Removal of Pb(II) by lignin-sodium alginate composite in a fixed-bed column

ABSTRACT A kind of adsorbent (Hydrogel-I) derived from sodium alginate and modified alkaline lignin (MAL) has been proved to possess a good adsorption performance for Pb(II)-loaded wastewater based on batch experiments. However, practical removal of Pb(II)-loaded-wastewater is a continuous and dynamic process. Herein, Hydrogel-I was further evaluated by packing it into a fixed-bed column. The breakthrough curves were established under different inflow rates (0.159–0.318 L/min), inflow directions (down-inflow mode and top-inflow mode), initial concentrations (5–20 mg/L) of Pb(II), and bed depths (20–60 cm). The results indicated that the slower inflow rate (0.159 L/min), down-inflow mode, lower initial concentration (5 mg/L), and higher bed depth (60 cm) prolonged breakthrough times (tb) and saturation times (ts). Compared to the top-inflow mode, the down-inflow mode guaranteed enough contact between Hydrogel-I and Pb(II). The values of adsorption capacity at tb, ts, and the removal efficiency under the down-inflow mode were higher than that under top-inflow mode by 2.33, 0.78, and 0.07 times, respectively. Hydrogel-I beads exhibited better adsorption performance than other adsorbents by comparing the rate constant (kAB) and the adsorption capacity (N0). The kAB and N0 of Hydrogel-I beads were calculated to be 0.0034 L/(mg·min−1) and 678 mg/L. Hydrogel-I beads showed good regeneration ability in a three-adsorption-desorption cycle. Meanwhile, FT-IR analysis showed that the groups of –NH/–NH2, C=S, and C–S were proved to be the adsorption sites. This study could prove valuable insight into the practical application of Hydrogel-I for dynamic removal of Pb(II) in an inflow-through column. GRAPHICAL ABSTRACT


Introduction
With the rapid development of industrialization, industrial pollution has increased dramatically, and a large amount of potentially toxic metals are discharged into the water and soil environment [1].Lead (Pb) is one of the most toxic metals announced by WHO.In the past 50 years, more than 800,000 tons of Pb have been released into the environment.Pb(II) belongs to heavy metals that can be easily taken into the human body through biological accumulation, causing damage to the nervous, hemopoietic, and digestive systems.Pb (II)-loaded-wastewater is mainly discharged from the industrial wastewater and acidic leachate in landfill [2].The concentrations of Pb(II) primarily depended on their sources.For example, it was reported that Pb(II) concentrations discharged from the storage battery factories were at the level of 5-15 mg/L [3,4].Other wastewaters such as the Chlor-alkali industry and smelting plants release Pb(II) with concentration corresponding to 2.2 [5] and 141.6 mg/L [6].These abovementioned concentrations in the industrial effluent exceeded the emission of 1.0 mg/g in Chinese promulgated integrated wastewater discharge standard [7].Thus, decontamination of Pb(II) loaded in the aquatic environment is imperative to guarantee human health and environmental safety.Until now, adsorption as one of the most efficient methods has been applied to remove heavy metals [8].Massive efforts have been spent on exploring new adsorbents, e.g.carbon-based materials [9], biomass scavengers [10], nano-adsorbents [11], bio-apatite-based materials [12] and biopolymers [13] for the removal of lead from both batch and column system.Although there are certain degrees of adsorption capacities for heavy metals by these powder adsorbents, they still exist some imperfections for the practical application including two disadvantages: (1) easy loss and (2) difficulty separation, which could lead to expensive operation costs and secondary pollution [14][15][16].Therefore, it is imperative to fabricate a kind of high-efficient adsorbents with low-cost and easy separation.
Hydrogel is a kind of polymer with a three-dimensional cross-linked network, having a conspicuous characteristic of swelling in an aqueous solution.This peculiarity of hydrogel could improve the foreign ions or molecules' fast access into its inner structure.In recent years, sodium alginate (SA) was proposed to be used for the host hybrid adsorbents attributing to its safety and good membrane-forming performances.The fabrication of 3D structures by SA could improve permeability and be conducive to adsorption.These hybrid adsorbents were fabricated from SA and were usually used for the removal of anion pollutants, e.g.phosphate [17,18], arsenic (arsenate/arsenite) [19,20], or fluoride [21].In the meantime, some studies also focused on SA hydrogel incorporated with powder adsorbents such as the modified lignin to remove metal cations [22].For example, Zhou et al. [22] reported a kind of 3D porous graphenelignin-sodium alginate adsorbent for the uptakes of Pb(II) and Cd(II).The maximum adsorption capacities (q m ) of Pb(II) and Cd(II) were obtained as 226.2 and 79.8 mg/g [22], respectively.In our previous work, a kind of hydrogel bead (named as Hydrogel-I) was fabricated by in situ embedding the modified alkaline lignin (MAL powders) into the matrix of SA.Based on the batch experiments, it proved that Hydrogel-I exhibited an ideal adsorption capacity for removing Pb(II).However, the exploration for the adsorption performance of Pb(II) by packing Hydrogel-I into a fixed-bed column to evaluate the practical application is demanded.
Based on the above backgrounds, the removal performance of Pb(II) by the fixed-bed column filled with the adsorbent of Hydrogel-I was conducted in this work.The breakthrough curves were established under different inflow rates, inflow directions, initial concentrations of Pb(II) and bed depths.The regeneration performance and adsorption mechanism were further evaluated.This work aims to provide valuable insight into the practical application of Hydrogel-I for dynamic removal of Pb(II) in an inflow-through column.

Preparation of Hydrogel-I
The modified alkaline lignin (MAL) powders and Hydrogel-I beads were prepared by the methods described in the reference reported by reference.The detailed steps were given in Text S1.The detailed steps of the modified alkaline lignin (MAL) powders and Hydrogel-I beadswere given in Text S1.

The fixed-bed column experiments
A description of the fixed-bed experiments was presented in Text S2.
Breakthrough curves were obtained by plotting C t /C 0 versus the running time.The parameter of C t (mg/L) was the concentration of Pb(II) ions in the effluent at t time (min), while C 0 (mg/L) the initial concentration of Pb(II) ions in the influent.As the maximum discharged limit of Pb(II) is 1.0 mg/L according to reference [7], the breakthrough time (t b ) in this study was determined as the time at which the Pb(II) concentration (C t ) in the effluent reached 1.0 mg/L.Once the ratio of C t /C 0 equaled to 0.95, the fixed-bed column experiments were over.The running time at C t /C 0 = 0.95 was defined as the saturation time (t s ), meaning that the treating ability of Hydrogel-I was completely lose [23,24].The breakthrough curves were established under different inflow rates, inflow directions, initial concentrations (C 0 ) of Pb(II) and bed depths (Text S2).
Three mathematical models of Dose-response, Thomas and Yoon-Nelson were used to fit the breakthrough curves.Especially, the column depth service time (BDST) model was adopted to simulate the breakthrough curves under different bed depths.The descriptions were listed in Text S2.

Adsorption performance
Three parameters including q, V eff , and Y were used to evaluate the adsorption performance of Pb(II) over Hydrogel-I in the column.The parameters of q and V eff stand for the adsorption capacity of Pb(II) over Hydrogel-I at a breakthrough time/saturation time (mg/g), and the effluent volume at a breakthrough time/saturation time (L).Y represents the removal efficiency at saturation time (%).The calculating equations were displayed in Text S3.

Regeneration experiment
To access the reusing ability of Hydrogel-I in the fixedbed column, three adsorption-desorption cycles were designed, in which 0.1 mol/L of HCl served as the regenerant to extract Pb(II) from the surface of Hydrogel-I.For each adsorption process, the breakthrough curves were plotted, while for each desorption process the separated and cumulative desorption efficiencies were obtained.The detailed information was seen in Text S4.

Adsorption and desorption characterization
Hydrogel-I beads were collected at regular intervals in each adsorption and desorption process.The fixedbed column was off for approximately 2 min in order to take out Hydrogel-I beads (the number was five).Noted that here the pause of two minutes and the number of five Hydrogel-I beads exerted no influence on the adsorption or desorption of Pb(II).The beads taken from the column were rinsed with ultrapure water to move the residual Pb(II).Then the beads were dried in a vacuum freeze drier (Shanghai Tianfeng Industrial Co., Ltd.) for 6 h at −60°C.The functional groups of Hydrogel-I and Hydrogel-I-Pb(II) complexes were analyzed by Fourier transform infrared spectroscopy (FT-IR) (Thermo Fisher Scientific, America).KBr powder and the beads were ground together and pressured into sheets for the measurement.The wavenumber for FT-IR was selected in the range of 500-4000 cm −1 .Scans' number and resolution corresponded to 16 and 4 cm −1 .

The breakthrough curves under different inflow rates
The effect of inflow rate on the breakthrough curve of Pb (II) over Hydrogel-I was displayed in Figure 1(a).Slope of the breakthrough curves exhibited a steeper tendency with an increase in the inflow rate.Earlier breakthrough and saturation times were found at higher inflow rates.Once the inflow rate was enhanced, the movement of Pb(II)-loaded wastewater in the fixed-bed column was correspondingly accelerated, therefore the residence time of Pb(II) in the column was insufficient [25][26][27].When the inflow rate ranged from 0.159 to 0.318 L/ min, the breakthrough time (t b ) reduced from 380 to 30 min while the saturation time (t s ) was from 881 to 397 min (Figure 1(b)).As mentioned above, t b referred to the running time at which the concentration of Pb (II) in the effluent was 1.0 mg/L since the maximum discharged limit of Pb(II) is 1.0 mg/L according to reference [7].The saturation time (t s ) stood for the running time at C t /C 0 = 0.95 [23,24], meaning that the treating ability of Hydrogel-I was completely lose.It was found that the value of t b at 0.159 L/min almost equaled to the value of t s at 0.318 L/min.Moreover, the ratio of t b to t s (r) was calculated to be 0.43, 0.15 and 0.08 at 0.159,, 0.255, and 0.318 L/min, respectively.These results demonstrated that the lower the inflow rate was, the more efficiently treating ability Hydrogel-I possessed.Parameters of the adsorption capacity (q) and V eff also verified such a viewpoint.
The adsorption capacity (q) of Pb(II) at t s and t b decreased from 35.7 to 24.7 mg/g and from 24.3 to 6.3 mg/g as the inflow rate increased from 0.159 L/min to 0.318 L/min (Figure 1(c)), respectively.The value of q at t b for 0.159 L/min was close to it at t s for 0.318 L/ min.The values of r 1 determined by q at t b accounted for 0.68, 0.33, and 0.26 of it at t s under 0.159, 0.255, and 0.318 L/min, respectively.The parameter of V eff referred to the effluent volume at a running time, in which V eff was obtained by the inflow rate multiplying t b or t s (Figure 1(d)).The difference among the values of V eff at t s was slight while at t b was large.It was found that 60 L of Pb(II)-loaded wastewater was treated under the situation of meeting the standard of 1.0 mg/L (GB8978-1996) when the inflow rate was 0.159 L/min, however, only 9.5 L for 0.318 L/min.As can be seen from the insert in Figure 1(d) and Table S1, a faster inflow rate resulted in a lower removal efficiency (Y ).Similar consequences were also reported by Han et al. [28] and Ai et al. [29].Since the inflow rate is an important factor that directly influences the adsorption performance of pollutants in the fixed-bed column, a relatively lower inflow rate of 0.159 L/min was selected for the subsequent experiments in this work.

Effect of the inflow direction
The effect of inflow direction was investigated by pumping Pb(II) solution throughout the fixed-bed column from the bottom (down-inflow mode) or from the top (top-inflow mode).Compared with the downinflow mode, the breakthrough curve of the top-inflow mode became steeper (Figure 2(a)).The possible reason was that the top-inflow mode led to a quicker passing of Pb(II) solution throughout the whole bed column without uniformly contacting with Hydrogel-I beads.Figure 2(b) and Table S1 display the values of t b and t s for the top-inflow mode equaled to 110 and 575 min, respectively.However, the values of t b and t s for the down-inflow mode were significantly enlarged to 380 and 881 min, respectively.The ratio of t b to t s (r) was obtained as 0.43 and 0.19 at down-inflow mode and top-inflow mode (Figure 2(b)), respectively.Figure 2(c) shows that the values of q were 24.3 and 35.7 mg/g at t b and t s in the down-inflow mode, respectively, obviously higher than those of the top-inflow mode of 7.3 and 20.0 mg/g.It was observed that the value of q at t b in the down-inflow mode was even better than that at t s in the top-inflow mode.The values of r 1 describing the ratio of q at t b versus t s were determined to be 0.68 and 0.37 for the down-inflow mode and the top-inflow mode, respectively.The insert of Figure 2(d) exhibits the removal efficiency (Y ) at t s was 50.9% and 43.7% corresponded to the down-inflow mode and top-inflow mode.As seen from Figure 2(d) and Table S1, it can be concluded that 60 L of Pb(II)-loaded wastewater was treated under the situation of meeting with the standard of 1.0 mg/L when the inflow direction was the down-inflow mode, much higher than 17.5 L obtained in the top-inflow mode.The breakthrough curves from down-inflow mode were different from top-inflow, which can be explained by the hysteresis phenomenon.As the down-inflow mode can strengthen the contact degree between Pb(II) and Hydrogel-I beads, this mode was selected in the subsequent experiments.

Effect of the initial concentration
Since a given mass of adsorbent removes a definite amount of pollutant, the initial concentrations (C 0 ) of Pb (II) are considered as one of the most important factors.In this study, the values of C 0 ranged from 5 to 20 mg/L. Figure 3(a) demonstrated that the breakthrough curves became more and more steeper when the initial concentration of Pb(II) increased.The times describing the breakthrough point (t b ) and saturation point (t s , Figure 3(b) and Table S1) were shorter under higher C 0 and followed the sequence of 20 mg/L < 10 mg/L < 5 mg/L.According to    3(c) shows the values of q at the saturation time were calculated to be 27.3, 35.7, and 62.0 mg/g when C 0 was 5, 10, and 20 mg/L, respectively, whereas 20.1, 24.3, and 48.7 mg/g at the breakthrough time.As presented in Figure 3(c), the values of r 1 were calculated to be 0.74, 0.68, and 0.79 for 5, 10, and 20 mg/L, respectively.The removal efficiency of Pb(II) (Y ) was obtained as 71.8% at C 0 of 5 mg/L (insert in Figure 3(d)), higher than that of 10 and 20 mg/L corresponding to 50.9% and 49.9%.The highest V eff of 79.5 L was obtained at t b for 5 mg/L and then followed by 60 L (10 mg/L) and 28.6 L (20 mg/L), which suggested that more volume of Pb(II) wastewater was treated and met with the emission standard of 1.0 mg/L (Figure 3(d) and Table S1).These results indicated that lower initial concentration extended the breakthrough curve [30].The values of V eff at t s were 152, 140, and 105 L corresponded to C 0 of 5, 10, and 20 mg/L.The higher concentration gradient arises a stronger mass transfer driving force [31] and faster solution transportation in the column [31], so that times to reach the breakthrough and the saturation points were quickened as well as V eff shrunk.

Effect of the bed depth
The influence of bed depth on the breakthrough curves was illustrated in Figure 4(a).
The values of t b was observed to be 125, 380, and 460 min when the bed depth was 20, 40, and 60 cm (Figure 4(b) and Table S1), respectively, while the values of t s were 480, 881, and 1024 min.These results suggested that a longer breakthrough time can be obtained by prolonging bed depth as the distance and service time for the mass transfer region between the two ends of the column were enlarged [30,33].In addition, the ratios of t b versus t s (r) were determined to be 0.26, 0.   [32] and modified beer lees [23] for k AB and N 0 .
fixed-bed column at the saturation time increased from 76.3 L to 162.8 L (Figure 4(d)) as well as 763-1268 mg (Table S1) corresponded to the bed depth of 20, 40, and 60 cm.When the treated volumes were evaluated at t b which stands for the emission standard of 1.0 mg/ L, the values of V eff increased from 19.9 mg/g to 73.1 mg/g as the bed depth was in the range of 20 cm to 60 cm.Although a bigger bed depth was beneficial for improving the removal efficiency (Y ) and treatment volume (V eff ) of Pb(II), the material cost also increased.So, the data of 40 cm was considered.
In order to further clarify the effect of bed depth, the column depth service time (BDST) model was employed [24] and the results were presented in Figure 4(e), in which the correlation coefficient (R 2 ) equaled to 0.8332.The rate constant (k AB ) and the adsorption capacity (N 0 ) were calculated to be 0.0034 L/(mg•min) and 678 mg/L (Figure 4(f)), respectively, higher than those of other adsorbents including Aeromonas hydrophila biomass (N 0 = 148.6 mg/L; k AB = 0.0002 L/ (mg•min) [32] and the modified beer lees (N 0 = 145.6 mg/L; k AB = 0.0012 L/(mg•min) [23].It is well known that higher N 0 represents faster mass transfer and bigger uptake efficiency, while higher k AB means smaller influence exerted by bed depth [34,35].Based on these data, it can be concluded that Hydrogel-I beads exhibited better adsorption performance compared to Aeromonas hydrophila biomass and the modified beer lees [34,35].

Modelling of breakthrough curves
Three models of Dose-response, Thomas and Yoon-Nelson were used to describe the adsorption behaviour of Pb(II) on Hydrogel-I packed in the column.As for the Dose-response model, it was originally adopted for predicting the adsorption kinetics of heavy metals in a biosorption process [2].Recently, this model was developed to be applied to various materials besides biosorption.The Thomas model is usually utilized to investigate such an adsorption process which follows the second-order reversible kinetics and Langmuir isotherm.The Yoon and Nelson model belong to a relatively simple model [36].It is noted that here the fitting curves of the Thomas model and Yoon-Nelson model were overlapping due to the same mathematical expression despite the model parameters being different [35].The experimental data fitted by Doseresponse model and the Thomas model were presented in Figures 5 and 6, respectively.The obtained parameters were listed in Table 1.The values of correlation coefficient (R 2 ) for the Thomas model (0.767-0.937) and Yoon-Nelson model (0.767-0.937) were higher than those of the Dose-response model (0.479-0.947), suggesting that the two former models better expressed the entire breakthrough curves.The calculated values of the adsorption capacity obtained from the Thomas model (q 0 ) were not close to the experimental data.The Thomas model is usually appropriate for the data correlation of symmetrical breakthrough curves but it may be inadequate and fail to describe the performance of fixed-bed columns with an unsymmetrical breakthrough [37].It was discovered that almost all the values of k TH were close to each other and approximately centered at 0.005 L/min•mg, demonstrating that the volume of Pb(II)-loaded-solution passed through the unit mass of Hydrogel-I beads within unit time was similar under different operational conditions.The parameter of τ from the Yoon-Nelson model exhibited a decreasing trend under the conditions of faster inflow rate, the topinflow mode, higher initial concentration of Pb(II) and lower bed depth.The reason could be assigned to the saturation of the column occurring more rapidly [23], which suggested that the contact between Hydrogel-I beads and Pb(II) solution was insufficient.

The breakthrough curves of three adsorption-desorption cycles
To further determine the regeneration performance of Hydrogel-I packed in the column, three consecutive adsorption-desorption cycles were conducted (Figure 7(a)).An aqueous solution of HCl (0.1 mol/L) was applied as the stripping agent.The adsorption process was expressed by plotting C t versus the running time and C t /C 0 versus the running time.The desorption process was expressed by plotting the desorption efficiencies of Pb(II) versus the running time.In the adsorption process, the breakthrough and saturation times were determined as the effluent concentration of Pb(II) were 1.0 and 9.0 mg/L, or the values of C t /C 0 reached 0.10 and 0.90 [7,23], respectively.In cycle 1, the values of q and V eff at the saturation time were 35.7 mg/g and 140 L, respectively.After 1040 min, the desorption was conducted in which 0.1 mol/L of HCl solution was used as the extractant.The separated desorption efficiency refers to an instantaneous desorption ability at a fixed time while the cumulative desorption efficiency represented a total desorption ability in a fixed time range.Figure 7(a) shows that in cycle 1 the separated desorption efficiencies increased sharply and reached as highest as 72% when the running time was 100 min, and then decreased a lot.After 150 min, there was no difference in the separated desorption efficiencies which were maintained at 0.5%.The cumulative desorption efficiency was obtained as 98% under the time scope of 0-1040 min.These results demonstrated that most of the desorption occurred in the initial 100 min.After 1040 min, Hydrogel-I beads filled in the column were washed with water until the effluent pH closed to neutral, and then the next adsorption-desorption cycle started.
Based on Figure 7(a), three phenomena were observed: (1) time to reach the saturation time shrunk when the reusing time increased; (2) the values of q and V eff at the saturation time showed a descending trend in which cycle 1 (35.7 mg/g and 140 L) > cycle 2 (33.1 mg/g and 99 L) > cycle 3 (28.1 mg/g and 90 L); (3) the separated desorption efficiency and the cumulative desorption efficiency of cycle 1 (72% and 98%) were the biggest then followed by cycle 2 (62% and 91%) and cycle 3 (48% and 91%).These results demonstrated that the adsorption performance of the regenerated Hydrogel-I decreased obviously when compared to fresh Hydrogel-I.

Adsorption mechanism
Figure 7(b) presents the photographic images of Hydrogel-I beads which were taken out from the fixed-bed column in the three adsorption-desorption cycles.As for cycle 1 during the adsorption process, the bead size of Hydrogel-I was discovered to gradually increase when the running time was prolonged from 0 to 1040 min.Oppositely, the bead size shrunk from 0 to 1040 min during the desorption process under the condition of HCl solution.Similar trends were also found in cycles 2 and 3.The possible reason was that Hydrogel-I exhibited hydrophilicity during the adsorption process, in which the solution pH was 6.0 therefore the swelling phenomenon occurred [38].However, in the desorption process, the solution pH was acidic as HCl was used so that the hydrophobicity of Hydrogel-I increased [38].
To clarify the adsorption-desorption mechanism, FT-IR spectra were used.As for fresh Hydrogel-I beads, five peaks at 3387, 1730, 1617, 1078 and 861 cm −1 were assigned to v(-NH) from -NH 2 [39][40][41], -C=O [42],  Inflow rate (L/min) a q 0 (mg/g) R 2 k TH (mL/(min•mg)) q 0 (mg/g) q e (mg/g) R 2 k YN (min  8(a)), respectively.In cycle 1, the three peaks centered at 3387, 1078, and 861 cm −1 weakened at the adsorption time of 8, 720, and 1040 min, confirming that these groups on the surface of Hydrogel-I were responsible for the uptake of Pb(II) and Hydrogel-I-Pb (II) complexes were further produced.According to Figure 8(b), the peak strength at 3387, 1078, and 861 cm −1 gradually increased when the desorption time ranged from 0 to 1040 min.These results suggested that the linkage between Pb(II) and these above-mentioned functional groups was broken.A similar trend was exhibited in cycles 2-3.Different from the peaks of 3387, 1078, and 861 cm −1 , the peak strength at 1617 cm −1 gradually decreased in the desorption process with the increase in desorption time for the three cycles.However, an opposite trend was found in the adsorption processes (Figure 8(a-e)).It could be deduced that during the desorption process the protons (H + ) of HCl probably combined with the -COOgroup carried by Hydrogel-I to form -COOH [44], while during the adsorption process the protons might be replaced by Pb(II).Interestingly, when comparing the peak of 1730 cm −1 to the peak of 1617 cm −1 , a similar change was observed.In all the desorption processes the peak strength at 1730 cm −1 was enhanced more and more along with the desorption time, however, in all the adsorption processes this peak disappeared.According to Fan et al. [42], the peak of 1730 cm −1 was attributed to -C=O.Based on the molecule structure of SA, -C=O should be as the part of -COOgroup.Therefore, same explanation for what happened at the peak of 1617 cm −1 can also be suitable for the peak of 1730 cm −1 .Based on these results, it can be concluded that the groups of -NH/-NH 2 , C=S, and C-S were the primary active sites for the adsorption of Pb(II).

Conclusions
The adsorbent of Hydrogel-I had been prepared in our previous work and used in the batch experiments.Different from batch experiments, Hydrogel-I was filled in a fixed-bed column in this study.The operation factors of inflow rates, inflow directions, initial concentrations of Pb(II) solution, and bed depths were studied in order to evaluate their influences on the breakthrough curves of Pb(II) over Hydrogel-I in the column.It was found that slower inflow rate, down-inflow mode, larger initial concentration, and higher bed depth were beneficial for not only prolonging the breakthrough and saturation times but also increasing the V eff and q values, meaning that more efficient adsorption performance was achieved.The inflow rate of 0.159 L/ min, down-inflow mode, initial concentration of 10 mg/L, and bed depth of 40 were determined as the running conditions.Three adsorption-desorption cycles were carried out, in which the technology of FT-IR was used to record each adsorption and desorption process.It proved that the functional groups of -NH/-NH 2 , C=S and C-S were considered to be the adsorption sites.The purpose of this work was to construct a theoretical foundation for the practical application of Hydrogel-I beads for Pb(II) removal.This study verified that Hydrogel-I could be practically applied through column experiments, and identified operational parameters that could be of concern for practical Pb-loaded-wastewater remediation, such as inflow  rates, inflow directions, initial concentrations of Pb(II), and bed depths.

Figure 1 .
Figure 1.The effect of inflow rates on (A) the breakthrough curves; (B) breakthrough time and saturation time; (C) the adsorption capacities (q) at breakthrough and saturation times; and (D) V eff at the breakthrough and saturation times, insert was the Y(%) at the saturation time.

Figure 2 .
Figure 2. (A) The effect of inflow mode on (A) the breakthrough curves; (B) breakthrough time and saturation time; (C) the adsorption capacities (q) at breakthrough and saturation times; and (D) V eff at breakthrough and saturation times, insert was the Y(%) at the saturation time.

Figure 3 (
Figure 3(b), the values of r for t b against t s were determined as 0.52, 0.43 and 0.27 corresponded to 5, 10 and 20 mg/L.Figure3(c)shows the values of q at the saturation time were calculated to be 27.3, 35.7, and 62.0 mg/g when C 0 was 5, 10, and 20 mg/L, respectively, whereas 20.1, 24.3, and 48.7 mg/g at the breakthrough time.As presented in Figure3(c), the values of r 1 were calculated to be 0.74, 0.68, and 0.79 for 5, 10, and 20 mg/L, respectively.The removal efficiency of Pb(II) (Y ) was obtained as 71.8% at C 0 of 5 mg/L (insert in Figure3(d)), higher than that of 10 and 20 mg/L corresponding to 50.9% and 49.9%.The highest V eff of 79.5 L was obtained at t b for 5 mg/L and then followed by 60 L (10 mg/L) and 28.6 L (20 mg/L), which suggested that more volume of Pb(II) wastewater was treated and met with the emission standard of 1.0 mg/L (Figure3(d) and TableS1).These results indicated that lower initial concentration extended

Figure
Figure 3(b), the values of r for t b against t s were determined as 0.52, 0.43 and 0.27 corresponded to 5, 10 and 20 mg/L.Figure3(c)shows the values of q at the saturation time were calculated to be 27.3, 35.7, and 62.0 mg/g when C 0 was 5, 10, and 20 mg/L, respectively, whereas 20.1, 24.3, and 48.7 mg/g at the breakthrough time.As presented in Figure3(c), the values of r 1 were calculated to be 0.74, 0.68, and 0.79 for 5, 10, and 20 mg/L, respectively.The removal efficiency of Pb(II) (Y ) was obtained as 71.8% at C 0 of 5 mg/L (insert in Figure3(d)), higher than that of 10 and 20 mg/L corresponding to 50.9% and 49.9%.The highest V eff of 79.5 L was obtained at t b for 5 mg/L and then followed by 60 L (10 mg/L) and 28.6 L (20 mg/L), which suggested that more volume of Pb(II) wastewater was treated and met with the emission standard of 1.0 mg/L (Figure3(d) and TableS1).These results indicated that lower initial concentration extended

Figure 3 .
Figure 3. (A) The effect of initial concentrations on (A) the breakthrough curves; (B) breakthrough time and saturation time; (C) the adsorption capacities (q) at breakthrough and saturation times; and (D) V eff at breakthrough and saturation times, insert was the Y(%) at the saturation time.
43, and 0.45 corresponded to deb depth of 20, 40, and 60 cm (Figure 4(b)).The adsorption capacity (q) at the breakthrough time was 16.5, 24.3, and 26.8 mg/g when the bed depth was 20, 40, and 60 cm (Figure 4(c)), respectively, whereas these values were obtained as 35.3, 35.7, and 36.6 mg/g at the saturation time.The values of r 1 were determined to be 0.47, 0.68, and 0.73 for 20, 40 and 60 cm, respectively.The V eff and the total amount (M ) of Pb(II) entering into the

Figure 4 .
Figure 4. (A) The effect of bed depths on (A) the breakthrough curves; (B) breakthrough time and saturation time; (C) the adsorption capacities (q) at breakthrough and saturation times; (D) V eff at breakthrough and saturation times, insert was the Y(%) at the saturation time; (E) the simulated data by BDST model; and (F) comparison of Hydrogel-I with Aeromonas hydrophila biomass[32] and modified beer lees[23] for k AB and N 0 .

Table 1 .
Parameters of Dose-response, Thomas and Yoon-Nelson models for Hydrogel-I.Parameters Dose-response model Thomas model Yoon-Nelson model

Figure 7 .
Figure 7. (A) Breakthrough curves, effluent concentrations and desorption efficiencies of Pb(II) in cycles 1, 2 and 3; and (B) outlooks of Hydrogel-I beads at different time in the three adsorption-desorption cycles.