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Abstract

Two novel adsorbents derived from shrimp shell were prepared and their adsorption performances on Congo red were investigated. The results suggested that treated shrimp shell powder exhibited a higher adsorption capacity than raw shrimp shell powder. The factors of initial concentration, solution pH, adsorption time, and temperature were investigated. The maximum adsorption capacity of treated shrimp shell powder calculated according to the Langmuir isotherm model was 288.2 mg/g, which is much higher than that of chitin. The adsorption behavior could be fitted well by the pseudo-second-order kinetic model. Intra-particle diffusion model was also used to study the adsorption process. The thermodynamic parameters indicated the spontaneous and endothermic nature of the adsorption. Shrimp shell powder exhibited enough advantages such as large adsorption capacity, low cost, simple processing methods and high specific gravity compared with chitin or chitosan. This work confirmed that the shrimp shell biosorbent had a potential to be applied in dye wastewater treatment area.

Keywords

Adsorption Congo red shrimp shell biosorbent wastewater

Introduction

Dyes have been widely used in several industries, such as textiles, leather, paper, printing, cosmetics and so on, which, however, lead to serious environmental problems (Ma et al., 2013). Especially, in printing and dyeing industries, a considerable amount of effluent is generated, which contains aromatic, polycyclic aromatic or heterocyclic dye, toxic, and carcinogenic compounds that are difficult to be removed and biodegraded (Crini, 2006). Therefore, dye removal is an important and challenging area in wastewater treatment (Ma et al., 2013).

Dyes are classified into three broad categories: (a) anionic: direct, acid, and reactive dyes; (b) cationic: all basic dyes and (c) nonionic: dispersed dyes (Purkait et al., 2007). Congo red (CR) named [1-naphthalenesulfonic acid, 3,3′-(4,4′-biphenylenebis (azo)) bis(4-amino-)disodium salt] is a benzidine-based anionic diazo dye, which has been known to cause an allergic reaction and to be metabolised to benzidine, a human carcinogen (Vimonses et al., 2009). The molecular structures of CR are various in aqueous solution with different pH (Spolnik et al., 2007). Especially, it is very sensitive to acids and its color will be blue instead of red in the presence of acids (below pH 5). The color variations may be attributed to resonance among charged canonical structures or the protonation of its amino groups (Purkait et al., 2007).

Various methods have been developed and used for the removal of dye contaminants from wastewater, including physical, chemical, and biological processes, e.g. adsorption, coagulation/flocculation, advanced oxidation, ozonation, membrane filtration and liquid–liquid extraction (Chakraborty et al., 2003; Vimonses et al., 2009). Adsorption, among them, is a kind of attractive method, which is considered to be superior to other techniques in terms of low initial cost, simplicity of design, ease of operation and insensitive to toxic substances (Dawood and Sen, 2012; Purkait et al., 2007; Sen et al., 2011). Activated carbon is the most efficient and popular adsorbent in removal of organic and inorganic pollutants, and has been used with great success (Namasivayam and Arasi, 1997). However, the use of this adsorbent is still limited because of its high cost and difficult regeneration (Vimonses et al., 2009). Thus, it is demanded to seek other adsorbents to replace or partly replace it. Kyzas et al. (2013) have reviewed some adsorbent materials which were attempted to replace the activated carbon in the treatment of dyeing wastewaters, including industrial waste products (waste carbon slurries, metal hydroxide sludge), agricultural solid wastes (orange bagasse, rice husk), natural materials (clay materials, bentonite, kaolinite), biosorbents (chitosan, peat), and others (starch, cyclodextrin). Chitin, the most widespread biopolymer in nature after cellulose, exhibits excellent adsorption capacities due to the presence of –OH and N-acetyl groups (Arbia et al., 2013; Daneshvar et al., 2014). Chitosan, a more excellent biosorbent, which can be obtained from chitin by a deacetylation process with a strong alkaline solution, has problems of difficultly dissolving and agglomeration (Chio et al., 2009). Although chitin and chitosan are recognized as superb adsorbents, their preparation process requires large amounts of acid and alkaline. Therefore, more environmental friendly adsorbents are still needed.

Shrimp shell, usually abandoned as an industrial by-product and food waste, may be an ideal candidate for environmental friendly adsorbents (Qin et al., 2016). The shrimp shell waste accounts for 40–66% of shrimp weight, and it mainly contains chitin, protein, and calcium carbonate. As reported, shrimp shell adsorbent has the advantages of low price, environmental protection and easy preparation (Rodde et al., 2008; Xu et al., 2013). Moreover, owing to the component of calcium carbonate, the settling of shrimp shell is much faster compared to chitin and chitosan, which facilitates the separation and recovery of the adsorbent.

This study aimed to investigate the adsorption performance of shrimp shell powder (SSP) from prawns in the removal of CR from water. The research mainly focused on the effect of process parameters on the adsorption capacity of SSP, including treatment of SSP, initial CR concentrations, initial solution pH, and temperature. Adsorption isotherms, kinetics, and adsorption mechanism were also evaluated and reported.

Materials and methods

Materials

Shrimp shells were obtained from a sea food processing factory, located in Zhoushan, China. Other chemicals used were of analytical grade without further purification, which were purchased from Aladdin Reagent (Shanghai, China) Co., Ltd. The molecular formula of CR is C₃₂H₂₂N₆Na₂O₆S₂, the structure of which is shown in Figure 1. The pH of the dye solution was adjusted by using 0.1 M hydrochloric acid (HCl) and 0.1 M sodium hydroxide (NaOH).

Figure 1.

The molecular structure of CR dye.

Preparation of adsorbents

Shrimp shells were separated from the collected shrimp waste and washed with tap water several times to remove unwanted particles or components, such as shrimp meat, sands, etc. The washed shells were dried in an oven (JINGHONG XMTD-8222) at 70°C for 12 h. Subsequently, they were smashed in a high-speed multifunction grinder (BLF-YB1000) to obtain the SSP. A part of it was washed with tap water to flush away the floating impurities. The other part was added into the mixed solution of NaOH (5 wt.%) and H₂O₂ (1 wt.%) under stirring at room temperature for 72 h, in order to remove protein and biological pigment. And then it was washed with enough water until the solution became neutral. After that, the two kinds of SSP with different treatments were filtered first and then dried at 70°C up to constant weight. Then they were smashed the same time in the grinder to get the final adsorbents, raw shrimp shell particle (RSSP) and treated shrimp shell particle (TSSP). Finally, they were collected in airtight plastic bags and used for analysis as well as for adsorption experiments.

Characterizations

A scanning electron microscope (SU8010, HITACHI) at an accelerating voltage of 3 kV was used for studying the surface micro-morphology of adsorbents. Thermogravimetric analysis (TGA) and differential thermogravimetry (DTG) were carried out using a TA-Q500 TGA instrument to test their thermal properties. The volume ratio of oxygen to nitrogen was 60 to 40 under the heating rate of 10°C/min. X-ray diffraction (XRD) analysis was carried out to determine the phases of the adsorbents, performed on SHIMADZU XRD-6000 [Cu (40 kV, 40 mA)] over the range of 10–80° 2θ for the solid powder samples. An FTIR spectrometer (Nicolet 5700)-based detector employing a KBr pellet method was used to determine the functional groups of the biosorbents. Particle sizes of RSSP and TSSP were measured by LS 13320 Laser Diffraction Particle Size Analyzer (Beckman Coulter, Inc., USA). The specific surface area and porosity were determined by N₂ adsorption–desorption isotherm measured at 77K in a TriStar II apparatus (Micromeritics Instrument Corp., USA).

Adsorption experiments

Batch experiments were performed to determine the effects of different process parameters such as initial dye concentration, temperature, adsorption time, solution pH, and processing methods of SSP on CR removal from aqueous solution. A desired amount (0.1 g) of adsorbent was mixed with 30 ml of CR solution in centrifuge tube (50 ml). The solution pH, measured by PHS-25 pH Meter (Shanghai Precision & Scientific Instrument Co., Ltd), was adjusted with 0.1 M HCl and 0.1 M NaOH. The mixture was agitated at 250 r/min in a shaking table (THZ-D constant temperature platform shaker, Jiangsu Shenglan Instrument Manufacturing Co., Ltd, China) at desired temperature. After adsorption equilibrium was achieved, the tubes were centrifuged at 10,000 r/min for 3 min in a centrifuge (Eppendorf Centrifuge 5804, Germany) to separate the supernatant from the adsorbent. The same amount of adsorbent was mixed with 30 mL dye solution and agitated at desired temperature for given recorded times to study the adsorption kinetics. The SHIMADZU UV-2550 UV/VIS spectrophotometer was used to determine the concentrations of CR solution. All experiments were performed three times in parallel experiments, and the average values were taken to minimize experimental errors.

The adsorption amount of adsorbent (q_t, mg/g) at time t (min), and the removal percentage (%) of CR were calculated by the following equations $q_{t} = \frac{(C_{0} - C_{t}) V}{m}$ (1) $% = \frac{100 (C_{0} - C_{t})}{C_{0}}$ (2)where C₀ (mg/L) is the initial dye concentration, C_t (mg/L) is the dye concentration at time t, V (L) is the volume of dye solution, and m (g) is the weight of the adsorbents.

Adsorption theory

Adsorption isotherm

There are four two-parameter equations—Freundlich, Langmuir, Temkin, and Dubinin-Radushkevich isotherms and three three-parameter equations—Redlich-Peterson, Toth, and Sips isotherms, which can be used to describe equilibrium adsorption data (Ho et al., 2002). Among them, the Freundlich and Langmuir isotherms are the most favorable and most frequently used (Chen et al., 2012). So Freundlich and Langmuir models were selected to explicate CR-SSP interaction in this study.

Freundlich Isotherm: The Freundlich adsorption isotherm, which assumes that adsorption takes place on heterogeneous surfaces (Sen et al., 2011), can be described by equation (3) $q_{e} = K_{F} C_{e}^{1 / n}$ (3)

The equation can be rearranged to obtain a linear form as follows $\log q_{e} = \log K_{F} + \frac{1}{n} \log C_{e}$ (4)where q_e (mg·g⁻¹) is the amount of dye adsorbed per unit of adsorbent at equilibrium, C_e (mg·L⁻¹) is the remaining concentration of dye solution after adsorbing at equilibrium, K_F and n are the Freundlich adsorption isotherm constants which indicate the extent of the adsorption and the degree of nonlinearity between concentration and adsorption, respectively (Sen et al., 2011; Vimonses et al., 2009). The value of 1/n, which is between 0 and 1 generally, shows the effect of concentration on adsorption capacity. And the values of K_F and 1/n can be calculated from intercept and slope of the linear plot between log C_e and log q_e.

Langmuir Isotherm: The Langmuir adsorption isotherm can be described as equation (5) $q_{e} = \frac{q_{m} K_{L} C_{e}}{1 + K_{L} C_{e}}$ (5)

The equation can be linearized as follows $\frac{C_{e}}{q_{e}} = \frac{C_{e}}{q_{m}} + \frac{1}{K_{L} q_{m}}$ (6)where q_m (mg·g⁻¹) is the maximum amount of monolayer adsorption on the adsorbent surface (Srilakshmi and Saraf, 2016), and K_L (L·mg⁻¹) is the Langmuir constant related to the energy adsorption. The Langmuir constants K_L and q_m can be calculated from intercept and slope of the plot between C_e/q_e and C_e.

For the Langmuir model, the effect of isotherm shape is used to predict a favorability of an adsorption system under specific conditions. Hall et al. (1966) pointed out that the essential characteristics of the Langmuir isotherm can be expressed in term of a dimensionless, R_L (the separation factor or equilibrium parameter), as given as equation (7) $R_{L} = \frac{1}{1 + K_{L} C_{0}}$ (7)where K_L is the Langmuir constant and C₀ (mg/L) is the initial CR dye concentration. The values of R_L indicate the shape of the isotherm: R_L > 1 (unfavorable), R_L = 1 (linear), 1 > R_L > 0 (favorable) and R_L = 0 (irreversible) (El-Shafey et al., 2012).

Adsorption kinetics

Adsorption kinetic models were used to interpret the experimental data and determine the controlling mechanism of dye adsorption from aqueous solution. Generally, the experimental data were analyzed using the pseudo-first-order, pseudo-second-order and intra-particle diffusion kinetic model (Srilakshmi and Saraf, 2016).

Pseudo-first-order model

The differential form of pseudo-first-order rate equation, which is widely used for the adsorption of an adsorbate from an aqueous solution, is described as follows (Vimonses et al., 2009) $\frac{{dq}_{t}}{dt} = k_{1} (q_{e} - q_{t})$ (8)

When q_t = 0 at t = 0 and q_t = q_t at t = t, equation (8) can be integrated into the following equation $ln (q_{e} - q_{t}) = \ln q_{e} - k_{1} t$ (9)where q_t (mg·g⁻¹) is the adsorption amount of dye per unit of adsorbent at time t, k₁ (min⁻¹) is the pseudo-first-order rate constant, and t (min) is the adsorption time. And the adsorption rate constant (k₁) can be calculated from the plot of ln (q_e − q_t) against t.

Pseudo-second-order model

Ho and McKay (1999) described the pseudo-second-order kinetic equation as follows $\frac{{dq}_{t}}{dt} = k_{2} {(q_{e} - q_{t})}^{2}$ (10)

Integrating equation (10) for the boundary condition t = 0 to t = t and q_t=0 and q_t=q_t, the obtained equation can be rearranged into a linear form $\frac{t}{q_{t}} = \frac{1}{k_{2} q_{e}^{2}} + \frac{t}{q_{e}}$ (11)where k₂ (g·mg⁻¹·min⁻¹) is the pseudo-second-order rate constant.

Furthermore, the constant k₂ is usually used to calculate the initial adsorption rate h (mg·g⁻¹·min⁻¹) as follows (Sen et al., 2011) $h = \underset{t \to 0}{\lim (} k_{2} q_{e}^{2})$ (12)

The values of h, k₂ and q_e can be calculated from the plot of t/q_t versus t using equation (11).

Intra-particle diffusion model

Intra-particle diffusion model is commonly used for identifying the adsorption mechanism for design purpose (Sen et al., 2011). And it can be described as the following equation (Vimonses et al., 2009) $q_{t} = k_{i} t^{1 / 2} + I$ (13)where k_i (mg·g⁻¹·min^−1/2) is the intra-particle diffusion rate constant. And the values of I represent the thickness of boundary layer, i.e. the larger the I, the greater the boundary layer effect (Vimonses et al., 2009).

Adsorption thermodynamic

The thermodynamic parameters of the CR adsorption (500 mg·L⁻¹) by SSP, including standard free energy change (ΔG⁰), enthalpy change (ΔH⁰) and entropy change (ΔS⁰) were estimated from the experimental data based on the following equations (Hou et al., 2012) $K = \frac{C_{s}}{C_{e}}$ (14) ${ΔG}^{0} = - RT1nK$ (15) $1n K= \frac{Δ S^{0}}{R} - \frac{Δ H^{0}}{RT}$ (16)where K is the adsorption equilibrium constant, C_s (mg·g⁻¹) is the amount of CR adsorbed per mass of SSP, R (8.314 J·mol⁻¹·K⁻¹) is the universal gas constant, and T (K) is the temperature.

Results and discussion

Characteristics of adsorbents

The particle size distribution of TSSP and RSSP is shown in Figure 2. Although they were smashed under the same condition, the average particle size of TSSP (105.0 μm) was much smaller than that of RSSP (145.4 μm). The easier pulverization of TSSP could be ascribed to its less organic matter content.

Figure 2.

Particle size distribution of TSSP and RSSP.

The SEM images of the two adsorbents are shown in Figure 3. As can be seen, the surface of TSSP was much rougher than that of RSSP, which could be due to the calcium carbonate crystals that clustered on the chitin after deproteinization (Tong and Yao, 1997). In Figure 3(a) and (b), it can be observed that the particle size of TSSP was smaller, which was consistent to the result of laser particle size data.

Figure 3.

SEM images of TSSP and RSSP.

N₂ adsorption-desorption isotherms and differential pore size distribution of TSSP and RSSP are shown in Figure 4. According to the IUPAC classification, both of them exhibited type IV isotherm, which represented typical characteristics of mesopores (Qin et al., 2016). The adsorption–desorption curves showed a hysteresis loop with a closure at a relative pressure (P/P₀) of 0.5, which reflected capillary condensation phenomena (Gorski and Stettler, 1975). TSSP exhibited a richer pore structure due to its much larger adsorption capacity of N₂. What’s more, the differential pore size distribution (the BJH method) of the mesopores is shown in Figure 4(c) and (d), and the pore characteristics of the two adsorbents are listed in Table 1. As compared to RSSP, TSSP exhibited a smaller average pore size (5.61 nm), higher BET surface area (66.35 m²/g) and larger pore volume (0.0968 cm³/g), which was due to the removal of protein and other organic matter in the shrimp shell. The BET results were in accordance with the image of SEM.

Figure 4.

(a, b) Nitrogen adsorption and desorption curves and (c, d) differential pore size distribution.

Table 1.

BET surface areas and pore volumes of TSSP and RSSP.

Adsorbents	BET surface area (m²/g)	Total pore volume (cm³/g)	Micropore volume (cm³/g)	Mesopore volume (cm³/g)	Average pore size (nm)
TSSP	66.35	0.0968	0.0029	0.0891	5.61
RSSP	3.618	0.0143	0	0.0113	12.8

BET: Brunauer-Emmett-Teller.

XRD is the basic technique to determine the bulk structure and composition of materials with crystalline structure. Figure 5 shows the XRD patterns of TSSP and RSSP. The picture indicated that the shrimp shell was mainly composed of highly crystalline calcite and α-chitin. The patterns can be fitted well by the standard diffraction diagram of calcite (JCPDS Card No. 24-0027) and the amorphous peaks at low angles (2θ of about 20°) corresponded to the α-chitin structure (Heredia et al., 2007). However, a strong diffused peak (2θ = 31–34°) was only found in the XRD pattern of TSSP, which could be indexed to Na₂CO₃·H₂O (JCPDS Card No. 08-0448). Sodium carbonate could be generated by the addition of NaOH and H₂O₂ during the preparation of TSSP.

Figure 5.

X-ray diffraction patterns of the adsorbents.

TGA is a technique to study the thermal properties and mass fraction of different materials. From Figure 6, the TGA curves can be divided into three stages. Stage I is the weight losing process of water, stage II belongs to the thermal decomposition of organic matters, and stage III is caused by decomposition of calcite. It also indicates that there was more weight loss for the adsorbents after adsorption due to the decomposition of CR dye. Besides, TSSP had shown less weight loss in stage II owing to its deproteinization. As calculated from the weight loss in the last stage, the calcite accounted for about 20% of the total weight of TSSP and RSSP.

Figure 6.

The TGA curves of adsorbents before and after adsorption (TSSP-1 and RSSP-1: before; TSSP-2 and RSSP-2: after).

The adsorption capacity of sorbents not only depends on porosity, but also relies on the chemical reactivity of the surface functional groups. RSSP is mainly composed of calcite, chitin and protein, and protein is not contained in TSSP because of the deproteinization. The FTIR spectra of TSSP and RSSP are shown in Figure 7. The broad and strong vibration around 3437.95 cm⁻¹ is indicative of the presence of amino and hydroxyl groups in α-chitin on the adsorbent’s surface (Kousha et al., 2015). The peak at 2960.95 cm⁻¹ and 2926.67 cm⁻¹ belongs to methyl C–H stretching vibration in the group of COCH₃ (Majtan et al., 2007). The peaks at 1655.08 cm⁻¹ and 1319.22 cm⁻¹ are assigned to stretching vibration of amide I and amide III, respectively. The peaks at 1070.86 cm⁻¹ and 1030.70 cm⁻¹ are related to C–O stretching vibration (Marchessault et al., 1960). The small peak at 1541.49 cm⁻¹ in the curve of RSSP is ascribed to the presence of protein (Kousha et al., 2015; Majtan et al., 2007). The sharp peaks observed at 1420.05 cm⁻¹ and 872.28 cm⁻¹ are assigned to calcite (Loftus et al., 2015).

Figure 7.

FTIR spectra of TSSP and RSSP.

Effect of initial dye concentration

The effect of initial CR concentration on the adsorption capacity and dye removal percentage was investigated under equilibrium conditions with 0.1 g adsorbents and 30 ml dye solution. The dye-binding capacity of an adsorbent is related to the initial dye concentration, which depends on the available binding sites on an adsorbent surface as well (Daneshvar et al., 2014). Therefore, as shown in Figure 8, the percentage of dye removal decreases with the initial concentration rising because of the saturation of adsorption sites on the adsorbent surface (Salleh et al., 2011). Besides, the higher adsorption capacity was obtained at the higher initial dye concentration. The incremental results were ascribed to the increased driving force of the concentration gradient (Salleh et al., 2011).

Figure 8.

Effect of initial dye concentration on removal of CR on TSSP and RSSP at different temperatures.

The adsorption capacity of TSSP and RSSP was compared in this experiment. As can be seen, TSSP had higher adsorption capacity and removal percentage under the same condition, and the difference became more obvious in high concentrations. For example, the equilibrium adsorption capacity was 225.9 mg/g for TSSP and 181.1 mg/g for RSSP at 1500 mg/L of CR at 303K. The better adsorption capacity of TSSP was due to its higher BET surface area and pore volume. However, the difference in adsorption capacity was not as much as that in BET specific surface area, because the big CR molecules could not enter the small pores on the adsorbent surface.

Effect of solution pH on dye adsorption

The pH of the dye aqueous solution is usually recognized as one of the important factors which may influence adsorption process and adsorption capacity (Hameed and Ahmad, 2009). In this section, the pH of the dye solution of CR was adjusted from 5.5 to 12 because the color of CR solution would turn blue when the pH was below 5. The effect of pH on the adsorption of CR at equilibrium by TSSP and RSSP is shown in Figure 9.

Figure 9.

Effect of pH on CR adsorption onto the adsorbents of TSSP and RSSP (initial concentration C₀ = 500 mg/L, temperature: 313K).

The adsorption capacity of both adsorbents was found to decrease with increasing pH. And the adsorption was more strongly pH-dependent when the pH was over 10.6. SSP contains high percentage of chitin or chitosan, and its activated functional groups such as multiple imine and amine groups are easily protonated under acidic conditions (Kousha et al., 2015). CR dye is an anionic dye, which exists in aqueous solution in the form of negatively charged ions. Consequently, the relatively high adsorption capacity observed at low pH values was due to the strong electrostatic attraction between positively charged surface of the adsorbents and negatively charged CR molecules (Namasivayam and Kavitha, 2002). The adsorption capacity decreased with pH values increasing, which was ascribed to the weakened electrostatic attraction as a result of the reduced positive charge on adsorbents. At high pH values, the adsorption decreased significantly mainly because the increased OH⁻ ions in solution competed with the anionic dye for adsorption site (Ge et al., 2017). The adsorption on RSSP was more affected by the pH values due to the existence of protein which would hydrolyze under alkaline conditions.

Adsorption isotherm

Analysis of adsorption isotherm is important to describe how adsorbate molecules interact with the adsorbent surface and establish the appropriate correlation with the equilibrium curve (Crini et al., 2008; Daneshvar et al., 2014).

Figure 10(a) and (b) shows the linear fitting of the experimental data to Freundlich adsorption isotherm model. The values of K_F, 1/n and correlation coefficient (R²) are listed in Table 2. As can be seen, the low values of R² indicate that this model is not able to well explain the relationship between the amount of adsorbed CR dye and its equilibrium concentration in the solution (Daneshvar et al., 2014).

Figure 10.

Freundlich (a, b) and Langmuir (c, d) adsorption isotherms for CR adsorption on TSSP and RSSP at three different temperatures.

Table 2.

Isotherm parameters for CR adsorption onto TSSP and RSSP at different temperatures.

Adsorbents	Temp. (K)	Freundlich			Langmuir
Adsorbents	Temp. (K)	1/n	K_F	R²	q_m (mg/g)	K_L (L·mg⁻¹)	R_L	R²
TSSP	303	0.2122	62.47	0.8818	232.0	0.0463	0.014–0.067	0.9998
	313	0.2568	55.26	0.9085	264.6	0.0379	0.017–0.081	0.9999
	323	0.2934	47.19	0.9495	288.2	0.0267	0.024–0.110	0.9986
RSSP	303	0.1523	66.87	0.9016	182.1	0.0407	0.016–0.076	0.9974
	313	0.1995	66.43	0.9073	234.7	0.0418	0.016–0.074	0.9964
	323	0.2534	52.70	0.9186	256.4	0.0313	0.021–0.096	0.9988

Figure 10(c) and (d) gives the results on Langmuir isotherm fittings. The values of q_m, K_L, R_L and correlation coefficient (R²) are all shown in Table 2. The maximum adsorption amount increased with increasing adsorption temperature for the two adsorbents. And TSSP owns higher values of q_m than RSSP at the same temperature. As shown in Table 2, R_L values are in the range of 0.014–0.11 for the adsorbent of TSSP, and 0.016–0.096 for the RSSP, indicating “favorable adsorption” and strong binding between CR and the two kinds of adsorbents. It is indicated from the values of R² (Table 2) that Langmuir isotherm can fit well for the experimental adsorption data, which confirms the monolayer coverage of CR onto SSP particles.

Adsorption kinetics

The adsorption kinetics was measured to investigate the mechanism of adsorption and its potential rate-controlling steps. The relationship between adsorption amount and time is shown in Figure 11. Little difference was observed in the equilibrium adsorption capacities between TSSP and RSSP at three different temperatures, which might be because the active sites of the adsorbent surface were not fully occupied. However, the adsorption rate all increased with the increasing of temperature due to the increasing mobility of dye molecules.

Figure 11.

Adsorption of CR on the adsorbents of TSSP and RSSP as a function of time at different temperatures (initial concentration C₀ = 500 mg/L).

The adsorption kinetics data for TSSP and RSSP were analyzed by the pseudo-first-order and pseudo-second-order kinetic models, the results of which are presented in Table 3. The linear fitting for the pseudo-first-order model exhibited low correlation values (R²) in all cases, showing a poor fit for the model. On the contrary, the pseudo-second-order model provided an ideal fitting for TSSP and RSSP at different temperatures, as proved by the extremely high R² values (R² > 0.99) and the accordance of the calculated equilibrium adsorption capacities (q_e2) and the experimental ones (q_e(exp)). The fitting result showed that the chemisorption was involved in the adsorption process through sharing or exchange of electrons between adsorbate and adsorbent (Ho, 2006). Besides, the values of k₂ and h increased with the rise of temperature, and they were larger for TSSP as compared to that for RSSP.

Table 3.

Pseudo-first-order and second-order rate constants of the kinetics of CR sorption at different temperatures.

Adsorbents	Temp. (K)	q_e(exp)(mg/g)	Pseudo-first-order model			Pseudo-second-order model
Adsorbents	Temp. (K)	q_e(exp)(mg/g)	k₁(min⁻¹)	q_e1(mg/g)	R²	k₂(g·mg⁻¹·min⁻¹)	q_e2(mg/g)	R²	h(mg·g⁻¹·min⁻¹)
TSSP	303	140.7	0.00517	71.44	0.9866	0.000277	143.5	0.9993	5.707
	313	141.9	0.00672	47.87	0.9570	0.000600	143.7	1.0000	12.39
	323	142.4	0.00543	34.99	0.8787	0.000814	143.5	1.0000	16.76
RSSP	303	133.0	0.00240	95.16	0.9817	0.000122	125.9	0.9928	1.935
	313	141.7	0.00429	89.48	0.9928	0.000168	144.7	0.9977	3.515
	323	140.0	0.00632	81.74	0.9854	0.000247	144.1	0.9994	5.138

q_e(exp): experimental equilibrium adsorption capacity; q_e1 and q_e2: calculated equilibrium adsorption capacity.

Adsorption mechanism

The dye adsorption rate is usually controlled by either the liquid phase mass transport rate or the intra-particle mass transport rate (Vimonses et al., 2009). In this study, the intra-particle diffusion model was used to identify the diffusion mechanism of the CR adsorption onto TSSP and RSSP, and the result is shown in Figure 12. It indicated that the whole time range could not be fitted by a straight line, but it could be separated into three linear regions, which implied that the CR adsorption might be affected by more than one process. Besides, none of the lines passed through the origin, indicating that the intra-particle diffusion was involved in the adsorption process but not the only rate-controlling step. Some other mechanisms such as complexation or ion-exchange might also control the rate of adsorption (Poots et al., 1978; Vimonses et al., 2009). Lorencgrabowska and Gryglewicz (2007) explained that the adsorption process included not only the intra-particle diffusion but also the boundary layer diffusion. The first part of the multi-linearity stages can be attributed to the boundary layer diffusion of solute molecules. The intra-particle diffusion is likely to occur in the last two stages as the rate-controlling process.

Figure 12.

Intra-particle diffusion kinetic for adsorption of CR on TSSP and RSSP at different temperatures (initial concentration C₀ = 500 mg/g).

Effect of temperature and adsorption thermodynamic

Temperature, an indicator of the adsorption nature, was investigated in this experiment. The curves in Figures 8 and 11 also exhibit that the adsorption capacity increased with increasing temperature, which suggested that the adsorption is an endothermic process in nature (Nandi et al., 2009; Salleh et al., 2011). For example, at the initial dye concentration of 1500 mg/L, the equilibrium adsorption capacity increased from 225.9 mg/g and 181.1 mg/g to 274.9 mg/g and 248.5 mg/g for TSSP and RSSP respectively when the temperature increased from 303 K to 323 K. This may be a result of the increase in the mobility of dye, the decrease in the retarding forces acting on the diffusion adsorbates, the improvement in the activity of functional groups on the surface of adsorbents and the swelling effect on the internal structure of the adsorbents with increasing temperature (Dawood and Sen, 2012; Hameed and Ahmad, 2009).

The values of ΔH⁰ and ΔS⁰ were determined by plotting ln K against 1/T based on equation (16). And the value of ΔG⁰ at a certain temperature was calculated in terms of equation (15). The determined values of ΔG⁰, ΔH⁰ and ΔS⁰ are given in Table 4. Spontaneous adsorption of CR by TSSP and RSSP in the range of 303–313 K can be proved by the negative ΔG⁰. The positive value of ΔH⁰ indicates that the adsorption process is endothermic, which is consistent with the result of increased adsorption amount at high temperature. The driving force for CR adsorption onto SSP is controlled by an entropy effect rather enthalpy change due to the positive values of ΔH⁰ and ΔS⁰ (Huo and Yan, 2012; Hou et al., 2012).

Table 4.

Thermodynamic parameters for the adsorption of CR onto TSSP and RSSP.

Adsorbents	Temp. (K)	ΔG⁰ (kJ·mol⁻¹)	ΔH⁰ (kJ·mol⁻¹)	ΔS⁰ (kJ·mol⁻¹·K⁻¹)
TSSP	303	−3.54	13.1	0.055
	313	−4.33
	323	−4.63
RSSP	303	−0.40	52.6	0.18
	313	−3.55
	323	−3.86

Comparison with other adsorbents

The maximum adsorption capacity of TSSP and RSSP obtained in this study is listed in Table 5, so as to make comparisons with many other common adsorbents. It can be seen that TSSP and RSSP have relatively high adsorption capacity among these adsorbents. The adsorption capacity of TSSP and RSSP is close to that of chitosan, and much higher that of chitin. The good adsorption performance of TSSP and RSSP may be ascribed to their abundant pore structures (Tong and Yao, 1997). More importantly, the adsorption operation with SSP can be greatly simplified because of its simple pre-treatment process and fast settling velocity owing to the presence of calcium carbonate. Furthermore, the dye-loaded TSSP has the potential to be used as pigments and fillers in other fields such as polymer because of the principle of “like dissolve like”. Most importantly, the utilization of SSP has a huge advantage of low cost due to the abundant resource of shrimp shell waste. Therefore, SSP can be considered as a promising candidate for the removal of CR from industrial wastewater.

Table 5.

Comparison of maximum adsorption capacity of various absorbents for CR removal.

Adsorbents	Maximum adsorptioncapacity (mg/g)	References
Red mud	4.05	(Namasivayam and Arasi, 1997)
Raw pine cone	19.18	(Dawood and Sen, 2012)
Clay materials	3.77–35.84	(Vimonses et al., 2009)
Activated carbon	300.00	(Purkait et al., 2007)
Fe₃O₄ particles	97.62	(Yu et al., 2014)
Ag-doped hydroxyapatite	458.08	(Srilakshmi and Saraf, 2016)
Hydroxyapatite/chitosan composite	769	(Hou et al., 2012)
Chitin	139	(Zuniga-Zamora et al., 2016)
Chitosan	320	(Zuniga-Zamora et al., 2016)
TSSP	288.2	This study
RSSP	256.4	This study

Conclusion

In this study, two novel adsorbents—raw shrimp shell powder (RSSP) and treated shrimp shell powder (TSSP)—were prepared from shrimp shell waste. The equilibrium adsorption capacity increases with the increased initial dye concentration and decreases with the increased pH, which is related to the driving force of the concentration gradient and electrostatic interaction, respectively. The Langmuir adsorption isotherm was found to perfectly evaluate the adsorption on TSSP and RSSP. And the maximum adsorption capacities (q_m) were 288.2 mg/g for TSSP and 256.4 mg/g for RSSP at 323K, which were greatly larger than the value of chitin. Kinetic studies showed that adsorption on the two adsorbents followed pseudo-second-order model with multi-step diffusion process. The values of thermodynamic parameters (ΔG⁰ < 0 and ΔH⁰ > 0) indicate the spontaneous and endothermic natural of the adsorption. The equilibrium adsorption capacity and adsorption rate increase with the rise of temperature. Besides, TSSP exhibits a higher adsorption capacity than RSSP, which can be attributed to its higher BET surface area and pore volume.

The SSP has been demonstrated to be a cheap and efficient adsorbent for the removal of CR dye in this study. Development of the SSP biosorbent contributes not only to relieving the environmental stress caused by dye effluent and shrimp shell waste, but also to decreasing the cost of industrial wastewater treatment and simplifying treatment process.

Footnotes

Acknowledgements

We would like to thank the State Key Laboratory of Chemical Engineering,Zhejiang University for the assistance in testing and analyzing samples.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research,authorship,and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research,authorship,and/or publication of this article: This study was supported by the National Natural Science Foundation of China (grant no. 529101-N11403).

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