#### Characterization of Sorbents

The values of proximate analysis and physico-chemical properties show gain in amorphous character (Table 1). The formation mechanism of MZBFA from BFA has been proposed as follows: dissolution of glass phase (aluminum-silicate), into the alkaline solution, decomposition of aluminosilicate gel as zeolite precursor, and crystallization of zeolite. In this study, mechanism of zeolization is mainly separated into three terms. The first step is dissolution of SiO_{2} and Al_{2}O_{3} into the alkaline solution at 0–15 min. After 15 min, the alumina and silicate ions are condensed to form an alumina-silicate gel, which is prematerial of zeolite crystal covering the outer surface of BFA particles. The intermediate gel begins to change into zeolite via dissolution-reprecipitation process at 40 min, and increase in Na_{2}O contents of MZBFA is caused by captured of sodium ions to neutralize the negative charge on aluminate in zeolite structure when zeolite crystal is occurred [19]. The particle size of both sorbents was determined by Mastersizer 2000 particle size analyzer. The particle size distributions specify that majority of particles lies below 58.55 µm (95%) in case of MZBFA, while in BFA majority of particles lies below 150.62 µm (95%). BET surface area for MZBFA (328.30 m^{2} g^{−1}) has been increased significantly after treatment as compared with virgin BFA (99.14 m^{2} g^{−1}). Point of zero charge (pH_{pzc}) for a given mineral surface is the pH at which surface has net neutral charge. The significance of this kind of plot show that a given mineral surface will have positive charge at solution pH values less than the point of zero charge and thus be a surface on which anions may adsorb. However, the mineral surface will have negative charge at solution pH values greater than point of zero charge. The pH_{pzc} values obtained by mass titration method are 8.18 and 9.09 pH for BFA and MZBFA, respectively.

Table 1. Physico-chemical properties of BFA and MZBFA.Characteristics | Obtained values |
---|

BFA | MZBFA |
---|

Proximate analysis |

Loss on drying (%) | 11.95 ± 0.2 | 13.74 ± 0.2 |

Moisture content (%) | 10.36 ± 0.3 | 12.25 ± 0.3 |

Ash content (%) | 72.85 ± 0.2 | 67.74 ± 0.2 |

Physico-properties |

Specific density | 1.888 ± 0.02 | 2.036 ± 0.02 |

Bulk density (g cc^{−1}) | 1.725 ± 0.02 | 1.983 ± 0.02 |

Dry density (g cc^{−1}) | 1.081 ± 0.02 | 1.225 ± 0.02 |

Void ratio | 0.747 | 0.662 |

Porosity, fraction | 0.428 | 0.398 |

pH_{pzc} | 8.18 ± 0.05 | 9.09 ± 0.05 |

BET Surface area | 99.14 | 328.30 |

Chemical constituents |

SiO_{2}% | 46.35 | 43.54 |

Al_{2}O_{3}% | 19.99 | 18.90 |

Fe_{2}O_{3}% | 5.89 | 2.99 |

CaO% | 4.97 | 3.17 |

MgO% | 4.83 | 4.12 |

Na_{2}O% | 4.17 | 7.24 |

K_{2}O% | 3.74 | 2.35 |

#### FTIR Analysis

FTIR spectra of BFA and MZBFA (Figure not shown) exhibit a broad band at about 3400 cm^{−1} indicating the presence of OH group of the silanol (SiOH). The band observed at 1033.97, 676.67, and 475.33 cm^{−1} can be ascribed to asymmetric, symmetric stretching vibration, and the bending vibration of internal tetrahedral, TO_{4} (where T = Si, Al), respectively. All these bands are more or less dependent on the crystal structure. The band at 1097.31 cm^{−1} of BFA was shifted to 1033.97 cm^{−1} in MZBFA confirms the tetrahedral coordination of aluminum in the zeolite framework. In MZBFA, the shifting symmetric stretching band 797.53 to 792.61 cm^{−1} of internal tetrahedral (TO_{4}) of amorphous aluminosilicates formed by the reaction of dissolved Si^{+4} and Al^{+3} confirm the formation of zeolite phases [18, 24]. The amount of improved tetrahedral sites of the aluminosilcate framework of the zeolite can be enlightened by decreased in frequency of asymmetric stretching vibration of tetrahedral. The band at about 1646.21 and 1456.34 cm^{−1} are belongs to the bending vibration of water molecules [18].

#### Sorption Isotherm Studies

The efficiency and nature of the sorption on the sorbent can be evaluated from the sorption isotherms. To obtain experimental equilibrium sorption data were then compared with the sorption isotherm models. Four models were used: Langmuir (Eq. (3)), Freundlich (Eq. (4)), Dubinin-Redushkwich (Eq. (5)), and Temkin (Eq. (8)) sorption isotherm. The models for characterization of equilibrium distribution relate the quantity *q*_{e} (mg g^{−1}) as a function of concentration at a fixed temperature.

- (3)

where *q*_{m} is the amount sorbed (mg g^{−1}), *C*_{e} is the equilibrium concentration of the sorbate (mg L^{−1}), and *Q*_{0} and *B* are the Langmuir constants related to maximum sorption capacity and energy of sorption, respectively.

The linear regression lines obtained for Langmuir isotherm graphs of *C*_{e}/*q*_{e} against *C*_{e}, gave highly significant correlation coefficient values closer to unity (Table 2). The value of dimensionless parameter, *R*_{L}, was less than unity (Table 2) which manifest that the sorption is favorable under the applied conditions. The comparatively smaller value of *R*_{L} for sorption by MZBFA than BFA indicates sorption to be more feasible.

- (4)

where *q*_{e} is the amount sorbed (mg g^{−1}), *C*_{e} is the equilibrium concentration of the sorbate (mg L^{−1}), and *K*_{f} and *n* are Freundlich constants related to sorption capacity and sorption intensity, respectively.

Table 2. Isotherm parameters for the sorption of PSM on BFA and MZBFA.Isotherms | Sorbents | Parameter values |
---|

Langmuir | | *q*_{m} (mg g^{−1}) | *B* (dm^{3} mg^{−1}) | *R*_{L} | *R*^{2} |

BFA | 34.483 | 0.023 | 0.261 | 0.968 |

MZBFA | 35.714 | 0.112 | 0.067 | 0.991 |

Freundlich | | *K*_{f} (dm^{3} g^{−1}) | *n* | | *R*^{2} |

BFA | 14.355 | 2.262 | | 0.993 |

MZBFA | 206.538 | 3.922 | | 0.927 |

D-R | | *X*_{m} (mg g^{−1}) | *β* (mol^{2} J^{−2}) | *E* (kJ mol^{−1}) | *R*^{2} |

BFA | 14.225 | 2.08 × 10^{−4} | 0.490 | 0.915 |

MZBFA | 27.716 | 1.64 × 10^{−5} | 1.746 | 0.903 |

Temkin | | *K*_{T} (dm^{3} mg^{−1}) | *B*_{1} | | *R*^{2} |

BFA | 0.228 | 7.757 | | 0.965 |

MZBFA | 2.821 | 5.646 | | 0.950 |

The Freundlich isotherm is derived by assuming a heterogeneous surface with a nonuniform distribution of heat of sorption over the surface. The Freundlich constants *K*_{f} and *n* were calculated from the slope and intercept of the linear plot ln *q*_{e} versus ln *C*_{e}. The values of heterogeneity factor, *n*, obtained from the slope were >1 would indicate conformity of the data to multilayer formation at the sorbent surface. Table 2 shows that the value of *n* for the sorption of PSM was higher for MZBFA than BFA demonstrate the higher sorption of PSM on MZBFA.

- (5)

where *q*_{e} is the amount sorbed (mg g^{−1}), *X*_{m} is D-R monolayer capacity, is the activity coefficient related to mean sorption energy, and *ε* is Polanyi potential, which is equal to

- (6)

where *R* is gas constant (J K^{−1} mol^{−1}), *T* is temperature (Kelvin), and *C*_{e} is the equilibrium concentration of the sorbate (mg L^{−1}). When ln *q*_{m} is plotted against *E*^{2}, a straight line is obtained. The slope of the plot gives the value of and the intercept yields the value of sorption capacity, *X*_{m}. The value of is related to sorption energy, *E*, via following relationship:

- (7)

The Dubinin Redushkwich isotherm model applied to test a pore filling mechanism in micropores of the sorbent, rather than layer-by-layer formation of a film on the walls of the pores. The sorption energy, *E*, of the process was calculated using the value of , from that it can be deduced that the sorption mechanism is either ion-exchange or physical in nature. The sorption process follows ion-exchange process when the magnitude of *E* is between 8 and 16 kJ mol^{−1}, while it is of a physical nature if the values of *E* < 8 kJ mol^{−1}. The observed values of sorption energy, *E*, were < 8 kJ mol^{−1} (Table 2), which manifests the sorption of PSM in the studied sorbate-sorbent systems is to be physical in nature.

Temkin isotherm contains a factor that explicitly takes into account sorbate species-sorbent interactions. This isotherm assumes that: (i) the heat of sorption of all the molecules in the layer decreases linearly with coverage due to sorbate species–sorbent interactions and (ii) sorption is characterized by a uniform distribution of binding energies, up to some maximum binding energy.

- (8)

where *B*_{1} = RT/*b* and *K*_{T} are the constants. *K*_{T} is the equilibrium binding constant (L mol^{−1}) corresponding to maximum binding energy and constant *B*_{1} is related to the heat of sorption. A plot of *q*e versus ln *C*_{e} enables the determination of the isotherm constants *K*_{T} and *B*_{1}.

Temkin considered the effects of indirect sorbate-sorbate interactions on sorption isotherms. The values of *K*_{T} and *B*_{1} obtained from the Temkin plots of *q*_{e} versus ln *C*_{e}, are shown in Table 2. The heat of sorption of all the molecules on the sorbent surface layer decreases linearly with coverage due to sorbent-sorbate interactions. So, the sorption of PSM on sorbent can be characterized by a uniform distribution of the binding energies, up to some maximum binding energy. The sorption equilibrium data of PSM on BFA follows the order of Freundlich > Langmuir ≥ Temkin > D-R and on MZBFA the order is Langmuir > Temkin > Freundlich > D-R.

#### Kinetic Studies

To define the sorption kinetics of PSM, the parameters for the sorption process were studied for contact times ranging between 0 and 24 h by monitoring the percent removal of the PSM by the sorbent. The data were then regressed against the Lagergren equation, which represents a pseudo-first-order kinetic (Eq. (3)) and against a pseudo-second-order kinetic (Eq. (4)).

- (9)

- (10)

where, *q*_{t} (mg g^{−1}) is amount of PSM sorbed at time *t* (min), *k*_{f} is the rate constant (min^{−1}) and *k*_{s} is the corresponding kinetic constant (g mg^{−1} min^{−1}) are the rate constants of the pseudo-first-order and pseudo-second-order kinetics equations, respectively. The slopes and intercepts of these curves were used to determine the values of *k*_{f} and *k*_{s}, as well as the equilibrium capacity (*q*_{e}). The calculated value of *q*_{e} (Table 3) from the pseudo-first-order kinetics model were well below the monolayer capacities found by Langmuir equilibrium isotherm model, suggesting the sorption process is not a true first order reaction. However, the linearized pseudo-second-order kinetics model (Figure 6 and Table 3), provided much better *R*^{2} values than those for the pseudo-first-order model. The results obtained reveal that the initial sorption rate (“*h*” value) was highest for sorption of PSM by MZBFA than that by BFA and a somewhat complex mechanism of sorption instead of single step process. As a result, the sorption system appears to follow pseudo-second-order kinetics.

Table 3. Kinetic parameters for the removal of PSM by BFA and MZBFA.**Pseudo-first order** |

| *q*_{e} (mg g^{−1}) | *k*_{f} (min^{−1}) | | *R*^{2} |

BFA | 7.261 | 6.610 × 10^{−3} | | 0.912 |

MZBFA | 8.318 | 6.425 × 10^{−3} | | 0.921 |

**Pseudo-second order** |

| *q*_{e} (mg g^{−1}) | *k*_{s} (g mg^{−1} min^{−1}) | *h* (mg g^{−1} min^{−1}) | *R*^{2} |

BFA | 21.277 | 2.110 × 10^{−3} | 0.045 | 0.999 |

MZBFA | 32.258 | 2.155 × 10^{−3} | 0.070 | 0.999 |

**Bangham** |

| *α* | *k*_{0} (g) | | *R*^{2} |

BFA | 0.246 | 0.813 | | 0.933 |

MZBFA | 0.231 | 1.855 | | 0.944 |

**Intraparticle diffusion** |

| *k*_{id,1} (mg g^{−1} min^{−1/2}) | *I*_{1} (mg g^{−1}) | | *R*^{2} |

BFA | 0.997 | 7.032 | | 0.962 |

MZBFA | 1.243 | 15.698 | | 0.979 |

**Intraparticle diffusion** |

| *k*_{id, 2} (mg g^{−1} min^{−1/2}) | *I*_{2} (mg g^{−1}) | | *R*^{2} |

BFA | 0.185 | 16.339 | | 0.903 |

MZBFA | 0.231 | 26.744 | | 0.951 |

The kinetic data were further used to learn about the slow step occurring in the present sorbent system using Bangham's equation [34]. The applicability of Bangham's equation to the present pesticide sorption studies was tested. The values of constants *α* (<1) and *k*_{0} are given in Table 3. The double logarithmic plot according to above equation did not yielded perfect linear curves, indicate that the diffusion of sorbate into pores of the sorbent is not the only rate controlling step.

Intraparticle diffusion was characterized using the relationship between specific sorption (*q*_{t}) and the square root of time (*t*^{1/2}). This relation is expressed as follows:

- (11)

where *q*_{t} is the quantity of PSM sorbed at time *t* (mg g^{−1}), *k*_{id} is the intraparticle diffusion rate constant (mg g^{−1} min^{−1/2}) and *I* is the intercept (mg g^{−1}). Value of *I* (Table 3) give an idea about the thickness of the boundary layer, that is, the larger the intercept, the greater is the boundary layer effect [35]. The deviation of straight lines from the origin (Figure 6) may be because of the difference between the rate of mass transfer in the initial and final stages of sorption. Further, such deviation of straight lines from the origin indicates that the pore diffusion is not the sole rate-controlling step [36] as shown earlier by Bangham's equation. From Figure 6, it may be seen that there are two separate regions–the initial portion is attributed to the bulk diffusion and the linear portion to intra-particle diffusion [37]. The lower values of *k*_{id, 2} than *k*_{id, 1} (Table 3) signify that PSM diffuses into the pores of the sorbents. As the diffusion resistance increases with time the diffusion rate decreases, thus, sorption is a multistep process involving transport of PSM to the surface of the sorbents followed by diffusion into the interior of the pores.

#### Removal of Pesticide from Wastewater

To test the efficiency of removal organophosphorus pesticide from real environmental samples, an agrochemical wastewater sample was collected from the effluent of a rice field, near Ankleshwar city, India. This agro-wastewater was mainly contaminated with PSM and the concentration of pesticide in the original water was measured to be 147 mg L^{−1}. The contaminated water sample (50 mL) was placed in a 100 mL brown colored glass bottles. To study the application of the developed sorption system batch experiments, within optimized conditions were carried out with both the sorbents. It has been observed that 41 ± 1% and 69 ± 1% of PSM were removed by BFA and MZBFA, respectively. However, the amount of PSM removed from wastewater was less than the amount sorbed from aqueous solutions, which is due to competitive sorption of possible contaminants in wastewater. Briefly, the reported method is effective, selective, and sensitive for the removal of PSM, and it can be used for their removal from wastewater.