Abstract
To enhance the removal of mercury (Hg(II)) and cadmium (Cd(II)) from aqueous solutions, rice bran (RB) was reacted with epichlorohydrin and then modified with ethylenediamine and sodium chloroacetate to bear iminodiacetate functional groups. The modified rice bran (MRB) was characterized by Fourier transform-infrared spectroscopy (FT-IR), thermogravimetric analysis (TG), energy dispersive spectroscopy (EDS), back titration, and X-ray photoelectron spectroscopy (XPS). The adsorption properties of MRB for Hg(II)/Cd(II) ions were also evaluated in batch experiments. The sorption kinetic experimental data were best described by the pseudo-second-order model. The maximum adsorption capacity (163.9 mg/g for Hg(II) at pH 5.0 and 106.4 mg/g for Cd(II) at pH 6.0) was observed at 298 K, and the isotherm adsorption equilibrium of MRB was followed by Langmuir isotherm equation. The major adsorption mechanisms should be predominantly controlled by the formation of complexes between the functional groups of MRB and Hg(II)/Cd(II) ions as well as ion-exchange. The regeneration experiments showed that the MRB could be successfully reused for six cycles when 0.1 M HCl eluent was used.
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Enhanced Adsorption of Mercury(II) and Cadmium(II) from Aqueous Solution onto Rice Bran Modified with Chelating Ligands
Yan Zhou,a Jianping Zhang,b,c,* Xuegang Luo,b and Yajun Luo a
To enhance the removal of mercury (Hg(II)) and cadmium (Cd(II)) from aqueous solutions, rice bran (RB) was reacted with epichlorohydrin and then modified with ethylenediamine and sodium chloroacetate to bear iminodiacetate functional groups. The modified rice bran (MRB) was characterized by Fourier transform-infrared spectroscopy (FT-IR), thermogravimetric analysis (TG), energy dispersive spectroscopy (EDS), back titration, and X-ray photoelectron spectroscopy (XPS). The adsorption properties of MRB for Hg(II)/Cd(II) ions were also evaluated in batch experiments. The sorption kinetic experimental data were best described by the pseudo-second-order model. The maximum adsorption capacity (163.9 mg/g for Hg(II) at pH 5.0 and 106.4 mg/g for Cd(II) at pH 6.0) was observed at 298 K, and the isotherm adsorption equilibrium of MRB was followed by Langmuir isotherm equation. The major adsorption mechanisms should be predominantly controlled by the formation of complexes between the functional groups of MRB and Hg(II)/Cd(II) ions as well as ion-exchange. The regeneration experiments showed that the MRB could be successfully reused for six cycles when 0.1 M HCl eluent was used.
Keywords: Adsorption; Modified rice bran; Mechanisms; Mercury; Cadmium Contact information: a: Mianyang Normal University, Mianyang, 621000, Sichuan, China; b: Engineering Research Center of Biomass Materials, Ministry of Education, Southwest University of Science and Technology, Mianyang, 621010, Sichuan, China; c: The Eighth Research Institute, China Academy of Engineering Physics, Mianyang, 621900, Sichuan, China;
* Corresponding author: zjpzy@126.com
INTRODUCTION
Heavy metals, such as mercury (Hg) and cadmium (Cd), are widely used in industrial production processes, and when heavy metals are converted into cationic form, there is a high probability of their discharge into the environment. Due to their toxicity and mobility in surface and ground water, these heavy metal ions are non-biodegradable and tend to accumulate in biological species, causing great harm to human health and ecosystems through the food chain (Li et al. 2015). Therefore, it is necessary for public health and ecological safety to remove mercury and cadmium from wastewater. Numerous techniques have been applied for the removal of hazardous heavy metal ions from aqueous solutions, such as chemical precipitation (Blue et al. 2010), ion exchange (Noh and Komarneni 2011), membrane separation (Cui et al. 2014), reduction (Ketir et al. 2008), electrochemical treatment (Pikna et al. 2015), and adsorption. Adsorption is one of the most studied methods due to its high efficiency, easy handling, low cost of the materials, and reversibility. Various materials have been developed for the adsorption of Hg(II) and Cd(II), including activated carbon (Gonzalez and Pliego-Cuervo 2014), clay minerals (Li et al. 2012; Tran et al. 2015), zeolites (Fardmousavi and Faghihian 2014; Nguyen et al. 2015), and biomaterials (Kushwaha et al. 2015; Yu et al. 2015). Moreover, bio-sorbents like chitosan (Kandile et al. 2015; Karthik and Meenakshi 2015), algae (Carro et al. 2013), bacteria (Huang and Liu 2013), and agricultural by-products (Liu et al. 2010; Anagnostopoulos et al. 2012; Huang and Lin 2015) have excellent adsorption capacity for these toxic ions.
Rice bran (RB) is an agricultural by-product resulting from rice processing and represents approximately 10% of the entire rice grain weight. A large amount of this by-product is used as animal feed or discarded as waste. RB contains vitamins, carbohydrates, nitrogen, and phosphorus compounds, which have carboxyl groups, hydroxyl groups, and imino groups (Montanher et al. 2005). Due to its granular structure, chemical stability, insolubility in water, and local availability, RB is a potential absorbent for removing toxic metals from wastewater. RB can remove Cd(II), Cu(II), Pb(II), Zn(II), Cr(III), Cr(VI), and Ni(II) from water (Wang et al. 2008; Chen et al. 2012). However, the adsorption capability of RB for these heavy metal ions is relatively poor due to its low content of active functional groups. Improving the adsorption properties of RB via carbonization (Zhang et al. 2014) or chemical modification (Fatima et al. 2013) is thus important.
It has been reported that the presence of –COOH or –NH2 groups can greatly improve the adsorption capacity of a sorbent on Hg(II)/ Cd(II) (Cui et al. 2015). The purpose of this study was to enhance the Hg(II) and Cd(II) adsorption capacities of RB by chemical modification using bearing iminodiacetate function on RB as chelating ligands. The characteristics of the resulting adsorbent were analyzed. The adsorption properties for Hg(II) and Cd(II) on the modified rice bran (MRB) from aqueous solution were evaluated at different pHs, contact times, temperatures, heavy metal ion concentrations, and ionic strength. The regeneration of the MRB was also investigated in its application.
EXPERIMENTAL
Materials
Rice bran was obtained from a local farmer’s market (Mianyang, China), thoroughly washed with distilled water, dried, crushed, and screened by a 100-mesh sieve. 2-(5-Bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP) (98%) and sodium chloroacetate were procured from Aladdin Chemical Ltd. (Shanghai, China). Mercury (II) chloride, cadmium nitrate, epichlorohydrin, ethylenediamine, sodium hydroxide, and other chemicals were all analytical grade and purchased from Kelong Chemicals Company (Chengdu, China).
Preparation of MRB
Preparation of the MRB adsorption material included the epoxidation of RB, ammoniation of the epoxidized RB, and carboxymethylation of aminated RB. For the epoxidation reaction, 5 g of RB was dispersed into 100 mL of sodium hydroxide (4%) with vigorous stirring. The reaction vessel was immersed into a water bath at 70 °C, and 30 mL of epichlorohydrin was added. After 3 h at 70 °C, the mixture was cooled to 25 °C and adjusted to pH 6 with HCl. The product was precipitated with acetone, filtered thoroughly, washed with ethanol (95%), and dried overnight at 90 °C. For the ammoniation reaction, 5 g of epoxidized RB, 25 mL of ethylenediamine, and 0.1 g of sodium carbonate were added to a round-bottomed flask and refluxed with magnetic stirring in a water bath at 60 °C for 2 h. The product was thoroughly washed with distilled water and dried. Next, 4 g of this product was mixed with 100 mL of sodium chloroacetate (25%) and 50 mL of sodium hydroxide (30%) at 60 °C for 2 h and then heated continuously in a boiling water bath for 4 h. The mixture was filtered and washed with distilled water until the filtrate was colorless and the pH of the filtrate was near neutral range. The modified rice bran (MRB) sorbent was dried in a convection oven at 105 °C for 6 h. The preparation of MRB is illustrated in Fig. 1.
Fig. 1. Synthetic scheme of MRB and the adsorption mechanism of heavy metal ion on MRB
Characterization
The concentration of carboxyl groups in MRB was determined by back titration method (Huang et al. 2012). One gram of MRB was mixed with 100 mL of deionized water. To transform the -COONa groups of the MRB into -COOH groups (Liu et al. 2011), the pH of the suspension was adjusted to 2.0 with hydrochloric acid. After stirring for 2 h, the suspension was separated by filtration, and the obtained solid was dried at 353 K until constant weight was attained; 0.1 g of dried solid was dispersed in 100 mL of aqueous solution of NaHCO3 (0.01 mol/L) and stirred for 2 h under argon atmosphere. Then the mixture was filtered, the obtained filtrate was divided into three aliquots (25.0 mL) and titrated with aqueous HCl (0.01 mol/L) using a pH meter (PHS-3C, Jiangfen Instruments, Jiangsu, China), and the results were given by average. The described procedure was run on the MB in the same manner. The concentration of carboxylic groups in the sample was calculated by the following equation,
(1)
where CNaHCO3 and CHCl (mol/L) are the concentration of NaHCO3 solution and HCl solution, respectively; VNaHCO3 and VHCl (L) are the volume of NaHCO3 solution and HCl solution, respectively; and W (g) is the weight of the RB or MRB.
Specific surface areas of samples were determined with N2 adsorption at a liquid nitrogen temperature (77 K) on a surface analyzer (SSA-4200, Beijing Builder Electronic Technology Co.). Fourier transform-infrared (FT-IR) spectra of RB and MRB in KBr discs were recorded using a Nicolet 5700 FT-IR spectrometer (ThermoScientific, Waltham, USA). Thermogravimetric (TG) analyses were performed with a TGA Q500 instrument system (TA Instruments, New Castle, USA) at prescribed heating rates of 10 °C/min up to 800 °C under a steady flow of N2. The elemental contents of MRB before and after adsorption of Hg(II) and Cd(II) were determined in randomly selected areas on the Au-coated solid surfaces. These areas were analyzed by energy dispersive spectroscopy (EDS) in conjunction with scanning electron microscopy (X-MaxN20; Oxford Instruments, UK). X-ray photoelectron spectroscopy (XPS) was performed on a spectrometer (XSAM-800, KRATOS Co, Japan) with an Al K X-ray source (1486.6 eV).
Sorption Experiments
Batch experiments were carried out by placing 0.1 g of MRB in 100 mL of aqueous solutions containing Hg(II) or Cd(II) ions at a predetermined concentration, desired pH, and ionic strength. The suspensions were shaken at 120 rpm in an electrically thermostatic reciprocating shaker (SHZ-B, Hede Instruments, Shanghai, China) for a predetermined period of time at the appropriate temperature. The solid and liquid phases were separated by centrifugation. The final concentrations of Hg(II) or Cd(II) in the supernatant were measured spectrophotometrically using 5-Br-PADAP chromogenic reagent at the wavelengths of 563 nm and 570 nm, respectively (Zhou et al. 2013, 2014). The initial pH of the metal ion solutions was adjusted by micro-additions of HCl or NaOH, using a pH meter; the ionic strength of the solution was adjusted with NaCl or KNO3. All experiments were performed in triplicate, and the mean values of the results were used for data analysis. The standard deviations were less than 5%. Adsorption was quantified by the adsorption capacity q (mg/g),
(2)
where C0 (mg/L) and Cf (mg/L) were the initial and final concentration of the metal ion in the solution, respectively. V was the volume (L) of the solution, and w was the weight (g) of the adsorbent.
Regeneration of MRB
The reusability of MRB was studied by static experiments. After adsorption of metal ions by MRB, 0.6 g of the loaded MRB was re-suspended in 250 mL of 0.1 mol/L HCl and stirred magnetically for 2 h at room temperature. Prior to the next adsorption-desorption cycle, the adsorbent was treated with 0.1 mol/L NaOH and thoroughly washed with distilled water until the filtrate was neutralized. The neutralized sorbent was dried at 105 °C for 6 h. The regenerated MRB was then used in the next adsorption process. The adsorption-desorption process was repeated 6 times.
RESULTS AND DISCUSSION
Characterization
The efficiency of introducing iminodiacetate function into the RB was estimated by comparing the carboxyl groups before and after chemical modification, and the results are shown in Table 1. Compared to the RB, the carboxyl concentration of the MRB increased from 0.032 mmol/g to 0.896 mmol/g in the test sample as the result of surface functionalization. There were no other carboxyls except that sodium chloroacetate was brought in during the whole synthesis process, and then it indicated that the iminodiacetate function was well introduced into the RB. The specific surface areas (BET surface) of RB and MRB are given in Table 1. The increment in carboxyl content in the MRB produces a slight increase (from 5.18 m2/g to 5.43 m2/g) in the specific surface area which improves the loading capacity of the MRB.
Table 1. Carboxyl Group Contents and Surface Area of RB and MRB
a Obtained by back titration. b Obtained by surface area.
The FT-IR spectra for RB and MRB are shown in Fig. 2. The appearance of a new strong absorption peak at 1608 cm−1 on MRB (which is in the range of stretching vibrations from –COO– groups), the moving of –NH/–OH stretching vibration from 3380 cm−1 to 3425 cm−1, and the decreasing in the absorption peak strength of the broad overlapping peaks confirmed the carboxymethylation reaction on RB (Zhang et al. 2011). Additionally, this result confirmed the formation of iminodiacetic acid groups.
Fig. 2. FT-IR spectra of RB and MRB
The thermogravimetric decomposition curves for virgin RB and MRB were evaluated, and results are shown in Fig. 3.
Fig. 3. TG and DTG curves of RB and MRB
For RB and MRB, the peaks from 50 to 150 °C were attributed to moisture elimination, and the relatively larger weight loss of MRB than RB (6.9% and 2.7%, respectively) implied that the introduction of iminodiacetic acid groups made MRB more hydrophilic. The mass loss from 220 through 450 °C corresponded to the loss of organic materials (Kyzas et al. 2014; Niticharoenwong et al. 2013). At 800 °C, the residual mass percentage was 20.6% for RB and 32.4% for MRB, which may be due to sodium oxide. The disappearance of the shoulder between 250 °C and 340 °C for the RB after modification and the change of maximum temperature of weight loss from 371 °C for RB to 307 °C for MRB suggested that the reactions shown in Fig. 1 were carried out as expected. Although the thermal stability of MRB was relatively low compared with RB, it was judged to be adequate for most needs in wastewater treatment.
The EDS spectra of MRB, Hg-sorbed MRB, and Cd-sorbed MRB are depicted in Fig. 4. The MRB spectrum indicated the presence of C, O, and Na but did not show the characteristic signals of Cd(II)/Hg(II) ions. Clear Cd(II) ions signals were observed at 2.77, 2.96, 3.13, 3.32, and 3.95 KeV on the surface of Cd-sorbed MRB, which was quite similar to Cd(II) ions observed on another Cd-loaded sorbent (Wang et al. 2010). The Hg(II) ions signals at 1.71, 2.2, 2.49, 8.72, 8.9, 11.82, and 11.92 KeV were clearly present on the surface of Hg-sorbed MRB. None of the samples show the presence of N, as the dispersive energy of N elements is 0.40 (Areco et al. 2013). These findings confirmed the adsorption of Hg(II) and Cd(II) ions on the surface of MRB. The total disappearance of the Na peak in Cd-sorbed MRB and Hg-sorbed MRB suggested that surface chelation and ion exchange were the main processes involved in adsorption.
Fig. 4. EDS analysis of (a) MRB, (b) Cd-sorbed MRB, and (c) Hg-sorbed MRB
Figure 5 shows the XPS spectra for the RB and MRB surfaces before and after adsorption. The elemental surface composition of RB and MRB were calculated based on the peak area of each element. The concentrations of C, O, and N for RB were 47.9%, 48.1%, and 1.2%, respectively; the relative concentration of C, O, and N for MRB was 38.1%, 52.4%, and 3.2%, respectively. Compared with the XPS spectra of original RB and MRB, a new Na peak was observed. In MRB, the N and O contents increased while the content of C decreased, which indicated that iminodiacetic acid groups were successfully introduced to the RB surface. The presence of Hg 4f and Cd 3d in Hg-sorbed MRB and Cd-sorbed MRB spectra, respectively, confirmed the successful adsorption of Hg(II) and Cd(II) on MRB. The O 1s peak shifted to higher binding energies (531.76 eV for MRB, 531.80 eV for Cd-sorbed MRB, and 531.83 eV for Hg-sorbed MRB). The N 1s peak showed a similar tendency before and after adsorption, suggesting the strong combination between Hg(II)/Cd(II) and the surface functional groups of MRB. Nevertheless, the disappearance of Na 1s in Hg-sorbed MRB and Cd-sorbed MRB illustrated that both chelation and ion exchange occurred between heavy metal ions and MRB. These results are consistent with the EDS data, and they also are complementary to the reported results of Hg(II)/Cd(II) bound to nitrogen containing groups or carboxyl groups by complexation or coordination (Guo et al. 2014).
Fig. 5. XPS spectra of (a) RB, (b) MRB, (c) Cd(II)-loaded MRB, and (d) Hg(II)-loaded MRB
Adsorption of Hg(II) and Cd(II) on MRB
Effect of pH and adsorption mechanism
The effect of pH on the adsorption of Hg(II) and Cd(II) onto MRB was investigated from pH 2 to 6 at an initial metal ion concentration of 300 mg/L and 25 °C (Fig. 6). The initial solution was set to a pH below 6 to avoid precipitation. The adsorption capacities increased with the pH in the experimental conditions used, and the maximum adsorption capacity was achieved at pH values around 6 for Cd(II) and pH of 5 for Hg(II). Because the main functional groups of MRB are carboxyl and nitrogen-containing groups, there is no doubt that the protonation and deprotonation of these groups on the surface of MRB (which are attributed to the complexation and electrostatic interaction for adsorption mechanism) were affected by pH changes. With decreasing pH, more nitrogen-containing groups are protonated, and the surface matrix of MRB becomes more positively charged (–COO– protonated into –COOH which restrain the formation of coordination compounds). Thus, the repulsive electrostatic interaction between surface sites of MRB and metal ions increases. Furthermore, the greater number of hydrogen ions compete with the metal ions for the sorption sites in the sorbent, resulting in the reduced adsorption of Hg(II) and Cd(II). At higher pH, the number of negatively charged–COO– groups on the adsorbent matrix probably increases; the greater number of negatively charged groups on MRB favors electrostatic attraction between cationic species. Figure 1 illustrates the possible mechanism for Hg(II)/Cd(II) adsorption on MRB. The lone pair electrons of oxygen and nitrogen atoms of functional groups (N- and O-containing) on MRB could be supplied to the empty atomic orbital of Hg(II)/Cd(II), which forms O, N–Hg(II)/Cd(II) complexes in the active sites of the biosorbent. The lower complexing capacity in a strongly acidic medium may be attributed to the partial protonation of the active groups on the surface of MRB. These mechanisms were partially confirmed by the EDS and XPS analyses of MRB and Hg(II)/Cd(II)-loaded MRB. Therefore, in further adsorption experiments, the optimum pH for Hg(II) and Cd(II) were selected as 5.0 and 6.0, respectively.
Fig. 6. Effect of pH on adsorption of Hg(II)/Cd(II)
Effect of contact time and adsorption kinetics
The effects of contact time on the extent of Hg(II) and Cd(II) adsorption on MRB were investigated at an initial metal ion concentration of 300 mg/L at 25 °C (pH 6.0 for Cd(II) and pH 5.0 for Hg(II)). As shown in Fig. 7, the adsorption capacities of the two metal ions increased rapidly with the contact time during the first 30 min. The variation reflects the vacant surface sites available for adsorption and the relatively high solute concentration during the initial stages. Thereafter, the adsorption rates declined and then slowed until the adsorbed metal ions reached the equilibrium value because of the decrease in metal ion concentration and the repulsive forces between metal ions adsorbed on MRB and the solution phases. To ensure that adsorption equilibrium was achieved, the contact time for Hg(II) and Cd(II) adsorption were maintained at 5 h for the following experiments.
Fig. 7. Effect of contact time on adsorption of Hg(II)/Cd(II) on MRB
Adsorption kinetics are important in designing a sorption system. Thus, the mechanisms of Hg(II) and Cd(II) biosorption on MRB, including the chemical reactions involved and their potential rate-limiting steps, were investigated. In this work, pseudo-first-order (Eq. 3), pseudo-second-order model (Eq. 4), and intraparticle diffusion (Eq. 5) were used to test the experimental data.
(3)
(4)
(5)
In these equations, t is the contact time (h), qt and qe are the amounts of metal ions absorbed at time t and at equilibrium (mg/g), respectively, and k1 (h-1), k2 (g/mg∙h), and ki (mg/h1/2 g) are the rate constants of pseudo-first-order, pseudo-second-order, and intraparticle diffusion models, respectively. C (mg/g) is the constant that provides information about the thickness of the boundary layer. For example, when C is 0, intraparticle diffusion is the rate-controlling step, and when C is not equal to 0, intra-particle diffusion is not the only rate-controlling step.
Table 2. Kinetic Parameters for the Adsorption of Cd(II) and Hg(II) on CRB
The kinetic rate constants k1, k2, and qe(cal) were obtained by plotting ln(qe–qt) and t/qt against t. ki and C were computed from the slope and intercept of the plots of qt versus t1/2. The estimated parameters are presented in Table 2. The calculated equilibrium adsorption capacities (qe,(cal)) of Hg(II) and Cd(II) on MRB were much closer to the experimental values (qe,(exp)) for the pseudo-second-order adsorption model than the pseudo-first-order model. The R2 correlation coefficients of pseudo-second-order kinetic models were relatively high (> 0.999). Therefore, the adsorption systems of Hg(II) and Cd(II) on MRB all obeyed pseudo-second-order kinetics. The plots of Hg(II) and Cd(II) adsorbed (qt) versus the square root of time (t1/2) presented two straight lines, but these lines did not pass through the origin. These results implied that adsorption involved intraparticle diffusion, but it was not the only rate-controlling step. The positive values of C in this work suggested that a chemisorption mechanism might have played a significant role in biosorption (Anagnostopoulos et al. 2012).
Effect of concentration and isotherm model
Solutions of different initial Hg(II)/Cd(II) concentrations (50 to 300 mg/L) were used to investigate the adsorption isotherms after 5 h with 1 g/L MRB at 25 °C and pH 5.0 for Hg(II) or pH 6.0 for Cd(II). As shown in Fig. 8, the equilibrium adsorption capacities of MRB increased with increasing the equilibrium concentration of Hg(II)/Cd(II) and reached almost a constant value in the higher equilibrium concentration range. The increase in the adsorption capacity of MRB with regard to Hg(II)/Cd(II) concentration is probably due to the higher driving force to overcome the mass transfer resistance of metal ions between the solid and aqueous phases. The greater number of metal ions adsorbed on MRB, the larger repulsive electrostatic interactions involved between the absorbed metal ions and the metal ions in solution. Therefore, the increase of adsorption capacities is reduced in the higher equilibrium concentration range, and the saturation adsorption will be obtained until equilibrium is attained.
Fig. 8. Adsorption isotherms of MRB
To analyze the interactive behaviors between MRB and metal ions, two well-known and widely used adsorption isotherm models—Langmuir (Eq.(6)) and Freundlich (Eq.(7))—were adopted to describe the nature of adsorption using the equilibrium data.
(6)
(7)
where Ce (mg/L) is the metal ion concentration in solution at equilibrium and qe is the adsorption capacity (mg/g) at equilibrium. The constants qmax and b are the theoretical maximum monolayer adsorption capacity of the adsorbent (mg/g) and Langmuir constant relating the free energy of adsorption (L/mg), respectively. KF and n are Freundlich constants representing the multilayer adsorption capacity ((mg/g)(mg∙L)1/n) and empirical parameter related to the intensity of adsorption, respectively. The constant values qmax and b were determined from the slopes and intercepts of the linear plots of Ce/qe versus Ce; n and KF were determined from the plot of logqeversus logCe. The fitted constants and regression coefficients (R2) are represented in Table 3.
Table 3. Isotherm Parameters for the Adsorption of Cd(II) and Hg(II) on MRB
Based on the coefficient of determination values (R2), the equilibrium data of Cd(II) and Hg(II) onto MRB were best described by the Langmuir isotherm equation rather than the Freundlich model. In the Langmuir isotherm, the calculated maximum adsorption capacities (qmax(cal)) (106.4 mg/g and 163.9 mg/g for Cd(II) for Hg(II) ions, respectively) were very close to the experimental values. These results indicated that the adsorption of Cd(II) and Hg(II) onto MRB are typical monolayer adsorptions of the Langmuir type. In the Langmuir equation, a dimensionless constant known as the separation factor (RL) is used to evaluate the favorability of adsorption, as defined by Eq. 8,
(8)
where b is the Langmuir constant (L/mg) and C0 is the initial metal ion concentration in the solution (mg/L). An RL value greater than 1 indicates unfavorable adsorption; RL equal to 1 reflects linear adsorption. An RL between 0 and 1 specifies favorable adsorption, and RL less than 0 indicates irreversible adsorption (Gonzalez and Pliego-Cuervo 2014). In the current study, the calculated RL values for the adsorption of Cd(II) and Hg(II) at 25 °C ranged from 0.101 to 0.018 and 0.046 to 0.008, respectively, for the initial metal ion concentration of 50 to 300 mg/g. These findings indicate that Cd(II) and Hg(II) adsorption on MRB were favorable. Similarly, the Freundlich constant values of n were 5.1 and 4.8 for Cd(II) and Hg(II), respectively, and the KFvalues were 40.1 and 65.7, respectively. This data indicated that adsorption was favorable.
Effect of temperature
The variation of Cd(II) and Hg(II) sorption was investigated from 25 °C to 65 °C under the optimized conditions (Fig. 9). The adsorption capacity of MRB increased for both Cd(II) and Hg(II) with increasing temperature. However, the increments were not noticeable (only about 20 mg/g) for the temperature range investigated. These results indicated that Cd(II) and Hg(II) adsorption on MRB is endothermic but not sensitive to temperature. This data could greatly increase the convenience in practical application. Similar behavior has been reported previously (Doskočil and Pekař 2012; Khoramzadeh et al. 2013). Because temperature had a negligible effect on adsorption, subsequent experiments were performed at ambient temperature (25 °C).
Fig. 9. Effect of temperature on Hg(II)/Cd(II) adsorption (initial metal ion concentration: 300 mg/L, contact time: 5 h, pH: 6.0 for Cd(II) and 5.0 for Hg(II), respectively)
Effect of ionic strength on adsorption
Industrial wastewater contains not only heavy metal pollutants but also various salts in a wide range of concentrations depending on the source. These coexisting ions often inhibit the removal of heavy metals because they compete for binding on adsorbents and decrease in the activity of heavy metal ions (Tran et al. 2015). The effects of the common salts, NaCl and KNO3 on the decontamination of Hg(II) and Cd(II) by MRB were examined (Fig. 10). Increasing the ionic strength from 0 to 1 mol/L reduced the adsorption capacities of Hg(II) and Cd(II) on MRB. The negative effects of NaCl were much more obvious than those of KNO3, especially the concentration was greater than 0.1 M. The decrease in adsorption with increasing electrolyte concentration reflects the increasing competition between mercury/cadmium species and K+/Na+ions. Secondly, the activity of heavy metal ions was reduced by enhanced electrostatic repulsion and a greater tendency to form metal-anion complexes such as CdOH+, CdCl+, CdNO3+, HgCl3–, and HgCl42-. As reported previously (Kim and Lee 2014; Hu et al. 2014), the coordinating ligands of Cl are much greater than NO3– with Hg(II) and Cd(II), which may explain the more negative effect of NaCl.
Fig. 10. Effect of ionic strength on Hg(II)/Cd(II) adsorption of MRB (initial metal ion concentration: 300 mg/L, contact time: 5 h, pH: 6.0 for Cd(II) and 5.0 for Hg(II), temperature: 25 °C, respectively)
Desorption and regeneration studies
In industrial wastewater treatment, the adsorption and desorption of metal ions coupled with the regeneration and recovery of spent adsorbents are crucially important to make the process more attractive and economical. As the adsorption behavior of Cd(II) and Hg(II) onto MRB are highly dependent on pH, the Cd(II)/Hg(II)-adsorbed MRB was eluted by adjusting the solution pH with 0.1 M HCl.
Fig. 11. Cd(II) and Hg(II) absorbed by regenerated MRB for six adsorption-desorption cycles (initial metal ion concentration: 300 mg/L, contact time: 5 h, pH: 6.0 for Cd(II) and 5.0 for Hg(II), temperature: 25 °C, respectively)
The regenerated adsorbent was recovered by neutralizing the carboxyl groups and the ammonium salt on the surface of MRB using 0.1 M NaOH. As shown in Fig. 11, the adsorption capacities decreased gradually with increasing cycle number, but MRB still retained considerable adsorption capacity even after six consecutive adsorption-desorption cycles. The adsorption of Cd(II) and Hg(II) on recovered MRB was decreased by only 18.0% and 18.7%, respectively. These results suggested that MRB is an economical and efficient adsorbent for practical applications in wastewater containing Cd(II) and Hg(II) ions.
CONCLUSIONS
- Rice bran (RB) modified with iminodiacetate chelating ligands was an excellent adsorbent for Hg(II) and Cd(II). The maximum adsorption of Hg(II) and Cd(II) on modified rice bran (MRB) reached 163.9 and 106.4 mg/g at 298 K, respectively.
- Ionic strength, but not temperature, had a significant effect on Hg(II)/Cd(II) uptake. The experimental data fit both the pseudo-second-order kinetic model and the Langmuir isothermal model. EDS and XPS studies showed that the adsorption mechanisms were chelation and ion exchange between Hg(II)/Cd(II) and MRB.
- The adsorption capacity of regenerated MRB remained relatively high even after six cycles of regeneration.
ACKNOWLEDGMENTS
This project was financially supported by the Science and Technology Foundation of the Education Department of Sichuan Province, China (16ZB0315).
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Article submitted: April 4, 2016; Peer review completed: June 12, 2016; Revised version received and accepted: June 28, 2016; Published: July 12, 2016.
DOI: 10.15376/biores.11.3.7145-7161