Successive Extraction of As(V), Cu(II), and P(V) Ions from Water Using

Research Article pubs.acs.org/journal/ascecg

Successive Extraction of As(V), Cu(II), and P(V) Ions from Water Using Surface Modified Ghee Residue Protein Linlin Hao,†,‡ Masoom Kartik Desai,† Peng Wang,‡ and Suresh Valiyaveettil*,† †

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 State Key Laboratory of Urban Water Resource and Environment School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin, People’s Republic of China 150090

‡

S Supporting Information *

ABSTRACT: Renewable adsorbents are interesting for water purification owing to easy access and presence of multiple functional groups for extraction of pollutants. The biowaste, ghee residue from the milk industry was washed to remove fat contents, coated with polyethylenimine (PEI) and Fe(III) ions (ProtPEI-Fe) was synthesized for the successive adsorption of As(V), Cu(II) and P(V) ions from spiked water samples. The pretreated ghee residue was characterized using FTIR, SEM, ζ-potential and elemental analysis. Batch mode experiments and kinetic regression results showed that the adsorption processes of As(V) and P(V) anions were more accurately described by a pseudo-secondorder model, whereas the adsorption of Cu(II) ions followed a pseudo-firstorder model. The maximum adsorption capacities estimated by Langmuir model for As(V), Cu(II) and P(V) ions were 45.1, 80.7 and 21.7 mg/g, respectively. The successive adsorptions of As(V), Cu(II) and P(V) ions were achieved through electrostatic attraction, which was demonstrated by the changes in ζpotentials of the adsorbent after each experiment. The dynamic column adsorption behavior of the adsorbent was described by Thomas model. The simulated results showed good agreement between the theoretical values (q0) generated by using Thomas model and the experimental values (qexp). The results presented in this paper could be used for developing new adsorbent from the renewable waste materials for water purification. KEYWORDS: Adsorbent, Biowaste, Ghee residue protein, Heavy metal ions, Arsenic pollution, Successive Adsorption

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position.19 Impregnation, blending, doping and chemical modification of chitin/chitosan have been applied for removing pollutants from water.20−22 In this study, a common biowaste, ghee residue from milk industry was collected, modified as an efficient biosorbent to successively remove As(V), Cu(II) and P(V) from water. Modified protein adsorbents are of interest due to their good biocompatibility and biodegradability.23,24 Moreover, the successive extraction of As(V), Cu(II) and P(V) is achieved through the mechanism of electrostatic attraction of net charges on the surface of the adsorbent and oppositely charged adsorbate in solution, which is demonstrated by the changes in ζ-potentials after each adsorption. Such layer-by-layer adsorption was seldom applied for water purification.

INTRODUCTION Arsenic, copper and phosphate are usual contaminants in potable water with geological and anthropogenic origins. Contamination with arsenic and copper can cause detrimental health effects in living organisms.1,2 High concentration of phosphate is known to deteriorate natural ecosystems, natural water quality and responsible for eutrophication problem of surface water.3 Chemical precipitation/flocculation,4 ion exchange5 and membrane filtration6 are some of the frequently used techniques to remove pollutants from aqueous solutions. Besides such methods, extraction of pollutants using engineered adsorbents has been considered as an effective and simple method for the removal of pollutants from water.7−9 In previous work, biomass adsorbents with high adsorption capacity are prepared with natural materials to adsorb various pollutants from wastewater.9−12 Natural biomass materials are inexpensive, highly abundant, renewable and efficient biosorbents for many contaminants in water.13,14 Even though, a few cellulose based adsorbents have been tested for extraction of contaminants from water,15,16 but protein based biowastes are usually not explored owing to low stability.17 Chitin is a second most abundant biomacromolecules after cellulose18 in Nature. Chitin and chitosan have similar chemical structure with cellulose with N-acetyl or amino functional groups at C-2 © 2017 American Chemical Society

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EXPERIMENTAL SECTION

Synthesis of Modified Protein Adsorbents. The raw ghee residue was collected from local milk industry and prewashed to remove adsorbed fat content using the successive extraction of hexane, diethyl ether, CH2Cl2 and ethanol. Each organic solvent used is 150 Received: September 9, 2016 Revised: March 13, 2017 Published: April 2, 2017 3742

DOI: 10.1021/acssuschemeng.6b02152 ACS Sustainable Chem. Eng. 2017, 5, 3742−3750

Research Article

ACS Sustainable Chemistry & Engineering mL and stirred for 20 min. The prewashed protein powder (5 g) was stirred with 10% polyethylenimine (PEI) solution (200 mL) for 2 h, followed by cross-linking with glutaraldehyde (0.5 mL, 25%) solution. The product was washed with deionized water to remove excess reagents, dried in air and named as Prot-PEI. To increase the adsorption affinity toward arsenic anion, the Prot-PEI (5 g) was stirred in aqueous Fe(NO3)3 solution (250 mL, 500 mg/L) for 4 h, filtered, washed with water and dried to get the adsorbent Prot-PEI-Fe. The PEI and iron loading on the adsorbent were determined using elemental analysis. Batch Adsorption Experiments. The As(V) stock solution was prepared by using Na3AsO4·12H2O. The initial concentrations of 1, 5, 10, 50, 75, 100 mg/L As(V) solutions (100 mL) were used for arsenic adsorption and 0.5 g/L suspended adsorbents were added. The deionized water was used for all adsorption experiments without adjusting the solution pH of 7.1. The adsorption capacity of arsenic was determined using the equation below: q=

(C0 − C t)V madsorbent

Fe (1 g) was packed into the column and As(V) salt solution (1 mg/ L) was added dropwise into the column from the top using a separating funnel. The flow velocity was kept at 10 mL/min for 800 min and samples were collected every 20−30 min to determine the concentration of arsenic present in the filtered solution. After about 800 min running of As(V) ion adsorption, the feed solution was changed to Cu(II) ion solution (1 mg/L) and the column was run for 1700 min. Effluent was collected every 20−30 min to determine the concentration of copper remained in solution. After about 1700 min running of Cu(II) ion adsorption, the feed solution of P(V) ions (1 mg/L) was added into the column and the procedure was repeated for 800 min and the effluents were collected every 20−30 min for analyses. Several models were used to simulate the column tests data, i.e., Thomas,27 Yoon-Nelson,28 Wolborska29 and Adams−Bohart models.30 In this study, Thomas model (as can be seen from the Supporting Information) was used to simulate the column test data, assuming the second-order reversible reaction kinetics and the data followed the Langmuir isotherm model. Characterization of Adsorbents and Determination of Concentrations of Adsorbates. Fourier transform infrared spectrometer (FTIR) of a UV2550 instrument (Bruker ALPHA) using potassium bromide(KBr) matrix over the range of 450−4000 cm−1 was employed to follow up the changes of functional groups on prewashed ghee residue protein due to chemical modification. Scanning electron microscopy (SEM, JEOL JSM-6701F field emission scanning electron microscope) was used to observe the changes in morphology of ghee residue powder before and after modifications. Total As, Cu, P and Fe ion concentrations in solutions were determined using an inductively coupled plasma-optical emission spectroscopy instrument (Dual-view Optima5300 DV). Elemental analyses were done using an Elementar Vario Micro Cube instrument. ζ-Potential measurement was done using Malvern Zeta sizer NanoZS90.

(1)

where q is the amount of As adsorbed per unit mass of adsorbent (mg/ g) at a given time of t; C0 is the initial concentration of As (mg/L); Ct is the concentration of As (mg/L) at a given time t; madsorbent is the mass of the adsorbent (g). After arsenic adsorption, the adsorbent Prot-PEI-Fe-As was filtered and collected for Cu(II) adsorption. The initial concentrations of 1, 5, 10, 50, 75, 100 mg/L Cu(NO3)2 solutions were used to study the adsorption capacities for Cu(II) and extraction capacity of Cu(II) ions was determined using eq 1. After Cu(II) extraction, the adsorbent designated as Prot-PEI-Fe-As-Cu was filtered and collected for phosphate anion extraction. The initial concentrations of 1, 5, 10, 50, 75, 100 mg/L NaH2PO4 solutions were used to study the adsorption capacities for P(V) ions. The final adsorbent was designated as Prot-PEI-Fe-As-Cu-P. For all the batch adsorption experiments, the shaking speed was kept at 250 rpm, the temperature is 25 °C, the total volume of every sample is 100 mL. The pH effects on the adsorption was conducted under pH 2.0, 5.0 and 7.0, with the initial concentration of 50 mg/L for As(V), Cu(II) and P(V), respectively. The pH values were adjusted by dilute HNO3 and NaOH solution. In all cases, the adsorption efficiency was calculated using elemental analyses of the samples before and after experiment. The Langmuir and Freundlich models are frequently used to simulate the adsorptive isotherms data. The details of the Langmuir and Freundlich models are discussed in the Supporting Information. Kinetic Studies. Prot-PEI-Fe (0.5 g/L) was dispersed in As(V) ion solution (10 mg/L, 2 L) to determine adsorption capacity at certain time intervals keeping a constant pH of ∼7.1. The kinetics study was done using an orbital shaker operating at 250 rpm under room temperature (25 °C). Time point collections of samples were done at 3, 5, 10, 15, 20, 30, 40, 60, 90, 120, 180, 240, 360 and 1440 min. Arsenic concentrations in solution were detected by an inductively coupled plasma-optical emission spectroscopy (ICP-OES). The Asadsorbed material was collected by a 0.45 μm of membrane for the kinetic experiment with Cu(II) ions. The initial concentration of Cu(NO3)2 solution was kept at 10 mg/L and the experimental procedure was repeated. After that, the Prot-PEI-Fe adsorbed with As(V) and Cu(II) ions was collected again via filtration and used for the kinetic experiment for the extraction of P(V) ions with an initial concentration of 10 mg/L. In all cases, three consecutive runs are conducted and the solution samples and saturated adsorbents were collected for elemental analysis (EA). The average of the three values are reported in the final results. The pseudo-first-order and pseudosecond-order kinetic models are commonly used to analyze the As adsorption kinetics on adsorbents,25,26 as shown in the Supporting Information. Column Test. Dynamic adsorption experiments were conducted using a polyethylene column with 10 cm in height and 2.0 cm inner diameter. Cotton (0.1 g) was placed at the bottom of column to prevent the discharge of adsorbent particles from the tubes. Prot-PEI-

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RESULTS AND DISCUSSION Characterization of Surface Modified Ghee Residue. Pretreated and modified ghee residue powder was fully characterized using a range of techniques as discussed below, before employing for adsorption studies. Fourier Transform Infrared Spectroscopy (FTIR). The changes in functional groups in the prewashed ghee residue protein powder before and after modification can be seen from FTIR spectra (Figure 1). Strong contributions from amides I, II and III peaks, C−H stretch, carboxylic acid derivatives, disulfide stretches and C−N stretches of proteins were observed in the spectrum on the prewashed ghee residue protein. Bands of the amide I region between 1665 and 1645 cm−1 have previously

Figure 1. FTIR spectra of pretreated ghee residue powder before and after modification with PEI and Fe ion adsorption. 3743


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ACS Sustainable Chemistry & Engineering

Figure 2. SEM images of raw protein (A), protein powder after washing with organic solvents (B), Prot-PEI-Fe (C), Prot-PEI-Fe after successive adsorption of As(V), Cu(II) and P(V) (D).

been assigned to the α-helix, but the β sheet of amides (I, II) at about 1615−1630 cm−1 was not observed31−33 for the protein adsorbent. There are also peaks from amino acid side chain such as phenylalanine at about 1550 cm−1 and proline at about 1452 and 1435 cm−1.31,34 The peaks at 1083 and 1256 cm−1 correspond to symmetric and asymmetric vibrations of PO2− groups in phospholipids.35 The N−H bending of primary amine in PEI at about 1574 cm−1 is overlapped with α-helix absorption peaks of protein.36 For the Prot-PEI-Fe adsorbent, the peak at 1338 cm−1 maybe assigned to the Fe−OH vibration,37 because Fe(III) ions can form Fe(OH)3 during the drying procedure. For the small peak in the IR spectrum of ProPEI-Fe at 1828 cm−1, maybe it is assigned to the Fe−O vibration because of the iron loading modification. SEM Images and Elemental Analysis. Figure 2 depicts the SEM images of the raw protein and modified protein powders. The average particle size of the raw protein was at ∼1 μm. It is obvious that the raw protein powder surface is smooth and small particles are connected together by the fat components adhered on the surface (Figure 2A). After prewashing with organic solvents, the particles became small, and the flat surface of the particles can be observed (Figure 2B). After adsorption and cross-linking of PEI, followed by adsorption of Fe(III) ions, the surface of pretreated protein became rough (Figure 2C, S1). After successive adsorption of As(V), Cu(II) and P(V), the protein surface becomes more coarse and small pores were observed on the surface (Figure 2D). As expected, elemental analysis (Table 1) showed that nitrogen content increased from 8.93% for the prewashed adsorbent to 12.8% after cross-linking with PEI. No significant leaching of PEI from the adsorbent into water was observed during the washing and extraction studies, as nitrogen content in the aqueous washings was nondetectable using elemental analysis. Elemental analysis also showed that iron content in the Prot-PEI-Fe was around 1%. Measurements of ζ-Potentials. To estimate the change of surface charge during the consecutive adsorption, the ζ-

Table 1. Elemental Analysis of Raw Protein, Pretreated Protein and Prot-PEI-Fe Powders (% based on dry weight) and ζ-Potentials of Different Materials Prepared and Characterized in This Study materials

elemental analysis (wt %)

ζ-potentials (mV)

pretreated protein

C, 45.8; H, 6.55 N, 8.93; Fe, N.D.a C, 51.1; H, 7.36 N, 12.8; Fe, N.D.a C, 47.9; H, 6.93 N, 12.5; Fe, 0.99

−14.8

Prot-PEI Prot-PEI-Fe Prot-PEI-Fe after As(V) adsorption Prot-PEI-Fe after As(V) and Cu(II) adsorption Prot-PEI-Fe after As(V), Cu(II) and P(V) adsorption a

−10.2 −6.83 −21.9 21

10.9

N.D.: Not Detected.

potentials of all adsorbents, pretreated protein, Prot-PEI, Prot-PEI-Fe, Prot-PEI-Fe-As, Prot-PEI-Fe-As-Cu(II) and Prot-PEI-Fe-As-Cu-P were measured, and the results are given in Table 1. The pretreated protein shows a ζ-potential of −14.8 mV, indicating the surface of protein is negatively charged. After modification with PEI, the ζ-potential was increased to −10.2 mV because of the protonation of amine groups on the surface. The ζ-potential was further increased to −6.83 mV after adsorption of Fe(III) ions on the surface. As expected, the adsorption of As(V) ions resulted in the reduction of ζ-potential to −21.9 mV and again increased to +21 mV after Cu(II) cation extraction. Similarly, adsorption of P(V) anion on the surface reduced the ζ-potential to +10.9 mV. The consecutive increase and decrease in ζ-potentials indicated 3744


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ACS Sustainable Chemistry & Engineering the layer-by-layer adsorption of cations and anions from solution. Comparison of Pretreated Protein, Prot-PEI and ProtPEI-Fe for As(V) Adsorption. The comparison of As(V) adsorption on pretreated protein powder, Prot-PEI and ProtPEI-Fe are shown in Figure 3. The adsorption ability was

Figure 3. As(V) adsorption on pretreated protein powder, Prot-PEI and Prot-PEI-Fe. The adsorbent concentration is 0.5 g/L, the initial pH is 7.1, at room temperature.

significantly enhanced after adsorption of PEI, followed by iron loading step. The initial concentration of As(V) anion used was 1, 5, 10, 50 mg/L in solution. The adsorption capacity of As(V) ions on Prot-PEI and Prot-PEI-Fe is increased significantly with an increase of initial concentration of the solutions. The pretreated protein powder showed a maximum adsorption capacity of 0.49 mg/g for As(V) ions; however, the maximum adsorption capacity of Prot-PEI is 24.8 mg/g whereas that of Prot-PEI-Fe was 43.3 mg/g at the initial As(V) ion concentration of 50 mg/L. The protonated amine groups of PEI were responsible for the increased As(V) anion extraction by Prot-PEI via electrostatic attraction. Similarly, the loading of Fe(III) cations on Prot-PEI surface significantly enhanced As(V) anion adsorption to 43.3 mg/g, not only because the Fe(III) ion loading could increase the net positive charge on Prot-PEI surface, but also Fe(III) ion on the surface can form Fe−As complex with As(V) anion.38 Therefore, incorporation of PEI and Fe on the surface contributed to the enhancement of As(V) anion extraction by the treated protein powder (Figure 3). Successive Adsorption of Arsenate, Copper and Phosphate Ions on the Prot-PEI-Fe Adsorbent. Figure 4 shows the successive extraction of As(V), Cu(II) and P(V) ions on Prot-PEI-Fe adsorbent. It can be seen that As(V) ion extraction on Prot-PEI-Fe increased from 1 to 45 mg/g with an increase in initial concentration of As(V) ions from 1 to 100 mg/L. Meanwhile, Cu(II) ion extraction capacity also increased with an increasing initial concentrations, and the maximum extraction capacity reached 81 mg/g. After As(V) anion adsorption, the effective negative surface charge of Prot-PEIFe-As favors the adsorption of positively charged Cu(II) ions from solution. Similarly, adsorption of Cu(II) cations on the surface of adsorbent enhances the net positive charges on the surface and facilitate the adsorption of negatively charged P(V) ions; the adsorption capacity was about 22 mg/g when the initial P(V) anion concentrations used was 100 mg/L. Such layer-by-layer adsorption of alternating metal cations and anions was achieved through electrostatic attraction between the adsorbents and adsorbates. The pH effect on the successive adsorption was shown in Figure S2. The highest adsorption capacities for three ions was obtained at pH around 5.0,

Figure 4. (A) Adsorption isotherms for As(V), Cu(II) and P(V)ions on Prot-PEI-Fe. (B) Langmuir isotherm plot for As(V), Cu(II) and P(V)adsorption of on Prot-PEI-Fe. (C) Freundlich isotherms plot for As(V), Cu(II) and P(V) adsorption of on Prot-PEI-Fe. (⧫) As(V), (■) Cu(II), (▲) P(V) ions.

indicating the weak acidic environment was beneficial for the successive adsorption. When the pH was lower, such as pH 2.0, abundant of H+ can compete with Fe(III) and Cu(II), so as to decrease the adsorption capacities. When the solution pH was increased to around 7, the protonation effect of amino groups was significantly weakened, and the positive charge of the adsorbent surface was reduced, thus reducing the adsorption capacities for three ions. Langmuir and Freundlich isotherm models were used to describe the adsorption isotherms as shown in Figure 4b,c; the fitting parameters are summarized in Table 2. For As(V) and P(V) adsorption, the Langmuir model showed a better fit with the adsorption isotherms (R2 > 0.99) than Freundlich model (R2 between 0.88 and 0.91). For Cu(II), the Freundlich model fitted much better than Langmuir model. The values of n > 1 represent the favorable adsorption of adsorbate onto adsorbent, and higher values of kL and n indicated the easier uptake of P(V) than As(V). The maximum adsorption capacity of As(V), Cu(II) and P(V) ions estimated by Langmuir model was calculated to be 45.1, 80.7 and 21.7 mg/g, respectively. A comparison of Langmuir constants indicated that the adsorption affinity toward As(V) and P(V) was stronger than Cu(II) ions, owing to the surface complex with Fe(III).39 3745


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Table 2. Parameters of Isotherm Models for As(V), Cu(II) and P(V) Adsorption on Prot-PEI-Fe in Deionized Water at pH 7.1 models

Langmuir

Freundlich

species

KL (L/mg)

Qmax (mg/g)

RL

R2

KF (mg/g) (L/mg)−1/n

n

R2

As(V) Cu(II) P(V)

0.213 0.022 0.359

45.05 80.65 21.65

0.045 0.313 0.027

0.992 0.497 0.992

7.446 3.053 6.136

2.1 1.6 2.9

0.886 0.921 0.910

shows a plateau. It is conceivable that two different mechanisms may be involved for the adsorption of As(V), P(V) and Cu(II) on the surface of adsorbent. To understand the kinetics of arsenate adsorption onto Prot-PEI-Fe surfaces, the data were analyzed using both pseudo-first- and pseudo-second-order kinetic models. The parameters of these two models were summarized in Table 3. As can be seem from Table 3, the kinetics involved in the extraction of As(V) and P(V) anions can be described by pseudo-second-order model with the correlation coefficient R2 of 0.990 and 0.998, respectively, indicating the adsorption process is chemisorption in nature. The k2 value of 0.049 for P(V) ions is much higher than that of 0.018 for As(V) anions, indicating that Prot-PEI-Fe exhibited a stronger affinity toward P(V) than As(V) anions. Moreover, it was observed that As(V) ions leaching into the solution increased during P(V) ion adsorption process, indicating a competitive adsorption between As(V) and P(V) ions was occurred. The process of mass transfer may be controlled by four independent processes: (i) bulk transport, (ii) film diffusion involving mass transfer across the external boundary, (iii) intraparticle diffusion such as diffusion within the pores of the adsorbent and (iv) chemical reaction along with adsorption at a special site on the surface facilitated by electrostatic interaction. The process of (i) bulk diffusion was not assumed to be the adsorption rate limiting step because of rapid mechanical mixing in the reactor. Given that the main functional groups such as amide, carboxyl, hydroxyl groups and the grafted amino groups from PEI on the adsorbent surface are hydrophilic functional groups and interact strongly with charged ionic species, it is reasonable to deduce that the film diffusion is also not the rate limiting step. The kinetic data could be best described by the pseudo-second order model, indicating both intraparticle diffusion and chemical reaction are the ratedetermining steps during arsenic adsorption process40 To investigate the mechanism of mass transfer and resistant of mass transfer, the kinetic data are divided in to two phases: rapid adsorption phase and slow adsorption phase, and simulated using the Weber and Morris intraparticle diffusion model41 that is shown below:

Moreover, it was observed that As(V) leaching into the solution increased during P(V) adsorption process, indicating a competitive adsorption between As(V) and P(V) occurred. The adsorbed arsenic anions can be partially desorbed by P(V) ions because of the competition for the same adsorption sites on Prot-PEI-Fe. As can be seen from Figure 5, the adsorption

Figure 5. Arsenic and copper leaching during the adsorption of P(V) ions on Prot-PEI-Fe in deionized water. Initial concentration of P(V) ions is 5 mg/L, adsorbent dose is 1 g/L at room temperature.

curve of P(V) ions is similar in shape with that of the leaching curve of arsenic anions. It was further understood that P(V) ions is able to replace the adsorbed As(V) anions from the surface of Prot-PEI-Fe adsorbent. Adsorption Kinetics. For the adsorption of As(V) and P(V) anions on the adsorbent surface (as shown in Figure 6), the initial adsorption rates within the first 25 min are relatively fast, followed by a slower extraction rate, and the adsorption equilibrium was achieved within 75 min. For Cu(II) ions, the adsorption rate is much faster owing to strong interaction between copper ions and amines on the surface, and the rate remains constant for the period of up to 400 min and then

qt = k id × t 1/2 + C

(2)

where qt is the adsorption capacity of solute on adsorbent at t (mg/g), kid is the intraparticle rate constant (mg/g/min), C represents the boundary layer effect; greater is the value of C, greater is the thickness of boundary layer, and t is the reaction time in minutes. The model parameters were determined by the slopes and intercepts by plotting qt versus t1/2 as shown in Figure S3. The parameters are shown in Table 3. As shown in Figure S3, the plot of qt against t1/2 for As(V) and P(V) presented multilinearity relation, which indicated that more than one process affected the adsorption.42 The rapid phase correlated to the boundary layer diffusion of adsorbate. The slow phase is

Figure 6. Kinetic curves of As(V), Cu(II) and P(V) adsorption on Prot-PEI-Fe in deionized water. (■) As(V), (●) Cu(II), (▲) P(V) ions. The black solid line() means the pseudo-second-order fitting plot, the black dash line (----) means the pseudo-first-order fitting plot. The experiments were done using the initial concentration for each element of 5 mg/L and adsorbent dose of 1 g/L, in room temperature. 3746


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ACS Sustainable Chemistry & Engineering Table 3. Parameters of Kinetic Models for As(V), Cu(II) and P(V) Adsorption on Prot-PEI-Fe pseudo-first-order model

pseudo-second-order model

intraparticle diffusion model

species

k1 (g·mg−1·min−1)

R2

k2 (g·mg−1·min−1)

v0 (mg·g−1·min−1)

R2

kid (g·mg−1·min−1)

C

R2

As(V)

0.013

0.749

0.018

4.49

0.990

Cu(II) P(V)

0.014 0.014

0.987 0.688

0.01 0.049

0.6 7.9

0.534 0.998

3.457 0.147 0.240 3.676 0.025

0.634 13.18 0.674 0.671 12.05

0.975 0.973 0.985 0.956 0.738

To investigate further the mechanism of mass transfer, the column data were simulated using the models proposed by Fulazzaky43 in 2011. It is briefly introduced as follows:

attributed to the gradual adsorption, where intraparticle diffusion is rate-controlling. According to the calculation of parameters of the intraparticle diffusion model, the kid values of As(V) ions decreased from 3.46 to 0.15 g·mg−1·min−1, and P(V) ions decreased from 3.68 to 0.03 g·mg−1·min−1, from the first rapid adsorption phase to slow adsorption phase, indicating the mass transfer of intraparticle diffusion is becoming more and more difficult. The observed significant increase in C values from 0.634 to 13.2, and from 0.671 to 12.1 for As(V) and P(V) ions, respectively, suggested the increase in the thickness of boundary layer. For Cu(II) ions, the correlation coefficient R2 of 0.99 indicated the intraparticle diffusion is one of the main resistances of mass transfer or rate-determining step for the adsorption of Cu(II) ions on Prot-PEI-Fe-As surface. Dynamic Column Tests. Batch experiments help to evaluate the equilibrium capacity of adsorbent for adsorbate present in water. But in practical industrial water treatment processes, adsorption in fixed bed columns is preferable, and the experimental data obtained from the laboratory scale fixed bed columns are helpful for checking the applicability at the designed industrial scale. Column adsorption offers a more realistic simulation by replicating the batch treatment. Therefore, a dynamic column adsorption experiment was conducted to determine various types of column parameters. As shown in Figure 7, the duration to reach the breakthrough point for 1

⎛C ⎞ ln⎜ 0 ⎟ = [kLa]f × t ⎝ Cs ⎠

(3)

where C0 is the initial concentration of adsorbate (in mg/L), Cs is the concentration of adsorbate to depart from the column at t (in mg/L), [kLa]f is the external mass transfer factor or film mass transfer factor (1/h), t is the reaction time (in h). Because it is impossible to determine the variations of [kLa]f before breakthrough point occurred due to the ratio of C0/Cs is infinite. Monitoring Cs at outlet of the column after the breakthrough point is important for modeling. [kLa]f = [kLa]g × e−β × ln(q)

(4)

where [kLa]g is the global mass transfer factor in 1/h. By substituting eq 4 into eq 3 yields a continuous equation valid in determining the variation of mass transfer factor. ⎛C ⎞ ln⎜ 0 ⎟ = [kLa]g × e−β × ln(q) × t ⎝ Cs ⎠

(5)

A deduction of eq 5 mathematically gives a linear expression below: ⎧ 1 ⎪ ln q = B + × ln t β ⎪ ⎨ ln([kLa]g ) − ln{ln(C0/Cs)} ⎪ ⎪B = β ⎩

(6) (7)

where q is the cumulative quantity of solute on adsorbent (in mg/g), which can be calculated by the following formula: q= Figure 7. Breakthrough behavior of the column runs for adsorption of As(V), Cu(II) and P(V) ions on Prot-PEI-Fe. Initial concentration of As(V), Cu(II) and P(V) ions was 1 mg/L; pH 7.0 ± 0.2, flow velocity was 10 mL/min. (⧫) As(V), (■) Cu(II), (▲) P(V) ions.

∫0

V

(C0 − Cs)dV m

(8)

β is the adsorbate−adsorbent affinity parameter (in g h/mg). [kLa]d = [kLa]g − [kLa]f

(9)

where [kLa]d is the internal mass transfer factor (in 1/h). A plot (Figure S5) of ln(q) versus ln(t) gives a straight line with an intercept at B and 1/β as slope with the correlation coefficient above 0.973, indicating that the use of parameter B and β is reasonable for investigating mass transfer potential and adsorbate−adsorbent affinity for As(V), Cu(II) and P(V) ions on Prot-PEI-Fe adsorbent. The variations of global, external and internal mass transfer factors are shown in Figure 8. The curves of [kLa]g, [kLa]f, [kLa]d for As(V) and P(V) ions descended progressively with increase in running time. Finally, the values of [k L a] f

mg/L for As(V) ions, needs 420 min at a flow velocity of 10 mL/min, whereas it needs 230 min to reach the breakthrough point for 1 mg/L for P(V) ions at the same flow velocity. For Cu(II) ions, it needs a much longer time of 500 min to reach the breakthrough point. Thomas model was used to fit the data from tests of column adsorption (Figure S4, Table S1). The results of column adsorption capacity indicated a good agreement between the values (q0) generated by using Thomas model and the experimental values (qexp). 3747


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Over 80% of efficiency for As(V) anions, 75% for Cu(II) cations and over 60% of efficiency for P(V) anions were obtained in one time adsorption−desorption cycles, demonstrating that the adsorption of As(V), Cu(II) and P(V) ions on Prot-PEI-Fe adsorbent was a reversible process and the synthesized sorbent was cost-effective.

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CONCLUSIONS In summary, renewable low-cost biowaste, ghee residue protein was developed as an efficient adsorbent for the removal of As(V), Cu(II) and P(V) ions from water. The raw protein powder was washed thoroughly with different organic solvents to remove impurities adhered on the surface. Iron was adsorbed on the PEI coated protein surface to facilitate successive adsorption of As(V), Cu(II) and P(V) ions using mainly changes in net surface charges after the adsorption of each ions. The maximum experimental adsorption capacities of Prot-PEIFe were 45.1, 80.7 and 21.7 mg/g for As(V), Cu(II) and P(V) ions, respectively. The kinetic study using the mixture of three ions in the same solution suggested Prot-PEI-Fe exhibited the strongest affinity toward Cu(II) ions through electrostatic attraction and metal coordination, followed by P(V) and As(V) ions. The column data showed a good fit with Thomas model and a strong agreement between the simulated and experimental data. The mass transfer model revealed that the internal diffusion is the rate-determining step for As(V), Cu(II) and P(V) ions adsorption on Prot-PEI-Fe. Our study concludes that the treated ghee protein residue is an interesting renewable adsorbent for the removal of various pollutants from contaminated water. Besides the As(V), Cu(II) and P(V) ions, Prot-PEI-Fe can also be used as a potential adsorbent for the removal of other dissolved pollutants.

Figure 8. Variations of global ([kLa]g), external ([kLa]f) and internal ([kLa]d) mass transfer factors pursuant to the running time for the adsorption of (A) As(V), (B) Cu(II)and (C) P(V) ions on the adsorbents. (■) [kLa]g, (▲) [kLa]f, (⧫) [kLa]d.

approached to zero when the Prot-PEI-Fe surface was saturated through adsorption. For Cu(II) ions, the [kLa]f is almost horizontal, means the rate of film mass transfer is stable. But the curves of [kLa]g and [kLa]d are convex, the intraparticle diffusion rate reaches a maximum and then decreases. For these three ions, the internal mass transfer [kLa]d constituted a larger proportion of the global mass transfer [kLa]g, and the resistance of mass transfer is dependent on the [kLa]d. The chemical interactions between As(V)/P(V) anions and the Fe(III) cations on the surface are relatively important for enhancing the As(V)/P(V) ion adsorption. As(V) and P(V) ions tend to form inner-sphere complex with iron oxyhydroxide through mechanism of ligand exchange.44,45 Such chemisorption is also indicated by the simulation with the pseudo-secondorder model. Cu(II) ions form the expected copper-amine complex on the Prot-PEI-Fe surface. Conversely, the external mass transfer [kLa]d for the three ions constituted a much smaller proportion of the global mass transfer [kLa]g, indicating the film transfer is not the main resistance for As(V), Cu(II) and P(V) ions adsorption on the surface of Prot-PEI-Fe adsorbent. Regeneration of the Adsorbent. In the practical application, the stability and reuse of the adsorbents are usually an important factor because of the economic necessity. In contrast to the energy intensive and degradative regeneration processes of expensive adsorbents, As(V), Cu(II) and P(V) ions are easily removed from Prot-PEI-Fe surface by rinsing with 5% NaOH solution (100 mL) and 5% H2SO4 solution (100 mL), respectively at room temperature. The desorption process occurred by ionization and replacement of As(V) and P(V) ions by OH− ions, and replacement of Cu(II) ions by H+ ions. Data from desorption studies are shown in Figure S6.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02152. Equations of Langmuir and Freundlich models, kinetic model, Thomas model, Thomas model parameters, SEM images of Prot-PEI, pH effect on adsorption, Weber and Morris intraparticle diffusion plot, linear Thomas model fit of breakthrough data, linear regression analysis for the adsorption, regeneration data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S. Valiyaveettil). ORCID

Peng Wang: 0000-0003-4465-6207 Suresh Valiyaveettil: 0000-0001-6990-660X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

The authors thank the technical support from the department of chemistry and National University of Singapore. L.H. thanks China Scholarship Council for financial support for the joint PhD programme. 3748


Research Article


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Successive Extraction of As(V), Cu(II), and P(V) Ions from Water Using

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