Cu(II) removal from aqueous solution in trickle tray adsorber using non-woven fabric modified via radiation-assisted functionalization | Scientific Reports

Scientific Reports volume 14, Article number: 27076 (2024) Cite this article

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An investigation into Cu(II) adsorption from contaminated water utilizing a trickle tray column that has been upscaled from batch-scale adsorption was performed to understand the efficacy of the adsorbent when used in a continuous system—which is more common in actual use in an industry. The size of the functionalized fabric adsorbent selected in a pilot-scale is about four times larger than a batch-scale. The continuous Cu(II) adsorption was analyzed using three parameters: initial Cu(II) concentration in solution; inlet solution flow rate and number of adsorbent sheets in the column to estimate the adsorption process’s breakthrough curve results. The breakthrough curve results were fitted with three mathematical models by Thomas, Yoon-Nelson and Adams-Bohart to comprehend the mechanisms of the adsorption process. All three mathematical models can be used to reliably describe the continuous adsorption process using diverse conditions applied in this study. Overall results showed that PBMEP-g-PE/PP adsorbent using the trickle tray column demonstrated as high as 97% efficiency of Cu(II) adsorption during the first 6 h. Three-cycle reutilization of the adsorbent through a process of adsorption-desorption revealed that Cu(II) removal efficiency surpassed 70% after 6 h of each cycle, confirming the suitability of this system for practical application.

With increasing industrial demand over the past decade, the environment has been polluted with heavy metals in wastewater. In spite of the fact that the wastewater is treated before being released into the environment, a small amount of metal ions is still dissolved. Among various metal ions, Cu(II) is different from other pollutants. Even small amounts can cause physiological or neurological effects on humans and aquatic life1,2,3,4. There are several methods for removing Cu(II) from wastewater, such as precipitation5, membrane filtration6, ion exchange7 and adsorption8. Each process has its own advantages and disadvantages. However, adsorption is another interesting method as it provides high quality treated water.

In general, there are two types of adsorption process: batch and continuous adsorption. These methods have been studied by several researchers but most of these studies were batch based and required many evaluation steps to be industrialized. A simple method adapted to industry is the continuous flow column adsorption process. In this process, columns are assembled with adsorption materials used in adsorption processes, for examples packed-bed adsorption column9,10, moving bed adsorption column11, and fluidized bed adsorption column12. Trickle bed column is a packed bed column, using gravity-assisted flow through a packing material. This flow occurs when liquid occasionally contacts the packed bed as minute droplets or thin films and cascades down the bed. The advantage is that the solution disperses well increasing the contact time between the solution and the adsorbent. Trickle bed reactor (TBR) occasionally drops the effluent through the packed bed, providing a large surface area for contacting and interacting of the effluent with the adsorbent when the liquid trickles down through the bed. This column is particularly beneficial in wastewater treatment, hydrotreatment in refineries, and chemical oxidation processes13,14,15. The flow of liquid in a TBR promotes effective mass transfer and adsorption, particularly in the pore of the fabric adsorbent. Moreover, the system of this column type is easily scalable, making them suitable for large industrial applications. TBR is also relatively simple and cost-effective for operation13,15. Overall, the benefits of TBR, such as high reaction efficiency, effective mass transfer, and scalability, make them highly proper for industries where large volumes of effluents need to be treated efficiently and cost-effectively.

The result of continuous adsorption is often expressed as a breakthrough curve. This curve analysis uses mathematical models to analyze and predict curve characteristics. Examples of some of the most widely used mathematical models are Thomas model16, Yoon-Nelson model17 and Adams-Bohart model18. These mathematical models are used to analyze the adsorption mechanism of the sorbent inside the column and relationships between various parameters affecting the adsorption process.

Adsorbent is a material put inside the adsorption column. Currently, different materials are used as trunk materials for the adsorption of heavy metal ions, such as fibers19,20, silica gel21,22 and non-woven fabric23,24,25. Polyethylene cached polypropylene non-woven fabric (PE/PP NWF) offers a number of advantages due to its high surface area and resistance to acid and alkali. These benefits make it practically possible to introduce desired functions onto its surface. Radiation-induced graft polymerization (RIGP) is a convenient technique to improve these polymeric trunk materials. The highlights of this technology were reported by Bhattacharya26. Unlike chemical or thermal treatments, radiation processing can induce graft polymerization (i.e. functionalization) without the use of catalysts, initiators and heat, thus making it a relatively green technology27. When graft polymerization is induced using radiation, the absence of catalysts and initiators results in impurities-free product, whereas the absence of heat makes it safe for application with temperature-sensitive trunk polymers and monomers as well as making the process comparatively energy-saving27. Many countries are taking advantages offered by radiation processing in industrial scale. Japan is one of these countries, with a number of commercial products, including adsorbents, developed using radiation technology. Japanese researchers have many times proved that industrialization of adsorbents via radiation processing is practically and economically feasible28. One of the most interesting advantages of RIGP is the freedom to choose suitable monomers for desired functionalization. With the right monomer, the functionalized adsorbent can offer higher efficacy for adsorption. Proper monomers with well-designed functional groups can lead to active sites capable of capturing targeted adsorbates (metal ions). One of the monomers that can offer practical functions is bis[2-(methacryloyloxy) ethyl] phosphate (BMEP). This monomer is composed of two methacrylate groups and a phosphate group in the middle. The BMEP monomer is hydrophilic, with the short carbon chain enhancing its wettability. Both methacrylate groups can also undergo polymerization and cross-linking processes. Therefore, it is a suitable monomer to improve or increase the capacity of the trunk polymer29.

In the previous study30, BMEP was chosen as a vinyl monomer to modify the surface of PE/PP NWF. With two polymerizable methacrylate groups in its chemical structure, BMEP can undergo graft polymerization onto the surface of PE/PP NWF. The prepared adsorbents offered effective performance for Cu(II) adsorption in a small batch system. The major goal of this study is to investigate the efficiency of the prepared adsorbent in a continuous adsorption process by applying the adsorbent in a trickle tray adsorber column. This research focuses on studying results from the following parameters: (1) effects of flow rate, (2) effects of initial Cu(II) concentration and (3) effects of the number of adsorbent sheets. In this study, experiments were performed to identify an optimum condition and a suitable mathematical model for the continuous process of Cu(II) adsorption. Additionally, desorption of Cu(II) from the adsorbent and reusability of the adsorbent were also analyzed.

Polyethylene/polypropylene non-woven fabric (PE/PP NWF) was purchased from Kurashiki Textile MFG Company Limited (Japan). Bis [2-(methacryloyloxy) ethyl] phosphate (BMEP) was bought from Sigma-Aldrich (Japan). Copper (II) chloride dihydrate (CuCl2.2 H2O) was procured from BDH Laboratory Supplies (Belgium). Polyethyleneimine (PEI, branched, M.W. 10,000, 99%) was purchased from Alfa Aesar, United States. Other chemicals including sulfuric acid (H2SO4) and methanol (> 99.9%, Merck KGaA) were analytical grade reagents.

PE/PP NWF sheets were cut into circles with diameter of 4.2 cm. The sheets were functionalized by RIGP of BMEP monomer in a gamma irradiator (60Co, Gamma chamber 5000, BRIT, India) at 1 kGy of radiation dose under an optimum condition at 2.5% BMEP in an aqueous solution containing PE/PP NWF sheet. RIGP of BMEP onto PE/PP NWF was reported and described in details in our previous work30. Henceforth, the prepared adsorbent will be abbreviated as PBMEP-g-PE/PP.

PBMEP-g-PE/PP prepared as an adsorbent was characterized by Fourier transform infrared spectrophotometer (FTIR) and scanning electron microscope (SEM). Visually, color changes from Cu(II) adsorption on the fabric surface was observed and photographed. FTIR (Bruker Tensor 27, equipped with DigiTect™ system, High sensitivity DLATGS detector) was operated in attenuated total reflectance (ATR) mode in a frequency range of 4000–650 cm− 1. Morphological micrographs of the prepared adsorbent without/with Cu(II) adsorption were taken by SEM (JSM-6480LV). Sample coating using a thin palladium film was applied prior to analysis by SEM.

For the continuous flow mode of the adsorption process of this study, the adsorption apparatus includes the following components: (1) a power system (2) a diaphragm pump and (3) a trickle tray adsorption column, which comprises three main parts: the column itself, a cover with a rubber gasket and an adsorbent holder and (4) a three-tank system for the storage of contaminated solution, deionized water, and treated solution.

Figure 1 shows the flow path and parameter study of the continuous adsorption process. Flow rate of the system can be adjusted in the range of 0–18.9 mL/min. The Cu(II) solution flows through the adsorbent installed in a trickle tray column from the top to the bottom of the column. The effects of number of adsorbent sheets on column efficiency were investigated. The concentrations of Cu(II) solution were analyzed by UV-Visible spectrophotometer using PEI as a complexing reagent31. After the adsorption test, Cleaning in Place (CIP) process was performed by adding 4 L of ultra-pure water into the deionized water tank and pumping in the top of the adsorber column at a flow rate of 18.9 mL/min (Stroke length 100%).

Figure 2 displays the operation unit of the trickle tray adsorber. At the beginning of the operation, the Cu(II) solution was prepared and added to the tank storing the contaminated solution. The adsorbent sheets were packed inside an adsorbent holder. The adsorbent holder was then placed inside the trickle tray column. One channel of the adsorbent holder can accommodate a maximum of ten adsorbent sheets.

Schematic diagram of continuous adsorption process.

Unit operation of the trickle tray adsorber.

The breakthrough of the adsorbate using Cu(II) and adsorbent using PBMEP-g-PE/PP was investigated by controlling parameters of the initial Cu(II) concentration, flow rate and the number of adsorbent sheets. The system was designed and set up as shown in Fig. 2. Then the effects of operating parameters were performed under various conditions for four experimental runs as shown in Table 1. The quantity of adsorbent was basically estimated from the amount of phosphate group attached on the adsorbent, ensuring to remove Cu(II) in the effluent. The bed height of the holder in the column was calculated from the amount of required adsorbent. For the dimension of the fabric sheet, it was estimated to offer ease of operation during gamma irradiation and also to make sure that they will be physically suitable for the trickle tray column. The diameter of the fabric sheet used for the adsorbent was designed to be 4.2 cm at a thickness of 0.1 cm. 2.5 g of fabric sheets containing phosphate group with 250% degree of grafting were weighed and placed in the holder, at 12 cm of bed height and 4.2 cm of inner diameter, inside the column.

To find the optimum condition for Cu(II) adsorption in the trickle tray continuous flow process, the results of this study were presented in the form of a breakthrough curve correlating the ratio of the outlet and the inlet of solute concentration in terms of Ct/C0 as a function of time, where Ct is the effluent concentration (mg/L) at time t and Co is the initial concentration (mg/L), focusing on three major parameters.

The breakthrough curve of Cu(II) on PBMEP-g-PE/PP adsorbent in the trickle tray column was obtained to understand the mechanisms of adsorption and determine the optimum operation parameters such as flow rate, initial concentration of Cu(II) aqueous solution and the number of PBMEP-g-PE/PP adsorbent sheets. The calculations used to observe the adsorption in the studied column are shown in Eqs. (1–3) and C0 and Ct were read from the breakthrough curves obtained using different parameters.

The amount of total metal adsorbed on the surface, qtotal (mg), can be written as:

Where Q is the volumetric flow rate (ml/min) and ttotal is the flow time (min). Cads is the amount of solute adsorbed on the adsorbent (mg/L), calculated from Cads = C0 – Ct, Cu(II) concentration is measured in the unit of mg/L. The integral term of the Eq. 1 was calculated from the area under the breakthrough curve in Fig. 6 using the trapezoidal method calculated in Excel.

The equilibrium metal ion uptake or the adsorption capacity of the trickle tray column, qeq (mg/g), is calculated as follows:

Where m is the dry mass (g) of the adsorbent in the trickle tray column.

The total metal removal percentage, Y(%), can be written as:

From the breakthrough curve results, the experiment data of Cu(II) adsorption in a trickle tray column were fitted with three mathematical models to determine the efficacy and operation of the studied system.

Thomas model16 is one of the most common models used to study the efficiency of a trickle tray adsorption columns. The model can also be used to determine the adsorption capacity of adsorbent in the process. It is based on the following assumptions: (1) adsorption equilibrium according to Langmuir-type isotherm, (2) constant void fraction in the column, (3) negligible radial and axial dispersion in the bed, (4) isobaric and isothermal operation and (5) no signs of resistance against intraparticle and external diffusion32.

The nonlinear equation of the Thomas model is shown in the following equation33.

where kTH is the Thomas rate constant (mL/min.mg); q0 is the equilibrium Cu(II) uptake in the adsorbent (mg solute/g adsorbent); m is the amount of adsorbent (g) in the trickle tray adsorption column; C0 is the initial Cu(II) concentration (mg/L); Ct is the effluent concentration at time t (mg/L); Q is the flow rate (mL/min); V is the effluent volume at time t (mL).

Yoon-Nelson model17 is also a highly accepted model for breakthrough curve analysis, mostly due to its ease, simplicity and small number of parameters required. This model assumes that the rate of decrease in the probability of adsorption for each adsorbate molecule is proportional to the probability of sorbate sorption and the probability of adsorbate breakthrough on the adsorbent.

The nonlinear equation of the Yoon-Nelson model is as follows.

where kYN is the constant of Yoon-Nelson (1/min) and \(\:{\uptau\:}\) is the time required for 50% breakthrough (min).

Adams-Bohart model18 is suitable for describing the initial part of the breakthrough curve. The model assumes that the adsorption rate is proportional to both the residual capacity of the adsorbent and the sorbate concentration, which is largely determined by surface adsorption on the adsorbent surface position.

The nonlinear equation of the Adams-Bohart model is presented in the following equation34.

where kAB is the Adams-Bohart rate constant (L/mg.min); a0 is weight of solute uptake per unit volume of the bed (mg/L); x is the bed height (cm); u is flow rate per unit cross-sectional area of column (cm/min).

The adjusted coefficient of determination (Adj. R2) and root of mean squared error (RMSE) were used to evaluate the fitting quality of above mathematic models35, which are expressed as

Where \(\:{y}_{i}\) are the observed values (y = Ct), \({\mathop y\limits^{\prime } _{i} }\) are the predicted values, \(\bar{y}\) is the average value of all observed values, n is the number of data points, p is the number of the model parameters.

Adsorption experiment with a condition of initial Cu(II) concentration of 100 mg/L, flow rate 5.67 mL/min and 20 adsorbent sheets was selected to study the desorption and reusability of the adsorbent.

First, the used adsorbent sheets were removed from the trickle tray adsorption column. Next, the used adsorbent sheets were immersed in 150 mL of 1 M H2SO4 for 1 h. The treated solution was intermittently collected at 0, 2.5, 5, 7.5, 10, 15, 20, 25, 30, 40, 50 and 60 min to observe the amount of Cu(II) that desorbed from the surface of adsorbent sheets. The concentration of Cu(II) dissolved in H2SO4 solution was measured by UV-Visible spectrophotometer using PEI as a complexing reagent31. After that, the desorbed adsorbent sheets were washed with RO water until the pH of the sheets was in the range of 6–7, after which the sheets were dried in an oven at 60oC until their weight remained unchanged for further reuse study. The efficiency of desorption was calculated as follows:

where qdes is the amount of the desorbed Cu(II) (mg) from Cu(II) adsorbed- PBMEP-g-PE/PP. qtotal is the amount of total metal adsorbed on the surface using Eq. (1).

To observe the reuse of PBMEP-g-PE/PP, the samples of Cu(II) solution were collected from the effluence at the bottom of the trickle tray column at 6 and 12 h with the assumption of the suitable efficiency of Cu(II) adsorption on the PBMEP-g-PE/PP adsorbent. The samples were used to measure the Cu(II) concentration by UV-Visible spectrophotometer using PEI as a complexing reagent31 for calculating the percentage of Cu(II) removal by Eq. (3) and then the reused sheets were dried to study the next cycle of adsorption/desorption cycle or reusability with the same procedure.

The grafting of the functional phosphate group on the PE/PP fabric was confirmed by FTIR spectra in Fig. 3. Additionally, SEM, SEM mapping and color changes before and after Cu(II) adsorption on the surface of the fabric adsorbent were observed in Fig. 4.

FTIR spectra of PE/PP NWF and PBMEP-g-PE/PP adsorbent.

Figure 3 shows FTIR spectra for both original PE/PP NWF and PBMEP-g-PE/PP adsorbent. The original PE/PP NWF exhibited prominent peaks at 2916 and 2848 cm− 1 (CH stretching in –CH3 and –CH2– groups), 1463 cm− 1 (CH2 scissoring), 1375 cm− 1 (CH3 deformation), and 717 cm− 1 (CH2 rocking). Following graft polymerization, the PBMEP-g-PE/PP spectra displayed new peaks at 1717 cm− 1 (C = O stretch in ester), 1255 cm− 1 (P = O stretch in phosphates), 1161 cm− 1 (P–O stretch), 1058 cm− 1 (C–O stretch), and 975 cm− 1 (P–O–C antisymmetric stretch)36,37. The new peaks confirm the successful grafting of BMEP monomer onto PE/PP NWF, indicating integration of phosphate groups into the adsorbent.

Comparison of SEM, SEM mapping and color before Cu(II) adsorption in (a) PBMEP-g-PE/PP and after Cu(II) adsorption in (b) Cu(II) adsorbed PBMEP-g-PE/PP.

Figure 4 illustrates the application of SEM micrograph and SEM mapping to analyze Cu(II) adsorption on PBMEP-g-PE/PP from the operation in a trickle tray column. Phosphorus mapping confirmed the presence of PBMEP, while Cu(II) mapping demonstrated Cu(II) adsorption by the PBMEP-g-PE/PP adsorbent effectively. Visual inspection also indicated a color change from white to light blue on the adsorbent surface after the adsorption process, further validating Cu(II) uptake.

To investigate the continuous process of a trickle tray column, the breakthrough of the adsorbate and adsorbent in the column is required. This section presents results from the investigation of changes of behavior zone of the S-curve of the adsorption experiment using 20 sheets of adsorbent, flow rate of 5.67 mL/min and initial Cu(II) concentration of 100 mg/L. The results are shown in Fig. 5.

Breakthrough curve of PBMEP-g-PE/PP adsorbent: tb: breakpoint time, t0.5: time point at Ct/C0 = 0.5 and ts: saturated time.

The breakthrough curve presents the relationship between Ct/C0 and time. The closer the Ct/C0 value is to 0, the better the adsorption efficiency of the adsorbent. On the contrary, if the Ct/C0 value is approaching 1, it means that the adsorbent is no longer adsorbable. From observation in Fig. 5, the breakthrough curve shows S-curve adsorption with the break point at 6 h (360 min) with Ct/C0 < 0.02 and it reaches the equilibrium or saturated point at 49 h (2,940 min) for 0.8 Ct/C0.

The curve can be divided into four zones, depending on the change in slope that can differentiate the following dissimilar characters of adsorption.

Zone 1, before breakpoint (tb), was the initial adsorption zone with a very small value of Ct/C0, relatively high adsorption efficiency. The effluence with water with high purity flowed out from the adsorption system in this zone. The value of water purity was in accordance with World Health Organization (WHO) standards and hence the water can be discharged into water resources.

Zone 2, between tb and t0.5, had a high slope of adsorption. This zone was located between the breakpoint (tb with Ct/C0 = 0.02 of this observation) and t0.5 (or 50% adsorption), indicating that the adsorbent still had a good adsorption efficiency. In this study, the effluence flowing out from the trickle tray process contained Cu(II) with concentration slightly higher than that of the standard set by WHO (Cu(II) < 0.2 mg/L).

Zone 3, between t0.5 and ts (the time of saturated adsorption), was the beginning point of the decrease of adsorption efficiency. The effluence in this zone contained high concentration of Cu(II) contamination. The Cu(II) was adsorbed on the designed adsorbent at a rate higher than 80%, implying that the adsorbent surface had already been used and should be changed.

Zone 4, the saturated adsorption zone over the ts point, was the zone where the adsorbent reached the equilibrium of Cu(II) adsorption. The adsorption efficiency was very low because the surface of the adsorbent was almost completely used. When considering the breakthrough curve in this observation, the Ct/C0 value was not equal to 1, with the cause of the mass transfer mechanism due to the following 4 steps: (1) adjective transport of adsorbate from the solution to the film layer covering the adsorbent surface, (2) film transfer that the adsorbate penetrates and becomes trapped in an immobile water film, (3) adsorption of adsorbate on the adsorbent surface of the sorbent and (4) intraparticle diffusion, as the last step that is the movement of solute particles into the pores of the adsorbent38. Nevertheless, PBMEP-g-PE/PP had no internal pores for occurrence of the intraparticle diffusion step.

In this work, the adsorption mechanisms of the Cu(II) adsorption on the surface of PBMEP-g-PE/PP adsorbent can possibly occur from both physical (reversible) adsorption and chemical (irreversible) adsorption to reach the equilibrium between adsorption and desorption, ultimately resulting in Ct/C0 ≠ 1.

The varied parameters of this study were the flow rate of Cu(II) solution, Cu(II) initial concentration and the amount of PBMEP-g-PE/PP adsorbent sheets under four different conditions as shown on the breakthrough curve. For this study, the breakthrough point where Ct/C0 = 0.02 was defined as the breakthrough time (tb) and the saturation or exhaustion time (ts) was read from the breakthrough curve of X-axis when Ct/C0 of Y-axis = 0.95.

Breakthrough curve of four different conditions of varied initial Cu(II) concentration, Cu(II) solution flow rate and amount of PBMEP-g-PE/PP adsorbent sheets.

Figure 6 shows the breakthrough curves of the varied conditions of Cu(II) adsorption on PBMEP-g-PE/PP adsorbent in a trickle tray column. Two initial Cu(II) concentrations at 100 mg/L and 150 mg/L and the flow rates of 5.67 and 7.56 mL/min of solution were chosen to make sure that there is enough time to reach the equilibrium of adsorption with the number of PBMEP-g-PE/PP adsorbent at 10 and 20 sheets in the column. The adsorption values analyzed from four breakthrough curves were summarized in Table 2.

From the results, it was found that tb, t0.5 and ts presenting adsorption zones of the four different condition curves were obviously affected by all three parameters. Effects of the flow rate can be observed by comparing the green curve (flow rate = 7.56 mL/min) with the black one (flow rate = 5.67 mL/min), where the initial concentration was fix at 100 mg/L and the number of adsorbents was fixed at 20 sheets. The green curve has a much steeper slope than the black one. The green curve reached saturation point much faster, with ts at 10 h (600 min) which is much faster than the black curve (with ts at 49 h or 2,940 min). This is most likely due to the fact that at faster flow rate, the contact time of adsorption on the adsorbent surface is reduced. The flow rate is a parameter related to contact time, which is one of the key parameters controlling the continuous process39. Additionally, the shorter contact time can result in less axial dispersion of Cu(II) to the adsorbent in the column and high flow rate can cause an extra flow path within the adsorbent, thereby reducing the ability of Cu(II) removal. Cu(II) adsorbed on the adsorbent (qtotal) and removal percentage (Y) of the green curve were 97.33 mg and 23.31%, respectively, while those of the black curve were 971.88 mg and 67.08%, in that order. These results confirmed that, at lower flow rate, the contact time between the solution and the adsorbent was longer, hence resulting in higher adsorption rates and increasing qtotal and Y (%) values33.

Effects of number of sorbent sheets were analyzed by fixing the initial Cu(II) concentration at 100 mg/L and the flow rate at 5.67 mL/min. Results for 10 and 20 sheets of adsorbent are displayed in red and black curves, respectively. The results in Fig. 6 show that qtotal and Y values of the black curve (971.88 mg/g and 67.08%) were higher than those of the red curve (414.55 mg/g and 32.02%). This can be explained from the fact that, with higher number of adsorbents, the adsorbent has more active sites available and capable of removing the adsorbate40,41. In this study, the breakthrough curve showed a longer breakthrough point and saturation point, with higher number of adsorbent sheets.

Effects of initial concentrations were investigated at two different concentration of Cu(II) in solution: 100 mg/L (black curve) and 150 mg/L (blue curve), while the flow rate and the number of adsorbent sheets were controlled at 5.67 mL/min and 20 sheets, respectively. Results in Fig. 6 show that ts of the blue curve was 2,280 min, whereas that of the black curve was 2,940 min. This indicated that lower concentration of adsorbate resulted in shorter saturation time. From Table 2, qtotal and Y (%) values of the blue curve (qtotal = 964 mg/g, Y = 53.88%) were also less than those of the black curve (qtotal= 971.88 mg/g, Y = 67.08%). The description of this parameter is the driving force of mass transfer of adsorbate in the flow stream and the adsorbent. If the driving force of mass transfer is high, the solute diffuses more quickly to the surface of the adsorbent. This agreed very well with results reported in previous studies41,42. From these results, it is obvious that saturation time is also a highly important factor for the removal of metal ions using functionalized adsorbent in a trickle tray column.

These results implied that lower initial concentration, lower flow rate and higher number of adsorbent sheets led to higher efficiency of solute removal. However, column pressure and flooding effect must be taken into consideration, since they can be caused by flow rate and amount of packed adsorbent in the column. The active sites on the adsorbent surface should be taken into account as well. From these results, the optimum condition for this study was chosen to be the black curve at 100 mg/L Cu(II) initial concentration, 5.67 mL/min solution flow rate and 20 adsorbent sheets. This optimum condition resulted in highest efficiency during the first 6 h of the adsorption process and a concentration level of Cu(II) in the effluent lower than the value set by WHO standard allowance.

From the study of the continuous Cu(II) adsorption process, the breakthrough curves were generated and used to fit three widely recognized mathematical models; the Thomas model, the Yoon-Nelson model and the Adams-Bohart model, in order to evaluate the efficiency of the adsorption process with time. This analysis can facilitate the design of operating conditions to achieve desired adsorption efficiency in a given time. The models allow for a better understanding of the adsorption behavior of the fabric adsorbent in the trickle tray column, more clearly interpreting the experimental findings. The mathematic models were used to generate simulated breakthrough curves that can estimate the capacity suitable for scaling up a unit plant43. The fitting data of the mathematical model are shown in Fig. 7, while constant values with statistic values of each model are summarized in Table 3. All three models were fitted based on three key parameters: flow rate, number of adsorbent sheets and initial concentration of solute.

Data fitting with Mathematical models of (a) Thomas model, (b) Yoon-nelson model and (c) Adams-Bohart model.

Thomas model was a plot of non-linear equation, as shown in Fig. 7a1 to a3. All parameter values used for the fitting and their fitted results are tabulated in Table 3. From Table 3; Fig. 7a2, kTH value decreased with increasing number of adsorbent sheets. On the other hand, results from Table 3 along with those from Fig. 7a1 and a3 implied that kTH value increased with increasing Cu(II) initial concentration and flow rate. This can be explained from the fact that the higher the initial concentration, the greater the diffusion capacity between the layers of the adsorbent. The difference between high concentration at the outer layer of the bulk and low concentration at the surface site of the adsorbent drives the movement of adsorbate or solute from the bulk to diffuse through the layers. The higher the solute concentration, the faster the diffusion of the solute. In a similar way, with increasing flow rate, the driving force of diffusion also increases, leading to higher adsorption rate.

Conversely, q0 increases with increasing number of adsorbents due to longer contact time and higher interaction of surface area between adsorbate and adsorbent. Therefore, the saturation time of the adsorbent will be longer with increasing number of adsorbents, giving rise to a greater adsorption capacity40,44. The value of q0 decreased with an increase in initial concentration and flow rate, bringing about a faster adsorption time. In addition, a high flow rate can create a special flow path in such a way that the solution has more chance to flow through this specific flow path, thereby reducing an opportunity for the adsorbate to come into contact with all available active sites of the adsorbent. The effects of all these parameters on adsorption has also been reported by a previous work45.

Considering Adj. R2 ≈ 0.922, the experimental data were fitted with the Thomas model, indicating that the behavior of Cu(II) adsorption on PBMEP-g-PE/PP inside the trickle tray column obeyed the pseudo-second-order reversible reaction kinetics, with no limiting step of the internal and external diffusion46,47. Additionally, this experimental data was fitted to Thomas model that explains the mechanism, involving the electron sharing or exchange between the adsorbent and the adsorbate, matching very well with the principles of pseudo second-order kinetics48,49. For this case, the adsorption principally involves the interaction of divalent Cu(II) ions with two active sites of phosphate. The concept of this model implied that the adsorption mechanisms of PBMEP-g-PE/PP can be divided into three steps as previously described in the section titled "Breakthrough curve of Cu(II) on PBMEP-g-PE/PP adsorbent in the trickle tray column" , with the limiting step of Step 3, the adsorption of Cu(II) on the active site of PBMEP-g-PE/PP.

The Yoon-Nelson model was plotted to explain the kinetics and adsorption constants of kYN (the Yoon-Nelson kinetic constant (min− 1)) and τ (the time required for 50% adsorbate breakthrough (min)). The fitting plots are illustrated in Fig. 7b1 to b3, while constant values of Yoon-Nelson model are tabularized in Table 3.

From Table 3; Fig. 7b2, higher number of adsorbent sheets led to lower value of kYN, whereas results from Table 3; Fig. 7b1 and b3 indicated that higher value of initial concentration and flow rate brought about higher value of kYN. The value of kYN can be used to explain the adsorption rate. When the solution flows through the adsorbent at a rapid rate, the adsorbent will be swiftly saturated, as well. In contrast, τ increased with increasing numbers of adsorbents, causing a longer saturation period of adsorbent, whereas τ decreased with increasing Cu(II) initial concentration and flow rate, resulting in faster saturation of the adsorbent50. These results harmonized with those previously published by a number of researchers33,39,46.

Considering Adj. R2 ≈ 0.924 for this model, the fitting results suggested that the Yoon-Nelson model is suitable to describe this adsorption process. Based on this assumption, the behavior of Cu(II) adsorption of PBMEP-g-PE/PP in the trickle tray column is most likely due to the fact that the rate of decrease in the probability of adsorption for each Cu(II) was proportional to the probability of Cu(II) adsorption and the probability of Cu(II) breakthrough on the active surface of PBMEP-g-PE/PP adsorbent.

The Adams-Bohart model was plotted to determine values of kAB (the Adams-Bohart rate constant L/mg.min) and a0 (the weight of solute uptake per unit volume of the bed in mg/L) as well as to explain the relation between kAB and a0. Effects of the three parameters on the curves fitted using Adams-Bohart model are displayed in Fig. 7c1 to c3, while the values of their related kAB and a0 values are shown in Table 3.

Similar to results reported by other researchers39,47. kAB decreased with increasing number of adsorbent sheets, whereas kAB increased with increasing initial concentration and flow rate. The latter entailed that the overall system kinetics were dominated by external mass transfer in the initial period of adsorption in the column51. a0 value increased with increasing number of adsorbent sheets but decreased with increasing initial Cu(II) concentration and flow rate, corresponding to the fact that contact time between the adsorbent and the adsorbed was reduced.

Considering Adj. R2 ≈ 0.922 for this model, the Adams-Bohart model suggests that the initial portion of the adsorption process is controlled by surface reactions and that the adsorbent has a significant capacity for the target solute.

Overall results of the mathematic model fitting agree with all three models to indicate that a comprehensive understanding of the adsorption dynamics in this system involved the pseudo-second-order reversible reaction kinetics, mass transfer of Cu(II) diffusion, and the active surface of the adsorbent.

To investigate reusability of PBMEP-g-PE/PP adsorbent sheets, a desorption of Cu(II) from the adsorbent was conducted. From the breakthrough curve shown in Fig. 5, Cu(II) adsorption under the controlled condition at 100 mg/L of Cu(II) initial concentration, 5.67 mL/min of flow rate and 20 adsorbent sheets was used for the total operation time of 78 h (4,680 min) and designated as cycle 1. During the first 6 h (360 min), the adsorption of Cu(II), the value of Ct/C0 was 0.01 which was very close to 0. After 12 h (720 min), Ct/C0 equaled 0.086. Results from Fig. 5 showed that the first 12 h resulted in effective adsorption efficiency. Hence, 12 h was chosen as an appropriate time for desorption and reusability study of cycles 2 and 3. Cu(II) adsorption-desorption cycle on PBMEP-g-PE/PP adsorbent was performed for 12 h of each cycle for three cycles, as shown in Fig. 8.

Cu(II) desorption from the Cu(II)-absorbed PBMEP-g-PE/PP during the first three desorption cycles (a) the amount of Cu(II) released (mg) with time (b) efficiency of Cu(II) desorption (%).

The Cu(II) desorption from PBMEP-g-PE/PP adsorbent was analyzed using the quantity of Cu(II) released in each cycle as a function of time. Results shown in Fig. 8a. implied that desorption time of 15 min was sufficient. The amount of desorbed Cu(II) (qdes) was lower than the amount of adsorbed Cu(II) (qtotal). This stems from the fact that adsorption of Cu(II) on the PBMEP-g-PE/PP involves both physical (reversible) and chemical (irreversible) adsorption on the surface of the adsorbent. Chemical adsorption occurs due to large number of active sites with chemical bonding on the adsorbent surface, leading to stronger adsorption of Cu(II) which cannot be easily desorbed. Unlike chemical adsorption, physical adsorption is caused by the Cu(II) deposition in multiple layers on the surface of the adsorbent with weak interaction of Vander Waals force. The majority of desorption from the adsorbent surface is most likely due to physical desorption, hence causing lower efficiency of Cu(II) adsorption for reducing Cu(II) removal in the next cycle. Figure 8b shows an increase in the desorption efficiency, as calculated by Eq. (9). This is because the amount of active site on the adsorbent surface was lessened in each cycle, due to the use of chemical (irreversible) adsorption per cycle.

The total Cu(II) removal(%) in each cycle at 6 h and 12 h under the controlled parameters of 100 mg/L initial Cu(II) concentration, 5.67 mL/min flow rate and 20 reused adsorbent sheets.

Figure 9 presents the percentage of Cu(II) removal, reflecting the efficiency of the trickle tray column in conjunction with the internal adsorbent. It indicated that the higher the percentage of Cu(II) removal, the better the adsorption by the trickle tray column. The 1st cycle of adsorption showed a high percentage of Cu(II) removal at both 6 and 12 h of adsorption. The percent removal of Cu(II) decreased from 97.47% (at 6 h) and 94.05% (at 12 h) from the 1st cycle to 86.49% (at 6 h) and 84.63% (at 12 h) during the 2nd cycle. As for the 3rd cycle, the values reduced further to 74.89% and 68.51% at 6 and 12 h, respectively. These results indicated that the number of available active sites at 6 h was higher than that at 12 h, simply due to shorter contacting time. Therefore, available sites for adsorption were reduced in each cycle.

PBMEP-g-PE/PP adsorbent was prepared by simultaneous radiation-induced graft polymerization technique and loaded into a trickle tray column to remove Cu(II) from aqueous solution in a continuous adsorption column of a trickle tray process. Three major parameters, initial Cu(II) concentration, flow rate, and number of sheets of the adsorbent, were varied to investigate the efficiency of adsorption observed from the breakthrough curves and to find the optimum condition. The efficiency of Cu(II) adsorption on the PBMEP-g-PE/PP adsorbent in a continuous experiment demonstrated was as high as 97% during the first 6 h. After the experiment continued, the adsorption efficiency remained at 50% at 33 h. These results indicate that the adsorbent is effective for long-term Cu(II) ion removal. Results from the continuous adsorption process using different conditions in this study can be explained using all three mathematical models. From the adsorption-desorption cycles at 12 h of adsorption time, Cu(II) removal percentage decreased from 94.05 to 86.43% and 68.51% at cycle 1, 2 and 3 respectively. The adsorption time at 12 h proved to offer high efficiency of adsorption and reusability of the adsorbent. Moreover, this adsorbent can be reused easily by immersing it in 1 M sulfuric acid for 15 min. Based on the results of this experiment, the study shows that the adsorbent is effective for removing Cu(II) ions. This system (adsorbent, trickle tray column, continuous adsorption process) is suitable for practical, real-life applications due to its convenience, ease of operation, and reusability.

All data generated or analyzed during this study are included in this published article.

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The authors would like to gratefully acknowledge the financial support of the Thailand Research Fund (TRF), the National Research Council of Thailand (NRCT) and the Thailand Science Research and Innovation (TSRI) in cooperation with Thailand Institute of Nuclear Technology (Public Organization) (TINT) through the Royal Golden Jubilee Ph. D. Programme (RGJ) (Grant number PHD/0197/2561). Furthermore, this work was made possible by a generous grant from Thailand Institute of Nuclear Technology (Public Organization) (TINT) through the TINTtoUniversity65 programme.

Department of Chemical Engineering, Faculty of Engineering, King Mongkut’s University of Technology North Bangkok, Bangkok, 10800, Thailand

Yanisa Limsuwan & Thirawudh Pongprayoon

Center of Eco-Materials and Cleaner Technology, King Mongkut’s University of Technology North Bangkok, Bangkok, 10800, Thailand

Yanisa Limsuwan & Thirawudh Pongprayoon

Thailand Institute of Nuclear Technology (Public Organization), Nakorn Nayok, 26120, Thailand

Pattra Lertsarawut & Kasinee Hemvichian

Department of Advanced Functional Materials Research, Takasaki Institute of Advanced Quantum Science, National Institutes for Quantum Science and Technology, 1233 Watanuki-machi, Takasaki, 370-1292, Gunma, Japan

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Concepts for the study were cooperatively proposed by Y.L., K. H., and T. P. Y.L. was the key researcher responsible for experiments, procedures, analysis of collected data as well as manuscript preparation. P. L. offered highly supportive assistance with experiments and characterization. N. S., K. H. and T. P. provided insightful comments for manuscript draft. The latest draft of the manuscript was prepared by Y.L., K. H., and T. P.

Correspondence to Thirawudh Pongprayoon.

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Limsuwan, Y., Lertsarawut, P., Hemvichian, K. et al. Cu(II) removal from aqueous solution in trickle tray adsorber using non-woven fabric modified via radiation-assisted functionalization. Sci Rep 14, 27076 (2024). https://doi.org/10.1038/s41598-024-78450-y

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Received: 07 July 2024

Accepted: 30 October 2024

Published: 07 November 2024

DOI: https://doi.org/10.1038/s41598-024-78450-y

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