Functionalization of recycled polymer and 3D printing into porous structures for selective recovery of copper from copper tailings

Selective copper recovery from copper tailings reduces environmental pollution caused by mining activities and provides a valuable source of copper. Furthermore, polymer waste accumulation and handling small functional materials like resins remains challenging. In this study, a 3D-printed adsorbent for selective copper recovery from copper tailings was designed by functionalizing recycled polymer with a chelating resin and 3D-printing using selective laser sintering technique. The 3D-printed adsorbent was characterized, and its adsorption performance examined under varying conditions. The adsorption kinetics and adsorption isotherm fitted well to the pseudo-second-order kinetics and Langmuir isotherm models, suggesting chemical and monolayer adsorption processes. FTIR indicated coordination as the possible adsorption mechanism. Thermodynamics revealed an endothermic process. The 3D-printed adsorbent demonstrated an excellent Cu(II) adsorption, high selectivity towards Cu(II), and reusability. This work offers a promising 3D-printed adsorbent for selective Cu(II) recovery, while also addressing polymer waste and particle handling issues.


Introduction
Copper is a highly valued mineral all over the world due to its unique and advantageous properties (Liu et al., 2022;Wołowicz and Hubicki, 2020).It finds extensive applications across various industrial sectors, including battery manufacturing, pipe systems, agriculture and farming, electrical systems, and healthcare (Ryu et al., 2019;Wołowicz and Hubicki, 2020).Copper at trace concentrations is essential for the growth and development of both organisms and plants.However, deficiency or excess of it can have negative effects.Copper deficiency can lead to arthritis and stunted growth in humans, as well as slow growth and weak stems in plants (Wołowicz and Hubicki, 2020).On the other hand, excess intake can result in diabetes, liver, and kidney disease, and even death in humans (Gao et al., 2022;Wołowicz and Hubicki, 2020;Yan et al., 2011), while plants may develop thick roots and fewer branches, resulting in slow growth (Wołowicz and Hubicki, 2020).Due to the numerous applications of copper, several industrial wastewaters containing the metal are generated (Ryu et al., 2019;Wołowicz and Hubicki, 2020).The World health organization (WHO) has regulated copper concentration in natural waters and portable water to be below 3.0 and 0.05 mg/L, respectively (Gao et al., 2022) and a valuable amount of copper is possibly lost due to the high discharge of these wastewaters (Wołowicz and Hubicki, 2020).In addition to industrial wastewater, copper mining produces a substantial amount of waste in the form of tailings, which are the fine-grained residues generated after extracting copper from the ore (Lü et al., 2018;Wang et al., 2020;Zhou and Zhang, 2022).Copper tailings (CT) make up a significant percentage of mining waste, accounting for 46 % of the eight billion tons of tailings generated yearly (Adrianto et al., 2023).CTs are typically stored in tailings reservoirs without undergoing further processing or utilization.These tailings contain residual copper and other metals that were lost during extraction (Liu et al., 2022;Wang et al., 2020) and may seep into the nearby environment, leading to pollution and significant harm to the ecosystem (Liu et al., 2022;Zhou and Zhang, 2022).Therefore, it is important to treat wastewater and tailings containing copper to recover copper before disposal because copper is hazardous even at lower concentrations but simultaneously economically valuable due to its numerous industrial applications (Ryu et al., 2019).
Various techniques such as coagulation, ion exchange, membrane filtration, chemical precipitation, electrolysis, adsorption, etc. have been utilized for heavy metals removal (Edebali and Pehlivan, 2016;Gao et al., 2022;Ma et al., 2014;Siu et al., 2016;Soylak and Tuzen, 2006;Tapaswi et al., 2014).Adsorption and ion exchange are widely used due to their efficiency, ease of operation, and low cost (Ma et al., 2014).Ion exchange technique has been employed for selective recovery of copper from solutions (Botelho Junior et al., 2019;Gando-Ferreira et al., 2011).The chelating resins are extensively used as ionexchange materials (Gando-Ferreira et al., 2011) and numerous investigations have revealed the potential of these resins in selectively removing and recovering copper (Bleotu et al., 2015;Gando-Ferreira et al., 2011;Kołodyńska et al., 2014;Neto et al., 2016;Shen et al., 2016;Wołowicz and Hubicki, 2020).Chelating resins anchored with bispicolylamine groups emerge as highly appropriate for situations requiring selective copper removal, and Lewatit MonoPlus TP 220 and DowexM4195 are well-known examples.These resins contain three nitrogen atoms as donors with two of these situated within the aromatic pyridyl groups.The deprotonation of these nitrogen donor atoms persists even at very low pH as a result of the electron with drawing influence of the aromatic group which causes them to have low pKa values.This property possessed by this kind of resins facilitates easy formation of bidentate complexes with metal ions at acidic pH, even at pH below 2 (Botelho Junior et al., 2019;Kołodyńska et al., 2014;Neto et al., 2016).
Wastewaters generated from industries such as copper mines, copper refineries, electrolytic metal-acid refinery, etc. are unique because they are usually acidic (Kołodyńska et al., 2014;Onuaguluchi and Eren, 2012;Wołowicz and Hubicki, 2020).Copper is also a unique mineral because it exhibits strong attraction towards chelating polymers that solely possess nitrogen donor atoms, particularly in very acidic environments.DowexM4195 and Lewatit MonoPlus TP 220 resins have been successfully utilized to remove copper from acidic media (Kołodyńska et al., 2014).Neto et al. studied the selectivity of DowexM4195 towards copper by using a synthetic and real electronic waste solution comprising Cu(II), Fe(III), Zn(II), Pb(II), Ag(I), Ni(II), Al(III), and Sn(II).
Copper was selectively recovered with 99 % purity at pH 1.3 (Neto et al., 2016).Kołodyńska et al. compared various factors including pH, Cu(II) concentration, contact time, ion exchanger dose, and the presence of sulfate and chloride ions, to determine their effect on the sorption of Cu (II) onto the named resins, and observed that sorption was more effective when using Lewatit MonoPlus TP 220, particularly when lower Cu (II) concentrations were used (Kołodyńska et al., 2014).Wołowicz and Hubicki investigated the selectivity of Lewatit MonoPlus TP 220 to base metals including Zn(II), Co(II), Cu(II), Ni(II), and noble metals including Au(III), Pd(II) and Pt(IV).They found that the resin selectively recovered more noble metals compared to base metals.Though, when only the base metals were taken into account, the resin exhibited high selectivity for copper, with the order of selectivity also reported (Wołowicz and Hubicki, 2012).
Handling of ion exchange materials is challenging because they are small particles that can be readily lost during use.Thus, reusing them becomes difficult or almost impossible.Utilizing three-dimensional (3D) printing technology can help overcome these challenges (Lahtinen et al., 2017).3D printing has facilitated the creation of complex structures and geometries (Lawson et al., 2021;Mohd Yusoff et al., 2022), in various shapes, sizes and porosity (Lahtinen et al., 2018(Lahtinen et al., , 2017;;Lawson et al., 2021;Tijing et al., 2020).3D printing has also allowed the creation of objects and devices that possess physical and/or chemical functionalities (Lahtinen et al., 2018(Lahtinen et al., , 2017;;Su, 2021).With this technique, various functional materials can be incorporated into printed objects for use in various applications (Halevi et al., 2018;Su, 2021).Therefore, it presents a new approach towards developing advanced adsorption and ion exchange materials with controllable properties for various applications.The potential of this technology in developing advanced adsorption and ion exchange materials has been reported by several researchers (Fijoł et al., 2021;Kennedy et al., 2021;Kulomäki et al., 2019;Lagalante et al., 2020;Lahtinen et al., 2018Lahtinen et al., , 2017;;Li et al., 2019;Liakos et al., 2020;Sun et al., 2019;Xia et al., 2020).These materials were created by using printing techniques including stereolithography (SLA), selective laser sintering (SLS), fused deposition modeling (FDM), material jetting (MJ), etc. and printing polymers in form of resins, powders and filaments (Mohd Yusoff et al., 2022;Su, 2021).Moreover, production of polymer waste is rising globally and is generally harmful to the environment, generating pollution that hurts both humans and aquatic creatures.Some of the commonly used techniques for disposing of polymers have unfavorable side effects.Polymer recycling is one way to turn unwanted polymers back into valuable goods, with 3D printing emerging as a promising approach (Shanmugam et al., 2020;Shiferaw and Hailu, 2022;Zhong and Pearce, 2018).The waste polymers can serve as printing materials or as support for functional materials.This strategy minimizes the environmental issues caused by the buildup of polymer waste as well as increase the lifespan of the polymers (Shanmugam et al., 2020).
This work investigated the feasibility and effectiveness of using 3Dprinted porous structures for selective recovery of copper.This involved functionalizing recycled polymer (polypropylene), optimizing the 3D printing process to create porous structures, and evaluating the performance of the functionalized 3D-printed structures in synthetic solutions.Here, selective laser sintering (SLS) was chosen as the 3D printing technique.This powder-based 3D printing method involves sintering partially melted polymer powders, resulting in solid and porous objects with accessible pores that favor fluid flow (Lahtinen et al., 2018(Lahtinen et al., , 2017)).Porosity is desirable in sorption and SLS has an advantage in such application.The 3D-printed porous structures were tested under varying operating conditions and their ability to selectively recover copper was evaluated.The efficiency and selectivity of the materials were compared to traditional ion exchangers and other materials currently used for copper recovery.The development of functionalized polymer materials and their 3D printing into porous structures holds the potential to transform the areas of 3D printing, mining, and wastewater treatment, and address significant environmental challenges.

Materials
SLS grade polypropylene (PP) was obtained from AM Polymers (Rolaserit PP01, AM Polymers GmbH, Willich, Germany).Lewatit MDS TP 220 (noted as LTP) was received from Lanxess (Lanxess Deutschland GmbH, Leverkusen, Germany).The characteristics of Lewatit MDS TP 220 are given in Table S1.and calcium (CaSO 4 ⋅2H 2 O), were supplied from Sigma-Aldrich Finland Oy, Helsinki, Finland.Sodium hydroxide (NaOH) and hydrochloric acid (HCl) were purchased from Merck KGaA, Darmstadt, Germany.Aluminum (Al(NO 3 ) 3 ⋅9H 2 O) was obtained from VWR International Oy, Espoo, Finland.Deionized water (DI) was used throughout the experiments.The preparation of standard solutions of metal ions was performed by dissolving a weighed quantity of the corresponding metal salts in DI water.The working solutions were then obtained from the prepared standards.The pH levels during the experiment were adjusted through the gradual addition of drops of NaOH or HCl solutions.

Functionalization of PP powder
Lewatit MDS TP 220 (LTP) was subjected to overnight drying in an oven operating at 50 • C. Following this, the resin was milled utilizing a rotor mill (Pulverisette 14, Fritsch GmbH, Idar-Oberstein, Germany).Polypropylene (PP) was aged 3 times in the SLS printer to mimic recycled powder.The printing material for the 3D-printer was prepared by mechanically blending PP powder with 10 wt% LTP powder.The resulting powder mixture was sieved using fine sieves and loaded into the powder tank for printing.Herewith, LTP chelating resin with bispicolylamine functional group was selected as the functional material to be incorporated into PP owing to its higher copper selectivity, faster kinetics, and approx.25 % greater binding capacity compared to Lewatit MonoPlus TP 220.

Three-dimensional (3D) model design and 3D printing
Porous cylindrical structures were designed using SOLIDWORKS and sliced using Ultimaker Cura 4.10 software.3D printing was carried out utilizing ShareBot SnowWhite2 equipped with a 14 W CO 2 laser and operating via the SLS printing technique.Laser power and rate were set at 25 % (3.5 W) and 40,000 (1600 mm/s) respectively.Plate temperature, warming layers, layer thickness and waiting time after each layer were set at 115-116 • C, 25 layers, 0.1 mm, and 10 s, respectively.After printing, the structures were carefully cleaned to remove any unsintered powder before using in adsorption experiments.

Characterization
The functional groups in the samples were analyzed by Fourier transform infrared spectroscopy (FTIR) employing Perkin Elmer Frontier spectrometer with a universal ATR module (PerkinElmer Inc., Massachusetts, USA).The spectra obtained within 4000 to 400 cm − 1 wavenumber range using 4 cm − 1 resolution collected 4 scans for each spectrum.The scanning electron microscope with energy dispersive spectroscopy (SEM-EDS, Hitachi SU3500, Tokyo, Japan) operating at a voltage of 10-15 kV, a vacuum pressure of 100 Pa, and a variable magnification was used to examine the surface morphology and elemental composition of the samples.The analysis of particle size distribution of 3D-LTP was conducted by examining the SEM image using the ImageJ software.Subsequently, the Gaussian distribution function was employed to fit particle size histograms.SurPASS electrokinetic analyzer (Anton Paar GmbH, Graz, Austria) operating with an adjustable gap cell technique was used to determine with the zeta potential and subsequently the isoelectric point (IEP).

Copper adsorption tests
The sorption behavior of Cu(II) onto the 3D-printed sorbent containing 10 wt% LTP was studied through batch adsorption experiments.The experiment investigated the effect of initial pH, initial copper concentration, contact time, and temperature within the specified ranges of 1-6, 0-20 h, 0-950 mg/L, and 298-328 K, respectively, on the adsorption performance.3D-printed sorbents were agitated in 10 mL Cu(II) solution at 300 rpm for 20 h.After equilibrium, a 0.45 μm pore size polypropylene syringe filter was used to filter the sorbents from the solution.Additionally, control experiment with 3D-printed PP (3D-PP) was also conducted.The Cu(II) concentration in the solution before and after the adsorption experiment were analyzed by using inductively coupled plasma mass spectrometer (ICP-MS, Agilent, 7900).The adsorption efficiency and capacity of the 3D-printed sorbent was calculated using equations (1) and (2) (Liu et al., 2022): where R and q e represent the adsorption efficiency (%), and the adsorption capacity (mg/g), respectively.C i and C e represent the initial and equilibrium metal ion concentrations (mg/L) in the solution, respectively.m denotes the mass (mg) of the adsorbent and V, the volume (mL) of the solution.

Desorption and reusability
To examine the potential reusability of the 3D-printed sorbent, five adsorption-desorption cycles were executed.The adsorption process was carried out as described earlier.In each desorption cycle, the 3Dprinted sorbents were contacted with 10 mL eluent, followed by agitation for 20 h at 300 rpm.Then they were filtered from the eluent solution utilizing a 0.45 μm pore size polypropylene syringe filter, rinsed multiple times with DI water, after which the sorbents were subjected to drying at 50 • C for 24 h before starting the next adsorption-desorption cycle.The eluent used for desorption was 2 M NH 4 OH, which is supported by earlier research (Kołodyńska et al., 2014;Wołowicz and Hubicki, 2020).The efficiency of desorption in each cycle was calculated using equation (3) (Zhang et al., 2019): where C d is the metal ion concentration (mg/L) in the eluent and V d denotes the volume (mL) of the eluent solution.

Selectivity
The selectivity of the 3D-printed sorbent towards Cu(II) was investigated by performing adsorption experiments in the presence of other competing metal ions, including Fe(III), Al(III), Mn(II), Zn(II), Ca(II), Mg (II), and Cr(III), which are abundant in copper tailings (Guo et al., 2013).The experiment was performed at pH 1.5.The 3D-printed sorbent was contacted with 10 mL of multi-metal ions solution, followed by agitation at 300 rpm for 20 h, under ambient temperature conditions.Additionally, control experiment with 3D-PP was also conducted.

Functionalization and 3D printing
In this study, a blend consisting of 10 wt% LTP powder and PP powder was prepared through mixing, and this powder mixture was 3Dprinted into porous structures for selective recovery of copper.The 3D printing process was conducted via SLS technique.The visual representation of the fabrication process is shown in Fig. 1, while Fig. S1a presents a photograph of the porous 3D-printed sorbents (noted as 3D-LTP).This preparation method presents a straightforward and feasible way to functionalize, and 3D print polymers, producing unique sorbents with specific properties for potential use in adsorption, ion exchange and other applications.
The SEM images and corresponding EDS results of the 3D-LTP before Cu (II) adsorption and after are presented in Fig. 3a-c.The images reveal a macroporous structure made up of PP beads that are partially sintered  together.The mean particle diameter was calculated to be ± 71.49μ m, as illustrated in Fig. S2.EDS result confirmed the successful functionalization of PP with LTP by revealing the presence of nitrogen in the 3D-LTP.The EDS results also confirmed successful Cu(II) adsorption to the surface of the 3D-LTP, further confirming that PP was functionalized with the resin and adsorption was successful.
Fig. 3d depicts the zeta potential of the 3D-LTP under varying pH conditions.A three-layer film was 3D-printed with power mixture and used as test sample.The zeta potential of the sorbent decreased gradually from pH 3 to 8, and the IEP was observed at pH = 4.54.pH of a solution influences metal ion adsorption by changing their speciation and the sorbent's active site ionization.Adsorption mechanism depends on protonation-deprotonation behavior; therefore, the sorbent's surface charge can significantly influence the efficiency of adsorption processes.When the solution pH surpasses the IEP, the surface of the sorbent becomes negatively charged, facilitating interactions with positively charged copper species through electrostatic interactions.Conversely, at pH levels below the IEP, the surface of the sorbent becomes positively charged enabling interactions with anionic species thereby, repelling copper cations (Wołowicz and Hubicki, 2020).However, for adsorbents bearing nitrogen atoms, chelation or coordination becomes dominant under such pH conditions (Kołodyńska et al., 2014;Neto et al., 2016;Wołowicz and Hubicki, 2020).

Effect of initial pH
The pH level of a solution is a crucial aspect affecting the adsorption behavior of metal ions due to the competitive interactions between metal ions and hydrogen ions adsorption sites, particularly at certain pH levels (Elfeghe et al., 2022).pH edge experiments were executed in the pH range of 1-6, Cu(II) concentration of 50 mg/L, and at room temperature, to study its effect on Cu(II) adsorption onto the 3D-LTP.The results, as presented in Fig. 4, indicate that copper adsorption increased as the pH level increased from 1 to 5. The optimum pH was attained at pH ~ 5, corresponding to 90.60 % copper removal and q e of 1.24 mg/g.The increase in copper adsorption can be explained by the availability of all three nitrogen atoms in the functional material for Cu(II) binding at higher pH values.Above pH 4.21, all the nitrogen atoms are deprotonated, and below this pH, one or two nitrogen atoms are deprotonated (Kołodyńska et al., 2014;Wołowicz and Hubicki, 2012).However, copper adsorption decreased at pH > 5, which could be a result of Cu (OH) 2 formation leading to a reduced Cu(II) solubility within the aqueous solution (Gao et al., 2022).Fig. S1b shows a photograph of 3D-  LTP after Cu(II) adsorption at different pH.The cream-colored samples (Fig. S1a) turned blue after adsorption indicating that copper was successfully adsorbed onto the sorbent.The ensuing adsorption experiments were executed at the optimal pH, except for selectivity studies, which were performed at pH 1.5.3D-PP displayed insignificant adsorption towards the copper ions in solution, suggesting that the copper adsorption observed in 3D-LTP were solely due to the presence of LTP in the 3D-printed structure (Fig. S3).

Adsorption kinetics
An investigation on the effect of contact time on Cu(II) adsorption onto the 3D-LTP was evaluated, with the findings displayed in Fig. 5a.The process of Cu(II) adsorption happened rapidly during the initial 60 min, after which a gradual rise in adsorption rate was seen until the maximum adsorption capacity.The initial rapid adsorption can be ascribed to the availability of binding sites for copper ions to interact with.As the available sites became almost filled, the adsorption rate slowed until it attained a maximum when the 3D-LTP was saturated.In order to comprehend the underlying mechanism influencing Cu(II) adsorption, the pseudo-first-order and pseudo-second-order kinetic models were employed, and their respective equations are given below (Wei et al., 2015): (4) (5) here q t represents the adsorption capacity (mg/g), and t represents the time (min).k 1 and k 2 (in units of min − 1 and g mg − 1 min − 1 , respectively) represent the rate constants for the pseudo-first-order and pseudosecond-order models.The resultant kinetic parameters derived from the models are tabulated in Table 1, while the fitted curve is presented in Fig. 5a.The pseudo-second-order model showed better fitting to the experimental data, as evidenced by a higher correlation coefficient (R 2 ) compared to the pseudo-first-order model.This suggests that the ratecontrolling step in the adsorption process likely involves chemical adsorption.Kołodyńska et al., Neto et al., and Wołowicz and Hubicki, also reported similar findings regarding Cu(II) adsorption onto chelating resin having bis-picolylamine functional groups (Kołodyńska et al., 2014;Neto et al., 2016;Wołowicz and Hubicki, 2020).

Adsorption isotherms
The initial Cu(II) concentration was varied between 0 and 950 mg/L, to investigated the adsorption isotherm of Cu(II) onto the 3D-LTP.The adsorption mechanism was studied by applying two isotherm models: the Langmuir and Freundlich models.The models expressions are provided below (Wei et al., 2015): where the Langmuir constant k L (L/mg) signifies the adsorption energy, while q m (mg/g) represents the maximum adsorption capacity of 3D-LTP.The Freundlich constants, n and k F (L/g), respectively describe the adsorbent capacity and adsorption intensity.
As depicted in Fig. 5b, the adsorption capacity increased quickly with increase in initial Cu(II) concentration, but then leveled off at higher concentrations, which could be because there were more binding sites available at lower Cu(II) concentrations than Cu(II) in the solution.However, at higher concentrations, there were more Cu(II) in solution than there were binding sites available, resulting in competition for empty sites.The parameters obtained from the isotherm models are tabulated in Table 2.The Langmuir isotherm model showed better fitting to the experimental data compared to the Freundlich isotherm model, as evidenced by a higher R 2 value.It is particularly interesting to note the sharpness of the initial slope, signifying a high adsorption affinity which is especially important for retrieving metal ions from low grade tailings (Bediako et al., 2016;Wei et al., 2015).The Langmuir model generally describes monolayer adsorption process that occurs at uniform sites on the adsorbent's surface.Therefore, the mechanism of adsorption of Cu(II) onto the 3D-LTP is described as homogenous monolayer adsorption.The Langmuir equilibrium parameter and separation factor constant, R L was employed to explain the favorability of the adsorption process.Equation for calculating the R L is defined in equation (8) (Gao et al., 2022;Liu et al., 2022):  where k L (L/mg) represents the Langmuir constant and, C i (mg/L) the initial concentration of the Cu(II) in the solution.Adsorption process is described as favorable or unfavorable when R L is < 1 or > 1, respectively and described as irreversible or linear when R L is equal to 0 or 1, respectively.Analysis of the R L values for the 3D printed sorbent ranged from 0.01-0.52,signifying a favorable adsorption process.

Adsorption thermodynamic
An investigation of the effect of temperature on Cu(II) adsorption onto the 3D-printed sorbent was conducted by varying the temperature range between 298 and 328 K, and thermodynamic parameters such as enthalpy (ΔH 0 ), standard entropy (ΔS 0 ), and Gibbs free energy (ΔG 0 ) were calculated with these equations (Gao et al., 2022): where T stands for the temperature measured in Kelvin (K), while K and R denote the thermodynamic equilibrium constant and ideal gas constant (8.314J/mol K), respectively.By plotting a graph of ln K against 1/ T, the values corresponding to ΔH 0 and ΔS 0 were determined from the intercept and slope, as shown in Fig. S4, while Table 3 gives the obtained thermodynamic parameters, which describe the nature of the adsorption process.The positive ΔH 0 value indicates an endothermic adsorption process.Simultaneously, the positive ΔS 0 value suggests an increase in randomness occurring at the solid-liquid interface as Cu(II) becomes affixed on the active sites of the 3D-printed sorbent.Moreover, the positive values across all ΔG 0 indicate non-spontaneous adsorption process.Furthermore, the decrease in ΔG 0 with increasing temperature suggests a favorable adsorption at higher temperatures (Elfeghe et al., 2022;Gao et al., 2022;Shen et al., 2016).However, the small changes with temperature indicate its effect on the adsorption process is minimal.

Analysis of adsorption mechanism
After adsorption, significant shifts to higher wavenumbers were detected in the FTIR spectra, particularly peaks at 1591, 761, and 577 cm − 1 that represent the C = N vibrations of the pyridine groups, out-ofplane bending vibrations of pyridine ring and C-N bending vibrations, shifted to 1611, 768 and 609 cm − 1 , respectively.Other absorption peaks on 3D-LTP were almost the same before and after adsorption.Suwannahong et al. reported that the pyridine group shift to a higher frequency following Cu(II) adsorption could be due to the interaction between protonated nitrogen and a divalent metal (Cu(II)) (Suwannahong et al., 2022).Wołowicz and Hubicki also reported shifts to higher wavenumbers for pyridine and aliphatic amine bands, which they described as the formation of coordination bonds with Cu(II) (Wołowicz and Hubicki, 2020).The shifts observed after Cu(II) adsorption onto the 3D-LTP are related to vibrations of the aliphatic amine and pyridine functional groups, and thus indicate that these groups were involved in binding Cu(II), likely through the formation of coordination bonds or chelation as a result of the properties of the bis-picolylamine functional groups (Wołowicz and Hubicki, 2012).

Desorption and reusability
The recovery of adsorbed copper ions and reusability of sorbents are important in sorption processes (Xiong et al., 2012).3D-LTP was evaluated for its reusability by performing five consecutive adsorption-desorption cycles, and the findings are displayed in Fig. 6.The adsorption efficiency of the sorbent reduced from 100 to 63 % over these cycles.However, the desorption efficiency ranged from 73-95 %, indicating effective recovery of the adsorbed copper from the sorbent.This result suggests that the sorbent has good reusability for copper recovery.

Selectivity studies
The effect of competing metal ions on selective Cu(II) adsorption onto the 3D-LTP was studied in a multi-metal ions solution also containing Fe(III), Al(III), Mn(II), Zn(II), Ca(II), Mg(II), and Cr(III), in low (0.70 mM) and high (3.50 mM) concentrations.The metals ions studied are those that may be present in copper tailings.As presented in Fig. 7, the sorbent showed selective adsorption to copper in comparison to the other metals.The q e of Cu(II) was 1.60 and 2.50 mg/g for 0.70 mM and 3.50 mM multi-metal ions solutions, respectively, while those of the other metal ions were almost negligible and below 0.10 mg/g.3D-PP displayed insignificant adsorption and no selectivity towards the metal ions in the solution.This suggests that the copper adsorption and selectivity observed in 3D-LTP were solely due to the presence of LTP in the 3D-printed structure (Fig. S5).This selectivity of 3D-LTP to copper can be clarified by the strong attraction copper exhibits towards chelating polymers that solely possess nitrogen donor atoms particularly in very acidic environments (Kołodyńska et al., 2014).Furthermore, the higher absolute electronegativity of copper ions than the other competing ions could be the major factor that drove the selective adsorption of copper ions over the other competing ions in the multimetal system.Higher absolute electronegativity implies higher affinity between the copper ions and adsorption sites.For the close proximity between the ionic radii of copper and zinc which paints a unique scenario, ionic radii could also be a possible driving force for copper selectivity (Bediako et al., 2022;Li et al., 2011;Liu et al., 2008;Parr and Pearson, 1983;Pearson, 1988).

Conclusion
In this study, an effective method for reusing waste polymers and handling of functional materials with small particle sizes was developed by integrating 3D printing technology with ion exchange.A 3D-printed porous adsorbent (3D-LTP) comprising of recycled polymer and chelating resin was designed for selectively recovering copper from  copper tailings.SEM images revealed the porous morphology of the 3D-LTP, whilst FTIR characterization results revealed the functional groups of LTP, confirming successful functionalization.The designed material showed good Cu(II) adsorption under the optimum pH of 5, demonstrating a maximum adsorption capacity of 4.56 mg/g.The Langmuir isotherm and pseudo-second-order kinetics explained the adsorption isotherm and kinetics as monolayer, and chemisorption process.FTIR spectra before Cu(II) adsorption and after indicated that the potential adsorption mechanism could be by coordination bond forming or chelation.Thermodynamics findings indicated that adsorption capacity slightly increased with increasing temperature, suggesting an endothermic adsorption process.However, the small changes with temperature indicate its effect is minimal on the adsorption process.The 3D-LTP also exhibited high selectivity towards Cu(II) in the multi-metal ions solution at a pH of 1.5.Additionally, its good reusability was demonstrated with a desorption efficiency exceeding 85 % following five consecutive adsorption-desorption cycles.This work highlights the significant potential of 3D printing in fabricating advanced adsorption and ion exchange materials with customizable properties for diverse applications.The designed adsorbent presents numerous benefits, such as simple preparation, adjustable geometry through 3D printing, retained functionality for Cu(II) recovery post-printing, and easy separation from the solution.Additionally, the study addresses various challenges, including the accumulation of polymer waste and the handling of particles.

Declaration of competing interest
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.

Fig. 1 .
Fig. 1.Visual representation of the fabrication process for the 3D-printed sorbent.

Table 1
Parameters obtained from the kinetic models for Cu(II) adsorption onto 3D-LTP.

Table 2
Parameters obtained from the isotherm models for Cu(II) adsorption onto 3D-LTP.