Palm Kernel Shell as an effective adsorbent for the treatment of heavy metal contaminated water
PMCID: PMC6908638
PMID: 31831850
Abstract
Heavy metal contamination in water causes severe adverse effects on human health. Millions of tons of kernel shell are produced as waste from oil palm plantation every year. In this study, palm oil kernel shell (PKS), an agricultural waste is utilized as effective adsorbent for the removal of heavy metals, namely; Cr 6+ , Pb 2+ , Cd 2+ and Zn 2+ from water. Different parameters of adsorptions; solution pH, adsorbent dosage, metal ions concentration and contact time were optimized. The PKS was found to be effective in the adsorption of heavy metal ions Cr 6+ , Pb 2+ , Cd 2+ and Zn 2+ from water with percentage removal of 98.92%, 99.01%, 84.23% and 83.45%, respectively. The adsorption capacities for Cr 6+ , Pb 2+ , Cd 2+ and Zn 2+ were found to be 49.65 mg/g, 43.12 mg/g, 49.62 mg/g and 41.72 mg/g respectively. Kinetics of adsorption process were determined for each metal ion using different kinetic models like the pseudo-first order, pseudo-second order and parabolic diffusion models. For each metal ion the pseudo-second order model fitted well with correlation coefficient, R 2 = 0.999. Different isotherm models, namely Freundlich and Langmuir were applied for the determination of adsorption interaction between metal ions and PKS. Adsorption capacity was also determined for each of the metal ions. PKS was found to be very effective adsorbent for the treatment of heavy metal contaminated water and short time of two hours is required for maximum adsorption. This is a comprehensive study almost all the parameters of adsorptions were studied in detail. This is a cost effective and greener approach to utilize the agricultural waste without any chemical treatment, making it user friendly adsorbent.
Full Text
Palm kernel shell (PKS) of oil palm is useful material to be applied as an adsorbent for the removal of heavy metal ions, as the good quality of organic compounds in it capable of adsorption of metal ions through biosorption mechanisms mentioned above. PKS is sustainable agricultural waste produced in millions of tons every year. The disposal of large quantity PKS causes adverse effects to the environment as it disposed-off by burning causing a lot smoke. In this study, we utilized the PKS as an adsorbent for the treatment of heavy metal contaminated water and the effect of different parameters such as pH of the solution, concentration of metal ions, adsorbent dosage and contact time was determined. In addition to this, different kinetic models and isotherms were used for the determination of the kinetics and sorption process, respectively. PKS was found to be excellent adsorbent for the removal of heavy metal ions from water as it required very short time of 2 hours for the removal of Cr6+, Pb2+, Cd2+ and Zn2+ from water with percentage removal of 98.92%, 99.01%, 84.23% and 83.45%, respectively. Figure 1 shows the schematic representation of PKS in the treatment of heavy metal contaminated water.
Infrared spectroscopy is a useful technique for the determination of functional groups and any changes take place in the functional groups of any compound. Figure 2 shows the Fourier Transformed infrared (FTIR) spectra of the samples; PKS, PKS-Zn2+, PKS-Pb2+, PKS-Cd2+ and PKS-Cr6+. The PKS shows the characteristic band of -OH group at 3338 cm−1 and this band has been shifted to the lower wavenumbers, 3313 cm−1, 3324 cm−1, 3325 cm−1 and 319 cm−1 after the adsorption of metal ions; PKS-Cr6+, PKS-Cd2+, PKS-Pb2+ and PKS-Zn2+, respectively. The PKS after the adsorption of the process showed new twin bands for Cr6+ at 2328 cm−1 and 2279 cm−1, for at Cd2+ 2320 cm−1 and 2284 cm−1, for Pb2+ at 2316 cm−1 and 2287 cm−1 and for at Zn2+ 2320 cm−1 and 2292 cm−1. For PKS alone, these infrared bands are absent, as these new bands appear due the adsorption of metal ions on the carbonyl groups. The PKS alone shows the carbonyl band at 1701 cm−1 and this carbonyl band has been shifted to higher wavenumber of 1717 cm−1, 1718 cm−1, 1716 cm−1 and 1710 cm−1 after the adsorption of Cr6+, Cd2+, Pb2+ and Zn2+ ions, respectively. In addition to this, new carbonyl bands were also appeared at about 1655 cm−1 after metal ions adsorptions, which was absent in the PKS alone. The infrared band, due to the C-O stretching was appeared at 1043 cm−1 and this bands is shifted to the higher values, 1235 cm−1 after the adsorption of metal ions. The rest of the stretching and bending infrared bands e.g. CH, C-C and C=C etc., were not significantly shifted. There was a significant shift in the infrared bands of O-H, C=O, C-O after the adsorption process. In addition to this, new infrared bands at about 2200–2350 cm−1 and at 1650 cm−1 were also observed. The shifting and appearance of new infrared bands strongly suggests the successful adsorption of the metal ions on the PKS adsorbents and these results was further confirmed by the elemental analysis. Table 1 shows the detailed infrared bands of PKS before and after the adsorption of metal ions.
Note: In Table 1 dash line (−) represents the absence of band.
Figure 3 shows the morphology of the PKS before and after the adsorption of Cr6+ ions PKS-Cr6+, Zn2+ ions (PKS-Zn), Cd2+ ions (PKS-Cd2+) and Pb2+ ions (PKS-Pb2+). Before the adsorption process, PKS morphology is rough with layers, stacking on top of one another and similar morphology has also been reported for PKS our previous study. After the treatment with metal ions e.g. Cr6+, Zn2+, Cd2+ and Pb2+ contaminated water, morphology of PKS has changed slightly having a smoother surface as shown in the Fig. 3. The changes in morphology and the formation of pores may possibly be attributed to the process of adsorption via electrostatic and other interactions like chelation, surface adsorption and biosorption. Similar changes in morphology after the adsorption process have also been reported in the literature.
In adsorption studies, pH of solution plays pivotal role in the electrostatic interactions between adsorbates and adsorbents. In this study, the effect of pH on the removal of heavy metal ions was determined by varying the pH e.g. pH 3, pH 4, pH 5, pH6, pH7, pH8 and pH9 of each of the metal ions; Cr6+, Pb2+, Cd2+ and Zn2+. The maximum adsorption was observed in basic pH i.e. above pH7 as shown in Fig. 4. However, in basic conditions, formation precipitation of metal ions as their respective hydroxide can influence the adsorption results, therefore we have selected the maximum adsorption under acidic environment e.g. <pH 7. Lead ions showed the highest adsorption of about 95.20% at pH 4 and the remaining three metal ions Cr6+, Cd2+ and Zn2+, pH 6 was found to be the optimum with maximum adsorption of 90.20%, 75.50% and 67.30%, respectively. For the remaining batch experiments, pH 4 for Pb2+ ions and pH 6 were selected.
To determine the minimum possible dosage for the maximum adsorption of metal ions, the amount of adsorbent dosage was varied, 0.25 g, 0.50 g, 1 g, 1.50 g and 2 g in 100 mL of 20 ppm metal ion solution. These experiments were conducted under optimized pH conditions for each of the metal ion as discussed in the previous section. Figure 4 illustrates the effect of variation dosage on the adsorption metal ions. The adsorption was increased with the amount of dosage for the Cr6+ and Pb 2+ ions, which was found to be 59.30% % and 70.12% at an adsorbent dosage of 0.25 g and reached to above almost to about 100% removal of metal ions at 1.5 g of PKS for both these metal ions. The effect of the increase in the amount of PKS on the adsorption of Cd2+ and Zn2+ was like Cr6+ and Pb 2+ ions however the overall adsorption of these metals is lower. Adsorption of Cd2+ and Zn2+ ions were 44.08% and 40.13% at 0.50 g PKS and reached to the maximum value of 86.2% and 77.20% at 2.0 g, respectively. However, there was a very minor difference of 1–3% of adsorption for all the metal ions at 1.5 g and 2 g PKS and therefore for the further batch experiments 1.5 g was used. As can be observed in over trend of adsorption with the adsorbent dosage, the adsorption increases with the increase in dosage and reached to maximum value at 1.5–2 g. These, observations suggest that adsorption is almost directly proportional to the amount of the dosage.
Figure 4 shows the effect of initial metal ions concentration on PKS adsorption ability. The effect of initial metal ions concentration on the adsorption ability of PKS was determined by varying their concentrations; 5 ppm, 10 pm, 15 ppm, 20 ppm and 25 ppm, under the previously optimized parameters. For Cr6+ and Pb2+ adsorption trends are almost a straight line with percentage removal of 97.5% and 90.4%, respectively at the highest initial concentration of 25 ppm. However, Cr+6 adsorption was decreased from 98.53% (20 ppm initial concentration) to 90.44% (25 ppm initial concentration). The adsorption trends for Cd2+ and Zn2+ ions showed a continuous decline with the increase in the initial concentration, however the overall adsorption for Cd2+ and Zn2+ was found to be 82.71% and 60.17%, respectively at 25 ppm initial concentrations. Adsorption of Zn2+ at 20 ppm initial concentration was 75.07%, which is still a very high removal percentage. The overall adsorption efficiency decreases with the increase in the initial concentration metal ions, as the adsorption sites in PKS get saturated. The maximum adsorption was observed at 20 mg/L and at 25 mg/L the overall percentage of adsorption decreases. As the metal ions concentration increases, the adsorption decreases due to the saturation of sites for chelation or adsorption.
One of the important parameters that affect the adsorption process is contact time between the adsorbent and adsorbate. A series of experiments for each metal ion solution was conducted by varying the contact time; 15 minutes, 30 minutes, 60 minutes, 90 minutes and 120 minutes and keep the all other optimized parameters constant. Figure 4 also shows the effect of contact time on the adsorption of Pb2+, Cr6+, Cd2+ and Zn2+ on the PKS adsorbent. It can be seen in the Fig. 4, that Cr6+ and Pb 2+ions took 60 minutes to reach the equilibrium with adsorption of about 98%. Cadmium and zinc ions took 90 minutes and 120 minutes to acquire equilibrium with maximum adsorption of about 84% and 83%, respectively.
Figure 5 shows the kinetics fitting for different models; pseudo-first order, pseudo-second order and parabolic diffusion of all the metal ions; Cr6+, Pb2+, Cd2+ and Zn2+ adsorption on the PKS. For each of the metal ion, all three adsorption kinetic models were applied. It was found that the adsorption follows the pseudo-second order process for each of the metal ion, as the correlation coefficient, R2 was found to be 0.999 compared to the other two models; pseudo-first order and parabolic diffusion models. Table 2 represents the correlation coefficient, R2 value for each model and the value of rate constant, K2 for the pseudo-second order. The kinetics of any reaction which follows the Pseudo second order reaction indicates that the adsorption has occurred via chemisorption. As discussed in earlier, adsorption involves the chelation, adsorbents functional groups interactions with metal ions. The Pseudo Second order for the adsorption of metal ions by PKS strongly suggests that adsorption occurred via chemisorption.
The isotherm models of Freundlich and Langmuir are realized to determine the mode of adsorption and interaction between the heavy metal ions; Cr6+, Pb 2+, Cd2+ and Zn2+ and PKS adsorbent. The equations for Freundlich isotherms (Eq. 1) and Langmuir isotherms (Eq. 4) can be written as follows.where Ce and Qe is the equilibrium concentration of metal ions (mg/L) and the amounts of metal ions (mg/g) adsorbed, respectively. QM is the maximum amount of metal ions adsorbed (mg/g) on the PKS, b is a constant and Kf and 1/n are Freundlich coefficients.
Figure 6 shows the Freundlich and Langmuir isotherms fitting, and Table 3 shows the correlation coefficient and values of constants for both of isotherm models. The adsorption was found to be in the order of Cr6+ > Pb2+ > Cd2+ > Zn2+; however, no significant difference was observed in the adsorption of Cr6+ and Pb 2+. The isotherms study revealed that adsorption fitted well with Freundlich compared to Langmuir model, as the correlation coefficient (R) was found to be higher for Freundlich model which is 0.9 for lead, chromium, cadmium and lower in case of Zn as given in the Table 3. This suggests that the adsorbent (PKS) sites were uniformly spread over the surface and metal ions formed mono layer on the PKS surface. In addition to this, values of nf constant for Freundlich model were found to be between 0.1 and 1, suggesting favorable adsorption of metal ions on the PKS surface.
Absorption capacity (qe) of PKS adsorbent was determined by varying contact time and keeping the concentration of metal ions (i.e. 20 ppm/100 mL) and PKS adsorbent dosage (i.e.1.5 g) constant. Figure 7 shows the trends in the adsorption of metal ions on the PKS with respect to time. It can be observed that Cr6+ and Pb2+ adsorption took place rapidly till 60 minutes, followed by slower adsorption due to saturation of the adsorption sites. The adsorption of Cd2+ ions was fast till 90 minutes, followed by a slower one for the last 30 minutes and minor increase in adsorption occurred because saturation of adsorption sites and possibly due to the establishment of the equilibrium. However, for Zn2+ ions adsorption was increased gradually in a sharp manner till the first 90 minutes, followed by a relatively rapid adsorption in comparison to the Cd2+ and reached to the maximum adsorption at 120 minutes. Table 3 also shows the maximum adsorption capacity qe (mg/g) for each of the metal ions at the longest contact time, 120 minutes.
Different bio-adsorbents coconut coir, coconut husk, rape straw powder, Equisetum (EH) and Teucrium (TH) are applied for the removal of heavy metals like Cr6+, Pb2+, Cd2+ and Zn2+ from water namely. Table 4 shows the adsorption of capacity (qe) of different adsorbent for the removal of heavy metal ions. The comparative adsorptions as shown in Table 4 revealed that PKS adsorbent is much more efficient compared to the other bio-adsorbents mentioned above.
Sections
"[{\"pmc\": \"PMC6908638\", \"pmid\": \"31831850\", \"reference_ids\": [\"Fig1\"], \"section\": \"Introduction\", \"text\": \"Palm kernel shell (PKS) of oil palm is useful material to be applied as an adsorbent for the removal of heavy metal ions, as the good quality of organic compounds in it capable of adsorption of metal ions through biosorption mechanisms mentioned above. PKS is sustainable agricultural waste produced in millions of tons every year. The disposal of large quantity PKS causes adverse effects to the environment as it disposed-off by burning causing a lot smoke. In this study, we utilized the PKS as an adsorbent for the treatment of heavy metal contaminated water and the effect of different parameters such as pH of the solution, concentration of metal ions, adsorbent dosage and contact time was determined. In addition to this, different kinetic models and isotherms were used for the determination of the kinetics and sorption process, respectively. PKS was found to be excellent adsorbent for the removal of heavy metal ions from water as it required very short time of 2\\u2009hours for the removal of Cr6+, Pb2+, Cd2+ and Zn2+ from water with percentage removal of 98.92%, 99.01%, 84.23% and 83.45%, respectively. Figure\\u00a01 shows the schematic representation of PKS in the treatment of heavy metal contaminated water.\"}, {\"pmc\": \"PMC6908638\", \"pmid\": \"31831850\", \"reference_ids\": [\"Fig2\", \"Tab1\"], \"section\": \"Infrared spectroscopic analysis\", \"text\": \"Infrared spectroscopy is a useful technique for the determination of functional groups and any changes take place in the functional groups of any compound. Figure\\u00a02 shows the Fourier Transformed infrared (FTIR) spectra of the samples; PKS, PKS-Zn2+, PKS-Pb2+, PKS-Cd2+ and PKS-Cr6+. The PKS shows the characteristic band of -OH group at 3338\\u2009cm\\u22121 and this band has been shifted to the lower wavenumbers, 3313\\u2009cm\\u22121, 3324\\u2009cm\\u22121, 3325\\u2009cm\\u22121 and 319\\u2009cm\\u22121 after the adsorption of metal ions; PKS-Cr6+, PKS-Cd2+, PKS-Pb2+ and PKS-Zn2+, respectively. The PKS after the adsorption of the process showed new twin bands for Cr6+ at 2328\\u2009cm\\u22121 and 2279\\u2009cm\\u22121, for at Cd2+ 2320\\u2009cm\\u22121 and 2284\\u2009cm\\u22121, for Pb2+ at 2316\\u2009cm\\u22121 and 2287\\u2009cm\\u22121 and for at Zn2+ 2320\\u2009cm\\u22121 and 2292\\u2009cm\\u22121. For PKS alone, these infrared bands are absent, as these new bands appear due the adsorption of metal ions on the carbonyl groups. The PKS alone shows the carbonyl band at 1701 cm\\u22121 and this carbonyl band has been shifted to higher wavenumber of 1717 cm\\u22121, 1718 cm\\u22121, 1716 cm\\u22121 and 1710 cm\\u22121 after the adsorption of Cr6+, Cd2+, Pb2+ and Zn2+ ions, respectively. In addition to this, new carbonyl bands were also appeared at about 1655 cm\\u22121 after metal ions adsorptions, which was absent in the PKS alone. The infrared band, due to the C-O stretching was appeared at 1043\\u2009cm\\u22121 and this bands is shifted to the higher values, 1235\\u2009cm\\u22121 after the adsorption of metal ions. The rest of the stretching and bending infrared bands e.g. CH, C-C and C=C etc., were not significantly shifted. There was a significant shift in the infrared bands of O-H, C=O, C-O after the adsorption process. In addition to this, new infrared bands at about 2200\\u20132350 cm\\u22121 and at 1650 cm\\u22121 were also observed. The shifting and appearance of new infrared bands strongly suggests the successful adsorption of the metal ions on the PKS adsorbents and these results was further confirmed by the elemental analysis. Table\\u00a01 shows the detailed infrared bands of PKS before and after the adsorption of metal ions.\"}, {\"pmc\": \"PMC6908638\", \"pmid\": \"31831850\", \"reference_ids\": [\"Tab1\"], \"section\": \"\", \"text\": \"Note: In Table\\u00a01 dash line (\\u2212) represents the absence of band.\"}, {\"pmc\": \"PMC6908638\", \"pmid\": \"31831850\", \"reference_ids\": [\"Fig3\", \"Fig3\"], \"section\": \"Field emission scanning electron microscopy analysis\", \"text\": \"Figure\\u00a03 shows the morphology of the PKS before and after the adsorption of Cr6+ ions PKS-Cr6+, Zn2+ ions (PKS-Zn), Cd2+ ions (PKS-Cd2+) and Pb2+ ions (PKS-Pb2+). Before the adsorption process, PKS morphology is rough with layers, stacking on top of one another and similar morphology has also been reported for PKS our previous study. After the treatment with metal ions e.g. Cr6+, Zn2+, Cd2+ and Pb2+ contaminated water, morphology of PKS has changed slightly having a smoother surface as shown in the Fig.\\u00a03. The changes in morphology and the formation of pores may possibly be attributed to the process of adsorption via electrostatic and other interactions like chelation, surface adsorption and biosorption. Similar changes in morphology after the adsorption process have also been reported in the literature.\"}, {\"pmc\": \"PMC6908638\", \"pmid\": \"31831850\", \"reference_ids\": [\"Fig4\"], \"section\": \"Effect of pH on adsorption\", \"text\": \"In adsorption studies, pH of solution plays pivotal role in the electrostatic interactions between adsorbates and adsorbents. In this study, the effect of pH on the removal of heavy metal ions was determined by varying the pH e.g. pH 3, pH 4, pH 5, pH6, pH7, pH8 and pH9 of each of the metal ions; Cr6+, Pb2+, Cd2+ and Zn2+. The maximum adsorption was observed in basic pH i.e. above pH7 as shown in Fig.\\u00a04. However, in basic conditions, formation precipitation of metal ions as their respective hydroxide can influence the adsorption results, therefore we have selected the maximum adsorption under acidic environment e.g. <pH 7. Lead ions showed the highest adsorption of about 95.20% at pH 4 and the remaining three metal ions Cr6+, Cd2+ and Zn2+, pH 6 was found to be the optimum with maximum adsorption of 90.20%, 75.50% and 67.30%, respectively. For the remaining batch experiments, pH 4 for Pb2+ ions and pH 6 were selected.\"}, {\"pmc\": \"PMC6908638\", \"pmid\": \"31831850\", \"reference_ids\": [\"Fig4\"], \"section\": \"Effect of dosage of adsorbent\", \"text\": \"To determine the minimum possible dosage for the maximum adsorption of metal ions, the amount of adsorbent dosage was varied, 0.25\\u2009g, 0.50\\u2009g, 1\\u2009g, 1.50\\u2009g and 2\\u2009g in 100\\u2009mL of 20 ppm metal ion solution. These experiments were conducted under optimized pH conditions for each of the metal ion as discussed in the previous section. Figure\\u00a04 illustrates the effect of variation dosage on the adsorption metal ions. The adsorption was increased with the amount of dosage for the Cr6+ and Pb 2+ ions, which was found to be 59.30% % and 70.12% at an adsorbent dosage of 0.25\\u2009g and reached to above almost to about 100% removal of metal ions at 1.5\\u2009g of PKS for both these metal ions. The effect of the increase in the amount of PKS on the adsorption of Cd2+ and Zn2+ was like Cr6+ and Pb 2+ ions however the overall adsorption of these metals is lower. Adsorption of Cd2+ and Zn2+ ions were 44.08% and 40.13% at 0.50\\u2009g PKS and reached to the maximum value of 86.2% and 77.20% at 2.0\\u2009g, respectively. However, there was a very minor difference of 1\\u20133% of adsorption for all the metal ions at 1.5\\u2009g and 2\\u2009g PKS and therefore for the further batch experiments 1.5\\u2009g was used. As can be observed in over trend of adsorption with the adsorbent dosage, the adsorption increases with the increase in dosage and reached to maximum value at 1.5\\u20132\\u2009g. These, observations suggest that adsorption is almost directly proportional to the amount of the dosage.\"}, {\"pmc\": \"PMC6908638\", \"pmid\": \"31831850\", \"reference_ids\": [\"Fig4\"], \"section\": \"Effect of the initial metal ions concentration\", \"text\": \"Figure\\u00a04 shows the effect of initial metal ions concentration on PKS adsorption ability. The effect of initial metal ions concentration on the adsorption ability of PKS was determined by varying their concentrations; 5\\u2009ppm, 10\\u2009pm, 15\\u2009ppm, 20\\u2009ppm and 25\\u2009ppm, under the previously optimized parameters. For Cr6+ and Pb2+ adsorption trends are almost a straight line with percentage removal of 97.5% and 90.4%, respectively at the highest initial concentration of 25 ppm. However, Cr+6 adsorption was decreased from 98.53% (20 ppm initial concentration) to 90.44% (25 ppm initial concentration). The adsorption trends for Cd2+ and Zn2+ ions showed a continuous decline with the increase in the initial concentration, however the overall adsorption for Cd2+ and Zn2+ was found to be 82.71% and 60.17%, respectively at 25 ppm initial concentrations. Adsorption of Zn2+ at 20 ppm initial concentration was 75.07%, which is still a very high removal percentage. The overall adsorption efficiency decreases with the increase in the initial concentration metal ions, as the adsorption sites in PKS get saturated. The maximum adsorption was observed at 20\\u2009mg/L and at 25\\u2009mg/L the overall percentage of adsorption decreases. As the metal ions concentration increases, the adsorption decreases due to the saturation of sites for chelation or adsorption.\"}, {\"pmc\": \"PMC6908638\", \"pmid\": \"31831850\", \"reference_ids\": [\"Fig4\", \"Fig4\"], \"section\": \"Effect of contact time on adsorption\", \"text\": \"One of the important parameters that affect the adsorption process is contact time between the adsorbent and adsorbate. A series of experiments for each metal ion solution was conducted by varying the contact time; 15\\u2009minutes, 30\\u2009minutes, 60\\u2009minutes, 90\\u2009minutes and 120\\u2009minutes and keep the all other optimized parameters constant. Figure\\u00a04 also shows the effect of contact time on the adsorption of Pb2+, Cr6+, Cd2+ and Zn2+ on the PKS adsorbent. It can be seen in the Fig.\\u00a04, that Cr6+ and Pb 2+ions took 60\\u2009minutes to reach the equilibrium with adsorption of about 98%. Cadmium and zinc ions took 90\\u2009minutes and 120\\u2009minutes to acquire equilibrium with maximum adsorption of about 84% and 83%, respectively.\"}, {\"pmc\": \"PMC6908638\", \"pmid\": \"31831850\", \"reference_ids\": [\"Fig5\", \"Tab2\"], \"section\": \"Kinetics of adsorption studies\", \"text\": \"Figure\\u00a05 shows the kinetics fitting for different models; pseudo-first order, pseudo-second order and parabolic diffusion of all the metal ions; Cr6+, Pb2+, Cd2+ and Zn2+ adsorption on the PKS. For each of the metal ion, all three adsorption kinetic models were applied. It was found that the adsorption follows the pseudo-second order process for each of the metal ion, as the correlation coefficient, R2 was found to be 0.999 compared to the other two models; pseudo-first order and parabolic diffusion models. Table\\u00a02 represents the correlation coefficient, R2 value for each model and the value of rate constant, K2 for the pseudo-second order. The kinetics of any reaction which follows the Pseudo second order reaction indicates that the adsorption has occurred via chemisorption. As discussed in earlier, adsorption involves the chelation, adsorbents functional groups interactions with metal ions. The Pseudo Second order for the adsorption of metal ions by PKS strongly suggests that adsorption occurred via chemisorption.\"}, {\"pmc\": \"PMC6908638\", \"pmid\": \"31831850\", \"reference_ids\": [\"Equ1\", \"Equ4\"], \"section\": \"Adsorption isotherms analysis\", \"text\": \"The isotherm models of Freundlich and Langmuir are realized to determine the mode of adsorption and interaction between the heavy metal ions; Cr6+, Pb 2+, Cd2+ and Zn2+ and PKS adsorbent. The equations for Freundlich isotherms (Eq.\\u00a01) and Langmuir isotherms (Eq.\\u00a04) can be written as follows.where Ce and Qe is the equilibrium concentration of metal ions (mg/L) and the amounts of metal ions (mg/g) adsorbed, respectively. QM is the maximum amount of metal ions adsorbed (mg/g) on the PKS, b is a constant and Kf and 1/n are Freundlich coefficients.\"}, {\"pmc\": \"PMC6908638\", \"pmid\": \"31831850\", \"reference_ids\": [\"Fig6\", \"Tab3\", \"Tab3\"], \"section\": \"Adsorption isotherms analysis\", \"text\": \"Figure\\u00a06 shows the Freundlich and Langmuir isotherms fitting, and Table\\u00a03 shows the correlation coefficient and values of constants for both of isotherm models. The adsorption was found to be in the order of Cr6+ > Pb2+ > Cd2+ > Zn2+; however, no significant difference was observed in the adsorption of Cr6+ and Pb 2+. The isotherms study revealed that adsorption fitted well with Freundlich compared to Langmuir model, as the correlation coefficient (R) was found to be higher for Freundlich model which is 0.9 for lead, chromium, cadmium and lower in case of Zn as given in the Table\\u00a03. This suggests that the adsorbent (PKS) sites were uniformly spread over the surface and metal ions formed mono layer on the PKS surface. In addition to this, values of nf constant for Freundlich model were found to be between 0.1 and 1, suggesting favorable adsorption of metal ions on the PKS surface.\"}, {\"pmc\": \"PMC6908638\", \"pmid\": \"31831850\", \"reference_ids\": [\"Fig7\", \"Tab3\"], \"section\": \"Absorption capacity\", \"text\": \"Absorption capacity (qe) of PKS adsorbent was determined by varying contact time and keeping the concentration of metal ions (i.e. 20\\u2009ppm/100\\u2009mL) and PKS adsorbent dosage (i.e.1.5\\u2009g) constant. Figure\\u00a07 shows the trends in the adsorption of metal ions on the PKS with respect to time. It can be observed that Cr6+ and Pb2+ adsorption took place rapidly till 60\\u2009minutes, followed by slower adsorption due to saturation of the adsorption sites. The adsorption of Cd2+ ions was fast till 90\\u2009minutes, followed by a slower one for the last 30\\u2009minutes and minor increase in adsorption occurred because saturation of adsorption sites and possibly due to the establishment of the equilibrium. However, for Zn2+ ions adsorption was increased gradually in a sharp manner till the first 90\\u2009minutes, followed by a relatively rapid adsorption in comparison to the Cd2+ and reached to the maximum adsorption at 120\\u2009minutes. Table\\u00a03 also shows the maximum adsorption capacity qe (mg/g) for each of the metal ions at the longest contact time, 120\\u2009minutes.\"}, {\"pmc\": \"PMC6908638\", \"pmid\": \"31831850\", \"reference_ids\": [\"Tab4\", \"Tab4\"], \"section\": \"Comparative studies\", \"text\": \"Different bio-adsorbents coconut coir, coconut husk, rape straw powder, Equisetum (EH) and Teucrium (TH) are applied for the removal of heavy metals like Cr6+, Pb2+, Cd2+ and Zn2+ from water namely. Table\\u00a04 shows the adsorption of capacity (qe) of different adsorbent for the removal of heavy metal ions. The comparative adsorptions as shown in Table\\u00a04 revealed that PKS adsorbent is much more efficient compared to the other bio-adsorbents mentioned above.\"}]"
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