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Valorization of an electronic waste-derived aluminosilicate – Surface functionalization and porous structure tuning Chao Ning, Pejman Hadi, Carol Sze Ki Lin, and Gordon McKay ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01523 • Publication Date (Web): 03 May 2016 Downloaded from http://pubs.acs.org on May 7, 2016

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ACS Sustainable Chemistry & Engineering

Valorization of an electronic waste-derived aluminosilicate – Surface functionalization and porous structure tuning

Chao Ninga, Pejman Hadia, Carol Sze Ki Linb and Gordon McKaya,c*

a

Chemical and Biomolecular Engineering Department, Hong Kong University of Science and

Technology, Clear Water Bay Road, Hong Kong SAR b

School of Energy and Environment, City University of Hong Kong, Tat Chee Avenue, Hong

Kong SAR c

Division of Sustainable Development, College of Science, Engineering and Technology, Hamad

Bin Khalifa University, Qatar Foundation, Doha, Qatar *

Tel: +852 23588412, Fax: +852 23580054, E-mail: [email protected]

ACS Paragon Plus Environment


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Abstract This study involves the sustainable development of an ion exchange material with ultra-high heavy metal uptake capacity from a waste material, originally destined for landfills. In this study, a promising thermo-alkaline reaction has been employed to simultaneously alter the surface chemistry and tune the textural properties of the waste-derived aluminosilicate. The effects of several reaction variables on the formation of mesotunnels in the structure of the material have been examined. Also, the surface characterization of the functionalized aluminosilicate has demonstrated that the functionalization reaction results in the cleavage of the robust − − linkages (where = ) into − moieties, counter-balanced by an alkali metal cation, resulting in the coverage of the aluminosilicate surface with active ion exchange sites. Comparison of the ion exchange capacity of the functionalized aluminosilicate with the commercial ion exchange resins has proved exceptionally higher heavy metal uptake for the former. The ultra-high heavy metal uptake of this material is ascribed to the high concentration of developed counter-balancing cations on the material surface. The attractiveness of this innovative approach is manifested by the dual environmental benefit, i.e. sustainable upcycling of a waste formerly disposed of into landfills and its utilization for heavy metal-laden wastewater treatment.

Keywords: Sustainable development; Waste – derived aluminosilicate; Mesoporous structure; Ion exchange; Heavy metal removal; Adsorption; Functionalization


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Introduction Aluminosilicate materials have received ever-increasing attention due to their high internal surface area and pore volume, tunable pore size distribution, high thermal stability and unique ion exchange capability1. Fabrication of these materials with desired textural properties and their surface modification to tailor certain functionalities stand out as an utmost crucial area of research2, as they find a wide array of applications in electrochemistry3, catalysis4–7, support8,9, cement industry10, chromatography11, dehumidification12, antioxidant13, fertilizers14, sensors15 and sorbent16–19. Ion exchange has been one of the main areas of interest for aluminosilicate materials for catalysis20 and adsorption21. Several investigations have been carried out for the isomorphous substitution of trivalent aluminum by tetravalent silicon in the aluminosilicate skeleton during the synthesis process giving rise to the net negative charge on its surface. This negativelycharged framework is balanced with extra-framework alkali metal cations22,23. These cations can theoretically be exchanged with target metal species, such as heavy metals, either to tailor certain other properties to the aluminosilicate (such as in catalysis)24 or to remove charged pollutants (including heavy metal ions) from a medium (such as in wastewater treatment)25. With the recent advent of environmental awareness, numerous researchers have attempted to sustainably exploit the ion exchange ability of natural aluminosilicate-based minerals, such as clinoptilolite26,27, kaolin28,29 and mordenite30, as an alternative for unsustainable multi-stage production of synthetic aluminosilicate requiring the reckless utilization of material and energy resources. Although some progress has been made in this area, the inherent drawback with this so-called sustainable concept is the low ion exchange capacity of these minerals31, which hinders their commercial utilization in applications requiring high ion exchange capacity. Gupta and



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Bhattacharyya have summarized the ion exchange capacities of many different types of inorganic materials32. Their tabulated results show the extremely low metal uptake by natural inorganic minerals. Therefore, it is deemed necessary to develop new sustainable approaches to fabricate cost-effective ion exchange materials without sacrificing the efficiency. As part of our ongoing investigations, we believe that the low ion exchange capacity of aluminosilicates originates from the fact that the alkali cations, functioning as a chargecompensator for clusters, can only locally move along the lattice and hence their potential to actively participate in ion exchange reactions is masked by two phenomena: barrier against the diffusion of the alkali metal cations through the pores to be exchanged with heavy metals 1, and resistance against the electroneutrality disturbance in the course of ion exchange. Therefore, we ascribe the low ion exchange capacity of the natural and synthetic zeolites predominantly to the microporous nature of their architecture as well as the great electrostatic interaction between the charge-balancing metal cations and the zeolite structure. Alternatively, the alkali cations in the position of network modifier, bound to the non-bridging oxygens (), are more likely to play the role of ion exchange sites because of the small diffusion barrier and comparatively low electrostatic attraction33. Yet, in most of the studies, this distinction between the charge compensator cations and network modifier species has been disregarded. In the light of this hypothesis, increasing the number of and thus ion exchange sites is a more promising approach to boost the ion exchange capability of an aluminosilicate-based material. The aim of this study is to create mesotunnels covered with active ion exchange sites in a hydrophobic nonporous aluminosilicate material derived from an electronic waste source and transform it into a functionalized hydrophilic ion exchange material. Recently, there has been an enormous outcry on the environmental impacts of waste printed


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circuit boards (PCBs) 34,35. Since waste PCBs comprise a myriad of heavy metals, they cannot be disposed of into landfills 36–38. In addition to the toxicity issue, the low calorific value of these materials does not allow their incineration. On the other hand, PCBs can be acknowledged as a mine for concentrated metals 39,40 as well as a resource for aluminosilicates35,41, the recycling of which can contribute to the environmental mitigation and add value to a waste substance. Hence, the sustainable development of ion exchange material from a problematic waste source will not only divert the waste disposal practice into landfills, but can also be commercially used for heavy metal-laden wastewater treatment, doubling the stimulus for greener environment. In this study, the effects of the reaction parameters on the mesotunnel formation with functionalized walls via the partial cleavage of unreactive − − linkages (where is bridging oxygen () and , = ) into more reactive − … linkages (where is non-bridging oxygen () and is an alkali metal cation) using a thermo-alkaline reaction have been elucidates.

Experimental Surface modification of the NMF aluminosilicate Waste printed circuit boards (WPCBs) had been crushed into fine powders in a local recycling company and the metal and nonmetal fractions were separated by a corona electrostatic discharge process. Non-metallic fraction of WPCB (NMF), a hydrophobic powder-form material with an average particle size of 2 , was used as an inexpensive amorphous aluminosilicate source. NMF was impregnated with 1M potassium hydroxide solution (Sigma-Aldrich, >85%), as the activating agent, at a range of impregnation ratios (1 − 4 /) for 1 h until homogeneous dispersion was reached. In this context, impregnation ratio is defined as the weight ratio of the



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chemical reagent to the precursor. After the completion of the impregnation process, the surface functionalization reaction was carried out by heating the mixture in a reactor to a desired activation temperature (200-400˚C) and for specified reaction durations (1-7 h) under an inert nitrogen atmosphere. Subsequently, the surface functionalized material was washed several times with hot water to remove excessive unreacted activating agent, freeze dried overnight and kept in a desiccator for later use. Notably, minimum of 3 replications for each reaction were conducted to ascertain the accuracy of the results.

Surface characterization The surface morphologies of the virgin and functionalized aluminosilicate were investigated by a JEOL JSM-6390 scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDX). The samples were mounted on a carbon tape and coated with a thin layer of gold via sputtering technique. Random areas of the sample surfaces were selected for EDX elementary mapping. Fourier transform infrared spectroscopy (FTIR) was recorded on a FTS 6000 FTIR spectrometer with a nominal resolution of 2 at a range of 4000-400 for both virgin and functionalized samples to determine their surface functional groups. X-ray photoelectron spectroscopy (XPS, model PHI5600) was conducted with a monochromatic Al Kα source (excitation energy, hυ = 1486.6 eV) at a voltage of 10 kV and a current of 15 mA. Low resolution spectrum for the whole range was acquired at 70 eV pass energy, whereas high resolution narrow range spectra for elements of interest were acquired at 20 eV pass energy. The water contact angle measurements were determined using GBX Digidrop (model No SERIE 2850902D).


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Textural properties Nitrogen adsorption-desorption isotherms were obtained using a Beckman Coulter SA3100 surface area analyzer at a relative pressure range of "

!

= 0.01 − 0.99 and at the liquid

nitrogen temperature (77 K). The functionalized materials were outgassed at 150˚C for 90 min in a high vacuum prior to analysis in order to remove moisture and any other impurities potentially physisorbed to the pores of the materials. Specific surface area ( &'( ) was determined at the linear relative pressure range of 0.05 – 0.3 using the Brunauer – Emmett – Teller ()) equation42. The total pore volume (*+ ) was calculated at a relative pressure of 0.98. The t-plot method43 was employed to determine the micropore volume (*-./ ). The pore size distribution was obtained using the Barrett – Joyner –Halenda (01) equation44 at the desorption branch of the isotherm. Average pore diameter (2 ) was estimated considering the cylindrical pore geometry.

Adsorption experiments The batch adsorption tests were performed by adding 50 mg ion exchange material into bottles containing 50 metal solution with a range of concentrations (0.5 − 7 . 5 ). The solution pH level was adjusted to 4 using 0.5 M HNO3 and NaOH solutions to eliminate any potential effect of acidity/basicity. The solutions were agitated in an orbital shaker at a rate of 120 rpm and a constant temperature of 25˚C until equilibrium. The initial and final metal concentrations were determined by an inductively coupled plasma atomic emission spectrometer (ICP-AES, Optima 7300DV, Perkin Elmer). The metal uptake capacities of the functionalized materials were determined using the mass balance between the liquid and solid phases.



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Result and Discussion Tunneling effect Mesotunnels covered with active functional moieties has been developed in the waste-derived aluminosilicate by the cleavage of the tetrahedrally-coordinated cages, consisted of − − linkages connected via bridging oxygens (), into − clusters, where charge neutrality for the negatively-charged non-bridging oxygens () is established by an alkali metal cation. Therefore, higher tunneling is indicative of higher − 6 7 alkali metal cations and thus higher concentrations of active ion exchange sites, while − 6 7 metal cation has a passive role of network charge compensator only. Herein, the effect of various reaction parameters on the porosity of the aluminosilicate-based NMF, and thus its ion exchange capability, has been examined.

Effect of reaction temperature In order to investigate the effect of reaction temperature, which has an utmost significance in tuning the porosity of materials, the reaction time and impregnation ratio were retained constant at 3 h and 2 /, respectively. As shown in Table 1, as the reaction temperature increases up to 300˚C, the surface area and pore volume increase. This is related to the fact that the cleavage of the − − into − linkages is a very energy-demanding process due to the robustness of the former functional group. Thus, higher temperatures provide higher energy intensity for the rupture of the tetrahedral aluminosilicate cages and tunnel formation, as illustrated in Figure 1, explaining the drastic increase in the surface area and pore volume of the aluminosilicate. Further increase in the reaction temperature results in a sharp decrease in both surface area and pore volume which is ascribed to the distortion of the aluminosilicate network, rather than its


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partial bond cleavage, at such high temperatures. (Figure 1) (Table 1) The negatively-charged ions formed on the surface of the aluminosilicate ( − ) are counterbalanced by two distinct approaches; 1) the cations of the activating agent, 8 9 , form − … 8 9 clusters, and 2) when the Al-containing tetrahedral network is ruptured, the :;9 cations, already as the charge compensator for clusters, migrate to the aluminosilicate surface and take the role of the charge-balancing cation for − linkages. Both of these cations can play the role of ion exchange sites to uptake heavy metal ions from aqueous solutions. Therefore, higher surface area is an indication of enhanced bond cleavage and thus higher concentration of ion exchange sites. Figure 2 compares the nitrogen adsorption/desorption isotherms and pore size distribution curves for the aluminosilicate materials functionalized at various reaction temperatures. All the isotherm curves, regardless of the reaction temperature, indicate the type IV isotherms with distinct H3type hysteresis loops, according to the IUPAC classification, characteristic of the mesoporous materials. Despite the existence of a hysteresis loop for all the materials, the hysteresis loop areas are different (see Table S1). FD250 and FD300 exhibit larger hysteresis loop areas, which are translated into larger mesopore surface area, in close agreement with the quantitative data presented in Table 1. (Figure 2) The trend of the change in the pore size of the samples is relatively identical to that of the surface area, where an increase in the reaction temperature from 200˚C to 250˚C also increases the average diameter of the developed pores. Further increase in the reaction temperature to 300˚C



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results in a slight decrease in the average pore size of the material. This trend is attributed to the parallel occurrence of two phenomena during the functionalization process; the development of micropores and the development and/or enlargement of mesopores as a result of small pore mergence. In the first stage, the micropore development is relatively low and mesopore enlargement is the dominating factor. However, when the temperature is further increased, micropore development dominates the mesopore enlargement validated by a significant increase in the micropore surface area (see Table 1). At an activation temperature of 400˚C, the aluminosilicate network is totally distorted due to the pore collapse or pore contraction 15. Therefore, unlike carbonaceous materials which require pore formation temperatures as high as 800-1000˚C45–49, even slightly high temperatures are detrimental for the pore formation of the siliceous materials, resulting in the recombination of the developed active − moieties into undesirable formation of passive − − linkages.

Impregnation ratio Quantity of the activating agent is another key element which significantly affects the textural and surface properties of the aluminosilicate materials and also is a determining factor in the cost effectiveness of the functionalization process. For effective tunneling and functionalization, sufficient amount of activating agent should be available in the reaction medium to cleave the robust − − linkages in the tetrahedral structure of the aluminosilicate. Also, excessive amounts of the activating agent, not only is not economical in terms of the chemical cost and the energy required to heat the activating agent solution, but also may result in “over-cleavage” of the tetrahedral bonds and thus distortion of the structure. In order to evaluate the optimum quantity of the activating agent in the reaction, the effect of the impregnation ratio on the textural


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properties of the aluminosilicate has been studied, while the reaction time and temperature are fixed at 3 ℎ and 300˚:, respectively. The activated materials have been denoted as ?@A, where indicates the impregnation ratio (/) employed in the reaction. As shown in Table 1, when the impregnation ratio is 1 /, incomplete activation occurs resulting in very low surface area and pore volume. This is due to the inadequate amount of the activating agent to break the abundant concentration of the − − linkages. Higher porosity can be achieved by increasing the relative quantity of the activating agent to reach the impregnation ratio of 2 /. This is indicative of the cleavage of more − − linkages which facilitates the creation of mesotunnels and hence higher surface area and pore volume. However, when excessive activating agent amount is introduced into the reaction, an interesting change in the textural properties occurs; the surface area remains unchanged, but the pore volume drastically increases. Supposedly, the surface area and pore volume should increase upon the introduction of large amount of the activating agent due to the formation of more mesotunnels. Nevertheless, at such vigorous basic conditions, a parallel phenomenon concurrently occurs in addition to the tunnel formation, the removal of pore walls separating two adjacent tunnels. This competing phenomenon, schematically illustrated in Figure 3, apparently leads to a decrease in the surface area, while retaining the pore volume unchanged. The coherent add-up of these two phenomena, i.e. tunnel formation and pore wall removal, results in the observed eccentric change in the textural properties of the aluminosilicate. Considering the decrease in the surface area of the aluminosilicate and the increase in the pore volume at such excessive activating agent amount, it can be inferred that the pore wall removal is the prevailing mechanism although the tunneling effect still proceeds in the activation process. The drastic increase in the pore size of the aluminosilicate at excessive amounts of the activating agent also



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validates the pore wall removal phenomenon. In the case of ion exchange and chemisorption, pore wall removal is not considered as a desired phenomenon, as pore walls accommodate a large fraction of active site which are eliminated upon the removal of the walls. (Figure 3) The nitrogen adsorption-desorption isotherms and pore size distribution curves for aluminosilicates with various impregnation ratios has been presented in Figure 4. The isotherm curve for ?@A1 depicts the poorly-developed mesotunnels, verified by the very small hysteresis loop. Increasing the impregnation ratio to ≥ 2 / clearly increases both the total adsorbed volume of nitrogen into the pores and the hysteresis area, indicating higher pore volume and enhanced mesotunnel formation. Moreover, the pore size distribution of the ?@A4 exhibits a drastic pore widening effect. These isotherm curves are in good agreement with the aforementioned mechanism of mesotunnel formation and pore widening effects. (Figure 4)

Reaction time The effect of the reaction time on the activation intensity of the aluminosilicate has been presented in Table 1. It has been demonstrated that the reaction time up to 5 h does not have a significant effect on the textural properties of the aluminosilicate. Surface area, pore volume and average pore size are approximately identical for the aluminosilicate samples functionalized at reaction times of 1-5 h. Nevertheless, increasing the reaction time to 7 h leads to a dramatic structural collapse of the aluminosilicate, where the surface area and pore volume drastically decrease and pore size increases. This trend of the changes in the porous structure of the aluminosilicate at prolonged reaction times is very similar to that of the excessively high reaction


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temperatures. This suggests that the functionalization process takes place in a rapid rate. Therefore, a considerable amount of energy can be saved by minimizing the reaction time to achieve a high ion exchange efficiency. Nitrogen adsorption-desorption curves and pore size distribution diagrams, depicted in Figure 5, also exhibit a drastic decrease in the ultimate pore volume and hysteresis area and a slight inclination towards larger pore sizes when the reaction time exceeds 5 h. (Figure 5)

Optimum reaction conditions Overall, the optimum porous structure and thus ion exchange efficiency is achieved at moderate reaction conditions (reaction temperature: 300˚:, reaction time: 1 ℎ and impregnation ratio: 2 /). Higher reaction times and temperatures do not necessarily lead to any enhancement in the textural properties of the aluminosilicate and surface functional active sites, but may adversely affect the aluminosilicate structural properties and its ion exchange performance. Therefore, the surface of the material functionalized at optimum conditions will be thoroughly characterized to further investigate the developed functional moieties and to elucidate the reaction mechanism. Hereafter, this material will be denoted as A-NMF.

Surface characterization Figure S1 shows the SEM images of NMF and A-NMF. NMF is composed of randomly-oriented glass fibers (mainly calcium aluminosilicate) with carbonaceous materials covering the external surface of the fibers. The fibers exhibit no porous structure. However, after the thermo-alkaline reaction, the fibers are transformed into very porous disordered clusters.



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Figure 6 displays the FT-IR spectra of the NMF and A-NMF. The band at 470 corresponds to the bending mode of the − − linkages ( species)50. Apparently, the concentration of − − linkages does not significantly vary before and after the activation reaction. This is due to the abundance of the − − linkages both in the bulk and on the surface of the aluminosilicate. Although some − − groups on the aluminosilicate surface are broken into − linkages upon functionalization, a major fraction of the linkages forming the aluminosilicate tetrahedral framework remain intact. Hence, the intensity of the bending mode peak at 470 can be used as reference to compare the intensities of the other peaks altered during the functionalization process. This can give a realistic insight into the concentration of the developed groups. The broad absorption band in the range of 3600-3200 is attributed to the stretching vibrations of the hydrogen bonded silanol groups and adsorbed water51. Absence of any peak in the 3800-3700 region corresponding to free hydroxyl silanol groups52,53 indicates that no isolated hydroxyl moieties are present on the aluminosilicate surface, implying the high concentration of hydroxyl groups. Also, the band peaked at 1001 is ascribed to the stretching vibrations of the − linkages, where can be either bridging () or non-bridging oxygen ()54. Apparently, the intensities of the peaks at 3471 and 1001 have been significantly increased when compared with the intensity of the reference peak. This illustrates a substantial increase in the concentration of the developed non-bridging − sites upon activation. This also verifies the hypothesis of the − − cleavage into − linkages and tunnel formation, as discussed earlier. The peak at 1670 , attributed to the hydrogen-bonded hydroxyl group vibrations, intensified upon functionalization, mainly due to the interaction of the water molecules with the abundant silanol groups55. These results are in line with the contact angle measurements for the aluminosilicate samples (see Figure S2), where the NMF exhibits a


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hydrophobic behavior with a contact angle of 120˚, while the functionalized A-NMF is highly hydrophilic. This hydrophilicity is established by the development of water-friendly silanol moieties, making it a suitable candidate to be used in aqueous media. (Figure 6) X-ray photoelectron spectroscopy (XPS), as a powerful analysis tool for accurately investigating the atomic composition and environment on the surface layer, has been employed to further investigate the surface functional moieties developed on the A-NMF upon activation (see Figure 7). Substantial differences in the surface chemical composition of the original and functionalized materials were observed in the XPS survey spectra. The appearance of a peak at 292 eV, attributed to the K 2p, for A-NMF validates the creation of potential ion exchange active sites on the material surface56. The intensity of the C 1s peak at 285 eV significantly decreased due to the conversion of carbon impurities to CO2 at high temperatures during the activation process. Also, the intensity of the O 1s peak at around 530 eV has been considerably increased upon the functionalization process which is related to the transformation of the − − into two − linkages as well as the exposure of higher amounts of to the surface because of the tunneling effect. The high resolution spectra for the O 1s peak, as illustrated in the inset of Figure 7, reveals the considerable shift of the O 1s peak to a lower binding energy (from 533.1 eV to 530.8 eV) after the functionalization process. This shift can be explained by the proposed transformation mechanism of into as a result of the hydroxylation reaction57. The functionalization process results in the cleavage of the − − (where O is a bridging oxygen) into − … (where O is a non-bridging oxygen and = 8 9 :;9 ). Thus, the more electropositive 8 9 or :;9 will increase the electron density on the , resulting in the significant shift of the oxygen peak toward lower binding energy57.



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(Figure 7)

Ion exchange efficiency The aim of the functionalization process is to increase the concentration of the species and thus active ion exchange sites. The ion exchange efficiency of the functionalized aluminosilicate material (A-NMF) has been evaluated for several heavy metals and tabulated in Table 2. The heavy metal uptake capacities of the A-NMF for all the metals are much higher than those reported in other literature for other siliceous materials, including zeolites and silica. This exceptionally higher capacity is attributed to the fact that the distinction between the alkali metal cations functioning as charge compensator and network modifier is often disregarded in most literature. As addressed earlier, the alkali metal cations as charge compensators render charge neutrality for the clusters. In order to retain the electroneutrality in the aluminosilicate network, these alkali metal cations can only locally move along the network. Hence, their diffusion to the aluminosilicate surface is obstructed by the high electrostatic interaction between the clusters and the cations58, and as a result, the feasibility of ion exchange with these charge balancing cations is low. In this study, the concentration of the network modifying alkali metal cations bound to the species is significantly increased upon functionalization and some of the calcium cations, originally as charge balancing cations, migrate to the surface in the course of tetrahedral network cleavage and act as active ion exchange sites. The increase in the concentration of available ion exchange sites on the functionalized aluminosilicate material accounts for its ultra-high heavy metal uptake. (Table 2)59–85


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In order to evaluate the competitiveness of the heavy metal uptake performance of this wastederived functionalized aluminosilicate with the commercial ion exchange resins, the efficiencies of several well-known commercial resins under the same experimental conditions, i.e. heavy metal concentration, pH value, ion exchange dosage and temperature, have been examined. According to Table 2, it has been shown that the ion exchange capacity of the waste-based A-NMF is also higher than the commercial resins under similar conditions. The high ion exchange performance of A-NMF together with the facile inexpensive functionalization technique enhances the feasibility of its commercialization. The attractiveness of this innovative approach lies on the dual environmental benefit, i.e. sustainable value-added upcycling of a problematic waste initially destined to landfill and its utilization for heavy metal-laden wastewater treatment. In addition to the high efficiency of the ion exchange material produced at optimum conditions, its cost is also expected to be low due to several reasons; the initial precursor is a wastederived material and thus has low value; the yield of the material is as high as 70-80 wt%; the activation temperature is as low as 250℃; and the activation process is a simple practical one. All these factors lead to the cost-effectiveness of the resultant ion exchange material for industrialization purposes. However, it is of note that the functional moieties developed on ANMF is very sensitive to humidity and temperature and thus, should be kept at controlled environments to retain its high efficiency.

Conclusion A very efficient ion exchange material has been sustainably developed by upcycling a wastederived aluminosilicate material (NMF) as precursor. The hydrophobic nonporous NMF has been



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transformed into functionalized hydrophilic ion exchange material via the cleavage of passive

− − bonds into reactive − − linkages (where and denote bridging and non-bridging oxygens, respectively). This bond cleavage takes place using a thermo-alkaline reaction, where tunnels covered with functional moieties are developed on the aluminosilicate surface by tetrahedral structure rupture. In this study, the effects of several reaction variables, such as the amount of the activating agent, reaction time and reaction temperature, on the creation of these tunnels and surface functional groups have been examined. It has been found that when the reaction time and temperature exceeds 5 h and 300˚C, respectively, the aluminosilicate structure partially collapses, leading to a drastic decrease in the surface area. Also, when the amount of the activating agent employed in the reaction is not sufficient (impregnation ratio < 2 /), the tunneling effect is not properly achieved. The heavy metal uptake capacity of the functionalized aluminosilicate under the optimized reaction conditions has been shown to be exceptionally superior to those of either synthesized or natural zeolites and also higher than the commercial ion exchange resins. This superiority is related to the high concentration of surface ion exchange sites achieved by the functionalization process. The competitive ion exchange capacity of this material combined with the cost-effective functionalization technique employed to upcycle this waste siliceous material into highperformance ion exchange material makes it a suitable material for heavy metal-laden wastewater treatment.

ASSOCIATED CONTENT Supporting Information. SEM graphs, contact angle measurements, and hysteresis data. The information is available free of charge via the ACS Publication website at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Authors. *Phone: +852-23588412, Fax: +852-23580054, E-mail: [email protected]. Notes. The authors declare no competing financial interests.



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Table 1. Textural properties of the materials functionalized at different reaction conditions. Sample ID

FGHI

FJNO

PQ

ST

(JK . LM)

(JK . LM)

(OJR . LM )

(UJ)

Effect of reaction temperature (impregnation ratio= K V/V and reaction time = R W) FD200

317

0

0.615

7.8

FD250

358

4

0.832

9.3

FD300

383

18

0.851

8.9

FD400

282

0

0.586

8.3

Effect of impregnation ratio (reaction temperature= RXX℃ and reaction time = R W) IMP1

143

0

0.302

8.4

IMP2

383

18

0.851

8.9

IMP4

331

0

1.029

12.4

Effect of reaction time (reaction temperature= RXX℃ and impregnation ratio= K V/V) d1h

368

4

0.848

9.2

d3h

383

18

0.851

8.9

d5h

378

10

0.853

9.0

d7h

262

7

0.678

10.3



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Page 30 of 46

Table 2. Comparison of the ion exchange capacity of the A-NMF with those in the literature and the commercial ion exchange resins.

Maximum ion exchange (JJYZ. LM) Material

Ref.

Cu2+

Pb2+

Zn2+

Co2+

Ni2+

Cd2+

NMF

< 0.1

< 0.1

< 0.1

< 0.1

< 0.1

< 0.1

This work

A-NMF

2.82

3.21

2.00

3.12

3.11

2.10

This work

1.67

1.11

59

Siliceous Material Heavy Metal Uptake Capacities Natural Zeolite

1.18

Natural Clinoptilolite

0.84

0.48

0.19

EMR-derived Zeolite

0.13

60

2.19

61 62

1.50

Na-Y Zeolite

1.85

Synthetic Zeolite A Chitosan-zeolite Composites

0.81

Zeolite/Chitosan Monoliths

1.34

Acid-Immobilized polymer/bentonite

0.74

Activated carbon-Zeolite Composite

1.72

Modified SBA-15

0.92

63 64 65

0.29 0.68

66

0.58

2.65

1.20 0.4


0.32

0.36

1.44

67 68

Page 31 of 46

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


1.04

Electrospun PVA/Zeolite nanofiber 2.08

Cancrinite type Zeolite

2.13

1.15

1.24

0.77

70

1.53

71

2.63

Modified Coal Fly Ash Hydrothermal treated Coal Fly Ash

0.95

0.69

Functionalized Silica Nanotubes

0.49

Amino-functionalized Silica

0.94

Ti-Si-P Hybrid Material

0.45

69

72

0.62 1.77 2.95

0.89

73

1.69

74

0.45

75 76

Fly Ash Based Geopolymer

0.91

Hydrothermally Modified Fly Ash

0.89

Blast-furnace Slag

2.10

1.58

0.95

Synthetic Zeolite

0.80

0.50

0.34

Zeolite 4A

0.63

0.62

Modified Clay

0.40

0.23

0.78

0.29

77 78

0.45

79

0.20

80

0.58

81

Commercial Ion Exchanger Heavy Metal Uptake Capacities Dowex 50W

0.35

0.23

0.30

0.12

0.25

82

Amberlite IR-120

0.34

0.41

1.3

0.82

0.90

83

Amberjet 1500H

0.39

84



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Page 32 of 46

0.32

0.04

85

1.97

1.78

1.21

This work

1.70

2.10

1.88

1.69

This work

0.41

0.71

0.67

0.30

This work

Lewatit CNP 80

0.16

0.35

0.31

Lewatit TP207

1.98

1.78

1.63

Suqing D401

2.00

1.88

MCM-41

0.71

0.68


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Figure Legends Figure 1 ¦ Schematic representation of I − [ − I bond cleavage and tunneling effect via the thermo-alkaline reaction (Silicon, black; Aluminum, purple; oxygen, red; calcium, green; potassium, blue; hydrogen yellow). This figure illustrates the hypothesized functionalization reaction in which the passive robust − − linkages are cleaved in the course of thermo-alkaline reaction, resulting in the formation of ion exchange active sites. Some of the calcium ions, originally as charge balancing cations for clusters, migrate to the surface acting as network modifying cations.

Figure 2 ¦ (a) Nitrogen adsorption-desorption isotherm, and (b) pore size distribution for the samples prepared at different activation temperature. The effect of the thermo-alkaline reaction temperature on the textural properties of the materials depicts that all the prepared materials, irrespective of the reaction temperature, show a type IV isotherm curve with a distinct hysteresis. However, At moderate reaction temperatures (FD250 and FD300), the pore volume and surface area have the highest values.

Figure 3 ¦ Schematic representation of pore wall removal caused by aggressive reaction conditions (Silicon, black; Aluminum, purple; oxygen, red). The

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hypothesized removal of pore walls leads to a decrease in the surface area, while keeping the pore volume unchanged.

Figure 4 ¦ (a) Nitrogen adsorption desorption isotherm, and (b) pore size distribution for the samples prepared at different impregnation ratios. When the amount of the activating agent is not sufficient (impregnation ratio of 1 /), the pore formation is not well-developed. Also, very high amount of the activating agent in the reaction medium results in the removal of pore walls, thus drastically increasing the average pore size.

Figure 5 ¦ (a) Nitrogen adsorption desorption isotherm, and (b) pore size distribution for the samples prepared at different reaction times. This verifies that the thermo-alkaline reaction is a very fast process and very prolonged reaction time has an adverse effect on the textural properties of the material.

Figure 6 ¦ FTIR spectra for the precursor and functionalized aluminosilicate. The functionalization reaction results in a significant increase in the concentration of the NBO moieties, which are counter-balanced with alkali metal cations. These cations can play the role of ion exchange sites on the aluminosilicate.

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Figure 7 ¦ XPS spectra for the precursor and functionalized aluminosilicate. The appearance of the K 2p peak on the functionalized aluminosilicate material and the increase in the intensities of the Ca 2p and O 1s peaks are in line with the hypothesized reaction mechanism, discussed earlier. A considerable shift of the O 1s peak to a lower binding energy (as depicted in the inset Figure) upon the functionalization process suggests the increase in the electron density of the oxygen as a result of the transformation mechanism of into linkages.

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Thermo-alkaline reaction

Figure 1 ¦ Schematic representation of I − [ − I bond cleavage and tunneling effect via the thermo-alkaline reaction (Silicon, black; Aluminum, purple; oxygen, red; calcium, green; potassium, blue; hydrogen yellow). This figure illustrates the hypothesized functionalization reaction in which the passive robust − − linkages are cleaved in the course of thermo-alkaline reaction, resulting in the formation of ion exchange active sites. Some of the calcium ions, originally as charge balancing cations for clusters, migrate to the surface acting as network modifying cations.

36


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Figure 2 ¦ (a) Nitrogen adsorption-desorption isotherm, and (b) pore size distribution for the samples prepared at different activation temperature. The effect of the thermo-alkaline reaction temperature on the textural properties of the materials depicts that all the prepared materials, irrespective of the reaction temperature, show a type IV isotherm curve with a distinct hysteresis. However, At moderate reaction temperatures (FD250 and FD300), the pore volume and surface area have the highest values.

37



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Pore wall removal

Figure 3 ¦ Schematic representation of pore wall removal caused by aggressive reaction conditions (Silicon, black; Aluminum, purple; oxygen, red). The hypothesized removal of pore walls leads to a decrease in the surface area, while keeping the pore volume unchanged.

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Figure 4 ¦ (a) Nitrogen adsorption desorption isotherm, and (b) pore size distribution for the samples prepared at different impregnation ratios. When the amount of the activating agent is not sufficient (impregnation ratio of 1 /), the pore formation is not well-developed. Also, very high amount of the activating agent in the reaction medium results in the removal of pore walls, thus drastically increasing the average pore size.

39



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Figure 5 ¦ (a) Nitrogen adsorption desorption isotherm, and (b) pore size distribution for the samples prepared at different reaction times. This verifies that the thermo-alkaline reaction is a very fast process and very prolonged reaction time has an adverse effect on the textural properties of the material.

40


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Figure 6 ¦ FTIR spectra for the precursor and functionalized aluminosilicate. The functionalization reaction results in a significant increase in the concentration of the NBO moieties, which are counter-balanced with alkali metal cations. These cations can play the role of ion exchange sites on the aluminosilicate.

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Figure 7 ¦ XPS spectra for the precursor and functionalized aluminosilicate. The appearance of the K 2p peak on the functionalized aluminosilicate material and the increase in the intensities of the Ca 2p and O 1s peaks are in line with the hypothesized reaction mechanism, discussed earlier. A considerable shift of the O 1s peak to a lower binding energy (as depicted in the inset Figure) upon the functionalization process suggests the increase in the electron density of the oxygen as a result of the transformation mechanism of into linkages.

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Valorization of an electronic waste-derived aluminosilicate – Surface functionalization and porous structure tuning sc-2015-01523e

Chao Ninga, Pejman Hadia, Carol Sze Ki Linb and Gordon McKaya,c* a




Kong SAR c

Division of Sustainable Development, College of Science, Engineering and Technology,

Hamad Bin Khalifa University, Qatar Foundation, Doha, Qatar *




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Page 44 of 46

Network modifier ions as ion exchange sites

Developed mesotunnels

Thermo-alkali reaction

Inaccessible charge compensator ions

Si

Al

O

Ca

K


H

Page 45 of 46

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Valorization of an electronic waste-derived aluminosilicate – Surface functionalization and porous structure tuning sc-2015-01523e

Chao Ninga, Pejman Hadia, Carol Sze Ki Linb and Gordon McKaya,c* a




Kong SAR c

Division of Sustainable Development, College of Science, Engineering and Technology,

Hamad Bin Khalifa University, Qatar Foundation, Doha, Qatar *




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 46 of 46

Network modifier ions as ion exchange sites

Developed mesotunnels

Thermo-alkali reaction

Inaccessible charge compensator ions

Si

Al

O

Ca

K


H

Valorization of an Electronic Waste-Derived ... - ACS Publications

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