Alkali Lignin from Talahib Grass (Saccharum spontaneum L.) as An Adsorbent for Chromium (III) and Phenolphthalein: Analysis of the Adsorption Kinetics and Mechanism

The study aims to quantitatively assess the capability of lignin extracted from Saccharum spontaneum L. (talahib grass) as an adsorbent of chromium (III) and phenolphthalein in aqueous solution through kinetic methods. Results show that using lignin as an adsorbent, the percentage removal for both chromium (III) and phenolphthalein in aqueous solution are 31.70% and 74.98%, respectively, based on the optimum exposure time of the adsorbent, which is 6.5 hours for chromium (III) and 60 minutes for phenolphthalein. Results from the mechanistic analysis show that a pseudo-second order adsorption kinetic model fits better than a pseudo-first order model for both substrates. Also, among the six selected adsorption isotherm models used in this study, the Flory-Huggins isotherm can best predict the properties of the adsorption process of both phenolphthalein and chromium (III) at equilibrium. Further improvement of the extracted lignin through various physical and chemical modifications may improve its ability to remove the contaminants.


INTRODUCTION
Since the abovementioned industries produce large amounts of heavy metal and dyecontaminated wastes, there needs to be a remediation process where the necessary materials are easily sourced, environmentally sustainable, and can significantly reduce the concentration of contaminants. The key for development of more efficient adsorption methods is to generate an adsorbing material with the abovementioned characteristics for desirable results.

Lignin and Its Potential as an Adsorbent for Heavy Metals and Dye in Wastewater.
Lignin is the second most abundant natural polymer in the world after cellulose; it is usually concentrated in the middle lamella of the plant and bound with cellulose and hemicellulose. Usually, lignin is deposited in cell walls of specialized plant cells, which undergo secondary cell wall biosynthesis (Chen, 2014;Li, 2011). A complex heteropolymer, lignin is composed of three monomeric units, pcoumaryl alcohol, sinapyl alcohol, and coniferyl alcohol, which vary in degree of methoxylation at the -ortho, -meta, and -para positions. The different monomeric structures and their possible linkages are shown in Figure 1.
Currently, the most popular method for utilizing lignin is its conversion to activated carbon, which is then used for various inorganic and organic decontamination processes in aqueous solution, due to its high performance as an adsorbent (Babel and Kurniawan, 2004). Lignin is an organic material made up of various several phenylpropane monomers, forming a complex, rigid structure (Brebu & Vasile, 2010). The structure of lignin is very specific to the species, which means that different species have different relative compositions of the different phenylpropane monomers. Currently, the most popular method for utilizing lignin is its conversion to activated carbon, which is then used for various inorganic and organic decontamination processes in aqueous solution, due to its high performance as an adsorbent (Babel and Kurniawan, 2004). Lignin is an organic material made up of various several phenylpropane monomers, forming a complex, rigid structure (Brebu & Vasile, 2010). The structure of lignin is very specific to the species, which means that different species have different relative compositions of the different phenylpropane monomers.
The aim of this study is to determine the adsorption properties of lignin towards chromium (III) and phenolphthalein through: (1) determination of the optimum removal percentage of both substrates in aqueous solution, and (2) determination of the mechanism of adsorption through model fitting of the time-dependent adsorption data with selected adsorption isotherms.

Materials and Equipment. All reagents purchased
from Scharlab Chemicals (Philippines) and were used without further purification. For the analysis equipment, the UV-Visible Spectrophotometer used was a Shimadzu UV-mini 1240 Single beam, while the Atomic Absorption Spectrophotometer was a Varian SpectrAA 55D graphite furnace with a Chromium hollow cathode lamp. Both analyses were performed in the Analytical Services Laboratory, Institute of Chemistry, UPLB. The Saccharum spontaneum L. (Talahib Grass) was collected from Sta. Teresa, Iguig, Cagayan, Philippines. Chromium (III) nitrate was provided by the Institute of Chemistry, UPLB.

Sample Preparation.
Ground air-dried stem of Saccharum spontaneum L (Talahib Grass) was sieved through a 0.5 mm screen. The resulting powder was extracted using a cyclohexane-methanol solution (2:1 v:v), in a Soxhlet extraction set-up for 8 hours. The solvent-free sample was then air-dried.

Extraction and Characterization of Alkali
Lignin. Three grams of the talahib sample was refluxed in 60.0 mL of 3.53M NaOH solution for 90 minutes. The mixture was allowed to cool for 30 minutes before filtration. The lignin was precipitated in the filtrate with concentrated hydrochloric acid (until pH 2). The mixture after precipitation was heated for 40 minutes at 76 o C. The coagulated solid (raw lignin) was recovered through filtration, where it was purified by washing with 15 mL of 10 % NaCl solution, 70% ethanol, and water. The procedure is repeated until the necessary amount of extract is produced.
Isolated lignin was characterized through UV-Vis spectrophotometry with 1.0 M of NaOH solution as the solvent (Xu, et al., 2005). A broad but prominent peak at 282 nm confirms the presence of lignin in the product. Absence of other peaks in the 220-600 nm range ensures purity of the extracted lignin.

Chromium (III) and Phenolphthalein
Adsorption. This is a modified procedure of Guo, Zhang, &Shan, 2008 andNair, Panigrahy, &Vinu, 2014. Isolated alkali -lignin will be used for adsorption. Ten portions of 250 mg alkali-lignin were weighed. Each portion was then added to ten 25-mL aliquots of 6.0 ppm Chromium (III) nitrate. The resulting solutions were stirred for varying hours (1.0 hour to 10.0 hours, with a 1-hour interval), where the resulting mixtures are filtered. The filtrates were analyzed for chromium using atomic absorption spectroscopy (AAS). The initial and final concentrations of chromium in the samples were determined based on a standard curve.
The same portions of lignin were each added to ten 50-mL aliquots containing 10 mg/L phenolphthalein. The resulting solutions were stirred at varying times (15-150 minutes, with 15 minutes variation). The resulting mixtures were then filtered and adjusted to pH 10 by gradually adding 0.1 M NaOH solution. The alkaline filtrates were then analyzed for phenolphthalein concentration using a UV-Visible spectrophotometer. The initial and final concentrations of phenolphthalein in the samples were determined based on a standard curve.
For both substrates, the adsorption rate kinetics were analyzed using both the pseudofirst order and pseudo-second order analysis, while the properties of adsorption at equilibrium were analyzed through modelfitting using the Langmuir, Freundlich, Dubinin-Radushkevich (D-R), Breunner-Emmett-Teller (BET), Flory-Huggins, and Temkin isotherms.

Extraction and Characterization of Alkali
Lignin. The extractions were done in duplicates per day, for a total of 3 days to check for the repeatability of the experimental runs. One-way analysis of variance was employed in the procedure. The ANOVA showed that at 95% confidence level the mean response of 15.19% is within interval limits. Therefore, there is no significant difference between the yields of each daily experiment at 95% confidence level.
The 15.19% yield is also found to be the maximum yield within the range of the conditions of the experiment. The process was optimized considering the analysis cost, extraction time, and the equipment requirements.
The isolated lignin was analyzed through UV-Vis spectrophotometry. Low, mid, and high concentrations of lignin, prepared under basic medium (0.1M NaOH) were prepared for the analysis. Although the wavelength of maximum absorbance is near 220 nm (cellulosic peak), the peak involving the phenolic groups present in the lignin-matrix is near 280 nm. Using the method of Tian, et al. (2015), the considered wavelength of maximum absorbance for the isolated lignin is 282 nm.   It is observed in Figure 3 that phenolphthalein adsorbs readily into the lignin even at low contact time for the adsorption process. The equilibrium was achieved at around 60 minutes, wherein the experimental qe is 1.46 mg per gram lignin.

Figure 4. Effect of Contact Time on the Adsorption of Chromium (III) on Lignin
Contrary to that of the phenolphthalein, the adsorption of chromium (III) ion lignin needed 6.5 hours to attain its equilibrium. The experimental qe is equal to 0.18 mg per gram lignin.
Kinetic analysis of adsorption of phenolphthalein on lignin shows that it follows a second-order rate equation, where the calculated coefficient of determination of the second-order analysis is 0.9994, as compared to 0.5006 for the first-order analysis. The positive value of the reaction rate implies that the adsorption of the phenolphthalein in lignin is favorable. This explains the high removal rate of the substrate in aqueous solution, as well as the low required contact time.
The adsorption of chromium (III) ion follows the second-order rate equation as well (coefficient of determination of second-order 0.9701 is greater than the first-order coefficient of determination value of 0.0018). Table 3 shows the regression data for the kinetic analysis for chromium (III).
It is important to note that even though the mechanism is a pseudo-second order, the calculated reaction rate is negative. This implies that the formation/interaction of the substrate to the adsorbent is unfavorable, which explains the low percentage removal of chromium and its longer required contact time. This could also imply that too much contact time will result in the desorption of the chromium substrate from the lignin, causing it to re-dissolve into the aqueous medium.

Determination of the Adsorption Isotherm.
Adsorption Isotherms are models that help predict the concentration of the free and adsorbed substrates at equilibrium. If    applicable, one or more of these models could predict the stability and thermodynamic favorability of the adsorption process itself. Table 4 shows the various isotherms used in this study.
For the adsorption of phenolphthalein and chromium on lignin using Langmuir analysis, the coefficient of determination is equal to 0.9919 and 0.9913. To check the favorability of the isotherm a dimensionless separation factor, RL can be used (Bulut & Aydin, 2006).
The calculated separation factors for the adsorption of phenolphthalein and chromium (III) ion on lignin are -0.1249 and 1.0133, respectively. An RL value between 0 and 1 is favorable; otherwise, the model cannot be used to predict the adsorption process, which is the case for both substrates.
Using the Freundlich model, the calculated coefficients of determination for the adsorption of phenolphthalein and chromium (III) are 0.9298 and 0.9950 respectively, while the calculated values of (1/n) are -2.04, and -0.56, respectively. Both (1/n) values are beyond the applicable range of the model, thus, the Freundlich model cannot be used to predict the adsorption equilibrium.
Using the D-R model, the coefficients of determination for the analysis of both phenolphthalein and chromium (III) adsorptions are 0.9271 and 0.9971 respectively. In the model equation, the Polanyi potential e is related to mean adsorption energy, E (J/mol) by the equation: The mean adsorption energy provides pertinent information regarding the adsorption process (Argun, Dursun, Ozdemir, & Karatas, 2005). Values less than 8.00 kJ/mol suggest a physiosorptive mechanism, while values greater than 16.00 kJ/mol suggest a particle diffusion mechanism. However, adsorption energy values between 8.00 kJ/mol and 16.00 kJ/mol suggest a chemisorptive mechanism.
The calculated adsorption capacity for phenolphthalein and chromium (III) are 2.26 mg/g lignin and 1.18 mg/g lignin, respectively, suggesting a physiosorptive mechanism for both substrates. The calculated mean adsorption energy values are 3748.58 J/mol and 2047.21 J/mol. The larger percentage removal of phenolphthalein in the system can also be explained by its larger *models deemed not valid based on the values of their other parameters adsorption capacity according to the DR isotherm, which is about 1.92 times greater than that of chromium (III). Note that the removal of phenolphthalein is also greater by about 2.37 times that of chromium (III).
From the BET isotherm, the calculated coefficients of determination are 0.9971 and 0.8327 for phenolphthalein and chromium (III), respectively. The low coefficient for chromium (III) means the BET model is not a satisfactory model for its adsorption process on lignin.
From the FH isotherm, the calculated coefficients of determination are 0.9997 and 0.9994 for phenolphthalein and chromium (III), respectively. The KFH values for phenolphthalein and chromium (III) are 1.072 and 1.030, respectively, which suggest that both substrates adsorb on lignin spontaneously at 298.15K.
Finally, from the Temkin isotherm, the calculated coefficients of determination for phenolphthalein and chromium (III) are 0.9299 and 0.9997, respectively. The value of B for a Temkin isotherm is related to its enthalpy of adsorption (Nechifor, Pascu, Traistaru, & Albu, 2015). If value of B is negative, then the process is exothermic, but endothermic if the value is positive. The calculated values of B for both phenolphthalein and chromium (III) are -0.70 J/mol and -0.37 J/mol. Thus, for both substrates, the process is exothermic.
In conclusion, only the BET, FH, and Temkin models can be used to predict the adsorption characteristics of chromium (III) and phenolphthalein. Among the 3 applicable models, the Flory-Huggins isotherm shows the strongest linear correlation, and thus best describes the adsorption characteristics of both substrates.
The suitability of the FH isotherm is due to its ability to account for the presence of solvent particles in the system. In the FH model, the substrate substitutes the solvent molecules surrounding the lignin adsorbent, as shown by the equilibrium reaction below: lignin(solvent) + substrate ⇌ lignin (substrate) + solvent Based on the KFH values, the displacement of the solvent particles by the substrates is slightly more favored than the displacement of the substrates by the solvent particles in the system.
The adsorption process is more effective for phenolphthalein since their structures are more similar compared to that of chromium (III). The aromatic structures in the lignin adsorbent is similar to that of phenolphthalein, which could cause a more stable interaction due to the ring current which is known for aromatic compounds. Chromium (III), on the other hand, contains a strong positive charge, which would repel other incoming substrates once it adsorbs in the lignin surface, which explains the lower percentage removal.

CONCLUSIONS
Lignin was extracted from Saccharum spontaneum L. using the alkaline method. It was found that the yield of the extraction is consistent at an average of 15.19% by mass, which is within the range of the conditions of the experiment.
Quantitative assessment of the adsorption capabilities of lignin towards chromium (III) and phenolphthalein was achieved through (1) determination of the optimum percentage removal of chromium (III) and phenolphthalein in the prepared aqueous solutions, and (2) determination of the kinetics of the adsorption process through model-fitting with selected adsorption isotherms based on the time-dependent adsorption data.
The adsorption mechanism of lignin towards phenolphthalein and chromium (III) was assessed through the pseudo-first order and the pseudo-second order analyses. The latter showed more satisfactory results based on the comparisons on the values of the coefficients of determination. However, the results for chromium (III) indicate that the adsorption is unfavorable, which explains the lower percentage removal and higher contact time (31.70% and 6.5 hours, respectively) compared to that of phenolphthalein (74.98% and 60 minutes, respectively).
In the determination of the appropriate adsorption isotherm, it was found that the Flory-Huggins isotherm model best describes the equilibrium adsorption of both substrates, based on the comparison of the coefficients of determination (Table 5). Results from the FH analysis also show that the KFH values for phenolphthalein and chromium (III) are 1.072 and 1.030, respectively, which suggests a spontaneous mechanism of adsorption for both substrates.
It is recommended that alternative methods of extraction be done to determine the effect of the differences in the lignin structure to its capability as an adsorbent. Also, it is recommended to study the adsorption characteristics of substrates other than phenolphthalein and chromium (III).