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Design of a selective regenerable cellulose microcolumn for selenium efficient recovery and economic determination
Accepted Manuscript Title: Design of a selective regenerable cellulose microcolumn for selenium efficient recovery and economic determination Author: Min Min Chen Shen Lei Fang Bingren Zhu Jionghui Li Lanying Yao Yujian Jiang Chunhua Xiong PII: DOI: Reference:
S0263-8762(16)30465-8 http://dx.doi.org/doi:10.1016/j.cherd.2016.11.032 CHERD 2504
To appear in: Received date: Revised date: Accepted date:
14-12-2015 20-8-2016 27-11-2016
Please cite this article as: Min, Min, Shen, Chen, Fang, Lei, Zhu, Bingren, Li, Jionghui, Yao, Lanying, Jiang, Yujian, Xiong, Chunhua, Design of a selective regenerable cellulose microcolumn for selenium efficient recovery and economic determination.Chemical Engineering Research and Design http://dx.doi.org/10.1016/j.cherd.2016.11.032 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Design of a selective regenerable cellulose microcolumn for selenium efficient recovery and economic determination Min Mina, Chen Shena, Lei Fangb, Bingren Zhua, Jionghui Lic, Lanying Yaoa, Yujian Jianga, Chunhua Xionga* [email protected] a
Department of Applied Chemistry, Zhejiang Gongshang University, No. 149 Jiaogon
gRoad, Hangzhou, 310012, PR, China. b
Department of Food Science and Human Nutrition, University of Florida, Bldg 475
Newell Drive, Gainesville, FL 32611, USA c
School of Environmental Science and Engineering, Zhejiang Gongshang University,
Hangzhou, 310012, China. *Corresponding author. Tel: +86-0571-88071024-7571.
Graphical Abstract
Proposed synthesis routes to TMA-C and structure changes before and after synthesis
Highlights
Functionalized regenerate cellulose (TMA-C), was synthesized via one-step reaction
TMA-C can effectively recovery the toxic selenium compounds from waste waters
TMA-C -Se(IV) could be easily eluted and can be reused many times
Set up an coupling technique for economic and effective determination of
Se(IV)
Abstract Recovery the toxic selenium compounds selenite and selenate from waste waters before discharging is imperative in industrialized countries. In this study, A functionalized regenerable cellulose, based on the chemical modification of cellulose grafted thiomalic acid (TMA-C), was prepared simply with an efficient methodology for removing and recovering selenium(IV) effectively. Optimization of the TMA-C synthesis was carried out at different temperatures, different molar ratios, and dosage of catalyst using response surface methodology. The surface functionalization effect was detected with FTIR, SEM, zeta potential, EDS and TGA technique. And under optimum adsorption conditions (pH = 4.5, T = 318 K), TMA-C can remove toxic selenium compounds more than 99.9%, and have high adsorption capacity for Se (IV), which is 101.2 mg/g. The adsorption isotherms data fitted the Langmuir model well, and Pseudo-second-order kinetics showed better accordance. In addition, in the batch desorption experiments, the TMA-C could be regenerated completely with 3.0 mol/L
HCl
and
2%
KClO3
solutions.
Finally
re-generable
cellulose
microcolumn-UV-spectrophotometry was developed for effective and economic determination of trace Se(IV).
Keywords: Regenerative cellulose; Selenium (IV); Adsorption; Recovery; Economic detection
1.
2.
3. Introduction With the consumption of valuable resources, growth of energy prices, industries urgently need to look for economic and improved processes to harmful and valuable compounds from waste waters because of tighter environmental policies. Selenium (atomic number 34) is widely used for manufacturing
industrial
products,
such
as
glass,
rubber,
electronic
components, pigments (Zhou et al, 2012). But it also is a common contaminant released
from
mining,
metal
smelting
and
coal-fired
power
plants
industries(Ling et al, 2015), Selenium exists in water as oxy-anionic species, selenite (SeO32-) and selenate (SeO42-).(Zhang, Ge, Yao, Liu, Xie & Fang, 2015) And selenium in the form of Se (IV) is more toxic than Se (VI) .(Kongsri, Janpradit, Buapa, Techawongstien & Chanthai, 2013). Looking deeper into national eating patterns, the nutritional range of Se in food product between essentiality and toxicity is relativelty lower in comparison with the other essential trace elements. (Letavayova, Vlasakova, Spallholz, Brozmanova & Chovanec, 2008). Also, selenium is difficult to form independent deposits as a kind of typical scattered elements. Regardless of good selenium nutrition is antioxidant protection and for other health reasons, some confusing and contradictory research shown that taking high amounts of selenium may result in hair and nail
changes, neurologic manifestations and gastrointestinal disorders. Therefore, it is vital to develop a highly sensitive and economic method for detecting and recovering selenium present in sewage waters. Over the past few decades, a variety of technologies including, ion-exchange(Pakzadeh & Batista, 2011), membrane filtration,(Mavrov, Stamenov, Todorova, Chmiel & Erwe, 2006) reverse osmosis,(Lin, Xu, Papelis, Cath & Xu, 2014) and chemical precipitation, (Geoffroy & Demopoulos, 2011) have been developed for the removal and recovery of selenite and selenate. The disadvantages of these methods are that they cannot 100% eliminate selenium from water. Considering large volumes of mass sludge are generated daily, the high cost of reagents and the removal process is time consuming and economically unsatisfied. (Mavrov, Stamenov, Todorova, Chmiel & Erwe, 2006) Recently, removal of selenium by adsorption on a wide variety of adsorbents has been reported in several studies.(Karatchevtseva, Astoux, Cassidy, Yee, Bartlett & Griffith, 2010; Szlachta & Chubar, 2013; Tang, Huang, Zeng & Zhang, 2014) Among these, majority studies have been documented to utilize various inorganic adsorbents for the adsorptive recovery of selenite; however, the adsorption capacity of these inorganic adsorbents are relatively low.(Chubar, 2014; Jordan, Ritter, Scheinost, Weiss, Schild & Hubner, 2014) Today, biopolymer materials gradually becomes hot topic in the field of metal pollution control because of its greater environmental compatibility, lower toxicity and higher biodegradability than synthetic surfactants.(Zhu et al., 2013)
Cellulose (Cel) is the principal structural component of plants and the contemporary strive for sustainable environment has amounted to a renewed scientific
interest
as
a
source
for
commodity
chemicals
and
new
materials(Filpponen et al., 2012). Cellulose widely exists in environment and exerts several characteristics of high mechanical strength, renewability and biodegradablilty. (Yu et al., 2013) Notably, it is able to bind trace elements ions in solution with the existence of abundant polar –OH groups which would also be replaced by specific functional groups. Recently, A study reports that modified/unmodified cellulose have been used for removing trace elements or heavy-metal ions. The cellulose nitrate membrane has been prepared and applied for the efficient pre-concentration of nickel(II), cadmium(II), copper(II), cobalt(II). The levels of the analytes were determined by FAAS (Flame Atomic Absorption Spectrometry ) (Narin & Soylak, 2003) Moreover, according to another study, the adsorbents prepared by chemical modification of cellulose have shown promise for pre-concentration and separation of precious-metal ions.(Gurung, Adhikari, Gao, Alam & Inoue, 2014) Hence, the usage of cellulose to fabricate selective adsorption materials is a promising concept for the recovery and recycling of secondary resources as well as for maintaining a sustainable environment. In this study, TMA-C (cellulose modified thiomalic acid) is synthesised and the TMA-C microcolumn is prepared for Se(IV) separation and enrichment from total Se aqueous media, meanwhile UV-spectrophotometry is used for
economic determination. The response surface methodology was opted for optimizing synthesis process parameters. Se(IV) adsorption onto the TMA-C was investigated, including adsorption condition optimization, adsorption isotherms and kinetics in batch system and evaluation of breakthrough curves. The regenerable cellulose microcolumn - UV- spectrophotometry was set up for the determination of trace Se(IV) in food and waste water samples .
4. Experimental
2.1 Reagents and Apparatus Cellulose powder was purchased from LanBo Co., Hangzhou, China (Φ= 50 μm). The standard solution of selenium(IV) were prepared by dissolving analytical grade selenium powder in appropriate amount of concentrated HNO 3, KClO4 and HCl and then diluted with ultra-pure water in a 1000 ml volumetric flask. Se(IV) and Se(VI) stock solutions were prepared from analytical grade sodium selenite and sodium selenate, respectively. The pH of each test solution was adjusted with HCl solution to pH 0.5-2.5, citric acid/Na2HPO4 buffer solution to pH 3.5-6.5, NH4Cl/NH3·H2O buffer solution to pH 8.5–10.5, and Na2HPO4/NaOH solution to pH 11.5. All other reagents and solvents were of analytical grade and used as received without further purification. Infrared spectra of the TMA-C were scanned in the region of 400-4000 cm −1 in KBr pellets on NICOLET 380 FT-IR spectrophotometer; the elemental analysis was carried out using a Vario EL III Elemental Analyzer; the
thermo-gravimetric analysis was investigated using Mettler TGA/DSC1 simultaneous thermal analyzer. The shapes and surface morphology of TMA-C were observed by means of a HITACHI S-3000N scanning electron microscope (SEM). The concentrations of Se(IV) and Se(IV) were determined by Inductively coupled plasma optical emission spectrometer (ICP-OES) and UV-2550. The respective zeta potentials of TMA-C were determined by a zeta potential analyzer (Malvern Zetasizer NanoZS90).
2.2 Synthesis of TMA-C The preparation of TMA-C was carried out according to the following procedures: 1.0 g Cel was fully soaked with 50 mL reaction solvent in a 100 mL three-necked flask, and swelling overnight. Then, a certain amount of TMA and a small amount of phosphotungstic acid used as catalyst were added to the flask. The mixture reacted for 10 h at 75 ℃with 300rmp stirring speed under nitrogen atmosphere. The solid product was carefully washed thoroughly with ultra-pure water until the washing liquid was colorless. After that, the obtained TMA-C was dried in vacuum at 50 ℃.
2.3 Experimental design for RSM study Response surface methodology (RSM) is a collection of mathematical and statistical techniques for designing experiments, building models, and analyzing the interactive effects of independent factors.(Srivastava, Sharma & Sillanpää,
2015) In this study, the variables, such as temperature, molar ratio, and dosage of catalyst, are selected process parameters on the functional group capacity of TMA-C. And independent variables, experimental range and levels for the production of TMA-C were tabulated in Table 1.
2.4 Batch adsorption studies Batch adsorption experiments were performed in the conical flasks containing 30 mL adsorption solution and 20.0 mg TMA-C. Adsorption of Se(IV) from aqueous solution to the adsorbents was studied at various pH values. After 24 h, a required amount of standard solution of Se (IV) was put in. Adsorption experiments were conducted in a shaker at 100 rpm at 25 . The upper layer liquid was analyzed by UV spectrophotometer until adsorption equilibrium reached. The adsorption capacity (Q, mg g-1) and distribution coefficient (D, mL g-1) were calculated with the following formulae: Q
(Co Ce )V m
(1)
D
Q Ce
(2)
where C0 is the initial concentration in solution (mg mL-1); Ce is the equilibrium concentration in solution (mg mL-1); V is the volume of solution (mL); and m is the TMA-C dry weight (g). Desorption experiments were performed immediately after the completion of the adsorption experiments. At the end of the adsorption experiments, The TAMC-Se(IV) was placed in the iodine flasks, and then 60 mL of thiourea was
added. The NaOH, HCl, KCl, NH4Cl, mixture of HCl and KClO 3 with different concentrations were further stirred at 100 rpm for 24 h at 298 K, respectively. After desorption, the final HSeO3- concentrations in the aqueous phase were determined by ICP-OES. The desorption ratio (E) was calculated from eq 3:
E (%)
CdVd 100 C0 Ce V
(3)
where Vd is the volume of the desorption solution, and C 0, Cd are the concentration of the solutes in the desorption solutions, Ce and V are the same as defined above.
2.5 Microcolumn experiment studies The fixed-bed experiments were carried out at in a water jacketed glass column with an inner diameter of 3.0 mm and a full Length of 30 cm. An aliquot of the fresh TMA-C (200 mg) was putted into the bed. The TMA-C in the column was pre-soaked for 24 h before starting the experiment. The aqueous solution with known concentrations of Se(IV) ions was then fed to the top of the bed at a desired flow rate until the breakthrough curve was completed. The column studies were performed at the optimum pH value and temperature determined from batch studies. The samples in the outlet were taken at the present time intervals and the concentrations of Se(IV) ions were similarly determined as above, In addition, dynamic desorption procedures were also carried out.
4.6 Preparation of samples The waste water: The water samples were filtered using a filter membrane with a pore size of 0.45 µm. Then, the pH of filtrate was adjusted to 4.5 and the filtrate was used for the procedure of separation, preconcentration, and determination of Se(IV). If the sample of water contained a visible precipitate or mechanical admixtures, the sample should have been filtered through the filter paper first; When measuring total Se, 6mol/L HCl was added into samples for the reduction of Se(VI) into Se(IV) Preservation of water sample: The samples were acidified with HNO3 to avoid the co-precipitation of the metal and stored in pre-cleaned polyethylene bottles (Agrawal, Singh Patel & Shrivas, 2009) The rice: The rice were washed with tap water, deionized water respectively for several times to remove pesticides. And the sample were placed into thermostatic oven for 72h at 50 , then dried and crushed. The sample were further heated for 24h at 80 . An accurately weighed amount of 2g sample, 10.0 mL mixed acid (HNO3: HClO4 = 9:1) and a few glass beads were added to the digested bottle, cold digestion overnight. A certain amount of HNO3 was added in time sequentially to rice sample, mixed and heated. The sample solution was evaporated to 2 mL when it is clear and produce white smoke. (Add 5.0 mL 6mol / L HCl to the solution after it is cool, repeat the heating step)1 After Cooling, the solution was transferred into 50 mL volumetric flask. (GB 5009.93—2010: National food safety standard Determination of selenium in foods)
1
HCl is added in order to reduce Se(VI) to Se(IV) , so we can Recovery and measure amount of total selenium; We need not added HCl if we Measure amount of Se(IV)
2.7 Procedure The sample solution of 10ml containing the species of selenium was divided into two halves, which were named sample 1 and 2, Meanwhile, 0.5 mL of 6mol/L HCl was added into sample 2 for the reduction of Se(VI) into Se(IV). The sample 1 was moved in TMA-C column and passing the column with 1.0ml/min, The column retained Se(IV) was eluted with 3.0 mol/L HCl -2% KClO3, and the Se(IV) in eluate were determined by ICP-OES. The sample 2 (for total Se) were processed according to same procedure mentioned above. The concentration of Se(VI) in the sample solution were calculated by subtracting Se(IV) from total Se.
5. Results and Discussion 3.1 Synthesis of TMA-C 3.1.1 Analysis of variance (ANOVA) and development of regression mode The optimum quantities of the chosen changeable were obtained by regression equation and by distinguished the RSM contour plots. The optimization step for the synthesis of TMA-C was carried out using Box–Behnken design (BBD). The predicted response M for sulphur content was obtained and is given as: M=-15.42919+0.35220A+2.86850B+0.99175C+2.13333E-003AB-2.07500E-0 03AC-0.015625BC-2.21333E-003A2-0.49700B2-0.078062C2
(4)
In this equation M is the sulphur content(%), A, B and C are the independent variables in coded units, i.e., Temperature ( ), molar ratio and Dosage of catalyst(mass ratio)(%), respectively. The results from ANOVA (Table S1) display the proportional of equation represented the concrete relevance between the principal changeable and the response. There is excellent relevance between the predicted and observed quantity as shown by nearly between R 2 and adjusted R2 value. The pattern is regarded to be actuarial basic because the affiliated Prob.> F quantity for the pattern is lower than 0.05. In this model A; B; AC; A 2; B2 and C2 are significant model terms. Analysis of variance (ANOVA) for response surface quadratic model gave F-value 303.47, R2 value 0.9974 (the predicted R2 represents the prediction of a response value estimated by the model), probability < 0.0001 and coefficient of variation (C.V. = 0.77%) signifying that model is highly significant and experiments are highly accurate and reliable.(Liu, Yan, Zhang & Du, 2011) Moreover, the insignificant lack-of-fit test (F-value is 0.25) also indicated that the model was suitable to represent the experimental data using the designed experimental date.(Xu, Yin, Liu, Tang, Qu & Xu, 2013) Adequate precision is an estimation of the signal to noise ratio. A ratio greater than four is desirable. The ratio of 48.987 implies an adequate signal. This model can be used to navigate the design space.(Mourabet, El Rhilassi, El Boujaady, Bennani-Ziatni, El Hamri & Taitai, 2012)
3.1.2 The optimization of synthesis conditions The results of the sulphur content affected by temperature, molar ratio, and mass ratio of catalyst are shown in panels a, b, and c of Fig. 1, respectively. These types of plots show the effects of two factors on sulphur content while the other factor was kept at level zero. From the 3D response surface plots, the optimal values of the parameters could be observed. The RSM-guided optimization demonstrated that the optimal synthesis conditions of TMA-C are: temperature of 78.64 , molar ratio of 2.98, and dosage of catalyst of 5.01, and the predicted sulphur content reached 5.17303%. Three synthesis experiments of TMA-C were implemented under the predicted optimum conditions in order to validate the mathematical models developed. The mean value of sulphur concent was 5.101%, which obtained from real experiments what were in close agreement with the predicted value, indicating that the developed model was adequate for predicting the synthesis conditions of TMA-C.
3.2 Characterization of TMA-C The structures of Cel, TMA, mixture of Cel and TMA, TMA-C were confirmed by FT-IR spectroscopy and the results are shown in Fig. 2(a). The main bands and their assignments in TMA are as follows: stretching vibrations of the carboxylic acid ʋ(CO) at 1712.3 cm-1, (Can, Bulut, Örnek & Özacar, 2013) it is clear that the presence of a weak band at 2569.2 cm-1 can be due to the presence of sulfhydryl groups in the TMA. The broad and strong bands at
3373.4 (Fig. 2(a), cellulose), 3388.4 (Fig. 2(a),TMA-C) and 3363.2 cm−1 (Fig. 2(a), TMA-C-Se(IV)) are all due to the characteristic stretching vibration of hydroxyl group, The peaks at 1063.9 cm−1 (Fig. 2(a), cellulose), 1055.7 cm−1 (Fig. 2(a), TMA-C),1035.2 cm−1 (Fig. 2(a), TMA-C-Se(IV)) are due to the C-O stretching vibration in glucose rings.(Yang, Li, Wang, Zhang, Ma & Ye, 2010) But it is observed that there are significant changes from the IR spectra of Cel to TMA-C. Comparing the curve of Cel with TMA-C, The absorption band observed at 2909.4 cm-1 is due to the C-H vibration stretch of the -CH2 groups from primary alcohols of Cel. This band practically appears, at 2911.9 cm-1 represent the C-H stretching from the -CH2 group of TMA-C.(Anirudhan, Rauf & Rejeena, 2012; Xiong, Chen & Liu, 2012)
The appearance of a new band is
associated to the carbonyl stretching vibration of ester groups linked to methylene at 1721.4 cm-1, (Can, Bulut, Örnek & Özacar, 2013) and new band appears at 2571.2 cm-1, which is the characteristic absorbance of -SH group. Appearing and shifting these bands TMA-C spectra can be explained by the dehydration reaction between the CH2-OH and-COOH, Which indicated that the TMA ligands have been grafted onto the Cel surface successfully. Compared with peaks of TMA-C absorption before and after, the relevant changes in FTIR spectrum of TMA-C-Se(IV) suggested that the adsorption mechanism, the stretching vibration of -SH shifted from 2571.2 to 2573.6 cm-1, the peak at 3388.4 cm-1 related to -OH in the TMA-C, shifted to 3363.2 cm-1,
which indicate that -SH and -OH are involved in chelation adsorption of Se(IV). The surface and microstructure of Cel ,Cel grafted TMA and after Se(IV) adsorption were investigated using a scanning electron microscope with an accelerating potential of 5.0 kV and magnifications of 1000 and 5000 (Fig. 3). As expected the surface topography of Cel was extremely smooth, In addition, the topography of TMA-C was rough and heterogeneous, meanwhile, the surface appeared spots and light patches after modification, which is related to reaction between ligand and exposure sites on the surface of maternal, Comparing the surface of TMA-C with that of Se(IV)-loaded TMA-C, the surface of TMA-C turned more thicker and coarser with powder flake material, In order to confirm the presence of Se(IV) in TMA-C, the EDS spectrum of the TMA-C-Se(IV) is investigated in Fig. 3(EDS-(a)/(b)/(c)). The peak indicating the presence of sulphur and selenium can be clearly observed. Hence, the results revealed that sulfydryl have been grafted onto the Cel surface and Se(IV) was loaded on the surface of TMA-C successfully, which was consistent with the results of FTIR and SEM. The thermogravimetric analysis (TGA) curve of Cel was shown in Fig. 4. 80.41% weight loss from 335
to 385℃ indicates rupture of the skeleton
structure of Cel is caused by the heat. And the TG curve of TMA is characterized by only one wide temperature zones: 205–295 , almost 84.45% is lost as a result of decomposing the structure of TMA and losing crystallize.
And the TG curve of Cel mixed TMA shows the decomposition and weight loss occured in three different stages. There is a sharp weight loss up to 195
,
which attributed to the loss of TMA, and a similar decomposition step starting at about 295
is noted as that in the curves of the Cel. As for TMA-C, the
weight of ligand can be acounted for 23.911 % according to the sulphur content of TMA-C in the amount of 5.101 %. And there is two weight losses, the first weight loss occurred from 255
to 485 , due to the breaking of the covalent
bond between Cel and thioglycollic acid. The analysis TG of curve mentioned above indicate that the thioglycollic acid has been successfully grafted onto the Cel. Compared to the stability of Cel, the temperature beginning weightlessness of TMA-C-Se(IV) later than matrix-Cel significantly. The results mentioned above indicate that the thermal stability of the TMA-C-Se(IV) can be improved. And the higher residue (13.3%) of TMA-C-Se(IV) compared with that of TMA-C(8.08%) further revealed that Se(IV) ions were adsorbed onto TMA-C.
3.3 Determination of static adsorption capacity in different pH The pH of aqueous solution is a very critical parameter affecting the adsorption process. It can be easily seen in Fig. 5(a) that little of the species had quantitative adsorption on the untreated Cel in the studied pH range. On the contrary, the Cel modified TMA showed different adsorption characteristics towards selenium species in pH range of 0.5–11.5. And trace amounts of Se(IV) species was separated and preconcentrated from total Se at pH 4.5 by
TMA-C.
A variety of ions, such as Ca2+, Mg2+, Na+, Citrate-, ClO3-, HPO42-, Cl- and
HSeO3- were divided into two groups because ClO3 - and Cl- cannot co-exist under acidic conditions and were added individually to solutions. The concentrations of foreign ions were chosen to be close or higher to their contents in natural waters. The results are shown in Fig. 5(b). It can be noted that the adsorption capacity of TMAC for Se (IV) was far greater than for other ions, which suggest that Se (IV) ions can be easily separated from these interference ions using TMA-C, this associated with spatial effect and electronic effect by quantum chemical calculations. Fig. 5(c) shows the trend of Se(IV) removal as a function of pH resembled that of the zeta potential. The optimal pH for Se(IV) removal was approximately consistent with the pH = pHpzc, the pHpzc reported to be 4.5±0.1 in Fig. 5(c). The zeta potential of TMA-C was positive at weakly acidic conditions (when pH
repulsion of the electrostatic forces between the negatively charged surface of the TMA-C and selenite (Kongsri, Janpradit, Buapa, Techawongstien & Chanthai, 2013).
3.4 Performance of regeneration and recovery Whether an adsorbent is an appropriate material in removal of ions from aqueous solutions depends not only on its adsorptive capacity, but also on its regeneration ability (Xiong & Yao, 2009b). In this study, different concentrations of various eluent were used for desorbing the adsorbent after absorption. The influence of different types and concentrations of eluent on the elution rate is shown in the table below. As shown in Fig. 6(a), 3.0 mol/L HCl - 2% KClO3 solution can realize the eluention of TMA-C entirely. In order to show the reusability of the adsorbent, adsorption-desorption cycle of HSeO3– was repeated five times by using the same modified Cel (TMA-C). The adsorption capacities of TMA-C did not noticeably change (only a maximum 4% change was
observed
in
Fig.
6(b)).
Good
reusability
can
be realized to recycle selenium effectively.
3.5 Adsorption kinetics The kinetics of adsorption describes the rate of Se(IV) adsorption on TMA-C and it controls the equilibrium time. The kinetic adsorption curves were conducted to investigate the dynamics of the adsorption process. In the
present study, pseudo-first order model and pseudo-second order model were employed to evaluate the adsorption kinetic data. And, the Lagergren (pseudo-first-order)(Keskinkan, Goksu, Basibuyuk & Forster, 2004) rate equation is given By
lg(Qe Qt ) lg Q1
k1 t 2.303
(5)
The pseudo-second-order model(Ho & McKay, 1999) can be expressed as
t 1 t 2 Qt k 2Q2 Q2
(6)
K1 and K2 are the pseudo-first-order and pseudo-second order rate constant, respectively (g/mmol min), Qe and Qt are the amounts of Se(IV) adsorbed onto TMA-C at equilibrium and contact time, respectively (mmol/g). Fig. 7 shows the kinetic adsorption curve by means of plotting Se(IV) removal capacity (Qt) versus contact time (t) at 288 K, 298K, 308K and 318K. It is clear that the adsorption capacity increased rapidly during the first 6 hours and the equilibrium was then attained within 8 hours. Kinetic parameters and experimental values of Qe were evaluated and listed in Table 2. The values of the correlation coefficients (R22 > 0.97) for all the concentrations indicate that the adsorption process follows pseudo-second order kinetic model and the Se(IV) were adsorbed on the TMA-C via chemical interaction.
3.6 Adsorption Isotherms.
Two mathematical models proposed by Langmuir and Freundlich were used to describe and analyze the adsorption isotherm. The initial concentration of adsorbate is rang of 0.4mg/mL to 0.93mg/mL and 48h shaking time at 288K, 298K, 308K and 318K, respectively. And the linear Freundlich isotherm is commonly expressed as follows:
lg Qe
1 lg Ce lg K F n
(7)
Where KF is a constant related to the adsorption capacity, Ce is the amount of adsorbate in liquid phase at equilibrium (mol·L−1), Qe is the amount of adsorbate (mol·g−1 ) at adsorbent surface and 1/n is another constant related to energy or intensity of adsorption. The lgQe was plotted against lgCe. From the intercept and slope the values of KF and 1/n are compiled in Table 3. The Langmuir model assumes that a monolayer of adsorbate is covered on a homogenous adsorbent surface. The adsorption takes place only at specific sites of the adsorbent, which is valid for monolayer sorption onto a surface, given by
Ce 1 C e Qe Q0 K L Q0
(8)
where Ce is the equilibrium concentration of adsorbate in solution (mol·g−1 ) and Qe is the amount of adsorbate (mol·g−1 ) on adsorbent surface, Q 0 is the maximum amount of solute adsorbed corresponding to monolayer coverage of adsorbent surface, and K L is a constant related to the binding energy of solute. The plot of Ce/Qe versus Ce gives a straight line, indicating that the Langmuir adsorption is also followed by the adsorption data very well.
3.7 Dynamic adsorption and desorption The
fixed-bed
column
operation
is
more
effective
for
cyclic
adsorption/desorption than the batch process. And dynamic adsorption curve, that can reflect the adsorption equilibrium state directly and achieve the required time, adsorption kinetics and the mechanism of mass transfer, is a major basis for adsorption process design and control points.(Mthombeni, Mpenyana-Monyatsi, Onyango & Momba, 2012) Total adsorption capacity of Se(IV) Q (mg·g-1) in the column for a given feed concentration and flow rate is calculated from.(Tabakci & Yilmaz, 2008) V
(C 0 C t ) dV m 0
Q
(9)
where C0 and Ce are metal ion concentrations in the influent and effluent, respectively, m is the total weight of the absorbent loaded in the column, and v is the volume of metal solution passed through the column. The maximum adsorption capacity value Q was obtained by graphical integration as 102.7 mg/g. Successful design of a column adsorption process requires prediction of the concentration–time profile or breakthrough curve for the effluent. Traditionally, the Thomas model (Mathialagan &Viraraghavan, 2002) is used to fulfill the purpose. The model has the following form. Ce 1 C0 1 exp[ K T (Qm C0V ) / ]
(10)
where KT is the Thomas rate constant and θ is the volumetric flow rate. The linearized form of the Thomas model is as follows: C K Qm KT C0 ln 0 - 1 T V Ce
(11)
The kinetics coefficient KT and the adsorption capacity Q of the column can be determined from a plot of ln[(C0/Ce)-1] versus 1/θ at a certain flow rate, m is the mass of the TMA-C(g). The theoretical predictions based on the model parameters were compared with the observed data as shown in Fig. 8. It was shown that the experimental data were well fitted by the Thomas model with a high R2 value (0.990) and the calculated Q value was very close to the experimental data, which indicates that this model was successfully used for the prediction of the breakthrough curves and to determine the characteristics parameters of the column useful for process design. The residual HSeO3- in the TMA-C column was then desorbed with a 3.0 mol/L HCl - 2% KClO3 solutions selected in batch desorption studies. We achieved the desorption amount through a numerical integration of the desorption purge curve (effluent concentration (Ce) vs. bed volume (V)). As seen in Fig. 9, only 25.0ml bed volumes of eluent solutions were needed to completely regenerate the TMA-C column at a flow rate of 0.1 mL min-1. TMA-C was used in second adsorption-elution cycle and similar breakthrough curve and elution profile with first adsorption-elution cycle was obtained. This
is a direct proof that the adsorption capacity of the TMA-C column was fully recovered after regeneration with 3.0 mol/L HCl - 2% KClO3 solutions.
3.8 Verification tests The influence of Se(IV)/Se(VI) ratio on the recovery values of Se(VI) and total selenium results has been evaluated, and the results are given in Table 4. The results show that all species of selenium were completely separated and recovered quantitatively.
3.9 Application In addition to excellent recovery performance, TMA-C microcolumn can also be used for the determination of trace Se(IV) in sample analysis, which need to combine with ultraviolet spectrophotometry. Depending on the concentrations of HSeO3- in the studied samples (waste waters), 200 mL sample solutions was moved in TMA-C column and passing the column with 1.0ml/min. The column studies were performed at the optimum pH value and temperature determined from batch studies, 3.0 mol/L HCl -2% KClO3 was used to eluent Se(IV) loaded TMA-C column with 0.5 ml/min Meanwhile, the concentrations of effluent were measured at a wavelength of 335 nm (under the optimal conditions) by UV method, while using ICP-OES for measuring Se(IV) content in samples that was pretreated but not pass the column. Similar comparative test results showed that separation enrichment-ultraviolet visible spectrophotometry method can get basic consistent results with ICP-OES. The results
are shown in Table 5, it demonstrated the applicability of the TMA-C microcolumn UV-spectrophotometry method and indicated that the TMA-C essentially free from interferences when applied to the pre-concentration of waste water and food samples. In order to determine the accuracy of the method, the precision of the method under the optimum conditions was determined by seven times of parallel determination. The results shown that the relative standard deviations (R.S.D.) for Se(IV) studied were lower than 2%, which illustrated that the determination method has excellent reproducibility and high accuracy. The accuracy of the method was also evaluated by recovery study. The solution contained 5 µg L-1 Se(IV) was added to the samples. The standard solutions and treated sample solutions were introduced into the TMA-C separation and enrichment column system following the same procedure as described previously. The concentrations of Se(IV) were determined according to the proposed procedure. The analytical results (Table 6) show that the standard materials are in good agreement with the determined values. The results demonstrated that the analysis method have considerable application prospect in the detection of trace selenium.
Conclusions The regenerable cel(TMA-C) was successfully prepared and showed best performance in the adsorption and enrichment of Se(IV) which was separated and preconcentrated from total Se at desired pH values(pH4.5). And it has high
adsorption capacity for Se (IV), which is 101.2 mg/g. It is evident from the experimental data that the pseudo-second order model best describe the kinetics and the Langmuir model best fit equilibrium data, and the Thomas model fits the adsorption breakthrough curve well. The TMA-C was fully regenerated by purging with 3.0 mol/L HCl - 2% KClO3, which illustrate that the microcolumn has effective and high-quality recovery of selenium. Finally, the coupling technique (TMA-C microcolumn-UV-spectrophotometry) was developed for effective and economic determination of trace Se(IV) in waste waters, the results can get basic consistent results with ICP-OES.
Acknowledgments The work is supported by the National Science Fundamental Project of China (No.20972138), Ph.D. Programs Foundation of Ministry of Education of China (No. 20133326110006), and the Program of Science and Technology of Zhejiang Province，China (No.2015C3704).
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Figure 1. Three-dimensional plots showing effects of (a) molar ratio and temperature; (b) mass matio of catalyst and temperature, and (c)mass matio of catalyst with molar ratio interactions on the content of the functional group of TMAC.
Figure 2. FTIR spectroscopy comparison with Cel, TMA-C, Cel-g-TMA and TMA-C-Se(IV) (a) and synthesis routes for TMA-C(b)
Figure 3. SEM /ESDgraphs of Cel(a); TMAC(b); TMAC-Se(IV)(c)
Figure 4. The thermogravimetry (TG) curves for Cel, TMAC, Cel+TMAC, Cel-g-TMA and TMAC-Se(IV).
Figure 5. (a)The static adsorption capacity at pH ranging from 0.5 to 11.5; (b) Adsorption efficient of the TMAC for Ca2+, Mg2+, Na+, Citrate-, ClO3 -, HPO42- and HSeO3-; Adsorption efficient of the TMAC for Cl- and HSeO3-; (c)zeta potentials of TMAC
Figure 6 (a)Effect of different elution for TMAC after adsorption, (b) Influence of repeat time for TMAC recovery
Figure 7. Adsorption kinetics and capacity Q at different times and different temperatures (TMA-C200.0 mg, [HSeO3-]0 =0.8 mg/ mL, pH 4.5, 100 rpm)
Figure 8
Experimental and predicted breakthrough curves using Thomas
model forSe(IV) adsorption by TMA-C (TMA-C 200.0 mg, pH 4.5, C0 = 0.10 mg/mL, flow rate = 0.25 mL/min).
Figure 9 .Dynamic deadsorption curve flow rate=0.1 mL min-1.
Table 1
Experimental design of synthesis conditions of TMA-C
Symbols
Factors
-1
0
1
A
Temperature (℃)
60
75
90
B
molar ratio
2
3
4
3
5
7
C
Dosage of catalyst(mass ratio) (%)
Table 2
Kinetic parameters of TMA-C
First-order kinetic T(K)
Second-order kinetic
Qe(mg
K1(min-1
Q1(mg/g)
R1 2
K2(g mg-1
Q2(mg
g-1)
)
288
59.82
0.0053
178.57
298
85.93
0.0051
308
94.63
318
106.91
Table 3
Parameter for adsorption isotherms of Se (IV) by TMA-C
min-1)
g-1)
0.9531
4.80×10-6
79.54
0.9920
181.82
0.9217
5.49×10-6
126.50
0.9789
0.0062
243.90
0.9388
3.75×10-6
131.13
0.9720
0.0053
256.41
0.9248
4.41×10-6
154.13
0.9812
Langmuir T(K）
R2 2
Freundilich
Q0
KL
R1 2
n
KF((mg/g)/(
R2 2
(mg/g)
(mL/mg)
288
59.88
67
0.999
10.01
398
86.21
87
0.998
24.78
85.6643
0.586
308
96.15
120
0.996
30.03
95.4113
0.457
318
107.53
155
0.997
39.53
111.25
0.773
mg/mL)1/n) 59.1289
0.930
Table 4 Recovery values of Se(IV) and total selenium determined at different concentration ratios, as well as the calculated Se(VI) concentrations
Se(IV):Se(VI)
Se(IV)
Total selenium
Calculated Se(VI)
Added(μg/L)
Found(μg/L)
Recovery(%)
Found(μg/L)
Recovery(%)
10:1
100.0
98.6±1.8
98.6
108.9±2.3
99
10.3±0.5
5:1
50.0
49.1±1.1
98.2
58.7±0.8
100
9.6±0.3
1:1
10.0
10.1±0.2
101
19.2±1.0
96
9.1±0.8
1:5
10.0
9.7±0.4
97
59.3±1.9
98.8
49.6±1.5
1:10
10.0
9.9±0.1
99
109.5±1.2
99
99.6±1.1
Table 5
Selenium(IV) of Samples determination results with these two methods
S0263-8762(16)30465-8 http://dx.doi.org/doi:10.1016/j.cherd.2016.11.032 CHERD 2504
To appear in: Received date: Revised date: Accepted date:
14-12-2015 20-8-2016 27-11-2016
Please cite this article as: Min, Min, Shen, Chen, Fang, Lei, Zhu, Bingren, Li, Jionghui, Yao, Lanying, Jiang, Yujian, Xiong, Chunhua, Design of a selective regenerable cellulose microcolumn for selenium efficient recovery and economic determination.Chemical Engineering Research and Design http://dx.doi.org/10.1016/j.cherd.2016.11.032 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Design of a selective regenerable cellulose microcolumn for selenium efficient recovery and economic determination Min Mina, Chen Shena, Lei Fangb, Bingren Zhua, Jionghui Lic, Lanying Yaoa, Yujian Jianga, Chunhua Xionga* [email protected] a
Department of Applied Chemistry, Zhejiang Gongshang University, No. 149 Jiaogon
gRoad, Hangzhou, 310012, PR, China. b
Department of Food Science and Human Nutrition, University of Florida, Bldg 475
Newell Drive, Gainesville, FL 32611, USA c
School of Environmental Science and Engineering, Zhejiang Gongshang University,
Hangzhou, 310012, China. *Corresponding author. Tel: +86-0571-88071024-7571.
Graphical Abstract
Proposed synthesis routes to TMA-C and structure changes before and after synthesis
Highlights
Functionalized regenerate cellulose (TMA-C), was synthesized via one-step reaction
TMA-C can effectively recovery the toxic selenium compounds from waste waters
TMA-C -Se(IV) could be easily eluted and can be reused many times
Set up an coupling technique for economic and effective determination of
Se(IV)
Abstract Recovery the toxic selenium compounds selenite and selenate from waste waters before discharging is imperative in industrialized countries. In this study, A functionalized regenerable cellulose, based on the chemical modification of cellulose grafted thiomalic acid (TMA-C), was prepared simply with an efficient methodology for removing and recovering selenium(IV) effectively. Optimization of the TMA-C synthesis was carried out at different temperatures, different molar ratios, and dosage of catalyst using response surface methodology. The surface functionalization effect was detected with FTIR, SEM, zeta potential, EDS and TGA technique. And under optimum adsorption conditions (pH = 4.5, T = 318 K), TMA-C can remove toxic selenium compounds more than 99.9%, and have high adsorption capacity for Se (IV), which is 101.2 mg/g. The adsorption isotherms data fitted the Langmuir model well, and Pseudo-second-order kinetics showed better accordance. In addition, in the batch desorption experiments, the TMA-C could be regenerated completely with 3.0 mol/L
HCl
and
2%
KClO3
solutions.
Finally
re-generable
cellulose
microcolumn-UV-spectrophotometry was developed for effective and economic determination of trace Se(IV).
Keywords: Regenerative cellulose; Selenium (IV); Adsorption; Recovery; Economic detection
1.
2.
3. Introduction With the consumption of valuable resources, growth of energy prices, industries urgently need to look for economic and improved processes to harmful and valuable compounds from waste waters because of tighter environmental policies. Selenium (atomic number 34) is widely used for manufacturing
industrial
products,
such
as
glass,
rubber,
electronic
components, pigments (Zhou et al, 2012). But it also is a common contaminant released
from
mining,
metal
smelting
and
coal-fired
power
plants
industries(Ling et al, 2015), Selenium exists in water as oxy-anionic species, selenite (SeO32-) and selenate (SeO42-).(Zhang, Ge, Yao, Liu, Xie & Fang, 2015) And selenium in the form of Se (IV) is more toxic than Se (VI) .(Kongsri, Janpradit, Buapa, Techawongstien & Chanthai, 2013). Looking deeper into national eating patterns, the nutritional range of Se in food product between essentiality and toxicity is relativelty lower in comparison with the other essential trace elements. (Letavayova, Vlasakova, Spallholz, Brozmanova & Chovanec, 2008). Also, selenium is difficult to form independent deposits as a kind of typical scattered elements. Regardless of good selenium nutrition is antioxidant protection and for other health reasons, some confusing and contradictory research shown that taking high amounts of selenium may result in hair and nail
changes, neurologic manifestations and gastrointestinal disorders. Therefore, it is vital to develop a highly sensitive and economic method for detecting and recovering selenium present in sewage waters. Over the past few decades, a variety of technologies including, ion-exchange(Pakzadeh & Batista, 2011), membrane filtration,(Mavrov, Stamenov, Todorova, Chmiel & Erwe, 2006) reverse osmosis,(Lin, Xu, Papelis, Cath & Xu, 2014) and chemical precipitation, (Geoffroy & Demopoulos, 2011) have been developed for the removal and recovery of selenite and selenate. The disadvantages of these methods are that they cannot 100% eliminate selenium from water. Considering large volumes of mass sludge are generated daily, the high cost of reagents and the removal process is time consuming and economically unsatisfied. (Mavrov, Stamenov, Todorova, Chmiel & Erwe, 2006) Recently, removal of selenium by adsorption on a wide variety of adsorbents has been reported in several studies.(Karatchevtseva, Astoux, Cassidy, Yee, Bartlett & Griffith, 2010; Szlachta & Chubar, 2013; Tang, Huang, Zeng & Zhang, 2014) Among these, majority studies have been documented to utilize various inorganic adsorbents for the adsorptive recovery of selenite; however, the adsorption capacity of these inorganic adsorbents are relatively low.(Chubar, 2014; Jordan, Ritter, Scheinost, Weiss, Schild & Hubner, 2014) Today, biopolymer materials gradually becomes hot topic in the field of metal pollution control because of its greater environmental compatibility, lower toxicity and higher biodegradability than synthetic surfactants.(Zhu et al., 2013)
Cellulose (Cel) is the principal structural component of plants and the contemporary strive for sustainable environment has amounted to a renewed scientific
interest
as
a
source
for
commodity
chemicals
and
new
materials(Filpponen et al., 2012). Cellulose widely exists in environment and exerts several characteristics of high mechanical strength, renewability and biodegradablilty. (Yu et al., 2013) Notably, it is able to bind trace elements ions in solution with the existence of abundant polar –OH groups which would also be replaced by specific functional groups. Recently, A study reports that modified/unmodified cellulose have been used for removing trace elements or heavy-metal ions. The cellulose nitrate membrane has been prepared and applied for the efficient pre-concentration of nickel(II), cadmium(II), copper(II), cobalt(II). The levels of the analytes were determined by FAAS (Flame Atomic Absorption Spectrometry ) (Narin & Soylak, 2003) Moreover, according to another study, the adsorbents prepared by chemical modification of cellulose have shown promise for pre-concentration and separation of precious-metal ions.(Gurung, Adhikari, Gao, Alam & Inoue, 2014) Hence, the usage of cellulose to fabricate selective adsorption materials is a promising concept for the recovery and recycling of secondary resources as well as for maintaining a sustainable environment. In this study, TMA-C (cellulose modified thiomalic acid) is synthesised and the TMA-C microcolumn is prepared for Se(IV) separation and enrichment from total Se aqueous media, meanwhile UV-spectrophotometry is used for
economic determination. The response surface methodology was opted for optimizing synthesis process parameters. Se(IV) adsorption onto the TMA-C was investigated, including adsorption condition optimization, adsorption isotherms and kinetics in batch system and evaluation of breakthrough curves. The regenerable cellulose microcolumn - UV- spectrophotometry was set up for the determination of trace Se(IV) in food and waste water samples .
4. Experimental
2.1 Reagents and Apparatus Cellulose powder was purchased from LanBo Co., Hangzhou, China (Φ= 50 μm). The standard solution of selenium(IV) were prepared by dissolving analytical grade selenium powder in appropriate amount of concentrated HNO 3, KClO4 and HCl and then diluted with ultra-pure water in a 1000 ml volumetric flask. Se(IV) and Se(VI) stock solutions were prepared from analytical grade sodium selenite and sodium selenate, respectively. The pH of each test solution was adjusted with HCl solution to pH 0.5-2.5, citric acid/Na2HPO4 buffer solution to pH 3.5-6.5, NH4Cl/NH3·H2O buffer solution to pH 8.5–10.5, and Na2HPO4/NaOH solution to pH 11.5. All other reagents and solvents were of analytical grade and used as received without further purification. Infrared spectra of the TMA-C were scanned in the region of 400-4000 cm −1 in KBr pellets on NICOLET 380 FT-IR spectrophotometer; the elemental analysis was carried out using a Vario EL III Elemental Analyzer; the
thermo-gravimetric analysis was investigated using Mettler TGA/DSC1 simultaneous thermal analyzer. The shapes and surface morphology of TMA-C were observed by means of a HITACHI S-3000N scanning electron microscope (SEM). The concentrations of Se(IV) and Se(IV) were determined by Inductively coupled plasma optical emission spectrometer (ICP-OES) and UV-2550. The respective zeta potentials of TMA-C were determined by a zeta potential analyzer (Malvern Zetasizer NanoZS90).
2.2 Synthesis of TMA-C The preparation of TMA-C was carried out according to the following procedures: 1.0 g Cel was fully soaked with 50 mL reaction solvent in a 100 mL three-necked flask, and swelling overnight. Then, a certain amount of TMA and a small amount of phosphotungstic acid used as catalyst were added to the flask. The mixture reacted for 10 h at 75 ℃with 300rmp stirring speed under nitrogen atmosphere. The solid product was carefully washed thoroughly with ultra-pure water until the washing liquid was colorless. After that, the obtained TMA-C was dried in vacuum at 50 ℃.
2.3 Experimental design for RSM study Response surface methodology (RSM) is a collection of mathematical and statistical techniques for designing experiments, building models, and analyzing the interactive effects of independent factors.(Srivastava, Sharma & Sillanpää,
2015) In this study, the variables, such as temperature, molar ratio, and dosage of catalyst, are selected process parameters on the functional group capacity of TMA-C. And independent variables, experimental range and levels for the production of TMA-C were tabulated in Table 1.
2.4 Batch adsorption studies Batch adsorption experiments were performed in the conical flasks containing 30 mL adsorption solution and 20.0 mg TMA-C. Adsorption of Se(IV) from aqueous solution to the adsorbents was studied at various pH values. After 24 h, a required amount of standard solution of Se (IV) was put in. Adsorption experiments were conducted in a shaker at 100 rpm at 25 . The upper layer liquid was analyzed by UV spectrophotometer until adsorption equilibrium reached. The adsorption capacity (Q, mg g-1) and distribution coefficient (D, mL g-1) were calculated with the following formulae: Q
(Co Ce )V m
(1)
D
Q Ce
(2)
where C0 is the initial concentration in solution (mg mL-1); Ce is the equilibrium concentration in solution (mg mL-1); V is the volume of solution (mL); and m is the TMA-C dry weight (g). Desorption experiments were performed immediately after the completion of the adsorption experiments. At the end of the adsorption experiments, The TAMC-Se(IV) was placed in the iodine flasks, and then 60 mL of thiourea was
added. The NaOH, HCl, KCl, NH4Cl, mixture of HCl and KClO 3 with different concentrations were further stirred at 100 rpm for 24 h at 298 K, respectively. After desorption, the final HSeO3- concentrations in the aqueous phase were determined by ICP-OES. The desorption ratio (E) was calculated from eq 3:
E (%)
CdVd 100 C0 Ce V
(3)
where Vd is the volume of the desorption solution, and C 0, Cd are the concentration of the solutes in the desorption solutions, Ce and V are the same as defined above.
2.5 Microcolumn experiment studies The fixed-bed experiments were carried out at in a water jacketed glass column with an inner diameter of 3.0 mm and a full Length of 30 cm. An aliquot of the fresh TMA-C (200 mg) was putted into the bed. The TMA-C in the column was pre-soaked for 24 h before starting the experiment. The aqueous solution with known concentrations of Se(IV) ions was then fed to the top of the bed at a desired flow rate until the breakthrough curve was completed. The column studies were performed at the optimum pH value and temperature determined from batch studies. The samples in the outlet were taken at the present time intervals and the concentrations of Se(IV) ions were similarly determined as above, In addition, dynamic desorption procedures were also carried out.
4.6 Preparation of samples The waste water: The water samples were filtered using a filter membrane with a pore size of 0.45 µm. Then, the pH of filtrate was adjusted to 4.5 and the filtrate was used for the procedure of separation, preconcentration, and determination of Se(IV). If the sample of water contained a visible precipitate or mechanical admixtures, the sample should have been filtered through the filter paper first; When measuring total Se, 6mol/L HCl was added into samples for the reduction of Se(VI) into Se(IV) Preservation of water sample: The samples were acidified with HNO3 to avoid the co-precipitation of the metal and stored in pre-cleaned polyethylene bottles (Agrawal, Singh Patel & Shrivas, 2009) The rice: The rice were washed with tap water, deionized water respectively for several times to remove pesticides. And the sample were placed into thermostatic oven for 72h at 50 , then dried and crushed. The sample were further heated for 24h at 80 . An accurately weighed amount of 2g sample, 10.0 mL mixed acid (HNO3: HClO4 = 9:1) and a few glass beads were added to the digested bottle, cold digestion overnight. A certain amount of HNO3 was added in time sequentially to rice sample, mixed and heated. The sample solution was evaporated to 2 mL when it is clear and produce white smoke. (Add 5.0 mL 6mol / L HCl to the solution after it is cool, repeat the heating step)1 After Cooling, the solution was transferred into 50 mL volumetric flask. (GB 5009.93—2010: National food safety standard Determination of selenium in foods)
1
HCl is added in order to reduce Se(VI) to Se(IV) , so we can Recovery and measure amount of total selenium; We need not added HCl if we Measure amount of Se(IV)
2.7 Procedure The sample solution of 10ml containing the species of selenium was divided into two halves, which were named sample 1 and 2, Meanwhile, 0.5 mL of 6mol/L HCl was added into sample 2 for the reduction of Se(VI) into Se(IV). The sample 1 was moved in TMA-C column and passing the column with 1.0ml/min, The column retained Se(IV) was eluted with 3.0 mol/L HCl -2% KClO3, and the Se(IV) in eluate were determined by ICP-OES. The sample 2 (for total Se) were processed according to same procedure mentioned above. The concentration of Se(VI) in the sample solution were calculated by subtracting Se(IV) from total Se.
5. Results and Discussion 3.1 Synthesis of TMA-C 3.1.1 Analysis of variance (ANOVA) and development of regression mode The optimum quantities of the chosen changeable were obtained by regression equation and by distinguished the RSM contour plots. The optimization step for the synthesis of TMA-C was carried out using Box–Behnken design (BBD). The predicted response M for sulphur content was obtained and is given as: M=-15.42919+0.35220A+2.86850B+0.99175C+2.13333E-003AB-2.07500E-0 03AC-0.015625BC-2.21333E-003A2-0.49700B2-0.078062C2
(4)
In this equation M is the sulphur content(%), A, B and C are the independent variables in coded units, i.e., Temperature ( ), molar ratio and Dosage of catalyst(mass ratio)(%), respectively. The results from ANOVA (Table S1) display the proportional of equation represented the concrete relevance between the principal changeable and the response. There is excellent relevance between the predicted and observed quantity as shown by nearly between R 2 and adjusted R2 value. The pattern is regarded to be actuarial basic because the affiliated Prob.> F quantity for the pattern is lower than 0.05. In this model A; B; AC; A 2; B2 and C2 are significant model terms. Analysis of variance (ANOVA) for response surface quadratic model gave F-value 303.47, R2 value 0.9974 (the predicted R2 represents the prediction of a response value estimated by the model), probability < 0.0001 and coefficient of variation (C.V. = 0.77%) signifying that model is highly significant and experiments are highly accurate and reliable.(Liu, Yan, Zhang & Du, 2011) Moreover, the insignificant lack-of-fit test (F-value is 0.25) also indicated that the model was suitable to represent the experimental data using the designed experimental date.(Xu, Yin, Liu, Tang, Qu & Xu, 2013) Adequate precision is an estimation of the signal to noise ratio. A ratio greater than four is desirable. The ratio of 48.987 implies an adequate signal. This model can be used to navigate the design space.(Mourabet, El Rhilassi, El Boujaady, Bennani-Ziatni, El Hamri & Taitai, 2012)
3.1.2 The optimization of synthesis conditions The results of the sulphur content affected by temperature, molar ratio, and mass ratio of catalyst are shown in panels a, b, and c of Fig. 1, respectively. These types of plots show the effects of two factors on sulphur content while the other factor was kept at level zero. From the 3D response surface plots, the optimal values of the parameters could be observed. The RSM-guided optimization demonstrated that the optimal synthesis conditions of TMA-C are: temperature of 78.64 , molar ratio of 2.98, and dosage of catalyst of 5.01, and the predicted sulphur content reached 5.17303%. Three synthesis experiments of TMA-C were implemented under the predicted optimum conditions in order to validate the mathematical models developed. The mean value of sulphur concent was 5.101%, which obtained from real experiments what were in close agreement with the predicted value, indicating that the developed model was adequate for predicting the synthesis conditions of TMA-C.
3.2 Characterization of TMA-C The structures of Cel, TMA, mixture of Cel and TMA, TMA-C were confirmed by FT-IR spectroscopy and the results are shown in Fig. 2(a). The main bands and their assignments in TMA are as follows: stretching vibrations of the carboxylic acid ʋ(CO) at 1712.3 cm-1, (Can, Bulut, Örnek & Özacar, 2013) it is clear that the presence of a weak band at 2569.2 cm-1 can be due to the presence of sulfhydryl groups in the TMA. The broad and strong bands at
3373.4 (Fig. 2(a), cellulose), 3388.4 (Fig. 2(a),TMA-C) and 3363.2 cm−1 (Fig. 2(a), TMA-C-Se(IV)) are all due to the characteristic stretching vibration of hydroxyl group, The peaks at 1063.9 cm−1 (Fig. 2(a), cellulose), 1055.7 cm−1 (Fig. 2(a), TMA-C),1035.2 cm−1 (Fig. 2(a), TMA-C-Se(IV)) are due to the C-O stretching vibration in glucose rings.(Yang, Li, Wang, Zhang, Ma & Ye, 2010) But it is observed that there are significant changes from the IR spectra of Cel to TMA-C. Comparing the curve of Cel with TMA-C, The absorption band observed at 2909.4 cm-1 is due to the C-H vibration stretch of the -CH2 groups from primary alcohols of Cel. This band practically appears, at 2911.9 cm-1 represent the C-H stretching from the -CH2 group of TMA-C.(Anirudhan, Rauf & Rejeena, 2012; Xiong, Chen & Liu, 2012)
The appearance of a new band is
associated to the carbonyl stretching vibration of ester groups linked to methylene at 1721.4 cm-1, (Can, Bulut, Örnek & Özacar, 2013) and new band appears at 2571.2 cm-1, which is the characteristic absorbance of -SH group. Appearing and shifting these bands TMA-C spectra can be explained by the dehydration reaction between the CH2-OH and-COOH, Which indicated that the TMA ligands have been grafted onto the Cel surface successfully. Compared with peaks of TMA-C absorption before and after, the relevant changes in FTIR spectrum of TMA-C-Se(IV) suggested that the adsorption mechanism, the stretching vibration of -SH shifted from 2571.2 to 2573.6 cm-1, the peak at 3388.4 cm-1 related to -OH in the TMA-C, shifted to 3363.2 cm-1,
which indicate that -SH and -OH are involved in chelation adsorption of Se(IV). The surface and microstructure of Cel ,Cel grafted TMA and after Se(IV) adsorption were investigated using a scanning electron microscope with an accelerating potential of 5.0 kV and magnifications of 1000 and 5000 (Fig. 3). As expected the surface topography of Cel was extremely smooth, In addition, the topography of TMA-C was rough and heterogeneous, meanwhile, the surface appeared spots and light patches after modification, which is related to reaction between ligand and exposure sites on the surface of maternal, Comparing the surface of TMA-C with that of Se(IV)-loaded TMA-C, the surface of TMA-C turned more thicker and coarser with powder flake material, In order to confirm the presence of Se(IV) in TMA-C, the EDS spectrum of the TMA-C-Se(IV) is investigated in Fig. 3(EDS-(a)/(b)/(c)). The peak indicating the presence of sulphur and selenium can be clearly observed. Hence, the results revealed that sulfydryl have been grafted onto the Cel surface and Se(IV) was loaded on the surface of TMA-C successfully, which was consistent with the results of FTIR and SEM. The thermogravimetric analysis (TGA) curve of Cel was shown in Fig. 4. 80.41% weight loss from 335
to 385℃ indicates rupture of the skeleton
structure of Cel is caused by the heat. And the TG curve of TMA is characterized by only one wide temperature zones: 205–295 , almost 84.45% is lost as a result of decomposing the structure of TMA and losing crystallize.
And the TG curve of Cel mixed TMA shows the decomposition and weight loss occured in three different stages. There is a sharp weight loss up to 195
,
which attributed to the loss of TMA, and a similar decomposition step starting at about 295
is noted as that in the curves of the Cel. As for TMA-C, the
weight of ligand can be acounted for 23.911 % according to the sulphur content of TMA-C in the amount of 5.101 %. And there is two weight losses, the first weight loss occurred from 255
to 485 , due to the breaking of the covalent
bond between Cel and thioglycollic acid. The analysis TG of curve mentioned above indicate that the thioglycollic acid has been successfully grafted onto the Cel. Compared to the stability of Cel, the temperature beginning weightlessness of TMA-C-Se(IV) later than matrix-Cel significantly. The results mentioned above indicate that the thermal stability of the TMA-C-Se(IV) can be improved. And the higher residue (13.3%) of TMA-C-Se(IV) compared with that of TMA-C(8.08%) further revealed that Se(IV) ions were adsorbed onto TMA-C.
3.3 Determination of static adsorption capacity in different pH The pH of aqueous solution is a very critical parameter affecting the adsorption process. It can be easily seen in Fig. 5(a) that little of the species had quantitative adsorption on the untreated Cel in the studied pH range. On the contrary, the Cel modified TMA showed different adsorption characteristics towards selenium species in pH range of 0.5–11.5. And trace amounts of Se(IV) species was separated and preconcentrated from total Se at pH 4.5 by
TMA-C.
A variety of ions, such as Ca2+, Mg2+, Na+, Citrate-, ClO3-, HPO42-, Cl- and
HSeO3- were divided into two groups because ClO3 - and Cl- cannot co-exist under acidic conditions and were added individually to solutions. The concentrations of foreign ions were chosen to be close or higher to their contents in natural waters. The results are shown in Fig. 5(b). It can be noted that the adsorption capacity of TMAC for Se (IV) was far greater than for other ions, which suggest that Se (IV) ions can be easily separated from these interference ions using TMA-C, this associated with spatial effect and electronic effect by quantum chemical calculations. Fig. 5(c) shows the trend of Se(IV) removal as a function of pH resembled that of the zeta potential. The optimal pH for Se(IV) removal was approximately consistent with the pH = pHpzc, the pHpzc reported to be 4.5±0.1 in Fig. 5(c). The zeta potential of TMA-C was positive at weakly acidic conditions (when pH
repulsion of the electrostatic forces between the negatively charged surface of the TMA-C and selenite (Kongsri, Janpradit, Buapa, Techawongstien & Chanthai, 2013).
3.4 Performance of regeneration and recovery Whether an adsorbent is an appropriate material in removal of ions from aqueous solutions depends not only on its adsorptive capacity, but also on its regeneration ability (Xiong & Yao, 2009b). In this study, different concentrations of various eluent were used for desorbing the adsorbent after absorption. The influence of different types and concentrations of eluent on the elution rate is shown in the table below. As shown in Fig. 6(a), 3.0 mol/L HCl - 2% KClO3 solution can realize the eluention of TMA-C entirely. In order to show the reusability of the adsorbent, adsorption-desorption cycle of HSeO3– was repeated five times by using the same modified Cel (TMA-C). The adsorption capacities of TMA-C did not noticeably change (only a maximum 4% change was
observed
in
Fig.
6(b)).
Good
reusability
can
be realized to recycle selenium effectively.
3.5 Adsorption kinetics The kinetics of adsorption describes the rate of Se(IV) adsorption on TMA-C and it controls the equilibrium time. The kinetic adsorption curves were conducted to investigate the dynamics of the adsorption process. In the
present study, pseudo-first order model and pseudo-second order model were employed to evaluate the adsorption kinetic data. And, the Lagergren (pseudo-first-order)(Keskinkan, Goksu, Basibuyuk & Forster, 2004) rate equation is given By
lg(Qe Qt ) lg Q1
k1 t 2.303
(5)
The pseudo-second-order model(Ho & McKay, 1999) can be expressed as
t 1 t 2 Qt k 2Q2 Q2
(6)
K1 and K2 are the pseudo-first-order and pseudo-second order rate constant, respectively (g/mmol min), Qe and Qt are the amounts of Se(IV) adsorbed onto TMA-C at equilibrium and contact time, respectively (mmol/g). Fig. 7 shows the kinetic adsorption curve by means of plotting Se(IV) removal capacity (Qt) versus contact time (t) at 288 K, 298K, 308K and 318K. It is clear that the adsorption capacity increased rapidly during the first 6 hours and the equilibrium was then attained within 8 hours. Kinetic parameters and experimental values of Qe were evaluated and listed in Table 2. The values of the correlation coefficients (R22 > 0.97) for all the concentrations indicate that the adsorption process follows pseudo-second order kinetic model and the Se(IV) were adsorbed on the TMA-C via chemical interaction.
3.6 Adsorption Isotherms.
Two mathematical models proposed by Langmuir and Freundlich were used to describe and analyze the adsorption isotherm. The initial concentration of adsorbate is rang of 0.4mg/mL to 0.93mg/mL and 48h shaking time at 288K, 298K, 308K and 318K, respectively. And the linear Freundlich isotherm is commonly expressed as follows:
lg Qe
1 lg Ce lg K F n
(7)
Where KF is a constant related to the adsorption capacity, Ce is the amount of adsorbate in liquid phase at equilibrium (mol·L−1), Qe is the amount of adsorbate (mol·g−1 ) at adsorbent surface and 1/n is another constant related to energy or intensity of adsorption. The lgQe was plotted against lgCe. From the intercept and slope the values of KF and 1/n are compiled in Table 3. The Langmuir model assumes that a monolayer of adsorbate is covered on a homogenous adsorbent surface. The adsorption takes place only at specific sites of the adsorbent, which is valid for monolayer sorption onto a surface, given by
Ce 1 C e Qe Q0 K L Q0
(8)
where Ce is the equilibrium concentration of adsorbate in solution (mol·g−1 ) and Qe is the amount of adsorbate (mol·g−1 ) on adsorbent surface, Q 0 is the maximum amount of solute adsorbed corresponding to monolayer coverage of adsorbent surface, and K L is a constant related to the binding energy of solute. The plot of Ce/Qe versus Ce gives a straight line, indicating that the Langmuir adsorption is also followed by the adsorption data very well.
3.7 Dynamic adsorption and desorption The
fixed-bed
column
operation
is
more
effective
for
cyclic
adsorption/desorption than the batch process. And dynamic adsorption curve, that can reflect the adsorption equilibrium state directly and achieve the required time, adsorption kinetics and the mechanism of mass transfer, is a major basis for adsorption process design and control points.(Mthombeni, Mpenyana-Monyatsi, Onyango & Momba, 2012) Total adsorption capacity of Se(IV) Q (mg·g-1) in the column for a given feed concentration and flow rate is calculated from.(Tabakci & Yilmaz, 2008) V
(C 0 C t ) dV m 0
Q
(9)
where C0 and Ce are metal ion concentrations in the influent and effluent, respectively, m is the total weight of the absorbent loaded in the column, and v is the volume of metal solution passed through the column. The maximum adsorption capacity value Q was obtained by graphical integration as 102.7 mg/g. Successful design of a column adsorption process requires prediction of the concentration–time profile or breakthrough curve for the effluent. Traditionally, the Thomas model (Mathialagan &Viraraghavan, 2002) is used to fulfill the purpose. The model has the following form. Ce 1 C0 1 exp[ K T (Qm C0V ) / ]
(10)
where KT is the Thomas rate constant and θ is the volumetric flow rate. The linearized form of the Thomas model is as follows: C K Qm KT C0 ln 0 - 1 T V Ce
(11)
The kinetics coefficient KT and the adsorption capacity Q of the column can be determined from a plot of ln[(C0/Ce)-1] versus 1/θ at a certain flow rate, m is the mass of the TMA-C(g). The theoretical predictions based on the model parameters were compared with the observed data as shown in Fig. 8. It was shown that the experimental data were well fitted by the Thomas model with a high R2 value (0.990) and the calculated Q value was very close to the experimental data, which indicates that this model was successfully used for the prediction of the breakthrough curves and to determine the characteristics parameters of the column useful for process design. The residual HSeO3- in the TMA-C column was then desorbed with a 3.0 mol/L HCl - 2% KClO3 solutions selected in batch desorption studies. We achieved the desorption amount through a numerical integration of the desorption purge curve (effluent concentration (Ce) vs. bed volume (V)). As seen in Fig. 9, only 25.0ml bed volumes of eluent solutions were needed to completely regenerate the TMA-C column at a flow rate of 0.1 mL min-1. TMA-C was used in second adsorption-elution cycle and similar breakthrough curve and elution profile with first adsorption-elution cycle was obtained. This
is a direct proof that the adsorption capacity of the TMA-C column was fully recovered after regeneration with 3.0 mol/L HCl - 2% KClO3 solutions.
3.8 Verification tests The influence of Se(IV)/Se(VI) ratio on the recovery values of Se(VI) and total selenium results has been evaluated, and the results are given in Table 4. The results show that all species of selenium were completely separated and recovered quantitatively.
3.9 Application In addition to excellent recovery performance, TMA-C microcolumn can also be used for the determination of trace Se(IV) in sample analysis, which need to combine with ultraviolet spectrophotometry. Depending on the concentrations of HSeO3- in the studied samples (waste waters), 200 mL sample solutions was moved in TMA-C column and passing the column with 1.0ml/min. The column studies were performed at the optimum pH value and temperature determined from batch studies, 3.0 mol/L HCl -2% KClO3 was used to eluent Se(IV) loaded TMA-C column with 0.5 ml/min Meanwhile, the concentrations of effluent were measured at a wavelength of 335 nm (under the optimal conditions) by UV method, while using ICP-OES for measuring Se(IV) content in samples that was pretreated but not pass the column. Similar comparative test results showed that separation enrichment-ultraviolet visible spectrophotometry method can get basic consistent results with ICP-OES. The results
are shown in Table 5, it demonstrated the applicability of the TMA-C microcolumn UV-spectrophotometry method and indicated that the TMA-C essentially free from interferences when applied to the pre-concentration of waste water and food samples. In order to determine the accuracy of the method, the precision of the method under the optimum conditions was determined by seven times of parallel determination. The results shown that the relative standard deviations (R.S.D.) for Se(IV) studied were lower than 2%, which illustrated that the determination method has excellent reproducibility and high accuracy. The accuracy of the method was also evaluated by recovery study. The solution contained 5 µg L-1 Se(IV) was added to the samples. The standard solutions and treated sample solutions were introduced into the TMA-C separation and enrichment column system following the same procedure as described previously. The concentrations of Se(IV) were determined according to the proposed procedure. The analytical results (Table 6) show that the standard materials are in good agreement with the determined values. The results demonstrated that the analysis method have considerable application prospect in the detection of trace selenium.
Conclusions The regenerable cel(TMA-C) was successfully prepared and showed best performance in the adsorption and enrichment of Se(IV) which was separated and preconcentrated from total Se at desired pH values(pH4.5). And it has high
adsorption capacity for Se (IV), which is 101.2 mg/g. It is evident from the experimental data that the pseudo-second order model best describe the kinetics and the Langmuir model best fit equilibrium data, and the Thomas model fits the adsorption breakthrough curve well. The TMA-C was fully regenerated by purging with 3.0 mol/L HCl - 2% KClO3, which illustrate that the microcolumn has effective and high-quality recovery of selenium. Finally, the coupling technique (TMA-C microcolumn-UV-spectrophotometry) was developed for effective and economic determination of trace Se(IV) in waste waters, the results can get basic consistent results with ICP-OES.
Acknowledgments The work is supported by the National Science Fundamental Project of China (No.20972138), Ph.D. Programs Foundation of Ministry of Education of China (No. 20133326110006), and the Program of Science and Technology of Zhejiang Province，China (No.2015C3704).
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Figure 1. Three-dimensional plots showing effects of (a) molar ratio and temperature; (b) mass matio of catalyst and temperature, and (c)mass matio of catalyst with molar ratio interactions on the content of the functional group of TMAC.
Figure 2. FTIR spectroscopy comparison with Cel, TMA-C, Cel-g-TMA and TMA-C-Se(IV) (a) and synthesis routes for TMA-C(b)
Figure 3. SEM /ESDgraphs of Cel(a); TMAC(b); TMAC-Se(IV)(c)
Figure 4. The thermogravimetry (TG) curves for Cel, TMAC, Cel+TMAC, Cel-g-TMA and TMAC-Se(IV).
Figure 5. (a)The static adsorption capacity at pH ranging from 0.5 to 11.5; (b) Adsorption efficient of the TMAC for Ca2+, Mg2+, Na+, Citrate-, ClO3 -, HPO42- and HSeO3-; Adsorption efficient of the TMAC for Cl- and HSeO3-; (c)zeta potentials of TMAC
Figure 6 (a)Effect of different elution for TMAC after adsorption, (b) Influence of repeat time for TMAC recovery
Figure 7. Adsorption kinetics and capacity Q at different times and different temperatures (TMA-C200.0 mg, [HSeO3-]0 =0.8 mg/ mL, pH 4.5, 100 rpm)
Figure 8
Experimental and predicted breakthrough curves using Thomas
model forSe(IV) adsorption by TMA-C (TMA-C 200.0 mg, pH 4.5, C0 = 0.10 mg/mL, flow rate = 0.25 mL/min).
Figure 9 .Dynamic deadsorption curve flow rate=0.1 mL min-1.
Table 1
Experimental design of synthesis conditions of TMA-C
Symbols
Factors
-1
0
1
A
Temperature (℃)
60
75
90
B
molar ratio
2
3
4
3
5
7
C
Dosage of catalyst(mass ratio) (%)
Table 2
Kinetic parameters of TMA-C
First-order kinetic T(K)
Second-order kinetic
Qe(mg
K1(min-1
Q1(mg/g)
R1 2
K2(g mg-1
Q2(mg
g-1)
)
288
59.82
0.0053
178.57
298
85.93
0.0051
308
94.63
318
106.91
Table 3
Parameter for adsorption isotherms of Se (IV) by TMA-C
min-1)
g-1)
0.9531
4.80×10-6
79.54
0.9920
181.82
0.9217
5.49×10-6
126.50
0.9789
0.0062
243.90
0.9388
3.75×10-6
131.13
0.9720
0.0053
256.41
0.9248
4.41×10-6
154.13
0.9812
Langmuir T(K）
R2 2
Freundilich
Q0
KL
R1 2
n
KF((mg/g)/(
R2 2
(mg/g)
(mL/mg)
288
59.88
67
0.999
10.01
398
86.21
87
0.998
24.78
85.6643
0.586
308
96.15
120
0.996
30.03
95.4113
0.457
318
107.53
155
0.997
39.53
111.25
0.773
mg/mL)1/n) 59.1289
0.930
Table 4 Recovery values of Se(IV) and total selenium determined at different concentration ratios, as well as the calculated Se(VI) concentrations
Se(IV):Se(VI)
Se(IV)
Total selenium
Calculated Se(VI)
Added(μg/L)
Found(μg/L)
Recovery(%)
Found(μg/L)
Recovery(%)
10:1
100.0
98.6±1.8
98.6
108.9±2.3
99
10.3±0.5
5:1
50.0
49.1±1.1
98.2
58.7±0.8
100
9.6±0.3
1:1
10.0
10.1±0.2
101
19.2±1.0
96
9.1±0.8
1:5
10.0
9.7±0.4
97
59.3±1.9
98.8
49.6±1.5
1:10
10.0
9.9±0.1
99
109.5±1.2
99
99.6±1.1
Table 5
Selenium(IV) of Samples determination results with these two methods