Chemicals and stock solution preparation

Analytical grade chemicals were purchased and used without further alteration. A stock solution of 1000 mg/L of Cr(VI) was prepared via dissolving 2.8255 g of K₂Cr₂O₇ salt in 1000 mL of distilled water from which 10, 20, 40, and 60 mg/L of Cr(VI) were prepared by diluting with distilled water. 0.1 M of either NaOH or HCl was used to adjust the solution pH.

Adsorbent preparation

Unwanted impurities were removed from collected hulls of Garcinia kola via thorough washing with tap water and rinsing with distilled water. The retained water was removed by sun drying for 7 days and subsequent oven-drying for 48 hours at 105°C. Particle size of less than 150 µm was obtained from the mechanically milled Garcinia kola hulls via an electric sieve shaker. Alkaline hydrolysis of the sieved particles (200 g) was conducted using 500 mL of 10% NaOH in a round bottom flask at 120 rpm for 3 hours. The particles were thoroughly washed with distilled water till neutrality, filtered and then dried in an oven at 105°C for 24 hours. The filtrate was treated with phenolphthalein to examine the neutrality level. The NaOH-modified Garcinia kola hull particles (cMGK-HP), used as adsorbent, was then stored in a desiccator.

Batch adsorption step

The batch adsorption process was conducted at different parameters’ levels (stated in Table 1) on a shaker (Stuart heat-stirrer SB162, China) operated at 160 rpm. Specified adsorbent dose was mixed with 100 mL of Cr(VI) salt solution in a 250-mL flask for the batch experiments. The filtrate and the residue were separated using Whatman filter paper and Cr(VI) concentration was measured via Atomic Adsorption Spectrophotometer (Buck scientific model 210 VGP, USA) operated at a maximum wavelength of 540 nm. A standard solution of 1,5-diphenylcarbazide in acetone was prepared. The percentage of Cr(VI) removed from aqueous solution (X, %) and adsorbent sorption capacity (q_e, mg/g) were determined using equations (1) and (2) respectively.

(1)

(2)

where C_o and C_e = initial and final concentrations of Cr(VI) (mg/L) respectively, V = solution volume (mL) and W = adsorbent weight (g).

Table 1.

Experimental parameters and their values at different levels.

Taguchi design of experiment

For model development and measurement of contributory parameters influence on Cr(VI) sequestration from aqueous solution by cMGK-HP, factorial Taguchi orthogonal array of Design Expert 12 was used. Table 1 presents the experimental parameters used and their values at different levels.²¹ In all, 16 experimental runs were conducted for 5 parameters having values at 4 different levels.

Regeneration and reusability of cMGK-HP

Regeneration of used cMGK-HP was conducted by washing the adsorbent thoroughly with distilled water until neutrality is attained. The washed cMGK-HP was then dried at 80°C for 6 hours. The reusability efficiency of cMGK-HP for Cr(VI) sequestration from aqueous solution was investigated via conducting adsorption-desorption experiments consecutively. For the first adsorption experiment, 0.5 g of Cr(VI) ions loaded cMGK-HP was added to 50 mL of 0.1 M HNO₃ solution in a flask for 2 hours at 80°C in order to increase the oxygen-containing functional groups of the regenerated adsorbent.²² The regenerated cMGK-HP was re-used consecutively for 5 times and the method prescribed by Giri et al²³ was used to calculate the desorption efficiency.

Characterization

The adsorbent was characterized before and after the hexavalent chromium removal using Brunauer-Emmett-Teller method (Nova 11.03A, USA version), scanning electron microscope (SEM/EDX-JEOL-JSM 7600F) and Fourier transform infrared spectrophotometer (Nicolet iS10).

Taguchi design of experiment, mathematical model, and statistical analysis

The response at different values of parameters as obtained from Taguchi experimental design is presented in Table 2. All the experimental runs revealed Cr(VI) ion sequestration from solution by cMGK-HP justifying the effectiveness of the adsorbent for the process. At pH, contact time, adsorbent dose, Cr(VI) initial concentration and temperature of 2, 120 minutes, 8 g/L, 10 mg/L and 60°C respectively, 97.27% of Cr(VI) sequestration from solution by cMGK-HP was observed. This favorable removal efficiency could be attributed to low initial concentration of Cr(VI) and maximum mixing time, temperature and adsorbent dosage. For prediction purpose, equation (3) was generated using the Taguchi orthogonal array and was used to determine the predicted values of Cr(VI) ion sequestration presented in Table 2. A correlation coefficient value of .9937 (close to unity), as shown in Figure 1, between the experimental and predicted values justifies the effectiveness of the model for prediction.

(3)

Table 2.

Design of experiment by Taguchi indicating parameters’ variables and response.

Figure 1.

Correlation plot of experimental and predicted values.

Statistical analysis by ANOVA

Table 3 presents the analysis of variance (ANOVA) result for the sequestration process. ANOVA revealed the developed model was significant. The examined parameters were significant to the study with pH exhibiting the highest significant term with P-value and F-value of .0065 and 39.81 respectively. A model term is significant when its P-value is less than .05. Model term becomes highly significant at lower P-value. The percentage contribution of each parameter is calculated using equation (4). The order of experimental parameter contribution to the Cr(VI) ion sequestration from solution by cMGK-HP was pH >adsorbent dose > initial concentration > contact time > temperature with respective percentage of 46.83, 35.19, 10.27, 4.10, and 3.60.

(4)

such that SS_i = Sum of square of significant parameter.

Table 3.

Analysis of Variance (ANOVA) for Cr(VI) sequestration by cMGK-HP.

Single factor influence on hexavalent chromium ion sequestration

The influence of time, adsorbent dose, pH, Cr(VI) ion initial concentration and temperature on hexavalent chromium ion sequestration are presented as Figure 2. The percentage of Cr(VI) ion removed increased from 78.38% to 98.91% (Figure 2a) when the contact time was increased from 20 to 120 minutes with the attainment of equilibrium after 60 minutes. The bulk of adsorption noticed before this time resulted from higher forces of attraction between the active sites on adsorbent surface and the Cr(VI) ions.¹⁰ As the adsorbent dose was increased from 2 to 12 g/L, the percentage of Cr(VI) ion removed increased from 84.81% to 96.25% (Figure 2b). Equilibrium was attained at adsorbent dose of 8 g/L due to the saturation of the available active sites on adsorbent surface. Increasing solution pH from 2.0 to 12.0 decreased the percentage of hexavalent chromium ion removed from 96.25% to 16.77% (Figure 2c). This observation resulted from active sites dissociation on adsorbent surface due to increase in the concentration of Cr₂O₇²⁻ as the pH deviates from acidic to basic.²¹ Increase in the initial concentration of hexavalent chromium ion from 10 to 60 mg/L causes a reduction in the percentage of hexavalent chromium ion removed from 91.91% to 49.03% (Figure 2d) by cMGK-HP due to reduction in the number of active sites on adsorbent surface. Increasing the temperature from 20°C to 60°C increases the percentage of Cr(VI) ion sequestration from 83.58% to 93.42% (Figure 2e) which could be linked to increase in the convective movement of adsorbent particles due to temperature increase.²⁴

Figure 2.

Influence of (a) time, (b) adsorbent dose, (c) pH, (d) Cr(VI) ion initial concentration, and (e) temperature on hexavalent chromium ion sequestration.

3D surface plot

Figure 3 depicts the combinatory 3D-surface plots of pH and contact time (Figure 3a), pH and adsorbent dose (Figure 3b), pH and initial concentration (Figure 3c), pH and temperature (Figure 3d), contact time and adsorbent dose (Figure 3e), contact time and initial concentration (Figure 3f), contact time and temperature (Figure 3g), adsorbent dose and initial concentration (Figure 3h), adsorbent dose and temperature (Figure 3i), and initial concentration and temperature (Figure 3j) on the percentage of Cr(VI) removed from solution using cMGK-HP. The percentage of Cr(VI) removed in solution remained constant as the temperature increases with pH (Figure 3a). The percentage of Cr(VI) removed increased as the adsorbent dose was increased but decreased with increase in solution pH (Figure 3b). Decrease in Cr(VI) removal was noticed as the initial concentration and pH were increased (Figure 3c). The percentage of Cr(VI) removed decreased with increase in pH (Figure 3d). As shown in Figure 3e, the percentage of Cr(VI) removed was constant with increase in contact time but increase with increase in adsorbent dose. A decrease in the percentage of Cr(VI) removed was noticed with increase in initial concentration but remain constant with increase in contact time (Figure 3f). The percentage of Cr(VI) removed remained relatively constant with increase in temperature and contact time (Figure 3g). Figure 3h revealed decrease in Cr(VI) removed from solution as the initial concentration was increased and vice versa for adsorbent dose. The Cr(VI) removed from solution remained relatively constant with increase in temperature but increased with increase in adsorbent dose (Figure 3i). The percentage of Cr(VI) was relatively constant with increase in temperature but decreased with increase in initial concentration.

Figure 3.

3D-Surface plot showing the combinatory effect of (a) pH and contact time, (b) pH and adsorbent dose, (c) pH and initial concentration, (d) pH and temperature, (e) contact time and adsorbent dose, (f) contact time and initial concentration, (g) contact time and temperature, (h) adsorbent dose and initial concentration, (i) adsorbent dose and temperature, and (j) initial concentration and temperature on the percentage of Cr(VI) removed from solution.

Generally, increase in the percentage of Cr(VI) removed with increase in adsorbent dosage resulted from increase in the number of active pores available for the heavy metal removal. The observed decrease in the Cr(VI) removed as the pH was increased could be linked to protonation reduction on the adsorbent surface by H⁺ ions causing precipitation of OH^- ions which in return blocked the active sites.²⁵ A decrease in the percentage of the hexavalent chromium ions removed as its initial concentration was increased resulted from the blockage of the constant number of the active pores available for the adsorption process.²⁶

Optimum Point for Cr(VI) sequestration

Figure 4 displays the Taguchi optimum predicted points for Cr(VI) sequestration using cMGK-HK where pH, contact time, adsorbent dose, Cr(VI) initial concentration and temperature were obtained at level 1 (2), level 1 (20 minutes), level 3 (8 g/L), level 1 (10 mg/L), and level 1 (20°C) respectively. The predicted Cr(VI) sequestration from solution by the adsorbent was 99.02%. At this point, laboratory experiment was conducted and 98.76% of Cr(VI) sequestration was observed. Ali and Saeed²⁷ revealed optimum removal efficiency of 90% at initial concentration, adsorbent dose, pH, and contact time of 3 mg/L, 4 g/L, 6, and 60 minutes respectively using alkaline-hydrolysed banana peels. Rambabu et al¹¹ reported maximum sequestration efficiency of 87.2% for hexavalent chromium ions applying Phoenix dactylifera coir biosorbent at initial concentration, adsorbent dose, pH, contact time, temperature, and agitation speed of 100 ppm, 0.3 g, 2, 60 minutes, 30°C, and 100 rpm respectively. Yusuff et al⁹ obtained optimum point at initial concentration, adsorbent dose, pH, and contact time of 10 mg/L, 0.3 g/L, 6, and 140 minutes respectively with 83.2% Cr(VI) sequestration using ZnCl₂-modified eucalyptus bark biochar.

Figure 4.

Optimum predicted points by Taguchi at different parameter levels.

Isotherm, kinetics and thermodynamics studies

Equations (5) and (6) represent the linearized forms of Freundlich²⁸ and Langmuir²⁹ respectively used for the isotherm study.

(5)

(6)

where q_e = adsorption capacity (mg/g), K_F = Freundlich isotherm constant (mg^1-1/n L^1/n g^–1), n = heterogeneity constant, C_e = adsorbate equilibrium concentration (mg/L), K_L = Langmuir constant (Lmg⁻¹) and q_max = Langmuir maximum adsorption capacity (mg g⁻¹).

Figures 5 and 6 represent the plots of log q_e versus log C_e (Freundlich) and C_e/q_e versus C_e (Langmuir) respectively. The results obtained revealed R² values of .9984 and .8780 for Langmuir and Freundlich respectively suggesting excellent fitness of Langmuir for the experimental data. A maximum adsorption capacity of 217.39 mg g⁻¹ was obtained for cMGK-HP. The result was similar to those presented by Ali et al³⁰ and Parlayici and Pehlivan³¹.

Figure 5.

Freundlich plot of logq_e versus logC_e.

Figure 6.

Langmuir plot of C_e/q_e versus C_e.

Equations (7) and (8) are the linearized pseudo first-order and pseudo second-order kinetic models used in this study.

(7)

(8)

where q_e = equilibrium experimental adsorption capacity (mg g⁻¹), q_e = adsorption capacity at time t (mg g⁻¹), k₁ = pseudofirst-order rate constant (min⁻¹) and k₂ = pseudosecond-order rate constant (g mg⁻¹ min⁻¹).

Figures 7 and 8 represent the plots of ln(q_t-q_e) versus t (pseudo first-order kinetic) and t/q_t versus t (pseudo second-order kinetic) respectively. A correlation coefficient (R²) of .9989 obtained for pseudo second order kinetic model suggested its fitness for Cr(VI) removal using cMGK-HP as against a value of .8941 obtained for pseudo first order kinetic model. The results presented by Parlayici and Pehlivan³¹ and Basnet et al¹⁰ were similar to those obtained in this study.

Figure 7.

Pseudo first-order kinetic plots of Cr(VI) sequestration by cMGK-HP.

Figure 8.

Pseudo second-order kinetic plots of Cr(VI) sequestration by cMGK-HP.

The change in standard enthalpy (∆H°) and standard entropy (∆S°) were determined as the respective slope and intercept via plotting ln K_L against 1/T (Figure 9) using equation (9). These values were substituted into equation (10) at different temperatures to estimate the values of ∆G° (change in standard Gibbs free energy) stated in Table 4. The result suggested adsorption to be spontaneous and exothermic (ΔH° and ∆G° were negative), occur by physisorption (∆G° values range) and possess high degree of disorderliness (∆S° was positive).

(9)

(10)

Where R = 8.314 Jmol⁻¹K⁻¹ (universal gas constant), K_L = Langmuir constant due to binding sites affinity and adsorption energy and T (°K) = absolute temperature.

Table 4.

Thermodynamic parameters for Cr(VI) sequestration from solution by cMGK-HP.

Figure 9.

Plot of ln K_L versus 1/T (K⁻¹) for Cr(VI) sequestration by cMGK-HP at 393–323 K.

Regeneration and reusability of cMGK-HP

The percentage of Cr(VI) removed decreased as the number of adsorption-desorption cycle was increased as depicted in Figure 10. The percentage of Cr(VI) removed from solution decreased from 96.29% to 91.05% after the fifth regeneration of cMGK-HP. This resulted from the reduction in the number of active sites on adsorbent surface during dissolution. The result suggests cMGK-HP could be used repeatedly for Cr(VI) sequestration from solution. After the fifth cycle, the percentage removal was greater than 90%.

Figure 10.

Cr(VI) Adsorption-desorption efficiency after 5 cycles using cMGK-HP.

Clean disposal of effluent and spent adsorbent

In this study, optimum removal of Cr(VI) from solution by cMGK-HP as shown by Taguchi was observed at a pH of 2 signifying a highly acidic condition which makes both the spent adsorbent and filtrate solution to be unsafe for disposal into open environment. These may leach into the soil and disrupt the ecosystem. Thus, the spent Cr(VI)-loaded cMGK-HP was incinerated for 6 hours at a temperature range 680°C to 700°C in a muffle furnace to get the ash, and a solid-liquid ratio of 1:5 was obtained by dissolving 5 mg of the spent adsorbent in 25 mL of deionized water.³² After stirring for 24 hours, the mixture was then filtered and Cr(VI) concentration was analyzed using atomic absorption spectrophotometer. Absence of Cr(VI) was noticed in the filtrate which signifies the effectiveness of the thermal treatment. The same process was repeated for the sediment obtained in the filtrate after the sequestration. The filtrate was first heated in an oven at a temperature of 120°C for 5 hours to remove the aqueous content. The ash product can be used to keep harmful bugs away at home, add traction to slippery walkways and soak up driveway spills.

Cost implication of Cr(VI) sequestration from solution by cMGK-HP

The high medicinal purpose of Garcinia kola has increased its demand by consumers and its marketing efficiency was estimated to be 1.19% indicating that for every unit cost incurred, the value added by marketing is 0.0119.³³ The hulls of Garcinia kola generated have been influenced by this value thereby, causing environmental pollution. This could be converted into an effective adsorbent for sequestration of harmful pollutants (such as Cr(VI)) from aqueous environment thereby, reducing the menace of solid waste disposal.

For any proposed treatment method such as adsorption of pollutants from aqueous solution, adsorbent cost is a major cost for consideration. Others are based on the operation process which include washing, drying, sieving, stirring, chemical modification, and incineration costs. The basis of calculation is the electrical energy cost incurred during the operations and 1 L of water. In this study, the method proposed by Das and Debnath³⁴ was used to evaluate the operational cost of Cr(VI) sequestration from solution by cMGK-HP presented in Table 5.

Table 5.

Cost estimation of Cr(VI) sequestration from aqueous solution by cMGK-HP.

The economic model (equation (11)) proposed by Das and Debnath³⁴, was used to calculate the total operational cost (T) of using cMGK-HP as adsorbent for Cr(VI) sequestration from solution.

(11)

where A₁ = Adsorbent price based on local market = 0.135 $/kg, A₂ = Practical loading which is a function of Cr(VI) uptake capacity = 0.77, 0.59, 0.42, 0.33, and 0.21 mole of Cr(VI)/kg for the first, second, third, fourth and fifth regeneration cycle, P = Adsorbent preparation cost = 0.411 $/mole, B = Batch adsorption process cost = 0.078 $/mole, and R = Adsorbent regeneration cost = 0.160 $/mole. For the 5 regeneration cycles, total cost was 5 times 0.160 which gives 0.80 $/mol.

Therefore, T = 0.175 + 0.411 + 0.078 + 0.160 = 0.824 $/mole Cr(VI) ions.

The operational cost of commercial activated carbon to remove Cr(VI) from solution has been estimated to be 142.15 Rs./g Cr(VI) ions which is equivalent to 90.413 $/mole Cr(VI) ions in 1 L of aqueous solution.³⁵ The study conducted by Sharma et al³⁵ applied adsorbents prepared from almond shells, discarded potato and petha via chemical modification with 0.1 N NaOH and 0.1 N H₂SO₄ for Cr(VI) ion removal from aqueous solution. The operational cost of Cr(VI) removal from 1 L of aqueous solution using chemically-modified almond shells, discarded potato and petha was estimated to be 6.78 Rs./g Cr(VI) ions, 20.29 Rs./g Cr(VI) ions and 10.99 Rs./g Cr(VI) ions which is equivalent to 4.312 $/mole Cr(VI) ions, 12.905 $/mole Cr(VI) ions and 6.990 $/mole Cr(VI) ions respectively. This suggests that the use of cMGK-HP as adsorbent for Cr(VI) sequestration from solution is less expensive and more economical than previous adsorbents. Rambabu et al¹¹ revealed a production cost of $1.09/kg using Phoenix dactylifera coir biosorbent for hexavalent chromium sequestration and a total cost of $4.36/m³ for scaling up a batch process wastewater treatment plant.

Adsorbent characterization

The surface area, total pore volume and average pore diameter of cMGK-HP were revealed to be 221.75 m²/g, 0.267 cm³/g, and 58.44 Å respectively. The alkaline hydrolysis of raw Garcinia kola hull particles with NaOH greatly improved its textural properties when compared with untreated Garcinia kola hull particles having surface area, total pore volume and average pore diameter of 52.93 m²/g, 0.092 cm³/g, and 18.11 Å, respectively.

Figure 11 represents the SEM micrographs of cMGK-HP before and after Cr(VI) sequestration from aqueous solution. Figure 11a depicts the surface morphology of cMGK-HP showing formation of more micropores due to chemical modification of the cellulosic compounds present in raw GK-HP. After Cr(VI) sequestration, the number of pores drastically reduced (Figure 11b) as a result of blockage by the Cr(VI) and makes the surface not to be too smooth.

Figure 11.

SEM micrograph of cMGK-HP (a) before and (b) after Cr(VI) sequestration.

Figure 12a and b represent the FTIR spectra of cMGK-HP before and after Cr(VI) removal. Peaks formed at 3441.12, 2928.04, 2355.16, 2023.97, 887.29, and 719.47 cm⁻¹ in the samples could be attributed to O-H stretching vibration resulting from hydrogen bonding present in cellulose and protein,³⁶ asymmetric stretching vibration of C-H in methyl group,³⁷ strong O=C=O stretching,⁶ medium C=C=C stretching,³⁸ O-Si-O asymmetric stretching³⁷ and deformation of C-H in cellulose³⁹ respectively. New peaks were formed at 1462.09 and 1359.86 cm⁻¹ on cMGK-HP spectrum after Cr(VI) removal suggesting the occurrence of sequestration.⁴⁰

Figure 12.

FTIR spectrum of cMGK-HP (a) before and (b) after Cr(VI) sequestration.

Mechanism of adsorption

The presence of bending of strong -OH stretching vibration on cMGK-HP surface was shown by FTIR. The surface becomes protonated (positively charged) when the pH is low with the formation of oxy-anions in solution. Adsorption rate is enhanced by the electrostatic attractive forces between (1) hexavalent chromium ions and hydroxyl functional group and (2) hydrogen ions and oxy-anions formed in solution. The adsorbent surface becomes negatively charged at high pH together with the formation of HCrO₄⁻ and CrO₄²⁻ in solution. Complexes are formed due to the competition of chromium compound oxy-anions with the negative charge which reduces the rate of hexavalent chromium ions removal.