Nitrogen adsorption and desorption studyThe nitrogen adsorption and desorption isotherms are depicted in Fig. 1. Table 1 showcases the specific surface area, pore volume, and average pore size associated with activated carbon and ammonium persulfate-modified activated carbon.Figure 1The adsorption and desorption isotherms of nitrogen.Table 1 Properties of AC and modified AC with ammonium persulfate.The reduction of the average pore size from 2.12 nm to 0.96 nm in modified activated carbon indicates that ammonium persulfate particles were introduced into the activated carbon pores. Also, for this reason, the specific surface area was reduced from 1099 to 974 m2 g−1.On the other hand, during the process of making the modified adsorbent, due to the use of 650 °C heat and the destruction of some surface bands, there is a slight increase in specific surface area. However, the effect of ammonium persulfate particles on the specific surface area change is greater. As a result, the specific surface area of modified AC is lower than that of AC.Infrared spectroscopy studyInfrared spectroscopy was used to investigate the effect of ammonium persulfate on granular AC. The FTIR spectra of granular AC and modified AC are shown in Fig. 2. In the modification process of AC, its surface is oxidized. Therefore, in infrared spectroscopy, the peaks of the oxygen groups already present on the surface increase. For example, the OH group in carboxylic acid structures (in the range of 3500 cm−1) and the C–O–C group in the ether structure (in the range of 1200 cm−1) are among these.Figure 2Infrared spectroscopy (a) granular AC and (b) ammonium persulfate-modified AC.Asymmetric and symmetric stretching vibrations of the methyl group (-CH3-) are also observed at wavelengths of 1379 cm−1 and 1469 cm−1. The peak created in 1633 cm−1 belongs to the carboxylic acid group (C = O). The peaks formed at 2851 and 2922 cm−1 are related to the C-H bond. The peak in the range of 3500 cm−1 is related to the O–H bond. However, the peak belonging to the OH-group in the phenolic structure (1462 cm−1) becomes two peaks, 1379 and 1462 cm−1. After increasing the oxidation time, the phenolic groups become lactone groups 30,31.Effect of pHThe influence of initial pH on the adsorption of Cr(VI) ions was investigated in the range of 2–5. The pH of the solutions was adjusted by a solution of 0.1 mol L−1 HCl and NaOH. The experimental conditions such as adsorbent dosage, shaker speed, initial Cr concentration and time were determined from other studies. The acquired results are displayed in Fig. 3. The results showed that with increasing pH, the adsorption of Cr(VI) decreased. Therefore, the highest Cr(VI) adsorption occurred at a pH of 2.Figure 3Effect of pH on the adsorption of Cr(VI) by ammonium persulfate-modified AC (Agitation time 3 h, Temperature: 298.15 K, Speed: 150 rpm, Cr: 100 mg L-1, Adsorbent mass: 0.03 g, and Solution volume: 10 mL).In aqueous solutions, Cr(VI) is present as CrO42−, Cr2O72−,and HCrO4− species. In acidic solutions with concentrations greater than 500 mg L−1, Cr2O72− is the predominant species. While CrO42− and HCrO4− species are predominant species at pHs 2 to 6 and concentrations less than 500 mg L−132,33.Therefore, according to the concentration and pH of the solution, the species in this range are CrO42- and HCrO4-. Possible ion exchange reactions between the AC surface and different species of Cr(VI) are the following equilibrium reactions34. The C is the symbol of AC in all of the following reactions:$$\underline {C} OH_{2}^{ + } + HCrO_{4}^{ – } \rightleftarrows \underline {C} HCrO_{4} + H_{2} O$$
(4)
$$\underline {C} OH + HCrO_{4}^{ – } \rightleftarrows \underline {C} HCrO_{4} + OH^{ – }$$
(5)
$$\underline {C} OH_{2}^{ + } + CrO_{4}^{2 – } \rightleftarrows \underline {C} CrO_{4}^{2 – } + H_{2} O$$
(6)
$$\underline {C} OH + CrO_{4}^{2 – } \rightleftarrows \underline {C} CrO_{4}^{2 – } + OH^{ – }$$
(7)
Adsorption reactions that can take place between new functional groups on the AC surface and different species of Cr(VI) are as follows17:$$\underline {C} – C = O_{\left( s \right)} + HCrO_{4}^{ – } + H_{{\left( {aq} \right)}}^{ + } \to \underline {C} – C = OH^{ + } \ldots HCrO_{4\left( s \right)}^{ – }$$
(8)
$$\underline {C} – COOH_{\left( s \right)} + CrO_{4}^{2 – } + H_{{\left( {aq} \right)}}^{ + } \to [\underline {C} – COOH_{2}^{ + } ]_{2} ….CrO_{4\left( s \right)}^{2 – }$$
(9)
$$\underline {C} – C = O_{\left( s \right)} + HCrO_{4}^{ – } + H_{{\left( {aq} \right)}}^{ + } \to \underline {C} – C = OH^{ + } ….HCrO_{4\left( s \right)}^{ – }$$
(10)
$$\underline {C} – C = O_{\left( s \right)} + CrO_{4}^{2 – } + H_{{\left( {aq} \right)}}^{ + } \to [\underline {C} – C = OH^{ + } ]_{2} ….CrO_{4\left( s \right)}^{2 – }$$
(11)
$$\underline {C} – OH_{\left( s \right)} + HCrO_{4}^{ – } + H_{{\left( {aq} \right)}}^{ + } \to \underline {C} – OH_{2}^{ + } ….HCrO_{4\left( s \right)}^{ – }$$
(12)
$$\underline {C} – OH_{\left( s \right)} + CrO_{4}^{2 – } + H_{{\left( {aq} \right)}}^{ + } \to [\underline {C} – OH_{2}^{ + } ]_{2} ….CrO_{4\left( s \right)}^{2 – }$$
(13)
The Cr(VI) removal process combines ion exchange and adsorption. Due to the equilibrium reactions of ion exchange and high adsorption, it can be concluded that the more acidic the environment, the higher the adsorption of Cr(VI) species. The obtained results are in good agreement with those above Therefore, other tests were performed at pH = 2.Kinetic study of adsorptionThe impact of time on the adsorption of Cr(VI) ions on the modified AC adsorbent is depicted in Fig. 4. To investigate the adsorption kinetics, Cr(VI) adsorption experiments were performed for a long time (1500 min). Pseudo-first-order kinetics models and pseudo-second-order kinetics models have been investigated to determine the degree of the adsorption kinetics equation. The phenomenon of Cr(VI) adsorption displays a gradual increase over time, followed by a subsequent decrease in the adsorption rate due to the progressive occupation of the active sites on the adsorbent until a state of equilibrium is attained. The findings indicate that a substantial portion, exceeding 90%, of the adsorption process occurs within the initial 300-min timeframe. Therefore, other tests were performed in 300 min.Figure 4Effect of time on the adsorption of Cr(VI) by ammonium persulfate-modified AC (Temperature: 298.15 K, Speed: 150 rpm, Cr: 100 mg L-1, Adsorbent mass: 0.03 g, and Solution volume: 10 mL).The kinetic information regarding the adsorption of Cr(VI) by the adsorbent was examined using the following equations, which employed the pseudo-first-order and pseudo-second-order approaches35,36,37:$${\text{q}}_{{\text{t}}} = {\text{ q}}_{{\text{e}}} { }\left( {1 – {\text{e}}^{{ – {\text{k}}_{1} {\text{t}}}} } \right)$$
(14)
$${\text{q}}_{{\text{t}}} { = }\frac{{{\text{q}}_{{\text{e}}}^{2} {\text{k}}_{2} {\text{t}}}}{{1 + {\text{q}}_{{\text{e}}} {\text{k}}_{2} {\text{t}}}}$$
(15)
where the adsorption capacity at time t and the adsorption capacity at equilibrium time, as well as the kinetic constant, are represented by qt, qe, and k, respectively. The graphs illustrating the pseudo-first-order and pseudo-second-order kinetic models can be observed in Figs. 5 and 6, respectively. Also, Table 2 presents the results and percentage of compliance with these models. The results show that the correlation of Cr(VI) adsorption kinetics with the pseudo-second-order model is much higher than the pseudo-first-order model.Figure 5The pseudo-first-order kinetic model for the adsorption of Cr(VI) on persulfate-modified AC (Temperature: 298.15 K, Speed: 150 rpm, Cr: 100 mg L-1, Adsorbent mass: 0.03 g, and Solution volume: 10 mL).Figure 6The second-order kinetic model for the adsorption of Cr(VI) on persulfate-modified AC (Temperature: 298.15 K, Speed: 150 rpm, Cr: 100 mg L-1, Adsorbent mass: 0.03 g, and Solution volume: 10 mL).Table 2 The parameters of the pseudo-first-order and pseudo-second-order kinetic model of Cr(VI) adsorption on ammonium persulfate-modified AC (Temperature: 298.15 K, Speed: 150 rpm, Cr: 100 mg L−1, Adsorbent mass: 0.03 g, and Solution volume: 10 mL).Thermodynamic study of adsorptionTo determine the thermodynamics of Cr(VI) adsorption with the prepared adsorbent, the values of enthalpy change (ΔHo), entropy change (ΔSo), and Gibbs free energy change (ΔGo) were obtained. The experiments were performed at temperatures of 298.15 to 338.15 K. The Van’t Hoff plots are shown in Fig. 7, and the thermodynamic parameters are obtained from the following Eqs.38,39,40:$$\text{ln}{K}_{d}=-\frac{\Delta \text{H}}{\text{RT}}+ \frac{\Delta \text{S}}{\text{R}}$$
(16)
$${\Delta \text{G}}^{o}={ \Delta \text{H}}^{o}-{\text{T }\Delta \text{S}}^{o}$$
(17)
where R, T, and Kd are the universal gas constant (kJmol-1K-1), the absolute temperature (K), and the distribution coefficient (cm3/g), respectively41. The ΔHo and ΔSo values are determined by drawing ln Kd vs. 1/T. The results are displayed in Table 3.Figure 7The logarithm variations of the distribution coefficient versus the 1/T for Cr(VI) adsorption on persulfate-modified AC (Agitation time 5 h, Speed: 150 rpm, Cr: 100 mg L-1, Adsorbent mass: 0.03 g, and Solution volume: 10 mL).Table 3 Thermodynamic parameters of Cr(VI) adsorption on persulfate-modified AC (Agitation time 5 h, Speed: 150 rpm, Cr: 100 mg L−1, Adsorbent: 0.03 g, and Solution volume: 10 mL).The value of ΔHo was calculated at 18.28 kJ mol-1. The positive value of ΔHo shows that Cr(VI) adsorption is endothermic, and the adsorption will enhance with increasing temperature. The value of ΔSo is 0.06 J K−1mol−1, which shows that the Cr(VI) adsorption is random 42. The ΔGo values in this temperature range are negative, and the adsorption reaction is spontaneous. The value of ΔGo is in the range of -20 to 0 kJ; it can be said that Cr(VI) adsorption on persulfate-modified AC is physical adsorption.Effect of ionic strengthThe impact of ionic strength on the adsorption capacity of Cr(VI) was investigated. The results are shown in Fig. 8. The results showed that the adsorption of the Cr(VI) ion on ammonium persulfate-modified AC did not change significantly with increasing ionic strength. As a result, sodium and increased ionic strength have almost no effect on the adsorption of Cr(VI) ions.Figure 8Effect of ionic strength on the adsorption of Cr(VI) by ammonium persulfate-modified AC (Agitation time 5 h, Temperature: 298.15 K, Speed: 150 rpm, Cr: 100 mg L-1, NaCl: 0.0001–0.1 mol L-1, Adsorbent mass: 0.03 g, and Solution volume: 10 mL).Effect of initial ion concentration (Isotherm Study)The effect of the initial concentration of Cr(VI) in the feed solution in the range of 5 to 600 mg L-1 on the adsorption was investigated. The Freundlich and Langmuir adsorption isdotherm models have been used to investigate the adsorption process. The mathematical expressions of the Freundlich and Langmuir adsorption isotherm models are expressed in the following Eqs.43,44,45:$${\text{q}}_{e} = {\text{K}}_{F} {\text{C}}_{e}^{\frac{1}{n}}$$
(18)
$${\text{q}}_{{\text{e}}} = \frac{{{\text{q}}_{{\text{L}}} {\text{K}}_{{\text{L}}} {\text{C}}_{{\text{e}}} }}{{1 + {\text{K}}_{{\text{L}}} {\text{C}}_{{\text{e}}} }}$$
(19)
where the qe and qL represent the adsorption capacity at equilibrium and the maximum adsorption capacity according to the Langmuir isotherm, respectively. KF and KL are the model’s constants. The extent of deviation from linearity in adsorption is indicated by n.The experimental results of Cr(VI) adsorption based on the Freundlich and Langmuir linear models of adsorption are shown in Figs. 9 and 10. The results of the equilibrium data modeling indicated that the data fitting using the Langmuir model (R2 = 0.98) is superior to that of the Freundlich model (R2 = 0.88). So, adsorption will be monolayer, and the adsorbent surface is homogeneous.Figure 9Freundlich adsorption isotherm plot for the adsorption of Cr(VI) on persulfate-modified AC (Agitation time 5 h, Temperature: 298.15 K, Speed: 150 rpm, Cr: 5–600 mg L-1, Adsorbent mass: 0.03 g, and Solution volume: 10 mL).Figure 10Langmuir adsorption isotherm plot for adsorption of Cr(VI) on persulfate-modified AC (Agitation time 5 h, Temperature: 298.15 K, Speed: 150 rpm, Cr: 5–600 mg L-1, Adsorbent mass: 0.03 g, and Solution volume: 10 mL).Variations of the dimensionless equilibrium parameter (RL) versus the initial concentration are shown in Fig. 11. The RL values between 0 and 1 indicate the desirability of the adsorption process. With the increase in ion concentration, the value of RL will be smaller, and the adsorption process will be more favorable. The findings presented in Table 4 showed that 108.69 mg g-1 is the maximal Cr(VI) adsorption capacity on persulfate-modified AC adsorbent.Figure 11Variations of the dimensionless equilibrium parameter (RL) versus the initial concentration (Agitation time 5 h, Temperature: 298.15 K, Speed: 150 rpm, Cr: 5–600 mg L-1, Adsorbent mass: 0.03 g, and Solution volume: 10 mL).Table 4 The Langmuir and Freundlich isotherm parameters for Cr(VI) adsorption with persulfate-modified AC (Agitation time 5 h, Temperature: 298.15 K, Speed: 150 rpm, Cr: 5–600 mg L−1, Adsorbent mass: 0.03 g, and Solution volume: 10 mL).Table 5 compares the adsorption of Cr(VI) by the persulfate-modified AC adsorbent and other types of AC adsorbents. This comparison shows that the adsorption properties are intensified with the modification of AC by persulfate. So, the ability to absorb Cr(VI) by this adsorbent is more than twice that of other AC adsorbents. High adsorption capacity and simple preparation method showed that ammonium persulfate modified-activated carbon adsorbent can be a promising Cr adsorbent.Table 5 Comparison of Cr(VI) adsorption by persulfate-modified AC adsorbent with other types of AC adsorbents.
Intensification of Cr(VI) adsorption using activated carbon adsorbent modified with ammonium persulfate
