffusion kinetic models was applied to fit the experimental data. The parameters obtained from the three models for adsorption kinetics of each metal ion were listed in Table S1. As seen from Table S1, the pseudo second-order adsorption model described the adsorption kinetics process better for all of the three metal ions, indicating that chemical adsorption was the main rate-limiting step in the whole adsorption process29. The uptake amounts of CCTS for the three metal ions were in the order of Ni2+ > Co2+ > Mn2+, suggesting that the affinity of CCTS for Mn2+ was weaker than that of Ni2+ and Co2+. Details of the reasons would be discussed in the following sections. In order to further study the adsorption isotherm, adsorption performances were carried out under different initial concentrations of the three metal ions in single solution and the adsorption isotherm results were displaced in the Fig. 1(c). Langmuir and Freundlich isotherm models were used to fit the experimental data and the parameters obtained from linear fitting of the two isotherm models were listed in the Table S2. It was obviously found that the Langmuir model was more suitable for describing the adsorption isotherm than Freundlich model for the three metal ions adsorption by CCTS, demonstrating that monolayer adsorption took place on the homogeneous surface30. The Langmuir constant values of the three metal ions followed the order of Ni2+ > Co2+ > Mn2+, which was consistent with the trend of the adsorption capacity, as higher Langmuir constant stood for stronger adsorption capacity of the same adsorbent for different


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adsorbate31. 3.2 Binary component adsorption Aiming at understanding the competitive adsorption performances of each two metal ions on CCTS in the binary metal species solutions, competitive adsorption kinetics and isotherms were explored with an equal initial concentration of each metal ion and the results were presented in Fig. 2 and 3. From Fig. 2(a), the adsorption of Co2+ uptake increased quickly within the first 2 h, which was attributed to that abundant adsorption sites were available on the CCTS at the beginning of the adsorption process. Subsequently, the adsorption process slowed down and the adsorption equilibrium of Co2+ was achieved after about 6 h adsorption as most of the adsorption sites were occupied. However, in binary system of Co2+/Mn2+, Mn2+ was found to be adsorbed initially and reached a maximum adsorption capacity. Subsequently the adsorption capacity decreased and finally reached an adsorption equilibrium. Obviously, the results showed different adsorption process in Co2+/Mn2+ competitive adsorption. In the beginning, the adsorption sites on the CCTS were enough to adsorb both Co2+ and Mn2+ metal ions, so the adsorption amount of the two metal ions increased quickly without an influence on each other. Afterwards, when a large number of the adsorption sites were captured by the metal ions from the aqueous solution, the Mn2+ adsorbed initially on the CCTS were gradually released into the solution, leading to the decrease of the uptake amount of Mn2+ ion. On the contrary, the Co2+ adsorption continued to increase until reaching adsorption equilibrium. Thus, the results showed that the earlier adsorbed Mn2+ on the CCTS might be displaced by the later adsorbed Co2+ from the aqueous solution. It was also found that the strong competitive and selective adsorption behavior of Co2+ over Mn2+ on the CCTS when the two metal ions coexisted in the same solution. In Fig. 2(b), it was interesting to note that the adsorption process for Ni2+/Mn2+ in the binary system showed the similar trend to that of Co2+/Mn2+ binary species systme. It also showed the strong competitive and selective behavior of Ni2+ over Mn2+ on the CCTS. In the binary solution, the recovery of cobalt, nickel and manganese could achieve 66.11% and 4.54% for cobalt and manganese, 96.01% and 7.62% for nickel and manganese with the initial concentration of 100 mg/L. Fig. 3 showed the adsorption isotherms of the three metal ions in binary systems. As seen in Fig. 3 (a) and 3 (b), it was clearly observed that both Co2+ and Ni2+ showed higher competitive



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adsorption behavior over Mn2+ under various equilibrium concentration. At low initial concentration, Mn2+ adsorption depended on the equilibrium concentration of Mn2+, but for higher concentrations, the Mn2+ adsorption reached a maximum amount and then dropped with a further increase of equilibrium concentration. Furthermore, the adsorption capacities of Co2+ and Ni2+ were much higher than that of Mn2+ under the same initial concentration and experimental conditions. The results further confirmed the phenomenon obtained by adsorption kinetics of the three metals in binary systems, suggesting that competitive and selective adsorption involved in the adsorption process of binary systems of Co2+/Mn2+ and Ni2+/Mn2+ solution. The distribution and separation coefficients, index of selectivity were associated with several factors like temperature, concentration and pH value. The parameters obtained from calculating the experimental data of adsorption isotherms of each binary systems were represented in Table S3. The separation coefficients of Co2+ or Ni2+ over Mn2+ increased with increasing the initial concentration first, and reached a maximum of 215.67 and 302.33, respectively at the initial concentration of 50 mg/L, followed by decreasing with a further increase of initial concentration. The results indicated high selectivity for Co2+ and Ni2+ against Mn2+ by CCTS. 3.3 Ternary component adsorption Fig. 4 indicated that the competitive adsorption kinetics and isotherms behavior of each metal ion in the ternary system solution. It can be seen that Co2+ and Ni2+ adsorption performance was similar to that of each single solution. While, Mn2+ adsorption increased quickly and then dropped gradually to reach adsorption equilibrium. That was to say, the adsorbed Mn2+ initially might also be displaced by the later adsorbed Co2+ and/or Ni2+ from the solution. The results proved that strong competitive and selective adsorption may be involved in the reaction between Co2+ and/or Ni2+ and Mn2+ onto the CCTS. Meanwhile, in the ternary solution, the recovery of cobalt, nickel and manganese could reach 74.47%, 84.22% and 4.06% with the initial concentration of about 66.7 mg/L, respectively. 3.4 Mutual displacement of cobalt, nickel and manganese ions The results in Fig. 2-4 indicated competitive adsorption of Co2+ and/or Ni2+ over Mn2+ and the possibility of the earlier adsorbed Mn2+ on CCTS being displaced by the later adsorbed Co2+ and/or Ni2+ from the aqueous solution. Therefore, displacement experiments among the three


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metal ions on CCTS were further conducted to verify this speculation and the results were expressed in Fig. 5. CCTS-Mn, (CCTS, loaded with Mn2+) from a Mn-containing solution, was placed in a Co2+ solution (denoted as (CCTS-Mn)-D-Mn), D means displaced), or in deionized water (denoted as (CCTS-Mn)-R-Mn), R means released). Subsequently, (CCTS-Mn)-R-Mn was added in a Co2+ solution (denoted as (CCTS-Mn)-w-D-Mn), w means deionized water) under the same conditions. The adsorption of Co2+ on the CCTS in the first displacement process was denoted as (CCTS-Mn)-D-Co,

and

that

in

the

second

displacement

process

was

denoted

as

(CCTS-Mn)-w-D-Co. In the first displacement process, as shown in Fig. 5(a), an amount of 5.75 mg/g Mn2+ ((CCTS-Mn)-R-Mn) was released into the deionized water, while 33.06 mg/g of Mn2+ was detected in the solution after the first displacement process by Co2+ solution. At the same time, about 23.93 mg/g of Co2+ ((CCTS-Mn)-D-Co) was adsorbed on the CCTS from the solution. In the second displacement process, another part of the adsorbed Mn2+ (about 23.63 mg/g) was released into the solution, meanwhile, 28.02 mg/g of Co2+ was adsorbed on the CCTS. Thus, all the initially adsorbed Mn2+ was almost displaced by the subsequent adsorbed Co2+. The details of the adsorption and released amount of the displacement process were listed in Table S4. In contrast, as seen from Fig. 5(b), the kinetic results on the displacement behavior of CCTS-Co in Mn2+ solutions noted that the Mn2+ could barely displace the Co2+ early adsorbed on the CCTS into the solution. The released amount of Co2+ by Mn2+ was even less than that in the deionized water. For Ni2+ and Mn2+ displacement experiments, the Ni2+ could displace some of the Mn2+ on the CCTS into the solution, but it was much less than that replaced by Co2+. Previous studies32-34 showed that the adsorption capacity of one adsorbent for metal ions was related to the characteristics of the metal ions such as ionic radius, electronegativity, hardness and hydrated radius. In the single system, the uptake amount of Ni2+ was higher than that of Co2+ onto CCTS, which was due to the higher electronegativity of Ni2+ than that of Co2+. However, in term of selective adsorption, covalent index 1 2 (Xm = electronegativity, r = Pauling ionic radius) was more suitable to explain the metal affinities to adsorbent. The 1 2 values of Co2+, Ni2+ and Mn2+

were 2.65, 2.52 and 1.99, respectively33,35-37. Thus, the amount of Mn2+ displaced by Ni2+ was less



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than that by Co2+ into the solution. What’s more, a great deal of Ni2+ was adsorbed on the CCTS. This phenomenon may prove that only part of the Ni2+ ion participated in the displacement adsorption process of Mn2+ through direct displacement. Furthermore, the rest Ni2+ ion was adsorbed onto the CCTS through other interaction mechanisms such as electrostatic attraction and hydrogen bonding, resulting in higher adsorption amount for Ni2+ ion38, 39. From the results obtained in the displacement experiments, it was identified that the Co2+ and Ni2+ showed strong displacement ability over Mn2+, which verified the phenomenon from the competitive adsorption of Co2+ and/or Ni2+ over Mn2+ solution in the binary system. 3.5 Adsorption behavior in pyrolusite leaching solution As seen from Fig. 6, great selective adsorption performance for Co2+ and Ni2+ removal from high Mn2+ solution was achieved with a removal percentage of about 98.26 % and 94.12 % for Co2+ and Ni2+, respectively. What is more, both the concentrations of Co2+ and Ni2+ in the solution after adsorption were below 1 mg/L, which met the requirement of high purity of Mn2+ solution. The results proved that the chitosan derivative was a promising adsorbent to be applied in pyrolusite leaching solution for high selectivity and efficient separation of Co2+ and Ni2+ from Mn2+-solution. 3.6 Adsorption mechanism 3.6.1 FTIR and XPS analyses for adsorption mechanisms In order to elucidate the selective adsorption mechanisms of each metal ion onto CCTS, FTIR spectra and XPS analyses were applied to examine the surface interactions in the adsorption process, whose results were showed in Fig. 7 and 8. From Fig. 7, the peak at 894 cm-1 remained unchanged during the competitive adsorption process, and all the IR spectra of CCTS after adsorption were similar with the CCTS before adsorption, indicating that the adsorption process did not destroy the structure of the CCTS40, 41. Compared with the FTIR of CCTS in Fig. 7(a) , the peaks at 1655 and 1580 cm-1 were attributed to the stretching vibration of N–H in primary amine and secondary amine. The IR spectra of CCTS after adsorbing different mixed metal ions in Fig. 7(b)~(d) represented the disappearance of the two peaks and the appearance of a new stronger peak at 1644 cm-1. This phenomenon suggested that the chemical bonds of the nitrogen atoms on the CCTS changed and formed a coordination bond with metal ions from the aqueous solution42, 43.


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Furthermore, the wavenumber at 1153 cm-1 assigned to the O–H stretching vibration (C3–OH) changed to about 1114 cm−1 after adsorption, indicating that the oxygen atom on the CCTS also involved in the coordination with metals44. At the same time, another wavenumber peak at about 617 cm-1 appeared, which was contributed to N–Metal or O–Metal, noting that the metal ions reacted with the adsorbents through forming chemical bonds between M2+ (M was Co2+, Ni2+ or Mn2+) and nitrogen or oxygen atoms on the CCTS. Unfortunately, from the FTIR spectra of the CCTS after adsorption at Fig. 7(b)~(d), no difference can be observed for the various selective adsorption conditions. To further verify the results of the FTIR analysis and explore the mechanism of competitive and selective adsorption, XPS analysis of CCTS before and after adsorption of different mixed metal ions was employed to distinguish the forms of elements and identify the typical elements on the surface of the adsorbent. The results were shown in Fig. 8, Fig. S1-S2 and Table S5. From the wide scans in Fig. 8, the conventional characteristic peaks at 285 eV, 399 eV, and 531 eV, corresponding to C 1s, N 1s, and O 1s, respectively, could be observed for all the materials under different experimental conditions44. However, in the binary species solution of Co2+ and Mn2+ adsorption, binding energy of element Co 2p was at about 794 eV, but no spectrogram of Mn on the adsorbent was detected, confirming the successful adsorption of Co2+ on the CCTS. This was mainly due to the strong competitive and selective adsorption of Co2+ over Mn2+ in the binary system. When the adsorption process reached an equilibrium, most of the active adsorption sites were occupied by the Co2+, as a result, the adsorption amount of Mn2+ on the surface of CCTS was too little to be discerned by XPS analysis. Similar phenomenon can also be discovered in the binary system of Ni/Mn (Fig. 8c) and ternary system of Co, Ni&Mn solutions (Fig. 8d). This results were in good consistence with the speculation of the adsorption kinetic and isotherm of binary and ternary solution, identifying that the selective adsorption was actually involved in the competitive adsorption of Co2+ and Ni2+ over Mn2+. To verify the results of FTIR analysis that the nitrogen and oxygen atoms were participated in the coordination of the metal ions, the spectra of N 1s, O 1s, Co 2p1/2, Co 2p3/2, Ni 2p1/2 and Ni 2p3/2 of the adsorbed materials were determined and the results were shown in Fig. S1-S2 and Table S5. The peaks at 397.4 and 399.0 eV were corresponding to the neutral amine –NH2 or –NH, and protonated amine –NH3+ or –NH2+, respectively. After adsorption, all the N 1s spectra



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

changed and moved to higher binding energy with an increase of about 1.2 eV as shown in Table S5. This was because of chemical bonds formation between the nitrogen atom on the CCTS and metal ion, resulting in that the nitrogen in the neutral amine transformed to a more oxidized state with a higher binding energy. The lone pair of electrons in the nitrogen atom was donated to the shared bond between the N and metal ions. Therefore, the changes of N 1s spectra indicated that nitrogen atom was involved in the coordination with metals ions. Furthermore, the peaks at 530.9 and 534.4 eV were assigned to C– OH and bond water, respectively. After adsorption, all the peaks of the adsorbed metal ions adsorbents at 530.9 changed to about 531.2 eV and a new peak at 532.1 eV appeared, which may be caused by the oxygen atoms forming coordination bonds with the metal ions so that the oxygen existed in a more oxidized state with higher binding energy to balance the electrical charges of the adsorbed metal ions. Thus, the results suggested that the oxygen on the CCTS were also participated in the adsorption with metal ions. XPS spectra of Co and Ni 2p after adsorption by CCTS were shown in Fig. S2 and Table S5. Compared to the XPS data of CoSO4 and NiSO4 at 784.0 and 856.8 eV, respectively, all the peaks of Co 2p or Ni 2p observed after adsorption of metal ions shifted towards lower binding energy46. This indicated that Co2+ or Ni2+ gained electron and formed complexes between Co2+ or Ni2+ and – OH, –NH2. Therefore, both FTIR and XPS analyses confirmed that the oxygen and nitrogen atoms on the CCTS were involved in the adsorption of metal ions from aqueous solution, no matter in the single, binary or ternary species solution, what’s more, the results of XPS analysis indicated that competitive and selective adsorption of Co2+ and/or Ni2+ over Mn2+ existed in the adsorption process. 3.6.2 Selective and displacement adsorption mechanisms Although it was conformed that: a. nitrogen and oxygen atoms on the CCTS chelating with metal ions was the main reaction in the adsorption process, b. the adsorption capacity for Co2+, Ni2+, and Mn2+ followed the order of Ni2+ > Co2+ > Mn2+ in each single solution, and c. competitive and selective adsorption behaviors for adsorption of Co2+ and/or Ni2+ over Mn2+ were observed through adsorption kinetic, isotherms and XPS analysis, the reason why the selective adsorption behaviors took place between Co2+ and/or Ni2+ over Mn2+ was still unknown. The


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adsorption selectivity of CCTS among Co2+, Ni2+, and Mn2+ can be caused by the different affinities between CCTS and Co2+, Ni2+, and Mn2+, which were related to the characteristic properties of the Co2+, Ni2+, and Mn2+ in aqueous solution13. The parameters including the ionic radius, electronegativity, hardness and hydrated radius were listed in Table 114–16,33. Both the ionic radius and hydrated radius of the Co2+ and Ni2+ were smaller than that of Mn2+, which would be easier for Co2+ and Ni2+ to be adsorbed on the surface or inner of CCTS to generate more stable complexes. The absolute electronegativity of Co2+ and Ni2+ were higher than that of Mn2+, suggesting that Co2+ and Ni2+ had stronger attractions to the lone pair electrons of the nitrogen atom in CCTS than that of Mn2+ to form dominant species, which was in agreement with the order of adsorption capacities obtained from single solution system. That was to say, the selective and competitive adsorption of Co2+ and Ni2+ over Mn2+ were mainly benefit from the stronger absolute electronegativity, smaller ionic and hydrated radius of Co2+ and Ni2+ than those of Mn2+. The results in Fig. 2-5 clearly showed that the initially adsorbed Mn2+ on CCTS were displaced by later adsorbed Co2+ and/or Ni2+ from the aqueous solution. However, the early adsorbed Co2+ and/or Ni2+ was barely displaced by the later adsorbed Mn2+ from the solution. The theory of hard and soft acids and bases (HSAB) can well explain the displacement adsorption mechanism. According to HSAB theory, Co2+ and Ni2+ were classified as intermediate acids, while Mn2+ was belonged to hard acid. The atoms of nitrogen and oxygen on the CCTS can be corresponding to intermediate base. Therefore, the CCTS would prefer to coordinate with Co2+ and Ni2+ to form a more stable compound. The adsorbed Mn2+ on the CCTS can be directly displaced by the intermediate acid Co2+ and Ni2+ to form relatively stable complexes.

4. Conclusions In this work, competitive and selective adsorption performance of Co2+ and Ni2+ over Mn2+ by triethylene-tetramine-modified cross-linked chitosan were conducted in batch experiments and FTIR and XPS analyses were used to determine the interactions between adsorbent and the three metal ions. The maximum adsorption capacities of Co2+, Ni2+ and Mn2+ on CCTS followed the order of Ni2+ > Co2+ > Mn2+ in single metal species system. In particular, CCTS displayed an excellent selectivity in the adsorption of Co2+ and/or Ni2+ over Mn2+ either in binary or ternary metal species solutions (Co/Mn, Ni/Mn, or Co, Ni/Mn presented). The results of displacement



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

experiments indicated that the initially adsorbed Mn2+ was displaced by the later adsorbed Co2+ or Ni2+, but not vice versa, which was consistence with the results of competitive experiments. FTIR results confirmed the adsorption mechanism of the three metal ions on the CCTS through forming coordination bonds with the atoms of nitrogen and oxygen atoms on the CCTS. XPS results presented that no Mn element was detected, which convinced the mechanism and this exhibited competitive and selective adsorption behavior of Co2+ and/or Ni2+ over Mn2+ on the CCTS. The difference in the absolute hardness, hydrated radius, and electronegativity of the three metal ions was identified as the main factor to the selective adsorption. It was speculated that the direct displacement was the main interaction mechanism for the results of the displacement experiments that the initially adsorbed Mn2+ was displaced by the later adsorbed Co2+ or Ni2+. Experiment in pyrolusit leaching solution with chitosan derivative for separating Co2+ and Ni2+ from high Mn2+-solution showed efficient separation of Co2+ and Ni2+. In general, the chitosan derivative was proved to be as an alternative adsorbent to remove metals (such as Co2+ and Ni2+), especially separate metal impurities from manganiferous wastewater for possible recovery and production of high-purity metal solution.

Acknowledgements This project is supported by the National Natural Science Foundation of China (NSFC-51304140 and NSFC-51374150) and Science and Technology Plan Projects of Sichuan Province, China (Grant No. 2015HH0067 and 2014SZ0146).

Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Five tables and two figures presenting the adsorption results are listed in the Supporting Information, including adsorption kinetics and isotherm parameters, separation factor parameters, adsorption amount of the displacement process and binding energies obtained from the XPS analysis.

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List of Tables Table 1. Characteristic properties of the Co2+, Ni2+, and Mn2+.

List of Figures Fig. 1 Co2+, Ni2+ and Mn2+ adsorption on CCTS in single metal species system: (a) solution pH effect (C0 =200 mg/L); (b) adsorption kinetics; (c) adsorption isotherms. Fig. 2 Adsorption kinetics of Co2+, Ni2+ and Mn2+ on CCTS in binary metal ions solution: (a) Co2+/Mn2+ binary system; (b) Ni2+/Mn2+ binary system. Fig. 3 Adsorption isotherms of Co2+, Ni2+ and Mn2+ on CCTS in binary metal ions solution: (a) Co2+/Mn2+ binary system; (b) Ni2+/Mn2+ binary system. Fig. 4 Adsorption kinetics and isotherms of Co2+, Ni2+ and Mn2+ on CCTS in ternary metal ions solution: (a) adsorption kinetics; (b) adsorption isotherms.


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Fig. 5 Displacement adsorption of Co2+, Ni2+ and Mn2+ on CCTS. (a, b) Co2+/Mn2+ binary system; (c, d) Ni2+/Mn2+ binary system. Fig. 6 Effects of pH value and reaction time on the selective adsorption of Co2+ and Ni2+ from high Mn2+-solution by CCTS: a, effect of pH value; b, effect of reaction time. Fig. 7 FTIR spectra of CCTS (a), CCTS-Co&Mn (b), CCTS-Ni&Mn (c) and CCTS-Co, Ni&Mn (d). Fig. 8 XPS wide scans of CCTS (a), CCTS-Co&Mn (b), CCTS-Ni&Mn (c) and CCTS-Co, Ni&Mn (d).

Table 1. Characteristic properties of the Co2+, Ni2+, and Mn2+. Parameters Metal ion

Absolute Absolute hardness

Ionic radius (Å)

Hydrated radius (Å) electronegativity

Co2+

8.2

0.73

25.28

4.23

Ni2+

8.5

0.69

26.75

4.04

Mn2+

9.3

0.82

24.4

4.38



100

a Adsorption capacity (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

80

60

40

Co2+ Ni2+ Mn2+

20

0 1

2

3

4

5

6

pH


7

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Page 23 of 30

100

100

c

80

80

60

60

Qe (mg/g)

Adsorption capacity (mg/g)

b

40

20

0

5

10

15

40

Co2+ 2+ Ni Mn2+

20

2+

Co Ni2+ 2+ Mn

0

0

20

25

30

0

20

40

60

Reaction time (h)

80

100

120

140

160

Ce (mg/L)

Fig. 1 Co2+, Ni2+ and Mn2+ adsorption on CCTS in single metal species system: (a) solution pH effect (C0 =200 mg/L); (b) adsorption kinetics; (c) adsorption isotherms.

30

b

a Adsorption amounts (mg/g)

Adsorption amounts (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25 2+

Co 2+ Mn

20

15

10

5

0

40

Ni2+ Mn2+ 20

0 0

5

10

15

20

25

0

5

Time (h)

10

15

20

25

Time (h)

Fig. 2 Adsorption kinetics of Co2+, Ni2+ and Mn2+ on CCTS in binary metal ions solution: (a) Co2+/Mn2+ binary system; (b) Ni2+/Mn2+ binary system.



40

b

a

35 30

40

25

Qe (mg/g)

Qe (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Co2+ Mn2+

20 15

Ni2+ Mn2+ 20

10 5

0

0 0

50

100

150

200

0

50

100

150

200

Ce (mg/L)

Ce (mg/L)

Fig. 3 Adsorption isotherms of Co2+, Ni2+ and Mn2+ on CCTS in binary metal ions solution: (a) Co2+/Mn2+ binary system; (b) Ni2+/Mn2+ binary system.


Page 25 of 30

30 35

a

b

25

30

20

25

Qe (mg/g)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Co2+ Ni2+ Mn2+

15

Co2+ Ni2+ Mn2+

20 15

10 10

5 5

0

0

0

5

10

15

20

25

0

20

40

60

80

100

120

140

Ce (mg/L)

Time (h)

Fig. 4 Adsorption kinetics and isotherms of Co2+, Ni2+ and Mn2+ on CCTS in ternary metal ions solution: (a) adsorption kinetics; (b) adsorption isotherms.



40

b

5

a



30

(CCTS-Mn)-R-Mn (CCTS-Mn)-D-Mn (CCTS-Mn)-D-Co (CCTS-Mn)-w-D-Mn (CCTS-Mn)-w-D-Co (CCTS-Mn

20 10 0 -10 -20

0 (CCTS-Co)-R-Co (CCTS-Co)-D-Co (CCTS-Co)-D-Mn (CCTS-Co)-w-D-Co (CCTS-Co)-w-D-Mn

-5

-30 -40 -200

0

200

400

600

800

1000

1200

1400

1600

0

200

400

600

800

1000

1200

1400

1600

Time (min)

Time (min) 50

8

c

30

Adsorption capccity (mg/g)

40


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(CCTS-Mn)-R-Mn (CCTS-Mn)-D-Mn (CCTS-Mn)-D-Ni (CCTS-Mn)-w-D-Mn (CCTS-Mn)-w-D-Ni CCTS-Mn

20

10

0

-10 -200

d

6

(CCTS-Ni)-R-Ni (CCTS-Ni)-D-Ni (CCTS-Ni)-D-Mn (CCTS-Ni)-w-D-Ni (CCTS-Ni)-w-D-Mn

4 2 0 -2 -4 -6

0

200

400

600

800

1000

1200

1400

1600

0

200

400

600

800

1000

1200

1400

1600

Time (min)

Time (min)

Fig. 5 Displacement adsorption of Co2+, Ni2+ and Mn2+ on CCTS. (a, b) Co2+/Mn2+ binary system; (c, d) Ni2+/Mn2+ binary system.


Page 27 of 30

100

a

b Removal percentage (%)

100

Removal percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

60

Co Ni

40

20

0

80

60

Co Ni 40

20

0

2

3

4

5

6

7

0

5

pH

10

15

20

25

Reaction time (h)

Fig. 6 Effects of pH value and reaction time on the selective adsorption of Co2+ and Ni2+ from high Mn2+-solution by CCTS: a, effect of pH value; b, effect of reaction time.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

N-H or O-H

C-H

d

CCTS-Co,Ni&Mn

c

CCTS-Ni&Mn

b

CCTS-Co&Mn

N-H2

paranoid ring N-M or O-M O-H

a CCTS

3000

2000

1000

Wavenumber/cm-1 Fig. 7 FTIR spectra of CCTS (a), CCTS-Co&Mn (b), CCTS-Ni&Mn (c) and CCTS-Co, Ni&Mn (d).


Page 28 of 30

O 1s

b

CCTS-Co&Mn

C 1s

30000 25000 20000

N 1s

N 1s

15000

10000

35000

CCTS

Intensity (Cps)

20000

Intensity (Cps)

40000

C 1s

a

O 1s

25000

15000 10000

5000

5000

0

0

0

200

400

600

800

1000

1200

0

200

400

600

800

1000

1200

Binding Energy (eV)

Binding Energy (eV) O 1s

d

CCTS-Co, Ni&Mn

30000

20000

10000

10000

0

0 0

200

400

600

800

1000

1200

N 1s

20000

Ni 2p

N 1s

30000

C 1s

Intensity (Cps)

C 1s

40000

0

200

Binding Energy (eV)

400

600

Ni 2p

CCTS-Ni&Mn

40000

Co 2p

c

O 1s

50000

50000

Intensity (Cps)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Co 2p

Page 29 of 30

800

1000

1200

Binding Energy (eV)

Fig. 8 XPS wide scans of CCTS (a), CCTS-Co&Mn (b), CCTS-Ni&Mn (c) and CCTS-Co, Ni&Mn (d).



Graphical Abstract: 100

80



30

60

40

Co2+ Ni2+ Mn2+

20

Single system

25

20

Co2+ Mn2+

15

Binary system 10

5

0 0

0

5

10

15

20

25

30

0

5

10

Time (h)

15

20

25

Time (h) 30


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 30

40

Ni2+ 2+ Mn

Binary system

20

20

Co2+ Ni2+ Mn2+

15

10

Ternary system

5

0

0 0

5

10

15

20

25

0

5

10

15

Time (h)

Time (h)


20

25

Efficient Separation and High Selectivity for Cobalt and Nickel from

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