Adsorbent propertiesTable 2 presents the textural properties of natural zeolite including surface area, volume of pores, and mean diameter of the pores.Table 2 The textural properties of natural zeolite.Table 3 provides the constituent elements of natural zeolite, according to the XRF test. As can be observed, the major constituents of natural zeolite are SiO2 and Al2O3.Table 3 The constituent elements of natural zeolite.The surface morphology of natural zeolite using SEM and X-ray energy diffraction spectroscopy (EDS) are shown in Figs. 2 and 3, respectively.Fig. 2Fig. 3The diffraction pattern obtained from XRD (Fig. 4) indicates that the natural zeolite clinoptilolite KNa2Ca2(Si29Al7)O72⋅24H2O is the major crystal phase constituting the sample.Fig. 4The effect of initial concentration on the efficiency of chromium (VI) adsorptionCr(VI) adsorption was examined at concentrations of 0.05, 0.15, 1, and 10 mg/m3 with a bed depth of 5 cm and flow rate of 3 L/min. the results obtained from this investigation are provided in Fig. 5. With the increase in the initial concentration, the time for reaching the breakthrough point and saturation was shortened. As shown in Fig. 5, at concentrations 0.15 and 0.05 mg/m3, following 350 h of sampling, the curve did not reach the breakthrough point. However, at concentrations 10 and 1 mg/m3, breakpoint was observed after 27 and 160 h, and its saturation occurred at 58.5 and 219 h, respectively. The results indicate that with increased initial concentration, the driving force of the adsorption process increases, which in turn causes faster saturation of adsorption sites, thereby shortening the time for reaching the breakthrough point44,45. At lower concentrations, the slope of the breakthrough point is smaller and the adsorbent surface becomes saturated after a longer time, suggesting a wide mass transfer zone (MTZ) at low concentrations. However, at higher concentrations, the breakthrough point has a larger slope and its mass transfer zone is smaller. In other adsorption studies, similar results have been obtained46,47. With the increase in the initial concentration of chromium in the adsorption bed, the removal percentage diminishes. However, the output of the bed and adsorption capacity (q) increase, as with elevation of concentration gradient, dispersion coefficient, and mass transfer coefficient, the mass transfer occurs more rapidly45,48,49.Fig. 5The effect of initial Cr(VI) concentration on its adsorption efficiency at the flow rate of 3 L/min and adsorption bed depth of 5 cm.The effect of input flow rate on the efficiency of Cr(VI) adsorptionTo determine the suitable flow rate of the flow introduced into the adsorption bed, flow rates of 1 and 3 L/min were tested (Fig. 6) (the adsorption bed depth is 5 cm and the initial concentration is 10 mg/m3). As can be observed in Fig. 6, the experiments at the flow rate of 1 L/min did not reach the breakthrough point. Thus, it was decided that the experiments would continue at the flow rate of 3 L/min.Fig. 6The effect of input flow rate on Cr(VI) adsorption efficiency at the adsorption bed depth of 5 cm and concentration of 10 mg/m3.The airflow rate is an important parameter in the design of the fixed adsorption column. The effect of airflow rate on the efficiency of Cr(VI) adsorption has been shown in Fig. 6. According to the results of this experiment, at the flow rate of 3 L/min, due to increased airflow rate, shortened retention time and diminished extent of entrapment of Cr(VI) ions by the adsorbent molecules, reaching the saturation point and saturation has occurred more rapidly, thus the slope of the breakthrough curve increases and the removal efficiency decreases. If the retention time of Cr(VI) ions in the adsorption column is not long enough, the ions leave the column before reaching equilibrium.The experiments progressed at the flow rate of 1 L/min up to 85 h, but the curve did not reach the breakthrough point. At lower flow rates, due to a longer contact time, the contaminant has more opportunity to bond with the adsorbent particles, thereby increasing adsorption efficiency. The input flow rate significantly influences the adsorption capacity, such that with an increased flow rate, the extent of adsorption and entrapment of Cr(VI) ions declines. This can be attributed to the fact that chromium retention time in the adsorption bed is short and part of it leaves the bed before reaching the saturation point. Similar results have been obtained by other researchers21,50,51,52,53,54.The effect of adsorption bed depth on the efficiency of Cr(VI) adsorptionCr(VI) adsorption was examined at heights of 2.5, 5, and 10 cm of zeolite adsorbent and with an initial chromium concentration of 10 mg/m3. As shown in Fig. 7, with the increase in the adsorption bed depth, its efficiency increases.Fig. 7Influence of adsorbent height in the column on efficiency of Cr(VI) adsorption in flow rate of 3 L/min and concentration of 10 mg/m3.The amount of adsorbent in the adsorption column determines the number of active and available adsorption sites. According to Fig. 7, the time required for reaching the breakthrough point in the bed with heights of 2.5, 5, and 10 cm is 15, 33, and 69 h, and the time required for reaching the saturation point is 22.5, 69, and 127.5 h, respectively. With elevation of the adsorption bed height, chromium retention time in the column and the time required for reaching the breakthrough point are prolonged, and in turn, bed saturation occurs later. Therefore, the specific surface area of the adsorbent increases, while the ratio of C (output concentration) to C0 (initial concentration) declines, thereby increasing adsorption capacity, which is due to increased specific surface area of the adsorbent and adsorption sites.At lower heights, the dispersion phenomenon is developed in mass transfer, and after the breakthrough point, a sharp rise occurs in the output concentration, and the adsorption capacity diminishes55. With the increase in the adsorption bed in the column, the time of contaminant contact with the adsorbent increases and the breakthrough curve slope decreases, suggesting a wide mass transfer zone at higher heights38,44,56. The results indicate that the optimal efficiency of the adsorbent is obtained with higher heights.As can be observed in Figs. 5, 6, and 7, all these models had a very high correspondence with experimental data for states in which the breakthrough curve started from the origin of coordinates. However, in cases where the curve has had an intercept, the accuracy of predictions diminished. For this reason, in the curves associated with bed height of 2.5 and 5 cm, the model prediction error has increased occasionally.Evaluation of different models of adsorption columnThe results obtained from the different models including the Yoon–Nelson adsorption model, Thomas adsorption model, Buhart–Adams adsorption model, and BDST adsorption model are shown in Tables 4, 5, 6 and 7.Table 4 The results of the Yoon–Nelson adsorption model.Table 5 The results of the Thomas adsorption model.Table 6 The results of the Buhart–Adams adsorption model.Table 7 The results of the BDST adsorption model.As shown in Table 4, with the increase in the initial concentration, KYN and q values increase due to the increased driving force of mass transfer. Further, τ and adsorption efficiency (R) diminished due to the increased rate of bed saturation. With elevation of concentration, more mass of chromium enters the adsorption column, resulting in an increased concentration slope. As a result, adsorption sites are occupied with chromium molecules more rapidly. With the increased height of the adsorption bed, KYN decreases, while τ, q, and R grow due to decreased bed saturation rate, which is in line with other studies35,44,46,55,57.According to Table 5, in the Thomas model, with increased depth of adsorption bed, due to more adsorption sites for the contaminant, KTh decreases, while q (including adsorption capacity of experimental data and adsorption capacity of the model) and R increase. With the increase in the initial concentration of Cr(VI) entering the adsorption bed, adsorption capacity increases, but efficiency and Thomas constant decreases. This is because with elevated concentration, concentration gradient and mass transfer driving force increase, thus adsorption capacity and breakthrough curve slope also increase. However, as the curve slope is obtained by multiplying the concentration by the Thomas constant, the Thomas constant decreases with increased concentration. The high correlation coefficient between experimental data and model prediction represents suitable correspondence between the model and study results. Therefore, it can be probable that according to the assumption of the Thomas model, adsorption has occurred in a monolayer form in this study. These results are in line with the findings of other researchers35,39.As can be seen in Table 6, with the increase in the initial concentration introduced into the adsorption bed, the concentration slope increases, while K and adsorption efficiency (R) diminish due to the increased bed saturation rate. Further, N0 (adsorption capacity per volume unit of the bed) and q increase due to elevation of mass transfer driving force. With the increase in the adsorption bed depth, K and N0 decrease due to the reduction of the breakthrough curve and elevation of the adsorbent volume, respectively, but q and R grow due to the increased number of adsorption sites and diminished saturation rate of the bed. These results are in line with the findings of studies conducted in this regard, which is dominant in the overall kinetics of the external mass transfer system44,55.According to Table 7, in the BDST model, with the increased adsorption bed depth, due to elevation of retention time of the contaminant in the bed and growth of adsorption sites, the coefficient of this model (K), which is a function of curve slope decreases, while q (including adsorption capacity of experimental data and model’s adsorption capacity), N0, and R increase. With the increased initial concentration of chromium (VI) into the adsorption bed, N0 and adsorption capacity increase due to the elevation of mass transfer driving force. However, BDST constant and efficiency decline, which is due to increased saturation rate of the adsorption bed. These results are in line with the findings of other researchers35,39.According to Tables 4, 5, and 7, the information of Yoon–Nelson, Thomas, and BDST models is matched with the data obtained from performing zeolite experiments with a correlation coefficient of 0.9933. These high values represent a high correlation and relationship between experimental data and the data predicted by the mentioned models. The information of the Buhart–Adams model corresponded with the experimental data obtained from zeolite with a lower correlation coefficient (around 0.6677), as it is used only for the first part of the breakthrough curve.Performance of natural zeolite in adsorption of Cr(VI)A summary of the performance of natural zeolite in the adsorption of Cr(VI) is shown in Table 8. It is obvious that with increasing the chromium concentration in the input flow to the adsorption bed, the mass of the consumed adsorbent and the adsorbent’s specific throughput decline. However, with the growth of adsorption bed depth, the specific throughput and mass ratio of chromium to the zeolite increase. Note that in adsorption problems, one of the main parameters regarding comparison of the adsorbent efficiency is adsorption capacity (q), with which one can have a better judgment. As shown in Tables 4, 5, 6, and 7, with the increase in the adsorption bed depth (due to prolongation of retention time and increased number of adsorption sites) and the chromium concentration in the input flow (due to dominance of driving force of adsorption process over resistance to mass transfer), the adsorption capacity grows.Table 8 A summary of the performance of natural zeolite in adsorption of Cr(VI).
