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ABSTRACT
In this study, the mechanochemical treatment with and without reagent (citric acid) was presented as a new modification method to improve the adsorptive capacity of corncob in the removal of Cu2+, Pb2+ and Zn2+ heavy metal ions from aqueous solutions. A high-energy planetary ball mill was used for modification. The parameters affecting both reagent-assisted (CA-MCM)/non-reagent (MCM) modifications and adsorption effectiveness were investigated via the response surface methodology approach. As a result of optimisation, over 95% heavy metal removals were achieved for both modified materials. To explain the adsorption mechanism; isothermal, kinetic, thermodynamic and activation parameters were examined. The pseudo-second-order and Langmuir isotherm models were found to be appropriate for all adsorbents and heavy metals. According to the thermodynamic parameters, the adsorption processes were spontaneous and endothermic. Desorption studies were conducted to determine the reusability of the adsorbents. Ternary adsorption system of metals was also investigated. Characterisation studies were carried out using Fourier Transform Infrared Spectrometer (FTIR), Scanning Electron Microscopy (SEM), Brunauer–Emmett–Teller (BET), and surface analysis (point of zero charge (pHpzc), Boehm titration) to investigate the effects of modifications on the adsorbents’ surface and chemical structure. In addition, the financial advantages of the modification methods were demonstrated by alternative selection and cash flow analyses. All the results revealed that this method as a low-cost and eco-friendly alternative modification treatment to increase the adsorption efficiency of agricultural waste of similar nature.
Abbreviations and symbols
| Analysis of variance | = | ANOVA |
| Attenuated total reflectance | = | ATR |
| Average costs | = | AC (€) |
| Average fixed costs | = | AFC (€) |
| Average variable costs | = | AVC (€) |
| Ball/Material ratio | = | B/RM (g/g) |
| Brunauer–Emmett–Teller surface area | = | BET |
| Box-Behnken design | = | BBD |
| Central composite design | = | CCD |
| Citric acid modified corncob | = | CA-MCM-CC |
| Corncob | = | CC |
| Degrees of freedom | = | DF |
| Flame atomic absorption spectrometer | = | FAAS |
| Fourier transform infrared spectroscopy | = | FTIR |
| Marginal expenses | = | TC (€) |
| Material/Reagent ratio | = | M/R (g/g) |
| Mean of squares | = | MS |
| Mechanochemical modification | = | MCM |
| Mechanochemical modified corncob | = | MCM-CC |
| Mechanochemical modification with citric acid | = | CA-MCM |
| pH value of point zero charge | = | pHpzc |
| Response surface methodology | = | RSM |
| Rotational speed | = | RS (rpm) |
| Scanning electron microscopy | = | SEM |
| Standard deviation | = | SD |
| Sum of squares | = | SS |
| Total of fixed expenses | = | FC (€) |
| Variable expenses | = | VC (€) |
| Variance inflation factor | = | VIF |
Symbols
| Absorbent amount | = | m (g) |
| Activation energy | = | Ea (kJ/mol) |
| Adjusted coefficient of determination: | = | R2 (adj) |
| Amount of adsorbed metal on the unit adsorbent | = | qe (mg/g) |
| Amount of solute sorbed per amount of sorbent | = | qe,cal (mg/g) |
| Boltzmann constant | = | kB (J/K) |
| Coefficients of the cut: | = | β0, βi: (i = 1, 2, . k) |
| Coefficient of determination | = | R2 |
| Dubinin–Radushkevich isotherm constant | = | K’ (mol2/J2) |
| Dubinin–Radushkevich maximum adsorption capacity | = | Xm (mg/g) |
| Enthalpy changes for activation | = | ΔH# (kJ/mol) |
| Entropy changes for activation | = | ΔS# (kJ/mol.K) |
| Free energy changes | = | ΔG° (kJ/mol) |
| Freundlich adsorption capacity | = | KF (mg/g) |
| Freundlich adsorption intensity | = | n |
| Gibbs energy changes for activation | = | ΔG# (kJ/mol) |
| Initial concentration of solution | = | Co (mM) |
| Input variables that are effective on the response | = | x1, x2 . xk |
| Langmuir isotherm constant | = | KL (L/g) |
| Langmuir maximum monolayer coverage capacity | = | Qm (mg/g) |
| Langmuir separation factor | = | RL |
| Linear, square and interaction constants | = | ii and ij (i = 1, 2,. . k; j = 1, 2,. . k) |
| Metal concentration in solution after adsorption | = | Ce (mg/L) |
| Month | = | Q |
| Planck constant | = | h (J/s) |
| Predicted coefficient of determination | = | R2(pred) |
| Pseudo first-order sorption rate constant | = | k1,ad |
| Pseudo second-order sorption rate constant | = | k2,ad |
| Probability value | = | P |
| Proportional to the boundary layer thickness | = | C’ |
| Random error | = | ε |
| Removal percentage | = | R % |
| Response | = | y |
| Rotation per minute: | = | rpm |
| Standard enthalpy changes | = | ΔH° (kJmol) |
| Standard entropy changes | = | ΔS° (kJ/mol) |
| Significance level in hypothesis test | = | α (alpha) |
| Temkin constant related to heat of sorption | = | B |
| Temkin isotherm equilibrium constant | = | AT (L/g) |
| The intraparticle diffusion rate constant | = | Ki (mg/g.min5) |
| Time | = | t (min) |
| Volume of metal solution | = | V (L) |
Acknowledgments
The authors are very grateful to the Scientific and Technological Research Council of Turkey (TUBITAK) for supporting this research [Grant Number 118Z038]. Thanks also the METU central laboratory for BET and SEM analyses.
Disclosure statement
No potential conflict of interest was reported by the authors.
Supplemental material
Supplemental data for this article can be accessed here.