environments Article
Optimizing Spacer Length for Positioning Functional Groups in Bio-Waste Revathi Iyengar *, Maria Faure-Betancourt, Saleh Talukdar, Jinting Ye and Abel E. Navarro
ID
Science Department, Borough of Manhattan Community College, The City University of New York, New York, NY 10007, USA;
[email protected] (M.F.-B);
[email protected] (S.T.);
[email protected] (J.Y.);
[email protected] (A.E.N.) * Correspondence:
[email protected]; Tel.: +1-212-220-8000 Received: 8 August 2018; Accepted: 30 August 2018; Published: 2 September 2018
Abstract: The goal of this study was to determine the optimal chain length needed for tethering functional groups on bio-wastes. The purpose of modifying the surface of bio-waste is to improve their affinity for phenols. To this end, four different aminated green tea leaves, with the amine group located at the end of 6, 8, 10, and 12 carbons were synthesized. Green approaches to functionalization lead to fewer reactive sites. Optimizing spacer length is one way to ameliorate this. The aminated tea leaves were prepared by a tosylation reaction followed by displacement with a diamine used in excess. The tea leaves with the amine at the end of six carbons proved to have the best ability to remove 2-chlorophenol (2-CP) from its aqueous solution. It was at least 3–4 times better than native spent tea leaves. The mechanism by which the phenol was removed proved to be primarily an acid–base reaction followed by H-bonding and dipole–dipole interactions. Because of the acid–base interactions, the relatively low-boiling 2-CP did not volatilize off the aminated tea leaves enabling recycling. On the other hand, with activated charcoal, the adsorbed 2-CP volatilized almost completely under ambient conditions. Keywords: chain length; spacer; tethering; phenol; bio-waste; 4-aminoantipyrine; spent tea leaves; tosylation and amination
1. Introduction Prospects in water treatment keep booming as freshwater availability declines. Phenolics constitute about 10% of the priority pollutant list for water issued by the EPA (Environmental Protection Agency, Washington, DC, USA). Activated carbon is a great adsorbent of organics. However, it needs to be heated for it to be in the active state required for adsorption [1]. Biomass is known to bind contaminants onto its cellular structure by a physicochemical process referred to as biosorption [2–4]. Bio-wastes, such as spent tea leaves, are largely untapped materials to develop inexpensive and effective tools for wastewater treatment. Phenols, being acidic in nature, need sites on the bio-waste to be basic enough for efficient removal. However, unmodified bio-wastes do not have enough of these strongly basic sites needed for phenol removal. Modifying surfaces to improve reactivity is a known idea implemented widely on artificial polymers [5]. The challenge with modifying a bio-based material is that the main functional group available for performing any reactions is the hydroxy group. The approach of modifying lignocellulosic material is currently explored by combining mechanical and chemical approaches, with a particular emphasis on green aspects [6,7]. The effort so far has been geared towards creating carboxyl groups. Carboxyl groups, being acidic in nature, are not expected to interact with the acidic phenolic group. Rather, a basic group, such as an amine, can remove the phenol by an acid–base reaction. Environments 2018, 5, 100; doi:10.3390/environments5090100
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Modification of lignocellulosic material is difficult due to the complexity of the bio-waste. Modification of lignocellulosic material difficult to the complexity of the and bio-waste. For For example, “spent” green tea leaves containisabout 20 due weight percent of phenolics saponins example, “spent” greenof teacrude leavesproteins contain [8] about 20 weighttopercent of phenolics saponins work, and 25 and 25 weight percent in addition the cellulose. In aand preliminary weight percent of crude proteins in addition to the the cellulose. In a developed preliminarybywork, aminated and thiolated leaves were[8] prepared following procedure Abel aminated et al. [9]. and thiolated leaves were prepared following the procedure by Abel et al.compared [9]. These These modified tea leaves exhibited significant adsorption of Cu(II)developed ions in aqueous systems tea leaves exhibited significant of Cu(II) ions in aqueous systems compared to tomodified the unmodified “spent” tea leaves (Tableadsorption 1). the unmodified “spent” tea leaves (Table 1). Table 1. Comparison of Cu (II) removal by modified tea leaves. Table 1. Comparison of Cu (II) removal by modified tea leaves. Name, Group
Name, Group
Chain Length
Chain Length
Thiol, -SH eight (six methylenes and two oxygens) Thiol, -SH and two oxygens) Amine, -NH2eight (six methylenes six methylenes
Amine, -NH2
six methylenes
Adsorption (%) Adsorption (%) Adsorption (%) Adsorption (%) (Unmodified) (Modified)
(Unmodified) 29 29 27 27
49 47
(Modified) 49 47
One reason for the enhanced interaction between the newly introduced functional group and the One reason for the enhanced interaction between the newly introduced functional group and Copper (II) ions is thought to be due to the chain length onto which the group is tethered. This has the Copper (II) ions is thought to be due to the chain length onto which the group is tethered. This been suggested even in the work of Abel et al. [9], where the maximum benefit for their application has been suggested even in the work of Abel et al. [9], where the maximum benefit for their (anti-bacterial activity) was observed when the chain length was 16 methylenes long. Amines are application (anti-bacterial activity) was observed when the chain length was 16 methylenes long. known to react with phenols forming the corresponding ammonium salts. Therefore, the idea was to Amines are known to react with phenols forming the corresponding ammonium salts. Therefore, the identify the optimal “spacer” for amines. idea was to identify the optimal “spacer” for amines. For this study, the optimum chain length was determined based on the effectiveness as a 2-CP For this study, the optimum chain length was determined based on the effectiveness as a 2-CP absorbent. There is considerable literature evidence that functional groups linked directly to a glucose absorbent. There is considerable literature evidence that functional groups linked directly to a ring without a spacer impose considerable steric hindrance. For example, if the thiol group is linked glucose ring without a spacer impose considerable steric hindrance. For example, if the thiol group is directly to the glucose ring, the abstraction of hydrogen from the SH by a polymeric radical [10] linked directly to the glucose ring, the abstraction of hydrogen from the SH by a polymeric radical [10] is inhibited. is inhibited. The goal of this project is to develop eco-friendly filters using raw solid wastes to effectively The goal of this project is to develop eco-friendly filters using raw solid wastes to effectively remove phenols as part of a water treatment process. Another advantage would be the easy remove phenols as part of a water treatment process. Another advantage would be the easy regeneration of the solid waste with a base to obtain the phenols back for reuse in other regeneration of the solid waste with a base to obtain the phenols back for reuse in other manufacturing processes. manufacturing processes. 2. Materials and Methods 2. Materials and Methods The synthetic scheme using hexamethylene diamine for placing the amino group at the end of The synthetic scheme hexamethylene diamine for placing thewith amino at theamines: end of a a six-carbon chain is shownusing in Scheme 1. The same scheme was used thegroup following six-carbon chain is1,10-diaminodecane, shown in Scheme 1. The same scheme was used the the following amines: 1,8-diaminooctane, and 1,12-diaminododecane. Thiswith tethered amine groups 1,8-diaminooctane, 1,10-diaminodecane, and 1,12-diaminododecane. This tethered the amine groups on the spent green tea leaves with varying chain lengths. This scheme extends work done by Abel et al. on the spent green tea leaves with varying chain lengths. This scheme extends work done by Abel et al. to modify cotton [9]. to modify cotton [9].
Scheme 1. Synthetic scheme for tethering the amine functionality. Scheme 1. Synthetic scheme for tethering the amine functionality.
2.1.Solvents Solventsand andReagents Reagents 2.1. Thewater waterused usedininall allexperiments experimentswas wasthe theMillipore Milliporefiltered filteredde-ionized de-ionized(DI) (DI)water. water.Solvents, Solvents, The such as pyridine and acetonitrile, were used as supplied without further purification in the synthetic such as pyridine and acetonitrile, were used as supplied without further purification in the synthetic steps.Pyridine Pyridine purchased Organics (Pittsburgh, ACS Certified steps. waswas purchased from from Acros Acros Organics (Pittsburgh, PA, USA).PA, ACSUSA). Certified acetonitrile acetonitrile from Fisher Scientific PA,and USA). Pyridinewere and used acetonitrile was obtainedwas fromobtained Fisher Scientific (Pittsburgh, PA,(Pittsburgh, USA). Pyridine acetonitrile in the were used in the tosylation and amination steps, respectively. ACS Certified analytical tosylation and amination steps, respectively. ACS Certified analytical grade ammonium chloridegrade was ammonium chloride was obtained from Acros Organics (Pittsburgh, PA, USA). Ammonium hydroxide was obtained from Fisher Scientific (Pittsburgh, PA, USA). The 4-Aminoantipyrine was purchased from Sigma Aldrich (Atlanta, GA, USA) and used without any further purification.
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obtained from Acros Organics (Pittsburgh, PA, USA). Ammonium hydroxide was obtained from Fisher Scientific (Pittsburgh, PA, USA). The 4-Aminoantipyrine was purchased from Sigma Aldrich (Atlanta, GA, USA) and used without any further purification. Potassium ferricyanide (for analysis, 99+%) was obtained from Acros Organics (Pittsburgh, PA, USA). ACS Grade 2-Chlorophenol from Acros Organics (Pittsburgh, PA, USA) was used in the studies. Solutions of 2-CP were always freshly prepared before use. Due to the photosensitive nature of 2-CP solutions, volumetric flasks containing its solution were always wrapped in aluminum foil. Amber colored glass tubes were used for equilibration studies to protect the 2-CP from any decomposition. Activated carbon (AC), NORIT™ SA 2, from Acros Organics (Pittsburgh, PA, USA) was used in the study. The biowaste used for the project was obtained from green tea bags. The bags were boiled in water repeatedly to remove all water-soluble compounds. The tea bags were then dried in the oven. The spent tea leaves were removed from the bags and subjected to Soxhlet extraction with a 1:1 mixture of ethanol and acetone (400 mL of solvent mixture per 20 g of spent tea leaves). The tea leaves were dried in the oven at 50 ◦ C till constant weight. Soxhlet extracted green tea leaves are abbreviated as SEGT. 2.2. Preparation of Tosylated Green Tea (TosGT) TsCl (253.8 g, 1.3 mol) was placed in a 1 L Wheaton bottle. Soxhlet Extracted Green Tea (SEGT) (31.5 g, 0.194 mol) was added to the tosyl chloride. For the calculations, SEGT was assumed to be 100% cellulose, and only one −OH group from an anhydroglucose unit (AGU unit, molecular weight (MW) = 162 g/mol) was considered to react. Pyridine (600 mL) was added to the bottle and sonicated for 20 min followed by magnetic stirring at room temperature for 96 h. The reason for a reaction time of 96 h can be found under results and discussion. The pyridine solution was then decanted off into a waste container. Ice cold water was added to dissolve the pyridine clinging to the surface of the tea leaves [11]. The tea leaves were washed five times with ice-cold water (~200 mL each time) to remove pyridine and other by-products. The washes were monitored using pH paper till neutrality. The tea leaves were then washed with acetone thoroughly. The purpose of the acetone washes was to remove un-reacted tosyl chloride. The acetone washes were monitored using a silica gel plate impregnated with a fluorescent material and UV lamp. The tea leaves were then vacuum-filtered. They were air dried to constant weight. The tosylation step produced 40.91 g of the product from 31.48 g of SEGT (Soxhlet Extracted Green Tea). Its purity was estimated to be at least 97%, based on the amount of chlorine present in the tosylated tea leaves, as determined by elemental analysis. The number of moles of tosyl group per gram of the tosylated material was estimated based on the increase of mass after the tosylation step. This was found to be 1.5 mmol/g of TosGT. 2.3. Preparation of the Various Aminated Green Teas (C6-AGT; C8-AGT; C10-AGT; and C12-AGT) The tosylated tea leaves were subjected to amination using four different diamines to produce four different kinds of aminated tea leaves. The diamines used were hexamethylene diamine (C-6 diamine), 1,8-octanediamine (C-8 diamine), 1,10-decanediamine (C-10 diamine), and 1,12-dodecanediamine (C12-diamine). This resulted in the displacement of the tosyl group by one of the amino groups in the diamine while the second amine group remained attached to the other end. This resulted in a family of aminated tea leaves with the amine functionality tethered at the end of varying chain lengths. Cross-linking was prevented by targeting a 1:8 mole ratio of tosylate functionality to the diamine. Each diamine (0.12 mol) was placed in a 500 mL bottle and dissolved in acetonitrile (250 mL). The C-10 diamine (1,10-decanediamine) and C-12 diamine (1,12-dodecanediamine) were distinctly less soluble than the C-8 diamine (1,8-octanediamine) and the C-6 diamine (hexamethylenediamine). Tosylated tea leaves (10 g) were then poured into the acetonitrile solution containing the varying diamines. The mixture was initially sonicated for 20 min followed by magnetic stirring at room temperature (19 h in the case of the C-8 diamine; 68 h for the other three diamines). The acetonitrile
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solutions containing the diamine were decanted off at that point. Ice cold aqueous NaOH solution Environments 2018, 5, x 4 of 15 (2 N, 500 mL) was added followed by magnetic stirring (5 min). Stirring with the base solution was carried to accomplish the conversion of the ammonium tosylate salt to as theshown diamine as shown outout to accomplish the conversion of the ammonium tosylate salt to the diamine in Scheme 2. in Scheme 2. that Note that Scheme 1 does the ammonium tosylate formation explicitly. Note Scheme 1 does not shownot the show ammonium tosylate formation explicitly. The tea werewere transferred to ato beaker andand distilled water (200 mL) was added Theleaves tea leaves transferred a beaker distilled water (200 mL) was addedtotoit.it.Five Fivewater water washes were performed to remove all traces of sodium hydroxide followed by acetone washes were performed to remove all traces of sodium hydroxide followed by acetone washing to washing remove tracesThis of diamines. This was monitored bythe spotting the acetone on apH wetpaper. remove tracestoof diamines. was monitored by spotting acetone washeswashes on a wet pH paper. The tea leaves were then vacuum filtered. In the case of the C-12 diamine, ethanol washes The tea leaves were then vacuum filtered. In the case of the C-12 diamine, ethanol washes were used were used initially before the acetone washes to aid in its complete removal. The four aminated tea initially before the acetone washes to aid in its complete removal. The four aminated tea leaves (each leaves (each weighing in the range of 8.7–9.2 g) were air dried to constant weight and bottled off till weighing in the range of 8.7–9.2 g) were air dried to constant weight and bottled off till further use. further use.
Scheme 2. Conversionofofammonium ammonium tosylate secondary amine. Scheme 2. Conversion tosylatetotothe the secondary amine.
2.4. Characterization 2.4. Characterization Elemental Analyses wereperformed performed at at the the Robertson Robertson Microlit Laboratories in Ledgewood, Elemental Analyses were Microlit Laboratories in Ledgewood, New Jersey. A Frontier Fourier transform infrared (FTIR) spectrometer (Perkin Elmer, Waltham, New Jersey. A Frontier Fourier transform infrared (FTIR) spectrometer (Perkin Elmer, Waltham, MA, USA) with a Universal Attenuated Total Reflectance (UATR) attachment was used for MA, USA) with a Universal Attenuated Total Reflectance (UATR) attachment was used for obtaining obtaining infrared spectra of the chemically modified tea leaves. Pictures of the morphological infrared spectra of the chemically modified teaelectron leaves.microscope, PicturesModel of theTM3000, morphological texture texture were obtained using a tabletop scanning from Hitachi were High-Technologies obtained using (Pleasanton, a tabletop CA, scanning microscope, Modelwithout TM3000, Hitachi USA). electron Samples were viewed directly goldfrom coating. High-Technologies (Pleasanton, CA, Samples were viewed directly without gold coating. Quantax 70, an optical accessory for USA). TM 3000, was used to obtain energy dispersive X-ray spectra Quantax 70,Itanwas optical accessory forBruker TM 3000, was used obtain energy X-ray (EDS). purchased from (Billerica, MA,toUSA). This was dispersive used to gauge thespectra relative(EDS). abundance of elements on the surface of chemically modified tea leaves. It was purchased from Bruker (Billerica, MA, USA). This was used to gauge the relative abundance of elements on the surface of chemically modified tea leaves. 2.5. Equilibrium Conditions for Adsorption
2.5. Equilibrium Amber Conditions glass tubesfor (8 Adsorption cm long with a diameter of 2.5 cm) with Teflon-lined screw caps were used for the equilibration experiments. Varying amounts of the aminated tea leaves and solutions Amber glass tubes (8 cm long with a diameter of 2.5 cm) with Teflon-lined screw caps were containing 2-CP at the appropriate concentrations were placed in these tubes. Targeted used for the equilibration experiments. Varying amounts of the aminated tea leaves and solutions concentrations of 2-CP solutions were prepared by making a stock solution of 1000 ppm and diluting containing 2-CP at the appropriate concentrations were placed in these tubes. Targeted concentrations it with Millipore filtered DI water to the desired concentration. of 2-CP solutions were prepared a stocksample solution of 1000 and diluting it with The tubes were placed by in making the “built-in” holders of ppm the incubator/shaker. TheMillipore I24 filtered DI water to the desired concentration. incubator shaker used in the study was purchased from New Brunswick Scientific (Edison, NJ, The were placed in the “built-in” sample holders of the incubator/shaker. The I24 USA).tubes Equilibration was performed at ambient conditions overnight at a setting of 250 rpm (revolutions minute). involved horizontal motions. Scientific (Edison, NJ, USA). incubator shakerper used in theAgitation study was purchased from circular New Brunswick Equilibration was performed at ambient conditions overnight at a setting of 250 rpm (revolutions per 2.6. Analysis: Phenol Estimation minute). Agitation involved horizontal circular motions. This was based on the spectrophotometric method described in the 21st edition of “Standard
2.6. Analysis: Methods Phenol for theEstimation examination of Water and Wastewater” [12]. According to the procedure, the sample, blank, and standards should all be at a pH of 10 ± 0.1 with the buffer used.
This was based on the spectrophotometric method described in the 21st edition of “Standard The microplate reader was purchased from Molecular Devices (San Jose, CA, USA). Absorbance Methods for the examination of Water and Wastewater” [12]. According to the procedure, the sample, measurements at 500 nm were obtained using SpectraMax iD3 from Molecular Devices (San Jose, blank,CA, andUSA). standards should all bemicroplate at a pH ofwith 10 ±960.1 with(well the buffer used. A UV-transparent wells capacity of 500 µ L) was used for The microplate reader was purchased from Devices (San CA, USA). Absorbance performing the absorbance measurements at 500Molecular nm. The absorbance of theJose, blank plate was initially measurements at 500 nm were obtained using SpectraMax iD3 from Molecular Devices (San Jose, CA, USA). A UV-transparent microplate with 96 wells (well capacity of 500 µL) was used for performing
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the absorbance measurements at 500 nm. The absorbance of the blank plate was initially obtained. The blank absorbance values of the respective wells were subtracted from the observed values with sample solutions. The 4-aminoantipyrine method [12] was adapted to be performed in Eppendorf vials (1.5 mL) as follows: to 910 µL of the buffer solution, 50 µL of the phenol solution and 20 µL each of the potassium ferricyanide and 4-aminoantipyrine solutions were added. After mixing, 200 µL of the mixture was used for plating the wells. Absorbance measurements were made within 15 min of combining the solutions. Analyses were performed in duplicate. The original phenol solutions used in the different studies were analyzed. Amount of decrease in absorbance as compared to the absorbance of the original solution is reported as a percentage. The reported percentage amount of 2-CP removed is the mean value for the two analyses. Mean absolute deviation is also included. 2.7. Desorption Studies Details of the adsorption/desorption pair of experiments are given in this section. 2.7.1. Details of Adsorption C6-AGT (0.5 g) and activated carbon (0.5 g) were each placed in two different 250 mL Wheaton bottles and mixed with a 2-CP solution (100 mL, 75 ppm). The bottles were wrapped in aluminum foil. They were placed in the shaker and allowed to equilibrate at 250 rpm overnight. Then, suspensions were gravity filtered and poured back in the bottles. This was repeated five times to ensure complete transfer of the solids on to the filter paper. They were allowed to air dry on the lab bench overnight. They had dried to constant weight at that point. The filtrate was analyzed to determine the percentage of 2-CP adsorbed. 2.7.2. Details of Desorption One hundred and fifty milligrams (150 mg) of C6-AGT and activated charcoal that had adsorbed the 2-CP (see Section 2.7.1) were each placed in three of the amber tubes. The three tubes were filled with aqueous hydrochloric acid (0.1 N, 30 mL), the sodium hydroxide solution (0.1 N, 30 mL), and Millipore filtered DI (de-ionized) water (30 mL). The solutions were placed in the shaker and equilibrated at room temperature at a speed of 250 rpm for 2 h. The solutions were then analyzed for 2-CP following the procedure described in Section 2.6 for Phenol Analysis. 3. Results and Discussion 3.1. Synthesis 3.1.1. Tosylation Step In the paper by Abel et al. [9], the tosylation was performed on cotton cloth, which is mostly cellulose. To determine the time needed for the reaction with SEGT, which has lignins and proteins in addition to the cellulose, an aliquot was removed at the 2 h period. At 2 h, the percent sulfur, an indicator of tosyl groups incorporated in the tea leaves, based on elemental analysis, was found to be 1.9%. That number rose to 2.7% when the reaction was allowed to proceed for 89 h. According to Abel et al. [9], the primary hydroxyl site in the glucose ring was expected to be functionalized quite specifically relative to secondary sites within the same molecule. The authors presumed the tosylation to occur at the 6-position of the carbohydrate unit without any reaction at the available 2and 3-position hydroxyl sites. However, as can be seen in the discussion in Section 3.1.2, the secondary hydroxyls appear to be tosylated. It is attributed to the extended reaction time employed for this step. Note that the molar excess used in the case SEGT was identical to that used in the case of cotton [9]. The percent sulfur
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jumped to 2018, 5.8%5, when Environments x
the reaction was scaled up from 1 g to 30 g. The only difference is that 6 ofthe 15 reaction was allowed to progress a little further; a reaction time of 96 versus 89 h. The other pertinent thethe useuse of very dry SEGT as the starting material.material. It is therefore change during duringscaleup scaleupwas was of very dry SEGT as the starting It is important therefore important oven dry the soxhletgreen extracted greentotea prior to performing the step. tosylation step. to oven drytothe soxhlet extracted tea prior performing the tosylation The water-holding water-holding capacity capacity of of green tea leaves is approximately 6% [8]. Thus, Thus, SEGT SEGT gained gained The significant significant weight weight under under ambient ambient humid humid conditions conditions when when left left out out on on the lab bench. ItIt is is therefore therefore important to not only to dry the SEGT but also to store it in an airtight bottle until future use. According to toAbel Abelet.al. et.al. tosylation on cloth be accomplished thepyridine. use of [9],[9], tosylation on cloth couldcould be accomplished withoutwithout the use of pyridine. Such tosylations their had been performedmedium in acetonitrile medium solid Such tosylations in their caseinhad beencase performed in acetonitrile with solid sodiumwith carbonate sodium carbonate present an acidmodified sink. Thecarbohydrate resultant modified carbohydrate exhibited present as an acid sink. Theas resultant materials exhibited thematerials same anti-bacterial the sameasanti-bacterial activity as those produced using base. In the case ofperformed tosylation activity those produced using pyridine as a base. In pyridine the case as of atosylation reactions reactions on the leaves, there was a significant difference rates of reaction. on the teaperformed leaves, there wastea a significant difference in rates of reaction.inPyridine increasedPyridine the rate increased rate of reaction significantly. For example,was when tosylation performed on20 a 1mL g of reactionthe significantly. For example, when tosylation performed onwas a1g scale using scale using 20 mL2 acetonitrile and 2 g sodium and the numberofofTsCl equivalents TsCl acetonitrile and g sodium carbonate and thecarbonate same number of same equivalents as in theof case of as in the case of as using as the solvent, the percent sulfur was only even after 24 h at room using pyridine the pyridine solvent, the percent sulfur was only 1.8 even after 1.8 24 h at room temperature. temperature. The reaction pyridine the solvent achieves this2stage The reaction with pyridinewith as the solventas achieves this stage within h. within 2 h. Using the thesodium sodiumcarbonate carbonate along acetonitrile the solvent also acreated a challenge. different along withwith acetonitrile as theas solvent also created different challenge. sodium carbonate using waterinresulted in basicthat solutions that to appeared to RemovingRemoving the sodiumthe carbonate using water resulted basic solutions appeared hydrolyze hydrolyze tosylate sodiumhardened carbonateand hardened formed a cake which thewere tea the tosylatethe group. Thegroup. sodiumThe carbonate formedand a cake in which theintea leaves leaves were trapped. it took longer fortothe to be mechanically of thecausing tea leaves trapped. Thus, it tookThus, longer for the leaves beleaves mechanically freed of the freed tea leaves the causing the hydrolysis to occur. hydrolysis to occur. 3.1.2. The The Basis Basis for for the the 1:8 1:8 Mole Mole Ratio Ratio of of Diamine Diamine to to Tosylate Tosylate 3.1.2. The main main challenge challenge in in the the synthesis The tosylate tosylate group group in in the the The synthesis was was to to avoid avoid cross-linking. cross-linking. The polymeric backbone backbone can can be be displaced displaced by by each each of of the the amine aminegroups; groups;one oneat ateach eachend endof ofthe thediamine. diamine. polymeric Scheme 33 shows shows this this using using structural structural drawing drawing where where the the Thus, a cross-linked structure could result. Scheme jagged lines represent the polymer backbone. jagged lines represent the polymer backbone. The 1:8 mole ratio was based on the author’s (RI) prior industrial experience. This This prior prior work work The involved trans-esterification of a diol. Less than 6 weight percent diester was obtained when a 6-fold involved trans-esterification of a diol. Less than 6 weight percent diester was obtained when a 6-fold molar excess of the diol was used.
Scheme 3. 3. Possible Possible Cross-linking Cross-linking Mechanism. Mechanism. Scheme
3.2. Characterization of SEGT, TosGT, and AGT Leaves 3.2. Characterization of SEGT, TosGT, and AGT Leaves 3.2.1. Elemental Analysis 3.2.1. Elemental Analysis Elemental analysis was performed in duplicate. The average of the two analyses is summarized Elemental analysis was performed in duplicate. The average of the two analyses is summarized in Table 2. Note that the percentage of nitrogen in SEGT corroborates well with the suggestion of 25% in Table 2. Note that the percentage of nitrogen in SEGT corroborates well with the suggestion of crude protein in green tea leaves by Ramdani et al. [8]. As expected, in the case of TosGT, the percentage 25% crude protein in green tea leaves by Ramdani et al. [8]. As expected, in the case of TosGT, the of sulfur increases dramatically and the percentage of nitrogen goes down. percentage of sulfur increases dramatically and the percentage of nitrogen goes down. Since the chlorine analysis was not performed in duplicate, the surface of the tosylated green tea (TosGT) was analyzed using energy dispersive X-rays. As can be seen in Figure 1, the surface
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Since2018, the5,chlorine Environments x
analysis was not performed in duplicate, the surface of the tosylated green tea 7 of 15 (TosGT) was analyzed using energy dispersive X-rays. As can be seen in Figure 1, the surface contains contains very little chlorine suggesting verycontribution, little contribution, any,TsCl fromtoTsCl the sulfur very little chlorine suggesting very little if any,iffrom the to sulfur valuesvalues seen in seen in Table 2. Table 2. p-toluenesulfonyl can hydrolyze hydrolyzeto to p-toluenesulfonic (p-TsOH). However, p-toluenesulfonylchloride chloride can p-toluenesulfonic acid acid (p-TsOH). However, p-TsOH p-TsOH is extremely water soluble and has most likely been removed by multiple water washes asin is extremely water soluble and has most likely been removed by multiple water washes as described described in the experimental section. Towhether determine the soxhlet extracted green for the experimental section. To determine thewhether soxhlet extracted green tea used for tea the used tosylation the tosylation reaction contained chlorine, EDS of the SEGT was obtained. As can be seen from reaction contained chlorine, EDS of the SEGT was obtained. As can be seen from Figure 2, no detectable Figure 2, no amount was sample. found in the starting sample. amount of detectable chlorine was foundofinchlorine the starting InIn Table 2, the general trend notednoted with the green tea leaves that theispercent Table 2, the general trend withaminated the aminated green tea(AGT) leavesis(AGT) that the nitrogen increases and the percent sulfur decreases when compared to the TosGT. Using the percent percent nitrogen increases and the percent sulfur decreases when compared to the TosGT. Using sulfur in the sulfur various aminated leaves,tea theleaves, density amineof groups was estimated to beto the percent in the varioustea aminated the of density amine groups was estimated 0.4–0.6 mmol/g. Interestingly, the tosylate groupgroup does not to be to completely replaced by theby be 0.4–0.6 mmol/g. Interestingly, the tosylate doesappear not appear be completely replaced amine. The most likelylikely explanation for the not not being fully displaced is the presence ofof the amine. The most explanation for tosylate the tosylate being fully displaced is the presence secondary tosylate groups [13]. AA displacement reaction atat a secondary carbon would require higher secondary tosylate groups [13]. displacement reaction a secondary carbon would require higher temperatures due to the higher energy of activation associated with the transition state involved. temperatures due to the energy of activation associated with the transition state involved. Thus, Thus, the secondary tosyl groups, mostdid likely, diddisplaced not get displaced ambient conditions. The the secondary tosyl groups, most likely, not get at ambientat conditions. The displacement displacement even of the primary groups might not have to completion. However, even of the primary tosyl groupstosyl might not have occurred to occurred completion. However, using higher using higher reaction temperatures was notinconsidered interest of the maintaining structural reaction temperatures was not considered the interestinofthe maintaining structuralthe integrity of the integrity of the tea leaves. tea leaves. Table Elemental analyses. Table 2.2. Elemental analyses.
Material % Carbon Material % Carbon SEGT 45.82 SEGT 45.82 TosGT 49.82 TosGT 49.82 C6-AGT 50.3 C6-AGT 50.3 C8-AGTC8-AGT 50.35 50.35 C10-AGT 50.37 50.37 C10-AGT C12-AGT ** C12-AGT ** 51.5 51.5
% Hydrogen % Hydrogen 6.25 6.25 5.68 5.68 6.10 6.10 6.47 6.47 6.49 6.49 6.48 6.48
% Nitrogen % Nitrogen 4.09 4.09 3.38 3.38 4.7 4.7 4.07 4.07 4.91 4.91 4.65 4.65
% Sulfur % Sulfur 0.085 0.085 5.81 5.81 3.5 3.5 4.54 4.54 3.923.92 3.813.81
% Chlorine % Chlorine *** *** 0.45 * 0.45 * *** *** *** *** *** *** *** ***
* Chlorine was not notobtained obtained duplicate. ** These numbers are the average of a triplicate * Chlorinenumber number was in in duplicate. ** These numbers are the average of a triplicate because of sample of variations. Not obtained. SEGT, Soxhlet Extracted Green Tea; AGT, aminated TosGT, tosylated because sample *** variations. *** Not obtained. SEGT, Soxhlet Extracted Greengreen Tea; tea; AGT, aminated green tea. green tea; TosGT, tosylated green tea.
Element Carbon Oxygen Nitrogen Sulfur Chlorine
Atom % 64.88 27.24 4.01 3.75 0.12
Figure Energy Dispersive X-ray spectra and Atom Percent tosylated green (TosGT). Figure 1. 1. Energy Dispersive X-ray spectra and Atom Percent of of thethe tosylated green teatea (TosGT).
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Element Element Carbon Carbon Oxygen Oxygen Nitrogen Nitrogen Sulfur Sulfur Chlorine Chlorine
Atom Atom% % 70.75 70.75 24.18 24.18 5.02 5.02 0.04 0.04 0.01 0.01
Figure Energy Dispersive X-ray spectra and Atom Percent SEGT. Figure 2.2.2. Energy Dispersive X-ray spectra and Atom Percent of SEGT. Figure Energy Dispersive X-ray spectra and Atom Percent ofof SEGT.
EDS the aminated leaves obtained to there were EDS spectra of the theaminated aminated tea leaves were obtained to determine determine whether there were EDSspectra spectra of of teatea leaves werewere obtained to determine whetherwhether there were significant significant amounts of N on the surface of the aminated tea leaves. A typical spectrum is shown in significant amounts of N on the surface of the aminated tea leaves. A typical spectrum is shown in3. amounts of N on the surface of the aminated tea leaves. A typical spectrum is shown in Figure Figure 3. As expected, the third most abundant element on the surfaces of the aminated tea leaves is Figure 3. As expected, third most abundant on the surfaces of the aminated is As expected, the thirdthe most abundant element element on the surfaces of the aminated tea leavestea is leaves nitrogen. nitrogen. nitrogen.
Element Element Carbon Carbon Oxygen Oxygen Nitrogen Nitrogen Sulfur Sulfur Chlorine Chlorine
Atom Atom% % 76.17 76.17 15.20 15.20 6.32 6.32 1.57 1.57 0.75 0.75
Figure 3.3.3. Energy Dispersive X-ray spectra and Atom Percent of C6-AGT. Figure Energy Dispersive X-ray spectra and Atom Percent C6-AGT. Figure Energy Dispersive X-ray spectra and Atom Percent ofof C6-AGT.
3.2.2. Fourier-TransformedInfrared InfraredSpectra Spectra 3.2.2. Fourier-Transformed 3.2.2. Fourier-Transformed Infrared Spectra Figures44 4and and55 5show showthe theFTIR FTIRspectra spectra SEGT and TosGT,respectively. respectively.Note Notethat that there Figures of SEGT and TosGT, there isisisaa a Figures and show the FTIR spectra ofof SEGT and TosGT, respectively. Note that there −1−1−1region. The presence of the significant decrease in the intensity of the O-H band in the 3500–3000 cm significant significant decrease decrease in in the the intensity intensity of the O-H O-H band band in in the the 3500–3000 3500–3000 cm cm region. region. The The presence presence of of −1 −1 −1 assigned tosylate group is is substantiated byby thethe peak at 1174 cmcm toto symmetrical stretching and the the tosylate group peak at symmetrical stretching and the tosylate group is substantiated substantiated by the peak at 1174 1174 cmassigned assigned to symmetrical stretching and −1 of −1 −1 asymmetrical stretching at 1364 cm the S=O group in organic sulfonates. Various strong S-O-C the the asymmetrical asymmetrical stretching stretching at at 1364 1364 cm cm of of the the S=O S=O group group in in organic organic sulfonates. sulfonates. Various Various strong strong −1 have −1 −1 region, stretching bandsbands are seen thein 1000–769 cm−1cm region, of which the bands at 817 S-O-C stretching are seen the of the at 817 and 667 S-O-C stretching bands arein seen in the 1000–769 1000–769 cm region, ofwhich which the bands bands atand 817667 andcm 667 cm cm−1−1 beenbeen highlighted [14].[14]. have highlighted have been highlighted [14].
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Figure 4. 4. FTIR FTIR of of SEGT SEGT (Soxhlet (Soxhlet extracted extracted green green tea leaves). leaves). Figure Figure 4. FTIR of SEGT (Soxhlet extracted green tea tea leaves).
Figure 5. FTIR of the Tosylated SEGT. Figure Figure 5. 5. FTIR FTIR of of the the Tosylated Tosylated SEGT. SEGT.
The FTIR of the aminated (C6) tea leaves shown in Figure 6 does not appear that different from The FTIR of the aminated (C6) tea leaves shown in Figure 6 does not appear that different from −1 region. The band at 1019 cm−1 is the tosylated leaves except (C6) for the theFigure 1000 cm The FTIRtea of the aminated tea shape leavesaround shown in 6 does not appear that different from −1 the tosylated tea leaves except for the shape around the 1000 cm region. The band at 1019 cm−−11 is − 1 most likely associated with the C-N stretch seen in aliphatic amines. On subtracting spectrum the tosylated tea leaves except for the shape around the 1000 cm region. The band the at 1019 cm of is most likely associated with the C-N stretch seen in aliphatic amines. On subtracting−1the spectrum of the tosylate from that of C6-AGT, anstretch additional at 1225 cm the is seen. This of is most likely associated with the C-N seen somewhat in aliphaticweaker amines.band On subtracting spectrum the tosylate from that of C6-AGT, an additional somewhat weaker band at 1225 cm−−11 is seen. This is also attributed to C-N vibrations from the amine functionality. Some of the other bands due to the tosylate from that of C6-AGT, an additional somewhat weaker band at 1225 cm is seen. This is also attributed to C-N vibrations from the amine functionality. Some of the other bands due to amines are broad and vibrations are most likely buried under the tosylate functionality that has due not been fully also attributed to C-N from the amine functionality. Some of the other bands to amines amines are broad and are most likely buried under the tosylate functionality that has not been fully displaced. Theare FTIRs the C8, C10, under and C12 tea leaves arethat very to the IRdisplaced. of the C6 are broad and mostoflikely buried theaminated tosylate functionality hassimilar not been fully displaced. The FTIRs of the C8, C10, and C12 aminated tea leaves are very similar to the IR of the C6 aminated tea leaves shown in Figure 6. The FTIRs of the C8, C10, and C12 aminated tea leaves are very similar to the IR of the C6 aminated aminated tea leaves shown in Figure 6. tea leaves shown in Figure 6. 3.2.3. Morphological Texture 3.2.3. Morphological Texture 3.2.3. Morphological Texture The microscopic view of the surfaces was obtained to determine whether the two chemical The microscopic view of the surfaces was obtained to determine whether the two chemical stepsThe used in modifying thetheleaves created any significant morphological Thesesteps are microscopic view of surfaces was obtained to determine whether thechanges. two chemical steps used in modifying the leaves created any significant morphological changes. These are presented in Figures Note that the structures seen in changes. SEGT in Figure 7a are also seen used in modifying the7–9. leaves created any“rosette” significant morphological These are presented in presented in Figures 7–9. Note that the “rosette” structures seen in SEGT in Figure 7a are also seen on the surfaces of the four aminated tea leaves in Figures 8a,b and 9a,b. This suggests that the Figures 7–9. Note that the “rosette” structures seen in SEGT in Figure 7a are also seen on the surfaces of on the surfaces of the four aminated tea leaves in Figures 8a,b and 9a,b. This suggests that the reactions performed SEGT did not8a,b change the morphology. Since the physical integrities of the the four aminated tea on leaves in Figure and Figure 9a,b. This suggests that the reactions performed reactions performed on SEGT did not change the morphology. Since the physical integrities of the four aminated tea leaves appear to be similar, their ability to interact with 2-CP is most likely not due four aminated tea leaves appear to be similar, their ability to interact with 2-CP is most likely not due to textural differences. to textural differences.
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on SEGT did not change the morphology. Since the physical integrities of the four aminated tea leaves appear to be similar, their ability to interact with 2-CP is most likely not due to textural differences. Environments 2018, Environments 2018, 5, x5, x 10 10 of of 15 15
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Figure 6. FTIR the of the C6 aminated leaves. Figure 6. FTIR aminated teatea leaves. Figure 6. FTIR of of the C6 C6 aminated tea leaves.
(a)(a) (a)
(b)(b) (b)
Figure 7. Scanning electron micrographs of (a) SEGT and TosGT. Figure 7. Scanning electron micrographs of (a) SEGT and (b)(b) TosGT. Figure 7. Scanning electron electron micrographs TosGT. Figure Scanning of (a) SEGT and (b) TosGT.
(a)(a) (a)
(b)(b) (b)
Figure 8. Scanning electron micrographs of (a) C6-AGT and C8-AGT. Figure 8. Scanning electron micrographs of (a) C6-AGT and (b)(b) C8-AGT. Figure Figure 8. 8. Scanning Scanning electron electron micrographs micrographs of of (a) (a) C6-AGT C6-AGT and and (b) (b) C8-AGT. C8-AGT.
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(a)
(b)
Figure Figure 9. 9. Scanning Scanning electron electron micrographs micrographs of of (a) (a) C10-AGT C10-AGT and and (b) (b) C12-AGT. C12-AGT.
3.2.4. 3.2.4. Confirmation Confirmation of of Amine Amine Functionality Functionality In the case caseof ofmodifying modifyinga apolycarbonate polycarbonate (PC) surface to introduce amine groups neither In the (PC) surface to introduce amine groups [15],[15], neither XPS XPS nor ATR-FT-IR spectroscopy definitively identified the presence of functional at the nor ATR-FT-IR spectroscopy definitively identified the presence of functional amines atamines the surface of surface of the treated PC samples.a Therefore, a surface-specific confirmofthe presence of the treated PC samples. Therefore, surface-specific test to confirmtest theto presence primary amine primary amine groups for their application. groups was devised forwas theirdevised application. As be seen seen from the data data presented presented thus the case case for for the the aminated aminated tea tea leaves leaves parallels parallels As can can be from the thus far, far, the the aminated surface analysis of the polycarbonate. The dye assay for surface analysis for the aminated surface analysis of the polycarbonate. The dye assay for surface analysis for primary primary amines chemical reaction picric acid acid or or acid acid orange orange 77 [16]. [16]. Rather Rather than than amines is is based based on on aa chemical reaction that that uses uses either either picric develop dye-specific test amines based based on on these these two two phenols, phenols, the the following following experiments experiments with with develop aa dye-specific test for for amines 2-CP provided evidence for the presence of the amine functionality. 2-CP provided evidence for the presence of the amine functionality. Preliminary evidence for for the Preliminary evidence the presence presence of of the the amine amine group group was was obtained obtained by by adding adding the the aminated aminated and solutions. After and native native tea tea leaves leaves to to varying varying concentrations concentrations of of 2-CP 2-CP solutions. After brief brief vortexing, vortexing, the the pH pH of of the the solution was gauged using litmus paper. The pH of a 2-CP solution (2%) rose from 4 to 6 at a 500 mg solution was gauged using litmus paper. The pH of a 2-CP solution (2%) rose from 4 to 6 at a 500 mg loading ofC6-AGT. C6-AGT.On Onthe the other hand, even at loadings inrange the range 1500 mg ofground finely loading of other hand, even at loadings in the of 940ofto940 1500tomg of finely ground SEGT, there was no change in the pH. SEGT, there was no change in the pH. Further, aminated GTs GTs have have shown shown aa significantly significantly higher higher ability ability to to pick pick up up phenol Further, aminated phenol compared compared to to native leaves. This results below. below. Also, Also, the the desorption desorption studies studies discussed native tea tea leaves. This is is shown shown in in the the results discussed below below confirm the presence presence of of “basic” “basic” functional functional groups groups in in the the aminated aminated tea tea leaves. leaves. confirm the
3.3. Phenol Analysis 3.3.1. Effect Effect of of pH pH 3.3.1. An initial initial study study using using SEGT SEGT and and C6-AGT C6-AGT was was performed performed to to determine determine whether whether the the removal removal of of An 2-CP would would be be sensitive sensitive to to the the pH pH of Based on on the the study, study, it it became 2-CP of the the solution. solution. Based became clear clear that that under under acidic pH conditions the amine group on the surface of the modified tea leaves most likely exists acidic pH conditions the amine group on the surface of the modified tea leaves most likely exists as + , making it unable to interact with the acidic phenolic group in 2-CP. as the ammonium ion R-NH 3 ⁺ the ammonium ion R-NH3 , making it unable to interact with the acidic phenolic group in 2-CP. Under basic conditions, the phenolate ion of 2-CP would not be able to interact with the basic amine Under basic conditions, the phenolate ion of 2-CP would not be able to interact with the basic amine functionality in AGTs. This result corroborated with the work done by Bernal et al. [17]. Their study functionality in AGTs. This result corroborated with the work done by Bernal et al. [17]. Their study confirmed the phenol was adsorbed on the basic sites of activated carbon best at pH values close to confirmed the phenol was adsorbed on the basic sites of activated carbon best at pH values close to neutral. The remaining two studies (effect of varying mass of adsorbent and concentration of 2-CP) neutral. The remaining two studies (effect of varying mass of adsorbent and concentration of 2-CP) were therefore performed using the 2-CP solution “as-such” without any pH adjustments. were therefore performed using the 2-CP solution “as-such” without any pH adjustments. 3.3.2. Effect of Varying the Mass of Material 3.3.2. Effect of Varying the Mass of Material The concentration of the 2-CP solution used in this study was 50 ppm. The materials included The concentration of the 2-CP solution used in this study was 50 ppm. The materials included in in the study were activated charcoal (AC), SEGT, C6 AGT, C10-AGT, and C12-AGT. The amounts of the study were activated charcoal (AC), SEGT, C6 AGT, C10-AGT, and C12-AGT. The amounts of adsorbent used were 50 mg, 75 mg, 100 mg, 150 mg, and 200 mg. Each of the amounts was studied in adsorbent used were 50 mg, 75 mg, 100 mg, 150 mg, and 200 mg. Each of the amounts was studied in duplicate for each material. Thus, 50 of the brown threaded tubes were labeled and charged with the duplicate for each material. Thus, 50 of the brown threaded tubes were labeled and charged with the appropriate amounts and type of adsorbent. The tubes were then filled with 30 mL of the 2-CP solution and allowed to equilibrate as described in Section 2.5. The solutions were then analyzed for
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appropriate amounts and type of adsorbent. The tubes were then filled with 30 mL of the 2-CP solution and allowed to equilibrate as described in Section 2.5. The solutions were then analyzed for 2-CP as described in Section 2.6. The percentages of 2-CP removed are based on the decrease in absorbance compared to that of the original solution (50 ppm). The results thus obtained are summarized in Table 3. For each entry in this table, the mean percentage removal (over two analyses) is shown along with the mean absolute deviation in parenthesis. Table 3. Percentages of 2-CP removed from a 50 ppm solution using varying masses of adsorbent. Material
50 mg
75 mg
100 mg
150 mg
200 mg
SEGT C6-AGT C10-AGT C12-AGT AC
5.3 (2.2) 21.8 (0.7) 20.7 (0.7) 14.7 (0.7) 89.3 (0.9)
4.8 (0.6) 24.7 (1.3) 21.1 (0.8) 17.2 (0.1) 89.3 (0.2)
6.0 (0.5) 25.8 (0.1) 24.2 (0.5) 16.3 (0.1) 89.4 (0.9)
8.7 (0.3) 32.5 (1.9) 26.7 (0.2) 16.7 (0.7) 89.2 (0.8)
12.0 (0.8) 32.1 (0.9) 27.6 (0.6) 22.8 (0.1) 90.5 (0.7)
AC, activated carbon.
The goal of this study was to determine the mass of adsorbent beyond which the 2-CP removal levels off for this particular configuration used in the experiment. The configuration is defined by the following parameters: the dimensions of the tube and the volume of the solution used, temperature, and the orbital agitation used at the speed studied. As can be seen from Table 3, the ability of activated carbon to remove 2-CP reached 90% even at 50 mg. The SEGT and the aminated tea leaves were used as such without creating fine particles. The C6-AGT removes 2-CP almost 3–4 times better than SEGT. Among the different AGTs, C6 appears to be the best. In the case of the C6-AGT, the removal capacity levels off at a loading of 150 mg of the adsorbent. Therefore, in the next study, when the effect of varying concentrations of the 2-CP solution was studied, the amount of adsorbent used was set to 150 mg. 3.3.3. Effect of Varying the Concentration of 2-CP Since activated charcoal and SEGT were at the two ends in their ability to remove phenol, one much better, and the other significantly worse than the AGTs, this study only focused on comparing the four AGTs. The varying initial concentrations used for the study were 50 ppm, 75 ppm, 100 ppm, 150 ppm, and 200 ppm solutions of aqueous 2-CP. Brown threaded tubes were loaded with 150 ± 1 mg of the material and each tube received 30 mL of each of the concentrations of 2-CP. Each of the concentrations was studied in duplicate for each material. The mean percentage of 2-CP removed for the varying concentrations is summarized in Table 4. For each entry in this table, the mean percentage removal (over two analyses) is shown along with the mean absolute deviation in parenthesis. Table 4. Percentages of 2-CP removed using 150 mg of adsorbent at varying concentrations of 2-CP. Material
50 ppm
75 ppm
100 ppm
150 ppm
200 ppm
C6-AGT C8-AGT C10-AGT C12-AGT
32.9 (0.1) 23.3 (0.8) 26.6 (1.1) 23.4 (1.0)
41.4 (2.3) 30.2 (0.4) 35.7 (0.1) 29.1 (0.5)
38.3 (0.2) 31.4 (1.0) 36.8 (0.5) 29.6 (0.4)
40.3 (0.7) 31.9 (0.6) 39.6 (0.7) 33.0 (0.1)
38.9 (0.7) 32.0 (0.3) 41.7 (0.4) 33.2 (0.4)
Based on these studies, it has been concluded that tethering the amine group at the end of six atoms from the substrate produces the best results. Forty percent (40%) of the 2-CP is removed from aqueous solutions at a concentration of 75 ppm when 150 mg of the adsorbent is used. The synthesis of C6-AGT has not been optimized. These results are therefore very encouraging because there is considerable scope for improvement. After having studied phenol removal, attention was focused on the desorption ability of these adsorbents.
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3.3.4. Desorption Studies To release the phenol easily so that it can be re-used in the industry is just as important as being 13 of 15 able to remove the phenol from the aqueous waste streams. 2-CP was expected to be removed by the amine groups in AGT a reaction theamine amineand andphenol phenoltotoform form the the ammonium ammonium phenolate amine groups in AGT viavia a reaction of of the phenolate entity. Therefore, using acid or base to release the sorbed phenol was expected In entity. Therefore, using acid or base to release the sorbed phenol was expected to to work work quite quite well. well. In Section 2.7.1, the experiment performed for removing 2-CP from water using activated charcoal and Section 2.7.1, the experiment performed for removing 2-CP from water using activated charcoal and C6-AGT isisdescribed. with the the 2-CP2-CP was was then desorbed using 0.1 N HCl, 0.1HCl, N aqueous C6-AGT described.The Theadsorbent adsorbent with then desorbed using 0.1 N 0.1 N NaOH solution, and DI water. Refer to Section 2.7.2 for the experimental details. Percentages phenol aqueous NaOH solution, and DI water. Refer to Section 2.7.2 for the experimentalofdetails. adsorbed and were measured according to measured the detailsaccording describedto inthe Section 2.6.described The results Percentages ofdesorbed phenol adsorbed and desorbed were details in obtained2.6. from desorption are summarized Table are 5. Note that the percentages are based Section Thethese results obtainedstudies from these desorption in studies summarized in Table 5. Note that on the amount ofare 2-CP thaton was the percentages based theadsorbed. amount of 2-CP that was adsorbed. Environments 2018, 5, x
Table 5. Percentages of 2-CP desorbed using aq. acid, aq. base, and neutral water. Table 5. Percentages of 2-CP desorbed using aq. acid, aq. base, and neutral water.
Solution 0.1 N HCl 0.1 N NaOH DI Water
Solution 0.1 N HCl 0.1 N NaOH DI Water
C6-AGT C6-AGT Activated Charcoal Activated Charcoal 4.5 55.5 55.5 4.5 9.9 70.3 70.3 9.9 27.0 27.0 3.4 3.4
DI, DI, de-ionized. de-ionized.
The reason reasonfor forthe the higher amounts of phenol released with thefor base, for charcoal and the higher amounts of phenol released with the base, charcoal and the aminated aminated tea, is most to the higherofsolubility of the sodium in phenoxide water. the tea, is most likely due likely to the due higher solubility the sodium phenoxide water. Oninthe otherOn hand, other acidic 2-CP, conditions, has a lower in water its underhand, acidicunder conditions, which 2-CP, has a which lower solubility in solubility water compared tocompared its sodiumtosalt, sodium salt,This is released. is shown is released. is shownThis in Scheme 4. in Scheme 4.
Scheme Scheme 4. 4. Showing Showing the the pH pH dependence dependence on on Phenol Phenol release. release.
The more important point to be realized from the results in Table 5 is the much lower desorption The more important point to be realized from the results in Table 5 is the much lower desorption in the case of the activated charcoal. This is most likely due to the 2-CP volatilizing off during the in the case of the activated charcoal. This is most likely due to the 2-CP volatilizing off during the drying of the adsorbent to get rid of the water. The 2-CP is adsorbed on to the surface of the charcoal is drying of the adsorbent to get rid of the water. The 2-CP is adsorbed on to the surface of the charcoal is most likely held in place by very weak physical forces, such as London dispersion forces. On the other most likely held in place by very weak physical forces, such as London dispersion forces. On the other hand, in the case of C6-AGT, the 2-CP is held in place by an acid–base reaction. Thus, the 2-CP could hand, in the case of C6-AGT, the 2-CP is held in place by an acid–base reaction. Thus, the 2-CP could not volatilize off. It was most likely released by the reactions shown in Scheme 4. not volatilize off. It was most likely released by the reactions shown in Scheme 4. In addition to the acid–base interactions, other weaker intermolecular forces also come into play In addition to the acid–base interactions, other weaker intermolecular forces also come into play in the case of C6-AGT. This is evident by noting that DI water was able to release a significant amount in the case of C6-AGT. This is evident by noting that DI water was able to release a significant amount of the 2-CP from C6-AGT. This is most likely due to the substances in the lignin cellulosic portions of of the 2-CP from C6-AGT. This is most likely due to the substances in the lignin cellulosic portions of C6-AGT. In other words, the amine groups in C6-AGT interact with 2-CP via ionic interactions while C6-AGT. In other words, the amine groups in C6-AGT interact with 2-CP via ionic interactions while the remaining biomass holds the 2-CP via H-bonding and dipole–dipole interactions. the remaining biomass holds the 2-CP via H-bonding and dipole–dipole interactions. Thus, the desorption studies clearly indicate that though charcoal removes a higher percentage of the 2-CP from aqueous solutions, it releases the phenol just as easily. Its method of removal is dependent on many “weaker” forces mainly because of its enhanced surface area. However, these weaker forces are “unzipped” more easily. The ability of the activated charcoal to give up the 2-CP so easily suggests that engineering controls would be needed in the case of activated charcoal. The amination step has not been optimized. Use of higher temperatures in the amination step
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Thus, the desorption studies clearly indicate that though charcoal removes a higher percentage of the 2-CP from aqueous solutions, it releases the phenol just as easily. Its method of removal is dependent on many “weaker” forces mainly because of its enhanced surface area. However, these weaker forces are “unzipped” more easily. The ability of the activated charcoal to give up the 2-CP so easily suggests that engineering controls would be needed in the case of activated charcoal. The amination step has not been optimized. Use of higher temperatures in the amination step needs to be explored. It also needs to be seen if one of the amine groups in the diamine can be converted to the ammonium group as shown by Abel et al. [9]. This will solve the cross-linking issue by the introduction of a protection-deprotection step. Thus, aminated tea leaves with an enhanced number of amine functional groups on the surface could offer the distinct advantage of easy and quantitative release of 2-CP compared to activated charcoal. 4. Conclusions In this study, among the four aminated tea leaves, C6-AGT, which has a chain of six methylenes, showed the best adsorption capacity. About 40% of the 2-CP is removed from a 75 ppm aqueous solution at a loading of 150 mg. The percentages adsorbed and desorbed can be further improved by optimizing the reaction conditions used for making these aminated tea leaves. Desorption using aqueous acidic or basic solutions released more than twice as much of the 2-CP compared to neutral water, strongly suggesting acid–base interactions. This confirmed our expectations that the amine group in the adsorbent would interact with the phenolic group in 2-CP via an acid–base reaction. Tethering the amine beyond six carbons does not appear to produce any additional benefit. The ultimate goal of this project is to prepare this C6-AGT using aqueous environmentally friendly reagents. The issues with this approach have been well-documented in Melo et al.’s review of chemical modifications of lignocellulosics [18]. It appears that the green approaches lead to fewer reactions sites on the surface. Future work will explore the use of spacers of around six atoms to ameliorate this issue. The ultimate goal is to use bio-waste modified in an environmentally friendly manner to remove pollutants from our current environment. Author Contributions: R.I. designed the synthetic schemes and supervised the project. M.F.-B. and S.T. worked on the synthetic experiments. J.Y. contributed to the quantitative experiments. A.E.N. developed the methodologies for phenol analysis and material characterization. R.I. and A.E.N. contributed to the writing and editing of the manuscript. All authors have read and approved the final manuscript. Funding: This research was funded by a PSC-CUNY Award # 60295-00 48 to R.I. The Program for Research Initiatives in Science and Math (PRISM) at John Jay College through the US Department of Education Title-V grant # P031C110174 supported M.F.-B. via stipends and also helped with the purchase of chemicals and analyses. The CSTEP project provided funding for S.T. and J.Y. Acknowledgments: R.I. would like to acknowledge K. Grohmann, Emeritus Professor, Hunter College, CUNY for discussions related to syntheses and Chiu Hong Lee for her technical support. Research Director Bach at BMCC and the Science Department at BMCC also played a key role in supporting this work. We would like to thank the anonymous reviewers for their critical reading of the manuscript and helpful comments. Conflicts of Interest: The authors declare no conflict of interest.
References 1. 2. 3.
Volesky, B. Sorption and Biosorption; BV Sorbex: Montreal, QC, Canada, 2003. Saka, C.; Sahin, O.; Kucuk, M.M. Applications on agricultural and forest waste adsorbents for the removal of lead (II) from contaminated waters. Int. J. Environ. Sci. Technol. 2012, 9, 379–394. [CrossRef] Desta, M.B. Batch Sorption Experiments: Langmuir and Freundlich Isotherm Studies for the Adsorption of Textile Metal Ions onto Teff Straw (Eragrostis tef ) Agricultural Waste. J. Thermodyn. 2013, 2013, 375830. [CrossRef]
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